We measure H 2 O dynamics in a well-defined synthetic hectorite clay by the neutron spin echo technique, in the temperature range from 240 to 347 K. The interlayer spaces of this anisotropic material contain two layers of confined water, corresponding to the so-called bihydrated state. We analyze the experimental data in light of parallel molecular dynamics simulations. Simulations demonstrate that H 2 O diffusion in the direction perpendicular to the clay layers is not negligible and has to be taken into account in the experimental data analysis. A diffusive model with only two fitting parameters D ⊥ and D ∥ is well adapted for such analysis. A clear physical meaning for the two fitting parameters exists, in view of the geometry of the system. Experimentally, the diffusion coefficients parallel to the clay layers D ∥ were estimated to be slowed down by a factor of 5 compared to bulk water. Further, the activation energy of the diffusion process is higher than in bulk water especially toward the lower temperatures within the range studied (20.3 kJ/mol above 300 K increasing to 28.4 kJ/ mol below 300 K). Simulations suggest that this is connected to the presence of the cations (1 cation per every 8 water molecules) rather than to the formation of hydrogen bonds between H 2 O molecules and the clay layers. However, improvements of microscopic force fields are necessary to achieve a full quantitative interpretation of the experimental water diffusion coefficients. We suggest the importance of polarizability in such endeavors.

In this joint experimental-theoretical work we study hydrodynamic interaction effects in dense suspensions of charged colloidal spheres. Using x-ray photon correlation spectroscopy we have determined the hydrodynamic function Hq, for a varying range of electrosteric repulsion. We show that Hq can be quantitatively described by means of a novel Stokesian dynamics simulation method for charged Brownian spheres, and by a modification of a many-body theory developed originally by Beenakker and Mazur. Very importantly, we can explain the behavior of Hq for strongly correlated particles without resorting to the controversial concept of hydrodynamic screening, as was attempted in earlier work by Riese et al. [Phys. Rev. Lett. 85, 5460 (2000)]. DOI: 10.1103/PhysRevLett.96.138303 PACS numbers: 82.70.Dd, 83.10.Mj, 87.15.Vv Suspensions of colloidal particles undergoing Brownian motion in a low-molecular-weight solvent are ubiquitous in chemical industry, biology, food science, and in medical and cosmetic products. For polar solvents like water, the particles are usually charged. At long to intermediate distances these particles interact electrostatically by an exponentially screened Coulomb repulsion originating from the overlap of the neutralizing electric double layers. A considerable effort has been devoted over the past years to study the dynamics of colloidal model suspensions of charged spheres at the microscopic level [1,2]. The dynamics is determined by a subtle interplay of direct interactions and solvent-flow mediated hydrodynamic interaction (HI). The latter dynamic interaction plays a pivotal role not only in unconfined colloidal systems, but also in microfluidic devices where narrow wall confinements or channels are present [3], and in sedimenting dispersions of large nonBrownian particles [4]. HI in unconfined suspensions is very long ranged. It decays with interparticle distance r like 1=r. An account of HI effects in theoretical and computer simulation studies is quite challenging due to its many-body nature, which must be accounted for in nondilute suspensions.An important measure of the strength of HI with regard to short-time particle diffusion caused by local density gradients is given by the hydrodynamic function [2]which is the sum of a wave-number-independent self-part and a q-dependent distinct part. Here, D s is the short-time self-diffusion coefficient, and D 0 is the particle diffusion coefficient at infinite dilution. In the limit of large q, Hq reduces to D s =D 0 . Without HI, Hq 1 and any variation in Hq is a hallmark of HI. It has a direct physical meaning [5] as the (reduced) mean particle sedimentation velocity for a suspension subject to a weak periodic force field, collinear to the wave vector q, and oscillating like cosq r. Experimentally, Hq can be determined as a function of wave number through a combination of static and dynamic scattering experiments [6,7]. Experimental findings [6] for the Hq of highly charged colloidal spheres at low volume fractions (typically <0:05) are in e...

We have reanalyzed our former static small-angle x-ray scattering and photon correlation spectroscopy results on dense solutions of charged spherical apoferritin proteins using theories recently developed for studies of colloids. The static structure factors S(q), and the small-wave-number collective diffusion coefficient D(c) determined from those experiments are interpreted now in terms of a theoretical scheme based on a Derjaguin-Landau-Verwey-Overbeek-type continuum model of charged colloidal spheres. This scheme accounts, in an approximate way, for many-body hydrodynamic interactions. Stokesian dynamics computer simulations of the hydrodynamic function have been performed for the first time for dense charge-stabilized dispersions to assess the accuracy of the theoretical scheme. We show that the continuum model allows for a consistent description of all experimental results, and that the effective particle charge is dependent upon the protein concentration relative to the added salt concentration. In addition, we discuss the consequences of small ions dynamics for the collective protein diffusion within the framework of the coupled-mode theory.

We have studied the mesoscopic shape fluctuations in aligned multilamellar stacks of 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine bilayers using the neutron spin-echo technique. The corresponding in plane dispersion relation tau-1(q//) at different temperatures in the gel (ripple, Pbeta') and the fluid (Lalpha) phase of this model system has been determined. Two relaxation processes, one at about 10 ns and a second, slower process at about 100 ns can be quantified. The dispersion relation in the fluid phase is fitted to a smectic hydrodynamic theory, with a correction for finite qz resolution. We extract values for the bilayer bending rigidity kappa, the compressional modulus of the stacks B, and the effective sliding viscosity eta3. The softening of a mode which can be associated with the formation of the ripple structure is observed close to the main phase transition.