Between two phases

A general prerequisite for the existence of a stable interface between two phases is that the free energy of formation of the interface be positive were it negative or zero, fluctuations would lead to complete dispersion of one phase in another. As implied, thermodynamics constitutes an important discipline within the general subject. It is one in which surface area joins the usual extensive quantities of mass and volume and in which surface tension and surface composition join the usual intensive quantities of pressure, temperature, and bulk composition. The thermodynamic functions of free energy, enthalpy and entropy can be defined for an interface as well as for a bulk portion of matter. Chapters II and ni are based on a rich history of thermodynamic studies of the liquid interface. The phase behavior of liquid films enters in Chapter IV, and the electrical potential and charge are added as thermodynamic variables in Chapter V. 

Thus Pi, surface potential jump X, the chemical potential p, and the Galvani potential difference between two phases A0 = are not. While jl, is defined, there is a practical dif-... 

Finally, the difference in Volta potential between two phases is the surface potential AV discussed in Chapter III ... 

Ideal adhesion simply means the adhesion expected under one or another model situation of uniform materials having intimate contact over a well-defined area. In these cases, the important quantity is the work of adhesion wab between two phases, which is given by... 

When two or more phases, e.g. gas, liquid or solid, are in equilibrium, the principles of internal equilibrium developed in section A2.1.5.2 apply. If transfers between two phases a and p can take place, the appropriate potentials must be equal, even though densities and other properties can be quite different. 

In addition to the case of a metal in contact with its ions in solution there are other cases in which a Galvani potential difference between two phases may be found. One case is the innnersion of an inert electrode, such as platinum metal, into an electrolyte solution containing a substance S that can exist m either an oxidized or reduced fomi tlirough the loss or gain of electrons from the electrode. In the sunplest case, we have... 

Furthermore, most physicochemical properties are related to interactions between a molecule and its environment. For instance, the partitioning between two phases is a temperature-dependent constant of a substance with respect to the solvent system. Equation (1) therefore has to be rewritten as a function of the molecular structure, C, the solvent, S, the temperature, X etc. (Eq. (2)). 

An equilibrium constant describing the distribution of a solute between two phases only one form of the solute is used in defining the partition coefficient... 

Extraction Between Two Phases When the sample is initially present in one of the phases, the separation is known as an extraction. In a simple extraction the sample is extracted one or more times with portions of the second phase. Simple extractions are particularly useful for separations in which only one component has a favorable distribution ratio. Several important separation techniques are based on simple extractions, including liquid-liquid, liquid-solid, solid-liquid, and gas-solid extractions. 

Earlier we learned that the partitioning of a solute between two phases is described by a partition coefficient. If the solute is initially in an aqueous phase and is extracted into an organic phase ... 

Because micelles are negatively charged, they migrate toward the cathode with a velocity less than the electroosmotic flow velocity. Neutral species partition themselves between the micelles and the buffer solution in much the same manner as they do in HPLC. Because there is a partitioning between two phases, the term chromatography is used. Note that in MEKC both phases are mobile. ... 

Static headspace GC/MS. The partitioning of volatile and semivolatile compounds between two phases in a sealed container. An aliquot of the headspace gas generated is injected onto a gas chromatographic column. This is followed by mass spectrometric analysis of compounds eluting from the gas chromatograph. 

The first point in developing the thermodynamic method is the observation that for equilbrium between two phases-say, a and 3-the chemical potential must be equal in both phases for all components ... 

Surfaces are formed in the transition from one state of matter to another, whether the two phases are chemically distinct or not. Thus, surfaces exist at interphases or interfaces between two phases of either the same or different materials. For example, the surface of an ice cube in a glass of water represents an interface between two phases that are identical in chemical composition. The surface of a straw in the same glass of water represents an example of an interface between chemically distinct materials. 

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only aline of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended soHd, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase iacreases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because iaterfaces can only exist between two phases, analysis of phenomena ia the L—L—S—G system breaks down iato a series of analyses, ie, surfactant solution to the emulsion, soHd, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed soHd. FiaaHy, the dispersed phases are ia equiUbrium with each other through their common equiUbrium with the surfactant solution. 

Gas Chromatography. Gas chromatography is a technique utili2ed for separating volatile substances (or those that can be made volatile) between two phases, one of which is a gas. Purge-and-trap methods are frequently used for trace analysis. Various detectors have been employed in trace analysis, the most commonly used being flame ioni2ation and electron capture detectors. 

Electrochemical systems convert chemical and electrical energy through charge-transfer reactions. These reactions occur at the interface between two phases. Consequendy, an electrochemical ceU contains multiple phases, and surface phenomena are important. Electrochemical processes are sometimes divided into two categories electrolytic, where energy is supplied to the system, eg, the electrolysis of water and the production of aluminum and galvanic, where electrical energy is obtained from the system, eg, batteries (qv) and fuel cells (qv). 

Figure 26-65 illustrates that Eq. (26-90) provides a linear approximation to the nonlinear relationship between two-phase specific volume and reciprocal pressure (v vs. P or vs. T ). For single components, me initial slope of the curve is found using me Clapeyron equation to give ... 

S = centre spacing between two phases in mm Skin effect... 

Geranylgeranyl pyrophosphate [6699-20-3 (NH4 salt)] M 450.5, pKEst(i)-<2, pKesk2) <2, pKEst(3) 3.95, pKes,(4) 6.26. Purified by counter-current distribution between two phases of a... 

A separation process that is achieved by the distribution of the substances to be separated between two phases, a stationary phase and a mobile phase. Those solutes, distributed preferentially in the mobile phase, will move more rapidly through the system than those distributed preferentially in the stationary phase. Thus, the solutes will elute in order of their increasing distribution coefficients with respect to the stationary phase."... 

This description of the dynamics of solute equilibrium is oversimplified, but is sufficiently accurate for the reader to understand the basic principles of solute distribution between two phases. For a more detailed explanation of dynamic equilibrium between immiscible phases the reader is referred to the kinetic theory of gases and liquids. 

The distribution coefficient is an equilibrium constant and, therefore, is subject to the usual thermodynamic treatment of equilibrium systems. By expressing the distribution coefficient in terms of the standard free energy of solute exchange between the phases, the nature of the distribution can be understood and the influence of temperature on the coefficient revealed. However, the distribution of a solute between two phases can also be considered at the molecular level. It is clear that if a solute is distributed more extensively in one phase than the other, then the interactive forces that occur between the solute molecules and the molecules of that phase will be greater than the complementary forces between the solute molecules and those of the other phase. Thus, distribution can be considered to be as a result of differential molecular forces and the magnitude and nature of those intermolecular forces will determine the magnitude of the respective distribution coefficients. Both these explanations of solute distribution will be considered in this chapter, but the classical thermodynamic explanation of distribution will be treated first. 

So far the plate theory has been used to examine first-order effects in chromatography. However, it can also be used in a number of other interesting ways to investigate second-order effects in both the chromatographic system itself and in ancillary apparatus such as the detector. The plate theory will now be used to examine the temperature effects that result from solute distribution between two phases. This theoretical treatment not only provides information on the thermal effects that occur in a column per se, but also gives further examples of the use of the plate theory to examine dynamic distribution systems and the different ways that it can be employed. 

The primary drawback to the application of XPS in adhesion science is associated with the limited spatial resolution of the technique. This can make it difficult to study processes that are highly localized, such as corrosion, or to accurately characterize certain types of failure surfaces where, for example, the locus of failure may pass back and forth between two phases. 

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