Relevance of Film Pressures to Interfacial Tension, Miscibility of Liquids, and Lewis Acid–Base Approach

Abstract

In this paper, we demonstrate the spreading pressure and interfacial film pressure to be profoundly relevant to interfacial tension, miscibility of liquids, and the Lewis acid–base approach. For immiscible liquid–solid and liquid–liquid systems, we prefer to employ Harkins' spreading model containing the equilibrium spreading pressure πe. With the inclusion of πe, we can also improve the Lewis acid–base approach for hydrogen-bonding, proposed by van Oss, Chaudhury, and Good. We establish an acidity–basicity scale for the initial surface tension by taking πeinto account, and we further calculate interfacial tensions for liquid pairs containing formamide or dimethyl sulfoxide (DMSO) with dispersion components cited in Fowkeset al.'s later publication. However, for initially immiscible liquid–liquid systems, the Harkins model does not apply, and we propose, instead, an adsorption model, which requires the interfacial tension to be varying and surface tensions of the bulk liquids at a distance from the interface to remain unchanged. Thus, the difference between the initial and equilibrium interfacial spreading coefficients (Si) equals the equilibrium interfacial film pressure (πi)e, and that difference also equals that of the two interfacial tensions. For liquid–liquid systems, we can choose either of these two models depending on the miscibility of liquids. According to our adsorption model, there should be two interfacial tensions. The initial (or calculated) interfacial tension can be positive or negative, while the equilibrium (experimental) interfacial tension can reach zero. The former has more predictive value than the latter. A negative, initial interfacial tension is delineated to favor miscibility or spontaneous emulsification, but it tends to revert to zero instantaneously. Furthermore, a slightly positive, initial interfacial tension can also lead to miscibility, if (πi)ecan help reduce it to a zero equilibrium interfacial tension. We have also found a new important relationship between (πi)eand πeon the basis of the Laplace equation. We also attempt to calculate with our model the (πi)e's for at least 34 liquid pairs, by assuming the published interfacial tensions to be reasonable equilibrium values. Finally, the equilibrium spreading pressure πefor the liquid–solid interface, or the (πi)efor the liquid–liquid interface, appears to be the missing link between the wetting thermodynamics and linear free energy solvatochromic relationship (LFER). We have shown the πeor (πi)eof the former to be the equivalent of the Π*, polarity/dipolarity parameter, of the latter. Our findings about the correlation between the surface tension component (STC) concept and LFER have provided a new support for the STC theory.