Adsorption and physical state of medium- and long-chain fatty acids in neutral aqueous buffer solution

Abstract

Varying amounts of radiolabelled fatty acids, lauric, myristic, palmitic, stearic, and oleic acids, were agitated with 5-ml volumes of phosphate buffer (pH 7.4) at 37°C in glass tubes, producing visibly clear solutions. A portion of the fatty acid was recovered from the aqueous phase, while the rest was adsorbed to the glass wall and the water surface. Stearate and oleate could hardly be demonstrated in the aqueous phase. The adsorbed amount, a, of the other fatty acids was related to the concentration, c, in the aqueous phase by a = βc α , where α was 0.87 for laurate, 0.73 for myristate and 1.17 for palmitate. The relation is identical to Freundlich's adsorption isotherm except that the numerical value of α, when dealing with adsorption equilibria, is less than unity. The observations can be explained if laurate and myristate are present in genuine aqueous solution in equilibrium with the adsorbed substances; palmitate is aggregated in the buffer phase as vesicles, liposomes, in equilibrium with monomer palmitate in lower concentration, the latter equilibrium as well as the adsorption obeying Freundlich's equation with α < 1. Even at a concentration as low as 10 −11 M, palmitate appears to be present mainly in an aggregated state.