Adhesion between two liquids

Adhesion between two liquids. The attraction exerted by one liquid across the interface on the other requires that work must be done to separate them. It may easily be shown that this work, called the adhesional work9 between the liquids, is equal to the sum of the surface tensions of the liquids singly, less the interfacial tension of the liquid-liquid interface. Suppose A and B (Fig. 1) are the liquids in a column one square centimetre in cross-section then when they are in contact the energy of the interface is yAB when they have been separated by a direct 

This important relation is due to Dupr. 1 It is so much easier to visualize the attraction WAB, which one liquid has for another, than the surface tension, or inward pull on the surface molecules, that an insight into the 

The work of adhesion cannot be measured Flo j directly, but must at present be deduced from 

The condition for complete miscibility of two liquids is simply that the interfacial tension between them should be zero or negative. If this is so, then the molecular forces no longer operate to keep the liquids apart, for each liquid attracts the molecules of the other as much as, or more than, they are attracted by their own liquid. WAB becomes equal to, or greater than, Ya+Yb and therefore molecules move across from one liquid to the other quite freely. 

Cohesional work. Harkins2 has introduced the term cohesional work , to denote the work required to separate a column of a liquid, one square centimetre in cross-section, into two since the result of this operation is to produce two square centimetres of liquid surface where no interface was before, the cohesional work of a liquid is 2y. Comparison of the cohesional work with the adhesional work to another liquid gives a fair comparison of the relative intensities with which the molecules of a liquid attract the molecules of the same and of another liquid. 

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It will be seen in Chapter IV that these adhesions between different chemical groups are in good agreement with the values deduced from measurements of the adhesion between two liquids. 

The work of adhesion between two liquids orientation at liquid-liquid interfaces. Much the clearest conclusions at present available as to the structure of the surfaces of liquids (except those covered by films of another substance) concern the interface between two liquids. They have been obtained by considering the work of adhesion between the liquids, or the perpendicular attraction across the interface. This work of adhesion can be ascertained by measurement of the surface tensions of the two liquids separately, and of the interfacial tension between them, and applying Dupre s equation (Chap. I, 8) ... 

Just as the work of adhesion between two liquids (Chap. IV, 2) gives information as to the orientation of molecules of an organic liquid in contact with water, so WSL, deduced by (3) from the contact angle, may indicate the orientation of the surface molecules in a solid. The problem is easier in one respect, for the surface molecules of a solid are not in constant motion with changes of orientation, as are those of a liquid. On the other hand, the soaking of the liquid into the surface layers of the solid may confuse the results in a way not likely to occur with the liquid surface also the surface of the solid may be difficult to clean. This last difficulty has, in the author s experience, proved surprisingly small. 

Derive the expression (in terms of the appropriate works of adhesion and cohesion) for the spreading coefficient for a substance C at the interface between two liquids A and B. 

Fig. XII-13. Dlustration of adhesion between two plates due to a meniscus of a wetting liquid.

The work of adhesion between two immiscible liquids is equal to the work required to separate unit area of the liquid-liquid interface and... 

This equation is basic for determination of interfacial tension and for explaining phenomena of wetting and adhesion [iii-v] including electrochemical experiments [vi]. For the work of adhesion between two immiscible liquids see -> Dupre equation. 

Several times in this section liquid bridges have been encountered, i.e. bodies of fluid with concave surfaces, connecting a solid object and a fluid. In connection with flg. 5.46 we shall briefly address these. In particular such bridges play important roles in the adhesion between two solid macrobodies, say between the particles in a powder. Depending on conditions, the Laplace underpressure in the bridge may result in a stronger adhesive force than that caused by Van der Waals forces. 

Polymers are involved in many practical adhesion problems. A polymer liquid can be present in the gap between the two media that adhere to one another in order to create strong attractive forces that strengthen the adhesion. In this context it is important to understand how polymer solutions interact with surfaces and how they create strong interactions between them [1]. The aim of this short review is to present rather qualitatively our understanding of the equilibrium thermodynamic properties of polymer solutions close to surfaces. This is clearly one of the important factors in understanding the adhesion between two surfaces mediated by polymers, but one must keep in mind that adhesion is a nonequilibrium process where energy dissipation plays a major role. This aspect will not be considered in this chapter. 

As is usually the case when theoretical and experimental science meet, it is necessary to make some simplifying assumptions in order to apply theories to practical systems. Good and Girifalco proposed an empirical approach to the problem based on the Berthelot principle that the interaction constant between two different surfaces or particles will be the geometric mean of the interaction constant for the individual surface units, an approach already introduced in Chapter 4. Good and Girifalco suggested that the work of adhesion between two different liquids could be expressed as a similar function... 

Because, in theory at least, file concept of ideal or thermodynamic adhesion applies equally well to liquid and solid phases, it is of interest to see how a calculation of such an ideal value compares with reality. The complete expression for the work of adhesion between two phases with each phase completely saturated by the other, denoted by A(B) and B(A) is... 

By analogy Good and Girifalco [10] represented the work of adhesion between two different liquids W u as the geometric mean of their respective works of cohesion,... 

Surface tension has been defined as the force per unit length in a liquid-vapour interface. The concept can be extended to two phases of different fluids providing they do not mix immiscible fluids. The surface tension between two liquid phases is called the interfacial tension, and that between a solid and a liquid the adhesion tension. There will also be a surface tension at a solid-gas interface. 

The work of adhesion between two solids, which is the sum of the surface tensions of the two solids minus the interfacial sohd-sohd tension, can be calculated by some of the methods we saw previously for liquid-liquid and hquid-sohd interfaees ... 

Mjcracalorimetry, also nanocalorimetry to follow the recent trends in thermal instrumentation and analysis, is a measuring technique that can be used to study interfacial phenomena occurring at the Solid-Liquid interface. Immersion of a solid in a pure liquid or a solution, wetting of a solid initially in contact with a gas or vapour by a liquid, adhesion between two condensed phases upon their molecular contact are examples of exothermic phenomena which are accompanied by significant heat evolvement. Competitive adsorption from solution is an important exception to the exothermicity of interfacial phenomena. This is because certain components of the... 

This chapter and the two that follow are introduced at this time to illustrate some of the many extensive areas in which there are important applications of surface chemistry. Friction and lubrication as topics properly deserve mention in a textbook on surface chemistiy, partly because these subjects do involve surfaces directly and partly because many aspects of lubrication depend on the properties of surface films. The subject of adhesion is treated briefly in this chapter mainly because it, too, depends greatly on the behavior of surface films at a solid interface and also because friction and adhesion have some interrelations. Studies of the interaction between two solid surfaces, with or without an intervening liquid phase, have been stimulated in recent years by the development of equipment capable of the direct measurement of the forces between macroscopic bodies. 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

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