Affinity of small-molecule solutes to hydrophobic, hydrophilic, and chemically patterned interfaces in aqueous solution

Abstract

Performance of membranes for water purification is highly influenced by the interactions of solvated species with membrane surfaces, including surface adsorption of solutes upon fouling. Current efforts toward fouling-resistant membranes often pursue surface hydrophilization, frequently motivated by macroscopic measures of hydrophilicity, because hydrophobicity is thought to increase solute-surface affinity. While this heuristic has driven diverse membrane functionalization strategies, here we build on advances in the theory of hydrophobicity to critically examine the relevance of macroscopic characterizations of solute-surface affinity. Specifically, we use molecular simulations to quantify the affinities to model hydroxyl- and methyl-functionalized surfaces of small, chemically diverse, charge-neutral solutes represented in produced water. We show that surface affinities correlate poorly with two conventional measures of solute hydrophobicity, gas-phase water solubility and oil-water partitioning. Moreover, we find that all solutes show attraction to the hydrophobic surface and most to the hydrophilic one, in contrast to macroscopically based hydrophobicity heuristics. We explain these results by decomposing affinities into direct solute interaction energies (which dominate on hydroxyl surfaces) and water restructuring penalties (which dominate on methyl surfaces). Finally, we use an inverse design algorithm to show how heterogeneous surfaces, with multiple functional groups, can be patterned to manipulate solute affinity and selectivity. These findings, importantly based on a range of solute and surface chemistries, illustrate that conventional macroscopic hydrophobicity metrics can fail to predict solute-surface affinity, and that molecular-scale surface chemical patterning significantly influences affinity-suggesting design opportunities for water purification membranes and other engineered interfaces involving aqueous solute-surface interactions.

Keywords: inverse design; membrane fouling; molecular simulation; solvation free energy; surface adsorption.

Fig. 1.

Binding free energies at interfaces are poorly correlated with bulk solvation free energies (A). Correlations improve when comparing binding free energies to octanol–water transfer free energies (B), but only for the methylated surface and only if boric acid and ammonia, the most polar solutes, are included. Diamonds represent fully hydroxylated interfaces, while squares are fully methylated.

Fig. 2.

Potentials of mean force (PMFs) for all solutes studied at (A) methylated and (B) hydroxylated interfaces. The distance to the interface is calculated from the solute heavy-atom centroid to the fixed sulfur atoms of the SAM chains. PMF values are relative to the bulk solvation free energies of solutes, which are shown as points at the furthest distances from the interface sampled. Error bars are those reported by pymbar (68).

Fig. 3.

Contributions to binding free energies as described in the text and defined in Eqs. 2 and 3 for the (A) methylated and (B) hydroxylated SAM surfaces. Summing the $\mathrm{\Delta }\mathrm{\Delta }{G}_{rep}$ and $\mathrm{\Delta }\mathrm{\Delta }{G}_{attr}$ yields the change in LJ interactions $\mathrm{\Delta }\mathrm{\Delta }{G}_{LJ}$. The repulsive component, which involves creating a cavity in which the solute may be inserted, is the predominant thermodynamic driving force for a solute’s preference for the interface over bulk solution. C and D show free energies of binding broken into differences between direct solute–system energetics $\left(\mathrm{\Delta }{\langle U_{sw}\rangle }_2\right)$ of solvation and relative entropies of solvation $\left(\mathrm{\Delta }{S}_{rel,1\to 2}\right)$, which for relatively rigid surfaces and solutes are dominated by water restructuring due to a solute.

Fig. 4.

(A) Minimum and maximum binding free energy surface representations from genetic algorithm optimizations of methane affinity. The Top images involve repatterning of hydroxyl and methyl groups, while the Bottom involve charged headgroups (quaternary ammonium and sulfonate) patterns on a methyl background. (B) Methane PMFs and Inset surface images for idealized spread (dashed) and patchy (solid) patterns of CH₃/OH headgroups. The Bottom panel shows methane PMFs for spread–patch (dashed) and patch–spread (solid) patterns of charged headgroups. The shaded gray regions represent areas between fully hydroxylated and methylated PMFs for methane shown in Fig. 2.