Glycolic acid contact time

The contact time for a glycolic acid peel, as well as for other AHAs, is the time between applying the peel and... 

AHAs may also limit or prevent the cross-linking of proteins in the extracellular matrix. Unlike TCA or phenol, AHAs do not bind with proteins and so are not neutralized by them. The type of action they produce depends on their concentration, the pH of the solution, the pK of the acids and the contact time with the skin before neutralization with a base solution. For example, a 70% glycolic acid solution (pH 0.5 for pK 3.83) in contact with the skin for 5 minutes will be more aggressive than a partially buffered (pH 3.5) 50% solution left in contact with the skin for 2 minutes before neutralization with a base solution (a saturated solution of sodium bicarbonate, for example). 

AHAs are not toxic to melanocytes, and can therefore be applied on dark skins and in all seasons, on condition that effective sun protection is used. Contact time (Table 8.1) depends on skin type, peel concentration and formulation, its pH, the method of application, preliminary skin preparation, etc. At an identical pH, a 50% concentration of glycolic acid will penetrate half as deep as a 70% concentration, and will take twice as long to do so. 

Table 8.1 Suggested contact times for glycolic acid ...

Condition Glycolic acid concentration (%) Contact time (min)... 

The effectiveness of a peel with glycolic acid in aqueous solution depends on the amount of time for which the acid is contact with the skin and therefore on how soon it is neutralized. Neutralizing too soon only allows a small proportion of the acid to act. 

The glycolic acid solution is applied quickly (in 15-20 seconds maximum), so that the contact time is the same for the whole face. The solution should be applied to the most resistant areas first (the forehead) and to the more sensitive areas last (the eyelids). It is important to develop application habits and always use the same system, both for applying and neutralizing the peel. 

Prior application of a glycolic acid mask, and close and difficult monitoring of the contact time and downtime are necessary. 

The reaction was performed under the fixed reaction conditions described in the Experimental section, while changing the contact time from 0.3 to 5.0 s. The main products detected were glyoxylic acid, formaldehyde, formic acid, and carbon oxides. The yields of these products are plotted as a function of the conversion of glycolic acid in Figure 1. The slope of lines from the origin indicate the selectivities to each product. 

The reaction was performed by changing the reaction temperature from 200 to 300°C and the contact time from 0.3 to 5 s, while fixing the feed rates as described in the Experimental section. Figure 2 shows the conversion of glycolic acid as a function of the temperature. The results indicate that the rate of reaction becomes about double as the temperature is raised by 20°C. 

The conversions of glycolic acid obtained at a short contact time of 0.62 s (amount of catalyst used = 2.5 g) are plotted as a function of the feed rate of oxygen in Figure 4. The rate of reaction increases with an increase in the feed rate of oxygen, but the oxgen dependency is very low far from first order dependency. 

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