A molecular twist on hydrophobicity

Abstract

A thorough exploration of the molecular basis for hydrophobicity with a comprehensive set of theoretical tools and an extensive set of organic solvent S/water binary systems is discussed in this work. Without a single exception, regardless of the nature or structure of S, all quantum descriptors of bonding yield stabilizing S⋯water interactions, therefore, there is no evidence of repulsion and thus no reason for etymological hydrophobicity at the molecular level. Our results provide molecular insight behind the exclusion of S molecules by water, customarily invoked to explain phase separation and the formation of interfaces, in favor of a complex interplay between entropic, enthalpic, and dynamic factors. S⋯water interfaces are not just thin films separating the two phases; instead, they are non-isotropic regions with density gradients for each component whose macroscopic stability is provided by a large number of very weak dihydrogen contacts. We offer a definition of interface as the region in which the density of the components in the A/B binary system is not constant. At a fundamental level, our results contribute to better current understanding of hydrophobicity.

Fig. 1. Densities (continuous lines) and derivatives of the densities (discontinuous lines) along the normal direction to the interface for the representative set of S. Vertical solid lines mark the boundaries of the interface. The complete set of plots is provided in Fig. S1 of the ESI.

Fig. 2. NCI surfaces at the S⋯water interfaces for the chosen subset. The complete set of plots is provided in Fig. S5 of the ESI.

Fig. 3. Dimers for the representative set of S⋯water pairs (orbital interactions for the entire set considered in this work are available in Fig. S6 of the ESI†). Explicit orbital interactions leading to the largest orbital interaction energies in kcal mol⁻¹ are shown. Two configurations for HTP⋯water are shown: the configuration at the interface (top left) and the configuration of the minimum for the isolated dimer (second structure at the top row). The water dimer is included as a reference. Donor orbitals are shown as solid surfaces, acceptor orbitals are shown as line surfaces. All calculations are done on the MP2/6-311++G(d,p) minima.

Fig. 4. Well defined heptane⋯water isolated dimers found in the MP2/6-311++G(d,p) potential energy surface. Binding energies in kcal mol⁻¹ and isomer populations, x_i (%), are included.

Fig. 5. Dimers for the representative set of S⋯S pairs in this study (orbital interactions for the entire set listed in Tables 1 and S1 are available in Fig. S7 of the ESI†). Explicit orbital interactions leading to the largest orbital interaction energies (there are many more) in kcal mol⁻¹ are shown. Donor orbitals are shown as solid surfaces, acceptor orbitals are shown as line surfaces. All calculations on the MP2/6-311++G(d,p) optimized global minima.

Fig. 6. Explicit donor (solid surfaces) to acceptor (line surfaces) orbital interactions leading to the extended NCI attractive surface in the global minimum of the MP2/6-311++G(d,p) potential energy surface of the isolated heptane dimer. The troughs of the reduced gradient, which yield only weak interactions, are also shown.