The Interface of Two Condensed Phases

At the interface between two condensed phases, the dissimilar molecules in the adjacent layers facing each other have potential energies greater than those of similar molecules in the respective bulk phases. This is due to the fact that cohesive forces between like molecules tend to be stronger than adhesive forces between dissimilar molecules. Thus the interfaciai tension is the force per unit length existing at the interface between two immiscible or partially miscible condensed phases and the interfaciai free energy is the work required to increase the interface by unit area. 

This is the fundamental relationship between the quantities which describe the electrical properties of the interface between two condensed phases and the gas phase. 

At the interface between two condensed, phases, the dissimilar molecules in the adjacent layers facing each other across the interface (Figure 5-1) also have potential energies different from those in their respective phases. Each molecule at the interface has a potential energy greater than that of a similar molecule in the interior of its bulk phase by an amount equal to its interaction energy with the molecules in the interior of its bulk phase minus its interaction energy with... 

The laws governing the interfacial phenomena between condensed phases and their vapor (or air) in single- and two-component systems, described in previous chapters, are largely applicable to the interfaces between two condensed phases, i.e., between two liquids, two solids, or between a solid and a liquid. At the same time, these interfaces have some important peculiarities, primarily related to the partial compensation of the intermolecular interactions. The degree of saturation of the surface forces is determined by the similarity in the molecular nature of the phases in contact. When adsorption of surfactants takes place at such interfaces, it may substantially enhance the decrease in the interfacial energy. The latter is of great importance, since surfactants play a major role in the formation and degradation of disperse systems (see Chapters IV, VI-VIII). 

Optical Alignment The most serious problem, especially for studying the electrochemical interface of two condensed phases, is detection sensitivity. Many problems are caused by the optical alignment related to the spectro-electrochemical cell and sample. Since the confocal microprobe Raman system has much higher sensitivity than other Raman systems, it is used most frequently for spectroelectrochemical studies. This will be taken as an example for the detailed discussion that follows. 

The Volta potential is defined as the difference between the electrostatic outer potentials of two condensed phases in equilibrium. The measurement of this and related quantities is performed using a system of voltaic cells. This technique, which in some applications is called the surface potential method, is one of the oldest but still frequently used experimental methods for studying phenomena at electrified solid and hquid surfaces and interfaces. The difficulty with the method, which in fact is common to most electrochemical methods, is lack of molecular specificity. However, combined with modem surface-sensitive methods such as spectroscopy, it can provide important physicochemical information. Even without such complementary molecular information, the voltaic cell method is still the source of much basic electrochemical data. 

Fig. 4-6. llie inner potential, 4, and the outer potential, tf, of two condensed phases A and B before and after their contact d4 )= inner (outer) potential difference between two contacting phases o = surface or interface charge dip = surface or interface dipole. 

Interphase — A spatial region at the interface between two bulk phases in contact, which is different chemically and physically from both phases in contact (also called interfacial region). The plane that ideally marks the boundary between two phases is called the interface. Particles of a condensed phase located near a newly created (free) surface are subject to unbalanced forces and possibly to a unique surface chemistry. Modifications occurring to bring the system to equilibrium or metastability generally extend somewhat into one of the phases, or into both. 

As seen previously, adsorption processes result in the decrease of surface energy. As mentioned in Section 1.3.1, adsorption is an increase in the concentration of a gaseous or dissolved substance at the interface of a condensed and a gaseous or liquid phase, respectively, due to the operation of surface forces. The main feature of the adsorption is that the adsorbed particles occupy the free sites of the interface. When two or more different substances are adsorbed, competitive adsorption occurs. In aqueous solution, competitive adsorption takes place in all cases because water molecules cover the total surface of the solid. The water concentration, however, is usually much greater than the dissolved substances, so the change in water concentration can be neglected. In the thermodynamic equations, the parameters characterizing water are included in other thermodynamic parameters. 

Atoms and molecules at surfaces and interfaces possess energies significantly different from those of the same species in the bulk phase. The term surface is usually reserved for the region between a condensed phase (liquid or solid) and a gas phase or vacuum, while the term interface is normally applied to the region between two condensed phases. 

Apparently, in the near future there will be developed (a) a detailed theory of surface excitons not only at the crystal boundary with vacuum but also at the interfaces of various condensed media, particularly of different symmetry (b) a theory of surface excitons including the exciton-phonon interaction and, in particular, the theory of self-trapping of surface excitons (c) the features of surface excitons for quasi-one-dimensional and quasi-two-dimensional crystals (d) the theory of kinetic parameters and, particularly, the theory of diffusion of surface excitons (e) the theory of surface excitons in disordered media (f) the features of Anderson localization on a surface (g) the theory of the interaction of surface excitons of various types with charged and neutral particles (h) the evaluation of the role of surface excitons in the process of photoelectron emission (i) the electronic and structural phase transitions on the surface with participation of surface excitons. We mention here also the theory of exciton-exciton interactions at the surface, the surface biexcitons, and the role of defects (see, as example, (53)). The above list of problems reflects mainly the interests of the author and thus is far from complete. Referring in one or another way to surface excitons we enter into a large, interesting, and yet insufficiently studied field of solid-state physics. 

Let us briefly address the basic laws governing adsorption phenomena at interfaces separating two condensed phases when the third component, surface active with respect to the said interface, is introduced into the system. According to the polarity equalization rule, originally formulated by Rehbinder [2], the surface activity of the introduced component is determined by its ability to compensate for the striking difference in polarities of two unlike substances with low mutual solubility. 

The boundary between two condensed phases is an interface. The interfacial tension yab is the free energy cost of increasing the interfacial area between phases A and B. If yab is large, the two media will tend to minimize their interfacial contact. Let s determine yab by using the lattice model for molecules of types A and B that are identical in size. 

