Cosurfactant/surfactant ratio

With increased surfactant/(cosurfactant + surfactant) ratio (0.22), the liquid crystal was formed immediately and the interface in the oil layer now lasted only 12 days, (Fig. 4). The formation of a liquid crystal impeded the transport of surfactant to the lower part. Fig. 5A. In this case, the surfactant concentration remained lower in the bottom layers during the entire duration of the experiment more than 2 months. The transport of cosurfactant to lower parts. Fig. 5B, and water from the layers below the liquid crystal. Fig. 5C, were not influenced to a great degree by the enhanced amount of liquid crystal. 

For the highest surfactant/(cosurfactant + surfactant) ratio (0.35), the liquid crystal also formed early. Fig. 6, in layers 5 and 6. This resulted in the concentration changes being focused towards the bottom layers the higher reservoir of surfactant and cosurfactant in the top layers resulted in small changes of their concentrations, Figs. 7A-C. The slowest process was the disappearance of the birefringence a time of 9 months was needed for that to happen. Fig. 6. 

For the 1 M NaCl system the solubility region was further reduced. Fig. 13, and the water solubilization maximum found at even higher surfactant/cosurfactant ratio. The series with the lower ratios of surfactant to cosurfactant showed an uptake of the aqueous solution somewhat similar to the series in the system with 0.5 M NaCl. The series with the surfactant/(cosurfactant + surfactant) ratio equal to 0.4 gave an initial liquid crystal formation lasting for 2-3 days folllowed by a middle phase lasting a longer time. The liquid crystalline and the middle phase layer were both more pronounced for the sample with initial salt concentration equal in the water and in the microemulsion. Fig. 14A, than for the sample with all the salt in the water. Fig. 14B. 

The conditions in the series with surfactant/(cosurfactant + surfactant) ratio (0.30), Fig. 18, were similar but the duration of the liquid crystalline phase was shorter. The concentration changes were also very similar. Figs. 19A-C. 

Table II tabulates the variation in CEES solubility with variation in cosurfactant/surfactant ratio from 0.67 1 to 2.33 1. It is clear from the data that in this system the ratio of cosurfactant to surfactant is not critical for solubilization...

In the methylsulfolane-containing system, it was found that neither incorporation of salts (as seawater) nor alteration of the cosurfactant/surfactant ratio seriously altered the solubilization of chlorethyl ethyl sulfide. Thus, it seems plausible that low concentrations of inorganic buffers and/or oxidants should not seriously degrade the solubilization effectiveness of microemulsions containing sulfones as cosurfactants. 

As said before, when entangled worm-like micelles are formed, viscoelastic behavior only follows Cates model at low and intermediate frequencies, indicating that other fast relaxation processes exist (see Figure 12.5). Moreover, at cosurfactant-surfactant ratios above the viscosity maximum, further addition of cosurfactant decreases viscosity and viscoelastic functions up to phase separation, where lamellar liquid crystal appears. As said above, this has been related to the fact that, after the maximum in viscosity, the decrease in spontaneous curvature produces branching of micelles before phase separation [9, 10). 

In this connection, it is interesting that vesicles can also form a cubic phase at high concentrations, if they are very monodisperse. This condition has been found to be fulfilled in a system of sodium oleate/octanol/water at a cosurfactant surfactant ratio of around 3 1 in the water-rich comer however, such cubic phases of vesicles are very rare, because the vesicles are not sufficiently monodisperse in most of the systems. These cubic phases of vesicles have similar rheological properties to the other cubic phases, and the values for Go and Gy are also considerably higher than for the normal vesicle phases. 

The main difference between the phases of these two limiting systems lies in the nature of the bilayer film, which for the dilute systemsl-5 usually contains cosurfactant in addition to surfactant molecules. From a geometric view point, the addition of cosurfactant (e.g. pentanol) molecules (with a cosurfactant/surfactant ratio of about 3/1) leads effectively to a significantly thinner membrane (5 = 20A) than the pure systems which are rigid. (For DMPC 6 35A). 

Viscosity studies have also been carried out to investigate the effect of the surfactant and cosurfactant concentrations as well as the surfactant-cosurfactant mass ratio on the hydration of the disperse-phase droplets for o/w ME systems [58], A... 

