Cosmetic products that contain water also need to include preservatives in order to kill microorganisms and prevent the growth of bacteria, mold or yeast. Broad-spectrum preservatives—which are effective against bacteria, mold and yeast—are recommended.

From parabens to ultra derivatives and isothiazolones, what are the commonly used preservatives? What are the natural alternatives and why are they controversial known antioxidants include:

1. Parabens. These are the most commonly used preservatives, and they are very effective against most bacteria and fungi. No direct evidence of a causal link between parabens and cancer has been discovered; however, there is research which shows that some parabens can have an estrogen-like effect.

2. Urea derivatives. These are effective against all types of microbes, including bacteria, fungi, and mold. They work in a wider pH range than parabens.

3. Phenol derivatives. They can be effective against a range of microbes, yet are not as effective as parabens and urea derivatives.

4. Alcohols. Benzyl alcohols, such as dichlorobenzyl alcohol, are effective preservatives. Like phenoxyethanol, benzyl alcohol should not be used in formulas that contain non-ionic surfactants because they may compromise its efficacy.

5. Quats. Quaternary ammonium compounds are typically used as hair conditioning agents. Many of them can also kill microbes.

6. Isothiazolones. They are effective at low use levels and are compatible in a variety of formulation systems.

However, due to the various concerns surrounding the use of these common synthetic preservatives and the increasing consumer demand for natural cosmetics, the cosmetic industry is continuously searching for new natural compounds that are able to limit the growth of microorganisms. So far, a few effective natural preservatives have found their way to the cosmetic market.

Natural preservatives come from organic matter and are often derived from plants, animals, fungi, and algae. Salt and sugar are both examples of natural preservatives, which also include plant extracts, chitosan, bacteriocins, bioactive peptides, and essentials oils. Natural preservatives have become an ostensibly better, safer alternative to synthetic preservatives. Since these preservatives are derived from plants, they do not exhibit the negative side effects of artificial preservatives.

Yet there has been some controversy surrounding the use of natural preservatives since most of the effective natural preservatives are also well-known antioxidants. Preservatives prevent microorganisms from growing and spoiling cosmetic products. Antioxidants slow down the process of oxidation and thereby prevent oils and other components from going rancid. As such, both preservatives and antioxidants are used to extend the shelf life of cosmetic products, but there are those in the industry who believe that these known antioxidants cannot function as preservatives.

However, researchers have proven that some antioxidants display antimicrobial properties which are effective against bacteria, fungi, protozoa and viruses, though it should be noted that most studies on the antibacterial effects of selected antioxidants have been on pathogens and indicator organisms, such as Salmonella spp., Staphylococcus aureus, Escherichia coli, Vibrio parahemolyticus, Clostridium perfringens and Clostridium botulinum.

Antioxidants such as polyphenols, vitamins, and carotenoids are the organic compounds commonly produced by plants in their living defense system; they are secondary metabolites (phytochemicals) synthesized by plants. Natural polyphenols are known to act as antioxidative, antibacterial, anticancer, anti-inflammation, and antiviral agents. As such, these phytochemicals may be able to replace synthetic antioxidants, some of which are suspected of causing damage to human health.

Known primarily as antioxidants, phenolic acids and flavonoids can directly affect bacterial growth and hinder their pathogenic activities, though the exact mechanisms by which these antioxidants act as antibacterial agents are still not fully understood.

Some research shows that these antioxidants’ antibacterial activity may involve three basic mechanisms: outer membrane permeability, cytoplasm leakage and inhibition of nucleic acid formation. Some demonstrate that the interaction of polyphenols with nonspecific forces such as hydrogen bonding, hydrophobic effects, lipophilic forces, and covalent bond formation is responsible for microbial adhesion.

The antibacterial activity of polyphenols may also be attributed to their capacity to chelate iron, since unchelated iron is vital for the survival of almost all bacteria. Polyphenols can also rupture bacteria cell walls, increase the permeability of cytoplasm membranes, and release lipopolysaccharides.

However, more recent studies on the antibacterial mechanisms of natural antioxidants have revealed that they are not as straightforward as previously assumed. For example, epigallocatechin gallate only induces clumping of FabG enzymes and does not display such an effect on other enzymes. In addition, it was discovered that flavonoids cause aggregation of bacterial cells. Clumping of bacterial cells due to treatment with flavonoids causes a reduction in the surface area of the bacterial population, which in turn reduces the oxygen consumption by bacteria. Reduction in the surface area of cells also decreases their nutritional uptake.

Although antioxidants tend to prevent bacterial growth at a slower rate, their actions are consistent and beneficial to human health, regardless of if they act directly or indirectly. As such, careful and continuous work to establish antibacterial profiles for these natural antioxidants can help institute these natural products as safer solutions—with negligible toxicity and less risk of bacterial resistance—against bacterial infections.

The downside of using natural preservatives is that their availability is limited. Naturally-sourced ingredients are sometimes challenging to acquire. Concerted R&D efforts by the cosmetic ingredient industry to develop advanced technologies to extract these molecules will hopefully improve this situation.

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Most common nonionic emulsifiers are ethoxylated alcohols, acids, or oils. The other common nonionic emulsifiers are polyol types, such as sugar ethers (e.g. alkyl glycosides) or sugar esters (e.g. sucrose esters, Figure 1, and sorbitan esters).

Compared to their ethoxylated counterparts, polyol types of nonionic emulsifiers do not lose water solubility with increasing temperature.

Furthermore, sugar esters display many other proven advantages, including robust emulsifying capacity, great stability under stress, and good consumer health.

Sugar esters also demonstrate better biodegradability versus their ether counterparts.

A representative structure for sucrose esters (left).

