Interaction potential, interfacial

The theoretical treatments of Section III-2B have been used to calculate interfacial tensions of solutions using suitable interaction potential functions. Thus Gubbins and co-workers [88] report a molecular dynamics calculation of the surface tension of a solution of A and B molecules obeying Eq. III-46 with o,bb/ o,aa = 0.4 and... 

The gradient model for interfacial tension described in Eqs. III-42 and III-43 is limited to interaction potentials that decay more rapidly than r. Thus it can be applied to the Lennard-Jones potential but not to a longer range interaction such as dipole-dipole interaction. Where does this limitation come from, and what does it imply for interfacial tensions of various liquids ... 

In the second picture, an interfacial layer or region persists over several molecular diameters due to a more slowly decaying interaction potential with the solid (note Section X-7C). This situation would then be more like the physical adsorption of vapors (see Chapter XVII), which become multilayer near the saturation vapor pressure (e.g.. Fig. X-15). Adsorption from solution, from this point of view, corresponds to a partition between bulk and interfacial phases here the Polanyi potential concept may be used (see Sections X-7C, XI-1 A, and XVII-7). 

Certainly these approaches represent a progress in our understanding of the interfacial properties. All the phenomena taken into account, e.g., the coupling with the metal side, the degree of solvation of ions, etc., play a role in the interfacial structure. However, it appears that the theoretical predictions are very sensitive to the details of the interaction potentials between the various species present at the interface and also to the approximations used in the statistical treatment of the model. In what follows we focus on a small number of basic phenomena which, probably, determine the interfacial properties, and we try to use very transparent approximations to estimate the role of these phenomena. 

It is important to propose molecular and theoretical models to describe the forces, energy, structure and dynamics of water near mineral surfaces. Our understanding of experimental results concerning hydration forces, the hydrophobic effect, swelling, reaction kinetics and adsorption mechanisms in aqueous colloidal systems is rapidly advancing as a result of recent Monte Carlo (MC) and molecular dynamics (MO) models for water properties near model surfaces. This paper reviews the basic MC and MD simulation techniques, compares and contrasts the merits and limitations of various models for water-water interactions and surface-water interactions, and proposes an interaction potential model which would be useful in simulating water near hydrophilic surfaces. In addition, results from selected MC and MD simulations of water near hydrophobic surfaces are discussed in relation to experimental results, to theories of the double layer, and to structural forces in interfacial systems. 

The theoretical study (2,3) of this interface is made inherently difficult by virtue of the complex, many-body nature of the interaction potentials and forces involving surfaces, counterions, and water. Hence, many models of the interfacial region explicitly specify the forces between colloidal particles or between solutes, but few account for the many-body interaction forces of the solvent. 

Equations 3-4 show that the form of the interaction potentials used in simulating interfacial water is critical. Of interest for interfacial systems are both the interaction potential between water molecules and that between the surface and a water molecule. 

The first MC (16) and MD (17) studies were used to simulate the properties of single particle fluids. Although the basic MC (11,12) and MD (12,13) methods have changed little since the earliest simulations, the systems simulated have continually increased in complexity. The ability to simulate complex interfacial systems has resulted partly from improvements in simulation algorithms (15,18) or in the interaction potentials used to model solid surfaces (19). The major reason, however, for this ability has resulted from the increasing sophistication of the interaction potentials used to model liquid-liquid interactions. These advances have involved the use of the following potentials Lennard-Jones 12-6 (20), Rowlinson (21), BNS... 

Surface Potentials. Consider the form of the surface-water Interaction potential for an interfacial system with a hydrophobic surface. The oxygen atom of any water molecule is acted upon by an explicitly uncharged surface directly below it via the Lennard-Jones potential (U j) ... 