The diffusion experiments for the nonsalt compositions (Fig. IE) showed a fast equilibration of the surfactant concentration with equal concentration of surfactant in the entire system after 30 days at the lowest surfactant/(cosurfactant + surfactant) weight ratio, 0.14, Fig. 2A. Thereafter the concentration in the lower part was higher than in the upper part, a fact that is to be viewed against the former low cosurfactant concentration. Fig. 2B, and its high water content. Fig. 2C. The increase in water content in the upper layers ceased after 20 days. Fig. 2C, at the time when the liquid crystal began to form in layer 6, Fig. 3. During the first 7 days the aqueous solution was turbid and an interface appeared within the oil phase. Fig. 3. This interface moved upwards in the oil phase and disappeared after 36 days. 

The hydrocarbon system was combined with water to form the W/0 microemulsions marked in Fig. 15. The surfactant/(cosurfactant + surfactant) weight ratio 0.25 gave a liquid crystal with initial fast extension for three days followed by a new fast growth between 10 and 17 days and a subsequent decline to zero in 40 days. Fig. 16. These changes were reflected in the concentration changes in the layers around the liquid crystal (Figs. 17A-D). The water concentration showed a rapid growth in layer 3, the first three days caused by a reduction of the surfactant, cosurfactant and the hydrocarbon content. 

Previous work, on the use of a reverse-micelle system for the production of tryptophan reported kinetic data obtained under various conditions (2). Both the water-to-surfactant ratio and the cosurfactant used influenced tryptophan production. The present work reports results from EPR studies of the effect of these parameters on both the water and the enzyme in the reverse micelle. EPR spectra of Mn(H20)g + were recorded to investigate the state of the water in reverse micelles. A nitroxide spin label that reacts with lysine residues was employed to probe the microstructure of tryptophanase in reverse micelles of different w0 values. 

An additional argument for a distinction between micelles and microemulsions is that in all the literature on the solubilization of hydrocarbons, dyes, and other substances in micellar solutions, the ratio of solubilized molecules to surfactant molecules very rarely exceeds, or even approaches, 2. Many microemulsion systems, on the other hand, have been described in which the dispersed phase surfactant (and cosurfactant) ratio exceeds 100 Because of the relatively low ratios of additive to surfactant obtainable in micellar systems, it is clear that there can exist no oil phase that can be considered separate from the body of the micelle. In many microemulsions, however, the size of the droplet and the high additive surfactant ratio requires that there be a core of dispersed material that will be essentially equivalent to a separate phase of that material. The seemingly obvious conclusion is that microemulsion systems (in the latter case, at least) possess an interfacial region composed primarily of surfactant (and cosurfactant), analogous to that encountered in macroemulsions. 

TABLE 1 Conductometricaily Estimated Water Pool Size Overall Droplet Dimension (R ), Surfactant Aggregation Number (A,), and Cosurfactant Aggregation Number (A,)per Droplet tor a Number of W O Microemulsions at 293 K at Constant Weight Fraction of Water = 0.2 and Surfactant Cosurfactant Weight Ratio = 0.5... 

It should be noted that the adsorption experiment described above involves systems that only contain surfactant, cosurfactant, brine and reservoir rock. There is no oil present in the system, and this is the reason why the ratio of cosurfactant to surfactant is higher than usually reported by other researchers. It was found that the higher ratio was necessary for complete dissolution of surfactants in the brine since a lower alcohol/surfactant ratio caused the solutions to become cloudy. This latter condition resulted in substantial increases in surfactant retention in flow experiments. The observation of the relationship between the solution condition and retention lead to experiments which could better define the phenomenon. 

In Figure 15.15, corrosion inhibition efficiency data are particularly shown for systems containing the surfactant ARIS in structurally distinct media. The metal surface affected was API5LX Gr X42 steel. The surfactant was previously dissolved in NaCl aqueous solutions, at 0.5 M or 1.0 M salt concentrations. The tests were carried out either using micellar or microemulsion systems, at 30°C and 60°C. The microemulsion system was prepared with the surfactant NaCl aqueous solutions, butan-l-ol as cosurfactant CIS ratio = 1.0) and kerosene as oil phase. 

