Adsorption from Surfactant Solutions

A recent design of the maximum bubble pressure instrument for measurement of dynamic surface tension allows resolution in the millisecond time frame [119, 120]. This was accomplished by increasing the system volume relative to that of the bubble and by using electric and acoustic sensors to track the bubble formation frequency. Miller and co-workers also assessed the hydrodynamic effects arising at short bubble formation times with experiments on very viscous liquids [121]. They proposed a correction procedure to improve reliability at short times. This technique is applicable to the study of surfactant and polymer adsorption from solution [101, 120]. 

There are numerous references in the literature to irreversible adsorption from solution. Irreversible adsorption is defined as the lack of desotption from an adsoibed layer equilibrated with pure solvent. Often there is no evidence of strong surface-adsorbate bond formation, either in terms of the chemistry of the system or from direct calorimetric measurements of the heat of adsorption. It is also typical that if a better solvent is used, or a strongly competitive adsorbate, then desorption is rapid and complete. Adsorption irreversibility occurs quite frequently in polymers [4] and proteins [121-123] but has also been observed in small molecules and surfactants [124-128]. Each of these cases has a different explanation and discussion. 

In the case of adsorption from solution, the surfactant layers are in equilibrium with the solution and will de-sorb on dilution. However, it would be very useful to produce adsorbed layers in both air and water, which will remain adsorbed. This can be achieved using the Langmuir-Blodgett deposition technique. The technique is based on the observation that if a surfactant, which is insoluble in water, is dissolved in a volatile, non-aqueous solvent and then spread on water, an insoluble monolayer of orientated surfactant molecules will remain at the air/solution interface. The effect of the spreading surfactant and its surface film pressure can be dramatically demonstrated by spreading hydrophobic talc powder on a clean water surface and then placing a... 

No discussion of adsorption from solution is anywhere near complete unless it includes some indication of its enormous practical applicability. As a matter of fact, the examples we briefly consider —detergency and flotation —encompass a wide variety of concepts from almost all areas of surface and colloid chemistry. We have chosen to stress principles rather than applications, however, so these subjects will receive an amount of attention that belies their actual importance. Following these traditional applications, two examples of new applications that are envisioned for surfactant layers deposited on solid substrates are discussed. 

Both adsorption from solution and micellization occur as a result of the hydrophobic effect. To test the correspondence between these two effects. Rosen assembled AG° values for adsorption at the air-water interface and for micellization of a number of linear and branched surfactants. The following is a selection of these data ... 

For our purposes, adsorption from solution is of more direct relevance than gas adsorption. Most, if not all, topics in the five volumes of FICS Involve one or more elements of it. In the present chapter, the basic elements will be introduced, restricting ourselves to low molecular weight, uncharged adsorbates and solid surfaces. Adsorption of charged species leads to the formation of electrical double layers, which will be treated in chapter 3. Adsorption at fluld/fluid Interfaces follows in Volume III. Adsorption of macromolecules will be Introduced in chapter 5. Between monomers, short oligomers, longer oligomers and polymers there is no sharp transition in the present chapter we shall go as far as non-ionic surfactants, but omit most of the association and micelle formation features, which will be addressed in a later Volume. There will be some emphasis on aqueous systems. 

J.S. Clunie, B.T. Ingram, Adsorption of non-ionic surfactants, in Adsorption from Solution at the Solid-Liquid interface (see sec. 2.10b), p. 105. 

Adsorption from Solution at the Solid/Liquid Interface, G.D. Parfitt, C.H. Rochester, Eds., Academic Press (1983). (Contains chapters on adsorption of smEill molecules (G.D. Parfitt and C.H. Rochester), adsorption from mixtures of miscible liquids (J.E. Lane) and adsorption of non-ionic surfactants (J.S. Clunle,... 

In section 6.2.4 we examined the case in which the surface of a solution containing an amphiphile became covered with a monomol-ecular film as a result of spontaneous adsorption from solution. The molecules in such films are in equilibrium with those in the bulk of the solution, i.e. there is a continuous movement of molecules between the surface and the solution below it. If, however, a surfactant has a very long hydrocarbon chain it will be insufficiently water-soluble for a film to be formed in this way. In such cases we can spread a film on the surface of the solution by dissolving the surfactant in a suitable volatile solvent and carefully injecting the solution on to the surface. The insoluble monolayer formed by this process contains all of the molecules injected on the surface there is no equilibrium with the bulk solution because of the low water solubility of the surfactant. Conse-... 

Another feature of adsorption from solution is the variety and complexity of molecules that may be involved in the processes. Indeed one can be interested either by a simple organic molecule, like benzene and its derivatives, or by much larger molecules like proteins, surfactants, or polymers, which bear many different chemical functions and may adopt a large number of conformations at the interface. For such molecules, a good knowledge of both the surface chemistry and the accessibility of porous materials are crucial to understand the adsorption phenomenon. 

