Lipid knowledge for the personal care industry

The function of emollients in skin care

Jari Alander2019

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Wikipedia: “complex mixtures of chemical agents specially designed to make the external layers of the skin (epidermis) softer and more pliable”

“Emollire” (Latin) – means to soften

Emollients are usually:the main component in emulsions after water (3–20% or more) and the dominating ingredient in many anhydrous formulationsvarious chemical structures including vegetable and mineral oils, esters and siliconesseen to have different polarities (hydrophilic – lipophilic)

Despite the definition in encyclopedias – emollient is not always equal to moisturizer

Emollient – definition

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Depends on applicationDepends on region/market

Which are the most common emollients in skin care?

Face careEU

Shea butter

Mineral oil

Dimethicone

Face care JP/KR

Mineral oil

Cyclopenta-siloxane

Squalane

Body care EU

Mineral oil

Shea butter

Dimethicone

Body care JP/KR

Dimethicone

Mineral oil

Shea butter

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Skin softening (emolliency)Occlusive moisturizationLubricationStructure formation in formulationDelivery system/solubilizer/carrierAesthetic and sensory modifier

Functions of emollients in cosmetic formulations

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The surface of the skin is a complex system comprising lipids, cells, proteins, microorganisms, water, natural moisturizing factors etc

Skin surface lipids are of two types– Epidermal lipids– Sebum lipids

Epidermal lipids (from keratinocytes)– Ceramides, cholesterol, free fatty acids– Typically extractable from skin surface: 5–10 μg/cm2

Sebum lipids (from sebaceous glands)– Triglycerides, squalene, free fatty acids, tocopherols– Typically extractable from the forehead: 150–300 μg/cm2

Emollients and skin

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Sensory properties, moisturization, ingredient absorption and many other properties are determined by the interaction between the stratum corneum and the emollients from the skin care product

Stratum corneum and emollients

Keratinocytes approx 20 micrometer wideEmulsion droplets 0.1 -5 micrometers in diameter

Skin lipids

Shear rate when applyinga cosmeticproduct:500–5000 1/s!

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Wetting of the stratum corneum by the emulsion– Surface & interfacial properties of emulsion droplets versus the

properties of the skin lipids

Rheology of emulsion– Determined by emollient properties and interaction with polymers

and emulsifiers

Solubility of emollient in stratum corneum and vice versa– Polarity of respective phases

Droplet-droplet interaction– Breaking of emulsion to create an oil film on the SC surface

Skin surface topology– Smooth versus wrinkled etc

Water release and evaporation from formulation

Physicochemical parameters affecting the interaction

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Wortel V.A.L, Wiechers JW – Skin sensory performance of individual personal care ingredients and marketed personal care products

Sensory attributes evaluated upondifferent steps of application

Before rubbing During rubbing After rubbing

Appearance:• Thickness• Cloudiness• Shine

Appearance:• Cloudiness• Shine

Appearance• Absorbency• Shine

Sensation:• Tingling• Cooling

Feel:• Absorbency• Thickness• Oiliness• Greasiness• Stickiness• Silkiness• Moisturization

Feel• Oily residue• Sticky residue• Silky residue• Amount of residue

Sensation:• Cooling• Warming• Tingling• Odor• Spreadability

Sensation:• Cooling• Warming• Moisturization• Stiffness or tightness of skin• Tingling• Coating

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Polarity is a 3-dimensional property (Hansen model)– Van der Waals interactions (Dispersive forces)

• Molecular size and shape– Hydrogen bonding

• Presence of –OH, -COOH, -NH2 etc groups with mobile hydrogen groups– Polarizability

• Presence of double bonds, ester and amide groups, aromatic rings etc• Measures the mobility of electron cloud around a molecule

Guiding principle – like dissolves like– If all three polarity dimensions match between solvent and solute

we have high solubility

Polarity determines interactionswith skin and between ingredients

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Qualitative comparison ofsome liquid emollients