Rather than fix the water/salt ratio as a brine pseudocomponent, another useful possibility is to fix the alcohol/ionic surfactant ratio as the pseudo-component (fix 8) and vary the salt concentration. An increase of the lyotropic salt concentration in ionic surfacant plus alcohol cosurfactant systems has the same effect as increasing temperature or salt in nonionic surfactant mixtures - a lipophilic shift is observed, and the phase behaviour progresses from 2 to 3 to 2 (15). If salt is placed in the position occupied previously by the cosurfactant in Figure 4.8, and the fixed ratio of alcohol/ionic surfactant placed as a combined pseudocomponent (fixed 5) at the surfactant position, at equal amounts of oil and water (a = 0.5) a plot of salt concentration (e) versus overall cosurfactant/surfactant concentration (y) also yields a fish -shaped phase diagram (45). Therefore, in either the case of fixed salt concentration (fixed ) or the case of fixed alcohol/ionic surfactant ratio (fixed 8), the optimally formulated microemulsions for the chosen fixed ratio of 8 or s, are found at X, where the tail and body of the fish meet (see Figure 4.8). Consequently, the phase behaviour of simple monodisperse ethoxylated alcohol surfactants in oil and water qualitatively mimics that of much more complicated mixtures containing ionic surfactants, cosurfactants and salt. Alcohol... 

Figure 10.9. Double logarithmic plots of the zero shear viscosity rjo of a mixture of C14DMAO and CiqOH, with different molar ratios of cosurfactant/surfactant, against the concentration of C14DMAO at 25°C...

In order to illustrate the phase behaviour of microemulsions, it is most convenient to consider the systems with nonionic surfactants of the ethylene oxide type. These have been studied extensively by Shinoda and Kunieda and co-workers and by Kahlweit and Strey and co-workers (for more recent reviews, see, e.g. refs (9) and (10)). At low surfactant concentrations, there is a general sequence of phase equilibria, often referred to as Winsor equilibria (11). The equilibrium conditions for the microemulsion phase, L, changes from equilibrium with excess oil (Winsor I) to equilibrium with excess water (Winsor II), via a three-phase equilibrium with excess water and oil (Winsor III). For nonionics, this sequence occurs when increasing the temperature, while for quaternary or ternary systems, it can be observed with increasing salinity or cosurfactant-to-surfactant ratio. 

The vesicle phase occurs within a wide range of the total surfactant and cosurfactant concentration but only within a small range of the molar ratio of cosurfactant/surfactant around 1 1. If the surfactant bilayers are charged by the addition of an ionic surfactant the phase diagram becomes somewhat simpler because some mesophases are suppressed by the charge. As can be seen from the Fig. 11.11, the vesicle phase is still found under these conditions but it is shifted towards higher cosurfactant concentrations. Thermodynamically stable vesicles have also been foimd in ternary systems of non-ionic alkylpolyglycol surfactants, cosurfactants, and water [10] and also in the binary system of the double-chain surfactant didodecyldimethylammoniumbromide (DDABr) and water [42]. 

Fig. 4.40 Pseudoternary-phase diagram of the system CyFisCOONa (1)-C3 F7CH2OH (2)-C6Fi4 (3) for a constant surfactant/cosurfactant weight ratio of 1 1 (molar ratio 1 2.2) at 23°C. Mi and M2 are the regions of respective W/0 and O/W microemulsions. (From Ref. 122. Reproduced by permission of Verlag Helvetica Chimica Acta.)...

The reverse microemulsion method can be used to manipulate the size of silica nanoparticles [25]. It was found that the concentration of alkoxide (TEOS) slightly affects the size of silica nanoparticles. The majority of excess TEOS remained unhydrolyzed, and did not participate in the polycondensation. The amount of basic catalyst, ammonia, is an important factor for controlling the size of nanoparticles. When the concentration of ammonium hydroxide increased from 0.5 (wt%) to 2.0%, the size of silica nanoparticles decreased from 82 to 50 nm. Most importantly, in a reverse microemulsion, the formation of silica nanoparticles is limited by the size of micelles. The sizes of micelles are related to the water to surfactant molar ratio. Therefore, this ratio plays an important role for manipulation of the size of nanoparticles. In a Triton X-100/n-hexanol/cyclohexane/water microemulsion, the sizes of obtained silica nanoparticles increased from 69 to 178 nm, as the water to Triton X-100 molar ratio decreased from 15 to 5. The cosurfactant, n-hexanol, slightly influences the curvature of the radius of the water droplets in the micelles, and the molar ratio of the cosurfactant to surfactant faintly affects the size of nanoparticles as well. 

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