Amphiphilic organic ion pairs in solution: a theoretical study.

The macroscopic manifestation of hydrophobic interactions for amphiphilic organic ion pairs (tetraalkylammonium-anion) has been shown experimentally by measuring their association constants and their affinity with the organic phase. Beyond a certain size, there is a direct relation between association constants and chain lengths in tetraalkylammonium ions. We propose to cast a bridge between these results and geometrical properties considered at the level of a single ion pair by means of quantum chemistry calculations performed on model systems: trimethylalkylammonium-pentyl sulfate instead of tetraalkylammonium-dodecyl sulfate. Two limiting cases are considered: head-to-head configurations, which yield an optimal electrostatic interaction between polar heads, and parallel configurations with a balance between electrostatic and hydrophobic interactions. All properties (geometries, complexation energies, and atomic charges) were obtained at the MP2 level of calculation, with water described by a continuum model (CPCM). Dispersion forces link hydrocarbon chains of tetraalkylammonium ions and pentyl sulfate, thus yielding (for the largest ion pairs) parallel configurations favored with respect to head-to-head geometries by solute-solvent electrostatic interactions. Given the small experimental association energies, we probe the accuracy limit of the MP2 and CPCM methods. However, clear trends are obtained as a function of chain length, which agree with the experimental observations. The calculated monotonic stabilization of ion pairs when the hydrocarbon chain increases in length is discussed in terms of electrostatic interactions (between ions and between ion pairs and water), dispersion forces, and cavitation energies.