Microemulsions nonionic systems

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

The rate constants for the reaction of N-dodecyl-3-carbamoyl-pyridinlum ion with cyanide in both cationic and nonionic o/w microemulsions have been measured as a function of phase volume. Added salt has no effect in the cationic system, but the rate constants in the nonionic system depend upon ionic strength as would be expected for a reaction between two ions. In order to compare the two microemulsions, the ionic strength in the reaction region has been estimated using thicknesses of 2-4A. The former produces values of the effective surface potential which yield... 

The ultralow interfacial tension can be produced by using a combination of two surfactants, one predominantly water soluble (such as sodium dodecyl sulfate) and the other predominantly oil soluble (such as a medium-chain alcohol, e.g., pentanol or hexanol). In some cases, one surfactant may be sufficient to produce the microemulsion, e.g., Aerosol OT (dioctyl sulfosuccinate), which can produce a W/O microemulsions. Nonionic surfactants, such as alcohol ethoxylates, can also produce O/W microemulsions, within a narrow temperature range. As the temperature of the system increases, the interfacial tension decreases, reaching a very low value near the phase inversion temperature. At such temperatures, an O/W microemulsion may be produced. 

Microemulsions were obtained using different types and concentrations of surfactant, cosurfactant, and styrene. An anionic surfactant, sodium dodecyl sulfate (C12H250SO3Na), and two types of nonionic surfactants, Emsorb 6916 (sorbitan monolaurate) and Neodol 91-5 (ethojqrlated alcohol), were used. The surfactant concentration was varied between 5 to 10% (w/w) for the anionic system and between 5 to 15% (w/w) for the nonionic systems. Either 2-pentanol or ethylene glycol monobutyl ether (butyl cellosolve, C4H90CH2CH20H) was used as the cosolvent with the anionic surfactant. The amount of cosurfactant used depended on the anionic surfactant concentration and varied form 12.5 to 25% (w/w). 

In the nonionic system observed under EVM, the initial microemulsion showed no tendency of gelation until it reached 60 C. After reaching 60<>C, the system gels and starts to polymerize after 10-12 hours. As polymerization proceeds, the water separates out. After about 20-24 hours, the gel starts to become a solid with an excess emulsion phase formed at the bottom. The polymerization is essentially complete after 36 hours. Due to different modes of polymerization in the anionic and nonionic surfactant systems, the mechanical properties of the solid are different. The polymers obtsuned from anionic microemulsions are brittle, while those obtmned from nonionic microemulsions are ductile. 

The decrease in Tg of nonionic microemulsion systems is due to the absence of electrostatic interactions between the surfactant and polystyrene. The nonionic surfactant behaves as a low molecular weight additive (i.e. plasticizer) which then lowers the Tg. In order to determine whether the cosurfactant 2-pentanol has any effect on Tg, a nonionic system was prepared containing 2-pentanol. The Tg of the polymerized solid was then determined. The Tg remained the same as in the nonionic system containing no cosurfactant. 

These systems have not been investigated as thoroughly as the W/O microemulsions have been. One determination (II) has been reported, and Prince has suggested (6) that the O/W microemulsions exist within a limited oil/emulsifier ratio in a sectorial solubility region emanating from the aqueous comer. This is tme only for nonionic systems (12) the combination ionic substance and alcohol gives a more complicated pattern. 

Solans, C., Garcia Dominguez, J. and Friberg, S.E. (1985) Evaluation of textile detergent efficiency of microemulsions in systems of water, nonionic surfactant and hydrocarbon at low temperature. /. Disp. Set. TechnoL, 6, 523. 

The first way to produce a microemulsion is to increase the temperature, which results in augmented molecular motion and debilitates the directional interactions. This method is used mostly in nonionic systems, but it works also with some ionic surfactants such as a-olefin sulfonates. 

Microemulsions have been achieved with supercritical fluids, particularly light hydrocarbons that can be solubilized in ionic systems [62,63], while carbon dioxide could be solubilized in only fluorocarbon surfactant and other nonionic systems [64]. 

Figure 17 Illustration of the fact that microemulsion structure is not simply a function of composition. Shown are partial ternary phase diagrams with nonionic and cationic surfactants at room temperature. For a similar composition (approximately 15% surfactant, 65 wt% water, and 20 wt% oil), the microstructures of the two systems are widely different, as shown by the ratio of the water and oil diffusion coefficients, Dn /Dhc where he here denotes oil (hydrocarbon). The nonionic system has an oil-in-water structure (D //)hc = 200), while the cationic system has a water-in-oil structure (D,/Z)h. = 1/200).