Since atoms or molecules in the vicinity of the surface of a condensed phase have different bonding from those in the hulk they have different thermodynamic properties. In this chapter the concept of the Gibbs dividing surface and the two fundamental quantities for describing the thermodynamic properties of surfaces and interfaces, surface energy (y) and surface stress (a ), are defined. The relations between y and the other thermodynamic variables for surfaces are established. Finally, methods for obtaining y and cr are described and representative values of both are presented. 

A gradient of properties is considered to be "steep" if the distance, X, over which the gradient exists is small compared to dimensions that can be observed conveniently in the laboratory. Historically, observation was specified to be by light microscopy, and so the practical condition was < 10 cm, for a region of space to be considered an interface. (Interfacial gradients less steep than this can be observed in special cases, such as a two-phase system near a critical point but when X 2 approaches macro-scopically observable dimensions, the word phase loses its meaning.) Regardless of ease of observation, X must be small compared to the dimensions of the two phases. Otherwise, the system would be better described as an adsorbed layer on a condensed phase, rather than as a system of two condensed phases. 

The principal thermodynamic properties of interfaces have been widely discussed. However, one property that is pertinent to adhesion has not been treated adequately, up to the present. This is the heat capacity per unit area of a layer of small but arbitrary thickness. (We do not refer, here, to the Gibbsian surface excess heat capacity of an interface between two condensed phases, which is, in general, small.) Formally, we define this property as follows Ca(A) is the heat capacity per unit interfacial area, for a... 

The physical properties of materials in a confined state have attracted considerable attention both due to their fundamental significance and to their primary importance for nanotechnology. The term confined state embraces a wide variety of systems the boundary layers at the interfaces between two bulk phases, the adsorption layers, wetting films, epitaxial structures, emulsions, free-lying nanoparticles and particles embedded in solid matrices, the substances condensed in pores, and so on. As a rule, the transition of a substance from the bulk state to the confined state is accompanied by an essential alteration of many physical properties. In particular, for practically all of the confined systems mentioned above, a shift of the first order phase transition temperature with respect to that in the bulk state was detected experimentally (see [1-4] for reviews). In nanoporous systems, not only a considerable (tens of degrees) depression of the freezing temperature can be observed, but the transition to solid state might even disappear. At the same time there are systems that demonstrate not a depression but an elevation of the solid/liquid phase transition temperature in pores. A similar situation occurs with small particles, adsorbed films and boundary layers at plane interfaces. 

An interface is the area that separates two phases. If we consider the solid, liquid, and gas phases, we immediately get three combinations of interfaces the solid-liquid, the solid-gas, and the liquid-gas interfaces. The term surface is often used synonymously, although interface is preferred for the boundary between two condensed phases and in cases where the two phases are named explicitly. For example, we talk about a solid-gas interface but a solid surface. Interfaces can also separate two immiscible liquids such as water and oil. These are called liquid-liquid interfaces. Interfaces may even separate two different phases within one component. In a liquid crystal, for example, an ordered phase may coexist with an isotropic phase. Solid-solid interfaces separate two solid phases. They are important for the mechanical behavior of solid materials. Gas-gas interfaces do not exist because gases mix. 

It is generally observed that the interface between two solids which results from the growth of one phase by condensation to form a film on die other is one in which the number of nearest neighbour bonds between the two phases is maximized. The close-packed planes tend to be found at the interface, which is consequently usually nearly atomically flat and this minimizes die interfacial... 

The scale of components in complex condensed matter often results in structures having a high surface-area-to-volume ratio. In these systems, interfacial effects can be very important. The interfaces between vapor and condensed phases and between two condensed phases have been well studied over the past four decades. These studies have contributed to technologies from electronic materials and devices, to corrosion passivation, to heterogeneous catalysis. In recent years, the focus has broadened to include the interfaces between vapors, liquids, or solids and self-assembled structures of organic, biological, and polymeric nature. 

A degree of stereoselective control of the course of a reaction, which is absent or different from that prevalent when the reaction is conducted in the absence of quaternary ammonium salts, may be achieved under standard phase-transfer catalysed reaction conditions. The reactions, which are influenced most by the phase-transfer catalyst, are those involving anionic intermediates whose preferred conformations or configurations can be controlled by the cationic species across the interface of the two-phase system. For example, in the base-catalysed Darzens condensation of aromatic aldehydes with a-chloroacetonitriles to produce oxiranes (Section 6.3), the intermediate anion may adopt either of the two conformations, (la) or (lb) which are stabilized by interaction across the interface by the cations (Scheme 12.1) [1-4]. 

A second classical method for making capsules from emulsions is to form the shell polymer in situ using interfacial polymerization (Morgan and Kwolek 1959 Wittbecker and Morgan 1959). This method is similar to the nylon rope trick often used as a demonstration, where a solution of diacid chloride in organic solvent (such as adipoyl chloride in hexanes) is layered in a beaker with a diamine aqueous phase (such as 1,6-hexadiamine in water Friedli et al. 2005). Because the two monomers meet only at the interface of the two phases, the condensation polymerization to form the polyamide occurs only at the interface. 

The fact that the polyreaction of diacetylenes is topochemically controlled is especially well documented by the polymerization behavior of the sulfolipid (22)23 . (22) forms two condensed phases when spread on an acidic subphase at elevated temperatures (Fig. 10). UV initiated polymerization can only be carried out at low surface pressures in the first condensed phase, where the molecules are less densely packed. Apparently, in the second phase at surface pressures from 20 to 50 mN/m the packing of the diyne groups is either too tight to permit a topochemical polymerization or a vertical shift of the molecules at the gas/water interface causes a transition from head packing to chain packing (Fig. 10), thus preventing the formation of polymer. 

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