Adsorption of Ionic Surfactants on Hydrophobic Surfaces

The adsorption of ionic surfactants on hydrophobic surfaces may be represented by the Stern-Langmuir isotherm [17]. Consider a substrate containing sites 

The rate of desorption is proportional to the fraction of surface covered 0, 

At equilibrium, the rate of adsorption is equal to the rate of desorption, and the ratio of (kads/ des) equilibrium constant K, that is 

The equilibrium constant K is related to the standard free energy of adsorption by, 

Equation (5.48) apphes only at low surface coverage (9 0.1), where lateral interaction between the surfactant ions can be neglected. 

The adsorption of ionic surfactants on hydrophobic surfaces may be represented by the Stern-Langmuir isotherm [1]. Consider a substrate containing Ns sites (mol m ) on which F moles of surfactant ions are adsorbed. The surface coverage 6 is (r/Ng) and the fraction of uncovered surface is (1 — 6). 

The rate of adsorption is proportional to the surfactant concentration expressed in mole fraction (C/55.5) and the fraction of free surface (1 — 0), i.e. 

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This chapter reviews results for the adsorption of ionic surfactants onto hydro-phobic surfaces. Even with this limitation there are many possible combinations. Both anionic and cationic surfactants are considered. The hydrophobic surfaces may be either uncharged or positively or negatively charged. Thus, the electrostatic interactions between the adsorbing surface and the surfactant may be either nonexistent, attractive, or repulsive. Finally, the adsorbing materials may be either a single species or a combination of surfactants. This review is not a comprehensive overview of all of the published material on this topic. Rather it presents representative results concerning the adsorption of ionic surfactants onto hydrophobic surfaces. 

The adsorption of ionic surfactants on hydrophobic polar surfaces resembles that for carbon black [8, 9]. For example, Saleeb and Kitchener [8] found a similar limiting area for cetyltrimethylammonium bromide on Graphon and polystyrene ( 0.4 nm ). As with carbon black, the area per molecule depends on the nature and amount of added electrolyte. This can be accounted for in terms of reduction of head group repulsion and/or counter ion binging. 

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... 

The adsorption of surfactants on solids is affected by the surface properties (hydrophilicity/hydrophobicity, surface charge) and by the surfactant properties (ionic/non-ionic, CMC, HLB). Figure 3.9 illustrates the possible configurations of non-ionic surfactants on hydrophobic (A) and on hydrophilic (B) solid surfaces as well as those of cationic surfactants on negatively charged oxide layers (C). The surface concentrations increases successively from row I (ideal gas) to row IV or V (saturated surface). The picture reveals a broad variety including monolayers. 

This instability can be avoided by adding a non-ionic surfactant to the surface of the latex, forming a hydrophilic layer (Triton x-405 of 30 units) on the surface of the latex [22]. In addition, this compound reduces the stacking effect by masking the hydrophobic domains (or properties) of the surface. Indeed, competition for adsorption between the ODN and the surfactant molecules can also lead to desorption. However, this effect was not observed in all reported studies, but it is in principle accessible by comparing the adsorption energies of ODN and the surfactant on the surface of the latex. 

These forces and hence the stability of the dispersions can be altered/controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers at the solid-liquid interface depends on the nature of the surfactant or polymer, the solvent, and the substrate. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. Such adsorption can alter the dispersion stability mainly by changing the double layer interaction, which depends on the extent of adsorption. Thus, it is seen that alumina suspensions are destabilized by the adsorption of SDS when the zeta potential is reduced to zero. At higher concentrations, bilayered surfactant adsorption can occur with changes in wettability and flocculation of the particles by altering the hydrophobic interactions. 

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