MD-simulation study of the hydrophobic hydration of nonionic surfactants

The hydrophobic hydration of nonionic surfactants of general structure H(CH2)(m)(OCH2CH2)(n)OH (abbreviated as CmEn) as well as their constituents, namely small alkane and ether molecules, in dilute aqueous solutions have been investigated. By an extensive series of classical molecular dynamics simulations the temperature dependent association of surfactant molecules and hydrophobic test-particles are studied. The simulations were performed at constant ambient pressure conditions and temperatures between 275 and 450 K. The hydrophobic interaction has been probed by Widom's particle insertion method. The temperature dependence of the association process can be described well by temperature independent enthalpies and entropies of transfer of the test particle from bulk to shell. These transfer properties can be reduced to group contributions. The resulting Gibbs free energy of transfer was used as a measure of the hydrophobicity and could be correlated with the experimental cloud point temperatures of binary aqueous mixtures of these surfactants. An empirical correction leads to a quantitative description of the experimental data. (C) 1999 Elsevier Science B.V. All rights reserved.