Sodium dodecyl sulfate-dodecanol-water

Chen [8] studied mixtures of the pure surfactants Ci2(EO)4 and sodium dodecyl sulfate (SDS) at 30 °C. At this temperature the former is a liquid which does not dissolve in water (see Fig. 3), and the latter is a solid. The SDS was doubly recrystallized from ethanol to remove n-dodecanol and other impurities. The solubility of SDS in pure Ci2(EO)4 at 30 °C was found to be approximately 9 wt. %. When small drops of an 8 wt. % mixture were injected into water at 30 °C, complete dissolution was observed, the time required being a linear function of the square root of initial drop radius. For instance, a drop having an initial radius of 70 (xm required approximately 100 s to dissolve, significantly more than the 16 s cited above for a slightly larger drop of pure Ci2(EO)6. Behavior was similar to that of nonionic mixtures below their cloud points discussed previously in that most of the drop dissolved rapidly, but the final small volume dissolved rather slowly with some observable emulsification. 

The third structure shown in Figure 18 is sodium dodecyl sulfate (SDS), often used by research workers for solubilizing proteins. It is a sulfate ester of the C12 alcohol dodecanol. Commercially, this alcohol is produced by reduction of coconut oil, and the resultant mixture is called lauryl alcohol (from lauric acid, the predominant fatty acid in coconut oU). The alcohol portion of sodium lauryl sulfate is a mixture of chain lengths, the approximate composition being 8% Cg, 7% Cjo, 48% C12, 20% Ci4, 10% C16, and small amounts of longer chains. In bakeries the most common use of sodium lauryl sulfate is as a whipping aid. The compound is added to liquid egg whites at a maximum concentration of 0.0125%, or to egg white solids at a level of 0.1 %. It promotes the unfolding of egg albumin at the air-water interface and the stabilization of the foam. 

When the surfactant was sodium dodecyl sulfate, some butanol had to be present to achieve the microemulsion. A typical run used 2 mmol aryl iodide, 0.5 g surfactant, 1 mL butanol, 10 mL water, and 2 mmol base. The butanol was unnecessary with nonionic surfactants, such as the one derived from 1-dodecanol with 23 eq of ethylene oxide. The advantages of such a system include the following (a) No organic solvent is needed. The substrate is the oil phase, (b) The microemulsions form without the need for vigorous agitation, (c) No excess base is needed, in contrast with some reactions in which phase-transfer catalysis is used, (d) The surfactants can be recovered and recycled. They are inexpensive and biodegradable. 

Whilst the occurrence of gel phases is commonly recognized for long chain dialkyl surfactants it is also a common occurrence for monoalkyl surfactants. Because of the lower packing order within the alkyl chain region in than the normal crystals, different chain length derivatives (usually up to four carbons) can mix within the Lp phase. Additionally, different head groups can also mix within the Lp phases. Indeed, Lp phases can occur for mixed systems where none is observed for the individual constituents. For example sodium dodecyl sulfate and dodecanol form a mixed gel phase where as none is observed for SDS alone [62]. (Note that dodecanol does form a stable Lp phase termed the a-crystalline phase with about 0.2 mol fraction of water [5, 63, 64].)... 

This exclusion of polymers from the interior of the vesicles results in an osmotic compression of the water layers and a decrease in the water layer thickness and lamellar phase volume. This effect allows the control of bulk properties such as viscosity and also provides a probe of water layer dimensions in lamellar dispersions. The lamellar surfactant system used in this study is the sodium dodecyl sulfate (SDS)/dodecanol (Ci20H)/water system that has been used to prepare submicron diameter emulsions (miniemulsions) from monomers for emulsion polymerization (5) and for the preparation of artificial latexes by direct emulsification of polymer solutions such as ethyl cellulose (4). This surfactant system forms lamellar dispersions (vesicles) in water at very low surfactant concentrations (< 13 mM). 

In an earlier investigation Vijayan and Woods (20) studied the stability of oil (benzene) drops at oil oil/water interface containing mixed surfactants (sodium dodecyl sulfate and dodecanol) around and below the critical micelle concentration. They measured both coalescence times and film drainage rates using interferometer-cinematography. For the range of concentration studied, they found uneven film drainage modes which finally lead the drops to coalescence. Also, anomalous coalescence time behavior was found at some concentration of the surfactant. (Note the comparison between this observation with the anomaly at 1.5% NaCl concentration in the current study.)... 

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