Acids/ pH and pKa relationship

Chemical peel definition:
A chemical peel is a skin treatment intended to visibly improve the structure of treated epidermal tissue by the application of a caustic solution. They are capable of  accelerating the natural processes of exfoliation, and can also completely destroy the epidermis as well as dermis via  protein coagulation or lysis.

Chemical peels are a well researched and common  method used to restore a more youthful appearance and improve defects of the dermis: acne, rosacea, melasma, photoaging, actinic keratosis, and postinflammatory hyperpigmentation.
Fig.1.Diagram of skin

Peels may be broken down into four categories described by their depth of penetration:
1) Very superficial: Its action is limited to the stratum corneum.
2) Superficial: It does not go past the basal layer of the  epidermis.
3) Medium: It reaches the papillary dermis.
4) Deep: It reaches the reticular dermis.

Fig. 2. Chemical Peel Skin Section

Pre Peel Care:
Alpha-hydroxy acid (AHA) peels produce the best results if  preceded by careful preparation and followed by long-term daily care. To  prepare the skin use of sunscreens, tretinoin, AHAs and tyrosinase inhibitors are beneficial. Hyper-pigmentation is of particular concern for those of darker complexion, however it remains true regardless of  phenotype.

Immediate pre peel:

Skin should be washed and disinfected with  alcohol and degreased with acetone. The degreasing products allow the peel solutions, to penetrate more deeply and evenly. They break down some of the proteins and phospholipids in the cell membranes, which enhances the action of the acids.

The acid is applied based on the % of tolerance and pH of the peel.. The length of time the peel is left  on is counted as  the time from beginning application till neutralizing it.Neutralizing an AHA peel consists of applying a base solution (usually pH 9–10) to the treated area. Sodium bicarbonate( baking soda) or sodium hydroxide (NaOH) is often used for this.  An effective neutralizing solution can be made by  simply mixing enough sodium bicarbonate ( baking soda) with water till the baking soda collects at the bottom of the bowl creating a saturated solution.

Depth of peel is affected by many factors, 1) overall dermal health, 2) degree of photoaging, 3) pre peel preparation, 4) acid strength (%)  5) buffered vs unbuffered  [ Free acid value ] and 5)  length of time till neutralizing.

Post peel care:

Sun protection: Any peel,  reduces the thickness of the stratum corneum that protects the skin against the effects of UV light.  The protection offered the outermost layers of the skin is no longer available, as a result the quantity of UV rays that penetrates the  skin increases. This  can cause actinic damage in cells that are usually physically protected by the thickness of the skin. Melanocytes are more strongly stimulated, and there is an increased likelihood of pigmentary change. All peels increase the rate of UV light penetration. This in turn will up regulate melanocytes and can induce hyperpigmentation. Wearing a sunscreen with an SPF 50 + and sun avoidance is essential post peel.

If skin flakes or scabs, it must not be picked at, as this will increase the likelihood of scaring. Keep the skin well moisturized and do not scratch as this may lead to infection. Use an antibiotic cream if necessary.

Depth of peel, strength of peel and length of time peel is left prior to neutralizing, as well as the conditon of the skin are co factors which will affect the relative efficacy of the peel. Peels do not need to be unnecessarily deep nor strong to have effect. The benefits of peels may be realized using more gentle peels. more frequently and over a longer time period.

Glycolic acid
Glycolic acid is extremely hydrophilic, a pure aqueous solution of glycolic acid, saturated at a concentration of about 80%, has a pH of 0.5. This pH, markedly lower than its pKa of 3.83, is a  solution that consists mostly of pure acid and is extremely effective for a peel. The pH of a glycolic acid solution determines its acidifying power on the skin: a 3% glycolic acid solution at pH 3 can acidify the first five layers of corneocytes, whereas at 10% and pH 3, it causes a deeper and more rapid acidification of the epidermis.  Note that ‘acidification of the epidermis’ does not mean ‘peeling effect’. Acidifying several layers of the skin does not necessarily mean destroying the cells. Up to a certain point, cells resist acidification.

