Oil continuous microemulsions

More recently an oil continuous microemulsion technique has been described,16 which allows the study of specific interactions between amino acid side chains and metal ions. Both the metal ion and amino acid are microencapsulated as aqueous droplets in a dispersed phase. The technique is of particular relevance to metalloprotein and metal-membrane interactions where the local dielectric constant can be considerably less than that of bulk water. 

However, often the identities (aqueous, oleic, or microemulsion) of the layers can be deduced reliably by systematic changes of composition or temperature. Thus, without knowing the actual compositions for some amphiphile and oil of points T, M, and B in Figure 1, an experimentalist might prepare a series of samples of constant amphiphile concentration and different oil—water ratios, then find that these samples formed the series (a) 1 phase, (b) 2 phases, (c) 3 phases, id) 2 phases, (e) 1 phase as the oil—water ratio increased. As illustrated by Figure 1, it is likely that this sequence of samples constituted (a) a "water-continuous" microemulsion (of normal micelles with solubilized oil), (b) an upper-phase microemulsion in equilibrium with an excess aqueous phase, (c) a middle-phase microemulsion with conjugate top and bottom phases, (d) a lower-phase microemulsion in equilibrium with excess oleic phase, and (e) an oil-continuous microemulsion (perhaps containing inverted micelles with water cores). 

Addition of salting-out type electrolytes to oil-water-surfactant (s) systems has also a strong influence on their phase equilibria and interfacial properties. This addition produces a dehydration of the surfactant and its progressive transfer to the oil phase (2). At low salinity, a water-continuous microemulsion is observed in equilibrium with an organic phase. At high salinity an oil-continuous microemulsion is in equilibrium with an aqueous phase. At intermediate salinity, a middle phase microemulsion with a bicontinuous structure coexists with pure aqueous and organic phases. These equilibria were referred by Vinsor as Types I,II and III (33). 

High water content, low-viscosity, oil continuous microemulsions and their use in cleaning applications... 

Figure 7 Droplet structure of oil-continuous microemulsions of the D20-n-decane-A0T system with varying droplet volume fraction. Bar = 2000 A. (From Ref 21.)...

The diffusion studies described in the above sections pertain to water-continuous and bicontinuous microemulsions. Chen and Georges [34] were the first to study diffusion in oil-continuous microemulsions using steady-state microelectrode voltammetry. Ferrocene was used to probe diffusion in an SDS-dodecane-1-heptanol-water system. The diffusion coefficient of the hydrophobic probe indicated the microviscosity of the oil rather than the bulk viscosity of the microemulsion. Owlia et al. [36] reported diffusion coefficient measurements of water droplets in an Aerosol OT [AOT, bis(2-ethylhexyl)sulfosuccinate] microemulsion using a microelectrode. Water-soluble cobalt(II) corrin complex (vitamin Bi2r) was used in an oil-continuous microemulsion containing 0.2 M AOT, 4 M water buffered at pH 3, and isooctane. The apparent diffusion coefficient decreased with the probe concentration in accordance with Eq. (13) as shown in Fig. 6 [36]. The water droplet size was... 

Garcia et al. [77,78] reported an electron transfer percolation threshold in highly resistive oil-continuous microemulsions. The Faradaic electron transfer is modulated by the amount of cosurfactant present in AOT-toluene-water microemulsions. Below a certain threshold concentration of the cosurfactant, the electron transfer between electroactive solutes in the water droplets and ultramicroelectrode is retarded or blocked. Electron transfer becomes facilitated, and a sharp increase in Faradaic current is observed above the threshold concentration. This effect was demonstrated for ruthenium hexamine reduction [77,78], ferrocyanide oxidation [77,78], acrylamide oxidation [77], and allQ lamide oxidation [77,79] with acrylamide, alkylamides, and acetonitrile as cosurfactants in AOT microemulsions. NMR results [80] suggest that there is an interfacial packing transition of the surfactant (AOT) at about the same cosurfactant concentration as the threshold transition observed electrochemically. 

Electrochemical catalysis constitutes a general synthesis route that is amenable to rate control and enhancement in microemulsions [5,6]. Owlia et al. [36] were the first to investigate electrochemical catalysis in oil-continuous microemulsions. The kinetics of reduction of several allQ l vicinal dibromides was studied in the presence of vitamin B12 as catalyst. The following reaction mechanism was suggested ... 

The change in electrical conductivity can be used to differentiate different types of microemulsions. For example, type II or type IV oil continuous microemulsions have a very low electrical conductivity (say less than 1 pS/cn type III or type IV bicontinuous microemulsions have a medium electrical conductivity (say in between 1 and 10 pS/cm) and type I or type IV water continuous... 

The phase transition from type IV oil continuous microemulsion to type IV water continuous microemulsion could be captured by observing changes in the viscosity (Watanabe et al. 2004). For example, the viscosity of type IV oil continuous (W/0) microemulsion rises slowly initially with the increase in water concentration. With further addition of water the viscosity begins to increase sharply. The increase in viscosity is mainly due to the transition from type IV oil continuous (W/0) microemulsion to type IV bicontinuous microemulsion. The viscosity reaches a maximum value at some water concentration. Upon further increase in water concentration, the transition of type IV bicontinuous microemulsion to type IV water continuous (0/W) microemulsion occurs resulting in a sharp decrease in viscosity. The viscosity of type IV water continuous (0/W) microemulsion continues to decrease with further increase in aqueous fraction (Watanabe et al. 2004). 

Generally speaking, hydrophilic surfactant is used to formulate water continuous microemulsions and lipophilic surfactant is used to formulate oil continuous microemulsions. The hydrophilicity of surfactant can be measured in terms of the HLB (Pasquali et al. 2008). The HLB value of a surfactant is dehned as follows based on Griffin s method ... 

The porosity of solid polystyrene produced by polymerization in a middle-phase (bicontinuous) microemulsion is greater than that obtained by polymerization in either water-continuous or oil-continuous microemulsion. The first account of a middle-phase microemulsion-based porous polymer was reported by Haque and Qutubuddin in 1988 [71]. The microemulsions were formulated with styrene, water, sodium dodecyl sulfate (SDS), and 2-pentanol or butyl cellosolve as the cosolvent. (Since butyl cellosolve has greater solubility than 2-pentanol in polystyrene, it increases the stability of SDS microemulsion.) Figure 3.14 shows the structure of polystyrene when obtained from middle-phase microemulsion polymerization at 60 °C for 36 h, the composition (wt%) before polymerization being SDS 10 %, 2-pentanol 25 %, styrene 40 %, and water 25 %. The polymerized stmcture shows pores in both micron and submicron ranges. The observed greater porosity of this solid compared to the solids obtained from polymerization of oil-continuous microemulsion (SDS 10 %, 2-pentanol 25 %, styrene 55 %, water 10 %) and water-continuous microemulsion (SDS 10 %, 2-pentanol 25 %, styrene 5 %, water 60 %) is apparently related to the fact that middle-phase microemulsions contain interconnected domains of both water-continuous and oil-continuous regions. 

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