Long-range dipolar order and dispersion forces in polar liquids

Abstract

Complex solvation phenomena, such as specific ion effects, occur in polar liquids. Interpretation of these effects in terms of structure and dispersion forces will lead to a greater understanding of solvation. Herein, using molecular dynamics, we probe the structure of polar liquids through specific dipolar pair correlation functions that contribute to the potential of mean force that is "felt" between thermally rotating dipole moments. It is shown that unique dipolar order exists at separations at least up to 20 Å for all liquids studied. When the structural order is compared with a dipolar dispersion force that arises from local co-operative enhancement of dipole moments, a strong agreement is found. Lifshitz theory of dispersion forces was compared with the structural order, where the theory is validated for all liquids that do not have significant local dipole correlations. For liquids that do have significant local dipole correlations, specifically liquid water, Lifshitz theory underestimates the dispersion force by a factor of 5-10, demonstrating that the force that leads to the increased structure in liquid water is missed by Lifshitz theory of van der Waals forces. We apply similar correlation functions to an ionic aqueous system, where long-range order between water's dipole moment and a single chloride ion is found to exist at 20 Å of separation, revealing a long-range perturbation of water's structure by an ion. Furthermore, we found that waters within the 1st, 2nd, and 3rd solvation shells of a chloride ion exhibit significantly enhanced dipolar interactions, particularly with waters at larger distances of separation. Our results provide a link between structures, dispersion forces, and specific ion effects, which may lead to a more robust understanding of solvation.

Extended coarse-grained dipole model for polar liquids: Application to bulk and confined water

“…We observe that h ∆ is longer-ranged than the radial distribution function and decays in an oscillatory manner consistent with the recent second-harmonic light scattering experiments [68,69]. This long-range behavior has also been observed by previous studies from molecular dynamic simulations [65,[70][71][72][73]. From AAMD simulations, we observe that there is a strong short-ranged correlation between dipoles of water molecules in the first solvation shell (∼3Å).…”

A systematic comparison of the structural and dynamic properties of commonly used water models for molecular dynamics simulations

“…The macroscopic dielectric constant contains information on the collective orientational correlations between molecules and their permanent dipole moments, where the dielectric constant is directly linked to the strength of dipolar forces between water molecules. 133 Computationally, the dielectric constant can be readily accessed from the total dipole moment of the system, M, via…”

Hydrogen‐Bond Structure and Low‐Frequency Dynamics of Electrolyte Solutions: Hydration Numbers from ab Initio Water Reorientation Dynamics and Dielectric Relaxation Spectroscopy

“…The application of these criteria to categorize water molecules in MgCl 2 solutions is based on the semi‐rigid hydration scheme, where water dynamics are „locked” in two directions: the orientation of the water dipole is mainly affected by cations; the O−H orientation is mainly affected by anions [87] . Previous experimental and simulation work provide evidence for orientation ordering of water in extended hydration shells around the ion [89–92] . Using this approach, the water molecules in the solution can be classified into 18 subpopulations, W abc , where a , b , c =1, 2, B.…”

“…[87] Previous experimental and simulation work provide evidence for orientation ordering of water in extended hydration shells around the ion. [89][90][91][92] Using this approach, the water molecules in the solution can be classified into 18 hydrogen in the bulk.The computational procedure used to categorize the water molecules in MgCl (aq) into different water subpopulation is presented in ESI, together with a detailed analysis of the process of water "exchange" between different subpopulations. Figure 8C reports the distribution of water molecules among the fifteen subpopulations in the 1.3mol kg À 1 MgCl 2 solution.…”