399 The Human Stratum Corneum
control actives such as the retinoid family of actives,108 AHAs (alpha hydroxyacids),109
niacinamide,110 and antioxidants such as Vitamin C.111 There are also several natural
alternatives for such benefits as Bakuchiol112 and Resveratrol.113 These actives may create
biological benefits by altering the structure of skin, and this would bring them under the
drug category. However, personal care companies limit their claims to cosmetic benefits of
such effects, which will allow them to market the products under the cosmetic category.
A detailed review of these ingredients and their impact on the SC structure is beyond the
scope of this paper, and details can be found in the refences.108–114
THE ROLE OF PH IN SKINCARE AND SKIN CLEANSING
The acid-mantle of the SC and the importance of maintaining it is well recognized in the
literature.115–118 Natural pH of a healthy SC is in the 4.5–5.0 range.119 Depending upon
the quality of skin, the pH of the SC can vary in the 4–6 range, with dry skin and atopic
skin in the 5.5–6.0 range. Infant skin at birth is close to pH 7.0 but comes to equilibrium
within a couple of weeks after birth.120
Cosmetic and personal care products in the marketplace vary in pH significantly, with
some of the acne and antiaging products at pH values as low as 3.5–4.0, to alkaline soap
bars at pH values as high as 10–11. This brings up the question of the ideal pH for leave-on
and wash-off products in skincare.
Elias and team have published extensively in this area, emphasizing the importance
maintaining the acidic pH of the SC and even suggesting that hyper-acidification of the
SC will help in rebuilding a heathier barrier.121 According to Elias and team, even a half a
unit increase in steady state pH of the SC can affect its barrier properties and the normal
regeneration of the SC barrier deleteriously. These include antimicrobial properties of
skin, enzymatic processes such as the conversion of glucosyl ceramides to ceramides, the
generation of NMFs, and the normal degradation of desmosomal linkages by proteolytic
enzymes in the SC. Changes in the antimicrobial properties of the SC can alter the skin
microbiome, which in turn can affect skin health. A renewed recognition of the importance
of the role skin pH in maintaining barrier health has led to the creation and evaluation of
products formulated under skin pH, or even hyper-acidified skin pH conditions.122
Elias et al. showed in their studies that taped-stripped mice skin recovered more rapidly
under hyper-acidification conditions compared to neutral pH conditions.121 Taking this
idea further, Blaak et al.,116 in their in vivo human studies, have further shown that the
corneum barrier in aged skin can be improved by treatment with a pH 4 lotion treatment.
In another study, Angelova-Fischer et al.122 showed that a pH 4 emulsion, compared to
a pH 5.8 emulsion, accelerated barrier recovery and enhanced the barrier integrity in
elderly subjects. In contrast to these studies, Buraczewska and Loden13 did not find any
difference between a pH 4 and a pH 7.5 leave-on cream on barrier recovery or TEWL on
SLS-challenged (sodium lauryl sulfate) skin. These findings are indeed interesting and raise
questions about the differences between SLS treated skin and naturally dry aging skin.
More studies are needed to fully validate the hypothesis that hyper-acidification indeed
will lead to superior skin barrier.
Regarding pH and skin cleansing, as mentioned earlier, the pH of products available in the
marketplace vary significantly from 3.5–11. Lately, there has been an increase in creating
cleansing products at skin pH and making implied claims that skin pH cleansers are better

400 JOURNAL OF COSMETIC SCIENCE
for the skin over neutral and alkaline pH cleansers. This aspect is reviewed in detail and
the key findings are summarized by Hawkins et al.124
Common soap bars with alkyl carboxylate-based surfactants are formulated at alkaline pH
conditions as high as 10–11.This is mainly because of the insolubility of soap molecules in
bar formulations below pH 9.0. By using organic counterions, such as tri-ethanol amine,
it is possible to lower the pH of soaps to about 8.4. With the introduction of neutral pH
syndet (synthetic detergent) bars in the mid-50s with acyl isethionate as the main surfactant,
neutral pH cleaning became popular. Importantly, Frosch and Kligman compared the
irritation potential of several alkaline soap bars to the isethionate based syndet bar in their
classic study using their soap chamber test, and showed that the neutral pH syndet bar was
significantly milder than alkaline soap bars.25 This has been confirmed with several less
aggressive cleansing protocols,26 including the popular FCAT methodology for assessing
the mildness of cleansing products.27
With the recent advances in our understanding of the role of pH in maintaining a healthy
SC barrier and the benefits of hyper-acidified leave-on products on skin barrier recovery
after tape striping, there has been an increase in formulating cleansing formulations at
skin pH.124 Contrary to expectations, Hawkins et al. showed that marketed syndet bars
with synthetic detergents such as alkyl sulfosuccinate as the main surfactant and prototype
bars formulated with acyl isethionate as the main surfactant under skin pH conditions
were harsher than the neutral pH acyl isethionate bars.124 Similarly, prototype liquid
formulations with sodium laureth sulfate as the main anionic surfactant formulated at skin
pH and neutral pH conditions also showed that the acidic pH cleansers were harsher than
their neutral pH counterparts. These results clearly show that simply formulating a product
at skin pH does not ensure enhanced mildness. The authors explained these differences due
to increased binding of anionic surfactants to the SC as the pH is lowered from neutral pH
to acidic pH conditions. The isoelectric point of SC is around pH 4.5. Lowering the pH of
the cleanser from neutral to skin pH conditions will increase the positively charged sites on
keratin, where anionic surfactants can bind strongly, leading to increased damage and skin
irritation. These results clearly show that it is important to understand the interactions
of surfactants and all other ingredients in a formulation with the SC under different pH
conditions to determine the relative benefits of skin pH versus other conditions for ensuring
mildness of a product. It is also important to recognize that these results do not imply that
it is not possible to formulate a skin cleansing product under skin pH conditions, but that
it requires surfactants that are mild to skin under low pH conditions.
The interest in formulating skincare and skin cleansing products under skin pH conditions
will continue in the coming years. It is therefore important for formulators to understand
the effect of pH on the interaction of their ingredients and the overall formulation with the
SC under skin pH conditions rather than assuming that any product formulated under skin
pH conditions will be mild to the SC.
CONCLUSION
Cosmetic and personal care formulations contain a wide range of ingredients including
surfactants, emulsifiers, penetration enhancers, emollients, occlusives, physiological and
nonphysiological lipids, and bio-actives ingredients. The interactions of these ingredients
with the SC will determine their ability to deliver the intended benefits without