Dynamics of Bound and Free Water in an Aqueous Micellar Solution : Analysis of the Lifetime and Vibrational Frequencies of Hydrogen Bonds at a Complex Interface

In order to understand the nature and dynamics of interfacial water molecules on the surface of complex systems, large scale molecular dynamics simulations of an aqueous micelle of cesium perfluorooctanoate surfactant molecules have been carried out. The lifetime and the intermolecular vibrational frequencies of the hydrogen bonds that the water molecules form with the polar headgroups of the surfactants, are calculated. Our earlier classification of the interfacial water molecules, based on structural and energetic considerations, into bound and free type is further validated by their dynamics. Lifetime correlation functions of the water-surfactant hydrogen bonds show the long lived nature of the bound water species. The water molecules that are singly hydrogen bonded to the surfactants have longer lifetime than those that form two such hydrogen bonds. The free water molecules that do not form any such hydrogen bonds behave similar to bulk water in their reorientational dynamics. A few water molecules that form two such hydrogen bonds are orientationally locked in for durations of the order of few hundreds of picoseconds. The intermolecular vibrational frequencies of these interfacial water molecules shows a significant blue shift in the librational band apart from a similar shift in the near neighbor bending modes, relative to water molecules in bulk. These blue shifts suggest an increase in rigidity in the structure around interfacial water molecules. This is in good agreement with recent incoherent, inelastic neutron scattering data on macromolecular solutions. The results of the present simulations should be relevant to the understanding of dynamics of water near any hydrophilic surface.Comment: 36 Pages including 7 Figures; Submitted to Phys. Rev.

Constraining the relativistic mean-field model equations of state with gravitational wave observations

The first detection of gravitational waves from the binary neutron star merger event GW170817 has started to provide important new constraints on the nuclear equation of state at high density. The tidal deformability bound of GW170817 combined with the observed two solar mass neutron star poses a serious challenge to theoretical formulations of realistic equations of state. We analyze a fully comprehensive set of relativistic nuclear mean-field theories by confronting them with the observational bounds and the measured neutron-skin thickness. We find that only a few models can withstand these bounds which predict a stiff overall equation of state but with a soft neutron-proton symmetry energy. Two possible indications are proposed: Circumstantial evidence of hadron-quark phase transition inside the star and new parametrizations that are consistent with ground state properties of finite nuclei and observational bounds. Based on extensive analysis of these sets, an upper limit on the radius of a $1.4M_\odot$ neutron star of $R_{1.4}\lesssim 12.9$ km is deduced.Comment: Matches with the published versio

Evidence for Bound and Free Water Species in the Hydration Shell of an Aqueous Micelle

Our atomistic molecular dynamics simulations reveal the existence of bound and free water molecules in the hydration layer of an aqueous micelle. The bound water molecules can be either singly or doubly hydrogen bonded to the polar head group on the surface of the micelle. The ratio of bound to free water is found to be approximately equal to 9:1 at 300 K.Comment: 7 pages and 3 figures. Submitted to Curr. Sci.(Ind. Acad. Sci.

Nonlinear hydrodynamic response confronts LHC data

Higher order harmonic flow $v_n$ (with $n\ge4$) in heavy-ion collisions can be measured either with respect to their own plane, or with respect to a plane constructed using lower-order harmonics. By assuming that higher flow harmonics are the superposition of medium nonlinear and linear responses to initial anisotropies, we propose a set of nonlinear response coefficients $\chi_n$'s, which are independent of initial state by construction. In experiments, $\chi_n$'s can be extracted as the ratio between higher order harmonic flow measured in the plane constructed by $v_2$ and $v_3$, and moments of lower order harmonic flow. Simulations with single-shot hydrodynamics and AMPT model lead to results of these nonlinear response coefficients in good agreement with the experimental data at the LHC energy. Predictions for $v_7$ and $v_8$ measured with respect to plane of lower order harmonics are given accordingly.Comment: 4 pages and 2 figures. Proceedings of Quark Matter 2015, Kobe, Japa

Shear viscosity to entropy density ratio in nuclear multifragmentation

Nuclear multifragmentation in intermediate energy heavy ion collisions has long been associated with liquid-gas phase transition. We calculate the shear viscosity to entropy density ratio eta/s for an equilibrated system of nucleons and fragments produced in multifragmentation within an extended statistical multifragmentation model. The temperature dependence of eta/s exhibits surprisingly similar behavior as that for water. In the coexistence phase of fragments and light particles, the ratio eta/s reaches a minimum of comparable depth as that for water in the vicinity of the critical temperature for liquid-gas phase transition. The effects of freeze-out volume and surface symmetry energy on eta/s in multifragmentation are studied.Comment: 5 pages, 5 figures, to appear in PR