Sorbitan esters are well-established products mainly used as leather and textile auxiliaries or as emulsifiers for food; their annual global consumption is about 20,000 tons.

Sucrose esters are relatively hydrophobic products with a market size of approximately 5,000 tons per year; they are used as emulsifiers for food and cosmetics.

In comparison, alkyl polyglycosides have a total market size of about 100,000 tons per year and are mainly used for cosmetic, manual dishwashing, and detergent applications.

Due to their natural raw material origin, good performance, and mildness, alkyl polyglycosides are the most successful sugar-based surfactants nowadays.
Sugar esters are nontoxic, compatible with skin, low or non-irritant, and generated from renewable resources, making them highly attractive to the cosmetic industry.

Although the melting point of sugar is high, the melting point of sugar esters can vary between 40 and 80 °C, depending on the degree of esterification.

Moreover, the length of the hydrophobic chain and the size of the hydrophilic group on sugar esters provide a wide range of HLB values. (High HLB values represent water-soluble surfactants, and low HLB values represent oil-soluble emulsifiers; the values can range between 0 and 16.)

Longer fatty acid chains and higher degrees of esterification result in lower HLB values.

For instance, if all eight hydroxyl groups in sucrose were to be esterified, the product would be highly hydrophobic and soluble in oil.

However, partial esterification would generate an amphiphilic sugar ester, which could be used as an emulsifier in the food, cosmetic, and pharmaceutical industries.

Amphiphilic sugar esters can form thermodynamically stable molecular aggregates called micelles in an aqueous solution. The sugar ester’s molecular structure and the experimental conditions influence the critical micelle concentration (CMC) value, which is the specific sugar ester concentration at which micelles start forming.

In general, increasing the alkyl chain length decreases the CMC value. The CMC value is highly relevant as it represents the amount of sugar ester required to solubilize hydrophobic compounds in water. Adding more sugar ester after the CMC is reached yields more micelles and promotes the growth of aggregates.

Sugar esters are capable of effectively reducing the surface tension of water, which is highly valuable for many industry applications. For instance, coconut milk, as an emulsion, is not stable, and its phases separate rapidly. Coconut milk emulsions have larger droplet sizes and lack good emulsifiers, which lead to unfavourable contact between water and oil.

Using emulsifiers capable of reducing surface tension improves emulsion stability. The greater the ability of an emulsifier to reduce surface tension, the greater the emulsion stability formed in coconut milk.

Research conducted on a series of sugar esters including fructose, sucrose, and lactose esters demonstrated that lactose esters are the best molecules among the group; they reduced surface tension from 52.0 to 38.0 N/m, displaying an emulsification index of 54.1%.

Decreasing surface tension is also relevant to generating foods which consist of foam, because the air/water interfacial area can be enlarged by decreasing surface tension.

The biodegradability of sugar esters is also a relevant factor as it helps determine if the concentration of sugar esters remains below levels that are detrimental to the environment. For example, household cleaning products, which contain surface active molecules such as sugar esters, are normally disposed of through the drain.

Therefore, the biodegradability of sugar esters becomes of interest as the detergent surfactants’ residues are linked to foaming incidents in sewage treatment plants.

However, the biodegradability of nonionic surfactants is more difficult to predict because of the wide variety of molecular structures and the lack of a common functional group.

In general, sugar esters are known for their excellent biodegradability and do not generate environmental pollution. Researchers have found that changes to the sugar head group size, the alkyl chain length, and the number of alkyl chains attached to one sugar head group had no significant effect on the biodegradability of sugar esters.

The only structural variation that did have a significant effect was the presence of side groups on the alkyl chain adjacent to the ester bond. These branched groups decreased the rate and extent of the biodegradation of the sugar esters.

This effect was greatest in the presence of an α-sulfonyl group. When α-ethyl or α-methyl groups were attached, the inhibitory effect was weaker but still significant.

In summary, sugar esters are relevant as emulsifiers to the scientific and industrial communities.

However, many questions remain for scientific research and applied studies regarding the nature of these compounds.

Their wide range of possible HLB values provides room for the design and synthesis of novel sugar esters for food, pharmaceutical, and cosmetic applications.

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Our skin protects against trauma and bacterial invasion, prevents dehydration, regulates body temperature, and provides the ability to sense temperature and touch.

The outmost epidermis and the associated stratum corneum form a waterproof barrier and contain pigment‐producing melanocytes.

The next layer, the dermis, contains fibrous and elastic tissues, hair follicles, nerve fibers, and sweat glands. The fibrous and elastic tissues provide skin its strength and flexibility.

Under the dermis is a layer of subcutaneous fat and connective tissues. Without proper preventative measures, environmental factors such as sunlight and air pollution will damage the skin, resulting in various signs of premature skin ageing.

Premature skin ageing can arise from exposure to many environmental factors such as ultraviolet (UV) radiation, high‐energy visible (HEV) light in the blue spectrum, infrared radiation, and environmental pollution.

These extrinsic factors cause the generation of reactive oxygen species (ROS) which initiate photoageing and DNA damage. DNA damage often causes skin cancers.

Novel skin protection strategies targeting a variety of environmental and energy aggressors, including UV radiation, HEV light in the blue spectrum, and infrared radiation, can be formulated by taking a page out of nature’s book.

Plants produce powerful molecules with robust energy absorbing abilities as well as effective mediation abilities against oxidation stresses and pollution stresses.

Identifying the right plant components and sustainably harnessing these functional phyto-compounds for skin protection purposes presents vast opportunities for the skincare industry.

All plants have antioxidant potential. This is because chloroplasts and mitochondria are the two main sites of ROS generation in plant cells.

