Interfacial acidity constant

The interfacial acidity constant or apparent acidity constant in the membrane phase Kam refers to the relative ratio of acidic and corresponding basic species in the membrane at a given (aqueous) pH-value ... 

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

A linear relationship was obtained between the logarithmic interfacial formation constant (log / ,) for PdL+-diazine derivative complexes and the logarithmic ratio of the distribution constant (A D) to the acid-dissociation constant (Ka) for the two groups. PdL+-pyridazine derivative complexes showed much higher stability at the interface than pyrimidine and pyrazine derivative complexes. This result suggests that pyridazine derivative complexes become more... 

To investigate the relationship between the interfacial rate constant and the diffiision layer thickness, the protonation reaction rate was measured by the CLM method. The schematic diagram of the CLM apparatus is shown in Figure 10.2. In the CLM method, the thickness of the dodecane phase in the aqueous hydrochloric acid phase can be controlled by the initial volume of each. When the thickness of the dodecane phase was changed from 53 to 132 pm, the observed interfacial diprotonation rate constant decreased from 9.9 x 10 s to 1.5 x 10 s". The experimental results were well reproduced by Equation (1), substituting 3 with the organic phase thickness [10]. 

The molecular recognition ability of Pd(II)-5-Br-PADAP for the isomers of diazine derivatives has been evaluated. Figure 10.14 summarizes the molecular structures of the diazines (Dzs or N) studied, the acid-dissociation constant Kf) and the distribution constant values between toluene and water Kd). PdLCl reacted with a neutral N with one or two nitrogen atoms, forming a PdLN" complex at the toluene/water interface. The interfacial formation constant (/6j) of the PdLN+ complex was determined as follows [52] ... 

In order to explain this experimental feature, Leodidis and co-workers developed a thermodynamic model where the chemical potential depends on the curvature of the film and on the bending moment or interfacial elastic constant C. The variation of Kx with curvature can be derived from the equality between the chemical potential of an amino acid in excess water and in the interface, as follows ... 

These two equations involve four interfacial variables (ao, era, tpo, iwo variables characterizing the medium (pH and electrolyte concentration C) and quantities specific to the surface (acidity constants /T and K and the total group density N ). 

Increased agitation of a given acid—hydrocarbon dispersion results in an increase in interfacial areas owing to a decrease in the average diameter of the dispersed droplets. In addition, the diameters of the droplets also decrease to relatively low and nearly constant values as the volume % acid in the dispersions approaches either 0 or 100%. As the droplets decrease in si2e, the ease of separation of the two phases, following completion of nitration, also decreases. 

SFA has been traditionally used to measure the forces between modified mica surfaces. Before the JKR theory was developed, Israelachvili and Tabor [57] measured the force versus distance (F vs. d) profile and pull-off force (Pf) between steric acid monolayers assembled on mica surfaces. The authors calculated the surface energy of these monolayers from the Hamaker constant determined from the F versus d data. In a later paper on the measurement of forces between surfaces immersed in a variety of electrolytic solutions, Israelachvili [93] reported that the interfacial energies in aqueous electrolytes varies over a wide range (0.01-10 mJ/m-). In this work Israelachvili found that the adhesion energies depended on pH, type of cation, and the crystallographic orientation of mica. 

The Gibbs equation allows the amount of surfactant adsorbed at the interface to be calculated from the interfacial tension values measured with different concentrations of surfactant, but at constant counterion concentration. The amount adsorbed can be converted to the area of a surfactant molecule. The co-areas at the air-water interface are in the range of 4.4-5.9 nm2/molecule [56,57]. A comparison of these values with those from molecular models indicates that all four surfactants are oriented normally to the interface with the carbon chain outstretched and closely packed. The co-areas at the oil-water interface are greater (heptane-water, 4.9-6.6 nm2/molecule benzene-water, 5.9-7.5 nm2/molecule). This relatively small increase of about 10% for the heptane-water and about 30% for the benzene-water interface means that the orientation at the oil-water interface is the same as at the air-water interface, but the a-sulfo fatty acid ester films are more expanded [56]. 

FIG. 25 Typical DPSC data for the oxidation of 10 mM bromide to bromine (forward step upper solid curve) and the collection of electrogenerated Br2 (reverse step lower solid curve) at a 25 pm diameter disk UME in aqueous 0.5 M sulfuric acid, at a distance of 2.8 pm from the interface with DCE. The period of the initial (generation) potential step was 10 ms. The upper dashed line is the theoretical response for the forward step at the defined tip-interface separation, with a diffusion coefficient for Br of 1.8 x 10 cm s . The remaining dashed lines are the reverse transients for irreversible transfer of Br2 (diffusion coefficient 9.4 x 10 cm s ) with various interfacial first-order rate constants, k, marked on the plot. (Reprinted from Ref. 34. Copyright 1997 American Chemical Society.)... 

M sulfuric acid to air [34]. As discussed above, for the aqueous-DCE interface, the rate of this irreversible transfer process (with the air phase acting as a sink) was limited only by diffusion of Bt2 in the aqueous phase. A lower limit for the interfacial transfer rate constant of 0.5 cm s was found [34]. 

FIG. 4 Thermodynamic equilibria for the interfacial distribution of a solute X which can be ionized n times, and X being its most acidic (or deprotonated) and its most basic (or protonated) forms, respectively. X and are the dissociation constants in the aqueous and organic phase, respectively, and P is the partition coefficient of the various species between the two phases. 

Ionizable molecules embedded in the surfaces of lipids, such as octanol (see Fig. 2.8), liposomes (see Fig. 5.2), or micelles, will have their apparent pKa values shifted. With neutral lipids, the pKa of an acid increases and the pKa of a base decreases. This is due to the effect of the decreased dielectric constant in the interfacial zone, as we have already discussed in various sections. 

The interfacial surface tension may be markedly affected by a surface reaction occurring in the presence of traces of impurities. A good example of such an effect is to be noted in the alteration of the interfacial surface tension between an oil containing a fatty acid and water containing acid or alkali. For acid solutions the interfacial surface tension remains constant and almost independent of the Ph of the aqueous phase. As the solution becomes more alkaline the carboxyl groups of the acid commence to dissociate and the interfacial surface tension falls rapidly. (Hartridge and Peters, Proc. Boy. Soc. A, Cl. 348, 1922, see p. 252.)... 

In acid solution as far as Ph = 5 the interfacial tension is constant but with increasing alkalinity it falls. In the case of fatty acids the tension becomes vanishingly small when the Ph exceeds 8 and the acid dissolves in the alkali in the form of micelles (see Ch. ix). 

Kinetics of Aromatic Nitrations. The kinetics of aromatic nitrations are functions of temperature, which affects the kinetic rate constant, and of the compositions of both the acid and hydrocaibon phase. In addition, a larger interifacial area between the two phases increases the rates of nitration since the main reactions occur at or near the interface. Larger interfacial areas are oblaincd by increased agitation and by ihc proper choice of the volumetric % acid in the liquid-liquid dispersion. The viscosities and densities of the two phases and the interfacial tension between the phases are important physical properties affecting the interfacial area. 

The MEMED technique has been used to study the hydrolysis of aliphatic acid chlorides in a water/l,2-dichloroethane (DCE) solvent system [3]. It was shown unambiguously that the reaction proceeds via an interfacial process and shows saturation kinetics as the concentration of acid chloride in the DCE increases the data were well fitted to a model based on a pre-equilibrium involving Langmuir adsorption at the interface. First-order rate constants for interfacial solvolysis of CH3(CH2) COCl were 300 150(n = 2), 200 100(n = 3) and 120 60 s-1( = 8). 

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