Effect of Degree, Type, and Position of Unsaturation on the p Ka of Long-Chain Fatty Acids

Abstract

Titration of a series of C 18 fatty acids yields p K a values that decrease with an increasing degree of unsaturation in the fatty acid chain. The p K a values of stearic, elaidic, oleic, linoleic, and linolenic acids were studied and compared to values of area per molecule in a spread monolayer of these acids. The decrease in p K a was found to relate to melting point temperature and area per molecule in the spread fatty acid monolayer. The p K a value was determined by first dissolving the fatty acid in a high pH solution (pH>10) and subsequently titrating the solution with HCl to obtain the characteristic S-shaped curves used to calculate the p K a values. The p K a values of stearic, elaidic, oleic, linoleic, and linolenic acids were found to be 10.15, 9.95, 9.85, 9.24, and 8.28, respectively. These p K a values were in the same order as area per molecule values of fatty acids in spread monolayers. This suggests that as area per molecule increases the intermolecular distance increases and p K a decreases due to reduced cooperation between adjacent carboxyl groups.