Microemulsion stability

Binks BP, Hetcher PDl, Taylor DJF (1998) Microemulsions Stabilized by lonic/Non-ionic Sm-factant Mixtures. Effect of Partitioning of the Nonionic Surfactant into the Oil. Langmuir 14 5324-5326... 

Robinson BH, Toprakcioglu C, Dore JC, Chieux P (1984) Small-Angle Neutron-Scattering Study of Microemulsions Stabilized by Aerosol-Ot.l. Solvent and Concentration Variation. J Chem Soc Faraday Trans 1 80 13-27... 

A. Bumajdad and J. Eastoe. Conductivity of water-in-oil microemulsions stabilized by mixed surfactants. J. Colloid Interface Sci., 274(l) 268-276, 2004. 

Holmes et al. (1998) performed two enzymatic reactions, the lipase-catalyzed hydrolysis of y>-nitrophenol butyrate and lipoxygenase-catalyzed peroxidation of linoleic acid, in w/c microemulsions stabilized by a fluorinated two-chained sulfosuccinate surfactant (di-HCF4). The activity of both enzymes in the w/c microemulsion environment was found to be essentially equivalent to that in a water/heptane microemulsion stabilized by Aerosol OT, a surfactant with the same headgroup as di-HCF4. The buffer 2-(A-morpholino)ethanesulfonic acid (MES) was used to fix the pH in the range 5-6. 

Fig. 11. Relative phase volume-corrected rate constants vs weight fraction water for the io-dosobenzoate-catalysed hydrolysis of a phosphate ester in water/hexadecane microemulsions stabilized by various surfactant/cosurfactant mixtures. Curve (a) Brij 96/1-butanol curve (b) CTAB/I-butanol curve (c) CTAC/dibutylformamide curve (d) CTAB/2-methylpyrrolidone and Adogen 464 (from [26])...

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

Fig. 2 Experimental and model rate versus conversion profiles for the polymerization of hexylmethacrylate in a microemulsion stabilized by the surfactant DTAB. The two curves are for initiator concentrations of 0.045 wt% (top) and 0.015 wt% (bottom) relative to the amount of monomer in the micro emulsion. The solid lines are predictions from the Morgan model [56]...

Formation and Characterization of Water-in-Oil Microemulsions Stabilized by A—B—A Block Copofymers... 

The earlier concepts of microemulsion stability stressed a negative interfacial tension and the ratio of interfacial tensions towards the water and oil part of the system, but these are insuflBcient to explain stability (13). The interfacial free energy, the repulsive energy from the compression of the diffuse electric double layer, and the rise of entropy in the dispersion process give contributions comparable with the free energy, and hence, a positive interfacial free energy is permitted. 

Fig. 3 (A) SANS data for water-in-hexane microemulsions stabilized by d-DDAB. The simultaneous fit to the Schultz core-shell model and error bars on the data are shown (B) SANS data for water-in-hexane microemulsions stabilized by fi-DDAB. The simultaneous fit to the Schultz core-shell model and error bars on the data are shown. (Reproduced from Ref. l)...

SANS studies and fluorescence correlation spectroscopy have been successfully combined to study the size of water-in-oil (heptane) microemulsions stabilized by sodium Z A-2-ethylhexyl sulpho succinate (AOT) with and without a fluorescently labeled peptide (phalloidin, a fungal toxin of mass 789 Da) and protein (ot-chymotrypsin, a serine protease of mass 25 kDa). In incorporation of the small peptide, phalloidin did not increase the size of the microemulsion droplets whereas the presence of ot-chymotrypsin significantly increased the size of the microemulsion droplets. Furthermore, the studies suggested that while all the phalloidin was in the disperse water phase, the a-chymotrypsin appears to be dispersed in the oil phase in monomeric form and protected from contact with the oil by a shell of surfactant. 

Several attempts have been made to explain the stability and structural aspects of various microemulsions (54-60). In this section, we would like to describe some of the important aspects of microemulsion stability. 

Stability of Microemulsions. The first attempt to describe the microemulsion stability in terms of different free energy components was made by Ruckenstein and Chi (55) who evaluated the enthalpic (Van der Waals potential, interfacial free energy and the potential due to the compression of the diffuse double layer) and entropic... 

Pseudoternary phase diagrams of the water-dodecane-SDS-pentanol and water-dodecane-SDS-hexanol systems have been investigated in detail. A great variety of new domains has been evidenced in the oil rich part of these diagrams including, one-, two-, three- and four-phase liquid regions. An interpretation of these diagrams is proposed it is shown that interactions between water domains play an important role in microemulsion stability. 

Holmes et al. reported the first enzyme catalyzed reactions in water-in-CO2 microemulsions (67). Two reactions, a lipase-catalyzed hydrolysis and a lipoxygenase-catalyzed peroxidation, were demonstrated in water-in-C02 microemulsions using the surfactant di(l/7,l/7,5/7-octafluoro- -pentyl) sodium sulfosuccinate (di-HCF4). A major concern of enzymatic reactions in CO2 is the pH of the aqueous phase, which is approximately 3 when there is contact with CO2 at elevated pressures. Holmes et al. examined the ability of various buffers to maintain the pH of the aqueous solution in contact with CO2. The biological buffer 2-(A-morpholino)ethanesulfonic acid sodium salt (MES) was the most effective, able to maintain a pH of 5, depending on the pressure, temperature, and buffer concentration. The activity of the enzymes in the water-in-C02 microemulsions was comparable to that in a water-in-heptane microemulsion stabilized by the surfactant AOT, which contains the same head group as di-HCF4. 

Eastoe J, Paul A, Downer A, Steytler DC, Rumsey E. Effects of fluorocarbon surfactant chain structure on stability of water-in-carbon dioxide microemulsions. Links between aqueous surface tension and microemulsion stability. Langmuir 2002 18 3014-3017. 

The pH inside microemulsion droplets is typically around 3 owing to the formation of carbonic acid, as determined with fluorescence (47) and absorbance (48) probes. Inorganic and organic bases and buffers, such as NaOH, can be used to control the aqueous pH in microemulsions stabilized by PFPE C00 NH4 from 3 up to 5 to 7. 

Engelskirchen, S., Eisner, N., Sottmann, T. and Strey, R. (2007) Triacylglycerol microemulsions stabilized by alkyl ethoxylate surfactants - A basic study Phase behaviour, interfacial tensions and microstructure. /. Colloid Interface Sci., 312, 114-121. 

Table 1. Aqueous Phase Critical Micelle Concentrations (erne s), Limiting Surface Tensions yeme s and Microemulsion Stability Pressures for Fluorinated Surfactants.

Figure 5. Pressure-temperature phase diagram ofW/C02 microemulsion stabilized by di-HCF4 with different additives Rh(CO) cac = 0.01 mmol Rh/TPPTS = 4, syngas = di-HCF4 = J.tfg, 7w/ 0.2Maqueous solution...

Microemulsion stability is influenced by environmental parameters such as temperature and pH. These parameters change upon microanulsion delivery to patients. 

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