ROS are also involved in maintaining a fine balance between energy linked functions and the level of ROS generation. Within the plant cell, ROS generation occurs mainly at photosystem I and II (PS I and PS II) of the chloroplasts, as well as complex I, ubiquinone, and complex III of the mitochondrial electron transport chain (ETC).

Under normal physiological conditions, there are electron slippages from PS I and PS II of the chloroplasts and the membrane of the mitochondrial ETC. These electrons later react with molecular oxygen to produce ROS.

Reactive nitrogen species (RNS) are also formed in various compartments of the cell, including the chloroplasts and mitochondria. These free radicals are constantly produced in the subcellular organelles of living cells.

Most of the time, the production of free radicals is genetically planned since they function as signalling molecules.

However, overproduction of free radicals can happen and damage biomolecules such as DNA, proteins, and lipids. Plants have efficient enzymatic and non-enzymatic antioxidant defence systems to avoid the toxic effects of free radicals.

The enzymatic systems include SOD, catalase (CAT), and glutathione reductase (GR). The non-enzymatic systems consist of low molecular weight antioxidants such as ascorbic acid, glutathione, proline, carotenoids, phenolic acids, flavonoids, etc., and high molecular weight secondary metabolites such as tannins.

There are two main reasons for the synthesis and accumulation of these non-enzymatic antioxidants by plants.

First, the genetic makeup of plants imparts them with an innate ability to synthesize a wide variety of phytochemicals to perform their normal physiological functions or protect themselves from herbivores and microbial pathogens.

Another reason for the synthesis of these phytochemicals is the natural tendency of plants to respond to environmental stress conditions.

Plants synthesize low molecular weight antioxidants as redox buffers to interact with numerous cellular components and to influence plant growth and development.

This is achieved by modulating processes from mitosis and cell elongation to senescence and death.

Plants also synthesize and accumulate a range of low and high molecular weight secondary metabolites. These secondary metabolites play important roles in ROS metabolism and avoiding uncontrolled oxidation of essential biomolecules.

Under normal physiological conditions, increases in free radical production are relatively small.

Common housekeeping antioxidant capacity is sufficient to maintain redox homeostasis.

However, the metabolic pathways of plants are sensitive to abiotic and biotic stress conditions such as high light intensity, heat, drought, anoxic conditions, and pathogen attack.

It is known that there is an approximately 3-to-10-fold increase in free radical production under stress conditions.

Some secondary antioxidant metabolites occur constitutively, while others are formed in response to biotic and abiotic stresses; the accumulation of phenolic compounds and enhancement of phenylpropanoid metabolism were observed under different environmental stress conditions.

For example, synthesis of flavonoids is known to be induced by UV stress, heavy metals toxicity, or low temperature and low nutrient conditions. This might explain flavonoids’ UV-absorbing, radical scavenging, and metal-chelating abilities.

UVB radiation was found to affect the production of various high molecular secondary metabolites such as tannins and lignin. In addition, plants growing in tropical and high-altitude conditions have been shown to contain a higher level of flavonoids than those growing in temperate conditions, likely due to an overexposure to light or UV radiation.

By thoroughly understanding the mechanisms of plant antioxidant generation, we can identify the correct plant or plant parts from which efficacious molecules can be harvested to provide both light shielding and radical- or pollutant-mediation functionalities.

The fundamental causes of plant antioxidant formation give us clues on where to search for these species; we must also research suitable ways to acquire these molecules sustainably and present them at a meaningful level to the skincare industry.

This is the key first step to achieving effective, natural skin protection.

Did you know that one of the themes at in-cosmetics Asia in Bangkok (5-7 Nov) is Skin Screen? Find out more.

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To incorporate nature and promote sustainability in the cosmetics industry, many companies have used natural and organic extracts from plants, flowers, and seeds as formulation ingredients. However, consumers are increasingly demanding even more sustainable products. As such, many in the industry have started to investigate upcycling materials that can be reused for cosmetics and that would have otherwise been discarded. These materials include coffee grounds, olive oil wastes, and fruit peels. By maximizing the usage of natural resources, we can help reduce the cosmetic industry’s impact on the environment.

Traditionally, recycling in the cosmetics industry has been focused on packaging materials. For most cosmetic products, packaging is a necessity. This is because many formulas are sold in water-based forms and, generally, in a liquid state. These formulas are susceptible to oxidation as well as contamination by bacteria, which makes airtight packaging essential to ensure the product’s safety during its intended shelf life. However, not all packaging materials are made equal, and not all of them are strictly necessary. Due to its widespread usage, plastic attracts a lot of negative attention with respect to its environmental impacts. As such, it is now generally agreed that the end of life for all plastic packaging materials should be clearly planned out before they are adopted for usage. In France, the Agec law stipulates a total ban on single-use plastic packaging by 2040, with an initial target to recycle all plastic packaging by 2025. Consequently, from March 2023, the cosmetics industry started to print the new Triman recycling logo, along with instructions on how to recycle it, on all plastic packaging materials.

In contrast, upcycling is the process of utilizing by-products, waste materials or discarded components for new functions and transforming them into new products. Food and beverage processing by-products represent a major source of materials that can be upcycled into beauty products, particularly natural and organic cosmetics.  Food and beverage by-products often contain ungarnered food-grade ingredients that have many beneficial properties for the skin. Since it recovers elements of agricultural production destined to be destroyed or devalued, upcycling is very conducive to environmental sustainability. It maximizes usage of existing products, thereby eliminating the consumption of additional resources.

