Biomedical subjects

Dirk Woywod

Publications and source records attributed to Dirk Woywod.

5 recordsLinked to original sources

PubMed2006

Topography of phase diagrams in binary fluid mixtures: a mean-field lattice density functional study.

We employ mean-field lattice density functional theory to calculate complete phase diagrams of binary fluid mixtures composed of molecules of equal size. We consider asymmetric binary mixtures in which the attraction strength between like molecules of either species differs as well as the attractivity between a pair of unlike molecules. Focusing on the topology of phase diagrams in the space spanned by the thermodynamic fields temperature , (mean) chemical potential , and incremental chemical potential (, are chemical potentials of pure mixture components A and B, respectively), we present an argument which precludes the existence of tricritical points (TCPs) in binary mixtures in general. This is a consequence of a purely geometrical argument based upon an analysis of the number of ways in which coexistence surfaces can be joined in the (Euclidian) space of , , and . However, we show that by the same token, TCPs may exist in cases where the mixture possesses some special symmetry. These latter results are in qualitative agreement with earlier works where, however, only special cuts through the complete phase diagrams were considered so that the important relation between existence of TCPs and symmetry properties of the mixture cannot be fully appreciated.

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PubMed2005

Phase behavior and local structure of a binary mixture in pores: mean-field lattice model calculations for analyzing neutron scattering data.

We investigate the phase behavior of an asymmetric binary liquid A-W mixture confined between two planar homogenous substrates (slit pore). Molecules of species W interact preferentially with the solid walls via a long-range potential. Assuming nearest-neighbor attractions between the liquid molecules, we employ a lattice-gas model and a mean-field approximation for the grand potential. Minimization of this potential yields the density profiles of thermodynamically stable phases for fixed temperature, chemical potentials of both species, pore width and strengths of attraction. This model is used to analyze experimental small-angle neutron-scattering (SANS) data on the microscopic structure of the binary system isobutyric acid (iBA)+heavy water (D2O) inside a mesoscopic porous matrix (controlled-pore glass of about 10 nm mean pore width). Confinement-independent model parameters are adjusted so that the theoretical liquid-liquid coexistence curve in the bulk matches its experimental counterpart. By choosing appropriate values of the pore width and the attraction strength between substrates and water we analyze the effect of confinement on the phase diagram. In addition to a depression of the liquid-liquid critical point we observe surface induced phase transitions as well as water-film adsorption near the walls. The temperature dependence of the structure of water-rich and iBA-rich phases of constant composition are discussed in detail. The theoretical predictions are consistent with results of the SANS study and assist their interpretation.

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PubMed2004

Confinement effect on the adsorption from a binary liquid system near liquid/liquid phase separation.

The preferential adsorption of one component of a binary system at the inner surfaces of mesoporous silica glasses was studied in a wide composition range at temperatures close to liquid/liquid phase separation. Confinement effects on the adsorption were investigated by using three controlled-pore glass (CPG-10) materials of different mean pore size (10 to 50 nm). For the experimental system (2-butoxyethanol+water), which exhibits an upper miscibility gap, strong preferential adsorption of water occurs, as the coexistence curve is approached at bulk compositions, at which water is the minority component. In this strong adsorption regime the area-related surface excess amount of adsorbed water decreases with decreasing pore width, while the shift in the volume-related mean composition of the pore liquid shows an opposite trend, i.e., greatest deviation from bulk composition occurring in the most narrow pores. A simple mean-field lattice model of a liquid mixture confined by parallel walls is adopted to rationalize these experimental findings. This model reproduces the main findings of the confinement effect on the adsorption near liquid/liquid phase separation.

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PubMed2004

Wetting of a selective solid surface by an asymmetric binary mixture.

We consider a lattice-gas model of an asymmetric binary mixture in which the attraction between a pair of molecules of species A exceeds that between a pair of molecules of species B. The interaction between two molecules of species A and B is chosen to promote the formation of demixed A-rich liquid bulk phases. Molecules interact with a selective solid wall, preferentially adsorbing molecules of species B. Positions of molecules are restricted to sites on a simple-cubic lattice. We invoke a mean-field representation of the Hamiltonian governing all intermolecular interactions and assume only nearest-neighbor attractions. Minimizing the grand-potential functional of the lattice gas numerically, phase diagrams for films wetting the solid substrate are obtained. One of our key findings concerns B-rich mixed or demixed films forming in the vicinity of the solid surface and coexisting with demixed A-rich films. The formation of B-rich films can be understood as a result of the competition between the asymmetry of the (bulk) mixture and the selectivity of the solid surface. The concentration of component B in B-rich mixed films shows a peculiar temperature dependence. It first increases with temperature T until an "inversion" temperature T(inv) is reached, and then declines for T>or=T(inv) until the critical point between (demixed) A- and B-rich films is reached.

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PubMed2003

Phase behavior of confined symmetric binary mixtures.

We employ mean-field lattice density functional theory to investigate the phase behavior of a binary (A-B) mixture confined to nanoscopic slit pores with chemically homogeneous walls. We consider only nearest-neighbor interactions in symmetric mixtures, where epsilon(AA)=epsilon(BB) not equal epsilon(AB) and epsilon is a measure of attraction between molecules of like (subscripts AA and BB) and unlike species (subscript AB), respectively. In addition, molecules are exposed to short-range attraction by the substrates separated by z lattice planes where epsilon(W) is the relevant coupling parameter. Moreover, the chemical potentials of both components are the same, that is, mu(A)=mu(B)=mu. In thermodynamic equilibrium (for fixed temperature T and chemical potential mu) the grand-potential density omega[rho,m] (rho identical with [rho(1),...,rho(z)], m identical with [m(1),...,m(z)]) assumes a global minimum which we find by minimizing omega numerically with respect to the order parameters rho(l) identical with rho(A)(l)+rho(B)(l) (total local density) and m(l) identical with (rho(A)(l)-rho(B)(l))/rho(l) (local "miscibility") at lattice plane l parallel to the pore walls. By varying epsilon(AB) three generic types of bulk phase diagrams are observed. On account of confinement (i.e., by varying epsilon(W) as well as z) one may switch between these different types of phase diagrams. This may have profound practical repercussions for experimental nanophase separation since depending on pore width and chemical nature of its walls a bulk gas mixture may undergo capillary condensation and form either a stable mixed or demixed liquid phase.

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