Solid/liquid surface tension

Two mechanisms were proposed for the strong upward and downward flows [135], The first was based on interplay of the liquid-liquid and solid-liquid surface tensions and the gravitational and inertia forces. The second is correlated with the neck formation, the droplet break-up and the retossing of the fluid. 

When the surface of the liquid is drawn up on the solid we say that the liquid wets the solid. This results from a substantial attraction between the liquid and the surface of the solid which is indicated by a small value ySL, the solid-liquid surface tension, and a large downward net surface tension force. A device that measures the force on a plate, such as shown in Fig. 6, is called a Wilhelmy balance. 

The quantity yLA cos was termed by Freundlich the adhesion tension , and is frequently used it is equal to the difference between the solid-air and solid-liquid surface tensions, that is, to WSL—yLA. 

One way to obtain ys from the measured difference ys — K.sv is described by Weiler et al.. who determined the surface tension of finely divided sodium chloride [88], They measured ys — ysv for ethanol and benzene up to saturating vapor pressure. Then they assumed that at saturating vapor pressure ysv is similar to the solid-liquid surface tension ysL- At saturating vapor pressure the surface of a solid is covered with a multilayer of the adsorbed vapor. Consequently the surface tension should not be significantly different from the solid-liquid interface. Taking ysL =171 mN/m from solubility experiments [204J and measuring ys - ysv = 56 mN/m for ethanol, they obtained ys = 227 mN/m. 

An alternative theoretical approach to the evaluation of the solid—liquid surface tension which is particularly useful at low pressure and coverage is based on an integration of the Gibbs adsorption equation (Eqn 2.60 in [7]) that can be written as... 

The lines of force acting along the line of intersection are the surface tensions of the liquid in equilibrium with the surrounding vapor (Ylv)> solid/vapor surface tension (y ) and the solid/liquid surface tension (Ysl)- Simple geometric considerations diagrammed in Fig. 1 lead to Young s equation... 

FIGURE 2.28. Liquid/vapor and solid/liquid surface tensions (both in mN/m) as functions of surfactant concentration. ... 

When we consider the formation of the MS (which we assume to be the closest to the sol phase), then the free energy gap AGy is lower than the most stable phase, referred to as SS. Thus, according to CNT (Equation 1.2), the only way that the phase MS can precipitate out at any temperature is to have such a lower surface tension that the nucleation barrier is lowest for MS. Because of lower solid-liquid surface tension leading to a lower nucleation barrier, MSs are kinetically favored. Thus, kinetics seems to play a very dominant role. 

Figure 7.14 Wetting of solids by liquids = liquid-air surface tension, Ks, = solid surface energy, Kj, =solid-liquid surface tension, 9 = wetting angle.

If the Hquid is not exposed to external forces (e.g. in a vacuum), it occupies a spherical shape as it gains the smallest surface at the given volume. On the surface of another liquid, it either forms a drop that has a lenticular shape, or can be spread over its surface. The liquid s behaviour at a given temperature depends on the size of the liquid-air surface tension, and on the interfacial liquid-liquid surface tension. On the sohd surface the liquid also either spreads out evenly or forms a drop. The shape and size of the drop depend on how the Kquid wets the solid surface, which is related to the liquid-air surface tension (Kig), surface energy of a sohd (Ksg) solid-liquid surface tension (y j) and temperature (Figure 7.14). 

In spite of its importance, available data on the solid-liquid surface tension YsI are scarce. Its experimental measurement is highly challenging and usually afflicted with large uncertainties. The computer simulations offer an alternative route to estimate yn and to interpret experimental results Several techniques have been developed for this... 

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