Lamellar phases hydrotropes

Often the phase diagram of an oil-water-surfactant system is such that the domain of a single isotropic phase contains both regions with normal micelles and those with inverse micelles. Inversion between the two microstructures is continuous and does not involve the formation of the lamellar phase when v lal is approximately equal to 1.0. Such behavior is facilitated by the presence of amphiphilic compounds with short hydrocarbon chains, such as short-chain alcohols and sodium xylene sulfonate. The latter compound and others with similar properties are called hydrotropes and have many practical applications (Friberg, 1997). 

The formulation of food systems as microemulsions is not easy, since addition of triglycerides to inverse micellar systems results in a phase change to a lamellar liquid crystalline phase. The latter has to be destabilized by other means than adding co-surfactants, which are normally toxic. An alternative approach to destabilize the lamellar phase is to use a hydrotrope, a number of which are allowed in food products. 

In compounds that have relatively small and weakly hydrophilic functional groups, the lamellar phase is very often the solubility-limiting phase. The same is true of many surfactants containing two or more lipophilic groups—irrespective of the hydrophilicity of the fiindlional groups present. Catanionic surfactants (ionic surfactants in vduch-both ions are amphiphilic [87]) are also reported to display the lamellar liquid-crystal solubility boundary. Dioctadecylammonium cumenesulfonate (mentioned earlier) may be regarded as a cationic surfactant in which the anion has a hydrotropic molecular structure [78]. It displays a very unusual lamellar liquid-crystal solubility boundary above the Krafft eutectic temperature. 

A change in the perception of their mechanism of action came in the sixties when Lawrence (7) pointed out that short chain surfactants would delay the gelling to a liquid crystalline phase which takes place at high surfactant concentrations. Friberg and Rydhag (8) showed that hydrotropes, in addition, prevent the formation of lamellar liquid crystals in combinations of surfactants with hydrophobic amphlphiles, such as long chain carboxylic acids and alcohols. The importance of this finding for laundry action was evident. 

The mechanism by means of which hydrotropes operate in surfactant solutions was elucidated by Friberg and coworkers (1970, 1971 Cox, 1981), who showed that it is due to the inhibition of the formation of surfactant liquid-crystalline phases by the hydrotrope. Since they have structures similar to those of surfactants, hydrotropes can form mixed micellar structures with surfactants. However, since their hydrophilic heads are large and their hydrophobic groups are small (their V)/ Icdo ratio [Chapter 3, Section II] is liquid-crystalline structures and thus inhibit the formation of the latter. This destruction or inhibition of the liquid-crystalline phase increases the solubility of the surfactant in the aqueous phase and the capacity of its micellar solution to solubilize material. Hydrotropic action occurs at concentrations at which the hydrotrope self-associates to form these mixed structures with the surfactant (Gonzalez, 2000). 

Figure 18.6. The effect of the hydrotrope sodium xylene sulfonate (SXS) on the phase behaviour for a system consisting of water (w), sodium dodecyl sulfate (SDS), pentanol (n-CsOU) and p-xylene I, o/w microemulsion 11, w/o microemulsion IIb, C5OH and p-xylene solution IV multiphase region with lamellar liquid crystal. (Reprinted from Guo, R. et al., J. Disp. Sci. Tech., 17, 493-507 (1996) p. 498-499, by courtesy of Marcel Dekker, Inc.)...

Figures 3 and 4 depict the phase behaviors of various triglycerides, ethox-ylated mono/diglycerides, and water in conjunction with hydrotropes such as ethanol, propylene glycol, and sucrose [13,18]. It can be seen from Fig. 3 that a w/o microemulsion (L2 phase) was easily formed at a ratio of 75 25 wt% of ethoxylated mono/diglycerides. In a certain region of the phase diagram, blue phase, droplets separated by lamellar liquid crystals were observed. Ethanol was reported to act synergistically with sucrose to destabilize...

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