Disjoining ionic-electrostatic component

Disjoining pressure and potential arise because molecules in a thin film reside in a different environment than those in a bulk phase of the same composition, temperature, and chemical potential. Several contributors to the disjoining pressure are known (see Table 1). The literature about molecular and ionic-electrostatic components is well reviewed by Sheludko (7). The structural contribution formally identified by Deryagin et al. (8) seems not to have been fully examined theoretically. All of the contributions have multicomponent versions (9) and are more or less additive. Their dependences on thickness are more complicated than the approximations in Table 1. It turns out that depending on the constitution of the three phases involved, the disjoining pressure (or potential) can vary with thickness in a variety of ways. 

The isotherm n = Alh - corresponds to thick P films of water, which are stabilized by the ionic-electrostatic component of the disjoining pressure [1], In this case, integration of Equation 2.66 at = 2 results in... 

The interfacial tension, o, affects directly the rate ratio in Eq. (89) through the capillary pressure, 2da. The addition of electrolyte would affect mostly the electrostatic component of the disjoining pressure (see Fig. 8a), which is suppressed by the electrolyte the latter has a destabilizing effect on OAV emulsions. In the case of ionic surfactant solutions the addition of electrolyte rises the surfactant adsorption and the Gibbs elasticity (see Fig. 5), which favors the stability of emulsion I. 

Tle(h) - Ionic molecules in water give rise to double layers at the fluid-fluid and at the fluid-solid interfaces that consist of a compact surface charged layer and a diffuse layer of counterions (see Figure 2). As a film thins, the counterions of the two double layers approach each other and repulsion of these double layers takes place. This repulsion stabilizes the film by preventing further thinning and creates the electrostatic component of the disjoining pressure. 

With increasing electrolyte concentration the film thickness decreases down to the critical value Cei, cr — 2 X 10 mol dm [7j. At Cei> Cei.a- the remains constant, close to 16 nm. The left-hand part of the h C ) dependence indicates that there is an electrostatic component of disjoining pressure while the plateau indicates the existence of non-DLVO forces due to the steric interaction between the adsorbed polymer layers. Similar are the h,v(Cei) curves of foam films stabilized by A-B-A copolymers, non-ionic surfactants, non-ionic phospholipids, and so on [1—4, 33). 

One concrete example of a situation with the sigmoidal ft/ h) dependence is when the dominating components of the disjoining pressure are those representing the molecular-and ionic-electrostatic molecular interactions, i.e. 

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