Bulkier surfactants

For much bulkier surfactants the orderly arrangements observed for simpler surfactants appear to disappear. Johal et al. [43] used sum-frequency spectroscopy to observe the coadsorption of a series of trichain surfactants and dodecanol. Their data indicated that the best model for the coadsorption was one consisting of an ordered dodecanol monolayer with scattered monomers or small clusters of disordered trichain molecules. 

As discussed by Israelachvili (1992), the shapes of surfactant aggregates can, to a first approximation, be anticipated based on the packing of simple molecular shapes (Tanford 1980). Figure 12-1 from Israelachvili illustrates this principle Conical molecules with bulky head groups attached to slender tails form spherical micelles cylindrical molecules with heads and tails of equal buUdness form bilayers and wedge-shaped molecules with tails bulkier than their heads form inverted micelles containing the heads in their interiors. A simple dimensionless molecular parameter that controls the shape of the aggregates is the molecular shape parameter here v is the volume occupied by the hydrocarbon... 

If the surfactant parameter is less than unity, a reversed bilayer can form, where the constituent surfactant molecules are placed head-to-head, rather than the chain-to-chain configuration characteristic of normal bilayers. If a chain-to-chain configuration occurs, the bilayer must be "blistered", to accommodate the bulkier head-groups (Fig. 4.6). 

Both changes lead to an increase in the solubilisation capacity, while the overall formulation is still the same [41]. Consequently, surfactants with bulkier groups on both sides are likely to be more efficient. For instance, in a system containing equal volumes of water and oil and a polyethoxylated alkylphenols an optimum formulation is found at EON = 5.1 for the nonyl species, and at EON = 8.3 for the dodecyl one at ambient temperature (solubilisation of 8 and 21 mL of oil and water per gram of surfactant). The value of the optimum solubility parameter changes with the alcohol content, as will be discussed next, but the trend is the same for all alcohols. 

Water is usually considered the most environmentally benign solvent however, the poor solubility of bulkier alcohols and molecular oxygen under ambient conditions has limited its application (in the absence of surfactants [25]). Pt nanoparticles supported on a water-soluble anion exchange resin exhibit excellent E-factors (12.8kilo waste per kilo product) in addition to very good activity and selectivity for alcohol oxidation [136]. Similarly, water has been used as solvent for, for example, benzyUc and primary alcohol oxidations over supported or stabilized Pd [137], Pt [138], and Au [139] clusters. 

For the CTAB complexes, the A, B and C values are less than 10.29, 7.48 and 5.60 A, respectively. Along the q direction the void space is connected as a column. This means that the value of C corresponds to the molar ratio between the smfactant and the aromatic compound. However, the A and B values indicate the size of the molecule included in the void space. The A and B values in the CPB complexes, especially in XXXIV, are larger than the threshold values of the CTAB complexes. This indicates that CPB can accommodate bulkier aromatic compounds than CTAB. All efforts to make a complex between CTAB and 9-anthracenecarboxyloc acid were in vain, probably because 9-anthracenecarboxylic acid is too bulky for the void space available in the CTAB complex, although a guaiacol complex with CTAB was made. These results suggest that it is possible to discriminate aromatic compounds by making complexes with different surfactants. 

The influence of the host phase was checked for some homologues of the anionic alkylammonium halides as well as the cesium and ammonium salts of perfluoiinated carbon acids which are cationic surfactants. The results can be summarized by two statements (i) the longer the alkyl chain of the surfactant, the smaller the twist and (ii) the bulkier the head group of the surfactant, the smaller the twist, see Figure 14.14. 

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