Density depletion at solid-liquid interfaces: a neutron reflectivity study

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.     

Interfacial water at hydrophobic and hydrophilic surfaces: depletion versus adsorption

The structure of the water-solid interface for widely varying surface properties is investigated with Monte Carlo simulations using the SPC/E water model. Of particular interest is the relation between the wetting coefficient as a measure of the hydrophobicity of the substrate and the density depletion close to the solid surface. The substrates are modeled as rigid ordered lattices of sites that interact with water molecules through an orientation-independent Lennard-Jones potential of varying strength. Hydrophilic character is obtained by addition of polar hydroxyl groups on the substrate surface, and the influence of density, spatial distribution, and angular orientation of the polar groups on the interfacial water structure is studied.     

The effect of preparation technique on the optical parameters of biological tissue

The absorption coefficient μ_a, the scattering coefficient μ_s, and the scattering anisotropy factor g of porcine liver were studied in vitro using the integrating sphere technique and inverse Monte Carlo simulation in the wavelength range 450 to 700 nm. A reference preparation technique was developed using a dermatome providing specimens of 200 to 800 μm thickness without pre-freezing the tissue. The optical parameters as measured applying the reference preparation were compared to those measured after cryo-homogenisation. We found significant deviations of the scattering coefficient and the anisotropy factor which were compensated when the reduced scattering coefficient μ_s ^′ was calculated. We also compared the effects of freezing reference specimens at -20 °C and at 77 K without homogenisation. For both freezing protocols noticeable deviations were found in all three optical parameters as well as in μ_s ^′. The impact of tissue storage at 4 °C was measured in the range 4 to 48 h post mortem and showed a clear reduction of μ_a and a significant increase of μ_s even after 24 h of storage. Short-time storage of the specimens in saline solution reduced all three optical parameters significantly. In conclusion, the tissue preparation must be controlled in order to provide in vitro optical parameters that sufficiently mimic the in vivo situation. Received: 29 June 1999 / Revised version: 1 October 1999 / Published online: 3 November 1999     

Debye-Hückel theory for slab geometries

The electrostatic interaction of charged particles through or at a low-dielectric slab, such as a lipid bilayer immersed in water or a self-assembled monolayer (SAM) on a metal substrate, is considered theoretically in the presence of salt within the Gaussian approximation using a generalized Green's formalism. A number of separate situations are discussed: i) The presence of a low-dielectric slab leads to pronounced interactions of a single charge with the slab via the formation of polarization surface charges. For SAMs on metal substrates, there is an intricate crossover from image-charge attraction to the metal substrate (for large distances) to image-charge repulsion from the SAM (for small distances) with a stable minimum at a distance of roughly 20 times the thickness of the hydrophobic film. For bilayers in water, the interaction of a single charge is always repulsive. ii) The surface potential of a SAM is calculated for the case when the hydrophobic layer contains dipole moments, which might explain the recently observed long-ranged repulsion of hydrophobic scanning tips from PEG-terminated SAMs on gold. iii) The interaction between charged particles through the bilayer is weakened. Oppositely charged particles still attract each other through the membrane. The free-energy minimum occurs as a result of the competition between self-repulsion from the slab and interparticle attraction and is located at a separation from the membrane surface which equals 15 times the membrane thickness. iv) Surface charges on the two surfaces of a bilayer attract each other through the bilayer unless the surface charge densities are the same, even if the signs are the same. v) All these effects are strongly influenced by the presence of salt. Received 25 January 2000     

One-component-plasma: Going beyond Debye-Hückel

Using field-theoretic methods, we calculate the internal energy for the One-Component Plasma (OCP). We go beyond the recent calculation by Brilliantov [N. Brilliantov, Contrib. Plasma Phys. 38, 489 (1998)] by including non-Gaussian terms. We show that, for the whole range of the plasma parameter , the effect of the higher-order terms is small and that the final result is not improved relative to the Gaussian theory when compared to simulations. Received 12 April 1999     

High-order time-integration applied to p-version finite elements within finite strain thermo-viscoelasticity

In this essay a fully coupled, monolithic thermo-mechanical coupled simulation using high-order finite elements based on hierarchical shape-functions in combination with a high-order time-integration scheme using diagonally-implicit Runge-Kutta schemes is presented. The constitutive model is based on a finite strain thermo-viscoelasticity relation of overstress-type. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Peptide chain dynamics in light and heavy water: zooming in on internal friction

