On the fluid-fluid phase separation in charged-stabilized colloidal suspensions

We develop a thermodynamic description of particles held at a fixed surface potential. This system is of particular interest in view of the continuing controversy over the possibility of a fluid-fluid phase separation in aqueous colloidal suspensions with monovalent counterions. The condition of fixed surface potential allows in a natural way to account for the colloidal charge renormalization. In a first approach, we assess the importance of the so called ``volume terms'', and find that in the absence of salt, charge renormalization is sufficient to stabilize suspension against a fluid-fluid phase separation. Presence of salt, on the other hand, is found to lead to an instability. A very strong dependence on the approximations used, however, puts the reality of this phase transition in a serious doubt. To further understand the nature of the instability we next study a Jellium-like approximation, which does not lead to a phase separation and produces a relatively accurate analytical equation of state for a deionized suspensions of highly charged colloidal spheres. A critical analysis of various theories of strongly asymmetric electrolytes is presented to asses their reliability as compared to the Monte Carlo simulations

Viscosity coefficient of dense fluid hydrogen

Evaluations of the viscosity of the dense hydrogen are presented in a region whese dissociation plays a major role. The viscosity is computed by a classical molecular dynamics model where the fraction of dissociated hydrogen is a priori given by the Ross model. A universal fit is given, based on scaling laws of inverse power potential

Sedimentation of charged colloids in the gravitational field : Relaxation toward equilibrium

The sedimentation of charged colloids in the gravitational field is studied. This paper deals with the dynamics of the process, i.e. the relaxation toward equilibrium. In the case of salt-free solutions without hard-sphere repulsion, it is found that electrostatic interactions change the apparent mass and the diffusion coefficient of the colloid. The problem can be studied in terms of relaxation times that have been analytically calculated. The result depends on the initial conditions and the spatial localisation. The hard-sphere repulsion, that has been treated at the Carnahan-Starling level, increases the diffusion forces so that the relaxation toward equilibrium is faster. On the opposite, the hydrodynamic interactions between the particle decrease the sedimentation velocity. We have studied the competition between these effects. It is found that the hydrodynamic interactions compensate for the hard sphere repulsion nearly exactly. The results are consistent with experimental studies

Individual adsorption of low volatility pheromones: Amphiphilic molecules on a clean water–air interface

International audienceEnvironmental conditions can alter olfactory scent and chemical communication among biological species. In particular, odorant molecules interact with aerosols. Thermodynamics variables governing the adsorption from air to water surface of bombykol, the most studied pheromone, and of three derivative molecules, bombykal, bombykoic acid, and bombykyle acetate, are computed by steered and un-biased molecular dynamics in order to compare the role of their polar head group on adsorption on aqueous aerosols. When adsorbed, the molecule center of mass stands at about 1.2 Å from the interface and oscillates on the same length scale, trapped in an energy well. Gibbs energy of adsorption and desorption time of bombykol are found to be 9.2 kBT and 59 µs, respectively. The following ordering between the molecules is observed, reading from the more to the least adsorbed: bombykoic acid [Formula: see text] bombykol [Formula: see text] bombykoic acetate [Formula: see text] bombykal. It originates from a complex interplay of entropy and enthalpy. The entropy and enthalpy of adsorption are discussed in the light of structural arrangement, H-bonding, and hydrophilic tail positioning of the molecules at the interface. Our results show that, when dispersed in the air, pheromones adsorb on aqueous aerosols. However, the individual residence time is quite short on pure water surfaces. Aerosols can, therefore, only have a decisive influence on chemical communication through collective effects or through their chemical composition that is generally more complex than that of a pure water surface

Experiments – Simulations – Theories: Multiscale approaches for solutions

The theory of complex media depends on various levels of description. At the macroscopic scale, hydrodynamic equations are used but they are not valid at small scales, the latter domain can be very important if slow processes occur in the system. At the microscopic scale, molecular dynamics describe the motion of the atoms individually. The two methods which correspond to different levels of description are complementary, but they are not the only ones. Intermediate methods based on continuous solvent model are able to take proper account of the solute dynamics and they can be applied at an intermediate time scale. We present such multi-scale descriptions in the case of solutions. Such an approach is all the more important in the context of of neutron science since neutron scattering experiments are now able to explore a large time domain: they are able to encompass the intermediate time domain for which intermediate methods are unavoidable