The above forms for the Lennard-Jones surface-water interaction potential have been used as models of hydrophobic surfaces such as pyrophyl1ite, graphite, or paraffin. If the intention of the study, however, is to understand interfacial processes at mineral surfaces representative of smectites or mica, explicit electrostatic interactions betweeen water molecules and localized charges at the surface become important. 

An interaction potential between the surface and ions may also be needed in simulating counterion diffusion for the smectite and mica surface models. The form of such an interaction potential remains to be determined. This may not pose a significant problem, since recent evidence (40) suggests that over 98% of the cations near smectite surfaces lie within the shear plane. For specifically adsorbed cations such as potassium or calcium, the surface-ion interactions can also be neglected if it is assumed that cation diffusion contributes little to the water structure. In simulating the interaction potential between counterions and interfacial water, a water-ion interaction potential similar to those already developed for MD simulations (41-43) could be specified. 

Two cases have been investigated. First, the solution of the Poisson-Boltzmann equation was obtained for an interfacial square-well potential for the anions, with W1 = lkT and d1 = 5 A. Second, a triangular well was employed for the potential (the interaction potential varying linearly form AW, = — IMF at x=0 to AW =0 at... 

Microemulsions consist of oil, water and an oil-water interfacial Him. DLS and SLS have been used to determine the translational diffusion coefficient and the interaction potential of microemulsions [45—47). The thickness of the inter-facial film and its curvature were measured by the contrast variation method in neutron scattering [48,491. This method is based on changing the scattering strength by changing the relative amount of light and heavy water in the microemulsion. 

Underlining that BCO and Diesel oil are not miscible a third component must be added to obtain a stable emulsion. This third component is called emulsifier (or surfactant). It changes the interfacial properties (namely interaction potential between droplets) of the system avoiding (or delaying) the emulsion s breaking. 

R-t,R) = interaction potential between an adsorbate molecule at r and the entire sohd surrounding the pore, while (f)FF(r,R) represents the corresponding potential if the bulk fluid was instead present in place of the surrounding sohd. Both these potentials incorporate the interfacial density profile. Using an analysis similar to that described in section 2.1, the equations for estimating the critical thickness are given by... 

In previous papers, the experimental values of B for several (jj/o microemulsions have been measured. As already pointed out, these values are function of both the radius of the micelles and the alcohol chain length (9-10). The attractive interactions between micelles increase as the micellar radius increases and as the alcohol chain length is shorter. We have proposed an interaction potential between w/o micelles which allows to account for the scattering results (10). This potential V(r) results from the possibility of penetration of the micelles. V(r) is proportional to the volume of interpenetration of micelles. The penetration is limited by the molecules of alcohol located inside the interfacial film, r is the distance between two micelles. 

From the positions of the maxima of the ionic density profiles relative to the minima of the ion-metal interaction potentials, they concluded that 1 is contact adsorbed and Li" " is not. Spohr [190] and later Perera and Berkowitz [191] obtained similar results by means of free energy calculations for 1 and simultaneous Li" " and 1 adsorption, respectively, on Pt(lOO), using the same interaction potentials. Eck and Spohr [77, 192] and Toth and Heinzinger [80] studied the adsorption of Li+ and several halide ions near the ab initio model of the mercury interface [40]. The liquid/ gas interface, contrary to metallic interfaces, is depleted in the interfacial region [193-195]. This is a consequence of the driving force towards fully hydrated ions. 

Spohr describes in detail the use of computer simulations in modeling the metal/ electrolyte interface, which is currently one of the main routes towards a microscopic understanding of the properties of aqueous solutions near a charged surface. After an extensive discussion of the relevant interaction potentials, results for the metal/water interface and for electrolytes containing non-specifically and specifically adsorbing ions, are presented. Ion density profiles and hydration numbers as a function of distance from the electrode surface reveal amazing details about the double layer structure. In turn, the influence of these phenomena on electrode kinetics is briefly addressed for simple interfacial reactions. 

