Adsorption of charged surfactants

Another quite important type of adsorption isotherms is given by a generalisation due to the electrical repulsion in adsorption layers of ionic surfactants. This was explained in the monographs of Davies Rideal (1961) and Lucassen-Reynders (1981). 

Narrowly defined, the main contributions to film pressure or interfacial tension decrease come from the osmotic term and the repulsion of the electrical double layers of ionic surfactants including the effects of counterions. Interactions in mixed adsorption layers are of broad interest for the description of the state of surfactant adsorption layers. For the clarification of the adsorption mechanism at liquid interfaces the replacement of solvent molecules, mainly water, has been intensively studied by Lucassen-Reynders(1981). 

In this book we are only concerned with the electrostatic repulsion aspect of the surface behaviour of charged surfactants, and then, as shown later, their electrostatic retardations effects. Such kinetic effects are in some cases decisive dynamic properties of a liquid interface and therefore significant for applications in colloid science and technology. 

Lucassen-Reynders (1981) derived the electrostatic term of the film pressure. Her model is that of a surface charge by long chain ions, without taking into consideration inorganic counterions. Starting from the equation of Davies (1951) for the electrostatic repulsion in surface films 

The coefficient P depends on temperature and the dielectric constant of the substrate, c, is the concentration of counterions in the electrical double layer. According to Lucassen-Reynders (1981) we obtain a dependence of it, on A (Fig. 2.11). 

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The spectra clearly show that the adsorption of charged surfactants at the CCLj/H20 interface at nanomolar aqueous phase surfactant concentrations results in a significant modification of the interfacial water behaviour. Wilhelmy balance surface pressure measurements [89] show that at these concentrations (headgroup areas of >4000 molecule ), the water molecules responsible for the observed spectral... 

An isotherm adsorption of charged surfactants on oxides is shown in Fig. 4.10. This isotherm is characterized by four regions, attributed to four different dominant mechanisms being operative in each region. Mechanisms involved in these regions may be viewed as follows ... 

The adsorption of charged surfactants on to adsorbents which possess ionic surface groups of opposite charge may cause flocculation of the suspension. Measurements of the changes in zeta potential of dispersed particles as the concentration of surfactant is increased [57,103,104] have indicated a charge... 

In general, the adsorption of a surfactant on particles with previously adsorbed polymer can be influenced by (i) a reduction of surface area available for adsorption as a result of the presence of adsorbed polymer, (ii) possible interactions between polymer and surfactant in the bulk solution or in the interfacial region (that is, surfactant with loops, tails or trains of adsorbed polymer molecules), (iii) the steric effect of adsorbed polymer, preventing approach of surfactant molecules for adsorption at the surface, or (iv) possible electrostatic effects if polymer and/or surfactant are charged species. 

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... 

Once the negative charges on the particle surface have been neutralised further adsorption of the surfactant occurs via the tail on to the hydrophobic patches of the surface and also by association of the hydrocarbon chains. Detailed adsorption studies have been reported on polystyrene latices (36). The additional adsorption provides a positive charge to the particles and restabilization occurs.. The sterpnasa of this phenomenon is... 

Fig. 11. Scheme of a two-step adsorption of a surfactant on a solid. 1) solid, 2) layer of charging ions a) free surface charge, b) surface charge with a counterion... 

System 1. Under ideal conditions the adsorption of a surfactant into the EDL proceeds as described in Chap. 3. The border of efficiency of anionic and cationic surfactants is IP or PZC, as follows from the correlation of e.g. adsorption density, potential and notability are dependent on pH [e.g. 44,129,167,174-176]. The course of such an adsorption is shown in Fig. 15. If H+ or OH" react with the surface of one mineral the released ions or their hydrolytic products can adsorb on unequally charged surface of the other mineral and cause an activated adsorption of the surfactant, or they can inhibit the adsorption, as shown on the schemes ... 

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. 

In what follows, one considers for illustration purposes the case in which the charge is generated on the surface of colloidal particles or droplets by the adsorption of a surfactant, namely sodium dodecyl sulfate (SDS). We selected this case because information about the adsorption of SDS on an interface is available in the literature, and as it will become clearer later the number of parameters involved is smaller than in the case of silica. A more complex calculation about the silica and the amphoteric latex particles will be presented in a forthcoming paper. It involves several kinds of surface dipoles and equilibrium constants. 

The inner part of the double layer may include specifically adsorbed ions. In this case, the center of the specifically adsorbed ions is located between the surface and the Stem plane. Specifically adsorbed ions (e.g., surfactants) either lower or elevate the Stem potential and the zeta potential as shown in Figure 4.31. When the specific adsorption of the surface-active or polyvalent counter ions is strong, the charge sign of the Stem potential will be reversed. The Stem potential can be greater than the surface potential if the surface-active co-ions are adsorbed. The adsorption of nonionic surfactants causes the surface of shear to be moved to a much longer distance from the Stem plane. As a result, the zeta potential will be much lower than the Stem potential. 

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. 

Figure 3.1. Examples of double layers (a) around a solid particle (b) at an Ionized monolayer of anionic surfactants, adsorbed at the oil-water Interface (c) on a hexagonal clay mineral particle at low pH (only the charge on the particle is drawn) (d) double layer generated by the adsorption of anionic surfactants on a hydrophobic surface. The pictures are schematic.

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