Liquid crystals hydrotropes

The hydrotropic action of a dicarboxylic acid is discussed against the general features of hydro-tropic action the liquid crystal/isotropic solution equilibrium. It is shown that the hydrotropic action of the dicarboxylic acid in question, 8-[5(6)-carboxy-4-hexyl-cyclohex-2-enyl] octanoic acid, depends on its conformation at an interface. 

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 hydrotropes in this era were short chain aromatic sulfonates, with the p-xylene sodium sulfonate as a typical example. Their action is preventing the formation of liquid crystals is easily understood from a direct comparison of their molecular geometry (Fig. 1). 

The molecular mechanism behind the destabilization of liquid crystals was subsequently clarified [25], The specific disordering promoted by the hydrotrope in the water-surfactant-oily liquid crystal was first determined, followed by an investigation into the conformation of the diacid molecule itself [92],... 

FIG. 2.14 Low-angle x-ray values for interlayer spacing in a lamellar liquid crystal (X) show the spacing is unchanged with the addition of the hydrotrope ( ) of Figure 2.9. Addition of a long-chain compound, oleic acid, gives the expected increase (A). 

Figure 2.13 is obviously the one encountered in the liquid crystal. As a comparison, the addition of oleic acid with one polar group located at the interface gives the expected increase in interlayer spacing, as shown in Figure 2.14. Destabilization of the lamellar liquid crystal is not only affected by the diacid it appears to be a general property shared by other hydrotropes, such as alkanols, short-chain...

In some cases, the oily dirt is less polar than the model system of Kielman and Van Steen [96], For less polar fatty oils the concept of hydrotropic breakdown of a liquid crystal is also useful [98],... 

The function of hydrotropes in detergency has been discussed as regards their interaction with surfactant colloidal association structures, especially lyotropic liquid crystals. The main activity of the hydrotrope as a part of a liquid detergent is to avoid gelation in both the concentrated package system and under the dilute conditions in the actual laundry process. 

Unlike structured liquids, these unstructured, low-viscosity, clear liquids can be developed only if the onset of the formation of liquid crystals is hindered or they are broken up. This can be accomplished by two different methods by the addition of hydrotropes and solvents which can disrupt or prevent any liquid crystal formation as well as aid in solubilizing the other components in the formulation or by increasing the water solubility of the individual components. More than likely a combination of both these techniques is used to develop a stable liquid. The respective costs of these approaches ultimately determine their usage in the final formulation. Some of the methods used to formulate stable, single-phase, clear unstructured liquids are summarized below. 

Hydrotropy When there are strong chain-chain and head-head interactions between surfactant molecules (due to long, straight chains and close-packed heads), either insoluble crystal formation (low Krafft point, p. 214) or liquid-crystal formation (Chapter 3, Section IIC) may occur. Since there is much less space available for solubilization in rigid liquid-crystal structures than in the more flexible types of micelles, the onset of crystal formation usually limits the solubilization capacity of the solution. The tendency to form crystalline structures can be reduced by the addition of certain nonsurfactant organic additives called hydrotropes. 

The diacid, 5-carboxy-4-hexyl-2-cyclohexene-l-yl octanoic acid, has a total chain length of 21 carbon atoms and the structure does not resemble the structure of traditional hydrotropes such as the short-chain aromatic sulfonates (see Figure 18.2). Despite the difference in structure, this diacid exhibits hydrotropic properties (15). However, it has been proven that the structure of the diacid when active as a hydrotrope will not be very different from the short and bulky conventional compounds. The conformation of the diacid within a lamellar liquid crystal is composed of a short loop where... 

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.)...

Aqueous solutions of hydrotropes have proven to be powerful systems for preparing vesicles. In this case, the vesicle-forming compound has been mixed with water and the hydrotrope, and dilution with water resulting in the vesicles (33). Vesicles formed by the nonionic surfactant Laureth 4 (Brij 30) from an aqueous solution of sodium xylene sulfonate (SXS) were more stable and smaller than vesicles prepared from the suspension of the lamellar liquid crystal in water. 

Guo. R., Compo, M. E., Friberg, S. E. and Morris, K., The coupling action of a hydrotrope and surface transition from lamellar liquid crystal to bicontinuous microemulsion, J. Disp. Sci. TechnoL, 17, 493-507 (1996). 

The addition of a hydrotrope, a more water soluble molecule with disordering action, supplemented the disordering and the liquid crystal range along the SDS/ W-C5OH axis disappeared (Figure 1.9) resulting in an excellent micioemulsion area [10]. 

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. 

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...

Other applications are as wetting agents in stearic acid-oleic acid separation ( crystallization), as a hydrotrope in liquid detergents, and in emulsion pol3merization. 

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