Tony O’Lenick poses the question, "What is the difference between an ester and a hydrolytically stable ester?" to John Imperante, esters expert and co-inventor of hydrolytically stable esters.
According to Imperante, esters are an important class of polar oils used in many personal care applications. However, esters are subject to hydrolysis, a retrograde reaction where water breaks the ester down into the starting alcohol and fatty acid. The rate of hydrolysis in esters is directly related to pH, limiting their usefulness to a pH level of 5–10.
Unfortunately there are common cosmetic applications reaching pH levels beyond this range, such as hair treatments including relaxers (pH 13) and hydroxy acid formulations (pH below 5). Hydrolytically stable esters are extremely hydrophobic materials and as such, water cannot react to hydrolyze them. They are derived from dimer alcohol and fatty acids.
Two patents1,2 describe these compositions, which unlike other esters are surprisingly stable to hydrolysis and consequently of interest in high and low pH applications. These products show an extraordinary resistance to hydrolysis both on the acidic and alkaline pH values. This is most easily seen when one attempts to run a saponification value. A saponification value is an analytical technique that allows the determination of the ester's molecular weight by breaking down the ester with base (KOH).
In standard esters, the amount of KOH consumed in the analysis is measured, which is equivalent to the molecular weight of the ester. Surprisingly, these esters only show 1% of the expected saponification value. Consequently, they can be used to provide conditioning, emolliency and barrier properties to hair and skin at these high and low pH values.
1. US Patent 6,537,531 issued March 25, 2003
2. US Patent 6,706,259 issued March 16, 2004