Surfactants double chained

Surfactants having an inverted tnmcated cone shape yield inverted spheroidal micelles. Many double-chain surfactants such as AOT fonn such inverted micellar stmctures. These kinds of surfactant also fonn inverted anisotropic liquid crystalline phases. 

The conditions for surfactants to be useful to form Hquid crystals exist when the cross-sectional areas of the polar group and the hydrocarbon chain are similar. This means that double-chain surfactants are eminently suited, and lecithin (qv) is a natural choice. Combiaations of a monochain ionic surfactant with a long-chain carboxyHc acid or alcohol yield lamellar Hquid crystals at low concentrations, but suffer the disadvantage of the alcohol being too soluble ia the oil phase. A combination of long-chain carboxyHc acid plus an amine of equal chain length suffers less from this problem because of extensive ionisa tion of both amphiphiles. 

Electrokinetic chromatography (EKC) using microemulsion is one of the most powerful tools for the rapid measurement of log P w with high reproducibility. Because it is relatively easy to manipulate the pseudostationary phases of EKC, a lot of phases have been reported for the measurement not only of physicochemical properties but also of the separation selectivity, such as polymer micelles (64) and double-chain surfactant vesicles (56-58,60,61). These phases are also interesting in terms of the correlation to bioactivity. 

Figure 9.7 Concentrations of monomers and micelles as a function of total concentration (schematic). Most single-chained surfactants containing 12-16 carbons per chain have their cmc in the range 10 -10 M, while the corresponding double-chained surfactants have much lower cmc values due to their greater hydrophobic-ity. (Adapted from Tanford, 1978). Some important cmc values are listed, as cac values, in Table 9.1.

Cast multibilayers prepared from cyclam-containing double-chain surfactants Size-quantized CdS particles generated in situ in cast bilayers... 

What are typical values for the parameters k and kl The bending rigidity of surfactant films is typically of the order of 1-20 kBT at room temperature. Factors that reduce k are short alkyl chains, cosurfactants, double-chain surfactants with unequal chains, and cis-unsaturated bonds. For the saddle-splay modulus only few measurements have been done. It tends to be negative with an amount much smaller than the bending rigidity for the same system. 

I. S. Barnes, P. J. Derian, S. T. Hyde, B. W. Ninham, and T. N. Zemb. A disordered lamellar structure in the isotropic-phase of a ternary double-chain surfactant system. J. Physique, 51(22) 2605-2628, 1990. 

Considerations of the packing parameter concept of Israelachvili et al. [1] suggest that double-chain surfactants, which form the basis of measurements described in this article, cannot readily form spherical micelles. With double-chain surfactants, a more likely aggregate structure is the formation of bilayer vesicles, which can be also thought of as a dispersed lamellar phase (La) as such the vesicular dispersed form is likely to be preferentially formed at low concentrations ( 1 mmol dm-3) of surfactant. Furthermore, it is necessary to consider the possibility, unlike in the case of micelles, that such vesicles, formed by self-assembly of surfactant monomers, will not be thermodynamically stable. The instability is then likely to be in the direction of growth to a thermodynamically-stable lamellar phase from the vesicles. This process will be driven, at least initially, by fusion of two vesicles. 

When a single chain anionic surfactant (such as sodium dodecyl sulfate, SDS) is used, it generally requires a cosurfactant for the formation of a microemulsion. A cosurfactant may not be needed to form a microemulsion if nonionic surfactant(s), certain types of cationic surfactants, or double-chain surfactants such as sodium l,4-bis(2-ethylhexyl)sulfosuccinate (Aerosol OT or simply AOT) are used. 

Branched or double chained surfactant More negative... 

If the chains are sufficiently flexible, and repulsive (hydration or other) forces between aggregates sufficiently large and the chains sufficiently hydrophobic, as for double-chained surfactants like phospholipids, single-walled vesicles can occur. 

The same progression can be observed if one adds progressively a single-chained surfactant (S) (v/al = 1/3) to a lamellar phase of a double-chained surfactant D v/al=l) of the same head-group area and chain length. Here it is the effective volume which changes according to. 

Figure 4.6 Schematic views of bilayer configurations as the value of the surfactant parameter, v/al, varies for a double-chain surfactant or lipid. The stippled regions denote polar regions (water plus head-groups). (Left ) v/al > 1, cross-section fiirough a pore of a saddle-shaped bilayer, whose mid-surface is a minimal surface (centre ) v/al = 1, a planar bilayer (right ) v/al < 1, a "blistered" bilayer, containing a vacuous region. In the last case, a reversed bilayer (Fig. 4.7) is favoured over the bilayer configuration iUustrated.

Furfiier evidence fiiat supports these calculations derives from studies of the ternary mixtures of the cationic double-chain surfactant DDAB (didodecyl dimethyl ammonium bromide), cyclohexane and water. Within the cubic mesophase region of fiiis surfactant-water-oil mixtiure, all the cyclohexane is adsorbed between the surfactant chains, so that the system is a pseudo-binary one, for which our theoretical analysis ought to hold. (The effective surfactant parameter for fliis surfactant in the presence of cyclohexane is slightly larger tiian unity.) Close scrutiny of the cubic phase region within this ternary phase diagram has revealed the presence of at least one - and... 

The formation of spheres exhausts the range of geometrically accessible monolayer structures, since the topology of spheres is lower than that of all other shapes. (In more familiar terms, spheres minimise the surface to volume ratio - thus soap bubbles form spheres. In other words, the volume associated with a unit surface area of a surface is maximised if that surface forms a part of a sphere. So we expect the surfactant interface to become spherical as the internal volume associated with each surfactant molecule becomes large.) This critical volume fraction is not particularly large for ty pical double-chain surfactants it lies between 20% and 50%. 

Double-chain surfactants with two sulfonate groups were proposed for micellar elec-trokinetic chromatography (MEC) analysis of phenolic pollutants in water °. CE with ESI-MS detection was applied to the analysis of phenolic compounds in olive mill wastewaters. Quantitative analysis was performed in the negative SIM mode, using p-chlorophenol as internal standard. LOD ranged from 1 pg for 4-hydroxybenzaldehyde and protocatechuic acid (27) to 386 pg for vanillic acid (38) °". A modified montmorillonite served for SPE preconcentration of phenols, followed by EtOH desorption and CE end analysis . 

The sign of ( is reversed from negative to positive at surfactant concentrations on the order of i0" -10 mol/dm . The logarithm of this concentration is a linear function of the number of carbon atoms in the surfactant molecule m a series of single chain surfactants and in a series of double chain surfactants (different slopes). 

This is the mechanism proposed by Datyner to account for CTAC antibacterial, and antifungal to a lesser extent, activity he reports [20]. Martins et al. invoke the ability of the surfactant to disrupt the cell membrane and form mixed micelles with its lipids to account for CTAB and SLS antibacterial activity [29], In contrast, the bacteria-killing efficacy of double-chain surfactants such as DHTDMAC is assigned to an alteration of the membrane protein function resulting from the adsorption of vesicles onto the bacterial membrane. This study was carried out under laboratory conditions (very low ionic strength), and the antibacterial efficacy of fabric softeners under realistic use conditions remains highly questionable. 

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