Partial solvation parameters

The latter (predictive approach) is based on the new concept of partial solvation parameters (PSPs) [39-43] and focuses on the prediction of thermodynamic properties of complex systems with minimal or almost no experimental data available. 

This chapter presents a quantum mechanics-based alternative to the vOCG approach whose predictive capacity offers a new way of addressing the previously mentioned controversial interfacial issues. The new approach is built upon the recently introduced molecular descriptors called partial solvation parameters (PSP) [12-16], The rationale and the working equations of partial solvation parameters are described in this series of recent papers [12-16] where the reader is referred to for the details. A short review is presented in Chapter 3. 

TABLE 11.4 Partial Solvation Parameters of Common Polymers Polymer r,y Op, ffca... 

A new theory for estimating interfacial tension using the partial solvation parameters (Panayiotou)... 

Panayiotou s method is based on the so-called PSPs (partial solvation parameters) which he has recently developed and applied in numerous cases (polymer-polymer miscibility, polymer-solvent interactions, solubility parameters, pharmaceuticals, phase equih-biia, etc.) (see Panayiotou, 2012b,c,d, 2013). The PSPs bear similarities with the Hansen solubility parameters presented in Section 3.4.2 but there are four distinct contributions due to dispersion, polarity, acid and base contributions ea, trca, < Gb and, moreover, there are predictive methods for their estimation. 

Nevertheless, as also discussed in this chapter, there are several problems even with the van Oss-Good theory and, while usefiil in many practical applications, it should be used with care and by experienced users who arc familiar with the interpretations of the results. Due to these problems, it is most likely that we have not as yet seen the last in the development of theories for the interfacial tension. The recent Panayiotou theory based on the partial solvation parameters may prove to be a promising tool for predictive calculations of interfacial tensions. 

Surface and interfacial energies of solids, adsorption of polymers, steric stabilization, surface energies and bulk properties, density functional theory (DFT), molecular simulation, new theories for interfacial tension based on the partial solvation parameters Adhesion, dynamic wetting, spectroscopic/microscopic analysis of surfaces AFM, ESCA Measurement of forces, "special" forces solvation, etc. 

The nonlinear character of log has not often been discussed previously. Nevertheless, Jorgensen and Duffy [26] argued the need for a nonlinear contribution to their log S regression, which is a product of H-bond donor capacity and the square root of H-bond acceptor capacity divided by the surface area. Indeed, for the example above their QikProp method partially reflects for this nonlinearity by predichng a much smaller solubility increase for the indole to benzimidazole mutation (0.45 versus 1.82 [39, 40]). Abraham and Le [41] introduced a similar nonlinearity in the form of a product of H -bond donor and H -bond acceptor capacity while all logarithmic partition coefficients are linear regressions with respect to their solvation parameters. Nevertheless, Abraham s model fails to reflect the test case described above. It yields changes of 1.8(1.5) and 1.7(1.7) [42] for the mutations described above. 

Partial molar entropies of ions can, for example, be calculated assuming S (H+) = 0. Alternatively, because K+ and Cl ions are isoelectronic and have similar radii, the ionic properties of these ions in solution can be equated, e.g. analysis of B-viscosity coefficients (Gurney, 1953). In other cases, a particular theoretical treatment which relates solvation parameters to ionic radii indicates how the subdivision could be made. For example, the Bom equation requires that AGf (ion) be proportional to the reciprocal of the ionic radius (Friedman and Krishnan, 1973b). However, this approach involves new problems associated with the definition of ionic radius (Stem and Amis, 1959). In another approach to this problem, the properties of a series of salts in solution are plotted in such a way that the value for a common ion is obtained as the intercept. For example, when the partial molar volumes of some alkylammonium iodides, V (R4N+I ) in water (Millero, 1971) are plotted against the relative molecular mass of the cation, M+, the intercept at M + = 0 is equated to Ve (I-) (Conway et al., 1966). This procedure has been used to... 

Now, if we assume that the active sites of these enzymes have a hydrophobic pocket at Sj as well as discrete subsites for substrate amino acids, we can explain these results by assigning different levels of importance to these different modes of interaction for the two enzymes. To account for the Pi specificity of FKBP, we not only assume a more prominent role for Pi-Si interactions but also that these interactions are characterized by dehydration of the Michaelis complex, E S, as it proceeds to the transition state, [E S]t. What we are suggesting here is that in E S, the Pi residue is not yet buried in Si and that the active site and the substrate are still at least partially solvated. As E S proceeds to [E S], the Pi residue becomes buried in the Si pocket and the residual water of solvation is expelled from the active site. This scenario can reasonably account for the large values of A/ft and ASt that we observe for reactions of FKBP, since the formation of hydrophobic contacts between apolar groups in aqueous solution is known to be accompanied by positive enthalpy and entropy changes (Nemethy, 1967). Likewise, to account for the lack of Pi specificity for CyP, we assume that subsite interactions play a more prominent role than do Pi—Si interactions. Thus, the Pi-Si hydrophobic interactions that dominate the thermodynamic parameters for FKBP have a smaller role for this enzyme. 

Nernst suggested in 1900 that could be found by adding to the solution a small known amount of a nonelectrolyte which was assumed to remain stationary in the electric field. The movement of solvent and of the various ion-constituents, relative to the inert nonelectrolyte, would then give co and r respectively. Early work on this principle was confined to aqueous systems but within the last decade it has also been applied to water-ethanoP and water-dioxan " mixtures. Unfortunately, however, the results are devoid of any simple physical mean-ingM58 because experimental evidence has accumulated to show that the basic assumption of the method is invalid. On reflection we can see why. The reference substances employed—rafflnose, fructose and the like—are polar, partially solvate the ions, and so are no more stationary or inert than the solvent itself. The Washburn numbers obtained are therefore not solvent transference numbers but simply convenient parameters for expressing certain experimental results. 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

Here Vij denotes the distance between atoms i and j and g(i) the type of the amino acid i. The Leonard-Jones parameters Vij,Rij for potential depths and equilibrium distance) depend on the type of the atom pair and were adjusted to satisfy constraints derived from as a set of 138 proteins of the PDB database [18, 17, 19]. The non-trivial electrostatic interactions in proteins are represented via group-specific dielectric constants ig(i),g(j) depending on the amino-acid to which atom i belongs). The partial charges qi and the dielectric constants were derived in a potential-of-mean-force approach [20]. Interactions with the solvent were first fit in a minimal solvent accessible surface model [21] parameterized by free energies per unit area (7j to reproduce the enthalpies of solvation of the Gly-X-Gly family of peptides [22]. Ai corresponds to the area of atom i that is in contact with a ficticious solvent. Hydrogen bonds are described via dipole-dipole interactions included in the electrostatic terms... 

---