According to a 2011 report by the Swedish Institute for Food and Biotechnology, roughly one-third of the food produced for human consumption globally is lost or wasted. A large percentage of this waste happens during food material processing. To prepare food stuff and transform it into a consumer-acceptable state, a large portion of the food and beverage components have to be discarded, often for taste and aesthetic purposes. However, this discard can be upcycled into natural cosmetics, benefitting both the cosmetic industry and the environment.

As validated by many pieces of academic research and cosmetic company practices, food waste is a valuable source of materials for the creation of natural and organic upcycled cosmetics. Many collaborations between food producers and cosmetic companies have catalyzed the upcycling of food waste stream materials with precious active functions for skin applications. In the same vein, cosmetic raw material companies have also started to focus their attention on developing plant-based ingredients obtained from various food processing by-products.

Examples of food waste that can be used to develop upcycled natural and organic cosmetics include olive oil processing wastes in the forms of water and solids, citrus extracts from the likes of orange and lemon peels, waste from brewing coffee and cacao, waste from making tea, etc. Dried fruit seeds discarded from the juice and jam industries can be cold pressed and transformed into essential oils. Extracts of grape waste from wine production can be used for their pigments and active ingredients. However, the successful recovery of these ingredients for cosmetic applications demands research and developmental efforts. Often these materials need to be freshly processed into cosmetic ingredients. They also need to be freed from germs and other possible contaminations.

Increased investment by the cosmetic industry, which is reinforced by consumers’ growing appetite for circular beauty products — products that are “good for people and planet” —, will keep the development of recycled and upcycled cosmetic technology highly active in the coming years. Explorations of additional waste material streams as well as advanced recovery technologies will bring novel ingredients and functions to the beauty industry while reducing its environmental footprint.

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Molecular mesophase behavior originates from two basic concepts. The first concept is the “closest packing” of simple-shaped mesogenic compounds. In these shape-driven mesophases, rod-like molecules are packed in an ordered liquid crystal phase. This ordered liquid crystal phase is energetically more stable than the randomly-packed isotropic phase. Most of the classical liquid crystals, notably the compounds used for traditional display devices, belong to this class: the monophilic liquid crystal.

The second concept is the “microphase separation” of two incompatible parts within one molecule. The most common incompatible characters are hydrophilicity and lipophilicity coexisting in one molecule, which is thereby amphiphilic. Amphiphilic molecules can form amphiphilic liquid crystals.

For example, glycolipids consisting of two incompatible parts—a polar sugar residue which is hydrophilic and a nonpolar paraffin chain which is lipophilic—display double melting behavior, which is characteristic of liquid crystal formation. The two incompatible parts of this molecule are connected covalently and cannot separate macroscopically. Instead, the molecules are forced to microscopically separate into a sequence of hydrophilic and lipophilic layers. These layers have the same geometry as the smectic A phase of the monophilic liquid crystals; as such, their physical properties are also the same. While monophilic liquid crystals and amphiphilic liquid crystals share many properties associated with their liquid crystallinity (e.g. textures of the smectic A phases, X-ray patterns in the mesophases, etc.), the chemical requirements for the formation of these two types of liquid crystals are completely different.

Many amphiphilic molecules can form mesophases both in melt (thermotropic liquid crystals) and in solution (lyotropic liquid crystals). Some researchers use the term amphotropic liquid crystals to express the ability of a compound to form both thermotropic and lyotropic mesophases.

Amphiphilic molecules with mesophases exhibit many useful properties for technological applications. Common applications include coating medicines to control their delivery, stabilizing hydrocarbon foam, and serving as primary solvents for topical medication.

Thermotropic properties of amphiphilic molecules are determined by several factors, though the amphiphilic character is the most important one.  Some of these factors are discussed below:

• A mesogen must consist of at least two different molecular parts which do not like to mix with each other; these parts can be hydrophilic and lipophilic, hydrophilic and fluorinated, siloxane and hydrophilic, etc. The contrast in attraction forces between these two molecular parts is one of the main driving forces for the formation of these mesophases.
• The sizes of the molecular parts are If we only consider the difference in hydrophilicity, methanol (CH₃OH) looks like an amphiphilic molecule. However, the polar and nonpolar moieties in methanol are too small. As such, the ratio between volume and surface of the microphases is too small to display any amphiphilicity.
• The balance between hydrophilicity and hydrophobicity in the molecular structure should be appropriately adjusted. There is an optimum lipid chain number for a given polar head group. For example, polar monosaccharides with 4 OH groups attain their highest clearing temperatures when attached to 16-carbon alkyl lipophilic chains.
• The relative positions of the two different parts within the molecule is important. Molecular fluidity such as that in the liquid phase (e.g. translations, rotations) should be possible in a mesophase, yet the two different molecular parts should be clearly separated. This is possible with rod-like or rope-like molecules, which form lamellar arrangements. It is also possible with wedge-shaped molecules, which form columnar phases. Many other possible arrangements of the two different parts may cause amphiphilicity but will not yield the formation of liquid crystals.
• There should be flexible parts of the molecule. For example, a mesogenic compound with only phenyl rings and a sugar moiety is hindered in the formation of thermotropic mesophases.
• We now know that the exact stereochemistry of the molecule influences its thermotropic properties; however, not all of the details have been thoroughly researched yet.

Lyotropic liquid crystal behavior is similar to that of thermotropic liquid crystals. However, its liquid crystal phase changes continuously with changes in the amount of solvent as well as temperature; therefore, a lyotropic system has one more variational vector than a thermotropic system. In addition, lyotropic liquid crystal behavior can be influenced by the types of solvents and their concentrations.

With lyotropic liquid crystals, the textures seen under a microscope with crossed polarized light are normally more complicated. The cubic mesophases tend to only display a “black” texture. As such, while microscopy is suitable for observing the thermotropic mesophases, X-ray analysis is commonly used to study lyotropic properties, especially the cubic phases.

