Fundamental measure theory of hydrated hydrocarbons

Recent developments in the theoretical, simulational, and experimental studies of the role of water hydrogen bonding in hydrophobic phenomena

Hydrophobic effects (hydrophobic hydration and hydrophobic interaction) constitute an important element of a wide variety of phenomena relevant to biological, physical, chemical, environmental, engineering, and pharmaceutical sciences, such as the immiscibility of oil and water, self-assembly of amphiphiles leading to micelle and membrane formation, folding and stability and unfolding of the native structure of a biologically active protein, gating of ion channels, wetting, froth floatation, and adhesion. On the other hand, the hydrogen bonding ability of water plays a major (if not crucial) role in hydrophobic phenomena. We present a review of most important and relatively recent experimental, simulational, and theoretical research on hydrophobic phenomena in various systems. With a particular interest we survey investigations clarifying the role of water hydrogen bonding therein, because it has been the main object of our own recent research. We have developed a probabilistic hydrogen bond (PHB) model that allows one to obtain an analytic expression for the number of bonds per water molecule as a function of its distance to a hydrophobe, hydrophobe radius, and temperature. Knowing that function, one can explicitly identify a water hydrogen bond contribution to the external potential whereto a water molecule is subjected near a hydrophobe. Combining the PHB model with the classical density functional theory (DFT), one can examine the contribution of water hydrogen bonding to the temperature and lengthscale effects on the hydration of particles and on their solvent-mediated interactions over the entire low-to-high temperature and small-to-large lengthscale ranges. We applied the combined DFT/PHB model to study a variety of hydrophobic phenomena such as (liquid) water in contact with a hydrophobic plate, solvation of spherical solutes of various radii in associated and non-associated liquids at various temperatures, the solvent-mediated interaction of spherical solutes and its temperature dependence, interaction of C60 fullerenes in water, temperature effect on the evaporation lengthscale of water confined between two hydrophobes, temperature dependence of the effective width of the solute-solvent transition layer and average density therein. These applications demonstrated that the DFT/PHB model can serve as a valuable tool in studying hydrophobic phenomena because it constitutes a balanced combination of simplicity, accuracy, and detail. The predictions of the combined DFT/PHB approach for the solvent density profiles and thermodynamic aspects of hydrophobic phenomena are generally in good agreement with experiments and simulations. For example, it predicts the small-to-large crossover lengthscale of its mechanism to be approximately in the range from 1nm to 4nm, and decreasing with increasing temperature. It also suggests that, in terms of the average fluid density in the solute-solvent transition layer, the transition layer for small hydrophobes (of radii ≲2 nm) becomes enriched with rather than depleted of fluid when both the solvent-solute affinity and hb-energy alteration ratio become large enough. The boundary values of these parameters, needed for the depletion-to-enrichment crossover, are predicted to decrease with increasing temperature.

Fluid transition layer between rigid solute and liquid solvent: is there depletion or enrichment?

The fluid layer between solute and liquid solvent is studied by combining the density functional theory with the probabilistic hydrogen bond model. This combination allows one to obtain the equilibrium distribution of fluid molecules, taking into account the hydrogen bond contribution to the external potential whereto they are subjected near the solute. One can find the effective width of the fluid solvent-solute transition layer and fluid average density in that layer, and determine their dependence on temperature, solvent-solute affinity, vicinal hydrogen bond (hb) energy alteration ratio, and solute radius. Numerical calculations are performed for the solvation of a plate and spherical solutes of four different radii in two model solvents (associated liquid and non-associated one) in the temperature range from 293 K to 333 K for various solvent-solute affinities and hydrogen bond energy alteration ratios. The predictions of our model for the effective width and average density of the transition layer are consistent with experiments and simulations. The small-to-large crossover lengthscale for hydrophobic hydration is expected to be about 3-5 nm. Remarkably, characterizing the transition layer with the average density, one can observe that for small hydrophobes, the transition layer becomes enriched with rather than depleted of fluid when the solvent-solute affinity and hb-energy alteration ratio become large enough. The boundary values of solvent-solute affinity and hb-energy alteration ratio, needed for the "depletion-to-enrichment" crossover (in the smoothed density sense), are predicted to decrease with increasing temperature.