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

Aqueous micelles are capable of solubilizing organic molecules with qu ite distinct polarities and degrees of hydrophobicity. Experimental K- s values for incorporation of neutral solutes in anionic sodium dodecy l sulfate micelles (SDS; 66 solutes), cationic hexadecyltrimethylammon ium (CTAB; 42 solutes) and dodecyltrimethylammonium bromide micelles ( DTAB; 39 solutes), and nonionic Brij-35 micelles (19 solutes) exhibite d excellent fits (multiple correlation coefficients greater than or eq ual to 0.98; standard deviations less than or equal to 0.2) to the lin ear solvation free energy relationship (LSER) proposed by Abraham (Che m. Soc. Rev. 1993, 22, 73): log K-s = c + a Sigma alpha(2) + b Sigma b eta(2) + s pi(2) + rR(2) + upsilon(V-x/100). The LSER is based on medi um-independent parameters for solute hydrogen bond acidity (Sigma alph a(2)) and basicity (Sigma beta(2)), excess molar refraction (R(2)), di polarity (pi(2)), and volume (V-x). For all four detergents, incorpora tion is dominated by the V-x terms (positive, reflecting the hydrophob ic effect) and the Sigma beta(2) terms (negative, implying that bulk w ater is a better hydrogen bond donor than the micellar solubilization site). The contributions of Sigma alpha(2) and R(x), though smaller, v ary in a chemically satisfying manner with detergent charge and struct ure. Incorporation is relatively insensitive to the solute dipolarity pi(2). These LSERs appear to provide a convenient framework for unders tanding the factors which contribute to the micellar solubility of org anic solutes and for developing quantitative structure-solubility rela tionships for organized media.