71-71 association forces

The present chapter is organized as follows. We focus first on a simple model of a nonuniform associating fluid with spherically symmetric associative forces between species. This model serves us to demonstrate the application of so-called first-order (singlet) and second-order (pair) integral equations for the density profile. Some examples of the solution of these equations for associating fluids in contact with structureless and crystalline solid surfaces are presented. Then we discuss one version of the density functional theory for a model of associating hard spheres. All aforementioned issues are discussed in Sec. II. 

Sec. Ill is concerned with the description of models with directional associative forces, introduced by Wertheim. Singlet and pair theories for these models are presented. However, the main part of this section describes the density functional methodology and shows its application in the studies of adsorption of associating fluids on partially permeable walls. In addition, the application of the density functional method in investigations of wettability of associating fluids on solid surfaces and of capillary condensation in slit-like pores is presented. 

In the case of computer simulations of fluids with directional associative forces a less intuitive but computationally more convenient potential model has been used [14,16,106]. According to that model the attraction sites a and j3 on two different particles form a bond if the centers of reacting particles are within a given cut-off radius a and if the orientations of two spheres are constrained as follows i < 6 i and [tt - 2 < The interaction potential is... 

The parameter is an analog of the equilibrium association constant. The value of A p itself, rather than the potential is used to characterize the strength of the associative forces. 

Let us consider a one-component system of hard spheres of unit diameter (7=1 with angular-dependent associative potential. The nonassociative potential is thus given by Eq. (35), whereas the associative forces are described by Eq. (61) with d = a = I, a = 1.05, and 6 = 21°. 

The calculations have been carried out for a one-component, Lennard-Jones associating fluid with one associating site. The nonassociative van der Waals potential is thus given by Eq. (87) with = 2.5a, whereas the associative forces are described by means of Eq. (60), with d = 0.5contact with an attracting wall. The fluid-wall potential is given by the Lennard-Jones (9-3) function... 

We report here some results for a simple model of a one-component fluid interacting via a slightly modified Lennard-Jones potential, with angular-dependent associative forces. The model is considered in contact with the adsorbing surface. The principal aim of the simulation is to investigate the... 

The association forces between juxtaposed surfactant monomers is physical, not chemical, so the motion of the hydrocarbon tails within a micelle is similar to the local motion in a sample of pure hydrocarbon. 

The effect of strong self-association of a proton-donating polymer on its com-plexation with proton-accepting polymers can be interpreted in terms of the competition of E p and the self-association force E j of the proton-donating polymer. Thus, the net driving force for interpolymer complexation is given by ... 

Here Dt is a positive proportionality constant ( diffusion constant for Et), Jfz is z-ward flow induced by the gradient, and superscript e denotes eigenmodt character of the associated force or flow. The proportionality (13.25) corresponds to Fick s first law of diffusion when Et is dominated by mass transport or to Fourier s heat theorem when Et is dominated by heat transport, but it applies here more deeply to the metric eigenvalues that control all transport phenomena. In the near-equilibrium limit (13.25), the local entropy production rate (13.24) is evaluated as... 

The first sum corresponds to the interaction of particle J with all (M - 1) MM atoms based on classical models. The associated force F f is in general composed... 

Experiments on crosslinking have been carried out in the author s laboratory (93). Solutions of isolated carp actomyosin or myosin in 0.6 M KC1 or suspensions in 0.05 M KC1 have been stored at -20°C. Samples were taken at intervals and homogenized with several different solutions. The solutions used were water (to test for nonspecific association forces), 0.6 M KC1 (to test for ionic bonds), 0.5 M e-mercaptoethanol (to test for disulfide bonds), 1.5 M urea (to test for hydrogen bonds), 8 M urea (to test for hydrogen bonds and nonpolar bonds), and 1 M K0H (to test for ionic bonds and others) (135,136). Combinations of these solvents were also tested. 

The term synthon is more familiar to organic chemists and describes fragments from which varions molecnles can be bnilt by synthetic procedures. Sometimes the term was also used to describe structural units within supermolecules which can be formed and/or assembled by known or conceivable synthetic operations involving intermolecular interactions To avoid confusion, the term tecton was introduced to describe the molecular units of supramolecular structures assembled through noncovalent forces. Thus, the tecton has been defined as any molecule whose interactions are dominated by particular associative forces that induce self-assembly of an organized network Here, we will use the term synthon for... 

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