Adsorption thermodynamics from surfactant solutions

In Section 3.6 it was mentioned that aqueous films on a substrate of organic solvents usually are thermodynamically unstable. A metastable asymmetric aqueous film from surfactant solutions can be obtained also when the electrostatic and/or the adsorption... 

For soluble surfactant adsorption layers the vertical mass transfer occurs under two different conditions, after the formation of a fresh surface of a surfactant solution and during periodic or aperiodic changes of the surface area. From the thermodynamic point of view the "surface phase" is an open system. The theoretical and practical aspects of this issues have been outlined in many classical papers, published by Milner (1907), Doss (1939), Addison (1944, 1945), Ward Tordai (1946), Hansen (1960, 1961), Lange (1965). New technique for measuring the time dependence of surface tension and a lot of theoretical work on surfactant adsorption kinetics under modem aspects have recently been published by Kretzschmar Miller (1991), Loglio et al. (1991), Fainerman (1992), Joos Van Uffelen (1993), MacLeod Radke (1993), Miller et al. (1994). This topic will be discussed intensively in Chapters 4 and 5. The relevance of normal mass exchange as a surface relaxation process is discussed in Chapter 6. 

Kronberg et al. have examined the thermodynamics of surfactant micellization and adsorption onto hydrophobic surfaces [47]. They considered two main contributions for the Gibbs free energy of transfer from the aqueous solution to the micelle or the surface ... 

K. Szczodrowski, B. Prdlot, S. Lantenois, J.-M. DouiUard, J. Zajac, Effect of heteroatom doping on surface acidity and hydrophilicity of Al, Ti, Zr-doped mesoporous SBA-15. Micro-porous Mesoporous Mater. 124(1-3), 84-93 (2009). doi 10.1016/j.micromeso.2009.04.035 J. Zajac, Mechanism of ionic and zwitterionic surfactant adsorption from dilute solutions onto charged non-porous and porous mineral oxides inferred from thermodynamic studies, in Recent Research Developments in Surface and Colloids, ed. by S.G. Pandalai (Research Signpost, Kerala, 2004), pp. 265-300... 

Scamehorn et. al. (20) also presented a simple, semi—empirical method based on ideal solution theory and the concept of reduced adsorption isotherms to predict the mixed adsorption isotherm and admicellar composition from the pure component isotherms. In this work, we present a more general theory, based only on ideal solution theory, and present detailed mixed system data for a binary mixed surfactant system (two members of a homologous series) and use it to test this model. The thermodynamics of admicelle formation is also compared to that of micelle formation for this same system. 

The primary mechanism for energy conservation is adsorption of surfactant molecules at various available interfaces. However, when, for instance, the water-air interface is saturated conservator may continue through other means (Figure 12.3). One such example is the crystallization or precipitation of the surfactant from solution, in other words, bulk phase separation. Another example is the formation of molecular aggregates or micelles that remain in solution as thermodynamically stable, dispersed species with properties distinct from those of an isotropic solution containing monomeric surfactant molecules (Myers, 1992). 

Adsorption isotherms represent a relationship between the adsorbed amount at an interface and the equilibrium activity of an adsorbed particle (also the concentration of a dissolved substance or partial gas pressure) at a constant temperature. The analysis of adsorption isotherms can yield thermodynamic data for the given adsorption system. Theoretical adsorption isotherms derived from statistical and kinetic data, and using the described assumptions (see 3.1), are known only for the gas-solid interface or for dilute solutions of surfactants (Gibbs). Those for the system gas-solid are of a few basic types that can be thermodynamically predicted81. From temperature relations it is possible to calculate adsorption and activation energies or rate constants for individual isotherms. Since there are no theoretically founded equations of adsorption isotherms for dissolved surfactants on solids, the adsorption of gases on solides can be used as a starting point for an interpretation. 

In the acidic route (with pH < 2), both kinetic and thermodynamic controlling factors need to be considered. First, the acid catalysis speeds up the hydrolysis of silicon alkoxides. Second, the silica species in solution are positively charged as =SiOH2 (denoted as I+). Finally, the siloxane bond condensation rate is kinetically promoted near the micelle surface. The surfactant (S+)-silica interaction in S+X 11 is mediated by the counterion X-. The micelle-counterion interaction is in thermodynamic equilibrium. Thus the factors involved in determining the total rate of nanostructure formation are the counterion adsorption equilibrium of X on the micellar surface, surface-enhanced concentration of I+, and proton-catalysed silica condensation near the micellar surface. From consideration of the surfactant, the surfactants first form micelles as a combination of the S+X assemblies, which then form a liquid crystal with molecular silicate species, and finally the mesoporous material is formed through inorganic polymerization and condensation of the silicate species. In the S+X I+ model, the surfactant-to-counteranion... 

