In 1997, Wiechers1 introduced the concept of relative performance measurement to compare the moisturization of several neat emollients. The capacitance of skin treated with test products was measured by a corneometer and compared with glycerin-treated skin (defined as 100%) and untreated skin (defined as 0%) at given intervals, normally 6 hr after application. As one might expect, this test showed that all emollients were not the same in their capacity to moisturize skin. Emollients offered either low moisturization performance (0–30% relative performance moisturization (RPM)), medium RPM (30–70%), high RPM (70–100%), or excellent RPM (> 100%).1 Two structurally similar emollients interestingly imparted completely different moisturization performance: isopropyl isostearate (IPIS), with an RPM of 100%; and isostearyl isostearate (ISIS) with a score of only 15%, as illustrated in Figure 1.
Molecular modelling of all emollients in the study revealed that the mechanisms of IPIS and ISIS to impart moisture did not adhere to the two generally accepted means of moisturizing skin—i.e., acting either as a humectant or sponge, such as a hygroscopic molecule like glycerin does; or working as a plastic cling wrap or Saran Wrapa, as substantive molecules in petroleum jelly do.2 This hinted at a possible third mechanism of skin moisturization.