2005 ANNUAL SCIENTIFIC SEMINAR THE CHEMISTRY OF PRESERVATIVES David C. Steinberg Steinberg & Associates, Inc. A preservatiYe is defined as a chemical that is added to a cosmetic to preYent the gro\\th of or destroy microorganisms that may grow in the product. Various countries haYe different regulations of preserYatiYes. Other speakers will cover this. 361 If you reyiew the chemistry of the frequently used preservati\'es, you will find that they fall into these chemical classes: acids, aromatic alcohols, N-methylol containing substances halogenated compounds, isothiazolinones. quaternia and l,2 diols. Understanding the chemistry of these groups enables the formulator to choose the appropriate preservatiYe with less guess-work and more science. l. Acids Organic acids have been used for preserying foods and cosmetics. These chemicals are actiYe in the acid form but not the salt form: R-COOH R-COO- + H + As organic acids arc weak acids, they exist in the acid or actiYe form depending on the pH. The lower the pH the more activity. Typical acids and their dissociations arc: pKa % ActiYe at each pH 3 4 5 6 7 Dchydroacctic 5 27 100 95 65 16 2 Benzoic .U8 94 61 13 1.5 0 Sorbic -U6 98 85 37 5.5 () Salicyclic 2.97 48 9 l 0 0 Formic ns 85 36 5 l () Propionic 4.87 99 88 43 7 l In general propionic. sorbic. salicylic and dchydroacetic acids arc stronger against fungi. while formic and bcnzoic acids show bcncr anti-bacteria action. 2. Aromatic nlcohols These represent the most important types of preservati,·es currently being used. All of these are substituted phenols. Phenol was once called Carbolic acid and found use as a hospital disinfectant. Trying to produce an odorless forrn of this, resulted in the discm·ery of parabens. our most frequently used prescn·atives. Like acids they are pH dependent:
362 JOURNAL OF COSMETIC SCIENCE HO Parabens arc para hydroxybenzoic acid esters: II Parabcns hme limited solubility in water md this limits the quantity you can use. Parnben Water. 25°C Water. 80°C Propylene Glycol Mcthylparabcn 0.25 2.0 22 Et..hylparabcn 0.17 0.86 25 Propy I parabcn 0.05 0.30 26 Butylparaben 0.02 US 110 Bcnzylparabcn 0.01 0.05 13 Parabcns arc strongest against Gram positi,·c bacteria and fungi and arc weakest against Gram ncgati,c bacteria. At a pH of 8 about half is dissociated into the inacti,·e sail form. Other phenolic prcscr. ath cs include Phcno.\! ethanol and Benzyl Alcohol. These arc much less acti,·e than parabens . . ,. N-methylol containing substances N-CHrOH I N-H I + HCHO This equilibrium between the secondary nitrogen and formaldehyde is dependent on the strength of this Nitrogen to Carbon bond. Many people refer to these as --rormaldehyde·· releasers.
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