A microscopic dipole model of water near an interface Ali Behjatian, Madhavi Krishnan Physical & Theoretical Chemistry Laboratory, University of Oxford, UK
Recent studies have shown that the anisotropic orientation of water molecules at interfaces in solution plays an important role in the anomalous experimentally observed phenomenon of attraction between like-charged colloidal particles in solutions 1-3 . Molecular dynamics (MD) simulations reveal that the orientation of water molecules at an interface significantly differs from bulk behaviour due to frustration at a discontinuity created by two different phases. Due to this symmetry-breaking effect of an interface, in general, the orientation of the water molecules near a charged interface can be anisotropic. These effects are not included in continuum electrostatics descriptions of interparticle interactions such as Poisson-Boltzmann (PB) theory that treat water as a featureless continuum. However, we have previously shown that in the calculation of the total interaction free energy between particles, a superposition of the PB electrostatic free energy, and an interfacial free energy due to water inferred from MD simulations does remarkably well in capturing various experimental trends 1-3 . While MD simulations provide detailed information on the molecular arrangement of water molecules at interfaces, an analytical model capturing the same behaviour would enable us to construct a self-consistent theoretical framework to describe interactions of macroscopic objects such as colloidal particles and polyelectrolytes. To this end, we present a statistical model that views water as an ensemble of noninteracting point-dipoles near a surface. We assume that the net molecular orientation near the interface is determined by the interplay of two types of potential energies: i) the electrostatic energy of dipoles in the electric field due to the charged particle, and ii) an additional short range symmetry breaking potential energy, which is non-zero within a shell of finite thickness in the vicinity of the interface. We incorporate this feature in a microscopic dipole description of the electric double layer system at an interface, and aim to show that by minimising the total free energy of the system, this behaviour of interfacial water can be explicitly and self-consistently implemented into the framework of a modified PB theory. Such a self-consistent model would be expected to improve upon the earlier superposition approach, and potentially enhance agreement of the model with experiments. Since virtually all biomolecules and colloidal particles in solution carry electrically charged groups, a molecular-level mean-field model of interparticle interactions would carry significant relevance in describing the profound effects of interfacial water in a wide range of particulate and molecular contexts.
A schematic representation of microscopic dipoles at a charged surface interacting with the electric field, and a symmetry breaking potential, u(x), close to the interface. 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. Wang, S.; Walker-Gibbons, R.; Watkins, B.; Flynn, M.; Krishnan, M. arXiv:2212.12894.
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