Model colloidal fluid with competing interactions: Bulk and interfacial properties

Abstract

Using a simple mean-field density functional theory theory (DFT), we investigate the structure and phase behaviour of a model colloidal fluid composed of particles interacting via a pair potential which has a hard core of diameter σ, is attractive Yukawa at intermediate separations and repulsive Yukawa at large separations. We analyse the form of the asymptotic decay of the bulk fluid correlation functions, comparing results from our DFT with those from the self consistent Ornstein-Zernike approximation (SCOZA). In both theories we find rich crossover behaviour, whereby the ultimate decay of correlation functions changes from monotonic to long-wavelength damped oscillatory decay on crossing certain lines in the phase diagram, or sometimes from oscillatory to oscillatory with a longer wavelength. For some choices of potential parameters we find, within the DFT, a λ-line at which the fluid becomes unstable with respect to periodic density fluctuations. SCOZA fails to yield solutions for state points near such a λ-line. The propensity to clustering of particles, which is reflected by the presence of a long wavelength ≫ σ, slowly decaying oscillatory pair correlation function, and a structure factor that exhibits a very sharp maximum at small but non zero wavenumbers, is enhanced in states near the λ-line. We present density profiles for the planar liquid-gas interface and for fluids adsorbed at a planar hard wall. The presence of a nearby λ-transition gives rise to pronounced long-wavelength oscillations in the one-body densities at both types of interface.

Theory for the phase behaviour of a colloidal fluid with competing interactions

“…From previous studies of the present model fluid [9,10,11,12,15,18,20,21,22,23,24,25,26], we know that the static structure factor S(k) plays an important role in characterising the microphase structuring displayed by the system. One finds that there is a large peak in S(k) at k = k c ≪ 2π/σ, where l c ≡ 2π/k c is the length scale associated with the density modulations in the system.…”

“…In Ref. [11] some of us developed a density functional theory (DFT) [33,34] for the present model. It was found that the DFT provided a good account of the structure and thermodynamics of the uniform liquid.…”

Effect of glycerol and dimethyl sulfoxide on the phase behavior of lysozyme: Theory and experiments

“…16 In particular, similar gas-liquid coexistence curves have been theoretically observed for colloidal dispersions with competing repulsive and attractive interactions, and have been attributed to critical fluctuations which become significant in an enlarged region of the phase diagram. 78 …”

Two-dimensional colloidal fluids exhibiting pattern formation

“…3 Self-consistent OrnsteinZernike approximation (SCOZA) integral equation theory for a model of such systems 4,5 showed that when the long range repulsion is not too strong, there is a large region of the phase diagram where the correlations in the fluid show significant fluctuation effects and where the compressibility increases significantly. The SCOZA theory (which is sophisticated and rather accurate) was also compared with results from density functional theory (DFT), 6 which showed good agreement between the theories for the liquid structure. When the long range repulsion is further increased, the SALR interaction between the particles gives rise to pattern formation in the fluid state, such as gathering to form clusters, stripes (lamellas), and holes (bubbles), referred to as microphase separation.…”