Functional Ingredients: Hydrating Substances

Difference between emollients and moisturizers

Emollients are substances used in cosmetics to soften the skin and to smooth the surface to the touch. 

Moisturizers are humectant, that allow water retention and hydration of the stratum corneum. 

Products for the treatment of dry skin are typically combinations of emollients and moisturizers in an emulsion, that is, a cream. That’s why we have oils and fats in the same cream formula as hydrating agents. 

In this article, I will list some hydrating (humectant, moisturizing) ingredients that we can use in our creams. 


NMF-related ingredients

Lactic acid and sodium lactate

Lactic acid and sodium lactate increase the water-holding capacity of the skin. Moreover, lactic acid is suggested to stimulate ceramide synthesis and improve skin barrier function. 

Lactic acid: mixture of 2-hydroxypropanoic acid, its condensation products, such as lactoyl-lactic acid and polylactic acids, and water. The equilibrium between lactic acid and poylactic acids depends on the concentration and temperature. It is usually the racemate ((RS)-lactic acid).
Appearance: colourless or slightly yellow, syrupy liquid.
Solubility: miscible with water and with ethanol (96%). 
Use: (1) As pH regulator: add dropwise until the desired pH value is reached. (2) As exfoliating agent: up to 10%, pH above 4. Be careful when using it: it’s an acid, use protective gloves and avoid contact of the pure ingredient with skin. 

Sodium lactate solution: solution of a mixture of the enantiomers of sodium 2-hydroxypropanoate in approximately equal proportions.
Appearance: clear, colourless, slightly syrupy liquid.
Solubility: miscible with water and with ethanol (96%)
Use: 0,5-5% in skincare products (face); up to 10% in body lotions; 2-3% in soaps. Add to the water phase (not heat-sensitive). It can affect the performance of electrolyte-sensitive thickeners (acrylates). 

[Related page: Preparation of sodium lactate – Lactic acid neutralisation]


PCA salts

2-pyrrolidone-5-carboxylic acid and its sodium and zinc salts .

Sodium PCA: can increase the water-holding capacity of the skin and reduce the skin dryness and flakiness. It is one of the major components of the NMF. Used also in haircare to enhance the appearance and feel of the hair and as antistatic agent.

Use: typically supplied as 50% aqueous solution, pH 6.8-7.4. Use level up to 5%, dissolve in the water phase (not heat-sensitive). If the formula includes electrolyte-sensitive thickeners (acrylates), add it in the cool down phase.

Zinc PCA: known for sebum-regulating properties.
Use: 0.1-1% (some suppliers suggest not more than 0.3% in emulsions). Add in the cool down phase and check that pH is below 7 before adding to the formula (otherwise it could precipitate).


Amino acids and proteins

Proteins: can bind water with the horny skin layer and its annexes, therefore they act as humectants. They can be used also as hair conditioning agents. Keep in mind that proteins have nothing to do with “hair reconstruction”: nothing can reconstruct hair.

To make them soluble in water-based products, they are typically supplied as hydrolized. The hydrolisis procedure consists of the cleavage of the protein molecule by disruption of some peptide bonds. This means that the protein we use in the DIY product is not the native protein form.
Use: use level depends on the specific protein. Examples: keratin 1-5%, rice protein 2-5%, collagen 0.5-2%. Some must be stored in the refrigerator. To be added in the cool down phase.

Amino Acids: amino acids are small molecules and the building blocks of proteins. They are part of the NMF and can be used as hydrating agents. Bering very hydrophilic, they don’t easily penetrate the stratum corneum.
Use: depends on the specific amino acid. Examples: (1) proline 0.5-2% (water phase, not heat-sensitive); (2) arginine 0.5-2% in skincare products, 1% in haircare products. It forms alkaline solutions in water and can be used to neutralise acids (exfoliating agents) and as pH regulator. Always check the pH when using it; (3) lysine 0,5-2%, add to the water phase. If supplied in the hydrochloride form, keep in mind that it could affect electrolyte-sensitive thickeners (acrylates). In this case, add to the cool down phase; (4) glycine 0.5-2%, add to the water phase.


Urea

Component of the NMF.
Appearance: white or almost white, crystalline powder or transparent crystals, slightly hygroscopic.
Solubility: highly soluble in water, soluble in alcohol, practically insoluble in methylene chloride.

At low percentage it is used as hydrating agent in moisturizers; at high percentage (starting from 10%) it is used in keratolytic products. Urea in solution slowly hydrolyses to ammonia and carbon dioxide.
Use: 3-10% as hydrating ingredient; up to 20% in keratolytic products. Add to the cool down phase. Keep the pH of the formulation below 6 and add a stabiliser (triethyl citrate 2-5%, or sodium lactate or sodium lactate/lactic acid buffer 0.2M/0.1M in same percentage of urea).


Other common hydrating ingredients

Glycerin (glycerol)

Appearance: syrupy liquid, unctuous to the touch, colourless or almost colourless, clear, very hygroscopic.

Solubility: miscible with water and with ethanol (96%), slightly soluble in acetone, practically insoluble in fatty oils and in essential oils.

Glycerol has many different uses in pharmaceutical technology, from oftalmic preparations, to intravenous formulations and suppositories. 

In cosmetics it is used as humectant in moisturizers. It attracts water and might have an influence in preserving the bilayer structure of the lipids. 

Use: typical range is 2-5% but can be found up to 10-20% in some formulas. Add to the water phase (not heat-sensitive). Sometimes it can be used as solvent for the dispersion of specific ingredients (for example for the dispersion of xanthan gum). 


Hyaluronan (sodium hyaluronate, hyaluronic acid)

Sodium hyaluronate is the sodium salt of hyaluronic acid, a glycosaminoglycan consisting of D-glucuronic acid and N-acetyl-D-glucosamine disaccharide units.

