Single chain surfactant

This inequality indicates the amphiphile adopts a shape essentially equivalent to that of a cone with basal area <3. Such cones self-assemble to fonn spheroidal micelles in solution or spheroidal hemimicelles on surfaces (see section C2.3.15). Single-chain surfactants with bulky headgroups, such as SDS, typify surfactants in this category. 

We have focused so far on single-chain surfactants with hydrocarbon chains, mostly with COOH or closely related head groups. Computer simulations have also been performed on a variety of other surfactants. We do not attempt here to exhanstively review all work, but describe some (hopefully) representative samples. 

Reactions of 2,4-dinitrochloro-benzene and -naphthalene are speeded by DDDAOH and the corresponding chloride -I- NaOH (Cipiciani et at., 1984). The rate/surfactant concentration profiles and the rate constants are very similar to those for reactions in solutions of the corresponding C16 single chain surfactants which form normal micelles. The spontaneous hydrolysis of 2,4-dinitrophenyl phosphate dianion is also speeded by DDDAC1 and rates reach plateau values in very dilute surfactant (Savelli and Si, 1985). 

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.

R. Leung and D. O. Shah. Solubilization and phase-equilibria of water-in-oil microemulsions. 2. Effects of alcohols, oils, and salinity on single-chain surfactant systems. J. Colloid Interface Sci., 120(2) 330-344, 1987. 

Spherical micelles are formed where the value of surfactant packing parameter is less than 1/3 (single chain surfactants with large head group areas such as anionic surfactants). The spherical aggregates are extremely small and their radius is approximately equal to the maximum stretched out length of the surfactant molecule. 

Microscopic observations afforded the calculation of the physical area of BLM, which, in combination with electrical measurements, led to values of BLM capacitances per unit area. Typical BLMs prepared from DODAC (both in the presence and in the absence of chlorophyll-a) had areas of 5.7 x 10-3 cm and 0.7 pF/cm capacitances. These values agreed well with those determined for BLMs prepared from phospholipids (capacitance 0.7-1.3 pF/cm ) and from single-chain surfactants (capacitance = 0.3-0.6 pF/cm ). 

Double-chain nonionic surfactant or lipid 9 Single-chain surfactant f> Emphiphilic or lipophilic drug Q Hydrophilic drug... 

Most single-chain surfactants do not sufficiently lower the oil-water interfacial tension to form MEs, nor are they of the right molecular structure (i.e., HLB) to act as cosolvents. To overcome such a barrier, cosurfactant/cosolvent molecules are added to further lower the interfacial tension between oil and water, fluidize the hydrocarbon region of the interfacial film, and influence the curvature of the film. Typically small molecules (C3-C8) with a polar head (hydroxyl or amine) group that can diffuse between the bulk oil and water phase and the interfacial film are suitable candidates [11],... 

Most single-chain surfactants do not lower the oil-water interfacial tension sufficiently to form microemulsions nor are they of the correct molecular structure, and short- to medium-chain length alcohols are necessary as cosurfactants. The cosurfactant also ensures that the interfacial film is flexible enough to deform readily around each droplet as their intercalation between the primary surfactant molecules decreases both the polar head group interactions and the hydrocarbon chain interactions. Medium-chain alcohols such as pentanol and hexanol have been used by many investigators as they are particularly effective... 

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. 

Many more common microemulsions are formed by single-chain surfactants, in which case the structural sequence on water dilution is reversed. However, the microstructures are identical. 

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). 

Escher and Oliveros systematically studied the effect of various parameters on fragrance adsorption onto fabrics [113], They found that the affinity of fragrance for fabrics is mainly determined by the type of fiber (cotton > polyacrylonitrile) and, to a lesser extent, by the type of single-chain surfactant (cationic > anionic and nonionic). These factors are interdependent (the effect of the type of surfactant on the affinity for polyacrylonitrile is weak). The effect of temperature and of surfactant concentration is less. 

Quaternary ammonium surfactants (dodecyl-, tetradecyl-, and hex-adecyltrimethylammonium or didodecyldimethylammonium) with Tp anion were prepared and characterized also by measurement of their Kraft temperatures and critical aggregation concentrations. NMR spectroscopy suggested that the single-chain surfactants containing the dodecyl ammonium form small aggregates in water. The X-ray structure analysis of the dodecyl derivative was performed.192Azolium poly(l,2,4-triazolyl)borate salts melt at relatively low temperatures to give thermally stable ionic liquids. Some of them exhibit conductivities close to that of KC1 in MeCN/H20... 

The solubility of the surfactant in decane is also quite small at 25°C, about 0.04 wt%, but over a narrow temperature range around 50°C it rises dramatically, as in the Krafft point range of a single-chain surfactant in water (11a). Such a phenomenon with a surfactant in a nonpolar solvent is not uncommon (35). Incidentally, the absence of a Krafft point range for the surfactant in water between 10 and 90°C argues for the absence of micelles in solution. Abrupt change in the slope of such a property as surface tension versus concentration (9) can be due to precipitation of a new phase as well as to onset of appreciable micelle formation, and so does not constitute conclusive evidence for the latter. 

Besides choline, the cationic character of phosphatides is created also by 2-aminoethanol, sphingosine and other aminoalcohols. Phosphatides with two hydrocarbon chains attached to the same head group are more lipophilic amphiphiles characterised by vesicular (liposomal) aggregation in water contrary to single-chain surfactants that typically form micelles. Synthetic monoalkylphosphorylcholines are essentially water-soluble zwitter-ionic surfactants [120] the distinction of which consist in the hindered compatibility with anionics in view of the end position of the cationic betaine group. 

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