Hydrophobic surfaces, ionic surfactant adsorption

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... 

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]. 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

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. 

Colloidal properties that influence RES uptake are particle size, surface charge, surface hydrophobicity, and the adsorption of macromolecules onto the particle surface. The surface of colloidal particles can be altered to avoid RES uptake by adsorption or grafting of a hydrophilic polymer onto the surface of a particle and thereby creating an energy barrier to particle interaction (e.g., the non-ionic surfactant Tween 20 can be adsorbed).Both biological and synthetic polymers have been used for RES masking of colloidal particles, for example, albumin,immunoglobulin car-... 

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

The adsorption of ionic surfactants onto hydrophobic polar surfaces resembles that for carbon black [24,25]. For example, Saleeb and Kitchener [24] found a similar limiting area for cetyltrimethyl ammonium bromide on Graphon and polystyrene ( 0.4nm ). As with carbon black, the area per molecule depended on the nature and amount of the added electrolyte. This can be accounted for in terms of the reduction in head group repulsion and/or counterion binging. 

As this subject was covered in detail in Chapter 5, only a summary will be provided at this point. Surfactant adsorption is usually reversible, and hence thermodynamics can be applied for deriving the adsorption isotherm. Eor example, the adsorption of ionic surfactants onto hydrophobic surfaces may be represented by the Stern-Langmuir isotherm [13]. Consider a substrate containing sites (molm ) on which F molm of surfactant ions are adsorbed. The surface coverage 0 is (F/NJ and the fraction of uncovered surface is (1 — 0). The Stern-Langmuir... 

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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