Figure 20 Double-water experiment, the aqueous analogy of the double-oil experiment, performed on an AOT microemulsion as a function of temperature. The polar solvent is a 5% A-methyl formamide (NMF) solution in heavy water (D2O). The ratio of the water (here measured as trace impurities of HDO) and NMF diffusion coefficients is monitored as a function of temperature (c). Also shown as (a) the individual self-diffusion coefficients of water (O). NMF ( ), and AOT ( ) and (b) the relative diffusion coefficient of water. Kq = 1.73 is the diffusion coefficient ratio in the pure water-NMF mixture and is indicated as a broken line in (c). The phase boundary at 75"C is indicated as a vertical broken line. The behavior with increasing temperature is completely analogous to that of the nonionic system (Fig. 19) and illustrates a transition from reverse micelles to a bicontinuous structure via growing droplets that become attractive. (Data from Ref 49.)... 

In subsequent work,Vasudevan et al. [23] conclusively showed that the hemoglobin was fully dissociated under the conditions presented in Ref. 18 and that only the heme (i.e., iron within porphyrin) was extracted into the microemulsion. Vasudevan and Wiencek [24] went on to prove that under very limiting circumstances, protein will partition into nonionic microemulsion liquid membranes. The underlying extraction mechanism is a weak electrostatic interaction between the trace impurities in the surfactant and the protein. Since the separation is based on an interaction between the surfactant (or some other interfacially active compound) and the solute, no stripping reaction is required, and the system is really an equilibrium microemulsion extraction system as described in Sec. II.B.2. 

Nonionics can function as cosurfactants in microemulsions. This cosurfactant role, which can also be performed by short-chain alcohols or glycol ethers, can make it possible to form microemulsions or other phases from anionic surfactant systems that would otherwise be unreachable in a pure anionic system. By one definition, a microemulsion system is one that achieves a zero or close-to-zero curvature [108]. This is easily achievable in nonionic systems, but very difficult in pure anionic surfactant systems due to the head group repulsions. Mixing the nonionic with the anionic makes these types of systems, which sometimes have greater cleaning ability, easily accomplished. 

Because there are no micelles in the excess phases, no structure can occur ia them. Thus, all the surfactant would be in the microemulsion phase. For nonionic systems, it was found that the water excess phase is indeed at the CMC, whereas the concentration in the excess oil phase can be much higher than in water because these surfactants do not form micelles easily in oil [43,44]. This peculiarity has been found responsible for producing complex phenomena like the selective partitioning of surfactant species [45-49]. 

When the system undergoes the transition from a triphasic to a biphasic case, the tie-triangle flattens and the microemulsion becomes a micellar phase and merges together with an excess phase. The flattened triangle thus vanishes into a tie-line, the so-called bottom tie-line in the seventies, recently called the CMC tie-line by some authors [51]. These authors studied the same kind of phase transition due to a temperatiue change in a nonionic system and probably wanted to point out that this line joins the representative point of the excess phases that are believed to be at the CMC in their respective solvents. 

Figure 7 depicts the case of a transition by cooling for a nonionic system, the so-called PIT emulsification method [64], because the formulation variable is temperature, and the FILD = 0 optimum formulation is attained at the phase inversion temperature. In this case the emulsion at a temperature above the PIT is W/O then as temperature decreases the microemulsion oil phase solubilizes more and more water and the water drops vanish. 

Results described in the literature have resulted in several patents, such as one for the improvement of the transport of viscous crude oil by microemulsions based on ether carboxylates [195], or combination with ether sulfate and nonionics [196], or several anionics, amphoterics, and nonionics [197] increased oil recovery with ether carboxylates and ethersulfonates [198] increased inversion temperature of the emulsion above the reservoir temperature by ether carboxylates [199], or systems based on ether carboxylate and sulfonate [200] or polyglucosylsorbitol fatty acid ester [201] and eventually cosolvents which are not susceptible for temperature changes. Ether carboxylates also show an improvement when used in a C02 drive process [202] or at recovery by steam flooding [203]. 

Surfactant Solutions New Methods of Investigation, edited by Raoul Zana Nonionic Surfactants Physical Chemistry, edited by Martin J. Schick Microemulsion Systems, edited by Henri L Rosano and Marc Clausse Biosurfactants and Biotechnology, edited by Naim Kosaric, W. L. Cairns, and Neil C. C. Gray... 

Dye-Doped Silica Nanoparticle Synthesis Using Nonionic Surfactant-Based Microemulsion Systems... 

Silica particles synthesized in nonionic w/o microemulsions (e.g., poly-oxythylene alkyl phenyl ether/alkane/water) typically have a narrow size distribution with the average value between 25 and 75 nm [54,55]. Both water and surfactant are necessary components for the formation of stable silica suspensions in microemulsions. The amounts of each phase present in the micro emulsion system has an influence on the resulting size of the silica nanoparticle. The role of residual water (that is the water that is present in the interface between the silica particle and the surfactant) is considered important in providing stability to the silica nanoparticle in the oil... 

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