AGIF(z) is the ligand-receptor interfacial interaction potential (section C2.14.7.1. equation (C2.14 52). equation (C2.14.53) and equation (C2.14.54)). and Q is the probability that a particle at q will return to b, equal to the ratio... 

Using the methods of semi-empirical quantum chemistry can be as instructive as it can be, quite frequently, illusive, when it is disconnected from the real dynamics and the global properties of the interface. The development of theoretical interfacial electrochemistry in the near future will proceed in the direction of a combination of ab initio quantum chemical methods and MD. Progress and obstacles in this direction were recently reviewed (320). MD of the solution part of the interface, based on empirical atom-atom interaction potentials, is often of great help for the verification of global models, rather than direct comparison with experiments. One obvious benefit... 

A correlation has been developed between the residual adhesion of bonded assemblies following accelerated ageing and the magnitude of acid-base interfacial interactions. The strength of interactions needed to avoid property loss under the chosen ageing conditions can be estimated. This capability represents a guideline to the selection of preferred silane additives for the adhesion of PU adhesives to substrates with known acid-base interaction potentials. 

The interaction forces and potentials between two charged surfaces in an electrolyte are fundamental to the analysis of colloidal systems and are associated with the formation of electrical double layers (EDLs) in vicinity of the solid surfaces. The charged surfaces typically interact across a solution that contains a reservoir of ions, as a consequence of the dissociation of the electrolyte that is already present. In colloid and interfacial sciences, the EDL interaction potential, coupled with the van der Waals interaction potential, leads to the fimdamental understanding of inter-siuface interaction mechanisms, based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [1]. In practice, the considerable variations in the EDL interaction, brought about by the variations in electrolytic concentration of the dispersing medium, pH of the medium, and the siuface chemistry, lead to a diverse natiue of the colloidal behavior. A fundamental understanding of the physics of EDL interactions, therefore, is of prime importance in... 

ABSTRACT. The paper examines the influence of interactions at polymer surfaces and interfaces on the properties of polymer systems, with emphasis on acid/base interactions. The method of inverse gas chromatography is used to evaluate the donor-acceptor interaction potential of components in polymer systems. The usefulness of the interaction parameters is established by their ability to rationalize diverse properties of polymer systems, including the adsorption of polymers on pigments, and the effectiveness of thermal stabilizers in pigmented polymers. Various strategies for controlling surface and interfacial interactions in polymer systems are reviewed, with emphasis placed on the ability of polymers to adopt various surface orientations and compositions. TTiese inherent surface modification effects are attributed to thermodynamic driving forces, and are shown to influence polymer adhesion, barrier and other properties dependent on surface and interfacial forces. 

Several contributions review the methods of investigation of interfacial interactions and surfaces. The IGC method to evaluate the donor-acceptor interaction potential of components, as well as the classical techniques such as SEM, Auger, SIMMS, ion scattering and X-ray photoelectron spectroscopy. Various surface FT-IR techniques are extensively discussed and explained with applications. A new qualitative method (induction time approach) to study the trans crystal layers is also introduced. 

Let us consider qualitatively the effect of the microemulsion composition on the sulfonate-microemulsion, sulfonate-adsorbent and microemulsion-adsorbent interactions. We assume that the interaction potential between a single sulfonate anion and its nearest-neighbor molecules and ions in the oil-brine interfacial region in the microemulsion is nearly constant within the appropriate series of microemulsions (e.g. A, B, C and D). As a first approximation therefore we consider the sulfonate-microemulsion interactions to be unaffected by microemulsion composition within such a series. 

This classical definition is for any interface. The interfacial energy is a consequence of the interaction between molecules and contains contributions from mutually attractive intermolecular forces due to combined effects of dispersions, dipole, induced dipole and hydrogen bonding interactions. At short distances molecular species are repulsive whereas at longer distances the molecules become attractive. A variety of different interaction potentials can be used however, the simplest is an inverse square law, in which case the interaction energy has the form... 

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