External Potential Modifies Friction of Molecular Solutes in Water

Abstract

Stokes's law for the friction of a sphere in water has been argued to work down to molecular scales, provided the effective hydrodynamic radius includes the hydration layer. In interpretations of experiments and in theoretical models, it is tacitly assumed that the solvent friction experienced by a solute does not depend on whether an external confinement potential acts on the solute. Using a novel method to extract the friction memory function from molecular dynamics simulations, we show that the solvent friction of a strongly harmonically confined methane molecule in water increases by 60% compared to its free-solution value, which is caused by an amplification of the slowest component of the memory function. The friction enhancement occurs for potential strengths typical of physical and chemical bonds and is accompanied by a significant slowing-down of the hydration water dynamics. Thus, the solvent friction acting on molecular solutes is not determined by solvent properties and solute-solvent interactions alone but results from the coupling between solute and solvent dynamics and thereby can be tuned by an external potential acting on the solute. This also explains why simulations of positionally constrained solutes do not reproduce free-solution diffusivities. Dynamic scaling arguments suggest similar effects also for macromolecular solutes provided the solution viscosity is sufficiently enhanced.

Properties of a nonlinear bath: experiments, theory, and a stochastic Prandtl–Tomlinson model

“…(Let us be reminded that this potential does not act on the bath particles in equation (9), in contrast to the analysis provided in [59]). This dependence has indeed been observed in computer simulations of molecular solutes in water in [47].…”

“…Combining experimental measurements, analytical computations, and simulations of a stochastic Prandtl-Tomlinson model, we investigated several equilibrium-properties of a colloidal particle suspended in a nonlinear bath. Additionally to displacements being non-Gaussian, a nonlinear bath shows up by unexpected properties of the coefficients of a linearized equation; For example, in a nonlinear bath, the effective friction memory of the bath can depend on the stiffness of a potential trapping the particle, as has been observed in molecular simulations [47], or on the mass or bare friction of the colloidal particle. These dependencies are observed in our experiments, so that the friction memory of the trapped particle varies by more than a factor of two for trap stiffnesses κ ranging from 1.89 μN m −1 ... 7.29 μN m −1 .…”

“…While, from equations (25) and (26), interesting behavior of the friction kernel upon varying the colloidal bare friction γ or κ is expected, we will restrict ourselves to varying κ, as this is easily done in our experiments, and also allows comparison to results of [47].…”

Macroscopic conductivity of aqueous electrolyte solutions scales with ultrafast microscopic ion motions

“…Through the Boltzmann relation F(r) = −k B T In(g(r)), we obtain the potential of mean force F(r) for anion-cation pairs ( Fig. 2c), where k B and T are the Boltzmann constant and temperature, respectively 45 . F(r) thus contains excluded volume effects of both water and ions as well as attractive intermolecular interactions.…”

Butane dihedral angle dynamics in water is dominated by internal friction

“…It is crucial that we extract without applying constraints along the reaction coordinate , as the pronounced differences in the barrier-crossing dynamics between free and constrained butane show that constraints decisively influence the system dynamics. In fact, recent simulation work demonstrated that friction is modified by positional constraints even for the simple system of a methane molecule that diffuses in water ( 52 ). in Eq.…”