Lactic acid
Lactic acid is the next shortest AHA after glycolic acid.  It has a pKa of 3.86, which is close to that of glycolic acid. At identical concentrations, lactic acid destroys the epidermis more slowly than glycolic acid. Concentrations of lactic acid of 10–20% or stronger begin to destroy the stratum corneum and stimulate skin regeneration, the renewal of epidermal cells. However, some studies have shown that cell renewal is not maintained uniformly during long-term treatments with 3% lactic acid at pH 3. It has been found that, cell renewal gradually falls off during the first 10 weeks of treatment with 3% glycolic or lactic acid at pH 3, decreasing to 29.3% and 28.3%, respectively. When applied regularly, lactic and glycolic acids (at 3%, pH 3) lose a significant percentage of their capacity to destroy corneocytes and renew the epidermis around the 12th week. Salicylic acid (a beta-hydroxy acid), however, retains its ability to destroy corneocytes much longer.At a concentration of 50–70%, lactic acid produces the same amount of exfoliation as glycolic acid.  Lactic acid is also a better hydrator than urea or glycerol. Some studies have shown that 3 weeks of daily application of 12% lactic acid would allow as much collagen to be deposited in the papillary dermis as applying 25% trichloroacetic acid (TCA) or phenol.

Salicylic acid
Salicylic acid has a long history of use and unlike glycolic acid it is lipophillic. Its pKa is 2.98. Salicylic acid is
frequently utilized in topical acne preparations at ph 3-4 because of its comedolytic effects. It is also often used in conjunction with other acids as it facilitates the penetration of other topical agents. Salicylic acid peeling can alter the  underlying dermal tissue without directly wounding the tissue or causing inflammation which makes its use more tolerable, however one must remain viligent in application. Salicylic acid has also been shown to have anti-inflammatory and antimicrobial properties.

To buffer or not to buffer, that is the question.
Free acid value is the % of acid available for the peel to have effect.
pH 3 is 10 times more acidic than pH 4
pH 3 is 100 times more acidic than pH 5
pH 0.5 is 1000 times more acidic than pH 3.5

Fig. 3. pH of common products and others.

Each acid has a pKa value which is unique to its chemical properties. That value allows one to create a lower or higher concentration with or without buffering depending on the desired outcome.When the
pH = pKa of the solution there is 50% Free acid available. However, for glycolic acid, in low concentrations, the pH of a glycolic acid solution varies with the concentration of acid due to the buffering capacity of the solution: an unbuffered glycolic acid solution, at less than 1%, has a pH of approximately 2.5. At 2%, the pH is 2.1. When the concentration reaches 5%, the pH goes down to 1.9. It is 1.7 for a 10% unbuffered glycolic acid solution. Higher concentrations, 50–80%, have a pH close to 0.5. At the same pH of 0.5, a solution with a concentration of 80% is more aggressive than a 50% solution.This must be neutralized with an alkaline solution.

When  pH = pKa there is 50 % Free acid available. Changes above and below the pKa have a significant effect on the species in solution.(Fig.4.) You must check the pH of the solution before applying. Do a patch test for tolerance.

Fig. 4. Species Distribution for Glycolic, Lactic, Salicylic acids.

PRUDENCE is strongly advised. If you are in any way uncertain about what you are doing.  
DO NOT DO IT. . Peels may be purchased at countless online retailers.

Or you can DIY it:
Glycolic acid 70% USP
SkinCare forum is available to ask and answer questions. SEA has been invited to participate at that venue.

Color Atlas of Chemical Peels, Antonella Tosti, Maria Pia De Padova, Springer; 2nd ed. 2006

Cosmetic Dermatology For Skin of Color, Murad Alam, MD, Ashish C. Bhatia, MD, FAAD, Roopal V. Kundu, MD, Simon S. Yoo, MD, Henry Hin-Lee Chan, MD, FRCP, The McGraw-Hill Companies, Inc., 2009
Textbook of Aging Skin, Farage, Miranda A.; Miller, Kenneth W.; Maibach, Howard I. (Eds.) 2nd Printing., 2010
Textbook of Chemical Peels: Superficial, Medium and Deep Peels in Cosmetic Practice By Philippe Deprez, Policlinica Estetica & Anti-Aging, Empuriabrava, Spain, 2007

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