Liquid-phase adsorptions from dilute solutions

The role of hydrogen bonds in liquid or vapour-phase adsorption processes has not hitherto been very clearly defined. The present paper is a brief review of some recent investigations in this laboratory, many of them unpublished, which it is believed may help to define the conditions favouring hydrogen bond adsorption especially from dilute solutions, by inorganic and organic substrates. 

IAS model for dilute liquid solution The IAS method was first proposed to accoimt for the adsorption of gas mixtures. It was later extended to multisolute adsorption from dilute liquid solutions [54]. Assuming that both the solution and the adsorbed phase are ideal, the following equation can be derived to calculate multi-solute equilibriirm composition [54]. 

There are certain conditions that must be fulfilled if Eqs. (2.2), (2.3) and (2.4) are to be used to calculate partition coefficients. The basic assumption is that the individual retention mechanisms are independent and additive. This will be true for conditions where the infinite dilution and zero surface coverage approximations apply or, alternatively, at a constant concentration with respect to the ratio of sample size to amount of liquid phase. The infinite dilution and zero surface coverage approximations will apply to small samples where the linearity of the various adsorption and partition isotherms is unperturbed and solute-solute interactions are negligible. The constancy of the solute retention volume with variation of the sample size for low sample amounts and the propagation of symmetrical peaks is a reasonable indication that the above conditions have been met. For asymmetric peaks, however, the constant concentration method must be employed if reliable gas-liquid partition coefficients are to be obtained [191]. It is difficult to state absolutely the conditions for which contributions to retention from the structured liquid phase layer can be neglected. This will occur for some minimum phase loading that depends on the support surface area, the liquid phase... 

The isosteric method can be considered, for adsorption from solution, in a similar manner as for gas adsorption (see Section 2.6.1). For example, by equating the chemical potentials of component 2 (the solute) in its adsorbed state and the liquid phase, by keeping the specific amounts adsorbed constant and by considering a dilute solution, so that the activity can be replaced by a molality (b2 = 1000 n2/mi), we obtain ... 

As a preparation to the following sections, we briefly discuss some aspects of measuring adsorption from fluid phases, including dilute solutions. For the sake of systematics, we divide the treatment into two parts (1) adsorption on disperse systems, sometimes poorly defined, and (ii) the same on well-defined, mostly smooth model surfaces. In case (1) adsorption is almost exclusively determined from solution analysis, i.e. by depletion, so that problems arise with the separation of liquid from solid and the accurate bulk composition determinations. In case (ii), adsorbed amounts can often be determined directly using typical surface analytical techniques. 

Another problem, that of adsorption at any of the several interfaces (such as gas-Uquid and gas-solid), may arise with minute samples. Martin found gas-chromatographic retention volumes to be markedly affected by such adsorptions and in some cases to be the cause of a larger contribution to retention volume than solution in the bulk stationary phase. He both measured and calculated the extent of such adsorptions for a variety of solutes. Adsorption at a gas-liquid interface is serious for polar stationary phases when surface areas are relatively high, when the ratio of stationary phase to solid support is low, and when the temperature is low. The effects of adsorption are minimized by the use of relatively highly loaded columns and nonpolar stationary phases and by avoiding solute-stationary phase pairs in which the infinite-dilution activity coefficients deviate markedly from unity. Ottenstein indicated that adsorption on a liquid surface can be considered negligible in packed columns when liquid loadings exceed 5% and activity coefficients are less than 10. 

Unfortunately, the study of phase equilibria in solution, e.g., liquid-solid adsorption, is not a highly popular area of research. Gas-solid adsorption and vapor-solution equilibria have been studied in far more detail, although most of the information available concerns the fate of single components in a diphasic system. Liquid-solid adsorption has benefited mainly from the extension of the concepts developed for gas phase properties to the case of dilute solutions. Multicomponent systems and the competition for interaction with the stationary phase are research areas that have barely been scratched. The problems are difficult. The development of preparative chromatography and its applications are changing this situation. 

For dilute solutions of surfactants then, the number of moles of surface-active solute adsorbed per unit mass of the solid substrate can be calculated from the concentrations of the solute in the liquid phase before and after the solution is mixed with the finely divided solid adsorbent and the mixture is shaken until equilibrium has been reached. Then n is plotted against C to yield the adsorption isotherm. A variety of analytical techniques are available for determining the change in concentration of the surfactant (Rosen, 1972). 

