Forces Between Surfaces

The discussion focuses on two broad aspects of electrical phenomena at interfaces in the first we determine the consequences of the presence of electrical charges at an interface with an electrolyte solution, and in the second we explore the nature of the potential occurring at phase boundaries. Even within these areas, frequent reference will be made to various specialized treatises dealing with such subjects rather than attempting to cover the general literature. One important application, namely, to the treatment of long-range forces between surfaces, is developed in the next chapter. 

The measurement of surface forces calls for a rigid apparatus that exhibits a high force sensitivity as well as distance measurement and control on a subnanometre scale [38]. Most SFAs make use of an optical interference teclmique to measure distances and hence forces between surfaces. Alternative distance measurements have been developed in recent years—predominantly capacitive techniques, which allow for faster and simpler acquisition of an averaged distance [H, 39, 40] or even allow for simultaneous dielectric loss measurements at a confined interface. 

SFA has been traditionally used to measure the forces between modified mica surfaces. Before the JKR theory was developed, Israelachvili and Tabor [57] measured the force versus distance (F vs. d) profile and pull-off force (Pf) between steric acid monolayers assembled on mica surfaces. The authors calculated the surface energy of these monolayers from the Hamaker constant determined from the F versus d data. In a later paper on the measurement of forces between surfaces immersed in a variety of electrolytic solutions, Israelachvili [93] reported that the interfacial energies in aqueous electrolytes varies over a wide range (0.01-10 mJ/m-). In this work Israelachvili found that the adhesion energies depended on pH, type of cation, and the crystallographic orientation of mica. 

J. N. Israelachvili, P. M. McGuiggan. Forces between surfaces in liquids. Science 247 795-800, 1989. 

Recently the wall-PRISM theory has been used to investigate the forces between hydrophobic surfaces immersed in polyelectrolyte solutions [98], Polyelectrolyte solutions display strong peaks at low wavevectors in the static structure factor, which is a manifestation of liquid-like order on long lengths-cales. Consequently, the force between surfaces confining polyelectrolyte solutions is an oscillatory function of their separation. The wall-PRISM theory predicts oscillatory forces in salt-free solutions with a period of oscillation that scales with concentration as p 1/3 and p 1/2 in dilute and semidilute solutions, respectively. This behavior is explained in terms of liquid-like ordering in the bulk solution which results in liquid-like layering when the solution is confined between surfaces. In the presence of added salt the theory predicts the possibility of a predominantly attractive force under some conditions. These predictions are in accord with available experiments [99,100]. 

In the presence of large amount of micelles, the total force between surfaces may oscillate due to the occurrence of oscillatory structural forces. Structural... 

S.S. Patel and M. TirreU Measurement of Forces Between Surfaces in Polymer Fluids. Annu. Rev. Phys. Chem. 40, 597 (1989). 

Historical deveiopment of van der Waais forces. The Lennard-Jones potentiai. intermoiecuiar forces. Van der Waais forces between surfaces and coiioids. The Hamaker constant. The DLVO theory of coi-loidal stability. 

The repulsive force between surfaces with end-grafted polymer chains has been fairly well studied both theoretically and experimentally and more details can be obtained from advanced and specialized textbooks such as Israelachvili (1991) and Napper (1983). The situation is much more complicated in the case of chains adsorbed on surfaces. In this case, the segments adsorbed on the surfaces are held on the surfaces by relatively weaker forces (in contrast to... 

Traditional Views of Forces Between Surfaces in Liquids... 

The second device with which surface forces can be measured directly and relatively universally is the atomic force microscope (AFM) sometimes also called the scanning force microscope (Fig. 6.8) [143,144], In the atomic force microscope we measure the force between a sample surface and a microfabricated tip, placed at the end of an about 100 //,m long and 0.4-10 //,m thick cantilever. Alternatively, colloidal particles are fixed on the cantilever. This technique is called the colloidal probe technique . With the atomic force microscope the forces between surfaces and colloidal particles can be directly measured in a liquid [145,146], The practical advantage is that measurements are quick and simple. Even better, the interacting surfaces are substantially smaller than in the surface forces apparatus. Thus the problem of surface roughness, deformation, and contamination, is reduced. This again allows us to examine surfaces of different materials. 

The force between surfaces which are coated with polymers is mainly determined by two factors. The first one is the quality of the solvent. In good solvents the force tends to be repulsive, in bad solvents attractive. Moreover, in good solvents polymer tends to remain in solution rather than adsorbing to surfaces. 

A fussier preparation that uses "atomic force microscopy" (AFM) demonstrates that there can be a lateral force between surfaces sinusoidally coated with gold.49 Sophisticates of earlier sections know that a probe such as AFM will "see" into a surface to a depth comparable with separation between probe tip and surface. When substrate structure varies with depth from surface, interpretation of van der Waals interactions can be problematic. Nevertheless, good success has been reported.50,51... 

Prieve, D. C., and E. Rnckenstein, The Surface Potential of and Double-Layer Interaction Force between Surfaces Characterized by Multiple Ionizable Groups, J. Theoretical Biol., in press. 

The van der Waals forces between surfaces covered with adsorbed layers can be calculated using the effective Hamaker constant A given by (18)... 

DiMarzio and Rubin s predictions (Rubin, 1965 DiMarzio and Rubin, 1971) confirm earlier results (Hoeve et al., 1965 Silberberg, 1967 Roe, 1965, 1966) and provide new ones. The entropy per segment is zero for xs < Xso but decreases considerably as chains lose configurational freedom upon adsorption for Xs > Xsc- The force between surfaces is monotonically repulsive for Xs < Xsc and increases as xs decreases. For Xs > Xsc. monotonic attraction is predicted, although its strength at fixed separation passes through a maximum and then falls as Xs increases. These predictions, however, do not allow for exchange of polymer with the bulk solution. 

Constant potential interactions the force between surfaces with a. Unlike potentials will be attractive at all separations. 

---