Adsorption Phenomena at the Surface of Silica Spheres in a Binary Liquid Mixture

Abstract

o Silica spheres of 1600-A diameter immersed in the binary fluid water-f 2-6-lutidine have been studied by light-scattering techniques. The scattered light was seen to be strongly dependent on temperature and concentration. This effect is due to the appearance of a lutidine layer on the spheres. The layer thickness dramatically increases near a transition line were the spheres aggregate as a result of attractive interactions. This transition exhibits all the features predicted for the prewetting transition between high and low adsorption of a fluid on a surface.PACS numbers: 68.45. Da, 64.70.Ja, 78.35. + c, 82.70.Kj Wetting phenomena on a wall appear in multicomponent systems when the components exhibit different interactions with the wall. Even in the simplest case of a binary fluid, a rich behavior is expected. 1 " 2 In the region where the system exhibits two phases, i.e., on the coexistence curve ( Fig. 1), a first-order transition between partial wetting (finite contact angle between the wall and the two phases) and complete wetting (one phase wets the wall and surrounds the other phase) has already been observed. 3 In the case of the water-lutidine (W-L) mixture, which presents a lower critical point at the L mass fraction C = 0.286 and at the temperature 7 1 = 34°C, 4 this "wetting transition" takes place for a silica wall at C w -0.07 and T w -50 °C, 5 with the L-rich phase wetting the silica. In the one-phase region of a binary mixture one expects a line of first-order "prewetting" transition which should separate a region of weak chemical adsorption from a region of large adsorption. This line ( Fig. 1) should begin at the wetting transition (C W ,T W ) and end with a prewetting critical point (CA>TA)-which should not be confused with the liquid-liquid critical point (C C ,T C ).Up to now, prewetting has remained undetected. However, the behavior of small silica spheres immersed in the W-L mixture provides, as we shall see, some evidence of the existence of such a prewetting line in the onephase region. (The anomalous adsorption which has already been detected near the bulk critical point 6,7 is related to the bulk critical region and is not connected to the prewetting transition.) Small spheres immersed in binary fluids have already been studied near the liquid-liquid critical point by microscopy 8 or dynamic light scattering. 9,10 In all cases anomalous mobilities have been observed, attributed to correlated layers 8,10 or to surface interactions. 9 Finally, interferometric measurements in the W-L mixture have already proved the existence of a Ladsorption layer on silica spheres. 11Experiments.-Our initial aim was to deduce the thickness of the L layer sticking on silica by measuring the optical cross section of small silica spheres (diameter 2a = 1600 A) immersed in the W-L mixture. For that purpose, we prepared W-L samples with water incorporating a small mass fraction C 0 (0.004 or 0.009) of monodisperse silica spheres prepared with use of the Storber method. 12 No shift of the bulk critical poi...

Alignment of cylindrical colloids near chemically patterned substrates induced by critical Casimir torques

“…Critical Casimir forces in colloidal suspensions have been discussed in the literature to drive colloidal aggregation phenomena [90][91][92]. Moreover, self assembly of spherical colloidal particles induced by critical Casimir forces has been observed experimentally in Ref.…”

Hydrodynamics in bridging and aggregation of two colloidal particles in a near-critical binary mixture

“…In real experiments, charges usually appear on the colloid surfaces in aqueous mixtures 14,15,[18][19][20][21][22]32 . Supposing weak ionization, we neglect the effect of charges on the adsorption-induced interaction.…”

“…For t < 0, we realized the equilibrium at fixed particle positions as a stationary solution of eqn (14). The correlation length ξ in the bulk is 0.09a in the final state.…”

Interplay of complete wetting, critical adsorption, and capillary condensation

“…On one side, this for the DMRG method unavoidable confinement of the system complicates the aforementioned crossover behavior due to finite-size effects and capillary condensation. On the other side, the interplay between complete wetting, critical adsorption, and capillary condensation do not appear to have been discussed before, although it might be of relevance for adsorption-induced colloidal aggregation in binary liquid mixtures [23,24]. Our approach is naturally suited for investigating interesting and relevant aspects of these important phenomena.…”