Appearance: white or almost white, very hygroscopic powder or fibrous aggregate.
Solubility: sparingly soluble or soluble in water, practically insoluble in acetone and in anhydrous ethanol.

It is present in the extracellular matrix in the stratum corneum. This polymer has different molecular weights depending on the production method and source. Because of the high molecular weight, it cannot penetrate the stratum corneum. However it can be used as hydrating substance as it binds water and forms a hydrated viscoelastic film on the skin.

Use:
In powder: as it is, 0.01-0.5% (some suppliers say up to 1%), add to the cool down phase.
In gel form (1% w/w): up to 5-10%, add to the cool down phase.

How to make the 1% w/w gel: prepare 99 g of preserved distilled water (preserved means containing the preservative of your choice); sprinkle the sodium hyaluronate into the water. Mix and wait for some hours (overnight is ok). The polymer will swell and on the day after, you will find your 1% gel ready to use.


D-Panthenol (Dexpanthenol)

Colourless or sightly yellowish, viscous hygroscopic liquid, or white or almost white, crystalline powder, very soluble in water, freely soluble in ethanol (96%). 

This alcohol is converted in tissues to D-pantothenic acid (vitamin B5). It can accelerate skin barrier repair and stratum corneum hydration in skin that was impaired by cleansers. 
Use: as 75% (w/w) aqueous solution, 0.5-5%. Add to the cool down phase.


D-Sorbitol

Appearance: white or almost white, crystalline powder.
Solubility: very soluble in water, practically insoluble in ethanol (96%).

Most commonly available as 70% aqueous solution. Normally used in pharmaceutical tablets and in candies with noncariogenic properties. In cosmetics it can be used as humectant and hydrating agent.
Use: up to 5% in leave-in products; up to 50% in rinse-off products. Add to the water phase.


Niacinamide (Nicotinamide)

Nicotinic acid and nicotinamide/niacinamide are the two main forms of vitamin B3 (niacin). 

It finds application in many conditions because of its numerous properties: it can be used in the treatment of photoaging (because it is a precursor of the co-factors NAD/NADH and NADP/NADPH, that have antioxidant functions), as enhancer of the epidermal barrier function by stimulating the synthesis of ceramides, as coadiuvant in the treatment of inflammatory and acne conditions, among other functions.

Use: as moisturizer, it is recommended between 2 and 4%. Product pH should not be lower than 5, as in acidic environment niacinamide can convert to nicotinic acid. Glamour cosmetics recommends adding it to the cool down phase; Making Cosmetics and Lotion Crafter indicate a use level range of 1-6% and adding it to the water phase; Alexmo cosmetics suggests 2-5%, either in cool down phase or in water phase. 


Allantoin

White or almost white, crystalline powder, sightly soluble in water, very slightly soluble in alcohol.

Allantoin is used in cosmetics for its protective and anti-inflammatory properties, but also as humectant. 

Use: 0,35% in emulsions, 0,4% in aqueous solutions; add to the water phase of the formula.


Stratum corneum lipids

Ceramides

Ceramides are the major components of the stratum corneum lipids. Several ceramide structures have been identified and they are distinguished according to the fatty acid chains and the sphingolipid base chains.

The name of a ceramide is typically “Cer” followed by an acronym that indicates the type of fatty acid and the type of sphingolipid base chain. 

  • Sphingolipid base chains:
    • S = sphingosine
    • H = 6-hydroxy sphingosine
    • DS = dihydrosphingosine
    • P = phytosphingosine
    • T = dihydroxy sphinganine
  • Fatty acid chains:
    • A = alpha-hydroxy acid
    • N = non-hydroxy fatty acid
    • O = omega-hydroxy fatty acid

An example of ceramide that we can find for use in DIY cosmetics is Ceramide NP (Glamour Cosmetics), that is, ceramide composed of phytosphingosine and non-hydroxy fatty acid; the one sold by Glamour Cosmetics is a blend of Ceramide NP and Olive Glycerides and it can be used at 0.5-2% (to be added in the cool down phase). 
Dragonspice offers Ceramide NP, too, and recommend a 0,05-1% use level. 
Making Cosmetics offers a blend of ceramides NP, AP, EOP and other formulation excipients (use level 1-15%). 
Lotion Crafter also offers a Ceramide complex blend that contains these three ceramides and recommends 3-5% use level. 

So basically, always check with your supplier which kind of ceramide they sell and which use level they recommend. In any case, they are typically added in the cool down phase and to be stored in refrigerator.


References

European Pharmacopoeia, 7th Edition (Monographs)

Lodén (2009), Hydrating Substances. In Handbook of cosmetic science and technology (eds. Paye, Barel, and Maibach)

Lodén (2016) Moisturizers: Treatment of Dry Skin Syndrome and Barrier Defects. In Cosmeceuticals and Active Cosmetics (eds. Sivamani, Jagdeo, Elsner, Maibach)

Hui and Al Dabagh (2016), Topical Niacinamide. In Cosmeceuticals and Active Cosmetics (eds. Sivamani, Jagdeo, Elsner, Maibach)

Sakamoto (2016), Amino Acids and Derivatives. In Cosmeceuticals and Active Cosmetics (eds. Sivamani, Jagdeo, Elsner, Maibach)

Moore and Rawlings (2017), The chemistry function and (phato)physiology of stratum corneum barrier ceramides, International Journal of Cosmetic Science, 39(4), 366-372

Secchi (2008), Role of protein in cosmetics, Clinics in Dermatology, 26, 321-325

Farber (2000), US patent US6329413B1

Technical sheets from suppliers: Glamour Cosmetics, Alexmo Cosmetics, Making Cosmetics, Lotion Crafter; Olionatura website.

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