Estimation of partitioning parameters of nonionic surfactants using calculated descriptors of molecular size, polarity, and hydrogen bonding

A study was carried out to verify the capacity of the Abraham's solute descriptors (R-2, an excess molar refraction; pi(2)(H), the dipolarity/ polarizability; Sigma alpha(2)(H) and Sigma beta(2)(H), the summation hydrogen-bond acidity and basicity; and V-x, the characteristic volume of McGowan) to predict the alkane-water partition coefficient (log P-alk) of nonionic surfactants, log P values were taken from the literature and examined for linear solvation energy-related (LSER) equations with calculated physicochemical descriptors. A stepwise multiple linear regression (MLR) analysis allowed us to derive LSER models which, unlike those previously published for a standard set of solutes, revealed that R-2 and Sigma beta(2)(H) are the major contributors to log P-alk. A minor contribution of V-x was also detected, whereas surprisingly Sigma alpha(2)(H) required a positive coefficient. The less relevant size effect seems to indicate that for nonionic surfactants the energy needed for creating a water cavity is largely compensated for by the favorable interactions of ethoxy groups in the polar chain with water, whereas the lag P-increasing effect of hydrogen-bond donor groups could be explained by considering the occurrence of folded, more lipophilic conformers, favored by the formation of intramolecular hydrogen bonds in the apolar phase. This hypothesis was substantiated by the calculation of the molecular lipophilicity potential on the water-accessible surface of extended and folded conformers of two representative surfactants, selected by high-temperature molecular dynamics. Besides MLR, a principal component analysis on a larger set of descriptors, comprising three solubility parameters (delta(D), based on dispersion forces; delta(H), including the contribution of hydrogen bonding; delta(O), overall solubility parameter) afforded the so-called principal properties, which were able to characterize nonionic surfactants and to satisfactorily assess their lipophilicity-related properties.