Adsorption isotherm of nonionic surfactants

FIGURE 4.29 Adsorption isotherms of nonionic surfactants on the polar and nonpolar solid surfaces. [Graph reconstructed from data by T. F. Tadros, Solid/Liquid Dispersions, Academic Press, New York, 1987.]... 

Fig. 9. Comparison of the adsorption isotherms of ( ) nonionic surfactant Ci2E4, ( ) AOT on OTS self-assembled monolayers (SAMs) on silicon, and (o) AOT adsorbed at the air-liquid interface.

The adsorption isotherms of nonionic surfactants on inorganic materials are similar to those shown in Fig. 4.65, but the difference in slope between regions II and III is usually less distinct, and they often merge into one region with approximately constant slope. 

The adsorption isotherm of nonionic surfactants are in many cases Langmuirian, much like those of most other highly surface active solutes adsorbing from dilute solutions, and the adsorption is generally reversible. However, several other adsorption types are produced [29], and these are illustrated in Figure 5.7. The steps... 

Examples of the Adsorption Isotherms of Nonionic Polymeric Surfactants... 

Zhu and Gu [7] used a simple mass action model to fit adsorption isotherms for nonionic surfactants. They noted that adsorption often increased with concentration in two steps, so they modeled adsorption with two binding constants. The first binding constant accounted for binding of the surfactant to the surface, and the second also accounted for attractive interactions between the adsorbed surfactant molecules. The latter are expected to be important at a high surface density of surfactant. 

The adsorption of nonionic surfactants on polar and nonpolar surfaces also exhibits various features, depending on the nature of the surfactant and the substrate. Three types of isotherms may be distinguished, as illustrated in Fig. 7. These isotherms can be accounted for by the different surfactant orientations and their association at the solid/liquid interface as illustrated in Fig. 8. Again, bilayers, hemimicelles, and micelles can be identified on various substrates. 

Cetyltrimethylammonium bromide CH3(CH2)i5N(CH 3)381 is a typical cationic surfactant. Dissociation of this compound results in amphiphilic cations and small hydrophilic anions. This classification of surfactants is similar to the classification of strongly interacting compounds (other than surfactants) in Sections II and III into cations (Section II A), anions (Section II B) and electroneutral and zwitterionic organic compounds (Section III). The adsorption of anionic surfactants is indeed enhanced, when the adsorbent carries high positive surface charge (at low pH for materials hsted in Tables 3.1, and 3.3-3.5), and adsorption of cationic surfactants is more pronounced at high pH, and the adsoiption of nonionic surfactants is often rather insensitive to the pH. However, the mechanisms of surfactant adsorption, and experimentally observed adsorption isotherms of surfactants are very different from the compounds discussed in Sections II and III. 

Spans, Tritons, and Tweens are series of nonionic surfactants. Some of these products do not represent specific chemical compounds but rather mixtures of similar compounds having different number of ethylene oxide segments and/or different lengths of hydrocarbon chain, and the apparent adsorption isotherm is a result of interaction of particular components of the mixture in solution and on the surface. 

Adsorption of nonionic surfactants on porous solids has been studied by Huinink et al. in a series of p ers [ 149,150]. They elaborated a thermodynamic approach that accounts for the major features of experimental adsorption isotherms. It is a very well known fact that during the adsorption of nonionic surfactants there is a sharp step in the isotherm. This step is interpreted as a change from monomer adsorption to a regime where micelle adsorption takes place. Different surfactants produce the step in a different concentration range. The step is more or less vertical depending on the adsorbate. The thermodynamic analysis made by Huinink et al. is based on the assumption that the step could be treated as a pseudo first order transition. Their final equation is a Kelvin-like one, which shows that the change in chemical potential of the phase transition is proportional to the curvature constant (Helmholtz curvature energy of the surface). 

Surfactant adsorption theories are based on different physical and geometrical models of the adsorbed layer, resulting in a variety of surface equations of state or equivalently in several different adsorption isotherms. The usual approach in the theoretical description of the adsorption of ionic surfactants is the generalization of an adsorption isotherm (or equation of state) of nonionic surfactants by incorporating the electrostatic contribution in the adsorption free energy [4, 5, 6, 7, 8]. The validity of the ionic models derived is usually tested by applying the models for the description of the surface... 

Gu, T., Zhu, B.-Y. The S-type isotherm equation for adsorption of nonionic surfactants at the silica gel-water interface. Colloids Surf. 1990, 44, 81-87. 

The presence of pre-adsorbed polyacrylic acid significantly reduces the adsorption of sodium dodecylsulfonate on hematite from dilute acidic solutions. Nonionic polyacrylamide was found to have a much lesser effect on the adsorption of sulfonate. The isotherm for sulfonate adsorption in absence of polymer on positively charged hematite exhibits the typical three regions characteristic of physical adsorption in aqueous surfactant systems. Adsorption behavior of the sulfonate and polymer is related to electrokinetic potentials in this system. Contact angle measurements on a hematite disk in sulfonate solutions revealed that pre-adsorption of polymer resulted in reduced surface hydrophobicity. 

The deviations from the Szyszkowski-Langmuir adsorption theory have led to the proposal of a munber of models for the equihbrium adsorption of surfactants at the gas-Uquid interface. The aim of this paper is to critically analyze the theories and assess their applicabihty to the adsorption of both ionic and nonionic surfactants at the gas-hquid interface. The thermodynamic approach of Butler [14] and the Lucassen-Reynders dividing surface [15] will be used to describe the adsorption layer state and adsorption isotherm as a function of partial molecular area for adsorbed nonionic surfactants. The traditional approach with the Gibbs dividing surface and Gibbs adsorption isotherm, and the Gouy-Chapman electrical double layer electrostatics will be used to describe the adsorption of ionic surfactants and ionic-nonionic surfactant mixtures. The fimdamental modeling of the adsorption processes and the molecular interactions in the adsorption layers will be developed to predict the parameters of the proposed models and improve the adsorption models for ionic surfactants. Finally, experimental data for surface tension will be used to validate the proposed adsorption models. 

The description of a mixed adsorption layer of ionic and nonionic surfactants requires the appropriate adsorption isotherms. For example, the Frumkin isotherm gives... 

S. Ikeda, M. Tsunoda and H. Maeda, Application of Gibbs adsorption isotherm to aqueous solutions of a nonionic cationic surfactant, J. Colloid Interface Sci. 67 (1978) 336-348. 

Finally, one must be aware of possible modifications of the sample during the adsorption process itself. This is what was shown to happen when adsorbing a nonionic surfactant (nonyphenoloxyethylene, with 9-10 ethoxy groups) on kaolin in the presence of 1% NaCl at 40°C. In this case one step only was visible in the normal L-shaped adsorption isotherm whereas two steps were seen in the microcalorimetric recording. The first was attributed to the displacement of water by the surfactant and the second to a partial opening and hydration of the sheet-like structure of kaolin under the action of surfactant and salt (Rouquerol and Partyka, 1981). 

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