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.

Molecular dynamics study of the interactions between a hydrophilic polymer brush on graphene and amino acid side chain analogues in water

“…Note that there is a similar but slightly different approach to decompose the Gibbs energy into the three parts. 11,102 In this type of method, MD simulations of purely repulsive solute molecules are required in addition to the MD simulations of the nonpolar solute molecules.…”

“…[4][5][6][7][8][9] Organic molecules, including proteins, often bind strongly to hydrophobic surfaces. [10][11][12] Brushes of hydrophilic polymers exhibit fouling resistance when the coverage is high enough on the hydrophobic surface. [13][14][15][16][17][18] This antifouling property has attracted great interest in many fields such as medical implants, biosensors, marine equipment, water purification, and desalination.…”

Dissecting the Conformational Free Energy of a Small Peptide in Solution

“…Such detailed insights require a dissection of the free energy into meaningful contributions, which include a multitude of enthalpic and entropic terms associated with intramolecular and intermolecular interactions and the corresponding degrees of freedom. Experimental and computational advances improve our ability to isolate individual free energy components, which can provide critical insights into the microscopic driving forces of protein conformational flexibility, , ligand binding, − and the interface association of ions and molecules. − …”

Understanding Hydrophobic Effects: Insights from Water Density Fluctuations

“…When a small non-polar solute binds a hydrophobic surface, water can no longer hydrogen bond around the solute; binding is thus accompanied by the release of constrained hydration waters, and ought to be favored by entropy (Figure 4e) [17]. However, the thermodynamic signatures of such processes can be sensitive to the binding context [38,124]. For example, Setny et al [125] found that the binding of methane to a concave pocket was favored by enthalpy, rather than entropy.…”