Theory of solvency

In the early phase of coating technology, due to the small number of binders, empirical selection of solvents had been adopted with the general rule of like dissolves like. But after around 1930, when a more diverse variety of synthetic binders was introduced, selection 

London dispersion forces, which arise from mutual induction of atomic dipoles due to the electromagnetic fieid between the nucleus and electrons of the atom, leading to attraction between molecules. In the case of non-polar molecules, these are the only intermolecular forces that exist. 

Dipole-dipole forces, which are the forces of attraction between molecules with a finite, permanent overali dipoie moment. 

Dipole-induced dipole forces, which arise from the interaction between a permanent dipole and another dipole induced by its proximity to the permanent dipole. 

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Bagley EF (1975) Theories of solvency and solution. In Carver JK, Tess RW (eds), Applied polymer science, ACS Organic Coatings and Plastics chemistry, Washington, DC chap 13... 

The power to dissolve resins is the foremost requirement of a solvent except in cases involving dispersions in nonaqueous solvents (NADs) or dispersions in water (latices, emulsions, and dispersions). Theories of solvency and solution are covered by Rider in the preceding chapter. The classic books by Hildebrand and Scott (18) and Hildebrand, Prausnitz, and Scott (19) discuss solubility and solutions in considerable depth. The monumental book by Doolittle ( covers both theoretical and applied aspects of solvents. Several chapters in the Mattiello series published in 1941-46 deal with solvents the chapter on lacquer solvents by Bogin... 

Perhaps the two most important properties of solvents are evaporation rate and solvent power. Solvent power is related to various fundamental solution parameters as discussed in the chapter on theories of solvency and solution. However, solvent power also influences viscosity and the orientation of molecules which in turn affects many other properties. 

An illustrative example is the work of Clark et al, on the conformation of poly(vinyl pyrrolidone) (PVP) adsorbed on silica 0). These authors determined bound fractions from magnetic resonance experiments. In one instance they added acetone to an aqueous solution of PVP in order to achieve theta conditions for this polymer. They expected to observe an increase in the bound fraction on the basis of solvency effects as predicted by all modern polymer adsorption theory (2-6), but found exactly the opposite effect. Their explanation was plausible, namely that acetone, with ability to adsorb strongly on silica due to its carbonyl group, would be able to partially displace the polymer by competing for the available surface sites. 

The effect of solvency for the polymer chain has been considered in the thermodynamic treatment of Flory and Huggins [6], usually referred to as the Flory- Huggins theory. This theory considers the free energy of mixing of a pure polymer with a pure solvent, in terms of two contributions, namely the enthalpy of... 

The interaction between particles containing adsorbed nonionic surfactants or polymer and the theory of steric stabihsation. Particular attention wiU be given to the solvency of the medium for the stabilising chains that determines the magnitude of steric repulsion. 

The results of Clayfield and Lumb relate entirely to the loss of configurational entropy of the polymer chains on close approach of the particles, due either to the presence of the impenetrable surface of the opposite particle or the polymer chains that are attached to that particle. In the early papers, the effect of the solvent on the conformation of the macromolecules was ignored but an attempt was made to include the role of solvency in some of the later publications. Notwithstanding this, essentially what Clayfield and Lumb calculated was the elastic contribution to Ae repulsive free energy of interaction between sterically stabilized particles. As such, their results are manifestly unable to explain the observed flocculation of sterically stabilized particles that is induced by decreasing the solvency of the dispersion medium. Even if only for this reason, the assertion by Osmond et al. (1975) that the Clayfield and Lumb theory was the best available at that time is clearly untenable. 

The early theories of steric stabilization reviewed in the previous chapter were primitive and phenomenologically incomplete. Specific ly, the quality of the solvency of the dispersion medium for the stabilizing moieties did not enter directly into these theories. This grave deficiency was remedied, however, in the second and later generation theories. In this chapter, we will consider the pseudo-a > initio theories of steric stabilization that incorporate the solvency of the dispersion medium. 

Feigin and Napper (1980b) predicted quite different behaviour with respect to the solvency of the system the better is the solvent for the free polymer, the larger are the free energy changes involved and thus the smaller are the values of V2 and V2. These predictions, however, only apply to naked surfaces. The situation is much more complicated when steric barriers are present, as they would be in the theory of Scheutjens and Fleer and in many practical situations. In that case, the steric repulsion is greater in the absence of free polymer in a better solvent. It is the subtle interplay of the effects of solvency change on both steric stabilization and depletion stabilization that determines the overall trend observed. At present there does not appear to be any data in the literature that permit a clear cut statement to be made of the effects of solvency on depletion stabilization in the presence of steric layers. 

As expected from theory, it was also shown that the adsorption of PVA increases with decrease of solvency of the medium for the chains [19], obtained either by increasing the temperature or addition of electrolytes such as KCl or Na2S04. 

To study the effect of solvency on adsorption, measurements were carried out as a function of temperature [30] and addition of electrolyte (Kd or Na2S04) [31]. Increasing temperature and/or addition of electrolyte reduces the solvency of the medium for the PVA chains (due to break down of the hydrogen bonds between the vinyl alcohol units and water). Figure 5.11 shows the adsorption isotherms for PVA with M = 65100 as a function of temperature. This shows a systematic increase of adsorption with rising temperature, i.e. with reduction of solvency (increase in x), as expected from theory. The results obtained in the presence of electrolyte are shown in Figures 5.12 and 5.13. In both cases, addition of electrolyte increases adsorption of PVA, again due to the reduction of solvency of the medium for the chains. 

The structure of the adsorbed layer is described in terms of the segment density distribution, p(z). As an illustration, Fig. 5 shows some calculations by Scheutjens and Fleer [17] for loops and tails with r = 1000,4>. = 10 , and X = 0.5. In this example, 38% of the segments are in trains, 55.5% in loops, and 6.5% in tails. This theory demonstrates the importance of tails which dominate the total distribution in the outer region of the adsorbed layer. As we will discuss in the next section on experimental techniques for characterization of the adsorption and conformation of polymers at the solid liquid interface, determination of the segment density distribution is not easy and usually assigns a value for the adsorbed layer thickness 6. This increases with increase of the molecular weight of the polymer and increase of solvency of the medium for the chains. 

The critical adsorption energy. A critical adsorption energy is predicted by many theories (2-6). Its value is dependent on conformational properties of the polymer and usually estimated as a few tenths ofkT (7). Yet, a method to determine x c experimentally has never been suggested. Displacement studies provide such a method. Inspection of Equation 5 bears out that Xsc is obtained from a displacement isotherm, provided that Xgd and the solvency terms vanish. This condition is met by taking as the displacer a molecule which is (nearly) identical to the repeating unit of the polymer, i.e. the polymer is displaced by its own monomer. Such... 

In this section we give a selection of theoretical and experimental results for homopolymer adsorption. For a meaningful comparison between theory and experiment it is mandatory that the experimental system Is well defined, with as many parameters known as possible (chain length and chain-length distribution, solvency, adsorbent properties, etc.). Wherever feasible, we shall discuss theoretical predictions In combination with experimental data. However, this correspondence cannot be malnteiined in all cases there are useful theoretical predictions that, as yet, cannot be checked experimentally (for example, the relative contributions of loops and tails), whereas for some measurable quantities no quantitative theory has yet been developed (for example, most kinetic data). 

The first theory to recognize clearly the prime importance of the solvency of the dispersion medium in steric stabilization was that published by Fischer (1958). Fischer considered the overlap of the steric layers attached to two spheres (see Fig. 10.4). The mixing free energy change S A G ) in the small volume element dV for one of the steric layers is given by... 

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