to determine the solubility of SC lipids such as fatty acids, cholesterol and ceramides in surfactant solutions, destruction of model bilayer membranes in the form of vesicles, removal of lipids from isolated SC, or changes in bilayer structure of isolated SC by vibrational spectroscopy. Such studies have also shown that the tendency of surfactants to damage lipids may not be the same as that for damaging SC proteins.4 For example, certain nonionic surfactants such as alkyl polyglucosides and amphoteric surfactants such as cocoamidopropyl betaine (CAPB) have minimal tendency to interact with proteins, but show higher tendency to interact with lipids. Mild cleansing requires mildness towards both proteins and lipids and therefore choosing surfactants that are mild towards both is important. A combination of anionic surfactant with amphoteric and/or nonionic surfactants can lead to such optimal conditions.The past work also showed that an amino acid surfactant, cocoyl glycinate, is as mild as the well-known syndet surfactant, cocoyl isethionate, commonly used in neutral pH syndet cleansing bars. These findings created the pathway for the introduction of an amino acid surfactant-based body wash by a major brand several years ago13 and the potential for other amino-acid surfactants in the future.Role of pH in Skin and Skin Cleansing It is well established that alkaline soaps are harsher than neutral pH syndet surfactant based cleansing bars.1-4 It is also well-known that the natural pH of SC is around 4.5 to 5.0.30 Korting et al have shown that use of alkaline soaps lead to a transient increase in pH of the order of almost 2 units and it returns to normal skin pH values with time.30 However, continued use of alkaline soaps can lead to a change in the steady state pH of skin and accompanying changes in the skin microflora.An elevation in the steady state pH has been observed also for abnormal different skin conditions. For example, it has been shown that extremely dry and atopic skin generally have higher than normal pH values, eg, around 5.5 to 6.0 or even higher.31 In such cases, it is not clear if skin pH change is the cause or the effect of skin conditions.Elias and team have shown that hyper acidification of SC can lead to improved SC cohesion, increased lipid synthesis, and enhanced antibacterial activity.32, 33 Subsequent work by several researchers has shown that intentionally lowering the pH of skincare lotions to values below the SC pH can lead to improved rates of recovery of tape stripped skin or even elderly dry skin.34, 35 Thus, there may be a case for lowering the pH of lotions and creams for improved skin benefits. More work is needed to validate the generality of these findings.In the cleansing arena, there have been attempts to make implied skin benefit claims by formulating skin cleansers under skin pH or even lower than skin pH conditions.10,36 In such cases, the claims have been based on just the pH of the product alone, rather than any experimental evidence of skin benefits for low pH cleansers.In order to test the effect of pH on skin cleansing systems, two identical syndet bar formulations, one under neutral pH conditions and the other under low pH conditions, were tested for their relative mildness in a typical forearm controlled application test (FCAT), which is commonly used to rank relative disruption to the skin’s barrier following exaggerated use conditions.10,37 The results showed that the neutral pH cleansing syndet bar was milder by lower transepidermal water loss (TEWL) and clinical visual dryness.10 Results of similar studies carried out with a typical SLES (sodium lauryl ether sulfate, 1 EO) and CAPB systems are shown in Figure 3.38 These results also show that the lower pH cleanser is harsher than the neutral pH cleanser in TEWL and visual dryness.The above study results suggest that neutral pH is better for mild cleansing. These are contradictory to the leave-on results mentioned earlier and can be explained as follows. The isoelectric point of keratin is around pH 5.39 The results for the zeta potential of human stratum corneum given in Figure 4 also shows that the IEP is around 5. Therefore, skin proteins have a net negative charge under neutral pH conditions. As the pH is lowered from neutral to acidic values, skin will have more positively charged sites than under neutral pH conditions and this in turn should promote increased binding of anionic surfactants to skin.10,11 Significant reduction in the zeta potential of SC in the presence of an anionic surfactant, SLES, is evident from the results given in Figure 5 and this is brought about by the binding of surfactants to the SC. Thus, in systems with predominantly anionic surfactants, there can be more residual surfactants left on skin under low pH conditions than under neutral pH conditions leading to more damage.The other question that arises when comparing neutral pH cleansing vs acidic pH cleansing is whether long-term use of a neutral pH cleanser elevates the pH of SC from its normal value. Previous reports show that transient pH change immediately after cleansing with a neutral vs a mild low pH cleanser is about the same and the skin pH values return to normal in less than an hour in both cases. Furthermore, even water wash is known to lead to a transient increase in pH that returns to normal values in less than one hour.38 Thus, unlike alkaline pH cleansing, neutral pH cleansing itself does not pose any harm to the SC.Note that the above arguments do not imply that it is not possible to create a low pH cleanser that is very mild. The current observation simply implies that conventional anionic surfactant rich-cleansers can be more irritating under low pH conditions.
