Surfactants Interactions with inorganic species

The generalized liquid-crystal-template mechanism focuses on the interaction between surfactant molecules and inorganic species. Surfactant molecules assemble with inorganic species to form the liquid-crystal-like mesophase. The three main interactions between surfactant and inorganic species are (i) electrostatic interaction (the charge-density match plays the key role), (2) hydrogen bond, in particularly for those neutral templates, and (iii) covalent bond. 

The interaction of inorganic species, such as those of calcium and aluminum that are normally present in reservoir fluids, with surfactants is found to produce precipitation of the surfactants followed by their redissolution above the critical micelle concentration. A maximum is often observed in the adsorption isotherm of surfactants on reservoir rocks. The contribution of the surfactant precipitation/dissolution phenomenon to the occurrence of adsorption maximum has been investigated in this study using the kaolinite/sulfonate system. The magnitude of the adsorption maximum is found to be minimized when the precipitation/redissolu-tion of the surfactant is taken into account, suggesting the important role of the latter phenomenon in determining the apparent adsorption. 

The preparation can also take place under acidic conditions below the isoelectric point of the Si-OH-bearing inorganic species (pH 2) then the silica species are positively charged, that is, protonated silanol groups (Si-OHT) in order to produce an interaction with cationic surfactants, it is necessary to add a mediator ion X- (usually a halide), which gives rise to the S+X I+ pathway (Fig. 3.6b). 

Figure 3.6 Interactions between the inorganic species and the head group of the surfactant with consideration of the possible synthesis pathway in acidic, basic, or neutral media. Electrostatic S+I, S+X-I+, S M+I, S 11 through hydrogen bonds S0 0/N0 0, S°(XI)°. (See color insert.)...

The direct interaction between surfactants and inorganic precursors was later found to be not the only pathway for the formation of mesophases. A major discovery following Mobil s work is the synthesis of mesophases through the assembly of cationic inorganic species with cationic surfactants in acidic solutions. Here, the interaction between cationic silica species and cationic surfactant headgroups is suggested to be mediated by halide anions. 

Figure 3 Schematic illustration of interaction between surfactants and inorganic species. (Reprinted with permission from Ref. 24. 1994 American Chemical Society)...

From a view point of reaction time, the typical preparation of mesoporous material can be divided three main steps (1) interaction between surfactant and silica (or other inorganic) species in solution and the formation of ordered mesostmcture (2) the further reaction (polymerization or condensation for silica) at a certain temperature for a time period. A possible phase transformation may occur (3) recovery of solid product by filtration, washing, and drying. The phase transformation may also occur in this step (4) removal of template from the solid product by calcination or extraction with solvent. The phase transformation is also possible even in this step. 

The resultant hydrolyzed, polar species can have a better interaction with the polar head group of the surfactant, resulting in the formation of ordered structures. Soler-illia et al. reported a modulation of hybrid interface approach, which relies on the addition of controlled quantities of water to the solution of the inorganic precursor and a nonionic surfactant in an organic solvent to obtain ordered mesostructures. 

These interactions are frequently ionic in character. The coulombic forces of interaction between macroions and lower molecular weight ionic species are central to the life processes of the cell. For example, intermolecular interactions of nucleic acids with proteins and small ions, of proteins with anionic lipids and surfactants and with the ionic substrates of enzyme catalyzed reactions, and of ionic polysaccharides with a variety of inorganic cations are all improtant natural processes. Intramolecular coulombic interactions are also important for determining the shape and stability of biopolymer structures, the biological function of which frequently depends intimately on the conformational features of the molecule. 

The cooperative self-assembly route is the most commonly used synthesis procedure for surfactant templated materials. It uses aqueous solutions at a much lower initial surfactant concentration than for the true liquid templating route, reducing the required amounts of expensive surfactant template. In these solutions, the surfactants are at a high enough concentration to form micelles, which may be spherical, elliptical, rod-like or vesicular, but do not form the ordered aggregates found in the final templated silica-surfactant composites. The solutions are in thermodynamic equilibrium so are stable at a given temperature until the silica precursors are added. Once added, a series of interactions between the inorganic species and the surfactant micelles occur, which involve simultaneous association of all components, hence the name, cooperative self-assembly. The result of the interactions is formation of the silica-surfactant composite, usually a precipitate, with an ordered nanoscale structure similar to those found in concentrated surfactant solutions. 

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