Microemulsions hydrotropes

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

In another microemulsion system, ° the colorant is incorporated into the ink as an aqueous pigment dispersion-based inkjet ink composition by formulating the ink to comprise at least one aqueous pigment dispersion and a microemulsion with at least one water-insoluble organic compound, one hydrotropic amphiphile, and water. This ink system was reported to improve waterfastness and bleed control, "" providing a fast drying ink. 

These disadvantages have recently been alleviated by the introduction of a method to prepare liposomes from microemulsions formed by a combination of Sl with a hydrotrope (H) [47]. The hydrotropes are short-chain molecules, usually with an ionic group [48,49]. The combination is highly advantageous as a precursor to the preparation of liposomes, as demonstrated in Fig. 4, which illustrates the pronounced difference from micellarly solubilized species as precursors (Fig. 2). The Sm/Sl ratio is typically approximately 2, and the H/Sl ratio is on the order of 0.25, which is one order of magnitude improvement. In addition, it should be realized that the surface activity of the hydrotrope molecule is far less than that of the surfactant, which is another advantage. 

Figure 4 The combination of a liposome-forming surfactant (Laureth 4, L4) and a hydrotrope (sodium xylenesulfonate, SXS) with water gives a huge microemulsion region (Li), from which a liposomal solution with high concentration of liposome may be made with a high ratio of liposomal surfactants to hydrotropes. The line from the water corner shows the maximum L4/SXS weight ratio.

The function of hydrotropes may vary in different systems. This has been observed in sun-screen microemulsions prepared with water, pentanol, sodium dodecyl sulfate (SDS) and a hydrotrope (13). Three different hydrotropes were tested, i.e. p-aminobenzoic acid (PABA), octyldimethyl-p-aminobenzoic acid (ODP) and 2-hydroxy-4-methoxy-5-sulfobenzophenone (HMSB). Their effects are presented in Table 18.2. 

Table 18.2. Function of three different sun-screen hydrotropes in microemulsion preparations...

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.)...

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 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. 

S.K. Hait, A. Sanyal, and S.P. Moulik 2002 Physicochemical studies on microemulsions. 8. The effects of aromatic methoxy hydrotropes on droplet clustering and understanding of the dynamics of conductance percolation in water/oil microemulsion systems, J. Phys. Chem. B 106, 12642-12650. 

Physiochemical studies on microemulsions. 7. Dynamics of percolation and energetics of clustering in water/AOT/isooctane and water/AOT/decane w/o microemulsions in the presence of hydrotopes (sodium salicylate, a-napthol, 3-napthol, resorcinol, catechol, hydroquinone, pyrogallol and urea) and bile salt (sodium cholate). J Phys Chem 105, 7145-7154 (b) Hait, S. K., Sanyal, A., and Moulik, S. P. (2002). Physiochemical studies on microemulsions. 8. The effects of aromatic methoxy hydrotropes on droplet clustering and understanding of dynamics of conductance percolation in water/oU microemulsions. J Phys Chem B 106,12642-12650. 

Fribeig S, Brancewicz C, and Morrison D. 1994. O/W microemulsions and hydrotropes - the coupling action of a hydrotrope. Langmuir 2945-2949. 

Uses Solubilizer, hydrotrope in maintenance chems., detergents, metal finishing soak sol ns., slat/conveyor lubricants, textiles, microemulsions, institutional, water treatment, cleaning in place, automatic dishwashing corrosion inhibitor Features Salt free low foam acid/alkali stable hard water stable compat. with cationics, anionics, and nonionics Reguiatory ADR nonhazardous for transport Properties Water wh. to pale yel. liq. 60% act. 

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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