The relationship between molecular shapes and their expected mesophase behavior is illustrated below, though, as previously discussed, the actual phase of a liquid crystal is further determined by its temperature and solvent environment.

Elongated amphiphiles with structures like 1, 2, and 3 tend to form a lamellar phase. Forked (4) or pie shaped (5) mesogens will most likely form hexagonal columnar phases. Most of the non-linear structures like 6 also form hexagonal columns. Banana-shaped amphiphiles (7) and elongated forks (8) are borderline between lamellar and columnar phases, and their exact textures are determined by environmental conditions. They may also manifest bicontinuous cubic phases.  The cone-shape molecule 9 normally displays discontinuous cubic phases. Star-like substituted molecules such as 10 will prefer columnar phases. If the star structure is dissymmetric as in 11, rectangular and tetragonal columnar phases are expected.

Lyotropic liquid crystals can form when compounds containing lipophilic (fat-loving) and hydrophilic (water-loving) structures known as amphiphiles dissolve into a solution. They are often present in water and oil interfaces and help lower the interface surface tension. Lyotropic liquid crystals thereby contribute to the formation of emulsions, which are the essence of most cosmetic formulations.

The micro-phase segregation of two incompatible parts in the same molecule results in different types of solvent-induced, extended anisotropic arrangements; the characteristics of these arrangements depend on the volume balances between the hydrophilic part and the lipophilic part. These anisotropic arrangements generate a long-range phase order, with the solvent molecules filling the space around the compounds to provide fluidity to the system.

Lyotropic liquid crystals have a unique ability to induce a variety of different phases at different concentrations. As the concentration of amphiphilic molecules increases, several different types of lyotropic liquid crystal structures occur in solution. Each of these different liquid crystal structures has a different extent of molecular ordering within the solvent matrix, from spherical micelles to larger cylinders, aligned cylinders, and bilayer and multiwall aggregates.

In most lyotropic liquid crystal systems, aggregation occurs only when the concentration of the amphiphile exceeds a critical concentration, variably referred to as the critical micelle concentration (CMC) or the critical aggregation concentration (CAC).

At very low amphiphile concentrations, the molecules are dispersed randomly and without any ordering. At slightly higher concentrations just above the CMC, self-assembled amphiphile aggregates exist as independent entities in equilibrium with monomeric amphiphiles in solution, but they exhibit no long-range orientational or positional order; the phases are isotropic and not liquid crystalline. These dispersions are referred to as micellar solutions, denoted by the symbol L1. The spherical aggregates within are known as micelles.

At higher concentrations, the assemblies become ordered. And when the concentration of amphiphile in water is increased beyond the point where the micellar aggregates are forced to be arranged regularly in space, lyotropic liquid crystalline phases form. The concentrations at which this formation occurs depends heavily on the chemical compositions of the amphiphiles, but it normally ranges from 5-30% by weight.

Liquid crystal structures are known to impact the stability and skin feel of cosmetic formulations. Therefore, they are fundamental to the production of physically stable emulsions that display desirable rheological profiles for pleasant skin application. We will detail the relationship between liquid crystal structures, emulsion stability, and skin feel in our future contributions; in this article, we will simply explain in principle how best to maximize the formation of liquid crystals when formulating an emulsion cosmetic product.

A typical phase diagram for an amphiphile emulsifier (as a function of concentration in water and temperature) is shown in Figure 1.  It is known that the lamellar structure, denoted by the symbol Lα or Lβ, is particularly beneficial to the stability and skin feel of the resulting emulsion (Figure 2). However, with most emulsifier molecules, the lamellar liquid crystal structure only forms at relatively high concentrations (> 5%) in water, as illustrated in Figure 1. Such high levels of emulsifier molecules severely limit the formulation flexibility of the overall product; this often causes cosmetic products to have poor performance or undesirable skin feel, not to mention the exorbitant cost usually associated with adding high amounts of emulsifiers.

Figure 1 – A simplified phase diagram of an emulsifier molecule.

Figure 2. Illustration of an emulsifier lamellar liquid crystal structure.

To overcome this challenge, a stepwise formulation make-up process can be used.  First, a stable intermediate is made with a concentration of the emulsifier molecule that is high enough to ensure the production of large quantities of lamellar liquid crystals. Then, appropriate amounts of oil and water are added to the intermediate to reach the target emulsion composition. This strategy is illustrated using a ternary isothermal phase diagram in Figure 3.

Figure 3. Ternary isothermal phase diagram that illustrates the stepwise formulation strategy.

In Figure 3, to reach the target composition (C) with eventual emulsifier loading of < 5%, we start with an emulsifier water solution with > 60% emulsifier molecule (A).  Oil is then added to this mixture to reach the intermediate (B). It is critical that the intermediate is within the lamellar phase so that the maximum amount of lamellar liquid crystals is formed. Finally, appropriate amounts of water and oil are added to the intermediate to reach the target composition (C).

To make this strategy work, the emulsifier molecules need to have the correct structure to form strong lamellar liquid crystals. Furthermore, the oil chosen should have a structure that does not negatively impact the lamellar liquid crystal arrangement. Lastly, the addition of water should ideally just increase the inter-layer space in the overall lamellar arrangement. The chemistry needed to satisfy all these requirements will be the topic of our future articles.

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At the onset of every summer, many organizations publish their newest lists of the best sunscreens for that season. These lists tend not to agree with one another, and so the associated debates and disagreements continue, one season after another.

The root cause of this situation appears to be the less-than-clear definition of sunscreen performance, as well as its related testing methods.

Accordingly, let us attempt to explain this situation in layman’s terms and in the hopes of advancing better ideas from dedicated professionals in this field.

