Soft-core interaction

T. Huber, A.E. Torda, W.F. van Gunsteren, Structure optimization combining soft-core interaction functions, the diffusion equation method and molecular dynamics, J. Phys. Chem. AlOl (1997), 5926. 

Instead of the hard-sphere model, the Lennard-Jones (LJ) interaction pair potential can be used to describe soft-core repulsion and dispersion forces. The LJ interaction potential is... 

More realistic treatment of the electrostatic interactions of the solvent can be made. The dipolar hard-sphere model is a simple representation of the polar nature of the solvent and has been adopted in studies of bulk electrolyte and electrolyte interfaces [35-39], Recently, it was found that this model gives rise to phase behavior that does not exist in experiments [40,41] and that the Stockmeyer potential [41,42] with soft cores should be better to avoid artifacts. Representation of higher-order multipoles are given in several popular models of water, namely, the simple point charge (SPC) model [43] and its extension (SPC/E) [44], the transferable interaction potential (T1PS)[45], and other central force models [46-48], Models have also been proposed to treat the polarizability of water [49],... 

Here, avaw is a positive constant, and and ctJ are the usual Lennard-Jones parameters found in macromolecular force fields. The role played by the term avdw (1 — A)2 in the denominator is to eliminate the singularity of the van der Waals interaction. Introduction of this soft-core potential results in bounded derivatives of the potential energy function when A tends towards 0. 

More refined continuum models—for example, the well-known Fumi-Tosi potential with a soft core and a term for attractive van der Waals interactions [172]—have received little attention in phase equilibrium calculations [51]. Refined potentials are, however, vital when specific ion-ion or ion-solvent interactions in electrolyte solutions affect the phase stability. One can retain the continuum picture in these cases by using modified solvent-averaged potentials—for example, the so-called Friedman-Gumey potentials [81, 168, 173]. Specific interactions are then represented by additional terms in (pap(r) that modify the ion distribution in the desired way. Finally, there are models that account for the discrete molecular nature of the solvent—for example, by modeling the solvent as dipolar hard spheres [174, 175]. 

The additive mixtures interact in a variety of ways, both in the bulk oil and on surfaces. Tribochemical interactions of additives in the oil formulation are discussed in Chapter 2. Surfactant molecules, when dissolved in base oil, are capable of self-organization to form aggregates such as soft-core reverse micelles (RMs). The polar or charged head groups of these molecules with the counter ions form the interior of the micelle (core), and the hydrocarbon chains made up its external shell. The most important factor governing the tribochemical reactions under boundary lubrication is connected with the action of soft-core and hard-core reverse micelles discussed in Chapter 3. 

ZDDP is known to interact with most other additives employed in these formulations e.g., ZDDP is solubilized by soft-core and hard-core micelles and has been considered for reduced antiwear performance (Inoue, 1993 Kapsa et al., 1981 Rounds, 1981 Shiomi etal., 1986 Willermet 1995a and 1995b Yin et al., 1997). Based on the tribofilm formation (polyphosphate) and the presence (or absence) of unchanged ZDDP in the film, we can conclude that the additives compete with the adsorption of ZDDP on the surface (Varlot et al., 2000 Yin et al., 1997a and 1997b). 

Solubilization of water. Detergency is defined as the ability of surfactant molecules to solubilize water molecules or polar substances in soft-core and hardcore RMs. Thus, micellization and solubilization are competitive processes. Any solubilized probe molecule causes a decrease in the CMC. Solubilization describes the dissolution of a solid, liquid or gas by an interaction with surfactant molecules. Addition of water has a dramatic effect on surfactant aggregation in hydrocarbons because hydrogen bonding has an appreciable stabilizing effect on reverse micelles. Solubilization for reverse micelles is phenomenologically similar to the adsorption processes (Eicke and Christen, 1978 Kitahara, 1980 Kitahara et al., 1976 Singleterry, 1955). 

Aggregation number Three main factors play a critical role in aggregation of soft-core reverse micelles the interaction between polar groups, the interaction of the hydrophobic (non-polar) part, and environmental factors. Compare aggregation numbers of some surfactants in low polar solvents specified in Table 3.1. 

Micellar interactions Write net equations for the following (a) carbonate-benzenesulfonate RMs solution with strong acid SH, (b) soft-core phenolate RMs and strong acid SH, and (c) soft-core carboxylate reverse micelles solution with carboxylic acid. 

Walls and Sternberg210 considered surface complementary for docking protein-protein complexes using a vdW-like soft core potential. The use of accessible surface area on binding and complexation has been discussed, for example, by Flower.205 Surface descriptors have been used to calculate solvation energies as part of the energetics of molecular association.144,188,211 Volume overlap was also used to quantify interactions of chemical compounds.201 The HINT exponential function and empirical atomic... 

Discrepancies between theory and experiment at high wave vector (e.g., the peak at 13 A i) are attributed to bond and angle vibrations. The PRISM theory simply ignores these. One more point is that even though attractive interactions and soft core repulsions do not appear in the theory, there is good agreement at k = 0, something that the authors consider simply fortuitous. 

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