Adsorption of Ionic Surfactants

Ionic surfactants are molecules which have hydrophobic and hydrophilic ends. The hydrophobic part is essentially a long chain allophatic 

Soda lime silica glass 1.00 NaaO 0.58 CaO 3.70 Si02 2-3 

The dissodation reaction for the carboxYlic add group is given by fiCOgH 5 fiCOa + (9.67) 

With respect to both dissociation and hydrolysis, the concentrations of the ionic species will depend highly on the pH of the solution and the ionic strength. The degree of ionization is measured by the equilibrium constant or more frequently the pK for ionization. T ical pK values of several ionizing groups used as ionic surfactants follow  

FIGURE 9.20 Hydrolysis, critical micelle concentration (CMC), and 0.01 x CMC for dodecylamine as a function of pH. For the hydrolysis cvirve 4 x 10 M dodecylamine was used with the reaction fJNHJ + OH RNH2 + H2O. From Ncmch and Ring [54]. Reprinted with permission from Langmuir [54]. Copsrright 1985 American Chemical Society, 

Assume a surface in contact with a solntion of an ionic surfactant in water containing low-molecular-weight electrolytes. Let, by way of example, the surfactant be sodium dodecyl sulfate (NaDS) and the low-molecular-weight electrolyte be NaCl. Then, at constant temperatnre. 

The chemical potentials of the strong electrolytes NaDS and NaCl in aqueous solution may be written as the sum of their ionic components, so that 

When the NaCl concentration is low such that Cnsds L Naci L Na  

If Naci NaDs. is negligible upon increase of Cnsds then the first 

Equation 3.92 is zero. It follows that for the adsorption of an ionic surfactant from a solution of high ionic strength, 

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Hydrophobic polar surfaces, adsorption of ionic surfactants on, 24 140-141 Hydrophobic precipitated silica, 22 399 Hydrophobic solvents, 16 413 Hydrophobic surfaces, 1 584-585... 

Adsorption of Ionic Surfactants in Presence of Inorganic Electrolytes. .. 34... 

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. 

Regression analysis developed on the basis of the proposed model described by Eqs. 36 and 38-40 has been applied for the adsorption of ionic surfactants. The regression analysis minimizes the revised chi-square,... 

The adsorption of ionic surfactants creates an adsorption layer of surfactant ions, a Stern layer of counterions and a diffusive layer distributed by the electric field of the charged surface. Every layer has its own contribution to surface tension. For example, the adsorption of dodecyl sulfate (DS") ions from the sodium dodecyl sulfate solution is described by the modified Frumkin isotherm as... 

Lajtar, L., J. Narkiewicz-Michalek, W. Rudzinski, and S. Partyka. 1993. A new theoretical approach to adsorption of ionic surfactants at water/oxide interfaces Effects of oxide surface heterogeneity, Langmuir 9, 3174-3190. 

The increase in temperature increases adsorption of non-ionic surfactants on solid surfaces since the solubility of non-ionic surfactants in water decreases with increased temperature. On the other hand, increasing temperature decreases the adsorption of ionic surfactants on solid surfaces because the solubility of ionic surfactant increases with increased temperature. Furthermore, the presence of electrolytes increases the adsorption of ionic surfactants if the solid surface has the same charge as the surfactant head groups. 

Hua, X.Y. and Rosen, M.J. (1982) Calculation of the coefficient in the Gibbs equation for the adsorption of ionic surfactants from aqueous binary mixtures with nonionic surfactants. /. Colloid Interface Sci., 87, 469. 

In general, the adsorption of ionic surfactants follows the Langmuir isotherm, as discussed in Section 4.1. The adsorption of the surfactants onto the solid surfaces is dependent on the orientation and the packing efficiency of the solid surfaces. The onset of the adsorption plateau may occur at the critical micelle concentration (c.m.c.) of the surfactant in water, as shown in Figure 4.28. If the adsorption isotherm... 

Adsorption of Ionic Surfactants onto Hydrophilic Surfaces... 