Let’s start by randomly quoting one of the latest best sunscreen lists: In CNN’s “The best sunscreens in 2023 tested by editors”, it is stated, “we didn’t test how well these sunscreens protect skin from the sun — for that, we relied on information from the US Food and Drug Administration and the experts we talked to.

Too many variables (skin type, time of day, the sun’s intensity, etc.) made it impossible to accurately measure efficacy in our real-world testing.

Rather, we had multiple testers with different skin types and tones test them for other variables such as feel, appearance on the skin, smell, ease of application and more.”

In other words, the performance and effectiveness of sunscreen, which are the primary reasons for using the product, have become so confusing that most people avoid the topic altogether.

Instead, they focus only on the sensory factors. Of course, sunscreens should be pleasant to wear so that people are willing to apply them. However, sensory factors should not replace effectiveness in the judgments of product quality.

We will focus on the performance aspect of sunscreen in our discussion here.

The metric most often used when describing sunscreen performance is SPF, the Sunscreen Performance Factor, which is a measurement of how long it takes for UV rays to hit and damage your skin. For example, SPF 30 means that when you wear this sunscreen, it will take you 30 times longer to burn compared to when you are not wearing any sunscreen at all.

Essentially, the sunscreen industry has adopted a performance metric that measures the UV ray dosage that reaches skin with sunscreen applied relative to the dosage that reaches skin without sunscreen.

Consequently, there are many variables that impact the measurable performance and apparent effectiveness of sunscreen. Skin type and irradiation source are commonly mentioned, as in the CNN article from earlier.

These two factors are important, but they are relatively easy to control. The skin type of a particular consumer is fixed, and when comparing the skin’s reaction to irradiation with and without sunscreen, the same sunlight source is normally used.

The largest variable influencing sunscreen’s effectiveness on a consumer’s skin is the amount of sunscreen applied and the quality of that application.

Due to the nature of UV prevention ingredients,  consumers cannot see with the naked eye the sunscreen that they have applied to their skin.

Therefore, they cannot easily judge if they have applied enough sunscreen or applied it evenly. However, the amount and uniformity of sunscreen on skin are the primary factors that determine the effectiveness of a particular sunscreen application.

Consumers are often told to use one ounce of sunscreen on their entire body; in other words, approximately enough sunscreen to fill a shot glass. This is obviously vague and not at all quantitative.

As for the quality of sunscreen application, which determines the uniformity of coverage on skin, this is even more difficult to perceive and is rarely conveyed to consumers.

The performances of sunscreens are professionally tested before the sunscreens are put on the market. There are several standard testing methods instituted by governmental and industrial regulatory organizations, but they are essentially the same in that they all measure the relative UV ray dosage that reaches the skin with and without sunscreen applied.

In these testing methods, the type of skin (substrate), the type of light source (irradiation), the amount of sunscreen (weight per area), and the end point (skin reddening or darkening) are carefully specified.

However, the quality of application (uniformity of coverage) is not carefully controlled or monitored. The end point determination is often a visual determination and is therefore not very quantitative.

These factors, coupled with the fact that tests are often measuring the last few percentages of the total irradiation reaching skin, make accurate sunscreen performance testing a well-known challenge.

For example, to tell the difference between SPF 50 (100/2) and SPF 100 (100/1) is to measure the difference between 2% and 1% of the original UV irradiation; consequently, small variations in measurements can lead to large differences in the calculated SPF.

As such, there have been many reported conflicts regarding the reliability of sunscreen efficacy tests.

In addition, it is logically challenging to test and label the performance of a sunscreen product when the product’s true performance is only on display after it is properly applied to a skin. It’s like trying to test a tube of oil paint to determine its degree of virtuosity, even though it can only attain virtuosity after being applied to canvas by a virtuoso.

The intended objective of applying sunscreen is to prevent harmful sun rays from reaching the consumer’s skin. Therefore, the only true test of a sunscreen’s effectiveness needs to be performed in real time, on the skin, and in a real-life scenario.

While an easy and convenient way to measure the real-life performance of sunscreen is not yet widely available, we hope that the obvious need for such a technology will soon motivate its invention.

Once a relevant performance metric becomes available to everyday consumers, they will be able to easily pick the best performing products. When that happens, the sunscreen industry will be fundamentally changed.

As we have detailed in past articles, sunscreen technologies that allow for pleasant and uniform coverage on skin are essential to the creation of sunscreen products that perform well in real life.

These products will enable every consumer to easily master the art of sunscreen application, thereby becoming the obvious number one pick of every best sunscreen list.

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Every artist uses color theory. By managing the amount and consistency of applied materials such as paint, clay, bronze, pastels, chalk, ink, etc., artists create their desired visual components. The ability to precisely overlap these elements is fundamental to an artist; it allows them to transmit their desired colors and effects to a canvas. Then they use their eyes to gauge and assess the results achieved.

Similarly, to enable optimum sunscreen performance, precise and consistent application of UV-protective ingredients to the skin surface is essential to shielding skin from UV radiation. However, while the naked eye can be used to monitor the amount and quality of a paint layer on canvas, the same eye cannot easily assess sunscreen performance. This is because most sunscreen ingredients that protect against UV radiation are not visible to the naked eye. UV-sensitive cameras have been used at times to enable humans to “see” UV active materials. However, UV cameras have failed to gain popularity in common consumer applications due to the inconvenience of carrying a camera only for seeing and applying sunscreen.

