Surface adsorption layer, molecular interaction

When a gas comes in contact with a solid surface, under suitable conditions of temperature and pressure, the concentration of the gas (the adsorbate) is always found to be greater near the surface (the adsorbent) than in the bulk of the gas phase. This process is known as adsorption. In all solids, the surface atoms are influenced by unbalanced attractive forces normal to the surface plane adsorption of gas molecules at the interface partially restores the balance of forces. Adsorption is spontaneous and is accompanied by a decrease in the free energy of the system. In the gas phase the adsorbate has three degrees of freedom in the adsorbed phase it has only two. This decrease in entropy means that the adsorption process is always exothermic. Adsorption may be either physical or chemical in nature. In the former, the process is dominated by molecular interaction forces, e.g., van der Waals and dispersion forces. The formation of the physically adsorbed layer is analogous to the condensation of a vapor into a liquid in fret, the heat of adsorption for this process is similar to that of liquefoction. 

The situation becomes most complicated in multicomponent systems, for example, if we speak about filling of plasticized polymers and solutions. The viscosity of a dispersion medium may vary here due to different reasons, namely a change in the nature of the solvent, concentration of the solution, molecular weight of the polymer. Naturally, here the interaction between the liquid and the filler changes, for one, a distinct adsorption layer, which modifies the surface and hence the activity (net-formation ability) of the filler, arises. Therefore in such multicomponent systems in the general case we can hardly expect universal values of yield stress, depending only on the concentration of the filler. Experimental data also confirm this conclusion [13],... 

The deviations from the Szyszkowski-Langmuir adsorption theory have led to the proposal of a munber of models for the equihbrium adsorption of surfactants at the gas-Uquid interface. The aim of this paper is to critically analyze the theories and assess their applicabihty to the adsorption of both ionic and nonionic surfactants at the gas-hquid interface. The thermodynamic approach of Butler [14] and the Lucassen-Reynders dividing surface [15] will be used to describe the adsorption layer state and adsorption isotherm as a function of partial molecular area for adsorbed nonionic surfactants. The traditional approach with the Gibbs dividing surface and Gibbs adsorption isotherm, and the Gouy-Chapman electrical double layer electrostatics will be used to describe the adsorption of ionic surfactants and ionic-nonionic surfactant mixtures. The fimdamental modeling of the adsorption processes and the molecular interactions in the adsorption layers will be developed to predict the parameters of the proposed models and improve the adsorption models for ionic surfactants. Finally, experimental data for surface tension will be used to validate the proposed adsorption models. 

It is noted that the molecular interaction parameter described by Eq. 52 of the improved model is a function of the surfactant concentration. Surprisingly, the dependence is rather significant (Eig. 9) and has been neglected in the conventional theories that use as a fitting parameter independent of the surfactant concentration. Obviously, the resultant force acting in the inner Helmholtz plane of the double layer is attractive and strongly influences the adsorption of the surfactants and binding of the counterions. Note that surface potential f s is the contribution due to the adsorption only, while the experimentally measured surface potential also includes the surface potential of the solvent (water). The effect of the electrical potential of the solvent on adsorption is included in the adsorption constants Ki and K2. 

The effect of alkyl alcohol on the surface adsorption and micellization of FC surfactant is noticeably different from HC surfactant. The molecular interactions between ROH and C7pNa in the surface layer are shown to be weaker (Smaler l jl-value) as compared with ROH-C, SNa system. 

By extending regular solution theory for binary mixtures of AEg in aqueous solution to the adsorption of mixture components on the surface (3,4), it is possible to calculate the mole fraction of AEg, Xg, on the mixed surface layer at tt=20, the molecular interaction parameter, 6, the activity coefficients of AEg on the mixed surface layer, fqg and f2s and mole concentration of surfactant solution, CTf=20 3t surface pressure tt=20 mn-m l (254p.l°C). The results from the following equations are shown in Table I and Table II. 

At present it is well established that the existence of the phase border between a polymer and any solid leads to the appearance of different types of micro- and macroheterogeneities at the molecular, supermolecular, and chemical levels78. It is established that, due to adsorption interaction at the interface, an essential decrease in molecular mobility takes place as a result of which the glass temperature of such systems increases79. At the same time, due to retardation of the relaxation processes in the surface layers, some loosening of packing takes place, whereas in pure adsorption layers some increase in density is observed80. ... 

One of the most recent observations in supramolecular surface chirality is the induction of homochirality on surfaces via cooperatively amplified interactions in molecular monolayers. As discussed in Sect. 2, adsorption-induced chirality leads to both mirror motifs. However, in the presence of additional chiral bias, lattice homo chirality can be installed in the entire molecular layer. Such bias comes from a chiral dopant, small ee or physical fields in combination with symmetry breaking of the surface. 