The values of absolute values of (po and <70, since the calculation from film thickness by the DLVO-theory does not give an estimation whether the potential is positive or negative. However, the direct experimental measurements provide information which are the ions adsorbed at the interfaces electrolyte solution/air and non-ionic surfactant solution/air, and it is possible to determined the potential sign (see below). This is valid also for adsorption of ionic surfactants. 

Electrocapillary Curves in the Presence of the Adsorption of Ionic Surfactant... 

Introduction of the polarized ITIES opened a new stage in the study of adsorption of ionic surfactants, which had been used from the early studies of the ITIES onward [78-81]. 

In passing It Is recalled that In systems where the surface charge Is caused by adsorption of ionic surfactants, the adsorption of these Ions can be anal3rtically determined the ensuing charge can be established If the molecules are fully dissociated. For systems in which both chau-ge-determlnlng ions and ionic surfactants adsorb (sodium dodecyl sulphate on oxides), both charges can be measured to obtain detailed double layer information. 

Recent progress in the studies of adsorption of ionic surfactants from aqueous solutions on mineral substrates... 

The intent of this chapter is to present a brief review of simple, fundamental physicochemical principles and experimental results which are necessary to understand both the mechanism of adsorption of ionic surfactants from aqueous solutions on oxide surfaces and the action of some simple, fundamental applications. It does not enter into details in the theoretical consideration, nor does it attempt to explain complex industrial uses. Both problems have been thoroughly treated in several review articles and monographs [e.g., 1-10]. Here emphasis is placed on the contribution the adsorption calorimetry makes to the improvement of current understanding of the interactions of ionic surfactants at the mineral-water interface. All experimental data, used for the illustrative purposes throughout this chapter, were obtained at the Laboratoire des Agregats Moleculaire et Materiaux Inorganiques. 

The adsorption of ionic surfactants at the solid-solution interface is strongly influenced by a number of factors ... 

Although large number of studies have been reported on the equilibrium adsorption of ionic surfactants at the interfaces, very little attention has been paid to the adsorption kinetics. Only a few attempts have been made to follow the time evolution of the process from the initial adsorption to the equilibrium configuration and to understand the role of the diffusion [24,25,31]. 

In reviewing a vast quantity of literature on the subject, it is possible to encounter some studies which are of little value because insufficient attention has been given to the control of important physical factors, to the potentiality of the experimental technique, or to the limitations of particular theoretical approaches. Therefore, Section 2 is exclusively devoted to description of the materials and experimental methods used in measuring the adsorption of ionic surfactant onto mineral substrates. Some experimental problems, encountered in the everyday laboratory practice, will be pointed out. Among different experimental methods, the adsorption calorimetry particularly deserves to be noted. 

Changes in the pH of the aqueous phase usually cause marked changes in the adsorption of ionic surfactants onto charged solid substrates. According to the site-dissociation-site-binding model [57,91], the surface contains surface hydroxyl groups M-OH which can take up or release a proton ... 

A pronounced influence of the pH on the adsorption of ionic surfactants requires special attention to be paid when carrying out the experiments. For example, if the experimental results obtained in different measurements are to be compared or correlate with one another, the same solid-to-solution mass ratio has to be maintained. 

There are several physical factors affecting the energetics of adsorption of ionic surfactants onto adsorbents with oppositely charged sites as well as its effectiveness, i.e., the amount adsorbed at surface saturation. The role of several important parameters, like the overall charge density, pH and temperature of the aqueous phase and the spatial distribution of surface charge (i.e., topography), has been discussed in the previous paragraphs. 

The adsorption of ionic surfactants onto mineral substrates is a complex phenomenon simultaneously controlled by the nature of the adsorbing species, the properties of the solid surface and the composition of the aqueous solution. As a consequence, there is no complete theoretical model which can describe, both qualitatively and quantitatively, all the experimental information available. Calorimetry of adsorption may be very useful in studying the nature of the interactions in the adsorption system, but alone it is not capable of solving satisfactorily many detailed problems which still remain to be explained. Hovewer, the evolved model of the phenomenon should be able to contain the experimental results from all the bulk, calorimetric and spectroscopic studies reported on the system. 

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