Anionic mesophases, formation

The basic mode of mesophase formation is as described above for CTAB. However, as one might expect, things are not quite so straightforward and there are various types of mesophase which can be formed from various types of surfactant. The surfactant structure can be varied so that fluorocarbon chains can be employed in place of the hydrocarbon variety, while anionic (e. g. -SOa") and the neutral (e.g. -(0CH2CH2) -0H) polar headgroups are often used. These surfactants can then form a variety of different mesophases as a function of (mainly) concentration in the solvent of choice (normally water). These phases are the lamellar (L ) phase, a simple bilayer phase, and variations on the cubic (li, I2, Vi, V2) and hexagonal (H, H2) phases. For these last phases, the subscript 1 implied a normal phase as found in a water-rich system, while the subscript 2 implied a reversed phase as found in an oil-rich system. For the cubic phases, the letter F implied a micellar phase (e. g. 

Further works in the field of nanostructured silica starting from SDA-CSDA ion pairs concern the formation of materials containing tethered anions. The formation of this type of material involves inverse ion pairs with respect to the synthesis of AMS mesophases, that is, anionic costructure-directing agent and cationic surfactant [78,106,107],... 

In recent studies, Friberg and co-workers (J, 2) showed that the 21 carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl-2-cyclohexene-1-yl octanoic acid (C21-DA, see Figure 1) exhibited hydrotropic or solubilizing properties in the multicomponent system(s) sodium octanoate (decanoate)/n-octanol/C2i-DA aqueous disodium salt solutions. Hydrotropic action was observed in dilute solutions even at concentrations below the critical micelle concentration (CMC) of the alkanoate. Such action was also observed in concentrates containing pure nonionic and anionic surfactants and C21-DA salt. The function of the hydrotrope was to retard formation of a more ordered structure or mesophase (liquid crystalline phase). 

So what about the cubic phase In polycatenar systems, it is possible to rationalize the formation of cubic phases on the basis of surface curvature alone, which will be considered in subsequent sections. However, it can be argued that, for calamitic systems, these arguments do not hold—at least on their own—and that other factors are important. For example, if cubic-phase formation is due to surface curvature, it is not possible to explain why an Sa phase (lamellar and with no surface curvature) is seen at higher temperatures. An important factor is the presence of specific intermolecular interactions and in the case of the silver systems, these are the intermolecular electrostatic interactions resulting from the presence of formally ionic groups. This is consistent with the observation of cubic phases in the biphenylcarboxylic acids and hydrazines (Fig. 29), as well as with other materials. However, it is also evident that this is not the only factor, as no cubic phase is seen with anion chains shorter than DOS, while other studies with fluorinated alkoxystilbazoles showed that the position of fluorine substitution could determine the presence or absence of the mesophase observed in the unsubstituted derivatives (56). Thus, structural factors are clearly not negligible. 

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. 

Mesostructure syntheses can be carried out under conditions in which the silicate alone would not condense to solid (at pH 12 14 and low silicate concentration) and the surfactant CTAB (concentration < 2%) alone would not form a lyotropic liquid-crystal phase. The rapid formation of MCM-41 when surfactant solution and silicate solution are combined indicates that there is strong interaction between the cationic surfactant and anionic silicate species in the formation of mesophases. 

S X I path takes place under acidic conditions, in the presence of halide anions X = Cl , Br ) and the S M I route is characteristic of basic media, in the presence of alkali metal ions (M = Na+, K+). Besides the syntheses based on ionic interactions, the assembly approach has been extended to pathways using neutral (5°) (87) or nonionic surfactants (iV°) (88). In the approaches denoted (S / ) and (A°/°), hydrogen bonding is considered to be the main driving force for the formation of the mesophase (Fig. 9.9). 

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