Solvation force measurements

Horn, R.G., Evans, D.F., and Ninhamt, B.W. (1988) Double-layer and solvation forces measured in a molten salt and its mixtures with water. J. Phys. Chem., 92, 3531-3537. 

Christenson H.K., Horn R.G. Solvation forces measured in non-aqueous Uquids. Chem. Scr. 1985 25 37 1... 

Oscillating forces have also been observed with other molecules such as tetra-chloromethane, benzene, cyclohexane, toluene, 2,2,4-trimethylpentane [1098,1115, 1116], n-alcohols [1100,1103,1104,1117], and ionic liquids [1118,1119]. In all cases, solvation forces were observed with periodicities corresponding to the spacing determined by X-ray diffraction of bulk liquid. As an example, the solvation force measured in propanol on mica with an AFM is plotted in Figure 10.2. In this case, the observed periodicity indicates that the molecules are preferentially oriented normal to the surfaces studied and are stabilized by a network of hydrogen bonds between hydroxyl groups. Alkanes have been studied extensively, by experiments [ 1120-1123], simulations, and theory [1069, 1124—1127], driven by their relevance as lubricants. n-Alkanes tend to orient parallel to surfaces and form layers of 0.4-0.5 nm thickness, which corresponds to the diameter of an alkyl chain. In branched alkanes, layering is reduced. 

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.20.8. 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].

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... 

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. 

This strnctnring of liqnids into discrete layers when confined by a solid surface has been more readily observable in liquid systems other than water [1,55]. In fact, such solvation forces in water, also known as hydration forces, have been notoriously difficult to measure due to the small size of the water molecule and the ease with which trace amounts of contamination can affect the ordering. However, hydration forces are thought to be influential in many adhesive processes. In colloidal and biological systems, the idea that the hydration layer mnst be overcome before two molecules, colloidal particles, or membranes can adhere to each other is prevalent. This implies that factors affecting the water structure, such as the presence of salts, can also control adhesive processes. 

Polarizability is a measure of the ease with which the electrons of a molecule are distorted. It is the basis for evaluating the nonspecific attraction forces (London dispersion forces) that arise when two molecules approach each other. Each molecule distorts the electron cloud of the other and thereby induces an instantaneous dipole. The induced dipoles then attract each other. Dispersion forces are weak and are most important for the nonpolar solvents where other solvation forces are absent. They do, nevertheless, become stronger the larger the electron cloud, and they may also become important for some of the higher-molecular-weight polar solvents. Large solute particles such as iodide ion interact by this mechanism more strongly than do small ones such as fluoride ion. Furthermore, solvent polarizability may influence rates of certain types of reactions because transition states may be of different polarizability from reactants and so be differently solvated. 

One naturally assumes that the larger the database used as a training set, the more reliable will be the calculation for structures not represented therein. But databases reflect what chemists are interested in and what they measure. Thus chemists may over-represent certain structural features (i.e., those which confer desirable properties), and evaluating these by regression analysis may give a skewed perspective of what solvation forces are operating. Examples of this problem appear later. 

In the constructionist method, aromatic carbons are evaluated from benzene, a solute which has been measured more times than any other. When the phenyl ring is fused to others, as in naphthalene or anthracene, or when it is joined to another, as in biphenyl, there is a change in the effective polarity of the pi electron cloud, and a slight but significant positive correction factor is introduced. Critics may disparagingly refer to these as fudge factors/ but these factors have been very helpful in distinguishing the solvation forces which operate in the two different phases (Taft, 1996). 

Israelachvili, J. 1987. Solvation forces and liquid structure, as probed by direct force measurements. Acc. Chem. Res. 20 415-421. 

Rau, D. C., and Parsegian, V. A. (1992b). Direct measurement of temperature-dependent solvation forces between DNA double helices. Biophys.J. 61, 260—271. 

O ince partitioning between immiscible solvents is an equilibrium process, it should be possible to calculate partition coefficients for any solute between any two given solvents if we had a measure of the solvation forces involved. At present these forces are not that well characterized, and the reverse approach can be more enlightening—i.e., a study of how the partition coefficient varies between the systems can yield valuable insight into the types and relative magnitude of the forces involved. 

Measurement of the oscillatory solvation force became possible after the precise surface force apparatus had been constructed. This apparatus allowed to measure the surface forces in thin... 

Experimental Results. The DLVO theory, which is based on a continuum description of matter, explains the nature of the forces acting between membrane surfaces that are separated by distances beyond 10 molecular solvent diameters. When the interface distance is below 10 solvent diameters the continuum picture breaks down and the molecular nature of the matter should be taken into account. Indeed the experiment shows that for these distances the forces acting between the molecularly smooth surfaces (e.g., mica) have an oscillatory character (8). The oscillations of the force are correlated to the size of the solvent, and obviously reflect the molecular nature of the solvent. In the case of the rough surfaces, or more specifically biomembrane surfaces, the solvation force displays a mono tonic behavior. It is the nature of this solvation force (if the solvent is water, then the force is called hydration force) that still remains a puzzle. The hydration (solvation) forces have been measured by using the surface force apparatus (9) and by the osmotic stress method (10, II). Forces between phosphatidylcholine (PC) bilayers have been measured using both methods and good agreement was found. 

The previously described measurements have been performed on lipids in aqueous solutions, but lipid bilayers also swell in some other solvents (12) and the results of such measurements compare quite well with the aqueous case. In addition, hydration (solvation) forces act between DNA polyelectrolytes (13) and polysaccharides (14). These facts make the interpretation of the forces even more complicated and it is no wonder that different approaches to explain the nature of this solvation force exist. So far no truly ab initio theory has been proposed. The existing theories include models based on the electrostatic approach, the free energy approach, and an approach based on the entropic or protrusion model. 

As with the electrostatic and van der Waals dispersion forces, a more precise characterization of the solvation force has come from direct experimental measurements of the net force per unit radius (energy per unit area) between opposing muscovite surfaces. Figure 6.6 shows this... 

Horn and Israelachvili measured the interaction between mica surfaces in octamethylcyclotetrasiloxane, an inert liquid with roughly spherical molecules of diameter 0.9 nm. They interpreted the oscillating force measured between the surfaces as contact was approached as a solvation force due to denser packing of the liquid molecules at separations of one, two, or three diameters, etc. This effect had been postulated by Abraham,Chan et al, and Snook et... 

As mentioned before, AFM can measure surface forces using two different operation modes, DC and AC. In the DC mode, one measures the deflection (AZ) of a cantilever as a function of the tip-sample distance (D) that is varied usually hy a piezoelectric transducer (Fig. 3). The tip-sample force is given by Hooke s law in terms of AZ, F = kAZ, where k is the spring constant of the cantilever. This force clearly depends on the tip-sample distance, D, which is given by D = Z — AZ, in the absence of sample deformation. Z in the expression is the displacement of the piezo and the one that can be controlled in the experiment. When the tip is far away from the sample (large D), the force is zero. When the tip approaches the sample, it experiences various forces, electrostatic, van der Waals, double-layer, solvation forces, and so on. This is the regime of interest in... 

When the tip approaches the substrate, it has to break the order or hydrogen-bonded network, so the measurement of solvation forces provides ordering information and discrete nature of solvent molecules near the solid surfaces. The most general type of solvation force is oscillatory, arising from the ordering of the solvent molecules into quasi-discrete layers near the surface. This solvation force can be approximately described by a cosine function with an exponentially decaying amplitude [45],... 

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