Danov, K.D., Adsorption from micellar surfactant solutions nonlinear theory and experiment, J. Colloid Interface ScL, 183, 223, 1996. 

The Langmuir Adsorption Isotherm A type of adsorption isotherm commonly observed in adsorption from solutions of surfactants is the Langmuir-type isotherm (Langmuir, 1918), expressed by... 

The fact that experimental adsorption data fit the Langmuir equation does not mean that the assumptions on which the Langmuir model is based are fulfilled. In the case of surfactants, these assumptions, particularly the absence of lateral interactions, are almost never valid. In spite of this, many surfactants show Langmuir-type adsorption from solution because of the mutual compensation of several factors that affect the shape of the Langmuir isotherm. Some of these factors and the manner in which they modify the shape of the isotherm are as follows (Kitchener, 1965) ... 

A so-called sacrificial flush is a solution containing cheap surfactant substitutes (e.g. lignosulphonates) likely to adsorb on the rock surface. Such a slug could be injected to prevent or reduce the adsorption from the surfactant slug, thus reducing losses (and cost) and formulation alterations. It also prepares the reservoir fluids to reach the optimal formulation in an easier and faster way. 

Adsorption from solutions onto solid surfaces is important in many industrial practices, such as dye or organic contaminant removal, edible oil clarification by activated carbon, and ion exchange, where the adsorption of ions from electrolyte solutions is carried out. Adsorption from solution is also used in analytical chemistry in various chromatography applications. On the other hand, surfactant, polymer and biological material adsorption on solids, to modify the surface of solid particles in stabilizing dispersions, are also very important industrial fields. 

A surfactant can be transported to a certain site at an interface by adsorption from solution and by lateral transport in the interface. In all cases it leads to an even distribution of surfactant over the interface and to a lowering of interfacial tension. These processes take time. 

When adsorption takes place at the surface of a highly porous solid adsorbent, the surface excess can be readily measured, e.g. by measuring the increase in the adsorbent weight in the case of adsorption from vapor, or by following the decrease in the adsorbate concentration during adsorption from solutions. Studies of the adsorption dependence on vapor pressure (or solution concentration) reveal T(p) (or T(c)) adsorption isotherms. In both cases the two-dimensional pressure isotherm can be established from the Gibbs equation (see Chapter II, 2, and Chapter VII, 4). Therefore, it is as a rule possible to establish the dependence between the two of three variables present in the Gibbs equation the surface tension isotherm, a(c), for mobile interfaces and soluble surfactants, the two-dimensional pressure, tt(c), isotherm for insoluble... 

The adsorption from solutions on finely dispersed powders and porous adsorbents is used for the removal of dissolved toxic components, as well as for concentrating and entrapping valuable substances from dilute solutions. In agreement with the polarity equalization rule, surface active substances dissolved in aqueous medium can be removed by adsorption on non-polar adsorbents (such as activated carbon), or on adsorbents that are capable of chemisorbing the surfactant polar heads. In order to increase the effectiveness... 

Adsorption from solutions is often used as a relatively simple method for the determination of the specific surface area of adsorbents [15]. The decrease in the surfactant concentration, Ac, is determined after a solution of known volume, V, has been equilibrated over an adsorbent of known weight, m, and the adsorption isotherm, T (c), is established (note that the adsorption in this case is expressed in moles per gram). The limiting adsorption, T max, can be obtained from the Langmuir equation, e.g. as c/T = a/T max + c/T max (see Chapter II, 2). Using independently acquired data for the area occupied by one surfactant molecule, on a similar solid substrate, one can obtain the specific surface area (in m2/g) ... 

The kinetics of adsorption from solutions of surfactant mixtures are described on the basis of a generalised Langmuir isotherm. The simultaneous adsorption leads to the replacement of less surface active compounds by those of higher surface activity, which are usually present in the bulk at much lower concentration. More general descriptions of the process are possible on the basis of the Frumkin and Frumkin-Damaskin isotherms, which include specific interfacial properties of the individual surface active species. Quantitative studies of such very complex models can be performed only numerically. 

The pseudophase-separation model for surfactant solutions (84, 132, 135) states that a surfactant solution above its critical micelle concentration (CMC) consists of two pseudophases in equilibrium with each other singly dispersed surfactant monomer molecules and micelles. When the surfactant solution is in contact with a solid, the adsorbed phase constitutes a third pseudophase (40). Strong experimental evidence suggests that surfactant adsorption takes place from the surfactant monomer phase but not from the micelles (84), behavior that leads to competition of both the micelles and the adsorbed phase for surfactant molecules from the monomer phase. Adsorption from a surfactant solution above the CMC then depends not only on the affinity of the surfactant for the solid surface but also on its tendency to form micelles. If a mixture can be formulated such that at least one of the surfactants is incorporated into micelles preferentially over the adsorbed phase, then the micelles act as a sink for the surfactant and thus prevent it from being adsorbed. 

