Partial molar volume of nonionic surfactants in aqueous solution studied by the KB/3D-RISM-KH theory

- KB/3D-RISM-KH theory predicts the PMV and self-assembly of amphiphiles in solution
- Reveals molecular picture of PMV as a thermodynamic quantity beyond the additive group approach
- Produces the PMV at infinite dilution (difficult for measurement due to a low critical micelle concentration).
- OPLS-AA force field gives the PMV in better agreement with experiment than OPLS-UA.
- PMV from the KB/3D-RISM-KH theory is in good agreement with experiment for ethylene glycols in water at 5-45 °C.

Description of self-assembly by means of atomistic models without coarse-graining and empirical adjustment is the most challenging problem in statistical mechanics of liquids. Partial molar volume (PMV) is a thermodynamic property related to effective solvation forces spontaneously driving self-assembly of amphiphilic molecules in solution. We calculate the PMV of several ethylene glycol derivatives, in particular, alkyl polyoxyethylene ethers H(CH2)m − 1(CH2OCH2)nCH2OH commonly known as CmEn nonionic surfactants, in aqueous solution at infinite dilution by using the Kirkwood-Buff (KB) equation and the three-dimensional reference interaction site model with the Kovalenko-Hirata closure relation (3D-RISM-KH) integral equation theory of molecular liquids. Special attention is paid to the infinite dilution case since direct measurement of PMV of monomeric surfactants is hindered by their very low critical micelle concentration (cmc). The PMVs obtained from the KB/3D-RISM-KH approach are in good qualitative agreement with experimental data for ethylene glycol derivatives in water at 5; 25; and 45 °C.