Showing papers on "Laplace pressure published in 1985"

Capillary condensation in systems of immiscible liquids

Abstract: Capillary condensation of water from undersaturated nonpolar liquids occurs around curved mica surfaces in contact. The capillary-condensed annulus of water leads to a large adhesion between the mica surfaces and the measured pull-off force agrees to within 5% with the force given by the Young-Laplace equation for the oil/water interface for water activities from ∼ 1 (saturation) down to 0.75. This indicates that the oil/water interfacial tension has its bulk value for menisci with a radii of curvature as low as 2 nm and vindicates the use of the Laplace pressure equation for such small radii. The results further show that, as for liquid/vapor systems, the adhesion force is independent of surface deformations and there is evidence of a solid-solid contribution to the adhesion force, showing up through an effect on the position from which the surfaces come apart. There are, however, systematic deviations from the Kelvin equation for liquid/liquid systems due to the presence of solute, probably from the mica surface, in the condensed annuli. At water activities above 0.8 the oscillatory solvation force found in dry nonpolar liquids is replaced by an attractive force that is observed to “switch on” at some critical distance. A possible explanation is the spontaneous formation of water bridges before the surfaces come into contact. Capillary condensation is also observed in several other systems of incompletely miscible liquids.

The mechanics and thermodynamics of rod-shaped micelles

Abstract: In Part I of this paper the detailed mechanics of a (hemi-)spherocylindrical rod-shaped surfactant micelle is derived by means of the pressure tensor. The anisotropy of the pressure tensor, particularly within the hydrocarbon core, is found to be of fundamental importance for the mechanical stability of a rod-shaped micelle, which requires that the Laplace pressure of the cylindrical part, γc/Rc, is equal in magnitude to the associated (negative) disjoining pressure, πc, that is chiefly due to the packing constraints imposed on the hydrocarbon-chain conformations. Our numerical calculations for sodium dodecyl sulphate (SDS) micelles indicate that this condition is actually fulfilled for a cylinder radius, Rc, which is significantly smaller than the extended hydrocarbon chain length.In Part II a thermodynamic analysis is carried out based on surface thermodynamics and the thermodynamics of small systems developed by T. L. Hill. Large length fluctuations arise, i.e. long, rod-shaped micelles can exist in appreciable amounts only when the work of forming the (homogeneous) central part of the micelle out of monomers at the chemical potential µ–2 in the surfactant solution (βkT per monomer) is close enough to zero (⩽10–2kT). Assuming a spherocylindrical shape it is established that Rc < Rs, which is consistent with the notion of the size distributions of small spherical and large rod-shaped micelles, respectively, being separated by a range of comparatively rare, elongated micelles of intermediate size and, furthermore, that the total free-energy excess of the end-caps, αkT, is larger than the corresponding quantity of the spherical equilibrium micelle. Finally, we show that the coexistence of spherical and rod-shaped micelles is largely geared by the relative proximity of the critical micelle concentration and β= 0 conditions.