INCORPORATION OF NONIONIC SOLUTES INTO AQUEOUS MICELLES - A LINEAR SOLVATION FREE-ENERGY RELATIONSHIP ANALYSIS

Aqueous micelles are capable of solubilizing organic molecules with quite distinct polarities and degrees of hydrophobicity. Experimental K-s values for incorporation of neutral solutes in anionic sodium dodecyl sulfate micelles (SDS; 66 solutes), cationic hexadecyltrimethylammonium (CTAB; 42 solutes) and dodecyltrimethylammonium bromide micelles (DTAB; 39 solutes), and nonionic Brij-35 micelles (19 solutes) exhibited excellent fits (multiple correlation coefficients greater than or equal to 0.98; standard deviations less than or equal to 0.2) to the linear solvation free energy relationship (LSER) proposed by Abraham (Chem. Soc. Rev. 1993, 22, 73): log K-s = c + a Sigma alpha(2) + b Sigma beta(2) + s pi(2) + rR(2) + upsilon(V-x/100). The LSER is based on medium-independent parameters for solute hydrogen bond acidity (Sigma alpha(2)) and basicity (Sigma beta(2)), excess molar refraction (R(2)), dipolarity (pi(2)), and volume (V-x). For all four detergents, incorporation is dominated by the V-x terms (positive, reflecting the hydrophobic effect) and the Sigma beta(2) terms (negative, implying that bulk water is a better hydrogen bond donor than the micellar solubilization site). The contributions of Sigma alpha(2) and R(x), though smaller, vary in a chemically satisfying manner with detergent charge and structure. Incorporation is relatively insensitive to the solute dipolarity pi(2). These LSERs appear to provide a convenient framework for understanding the factors which contribute to the micellar solubility of organic solutes and for developing quantitative structure-solubility relationships for organized media.