The Solubilization mechanism of benzyl esters in sodium dodecyl sulfate (SDS) micelles

Abstract

Micelles are aggregates of surfactant molecules used to increase the solubility and bioavailability of poorly soluble pharmaceutical compounds. Past studies have suggested that a micelle is an oil droplet surrounded by a layer of water. To further investigate the microenvironment of a micelle, the thickness of water penetration was not clearly understood. To study the mechanism of micellar solubilization, we have purchased and synthesized several benzyl esters derivatives. Alkyl chain length of the acid segment was increased from one as in benzyl acetate to ten carbons as in benzyl decanoate. Partition coefficients of each benzyl ester were determined from kinetic data using pseudo phase mathematical model. Degradation studies were conducted using borate buffer at pH 9 and constant temperature (40ºC). Surfactants (SDS) were dissolved in the buffer solution at varying concentrations to extend the concentration range below and above the critical micelle concentration. Reaction products were analyzed using high performance liquid chromatography HPLC and detected using UV detector at 254 nm. Hydrolysis reaction rate constants followed pseudo first order mechanism. Rate constants decreased as the surfactant concentrations have increased. Also, water solubility decreased as the alkyl chain length was increased which consequently caused the water-micelle partition coefficients to increase. To further understand the interaction between benzyl esters and micelles we introduced a linear interaction model where we proposed that each benzyl ester will fit into the micelle with linear orientation where the alkyl chain is pointed towards the core interacting with the hydrophobic region while the benzene ring is located in the hydrophilic region of the micelle.