Industry expert Tony O'Lenick explains the difference between sorbitan esters and sorbeth esters to help the personal care formulator differentiate between their INCI names, emulsion stabilities and feel on the skin.
Sorbitan, as shown in Figure 3, is a mixture of chemical compounds derived from the dehydration of sorbitol. The mixture can vary but usually consists of 1,4-anhydrosorbitol, 1,5-anhydrosorbitol and 1,4,3,6-dianhydrosorbitol. Reacting sorbitan with fatty acid and subsequently with ethylene oxide (EO) results in polysorbates, an important class of emulsifiers. The amount of EO reacted will alter solubility in water and the emulsification ability of the product.
When oleic acid is reacted with sorbitan, sorbitan monooleate (SMO) results. Ethoxylation of SMO with 5 moles and then 20 moles of EO results in the sorbitan monooleates PSMO-5 and PSMO-20, respectively. As EO is added, the SMO goes from water insoluble to water dispersible and to water soluble, as shown in Table 1. The ability to emulsify SMO also changes as the amount of EO is added. Sorbitol will dehydrate upon heating, resulting in cyclic materials present and the exact composition being a mixture of compounds.
If sorbitol is ethoxylated, the dehydration is minimized and sorbeth esters form. The empirical formula of sorbeth-30 is shown in Figure 4. These sorbeth esters are emulsifiers or oils depending upon the amount of EO present. For example, sorbeth-30 tetraoleate (liquid), sorbeth-40 tetraoleate (liquid) and sorbeth-60 tetraoleate (paste) are all excellent emulsifiers and solubilizers of high polar vegetable oils or esters. They provide stable emulsification in small quantities for various oils and esters.