Solvation force behavior

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. 

Raoult s law /rah-oolz/ A relationship between the pressure exerted by the vapor of a solution and the presence of a solute. It states that the partial vapor pressure of a solvent above a solution (p) is proportional to the mole fraction of the solvent in the solution (X) and that the proportionaUty constant is the vapor pressure of pure solvent, (po), at the given temperature i.e. p = PqX. Solutions that obey Raoult s law are said to be ideal. There are some binary solutions for which Raoult s law holds over all values of X for either component. Such solutions are said to be perfect and this behavior occurs when the intermolecular attraction between molecules within one component is almost identical to the attraction of molecules of one component for molecules of the other (e.g. chlorobenzene and bromobenzene). Because of solvation forces this behavior is rare and in general Raoult s law holds only for dilute solutions. 

Figure 1.3 Force as a function of separation between mica surfaces coated with (a) DOAB in OMCTS, displaying a reduced oscillatoiy solvation force as compared to uncoated mica (the dotted lines give the force envelope for uncoated mica), and with (b) CTAB in OMCTS, showing a jump in contact with no oscillatory behavior (reproduced with permission from Ref 13, copyright 1992, American Physical Society).

Forces mediated by dissolved, nonadsorbing macromolecules need not be purely attractive. They are attractive at short ranges and at low volume fraction of the dissolved species. At distances larger than the size of the dissolved molecules and at high concentrations, it can be repulsive [1439, 1441. 1442). In general, depletion forces can be treated like solvation forces, and oscillatory behavior is predicted [1432, 1433] and observed [1443,1444]. The interface induces a layered structure of the dissolved molecules. Once two surfaces get dose to each other, this layered structure is disturbed. This results in a force. We call it structural force. 

C—X, Cf, X- and C+ fX (see Fig. 2), the solvation energy increasing the driving force of these dissociations. It is possible that a coordination catalyst is not active in the C—X state but only in one or other of the ionized states. Such behavior blurs the distinction between ionic and coordination polymerization. 

QM calculations provide an accurate way to treat strong, long-range electrostatic forces that dominate many solvation phenomena. The errors due to the use of the approximate eontinuum solvation models ean be small enough to allow the quantitative treatment of the solute behavior therefore, this approach is widely used also for evaluating solvent extraetion equilibria of organic molecules [56]. 

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