Structural Component of the Disjoining Pressure

This component of disjoining pressure is caused by the orientatiou of water molecules in a vicinity of aqueous solution-solid interface or aqneous solnlion-air interface. Keep in mind that all water molecules can be modeled as an electric dipole. 

As a result of these two opposite trends, there is a formation of a finite layer, where the structure of water dipoles differs from the completely random bulk structure. This layer is frequently referred to as the hydration layer. If we now have two interfaces with hydration layers close to each of them (or even one of them), then at a close separation, comparable with the thickness of the hydration 

FIGURE 1.11 Formation of a hydration layer of water dipoles in the vicinity of a negatively charged interface. The darker part of water dipoles is positively charged, whereas the lighter part is negatively charged. 

Unfortunately, until now, there is no firm theoretical background on the structural component of the disjoining pressure, and we are unable to deduce theoretically those cases in which the structure formation results in an attraction and those in which it results in a repulsion. As a consequence, only a saniem-pirical equation exists, which gives a dependence of the structural component of disjoining pressure on the thickness of the liquid film [1]  

However, we are still far from a complete understanding of the preexponential factor K, which can be extracted on the current stage only from experimental measurements of the disjoining pressure. 

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Deryaguin, B, V. Churaev, N. V, (1977). Structural component of the disjoining pressure of thin layers of liquids. Croat. Chem. Acta 50, 187-195. 

In some cases, the growth of perturbations leads to the formation of spots of thinner metastable films (with thickness about 10 nm). The film at the spots is so thin that it appears black in reflected light. Such films are often referred to as black films. These objects are obliged by their origin to a sufficiently large value of the structural component of the disjoining pressure, which determines the existence of the second interval where dH/dh < 0 on the II(/i) curve. A rupture of the black films can also take place, but this mechanism is connected with a display of the vacancy instability [125]. 

First, let us consider the influence of the electrostatic potentials of the bubble and particle vj/p on the contact angle in absence of the structural component of the disjoining pressure. We denote by O, the larger potential between 1)/ and v /p, and by Oj the smaller one. 

In summary, the formation of colloid crystal structure and the corresponding positive structural component of the disjoining pressure in-... 

Figure 7. Sketch of the consecutive stages of the thinning of the liquid film containing spherical particles. The related oscillatory structural component of the disjoining pressure, Ilosc, vs. the film thickness, A, is plotted.

The molecular component of the disjoining pressure, IIm(/i), is negative (repulsive). It is caused by the London-van der Waals dispersion forces. The ion-electrostatic component, IIe(/i), is positive (attractive). It arises from overlapping of double layers at the surface of charge-dipole interaction. At last, the structural component, IIs(/i), is also positive (attractive). It arises from the short-range elastic interaction of closed adsorption layers. 

In Fig. 10.5b the barrier due to the stmctural component of the disjoining pressure is schematically represented for the case where its range of action is either smaller or greater than the thickness of the electric double layer. One of the ways of ensuring enhanced floatability in the presence of the structural component consists in the use of surfactants. Adsorption of... 

The isotherm FI(h) can be obtained experimentally or calculated on the basis of the surface forces theory. When using Eq. (10D.2) for wetting films of water or aqueous solutions, it is necessary to take into account at least three components of the disjoining pressure, i.e. the dispersion, electrostatic. If, and structural, If, contributions. 

At the transition to high electrolyte concentration (10 -10" M), electrostatic interaction is suppressed to some extent and a predomination of the electrostatic attraction force is questionable. It could preserve because the special water structure near a hydrophilic surface is destroyed by increasing salt concentration. More investigation is necessary to clarify which component of the disjoining pressure predominates as both electrostatic and structural components are suppressed to a certain degree at higher salt concentration. 

