Solvation, forces

A major advance in force measurement was the development by Tabor, Win-terton and Israelachvili of a surface force apparatus (SFA) involving crossed cylinders coated with molecularly smooth cleaved mica sheets [11, 28]. A current version of an apparatus is shown in Fig. VI-4 from Ref. 29. The separation between surfaces is measured interferometrically to a precision of 0.1 nm the surfaces are driven together with piezoelectric transducers. The combination of a stiff double-cantilever spring with one of a number of measuring leaf springs provides force resolution down to 10 dyn (10 N). Since its development, several groups have used the SFA to measure the retarded and unretarded dispersion forces, electrostatic repulsions in a variety of electrolytes, structural and solvation forces (see below), and numerous studies of polymeric and biological systems. 

The well defined contact geometry and the ionic structure of the mica surface favours observation of structural and solvation forces. Besides a monotonic entropic repulsion one may observe superimposed periodic force modulations. It is commonly believed that these modulations are due to a metastable layering at surface separations below some 3-10 molecular diameters. These diflftise layers are very difficult to observe with other teclmiques [92]. The periodicity of these oscillatory forces is regularly found to correspond to the characteristic molecular diameter. Figure Bl.20.7 shows a typical measurement of solvation forces in the case of ethanol between mica. 

Figure Bl.20.7. The solvation force of ethanol between mica surface. The inset shows the fiill scale of the experimental data. With pennission from [75].

Figure Bl.20.8. DLVO-type forces measured between two silica glass surfaces in aqueous solutions of NaCl at various concentrations. The inset shows the same data in the short-range regime up to D = 10 mn. The repulsive deviation at short range (<2 nm) is due to a monotonic solvation force, which seems not to depend on the salt concentration. Oscillatory surface forces are not observed. With pemiission from [73].

Wanless E J and Christenson H K 1994 Interaction between surfaces in ethanol adsorption, capillary condensation, and solvation forces J. Chem. Rhys. 101 4260-7... 

Elamrani et al. 1996] Elamrani, S., Berry, M.B., Phillips Jr., G.N., McCammon, J.A. Study of Global Motions in Proteins by Weighted Masses Molecular Dynamics Adenylate Kinase as a Test Case. Proteins 25 (1996) 79-88 [Elcock et al. 1997] Elcock, A.H., Potter, M.J., McCammon, J.A. Application of Poisson-Boltzmann Solvation Forces to Macromolecular Simulations. In Computer Simulation of Biomoleeular Systems, Vol. 3, A.J. Wilkinson et al. eds., ESCOM Science Publishers B.V., Leiden... 

Phase transfer catalysis succeeds for two reasons First it provides a mechanism for introducing an anion into the medium that contains the reactive substrate More important the anion is introduced m a weakly solvated highly reactive state You ve already seen phase transfer catalysis m another form m Section 16 4 where the metal complexmg properties of crown ethers were described Crown ethers permit metal salts to dissolve m nonpolar solvents by surrounding the cation with a lipophilic cloak leav mg the anion free to react without the encumbrance of strong solvation forces... 

Within the framework of Monte Carlo simulations, the relation between measurable quantities and the microscopic structure of confined phases can now be examined. An example of such a measurable quantity is the solvation force F h)/2 KR (see Sec. IIA 1). From a theoretical perspective and according to the discussion in Sec. IIA 3 its investigation requires the stress T zisz) exerted normally by a confined fluid on planar substrates [see Eqs. (19) and (22)]. Using Eqs. (11) and (53) one can derive a molecular expression for Tzz from... 

To illustrate the relationship between the microscopic structure and experimentally accessible information, we compute pseudo-experimental solvation-force curves F h)/R [see Eq. (22)] as they would be determined in SEA experiments from computer-simulation data for T z [see Eqs. (93), (94), (97)]. Numerical values indicated by an asterisk are given in the customary dimensionless (i.e., reduced) units (see [33,75,78] for definitions in various model systems). Results are correlated with the microscopic structure of a thin film confined between plane parallel substrates separated by a distance = h. Here the focus is specifically on a simple fluid in which the interaction between a pair of film molecules is governed by the Lennard-Jones (12,6) potential [33,58,59,77,79-84]. A confined simple fluid serves as a suitable model for approximately spherical OMCTS molecules confined... 

FIG. 5 The excess pressure f s ) ( , dashed line) and the solvation force per radius F h)/R (full line) as functions of s. and h, respectively, for a confined fluid composed of simple molecules (from Ref. 48). 

M. Schoen, T. Gnihn, D. J. Diestler. Solvation forces in thin films between macroscopically curved substrates. J Chem Phys 709 301-311, 1998. 

P. Attard, J. L. Parker. Oscillatory solvation forces A comparison of theory and experiment. J Phys Chem 9(5 5086-5093, 1992. 

I. K. Snook, W. van Megen. Solvation forces in simple dense fluids. J Chem Phys 72 2907-2913, 1980. 

P. Bordarier, B. Rousseau, A. H. Fuchs. Solvation forces and confinement-induced phase transitions of model ultrathin films. Mol Simul 77 199-215,1996. 

R. Kjellander, S. Sarman. A study of anisotropic pair distribution theories for Lennard-Jones fluids in narrow slits. II. Pair correlations and solvation forces. Mol Phys 74 665-688, 1991. 

M. Iwamatsu. A molecular theory of solvation force oscillations in nonpolar Uquids. J Colloid Interface Sci 204 374-388, 1998. 

It certainly does not seem that these interactions continue in solution, so that their magnitude is weaker than solvation forces. Theoretical explanation has suggested that the unused, filled, 6s-5dz2 hybrid (section 4.1) interacts with vacant 6px,py orbitals at right angles to the digonal bonds (Figure 4.49). 

Surface force apparatus has been applied successfully over the past years for measuring normal surface forces as a function of surface gap or film thickness. The results reveal, for example, that the normal forces acting on confined liquid composed of linear-chain molecules exhibit a periodic oscillation between the attractive and repulsive interactions as one surface continuously approaches to another, which is schematically shown in Fig. 19. The period of the oscillation corresponds precisely to the thickness of a molecular chain, and the oscillation amplitude increases exponentially as the film thickness decreases. This oscillatory solvation force originates from the formation of the layering structure in thin liquid films and the change of the ordered structure with the film thickness. The result provides a convincing example that the SFA can be an effective experimental tool to detect fundamental interactions between the surfaces when the gap decreases to nanometre scale. 

Subsequently, a lubrication theory considering the solvation force is deduced accordingly in such ultra-thin film lubrication. 

It is considered that the calculation described in this section agrees well with the experiments for the liquids that have a strong solvation force such as OMCTS and cyclohexane. It may be more difficult to apply this theory to the liquids that have a molecular shape far from spherical and exhibit weak solvation force. 

The normal pressure Pn In the fiuld confined between the walls varies with wall separation and Is not. In general, equal to the bulk pressure P3 of fiuld at the same chemical potential. The difference Pn - Pb Is the solvation force per unit area, fg, and can be calculated from the equilibrium density profiles by... 

It Is now well established experimentally that the solvation force, fg, of confined fiuld Is an oscillating function of pore wall separation. In Figure 4 we compare the theoretical and MD results for fg as a function of h. Given that pressure predictions are very demanding of a molecular theory, the observed agreement between our simple theory and the MD simulations must be viewed as quite good. The local maxima and minima In fg coincide with those In n y and therefore also refiect porewldths favorable and unfavorable to an Integral number of fiuld layers. 

Molecular dynamics results for solvation force versus pore width. 

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