j. Soc. Cosmet. Chem., 45, 247-256 (September/October 1994) A comparison of the rates of esterification of some hydroxy compounds ANTHONY J. O'LENICK, JR., and JEFF K. PARKINSON, Siltech Inc., Norcross, GA 30093. Received April 16, 1994. Synopsis Several types of hydroxy compounds were esterified by reacting them with stearic acid under standardized conditions. Included in the evaluation were fatty alcohols Guerbet alcohols polyoxyakylene glycol com- pounds polyoxypropylene glycol compounds, which are secondary alcohols dimethiconol dimethicone copolyol compounds having terminal hydroxyl groups and dimethicone copolyols having hydroxyl groups on the internal silicone groups. Rather than merely generating rate constants for these reactions, we chose to compare the rate of reaction under specific conditions and to attempt to determine the effect of (a) linearity of compound, (b) the number of hydroxyl groups present, (c) the presence of silicone groups in the molecule, and (d) the location of the branch in the molecule. The % reaction was followed by reduction in acid value. The resulting data were used to establish three reaction types, which include (a) silicone esters, based upon primary hydroxyl compounds, (b) non-silicone esters made with primary alcohols, and (c) non-silicone esters based upon secondary alcohols. Within each group the principal factor determining esterification rate is the molecular weight of the hydroxy-containing species. Specifically, the higher the equivalent molecular weight of the hydroxy compound being reacted with the stearic acid, the slower the reaction. This relates to the concentration of reactive groups per volume of reaction liquid. BACKGROUND Esters are a class of organic compounds that find applications in many diverse segments of the chemical industry, including the cosmetic market. There is little published data showing the effects of structural properties of the reactants upon the relative rates of esterification of the many types of esters of interest to the cosmetic chemist. Rather than merely determining rate constant for the esterification reactions, we un- dertook a study to determine which structural properties of the hydroxy compound, when reacted with stearic acid under standard conditions, resulted in the change in the rate of the esterification reaction. Included in our study are fatty alcohols, Guerbet alcohols, polyoxyethylene glycols, polyoxypropylene glycols, dimethiconol, terminal dimethicone copolyol (having the carbinol group at the terminal portion of the mole- cule), and comb dimethicone copolyols (having the carbinol internally located rather than terminal). 247

248 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Esters are prepared by reacting a hydroxy-containing compound with a carboxylic acid under elevated temperature, typically under reduced pressures, and with a suitable catalyst present. In the case where a fatty acid is used as the raw material, water is removed during the reaction as shown in equation 1 (1): O catalyst O R'OH + RCOH heat RCOR' + HOH (1) hydroxy carboxylic compound acid ester water The rate at which the esterification reaction occurs is dependent upon several factors, including: 1. Temperature 2. Pressure 3. Catalyst 4. Mole ratio of reactants 5. "Structure" of reactants Our study had as an objective determining the effect of structural properties upon the reaction rate esterification reaction under one condition: evaluating several classes of carbon-based hydroxy compounds (carbinols), including polyoxyalkylene glycols, Guer- bet alcohols, and two types of hydroxy silicone compounds, dimethiconols, also called silanols and dimethicone copolyols. We decided to include the silicone compounds in the study since they have been the topic of several recent U.S. patents (2-8), and to our knowledge there were no data available comparing the reaction rates of these compounds with the rates of carbinol compounds. We chose to maintain as constant the following factors: 1. Temperature: 195 ø C 2. Pressure: atmospheric 3. Catalyst: p-toluene sulfonic acid 4. Catalyst concentration: 0.1% catalyst 5. Mole ratio of reactants: 1.0:1.0 The following hydroxy compounds were esterified with 95% stearic acid under the conditions shown above. The % reaction was calculated by the change in acid value. The reduction in acid value was then divided by the original acid value to give the "% reaction." SIMPLE FATTY ALCOHOLS Simple fatty alcohols are linear compounds having even numbers of carbon atoms. Stearyl alcohol (98% purity) was chosen for this series of experiments.