Solvent Effects on Molecular Adsorption on Ag Surfaces: Polyvinylpyrrolidone Oligomers
T Balankura and X Qi and KA Fichthorn, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 14566-14573 (2018).
The solution-phase adsorption of solutes on solid surfaces is important in a number of applications that are currently being researched. However, most theoretical approaches describing this phenomenon fall short of accurately describing the solution environment. Herein, we use classical molecular dynamics simulations based on an accurate many body force field to quantify vacuum and solution-phase (ethylene glycol) adsorption free energies of polyvinylpyrrolidone (PVP) oligomers on Ag surfaces-a system studied experimentally for solution-phase nanocrystal growth. We find a favorable free-energy change when PVP adsorbs to Ag surfaces in the presence of solvent. However, the binding free energy for a PVP molecule in solution is significantly smaller than that for a PVP molecule in vacuum. In vacuum, the adsorbates lose considerable entropy upon adsorption to a solid surface because of a loss in their configurational degrees of freedom. In solution, adsorption entropies are a result of a solvent solute exchange process, in which the entropy loss of PVP solute is counterbalanced by the gain in entropy of the displaced solvent, so that the solution-phase system exhibits zero or slightly positive changes in entropy upon PVP adsorption. Solvent layering near solid surfaces can create free-energy minima near the surface, as well as free-energy barriers to adsorption. Our study underscores the importance of using explicit solvent, as well as extensive configurational sampling to quantify the thermodynamics of solution-phase adsorption. Such insight will be important in efforts to understand technologically relevant phenomena, such as crystal growth in solution, (electro)catalysis, and molecular sensing.
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