It is difTicult to find in the literature details of experiments that relate contact angle to the efficiency of the dispersion process. A correlation between contact angle and flotation rate for quartz in aqueous solutions of dodecylammonium acetate, with adsorption from solution and electrokinetic (zeta potential) data, was reported by Fuerstenau". In a later review Fuerstenau discussed the relevance of the adsorption process to flotation technology illustrating the direct relationship between the amount of surfactant adsorbed and the contact angle . Numerous values of 0 are published in the literature. Most have been measured on flat surfaces and even with these problems arise due to surface roughness" . For powders the measurement is more difficult, although a number of methods have been reported" and reviewed . 

There are different models developed in the past to describe the adsorption from mixed surfactant solutions, for example recently by Siddiqui and Franses (1997), Ariel et al. (1999), Mulqueen and Blankschtein (1999, 2000), Penfold et al. (2003). The simplest model is obviously a generalised Langmuir isotherm (for ideal behaviour in the bulk and at the interface) for mixtures of two surfactants 1 and 2 with similar partial molar surface area (O can be presented in the form (Fainerman et al. 2001)... 

Vlahovska, PM., Horozov, T., Dushkin, CD., Kralchevsky, P.A., Mehreteab, A., and Broze, G., Adsorption from micellar surfactant solutions nonlinear theory and experiment, J. Colloid Interface Sci., 183, 223, 1997. 

Trogus et al. (2) have also found that the adsorption of alkyl aryl sulfonates from brine on Berea and kaolinite increased with surfactant molecular weight. Lawson and Dilgren (3) have noted that sulfonate adsorption increased with brine salinity and therefore with decreased solubility. These results are in agreement with the theory that adsorption from solution depends in part on the magnitude of the solute-solvent interactions in the solution (4) in general, weak interactions, as manifested by low solubility, lead to large solute adsorption. 

In many biological and technological processes, adsorption from solution is of crucial importance. Most of these processes take place in aqueous media. Examples are the adsorption of toxic or other adverse components on active carbon in the production of potable water, adsorption of contaminants from the gastrointestinal tract on medical carbon, adsorption and desorption of herbicides and pesticides on plant and soil materials, and adsorption of surfactants to influence the wettability of surfaces. Applications of adsorption are given further attention in Section 14.5. 

In petroleum recovery [3J] and environmental soil remediation processes [158, 159], surfactant adsorption from solution onto solid surfaces most commonly occurs in porous media, either on the walls of pores or throats or else on fine particles in rock pores. This adsorption constitutes a loss of... 

Several important aspects of adsorption from solution have been omitted from this review and will be dealt with subsequently. Among these are the statistical mechanics of adsorption, and especially the problems of polymer adsorption, where much progress has been made in recent ye lrs. The phenomena associated with structuring at solid surface have been further studied, and increasing attention is being paid to the kinetics of adsorption from solution. One of the areas of greatest significance in colloid science is that of surfactant adsorption, and work in this area also continues and expands. 

The contact of a lubricating substance with a solid is particularly significant from a tribological point of view. Oxyethylated alcohols are nonionic surfactants, and their interactions with the surface are basically quite specific (hydrogen bonds). The contribution of universal (electrostatic) interactions is considerably smaller, as these are very weak dispersion interactions. In the solution in contact with a solid, one can distinguish the surface phase and the bulk phase. Due to adsorption from solutions, the surface phase is enriched with the component that has a stronger affinity for the surface. It is a characteristic of adsorption from solutions on a solid surface that individual components compete for free sites on the surface. At this point, one should not confuse adsorption with absorption, the latter of which may lead to penetration of the components into the solid. 

Adsorption from solution may be represented by a stoichiometric displacement reaction between molecules of the adsorbed solvent (water, component 2) and the solute (surfactant, component 1) [10-12] ... 

Dynamics of Adsorption from Micellar Surfactant Solutions.276... 

The overall rate of snrfactant adsorption is controlled by the slowest stage. If it is stage (1), we deal with diffusion control, while if stage (2) is slower, the adsorption occurs under barrier (kinetic) control. The next four sections are dedicated to processes under diffusion control (which are the most freqnently observed), whereas in Section 4.2.2.5 we consider adsorption under barrier control. Finally, Section 4.2.2.6 is devoted to the dynamics of adsorption from miceUar surfactant solutions. 

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