In a laboratory, the ability and quality of sunscreen layers in preventing UV light from transmitting can be precisely measured. The ideal sunscreen layer should allow no transmittance in the UV region that humans are exposed to on the earth’s surface (290-400nm). At the same time, it should immediately allow 100% transmittance at 400 nm and above (see Figure 1). This is because humans can see light with wavelengths longer than 400 nm; consequently, any materials interfering with light longer than 400 nm appear to have a color. A colored sunscreen is cosmetically unacceptable to most consumers.

Figure 1: Ideal transmission spectrum of a perfect sunscreen.

However, materials with an abrupt transmittance change from 0% to 100% at 400 nm (or any other wavelength) do not exist. The ability of a material to prevent light transmission is dependent on a collection of probabilities of its molecules’ interacting with photons. These probabilities do not change from 0% to 100% for all molecules at one wavelength. Therefore, even the best UV-protective materials can only achieve a transition of transmittance from 0% to 100% right before 400 nm (see Figure 2).  The idea is to make this transition occur during as narrow a wavelength range as possible.

Figure 2: Best transmission spectrum of a sunscreen.

To ensure the product appears colorless to human eyes, the transition from 0% to 100% transmission needs to start before 400 nm in order to achieve 100% at 400 nm. The region between the wavelength where the transmission starts to increase and 400 nm makes up an unavoidable “leakage” area. In this “leakage” area, harmful UV photons are allowed to reach human skin.

Nature has a way to handle such unavoidable leakages. Chlorophylls in the thylakoid membrane of plant leaves harvest sunlight and convert the energy to sugars, making life on earth possible. Chlorophylls are abundant in plants and absorb mainly red and blue wavelengths; they reflect green light and therefore appear green to humans. The number one insult to plant photosynthesis is the sun itself. Due to unavoidable variations in sunlight intensity and wavelength, undesired irradiation sometimes reaches chlorophyll molecules, creating oxidative stresses. One of the approaches plants use to protect themselves against excessive sun irradiation is to produce antioxidants capable of capturing free radicals and electrons, thereby scavenging any leaked reactive oxygen species. These molecules and their mechanisms were detailed in an earlier article. https://connect.in-cosmetics.com/formulation/plant-antioxidants-mechanisms-and-properties/

Incorporating these antioxidative molecules into sunscreens effectively mitigates the unavoidable irradiation leakages presented by the UV-protective ingredients. There are multiple ways to incorporate antioxidative functions into sunscreen formulations. We will explain these strategies in future contributions.

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Fragrances are volatile compounds capable of readily converting from a liquid state to a vapour state at room temperature. These molecules of vapourized liquid are what we perceive as smells; lighter than air, they drift into our noses and engage our olfactory receptors.

There are many types of fragrance molecules, but we will focus on essential oils, which are abundant in the natural world.

Essential oils are aromatic compounds produced by many plants, and they are of interest to us because of their fragrances: The smells of different essential oils can alter our brain chemistry, impacting our emotional and mental states.

Essential oils are complex mixtures of terpenes and other aromatic or aliphatic compounds, produced as secondary metabolites in specialized secretory tissues of aromatic plants.

Plants produce essential oils for a variety of biological purposes: to cool off by way of oil evaporation, to attract pollinators, to make themselves unpalatable to insects and animals, to ward off disease, even to make the soil around them toxic to other plants, thus reducing competition for sunlight, moisture, and nutrients. Essential oils may be present in the plants’ flowers, leaves, roots, or bark.

After the plants have been harvested, the essential oils can be extracted using different methods such as steam distillation, expression (physical crushing of essential oil glands situated in fruit rinds or the outermost waxy layer of fruit peels), microwave-assisted extraction, solvent extraction, or enfleurage (transfer of the essential oil from flower petals to fat).

In general, the most common method is steam distillation, but expression is the method used most frequently to obtain essential oils from the peels of citrus fruits such as oranges, lemons, or bergamot.

Essential oils have found their way into everyday life, notably into foods, beverages and confectionery items; into personal care products (soaps, toothpastes, mouthwashes, deodorants, bath lotions and shampoos), perfumes, other cosmetics, and pharmaceutical formulations. Essential oils are added to make such products more attractive or to mask the taste or smell of less pleasant ingredients.

Green consumerism and the “naturals” trend have provided a fresh impetus for the use of plant essential oils, particularly in the skincare beauty industry. In addition to their fragrances and natural marketing image, essential oils also bring comprehensive active compounds to modern skincare products. For example, essential oils can serve as natural preservative agents, due to their antimicrobial properties. They can also provide additional benefits to skin such as anti-acne effects, anti-aging effects, skin lightening, and sun protection.

Essential oils may contain anywhere from a few to more than 100 single molecular structures. However, essential oils are usually natural mixtures of about 20–70 components, with two or three of the major components being present at fairly high concentrations and other components present in trace amounts.

The contribution of a single compound to an oil’s fragrance does not solely depend on its concentration though; it also relies on its odor threshold, which is determined by its structure and volatility. Therefore, minor components derived from oxidation or degradation reactions may have a strong impact on an oil’s fragrance if their aroma values are high enough.

Essential oil components can be roughly classified into three families: lipophilic terpenoids, phenylpropanoids, and short-chain aliphatic hydrocarbons. Lipophilic terpenoids are the most frequent and characteristic constituents. Among them, allylic, mono-, bi-, tricyclic mono- and sesquiterpenoids make up a major part of essential oils.

In general, monoterpene hydrocarbons are less influential on the fragrance of the essential oil than their oxygenated counterparts, which are highly odoriferous. Having a concentration of about 90%, monoterpenes are the most abundant component in essential oils. They have a great variety of structures, but geraniol/nerol, linalool, citronellol, citronellal and citral are the most important terpenes to an oil’s fragrance.