Liquid oil

Van der Waals

interaction Hydrogen bonding Polarizability

C12-15 AlkylBenzoate X (X) XX Large polar group, highly

polarizable

Octyldodecanol X X X Exposed hydrogen bonds

Isopropyl palmitate X (X) X Medium polar, long FA chain, polarizable

Lipex Bassol C(Olus oil/Canola oil) XX (X) X Long chains, interacting with

each other, low polar

Lipex SheaLight(Shea butter ethylesters)

X (X) XLong FA chains, exposed ester group, high polarizability

Mineral Oil XX - -Non polar, no hydrogen bonds, very strong Van derWaals

Dicaprylyl Carbonate X (X) X(X)

Small molecule, more polar than other esters due to exposed carbonyl group and short chains

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Hansen’s Polarity ParametersLiterature data and approximations

VdW / Dispersive

Hydrogen bonding Polarizability

Mineral oil 16.3 0 0Cyclomethicone 12.9 1.3 1.0Dimethicone 12.5 0.8 0.8Lipex Bassol C 17.5 3.3 4.1Lipex SheaLight 16.2 3.8 4.5Skin 17.6 13.0 11.0Glycerol 17.4 11.3 27.2Water 15.5 16.0 42.3

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Lipex Shea™ dispersed in variousemollients (20% butter / 80 oil)

LipexBassol C

Caprylic/ caprictriglyce-

ride

LipexShea-Light

Dica-prylylether

Isopro-pyl

isostea-rate

Paraffin oil Castor oil

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Wetting of the skin or hair surface is important for functionality

Good wetting gives better film forming properties and more even coverage on the surface

Wetting ability is determined both by the surface and the properties of the liquid

Viscosity (rheology) determines speed of spreading while extent of spreading also depends on the polarity

Other aspects of polarity – spreading and surface interactions

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Contact angle on hydrophobic surface (teflon)Lipex SheaLight™ is uniquely polar

• Good spreadability on skin requires that emollient and skin polarity are matched

• Skin combines both hydrophilic and lipophilic properties -> balanced polarityis required

• Skin polarity varies on the body (face vs arm etc)

Paraffinum liquidum (21)

Dimethicone (45)

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Example 1: influence of oil phase on rheology of an emulsion

– Solid shea butter changes behavior when mixed with different emollients

Example 2: Influence of oil phase on the structure formation using beeswax in an anhydrous system

– Beeswax mixed with different oil phases gives changes in microstructure and solidification dynamics

Connecting the dots – polarity, solidity and behavior in formulation

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Effect on flow curve of varying emollientsEmulsion with Lipex Shea™ at 20°C

Very small differences in

behavior at 20°C

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Effect on flow curve of varying emollientsEmulsion with Lipex Shea™ at 35°C

Diffences in flowcurves due to differingsolubility of shea butter

at 35°C

70 60 50 40 30 201E-3

0,01

0,1

1

10

100

1000

10000

100000

1000000

1E7

1E8

G' [P

a]

Temperature [°C]

Castor oil Isopropylmyristate Lipex Bassol C Paraffin oil

Selection of oil phase determines microstructure

Beeswax in different carriers (15/85)

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Emollients offer great opportunities for optimizing formulation rheology, stability and skin feel

Consider chemical structure and physical properties for finetuning application properties:– Polarity (molecular size, hydrogen bonds, polarizability)– Viscosity/rheology

In complex emulsions, interactions are often more important than the effect of individual components– Do not evaluate emollients in pure form only as this will give

incomplete information

Conclusions

© 2019 AAK Sweden ABNo part of this presentation may be reproduced in any form or by any means without the prior written permission of AAK Sweden AB. Disclaimer: This presentation is intended for educational purposes only. AAK Sweden AB shall not be liable for any third-party claims or losses of any nature, including, but not limited to, lost profits, punitive or consequential damages.

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