Adsorption of Ionic Surfactants on Polar Surfaces

The adsorption of ionic surfactants on polar surfaces that contain ionisable groups may show characteristic features due to additional interaction between the head group and substrate and/or possible chain-chain interaction. This is best illustrated by the results of adsorption of sodium dodecyl sulphonate (SDSe) on alu- 

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The adsorption of ionic surfactants on polar surfaces that contain ionizable groups may show characteristic features due to additional interaction between the head group and substrate and/or possible chain-chain interaction. This is best illustrated by the results of adsorption of sodium dodecyl sulfonate (SDSe) on alumina at pH = 7.2 obtained by Fuerstenau [42] and shown in Fig. 3.11. At the pH value, the alumina is positively charged (the isoelectric point of alumina is at pH 9) and the counterions are Cr from the added supporting electrolyte. In Fig. 3.11, the saturation adsorption is plotted versus equilibrium surfactant concentration in logarithmic... 

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 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. 

The presence of an adsorbed surfactant layer on the surface of solid particles dispersed in an aqueous medium can affect the stability of the dispersion in several ways [95]. The adsorption of ionic surfactants on to non-polar surfaces imparts a surface charge to the solid surface which may increase stability through the repulsion of the electrical double layers. One of the ways in which the stability of a dispersion is increased by the adsorption of non-ionic polyoxyethylated surfactants is thought to be associated with the polyoxyethylene chains which extend into the solution. Interaction of the polyoxyethylene chains of the adsorbed layers on neighbouring particles would result in restriction in their movement and hence a decrease of the entropy - a process referred to as entropic stabilization. Stabilization of dispersions by surfactants is discussed in detail in Chapters 8 and 9. 

Adsorption of ionic surfactants on to polar substrates will be affected by pH. The relationship between the amount of NaDS adsorbed on to colloidal alumina and the stability ratio, W, of alumina dispersions is shown in Fig. 9.5 at two different pH values, 6.9 and 1,2, The zero point of charge of alumina is pH 9.1 thus the higher surface charge at pH 6.9 ensures that hydrocarbon chain interactions occur at a lower surfactant concentration than at pH 7.2. As the pH is lowered further this effect is accentuated [12]. The effect of pH on adsorption of lauryl sulphate ions and tetradecylpyridinium ion is shown clearly in Rupprecht s [6] results using colloidal titanium dioxide as the adsorbate (Fig. 9.6). 

Adsorption of surfactants at polar surfaces has been extensively investigated because of the importance of modifying particle surfaces for many industtial applications [70,71]. A number of models have been proposed, most of which are based on experimental and theoretical studies of the adsorption of ionic surfactants at the oxide/water interface [72-75]. All authors agree that at low surfactant concentrations, adsorption at charged surfaces (oxides, salts) is driven by... 

One important advantage of the polarized interface is that one can determine the relative surface excess of an ionic species whose counterions are reversible to a reference electrode. The adsorption properties of an ionic component, e.g., ionic surfactant, can thus be studied independently, i.e., without being disturbed by the presence of counterionic species, unlike the case of ionic surfactant adsorption at nonpolar oil-water and air-water interfaces [25]. The merits of the polarized interface are not available at nonpolarized liquid-liquid interfaces, because of the dependency of the phase-boundary potential on the solution composition. 

The adsorption of ionic or polar surfactants on charged or polar surfaces involves coulombic (ion-surface charge interaction), ion-dipole, and/or dipole-dipole interaction. For example, a negatively charged silica surface (at a pH above the isoelectric point of the surface, i.e., pH >2-3)... 

Information is obtained by modeling the competition between surfactant ions and other ions for adsorption sites on the floe surface. A statistical mechanical approach, as previously employed for couperative surface phasomena (e.g., hemimicelle formation), allows one to observe diet excessive concentrations of surfactant interfere with the flotation of paniculate material. This apparently results from the formation of a second bemimicelle of condensed surfactant on lop of the first, with the surfactant polar or ionic heads presented to the water this results in a hydrophilic surface (Fig. 17.2-4). The effects or ionic strength and surfactant hydrocarbon chain length on this behavior heve been modeled malhemeticaliy. 

The adsorption of anionic, cationic and non-ionic surfactants on to hydrophilic and hydrophobic surfaces in aqueous continuous phases has been considered in Chapter 1. Adsorption of surfactants on to non-polar surfaces occurs via hydrophobic interactions, the hydrocarbon chain adsorbing and lying close to the solid surface adsorption on to polar surfaces can occur by specific electrostatic interactions in which the surface is converted from a hydrophilic surface to a hydrophobic surface by the orientation of the alkyl chains of the surfactants outward into the water (Fig. 9.3). Adsorption of surfactants in this way frequently gives rise to multilayer adsorption by hydrophobic interactions between the primary and secondary monolayers, as shown in Fig. 9.3. Non-ionic surfactants based on polyoxyethylene ethers may also adsorb on to hydrophilic surfaces such as silica in this way. A representation of the orientation of non-ionic surfactants at a silica surface is shown in Fig. 9.3b. Adsorption isotherms for polar and nonpolar systems reflect these different possibilities as has been discussed previously (section 1.4). 

The change in surface wettability (measured by the contact angle) with concentration for the three surfactants is plotted in Fig. 2.54 (Zhang and Manglik 2005). The contact angle reaches a lower plateau around the CMC where bilayers start to form on the surface. Wettability of non-ionic surfactants in aqueous solutions shows that the contact angle data attains a constant value much below CMC. Direct interactions of their polar chain are generally weak in non-ionics, and it is possible for them to build and rebuild adsorption layers below CMC. The reduced contact an-... 

Electrostatic interactions occur between the ionic head groups of the surfactant and the oppositely charged solid surface (head down adsorption with monolayer structure) [56]. Acid-base interactions occur due to hydrogen bonding or Lewis acid-Lewis base reactions between solid surface and surfactant molecules (head down with monolayer structure) [57]. Polarisation of jt electrons occurs between the surfactant head group which has electron-rich aromatic nuclei and the positively charged solid surface (head down with monolayer structure) [58]. Dispersion forces occur due to London-van der Waals forces between the surfactant molecules and the solid surface (hydrophobic tail lies flat on the hydrophobic solid surface while hydrophilic head orients towards polar liquid) [59]. 

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