More rigorous thermodynamic relations valid for adsorption layers which undergo a phase transition could be derived based on the requirement that the chemical potentials in either phase should be equal to each other. The phases are represented by the surfactant bulk solution, the non-condensed (surface solution) and the condensed part of the surface layer. The dependence of p- on the composition of a surface layer is given by the Butler equation (2.7). The chemical potential of the i component in the condensed phase comprised of the given component only (f x = 1) can be derived from Eq. (2.7) as... 

Adsorption kinetics, mainly studied by dynamic surface tension measurements, shows many features very much different from that of typical surfactants (Miller et al. 2000). The interfacial tension isotherms for standard proteins such as BSA, HSA, (3-casein and (3-lactoglobulin were measured at the solution/air interface by many authors using various techniques. The state of the art of the thermodynamics of adsorption was discussed in Chapter 2 while isotherm data for selected proteins were given in the preceding Chapter 3. Here we want to give few examples of the dynamic surface pressure characteristics of protein adsorption layers. 

Adsorption is an entropically driven process by which molecules diffuse preferentially from a bulk phase to an interface. Due to the affinity that a surfactant molecule encounters towards both polar and non-polar phases, thermodynamic stability (i.e. a minimum in free energy or maximum in entropy of the system) occurs when these surfactants are adsorbed at a polar/non-polar (e.g. oil/water or air/water) interface. The difference between solute concentration in the bulk and that at the interface is the surface excess concentration. The latter... 

Because the extent of reduction of the surface tension of a solution depends on the substitution of surfactant for solvent molecules at the interface, the relative concentration of surfactant in the bulk and interfacial phases should serve as an indicator of the adsorption efficiency of a given surfactant and, therefore, as a quantitative measure of the activity of the material at the solution-vapor interface. For a given homologous series of straight-chain surfactants in water, CH3(CH2) —S, where S is the hydrophilic head group and n is the number of methylene units in the chain, an analysis based on the thermodynamics of transfer of a surfactant molecule from the bulk phase to the interface leads to the conclusion that the above-defined efficiency of adsorption is directly related to the length of the hydrophobic chain. The efficiency can be defined mathematically by the expression... 

Until recently, the fast rate at which a surfactant layer forms at the solid-liquid interface has prevented accurate investigation of the adsorption process. As a result, the mechanism of surfactant adsorption has been inferred from thermodynamic data. Such explanations have been further confused by misinterpretation of the equilibrium morphology of the adsorbed surfactant as either monolayers or bilayers, rather than the discrete surface aggregates that form in many surfactant-substrate systems.2 However, the recent development of techniques with high temporal resolution has made possible studies of the adsorption, desorption,25>38,4i,48-6o exchange rates of surfactants. In this section, we describe the adsorption kinetics of C ,TAB surfactants at the silica-aqueous solution interface, elucidated by optical reflectometry in a wall-jet flow cell. The adsorption of C jTAB surfactants to silica is the most widely studied system - and hence the adsorption kinetics can be related to the adsorption process with great clarity. For a more thorough review of adsorptions isotherms, the t5q)es of surfactant structures that form at the solid-liquid interface, and the influence of these factors on adsorption, the reader is directed to Reference 24. 

The thermodynamic equilibria of amphiphilic molecules in solution involve four fundamental processes (1) dissolution of amphiphiles into solution (2) aggregation of dissolved amphiphiles (3) adsorption of dissolved amphiphiles at an interface and (4) spreading of amphiphiles from their bulk phase directly to the interface (Fig. 1.1). All but the last of these processes are presented and discussed throughout this book from the thermodynamic standpoint (especially from that of Gibbs s phase rule), and the type of thermodynamic treatment that should be adopted for each is clarified. These discussions are conducted from a theoretical point of view centered on dilute aqueous solutions the solutions dealt with are mostly those of the ionic surfactants with which the author s studies have been concerned. The theoretical treatment of ionic surfactants can easily be adapted to nonionic surfactants. The author has also concentrated on recent applications of micelles, such as solubilization into micelles, mixed micelle formation, micellar catalysis, the protochemical mechanisms of the micellar systems, and the interaction between amphiphiles and polymers. Fortunately, almost all of these subjects have been his primary research interests, and therefore this book covers, in many respects, the fundamental treatment of colloidal systems. 

The SCLF method was developed by Koopal and coworkers [46-50] to describe adsorption of surfactant molecules at the solution-solid interface. The method derives from two earlier statistical thermodynamic lattice theories (1) the Flory and Fluggins [51 ] model describing properties of polymers in solution, and (2) the methods of Scheutjens and coworkers [52-55] developed to describe the properties of polymer molecules adsorbed at the solution-solid interface and in associated mesomorphic solution structures such as micelles and vesicles. 

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