When an ester is chosen by a formulator for a personal care product, the desired properties of the formulation must be considered since the ester choice will impact them. In applications such as lipsticks, solid esters are preferred whereas other applications may require liquid esters to achieve the desired aesthetics. Esters typically are formulated into personal care products as emollients.
The properties of the ester, including the melting point and cushion, are controlled by selection of acids and alcohols. Since alcohols and acids are used by cosmetic chemist to make esters, the personal care formulator should be aware of the differences between these materials; namely they are linear or branched, and saturated or unsaturated.
The use of linear saturated alcohols and acids results in esters with the highest melting point and the same number of carbon atoms in the acid and alcohol. On the other hand, unsaturated alcohols and acids, due to the presence of the double bond, may be thought of as "twisted molecules.” This double bond allows unsaturated alcohols and acids to have low freezing points, compared with straight chain linear substances of equal molecular weights.
However, the liquidity of unsaturated alcohols and acids is obtained at a price—oxidative stability. Branched saturated alcohols and acids have the advantage of greater oxidative, hydrolytic and color stability when compared to unsaturated materials. Branched alcohols also offer liquidity. Table 1 compares the physical properties of C18 acids and alcohols.
Therefore, a stearyl stearate ester with a melting point (MP) = 62–70°C will result from a combination of stearyl alcohol, MP = 58°C, and stearic acid, MP = 69.6°C; while isostearyl isostearate ester, MP = -5°C, results from a combination of isostearyl alcohol, MP = 0°C, and isostearic acid, MP = 5°C.