The interfacial layer is the inhomogeneous space region intermediate between two bulk phases in contact, and where properties are notably different from, but related to, the properties of the bulk phases (see Figure 6.1). Some of these properties are composition, molecular density, orientation or conformation, charge density, pressure tensor, and electron density [2], The interfacial properties change in the direction normal to the surface (see Figure 6.1). Complex profiles of interfacial properties take place in the case of multicomponent systems with coexisting bulk phases where attractive/repulsive molecular interactions involve adsorption or depletion of one or several components. 

Protein function at solid-liquid interfaces holds a structural and a dynamic perspective [31]. The structural perspective addresses macroscopic adsorption, molecular interactions between the protein and the surface, collective interactions between the individual adsorbed protein molecules, and changes in the conformational and hydration states of the protein molecules induced by these physical interactions. Interactions caused by protein adsorption are mostly non-covalent but strong enough to cause drastic functional transformations. All these features are, moreover, affected by the double layer and the electrode potential at electrochemical interfaces. Factors that determine protein adsorption patterns have been discussed in detail recently, both in the broad context of solute proteins at solid surfaces [31], and in specific contexts of interfacial metalloprotein electrochemistry [34]. Some important elements that can also be modelled in suitable detail would be ... 

The BCF theory discusses crystal growth in terms of the physical features of the molecular processes rather than in terms of the chemical changes which occur. If the adsorption interaction between the bulk ions and the crystal surface is slow, this may be the rate-determining step and the surface layer or adsorption layer may be considered to be in equilibrium with the crystal. In this situation, it is the chemical reaction between the solvated ions and the crystal surface which determines the rate of growth. 

The specific surface area of a ceramic powder can be measured by gas adsorption. Gas adsorption processes may be classified as physical or chemical, depending on the nature of atomic forces involved. Chemical adsorption (e.g., H2O and AI2O3) is caused by chemical reaction at the surface. Physical adsorption (e.g., N2 on AI2O3) is caused by molecular interaction forces and is important only at a temperature below the critical temperature of the gas. With physical adsorption the heat erf adsorption is on the same order of magnitude as that for liquefaction of the gas. Because the adsorption forces are weak and similar to liquefaction, the capillarity of the pore structure effects the adsorbed amount. The quantity of gas adsorbed in the monolayer allows the calculation of the specific surface area. The monolayer capacity (V ,) must be determined when a second layer is forming before the first layer is complete. Theories to describe the adsorption process are based on simplified models of gas adsorption and of the solid surface and pore structure. 

In the case of low-molecular-weight polar resins such as VE resins, relatively thin and dense adsorption layers can be assiuned. This should result in low viscosities due to low effective phase volumes of the dispersed phase and weak interparticulate interactions forces according to steric stabilization. However, addition of a solvent like styrene will influence the Hamaker constant of the liquid medium and of the adlayer and the structure of the adlayer in terms of swelling and/or multilayer formation. In particular, any multilayer formation could result in surface layer entanglement depending on the solvency of the liquid medium expressed in terms of the Flory-Huggins parameter % [11]. These effects should dramatically influence the viscosity and rest structure of the dispersion, as seen in the experiments. 

Li (2007) observed that the adsorption of a hydrophobically associating water-soluble polymer, AP-2, did not follow the Langmuir-type isotherm. Figure 5.37 shows that the adsorption increased to a maximum and then decreased as the polymer concentration was increased. The reason is probably that the hydrophobic polymer has an adsorption layer of multiple molecules on rock surfaces. When the polymer concentration is increased, the adsorption layer becomes thicker because of more adsorption. When the polymer concentration is further increased, the molecular interaction in the liquid is stronger than that between the adsorbed molecules and rock surfaces. Then the adsorbed molecules may leave the rock surfaces and redissolve into the liquid. Thus, the adsorption decreases. 

Special examples of mixture adsorption are competitive adsorption of the different forms of the same substance, such as pH-dependent ionic and undissociated molecular forms, monomers, and associates of the same substance, as well as potential-dependent adsorption of the same compound in two different orientations in the adsorbed layer. Different orientations on the electrode surface—for example, flat and vertical—are characterized with different adsorption constants, lateral interactions, and surface concentrations at saturation. If there are strong attractive interactions between the adsorbed molecules, associates and micellar forms can be formed in the adsorbed layer even when bulk concentrations are below the critical micellar concentration (CMC). These phenomena were observed also at mineral oxide surfaces for isomerically pure anionic surfactants and their mixtures and for mixtures of nonionic and anionic surfactants (Scamehorn et al., 1982a-c). 

The molecular component of the disjoining pressure, IIm(/i), is negative (repulsive). It is caused by the London-van der Waals dispersion forces. The ion-electrostatic component, IIe(/i), is positive (attractive). It arises from overlapping of double layers at the surface of charge-dipole interaction. At last, the structural component, IIs(/i), is also positive (attractive). It arises from the short-range elastic interaction of closed adsorption layers. 

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