Polarity of oils and emollients: what it means and how it affects the formula
There are many terms in the world of cosmetics and skin care that can seem complicated. One of them is the polarity of oils and emollients. Let's take a look at this concept and how it affects the creation of cosmetic formulas. But get ready, you'll have to read slowly and dive into the terminology of cosmetic chemistry...
What is Polarity
Polarity is a characteristic of molecules that is determined by the distribution of charges in molecules, their fragments or individual bonds and determines their dipole moment.
Polarity is the result of an uneven partial distribution of charge between different atoms in a compound. Atoms such as nitrogen, oxygen and halogens (fluorine, chlorine, bromine) are more electronegative, having partially negative charges. Atoms such as carbon and hydrogen tend to be more neutral or have partial positive charges.
Polar molecules must contain polar bonds due to the difference in electronegativity between the bonded atoms. Simply put, molecules that contain one or more bonds with electronegative atoms (i.e. polar bonds) are considered polar. Molecules that do not have such bonds, or that have symmetrical bonds that cancel each other out, are non-polar.
How Oils and Emollients Are Polar
Oil Polarity
Chemically, oils are classified as triglycerides, or fatty acid esters. These esters are formed by the condensation reaction of fatty acids with alcohols containing several OH groups. As a result, each oil molecule contains an ester (COO) bond, which is moderately polar because there is a high difference in electronegativity between the carbon and oxygen atoms.
However, the long hydrocarbon chains present in the alcohol molecule contain non-polar C-C and very weakly polar (almost non-polar) C-H bonds, which have a difference in electronegativity from zero to negligible.
In these long non-polar hydrocarbon chains, the minor effect of the polarity of the COO group is ignored. Therefore, in most cases, the oil molecule is considered non-polar.
The most common non-polar oils include:
- coconut oil;
- olive squalane;
- mineral oil;
- cocoa butter;
- palm oil;
- petrolatum.
However, many vegetable oils are complex mixtures containing non-polar and polar components. Let us consider olive oil as an example. The non-polar components of olive oil include triglycerides and sterols, and the numerous polar components include free fatty acids, aliphatic alcohols, tocopherols (containing a polar head and non-polar tails), phenolic compounds, etc.
The COOH and OH functional groups present in these polar components have significant dipole moment values, thereby making the olive oil molecule polar.
Below are some examples of other polar oils:
- avocado oil;
- jojoba oil;
- shea butter;
- linseed oil;
- castor oil;
- grape seed oil;
- argan oil, etc.
The polarity of emollients
Emollients are fat-soluble molecules of synthetic origin that differ from each other in their formula and structure, but like oils, emollient molecules also have an ester bond (COO). However, unlike oils, the hydrocarbon chains of the non-polar bonds of emollient molecules are not long enough to compensate for the polarity of the COO bond, which makes emollients polar substances.
Depending on the structural formula and the number of polar links, the polarity of the emolents varies from low to high. Below is a sign of the used components and their polarity values.
Торгова назва
INCI Name
Polarity
Cetiol® B
Dibutyl Adipate
High
Cetiol® AB
C12-C15 Alkyl Benzoate
High Octyldodecanol
High Cetiol® PGL
Hexyldecanol (and) Hexyldecyl Laurate
Middle
Capric/Caprylic Triglyceride
Middle Cetiol® J 600
Oleyl Erucate
Middle Cetiol® LC
Coco-Caprylate/ Caprate
Middle Cetiol® A
Hexyl Laurate
Middle Coco-Caprylate
Middle Caprylyl–Caprylate/Caprate
Middle Cetiol® CC
Dicaprylyl Carbonate
Средняя Plantasens Olive LD Hydrogenated Ethylhexyl Olivate, Hydrogenated Olive Oil Unsaponifiables Middle Cetiol® OE
Dicaprylyl Ether
Low
Cetiol® S
Diethylhexyl Cyclohexane
Low Isoamyl Laurate, Isoamyl Cocoate
Low Luvitol® Lite
Hydrogenated Polyisobutene
Low
How Polarity Affects Emulsion Structure
Of all the properties, polarity is probably one of the most important for emulsion stability. The polarity index of an oil can affect everything from viscosity, texture, and appearance to skin feel and shelf life.
There are two key factors that are influenced by the polarity of the fatty phase in an emulsion: the stability and solubility of the lipophilic crystalline materials. Polarity has a major influence on the stability of both oil-in-water and water-in-oil emulsions. It is well known that it is easier to obtain stable emulsions with non-polar emollients and vegetable oils than with polar ones of synthetic or natural origin. Thus, the higher the polarity of the composite oil phase, the more difficult it is to emulsify.
Very polar oils can also lead to Ostwald ripening, which is due to the diffusion of oil molecules through the aqueous phase from smaller oil droplets to more favorable oil droplets (for O/W emulsions). This process can lead to phase separation. Ostwald ripening can be reduced by adding oils of very low polarity to polar oils. This balances the polarity, making it easier to stabilize the emulsion.
How to create a stable formula considering the polarity and HLB of emulsifiers
Stability of emulsions with polar oils and emollients is achieved in two ways:
- Combining emollients of different polarity: in order to reduce the overall polarity of the fatty phase, it is necessary to add non-polar or low-polar oils to highly polar emollients (see the table above)
- Selecting the right emulsifier: polar oils require emulsifiers with a higher HLB value. Hydrophilic lipophilic balance (HLB) is the ratio of the polar part (hydrophilic groups) to the non-polar part (lipophilic groups) of a surfactant (emulsifier). HLB values of different emulsifiers range from 0 to 40 units. To achieve the required stability, you can combine several emulsifiers with different HLB values and their percentage of addition, taking into account the type of emulsion.
Today we have considered one of the important properties that affects the stability of emulsions – the polarity of oils and emollients. We have found out why it is necessary to combine oils of different polarity and have determined the pattern between the polarity of the fatty phase and the HLB index of the emulsifier. And how to choose the right emulsifier for a specific emulsion, based on its HLB value, we will tell you in the next article. Subscribe to our BEURRE telegram channel, through which you can receive up-to-date news about the publication of new articles, the release of new video tutorials and delicious discounts on the selected assortment of the BEURRE store.