The role of interfacial water in inter-particle interactions Rowan Walker-Gibbons and Madhavi Krishnan University of Oxford, UK
Molecular dynamics studies have demonstrated that molecular water at an interface displays anisotropic orientational behaviour in contrast to its bulk counterpart. This effect has been recently implicated in the like- charge attraction problem for electrically charged colloidal particles in solution 1 . Here, negatively charged particles in solution display a long-ranged attraction where continuum electrostatic theory predicts monotonically repulsive interactions, particularly in solutions with monovalent salt ions at low ionic strength. Anisotropic orientational behaviour of solvent molecules at an interface gives rise to an excess interfacial electrical potential which we suggest generates an additional solvation contribution to the total free energy that is traditionally overlooked in continuum descriptions of interparticle interactions in solution 2 . In the present investigation we perform molecular dynamics simulations to determine the interfacial potential at realistic model surfaces of silica and carboxylated polystyrene 3 . We discuss how a free energy contribution from interfacial water may strongly affect the interaction between charged entities in solution. We further consider how this interaction can be modulated in the presence of solvent mixtures or with the addition of a small concentration of zwitterionic osmolytes - showing that the resulting experimental behaviour can be qualitatively captured by our interfacial solvation model. The study carries broad implications for molecular-scale interactions, with the proposed interfacial mechanism finding relevance in explaining a range of phenomena such as biological phase segregation, crystallization and pH-induced gelation, and chromosome packing, or more generally in soft-matter and molecular biological systems that exhibit pH and salt concentration dependent attractive interactions between electrically charged entities.
Figure 1. ( a ) Molecular Dynamics snapshot of water molecules in contact with an electrically neutral silica surface. On average, a molecule close to the interface points its oxygen atom slightly towards the interface and hydrogen atoms towards the bulk, giving a net dipole moment (green arrow) whose z-component points away from the interface and towards the bulk. This symmetry-breaking in average orientation gives rise to an interfacial electrical potential (or solvation potential) with negative sign, i.e., φ 0 <0. ( b ) The excess interfacial potential as a function of surface charge φ int (σ) (left axis, dashed line and circle markers) and the corresponding excess free energy of interfacial solvation f int (σ) (right axis, solid curve).The blue and red arrows depict the impact of charge regulation on the interfacial free energy contribution in water. The approach of negatively charged particles is accompanied by a reduction in interfacial free energy implying an attraction (downward pointing blue arrow) that can mitigate and even overwhelm the electrostatic repulsion, ∆F el . The converse is true for the approach of positively charged particles, where the interfacial solvation contribution implies an additional repulsion that reinforces ∆F el (upward pointing red arrow). References 1. Kubincová, A.; Hü nenberger, P. H.; Krishnan, M. J. Chem. Phys. 2020, 152, 104713. 2. Behjatian, A.; Walker-Gibbons, R.; Schekochihin, A. A.; Krishnan, M. Langmuir 2022, 38, 786. 3. Walker-Gibbons, R.; Kubincová, K.; H nenberger, P. H.; Krishnan, M. J. Phys. Chem. B 2022, 126, 25, 4697–4710
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