So, in terms of cosmetics Humectants is defined as hygroscopic molecules which means they like and attracts water from surrounding or retains the moisture of the skin by providing occlusive layer when incorporated in cosmetics. Basically, Humectants attracts and retains the moisture.
Sometimes humectants and emollient are used for similar reasons since both shows conditioning property when incorporated into cosmetics to improve the look and feel of the skin. But due to their different molecular composition humectants and emollients perform this function in different way.
Humectants are both organic and inorganic, organic humectants are primarily used in cosmetics industry. Organic means an ingredient that contain hydrocarbon. In cosmetic and personal care products humectants plays major role.
HOW DOES A HUMACTANT WORKS ON THE SKIN?
Humectants tend to form hydrogen bonds with water. This contributes to moisturized and soft skin. They work by:
1. Attracting moisture from the environment onto the surface of your skin.
2. They also tend to pull moisture from the second layer (dermis) of your skin, keeping its top layer (epidermis) moisturized (1).
3. The epidermis is made-up of dry and dead skin cells that make it appear dull and flaky. A humectant works by reducing the appearance of dry skin.
4. While there are various humectants used to address dry skin and hair concerns, each of them work differently.
5. Even though all humectants are used to eventually moisturize the top layer of your skin and reduce frizzy hair, they are used in different strengths to specifically cater to your skin and hair type.
STRUCTURE OF HUMECTANT:
Basically, the molecular structure of humectants vary, and mainly constitutes hydroxyl group (-OH) or other hydrophilic sites that can interact through hydrogen bonding with water molecules. For the most part humectants are non-polar molecules compatible with water.
The way humectants work is that they attract water from either the atmosphere or from the body and bind it to itself via hydrogen bonding. This property known as hygroscopicity can be measured for any kind of humectant.
The amount of water that a material can bind at a specific humidity is called the equilibrium hygroscopicity. It is determined by placing a known quantity of a material in a fixed humidity chamber then measuring the change in mass of the material. When this test is done on a common humectant like Glycerin you find that it will absorb 25% of its weight in water when exposed to 50% humidity. Under the same conditions another common humectant, Propylene Glycol will absorb 20% its weight in water. In general, the more moisture a humectant will absorb, the better a humectant it makes for formulations.
CLASSIFICATION OF HUMECTANT
There are three types of humectants described below:
1. Inorganic Humectant - These are limited used in cosmetics. Calcium chloride is an example. It has compatibility problems and corrosive in nature. Hence it is not frequently used in cosmetics.
2. Metal Organic Humectant – These have limited use in cosmetics because of compatibility problems, corrosive nature and pronounced taste. The example of this is sodium lactate.
3. Organic Humectant – This includes polyhydric alcohols, their ester and ethers. These are widely used in cosmetics. The most commonly used organic humectant are glycerol, tri-ethylene glycol, ethylene glycol, polyethylene glycol, diethylene glycol, propylene glycol, glycerin, mannitol, sorbitol, glucose.
IDEAL PROPERTIES OF HUMECTANT:
High moisture absorption over a wide range of humidity.
Consistent moisture content even with changing humidity
Non-toxic and safe to use for cosmetics
Low odor and low color
Low viscosity to make compounding easier
Generally non-reactive with other cosmetic ingredients
Low cost and readily available
COMMON HUMECTANTS USED IN COSMETIC:
Glycerin - The most common humectant in cosmetics is glycerin or glycerol. This is a molecule that has three -OH groups on it. It is an odorless, clear liquid that can be derived from natural sources.
While it can be found in nature it is primarily manufactured as a bi-product of chemical reactions with fats and oils. It can also be synthesized from petroleum sources. Glycerin is perhaps the most versatile humectant in cosmetics and comes closest to being ideal. Its principle drawback is that at high levels is can feel sticky. When formulating with it you need to keep the levels low or find other materials to off-set the stickiness.
Propylene Glycol - This is another common humectant in cosmetics. It is non-toxic, low odor, low viscosity and compatible with many ingredients. It doesn’t absorb quite as much water as glycerin however, it still is excellent for this application. It also doesn’t have the stickiness problem of glycerin and is less expensive. Propylene glycol is similar to glycerin in chemical structure having three Carbon atoms. The main difference is that it only contain two -OH groups.
The primary drawback to propylene glycol is that it is synthetically produced from petroleum processing. It also has a bad reputation suffering from misinformation about it on the Internet. But from a formulation standpoint, it is excellent.
Sorbitol - This is a 6 Carbon sugar that has 6 -OH groups. It can be derived from glucose so can maintain a natural story. It is more hygroscopic than glycerin and doesn’t suffer from the stickiness problem. However, it is more expensive which is why it is not used as extensively as glycerin or propylene glycol.
Butylene glycol - This is a clear, low viscosity liquid that works well as a humectant. It is a 4 Carbon molecule that contains two -OH groups. It is similar in humectancy to propylene glycol and makes a good substitute if you are trying to move away from that material. It is also a more effective solubilizer than both Glycerin and Propylene Glycol.
PEG - There is a whole range of polymeric humectants based on the polymerization Ethylene Glycol. PEGs with an average molecular weight of 200 - 2000 are the ones that exhibit useful humectancy. When formulating, the higher the PEG molecular weight the less water soluble it will be.
Sodium PCA - This humectant is found naturally in human skin so it is often included as part of the Natural Moisturizing Factor. It makes for a good story. It is a highly effective humectant and can bind water 1.5 times better than glycerin. As humectants go, it is one of the best performing ingredients. However, its relatively higher cost has limited its application.
Hyaluronic acid – This helps to increase skin moisture and reduces the appearance of fine line and wrinkles. Sometime, it is also used with vitamin C to help lubricate dry skin.
What Are The Best Natural Humectants For Skin?
1. Aloe Vera
Aloe Vera is a wonder ingredient that works to treat almost any skin issue, like pimples, dry skin and dehydration. It also aids in skin hydration and moisture retention. Research suggests that aloe vera may be effective in soothing dry skin due to its humectant-like mechanism.
2. Honey
Honey is a great ingredient that is popularly used to address dry-skin concerns. It is also used in skincare products that offer deep hydration. It works as an effective humectant and emollient when applied to the skin. Apart from this, it is rich in vitamins and enzymes that contribute to soft and healthy skin.
3. Glycerin
Glycerin may be derived from natural or artificial sources. It is derived naturally from plant-based oils. Research shows that glycerin works as an effective humectant that lubricates and adds softness to the skin. It is used in skincare products and works as a skin-conditioner that improves texture. It is widely used as part of hair care products to reduce fizziness.
4. AHA (Alpha-Hydroxy Acids)
These are very popular and are naturally derived from fruits. They are prevalently used in anti-aging products. AHAs are also great exfoliators that help to remove dead skin cells and allow other ingredients to seep through your skin without any obstruction.
Alpha-hydroxy are usually recommended in 10-15 per cent concentrations. AHAs are also said to improve skin barrier functions by increasing the amount of ceramides in the skin.
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