Spreading of one liquid on another

When we place a drop of appreciably non-volatile low density liquid (1) on the surface of a high density sub-phase liquid (2) which is practically immiscible with liquid (1), there are three possibilities  

2 Liquid (1) spreads as a monolayer on the surface of the sub-phase liquid (2), if space on the surface permits. The thickness of the monolayer is so small that gravitational effects are negligible. 

In some instances, lens, monolayer and thick liquid him formation occurs simultaneously, depending on the strength of molecular interactions between these two immiscible materials and the availability of free surface. The surplus material (1) may remain as a lens in equilibrium with the monolayer. In general, a monomolecular him and some local small drops containing the excess material are seen when a pure substance is used to form the insoluble phase over water. However, much thicker hlms are formed for complex organic mixtures such as kerosene on water. 

It is possible to perform a physical analysis to predict either liquid lens or thick him formation, and the strength of adhesion between the two phases. In order to assess the adhesion strength, initially we need to formulate the work of cohesion and adhesion. In Section 2.1, we dehned the term cohesion to describe the physical interactions between the same types of molecule, so that it is a measure of how hard it is to pull a liquid (and solid) apart. In Section 3.5.3, we dehned, the work of cohesion, W), as the reversible work, per unit area, required to break a column of a liquid (or solid) into two parts, creating two new equilibrium surfaces, and separating them to inhnite distance. (In practice, a distance of a few micrometers is sufficient.) The work of cohesion required to separate liquid layers into two parts having unit area can obviously be expressed from the definition of surface tension as 

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For the spreading of one liquid on another S i 2 (in and eq) are accessible from the constituent three interfacial tensions (The tables for liquid-liquid interfacial tensions in appendix 1 refer to mutually saturated systems so from these only S(eq) is obtainable). Examples of initial and equilibrium spreading tensions... 

Interfacial tension is an important factor in chemical and physical processes involving a large interface, such as formation of emulsions, spreading of one liquid on another, wetting, foaming, enhanced oil recovery, and other processes of technical interest. Emulsification requires a low, near-zero, interfacial tension because a large interface has to be created. A low interfacial tension is a result of positive adsorption of the surfactant at the interface. The effect of a surfactant on interfacial tension depends on the extent of adsorption and the nature of the adsorbed film. 

The effect of mutual saturation on the L-V and L-L interfacial tensions is effectively illustrated by considering the spreading coefficient of one liquid on another using both the initial (unsaturated) and equilibrium values of 7. Use the following data to calculate Se/, (equilibrium) and S B/A (nonequilibrium) ... 

In the Report summarized in 167, Mr. Van der Mensbrugghe draws from our principle of the spontaneous transformation of lengthened liquid shapes, the explanation of a fact which is shown in certain cases of spreading out of one liquid on another for example, as Mr. Tomlinson has observed a droplet of lavender oil... 

The initial spreading coefficient does not consider the mutual saturation of one liquid with another for example, when benzene is spread on water,... 

If 0 = 0, then Wa = 2y that is, the work of adhesion between solid and liquid is equal to the work of cohesion of the liquid. Thus the liquid can spread indefinitely over the surface, since energetically the system is indifferent to whether the liquid is in contact with itself or with the solid. On the other hand, if 0 = 180°, cos 0 = — 1, and = 0. No Gibbs energy expenditure is required to separate the solid and the liquid. The liquid does not wet the solid and does not spread on it. The spreading coefficient f or one liquid on another is defined in the same way as for a liquid on a solid, Eq. (18.23), except that cos 0 = 1. Thus... 

The Harkins equation for the spreading of one liquid (e.g. oil) on another liquid (e.g. water) is also valid for the spreading of a liquid on a solid surface. 

Mr. Van der Mensbrugghe a great number of phenomena are due to differences in tension. - Centrifugal current and centripetal current, with the approach of a droplet of a volatile liquid. - If a drop of a liquid takes, on another liquid, a lenticular form, analytical condition of the equilibrium of this form case of spreading out - general Condition of the equilibrium of shape of a lanfinar cap resting on a liquid case where this liquid has a tension different from that of the film experimental checking. New facts on the substitution of one film for another. - Explanation of certain upward currents in water which slides on a tilted wall. - Explanation of the movements carried out, on water, by pieces of camphor and several other substances. 167... 

Another familiar property of liquids is surface tension, the resistance of a liquid to spread out and increase its surface area. Surface tension is caused by the difference in intermolecular forces experienced by molecules at the surface of a liquid and those experienced by molecules in the interior. Molecules at the surface feel attractive forces on only one side and are thus drawn in toward the liquid, while molecules in the interior are surrounded and are drawn equally in all directions (Figure 10.8). The ability of a water strider to walk on water and the beading up of water on a newly waxed car are both due to surface tension. 

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