Abstract. The stability of suspensions/emulsions is under consideration. Traditionally consideration of colloidal systems is based on inclusion only Van-der-Waals (or dispersion) and electrostatic components, which is refereed to as DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. It is shown that not only DLVO components but also other types of the inter-particle forces may play an important role in the stability and colloidal systems. Those contributions are due to hydrodynamic interactions, hydration and hydrophobic forces, steric and depletion forced, oscillatory structural forces. The hydrodynamic and colloidal interactions between drops and bubbles emulsions and foams are even more complex (as compared to that of suspensions of solid particles) due to the fluidity and deformability of those colloidal objects. The latter two features and thin film formation between the colliding particles have a great impact on the hydrodynamic interactions, the magnitude of the disjoining pressure and on the dynamic and thermodynamic stability of such colloidal systems. 

FIGURE 1.13 Calculated and experimentally-measured isotherms of disjoining pressure, n(h), of the films of water on a quartz surface at concentration of KCl C = 10" mol/l, pH = 7, and dimensionless < potential of the quartz surface equals to 6 [1]. (a) Within the region of large thicknesses dimensionless q potential of the film—air interface equals to 2.2 (curve 1), 1 (curve 2), and 0 (curve 3) (b) within the region of small thicknesses dimensionless q potential of the film-air interface equals to 2.2 (curve 1). The structural component, Ilj(/j), of the disjoining pressure isotherm and electrostatic component, n/h), are indicated by curves 2 and 3, respectively. Curves 4 in both part (a) and (b) are calculated according to Equation 1.13. 

Another option to reach an agreement between theoretical and experimental isotherms is provided by the assumption that the shift observed is due to structural interactions in the film which determines the structural component of disjoining pressure ns, [5,312], In that context it is interesting to estimate the function ln(nexp - ITiheor) on h, presented in Fig. 3.60. It is plotted at different NaCl concentrations under the assumption that at constant ( -potential and at Cei = 10 4 and 10 3 mol dm 3, the DVLO-theory is conformed with. 

The fact that the disjoining pressure in NBF does not contain an electrostatic component as well as the lack of a free aqueous core in the film structure allows to use the bilayer lattice model to explain the stability of NBF. This model accounts for the interaction between first neighbour molecules (see Section 3.4.4). 

Theoretical analysis of sheeting in the drainage of thin liquid films has been conducted in [359]. Sheeting dynamics and hole formation (i.e. black spot formation) was described by non-linear hydrodynamic stability analysis based on the equilibrium oscillatory structural component of disjoining pressure. The effect of stepwise thinning, accompanied by formation of holes , was described qualitatively. It is rather arguable whether the term holes for a black spot is appropriate since in 1980 holes in NBF were described as lack of molecules. The use the same term for two different formations is at least confusing. Besides, to have a hole in a CBF is almost as to have a hole in the sea water . 

Stratification of asymmetric aqueous films from NaDoS, CTAB and a commercial surfactant (alpha-olefin sulphonate) solutions at C > Ccmc on a decane substrate have been studied by Bergeron and Radke [236], They found three transitions by thickness in the metastable multilayer films. They observed also stratified CTAB aqueous films on glass. The n(/i) isotherms of stratified films were analysed considering also the oscillatory structural component of disjoining pressure. 

A semiempirical formula for the oscillatory structural component of disjoining pressure was proposed ... 

The disjoining pressure is a sum of several components (just as with soil water potential). The major components of the n(A)-isotherm in porous media are molecular, nm(h) electrostatic, ne(A) structural, ITS(A) and adsorptive IIa(A) ... 

Calculations give values of the contact angle of water on quartz of )/ 4° which is close to experimental data. For this calculation based on Eq. (10D.2), the third component, i.e. the repulsive structural force, was introduced into the equation for the disjoining pressure. Thus, the phenomenon of the incomplete wetting of silicate surfaces is connected to the influence of the electrical state of the film-gas interface. A small contact angle is caused by structural repulsive forces. A still higher effect can be provided by recharging the film-gas surface, which is corroborated experimentally (Zorin et al. 1979). 

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. 

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