The chemical compositions of essential oils are heavily dependent on physiological (plant organ, ontogenesis), environmental (soil composition, weather conditions), and genetic factors. Other factors such as geographical variation, plant characteristics (which species, whether cultivated or wild), harvest or postharvest conditions, production parameters (oil production methodology, distillation parameters), and other parameters (storage condition, storage time) also impact the compositions of essential oils.

Essential oils are classified as top, middle, or base notes according to their odorous characteristics, diffusion rate and volatility.

The top notes are the most volatile oils and are therefore the first perceptible odors. In other words, they are detected first and fade first. As such, top notes are responsible for a product’s first impression on customers. These are light scents, usually lasting 5–10 minutes, but they may remain for a maximum of 30 minutes. Bergamot, juniper, cinnamon, and gardenia essential oils are all top notes.

Middle notes tend to be spicy or floral and give body to blends; they can remain for up to one hour. Ylang-ylang, geranium, lavender, jasmine, and clove essential oils are all middle notes. The base notes give a fragrance depth and last the longest, remaining for up to several hours. Myrrh, vanilla, sandalwood, and frankincense essential oils are all base notes.

Most common essential oils have been well tried and tested; their safety levels have been determined. As such, it is known that some essential oils are more likely to cause adverse skin reactions than others.

The presence and concentration of a relatively potent allergen is a major factor in allergic contact dermatitis, and the oxidation of essential oil constituents can increase the risk of adverse skin reactions because the resulting oxides and peroxides are generally more reactive. Therefore, the proper storage of essential oils is important to the preservation of their effectiveness and the reduction of adverse reactions.

Photosensitization may occur when an essential oil with a photosensitizing constituent is applied to skin and then exposed to sunlight or ultraviolet light. For instance, furanocoumarins are photosensitizers that may be present in expressed citrus peel oils and angelica root, rue, parsley leaf, or marigold essential oils.

The most common furanocoumarins are psoralen and bergapten. Inflammatory skin reactions such as pigmentation, blistering, or even severe skin burns can occur when furanocoumarins are applied to skin and exposed to ultraviolet light. We have detailed this situation in an earlier article about phytotoxicity for safe, natural ingredients.

Want to know more about fragrances? Visit the Fragrance Zone at in-cosmetics Global

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Previously, we established that plant-based natural food materials are rich sources of skincare actives. In this article, we will dig deeper into these naturally occurring molecules by describing their antioxidative mechanisms and properties.

Antioxidation is important to cell survival. Normally, reactive oxygen species (ROS) are produced via neutrophil activation and are used by phagocytes to kill infectious agents. However, an overproduction of ROS can cause oxidative stress and cellular damage; it is thereby the leading cause of degenerative diseases. Human skin is particularly susceptible to ROS overproduction, which may be caused by both endogenous sources and environmental factors, as shown in Figure 1.

Since plants are also vulnerable to ROS overproduction, they, like humans, rely on antioxidants for cell protection. Antioxidant molecules are produced in many parts of plants and are closely associated with plant evolution. Plants use antioxidants to overcome two major threats to their survival: solar oxidative stresses and biological stresses.

To combat solar oxidative stresses, plant antioxidants use three distinguishable mechanisms:

1. The absorption mechanism, which is employed by antioxidant molecules that are light absorbers. These molecules absorb sunlight directly when the photosynthesis unit experiences excessive sunlight exposure.
2. The quenching mechanism, which is employed by antioxidant molecules that are quenchers. These molecules accept excited state energy from chlorophylls, thus preventing the chlorophylls from suffering solar stresses.
3. The neutralization mechanism, which is employed by antioxidant molecules that are neutralizers. When the first two mechanisms fail and ROS are produced as a result of oxidative stress in the photosynthesis unit, these molecules neutralize the ROS (usually by acting as reducing agents and being oxidized themselves) and therefore prevent ROS from harming the cell.

To combat biological stresses, plants produce antioxidants that make them less attractive to herbivores and pathogens. These antioxidants also prevent pathogens from reproducing on the plants. For example, neutralizers can create high chemical oxygen demand (COD) so that pathogens or other undesirable organisms cannot grow on or around the plants.

Like their antioxidative mechanisms, plant antioxidants can be grouped into three families: (1) phenolics, which include simple phenols and polyphenols, (2) terpenes and multi-terpenes, and (3) molecules without phenol or terpene structures.

Phenolic antioxidant molecules almost exclusively use the neutralization mechanism. They donate hydrogen atoms to reactive radical species and act as chain breakers to stop oxidation. Typical plant antioxidants in this family include hydroxytyrosol, α-tocopherol (a form of vitamin E), and catechin (Figure 2). Though not a phenolic structure, Vitamin C, also known as ascorbic acid, employs the same antioxidative mechanism because of the active hydrogens in its structure.

Terpenes are natural, unsaturated hydrocarbons with formulas (C₅H₈)_n, where n > 1, and are classified by number of carbons: monoterpenes (C₁₀), sesquiterpenes (C₁₅), diterpenes (C₂₀), triterpenes (C₃₀), and tetraterpenes (C₄₀), for example (Figure 3).

Individual terpene structures are easily oxidized at the unsaturated double bond and thus can function as antioxidants via the neutralization mechanism. Because of their low excited state energy, conjugated terpenes—such as vitamin A (retinol, β-carotene), lycopene, and lutein—can also use the quenching mechanism to protect cells. They quench the excited states of light-absorbing molecules as well as the singlet excited states of oxygen molecules.

Antioxidant molecules without phenol or terpene structures use the absorption mechanism. They absorb sunlight to avoid overwhelming the photosynthesis unit, thereby preventing the generation of oxidative stresses. When produced by plants, these molecules are typically based on cinnamate or coumarin structures, as shown in Figure 4.

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