Molecular dynamics dielectric response

In recent years, a class of methods has been developed for molecular dynamics simulations to be performed with an external pH parameter, like temperature or pressure [18, 43, 44, 70], These methods treat the solution as an infinite proton bath, and are thus referred to as constant pH molecular dynamics (PHMD). In PHMD, conformational dynamics of a protein is sampled simultaneously with the protonation states as a function of pH. As a result, protein dielectric response to the... 

Master curves are important since they give directly the response to be expected at other times at that temperature. In addition, such curves are required to calculate the distribution of relaxation times as discussed earlier. Master curves can be made from stress relaxation data, dynamic mechanical data, or creep data (and, though less straightforwardly, from constant-strain-rate data and from dielectric response data). Figure 9 shows master curves for the compliance of poly(n. v-isoprene) of different molecular weights. The master curves were constructed from creep curves such as those shown in Figure 10 (32). The reference temperature 7, for the... 

Once the molecular motions occurring in the glassy state have been characterised and assigned through dielectric relaxation, 13C and 2H NMR, it is interesting to investigate their effect on the dynamic mechanical response of Ar-Al-PA [60,61],... 

In Section VILA a strongly idealized picture was described. The dielectric response of an oscillating nonrigid dipole was found in terms of collective vibrations of two charged particles. Now a more specific picture pertinent to an idealized water structure will be considered. Namely, we shall briefly consider thermal motions of a dipole as (i) pure rotations in Fig. 56b and (ii) pure translations in Fig. 58a. Item (i) presents the major interest for us, since we would like to roughly estimate on the basis of a molecular dynamics form of the absorption band stipulated by rotation of a dipole. Of course, even in terms of a simplified scheme, the internal rotations of a molecule should also be accompanied by its translations, so the Figs. 56a and 56b should somehow interfere. However, in Section IX.B.l we for simplicity will neglect this interference. This assumption approximately holds, since, as will be shown in Section IX.B.2, the mean frequencies of these two types of motion substantially differ. 

K. Ando and S. Kato, Dielectric relaxation dynamics of water and methanol solutions associated with the ionization of /V,/V-dimcltiylanilinc theoretical analyses, J. Chem. Phys., 95 (1991) 5966-82 D. K. Phelps, M. J. Weaver and B. M. Ladanyi, Solvent dynamic effects in electron transfer molecular dynamics simulations of reactions in methanol, Chem. Phys., 176 (1993) 575-88 M. S. Skaf and B. M. Ladanyi, Molecular dynamics simulation of solvation dynamics in methanol-water mixtures, J. Phys. Chem., 100 (1996) 18258-68 D. Aheme, V. Tran and B. J. Schwartz, Nonlinear, nonpolar solvation dynamics in water the roles of elec-trostriction and solvent translation in the breakdown of linear response, J. Phys. Chem. B, 104 (2000) 5382-94. 

An innovative approach due to Haider et al. [113] may help to sidestep the challenges involved in explicit molecular dynamics simulation and obtain information on these slow dynamics. The authors use the results of dielectric reflectance spectroscopy to model the IL as a dielectric continuum, and study the solvation response of the IL in this framework. The calculated response is not a good description of the subpicosecond dynamics, a problem the authors ascribe to limited data on the high frequency dielectric response, but may be qualitatively correct at longer times. We have already expressed concern regarding the use of the dielectric continuum model for ILs in Section IV. A, but believe that if the wavelength dependence of the dielectric constant can be adequately modeled, this approach may be the most productive theoretical analysis of these slow dynamics. 

Recent molecular dynamics simulations of water between two surfactant (sodium dodecyl sulfate) layers, reported by Faraudo and Bresme,14 revealed oscillatory behaviors for both the polarization and the electric fields near a surface and that the two fields are not proportional to each other. While the nonmonotonic behavior again invalidated the Gruen—Marcelja model for the polarization, the nonproportionality suggested that a more complex dielectric response of water might, be at the origin of the hydration force. The latter conclusion was also supported by recent molecular dynamics simulations of Far audo and Bresme, who reported interactions between surfactant surfaces with a nonmonotonic dependence on distance.15... 

It was recently shown via molecular dynamics simulations14 that, in the close vicinity of a surface, water molecules exhibit an anomalous dielectric response, in which the local polarization is not proportional to the local electric field. The recent findings are also in agreement with earlier molecular dynamics simulations, which showed that the polarization of water oscillates in the vicinity of a dipolar surface,11,14 leading therefore to a nonmonotonic hydration force.15 Previous models for oscillatory hydration forces, based either on volume-excluded effects,18,19 or on a nonlocal dielectric constant,f4 predicted many oscillations with a periodicity of 2 A, which is inconsistent with these molecular dynamics simulations,11,18,14 in which the polarization exhibits only a few oscillations in the vicinity of the surface, with a larger periodicity. 

There are also a number of theories taking into account dipolar solvation dynamics. These theories use the solvent s dielectric response function as the dynamical input and also include effects due to the molecular nature of the solvent. The most sophisticated of these theories, by Raineri et al. [136] and by Friedman [137], uses fully atomistic representations for both solute and solvent and recent comparisons have shown it to be capable of quantitatively reproducing both the static and dynamic aspects of solvation of C153 [110]. In these cases the theoretical nature of solvation dynamics is fully understood. However, it must be remembered that much of the success of these theories rests on using the dynamical content of the complicated function, dielectric response function, determined from experiment. Although there... 

Two points should be mentioned here. First, the effect of solutes on the solvent dielectric response can be important in solvents with nonlocal dielectric properties. In principle, this problem can be handled by measuring the spectrum of the whole system, the solvent plus the solutes. Theoretically, the spatial dependence of the dielectric response function, s(r, co), which includes the molecular nature of the solvent, is often treated by using the dynamical mean spherical approximation [28, 36a, 147a, 193-195]. A more advanced approach is based on a molecular hydrodynamic theory [104,191, 196, 197]. These theoretical developments have provided much physical insight into solvation dynamics. However, reasonable agreement between the experimentally measured Stokes shift and emission line shape can be... 

In this, M is the molecular mass, p is density, is Avogadro s number, cq is the permittivity of free space, is the relative permittivity/dielectric constant and a is the molecular polarisability. For a full discussion of the dielectric behaviour of polymer-based materials, reference to the excellent works by Kremer (2003) and Jonscher (1983) is highly recommended. Nevertheless, simplistically, permittivity can be thought of in terms of the number and nature of the polarisable species present in the system, plus their dynamics. Since the dielectric response of a given moiety is affected by its environment, dielectric spectroscopy can provide local structural information. In practice, the relative permittivity of a material is a complex quantity ... 

Modem broadband dielectric spectroscopy is a very useful tool to interrogate the molecular dynamics of polymers because response over a broad frequency range from the milli- to giga-Hertz region is possible (10). Therefore, motional processes which take place in polymers on extremely different time scales can be investigated vs. temperature. Moreover, the motional process depends on the morphology of the system. 

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