Disjoining molecular component

With respect to the molecular interactions the simplest asymmetric films are these from saturated hydrocarbons on a water surface. Electrostatic interaction is absent in them (or is negligible). Hence, of all possible interactions only the van der Waals molecular attraction forces (molecular component of disjoining pressure) can be considered in the explanation of the stability of these films. For films of thickness less than 15-20 nm, the retardation effect can be neglected and the disjoining pressure can be expressed with Eq. (3.76) where n = 3. When Hamaker s constants are negative the condition of stability is fulfilled within the whole range of thicknesses. 

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. 

Mechanism of rupture. Black films. The mechanism of hydrodynamic instability of thin foam films was analyzed in [278, 279, 411], The stability of ultrathin films is governed by a competition between capillary forces and the molecular component of the disjoining pressure. An instability can arise when dU/dh > 0 and the capillary pressure is not too large. This is possible if... 

Consequently, for symmetric films the molecular component of disjoining pressure is always negative, which corresponds to a tendency of dispersion medium layer separating identical phases to decrease its thickness. At the same time, one should emphasize that in such systems in the absence of non-dispersion interactions the lower the value of complex Hamaker constant is, the more similar in nature the interacting phases (dispersed phase and dispersion medium) are. If contacting phases are essentially similar in structure and chemical composition, the value of A may be as low as 10 21 J or even much lower. The so low Hamaker constants result in changes in the nature of colloidal stability. 

If film thickness decreases solely due to the action of a molecular component of disjoining pressure, Ap = This allows one to determine... 

The electrostatic components of disjoining pressure and free energy of interaction in the film, given by eqs. (VII.21) and (VII.22), are positive, i.e. represent repulsion. These quantities may be compared with corresponding molecular components that are negative and describe attraction. This allows one to analyze according to the DLVO theory the stability of thin films, and consequently of disperse systems stabilized by adsorption layers. Carrying out summation of eqs. (VII.21) and (VII.22) with expressions (VII.9) and (VII. 10) one obtains ... 

The molecular component of the disjoining pressure, rtm, for a film of uniform thickness... 

This comparison shows that the disjoining pressure is of the same general magnitude as the adhesive force and its molecular component in air medium this... 

Sources of Disjoining Pressure. The disjoining pressure arises from several sources molecular (van der Waals) action of the solid phase on the boundary layer of liquid (molecular component) and the formation of an electrical double layer at the interface. The electrical component in turn includes ionic and diffusion components ... 

Let us now return to Eq. (VI. 18). We will compare the electrical and molecular components of disjoining pressure, considering, with certain assumptions, that these components are responsible for the full disjoining pressure, i.e.,... 

The modern theory of H,n was developed [1] based on a fluctuating electromagnetic field. The molecular component of the disjoining pressure, Hn, for a film of uniform thickness h between two semi-infinite phases (1 and 2) in... 

There are corresponding expressions for the molecular component of the disjoining pressure of films of non-polar liquids. Those expressions are presented in [1]. The functional dependency... 

Calculations were made for two nonpolar liquids with a different volatility decane and hexane. The well-known equation of the molecular component of the disjoining pressure, TlQi) = A/h, where A 10 erg [37], was used as the isotherm 11(h). The value of the Hamaker constant. A, was regarded as independent of the temperature [38]. Dependences of the total mass flow rate, Q, and of... 

The DLVO-theory considers only the molecular van der Waals and electrostatic interactions. A complete analysis of the theory can be found in several monographs [e.g. 3-6] where original and summarised data about the different components of disjoining pressure in thin liquid films, including in foam films are compiled. 

Fig. 4 The components of the surface energy measured on hydrogenated carbon overcoated thin film magnetic recording media (A) The dispersive component of the surface energy for PFPE Z and Zdol (B) the polar component of the surface energy for PFPE Zdol with molecular weight (M-w) of 1100 (A) 1600 (O), and 3100 ( ) g/mol and (C) the disjoining pressure as a function of film thickness for PFPE Zdol (Mw is 3100 g/mol).

The basic equations can be further simplified in the framework of the lubrication approximation, which can be applied to the case when the Reynolds number is small and when the distances between the particle surfaces are much smaller than their radii of curvature (Figure 5.31). There are two ways to take into account the molecular interactions between the two particles across the liquid film intervening between them (1) the body force approach (2) the disjoining pressure approach. The former approach treats the molecular forces as components of the body force, f... 

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) ... 

In conclusion, it should be noted that, in accordance with Eq. (VI. 18), the components of the disjoining pressure that we have been considering, particularly the molecular and ionic components, are manifested at comparatively short distances from the boundary of the solid phase (a few hundred angstroms). 

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