Frictional effects due to the chain itself, rather than the solvent, may have a significant effect on protein dynamics. Experimentally, such "internal friction" has been investigated by studying folding or binding kinetics at varying solvent viscosity; however, the molecular origin of these effects is hard to pinpoint. We consider the kinetics of disordered glycine-serine and α-helix forming alanine peptides and a coarse-grained protein folding model in explicit-solvent molecular dynamics simulations. By varying the solvent mass over more than two orders of magnitude, we alter only the solvent viscosity and not the folding free energy. Folding dynamics at the near-vanishing solvent viscosities accessible by this approach suggests that solvent and internal friction effects are intrinsically entangled. This finding is rationalized by calculation of the polymer end-to-end distance dynamics from a Rouse model that includes internal friction. An analysis of the friction profile along different reaction coordinates, extracted from the simulation data, demonstrates that internal as well as solvent friction varies substantially along the folding pathways and furthermore suggests a connection between friction and the formation of hydrogen bonds upon folding.     

Dielectric profile of interfacial water and its effect on double-layer capacitance

The framework for deriving tensorial interfacial dielectric profiles from bound charge distributions is established and applied to molecular dynamics simulations of water at hydrophobic and hydrophilic surfaces. In conjunction with a modified Poisson-Boltzmann equation, the trend of experimental double-layer capacitances is well reproduced. We show that the apparent Stern layer can be understood in terms of the dielectric profile of pure water.     

Entropy and enthalpy convergence of hydrophobic solvation beyond the hard-sphere limit

The experimentally well-known convergence of solvation entropies and enthalpies of different small hydrophobic solutes at universal temperatures seems to indicate that hydrophobic solvation is dominated by universal water features and not so much by solute specifics. The reported convergence of the denaturing entropy of a group of different proteins at roughly the same temperature as hydrophobic solutes was consequently argued to indicate that the denaturing entropy of proteins is dominated by the hydrophobic effect and used to estimate the hydrophobic contribution to protein stability. However, this appealing picture was subsequently questioned since the initially claimed universal convergence of denaturing entropies holds only for a small subset of proteins; for a larger data collection no convergence is seen. We report extensive simulation results for the solvation of small spherical solutes in explicit water with varying solute-water potentials. We show that convergence of solvation properties for solutes of different radii exists but that the convergence temperatures depend sensitively on solute-water potential features such as stiffness of the repulsive part and attraction strength, not so much on the attraction range. Accordingly, convergence of solvation properties is only expected for solutes of a homologous series that differ in the number of one species of subunits (which attests to the additivity of solvation properties) or solutes that are characterized by similar solute-water interaction potentials. In contrast, for peptides that arguably consist of multiple groups with widely disperse interactions with water, it means that thermodynamic convergence at a universal temperature cannot be expected, in general, in agreement with experimental results.     

Conformational dynamics and internal friction in homopolymer globules: equilibrium vs. non-equilibrium simulations

We study the conformational dynamics within homopolymer globules by solvent-implicit Brownian dynamics simulations. A strong dependence of the internal chain dynamics on the Lennard-Jones cohesion strength e \varepsilon and the globule size N _G is observed. We find two distinct dynamical regimes: a liquid-like regime (for e \varepsilon < e_s \varepsilon_{{\rm s}}^{} with fast internal dynamics and a solid-like regime (for e \varepsilon > e_s \varepsilon_{{\rm s}}^{} with slow internal dynamics. The cohesion strength e_s \varepsilon_{{\rm s}}^{} of this freezing transition depends on N _G . Equilibrium simulations, where we investigate the diffusional chain dynamics within the globule, are compared with non-equilibrium simulations, where we unfold the globule by pulling the chain ends with prescribed velocity (encompassing low enough velocities so that the linear-response, viscous regime is reached). From both simulation protocols we derive the internal viscosity within the globule. In the liquid-like regime the internal friction increases continuously with e \varepsilon and scales extensive in N _G . This suggests an internal friction scenario where the entire chain (or an extensive fraction thereof) takes part in conformational reorganization of the globular structure.