Transport of solute from a fluid phase to a spherical or nearly spherical shape is important in a vari of separation operations such as liquid-liquid extraction, crystallization from solution, and ion exchange. The situation depicted in Fig. 2.3-12 assumes that there is no forced or natural convection in the fluid about the particle so that transport is governed entirely by molecular diffusion. A steady-state solution can be obtained for the case of a sphere of fixed radius with a constant concentration at the interface as well as in the bulk fluid. Such a model will be useful for crystallization from vaqxtrs and dilute solutions (slow-moving boundary) or for ion exchange with rapid irreversible reaction. Bankoff has reviewed moving-boundary problems and Chapters 11 and 12 deal with adsorption and ion exchange. 

The second transition ion group contains Zn, Cd, and Hg, which are known to be strong poisons. Besides the works cited already. Huang and Rhoads [66] give curves for the removal of of different initial concentrations by different amounts of Y-AI2O3. Brady [28] shows the temperature dependence of Cd and Pb adsorption. The speciation of Zn and Cd in the liquid phase — necessary for mode calculations — is reported in Refs. 4 and 67 with remarkable differences, especially for the MeOH species. Liu et al. [4] observed a linear relation of the logarithms of adsorbed and dissolved amounts of 7n-+ and Cd ". Kalatsei et al. [68] report extremely slow equilibration of Zn with alumina in the presence of NH3. Lowson and Evans [53] studied the adsorption of Zn and Cd ions from very dilute solutions on a-Al203. 

Justification for giving prority to this interface is the fact that both experimental and theoretical studies of adsorption at the solid/liquid interface preceded those from the gaseous phase. Moreover, some equations of isotherms for adsorption at the solid/liquid interface, particularly, those referring to adsorption from the diluted solutions, are derived from the theoretical description of single gases and their mixtures on solid surfaces. 

Contaminants to water supplies are in low concentrations and knowledge of their behavior toward adsorbents is critical, hence the importance of the dilute solution. The main differences between adsorption from the gas phase and the liquid phase are as follows ... 

Because this adsorption may be present partially in the form of a diffuse double layer (thisCh 4 c p 78, Ch IV p 115) which may extend considerably into the solution phase, the notion of intermicellar liquid has to be defined more precisely. In a very dilute sol the composition of the liquid far away from every sol particle may be considered to be tliat of the intermicellar liquid In more concentrated systems the best definition is the liquid with which the sol can be in equilibrium when the sol particles are prevented from diffusing into it ... 

In a detersive system containing a dilute surfactant solution and a substrate bearing a solid polar soil, the first effect is adsorption of surfactant at the soil-bath interface. This adsorption is equivalent to the formation of a thin layer of relatively concentrated surfactant solution at the interface, which is continuously renewable and can penetrate the soil phase. Osmotic flow of water and the extrusion of myelin forms follows the penetration, with ultimate formation of an equilibrium phase. This equilibrium phase may be microemulsion rather than liquid crystalline, but in any event it is fluid and flushable from the substrate surface. This phase change effect explains the detersive behavior of sucrose fatty esters in admixture with alkylarenesulfonates (117). 

Tlie following recommendations should be tak(, n into account in order to obtain highly accurate activity coefficients from gas-liquid chromatography data (a) the adsorption at the gas-liquid interface should be avoided by measuring the retention volumes for several stationary phase/ support ratios (b) the vapour pressure of the solute should be accurately known. If these requirements are fulfilled, thou the activity coefficient at infinite dilution can be determined to an accuracy of 0.2 per cent [45] and the overall precision is higher than 0.5 per cent as compared to static measurements. 

The bubble point tests conducted in methanol/water mixtures were worked up to show properties of the three-phase interfaces along the complex contact line in SS304 LAD screens. In particular, the variation with F2 of the solid/vapor interfacial tension /sv differed from that of the solid/liquid interface j/sl- The data are consistent with the Langmuir isotherm description of the thermodynamics of adsorption. The result of the analysis is that the co-areas Amin are 0.32 nm /molecule for the SS304— vapor interface and 1.77 nm /molecule for the SS304—solution interface. This implies that that methanol molecules form a dense, liquid-like monolayer at the interface of SS304 with the vapor phase, while the methanol molecules are very dilute in the interface between SS304 and the solution of methanol/water. 

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