Surfactant cosurfactant system

Various initiation strategies and surfactant/cosurfactant systems have been used. Early work involved in situ alkoxyamine formation with either oil soluble (BPO) or water soluble initiators (persulfate) and traditional surfactant and hydrophobic cosurfactants. Later work established that preformed polymer could perform the role of the cosurfactant and surfactant-free systems with persulfate initiation were also developed, l90 222,2i3 Oil soluble (PS capped with TEMPO,221 111,224 PBA capped with 89) and water soluble alkoxyamines (110, sodium salt""4) have also been used as initiators. Addition of ascorbic acid, which reduces the nitroxide which exits the particles to the corresponding hydroxylamine, gave enhanced rates and improved conversions in miniemulsion polymerization with TEMPO.225 Ascorbic acid is localized in the aqueous phase by solubility. 

The interesting behaviour observed for aggregate systems of surfactant and BE requires a broader investigation. Systematic thermodynamic and ultrasonic absorption studies of systems where the number of EO units in the alcohol is increased are currently in progress. The information obtained should be of benefit in extending the commercial application of these surfactant-cosurfactant systems. 

Johnston et al. l also examined the solvatochromic shift of pyridine N-oxide in an ethane/CjEj (C = 10-13 E = 5) water-in-oil microemuision, also in equilibrium with a lower liquid phase. Contrary to the behavior exhibited by the AOT system, the nonionic microemulsions display a polar environment at low pressures, which becomes progressively less polar as pressure increases. At a pressure of only 50 bar, they reported that the probe s environment resembles that observed in bulk hexane. Added water increases the polarity somewhat, yet a cosurfactant (octanol) is required to produce an environment similar to that in bulk water. The polarity of the ethane/ water/surfactant/cosurfactant system remains essentially constant as pressure increases up to 350 bar. 

Solution Work. Results of measurements of enthalpies of the surfactants are shown in Figures 1 through 7. The observed critical micelle concentrations are tabulated in Table I. For several surfactant-cosurfactant systems, the surfactant was not sufficiently soluble to allow determination of the critical micelle concentration. 

With the surfactant-cosurfactant system, it has been observed (6) that the best oil displacement efficiency is achieved when the surfactant system spontaneously emulsifies with the oil, followed by rapid coalescence of the emulsified oil droplets (2). 

Miniemulsions are typically formed by subjecting the oil/water/surfactant/ cosurfactant system to a high shear field created by devices such as an ultra-sonifier, the Manton Gaulin homogenizer and the Microfluidizer. These rely tn mechanical shear and/or cavitation to break the oil phase into submicron size droplets. When monomer is used as the oil phase, free radical polymerizatim can subsequently be carried out by addition of an initiator (e.g. potassium persulfate, KPS). 

Batch miniemulsion polymerizations aiVAc/BA were investigated using SO/50 and 25/75 molar ratios of the two monomers, SHS/HD as the surfactant/ cosurfactant system and anunonium persulfate as initiator [6]. The polymeriza-ticxis were conducted at 60°C with 25% solids fiamulations. The miniemulsion droplets were created using ultrasonificatioiL The polymerizations were c iarac-terized their reaction kinetics, copolyma compositions and propoties. and final particle size distributions. Parallel conventional emulsion polymmzations (i.e. no cosurlactant, no high shear) were conducted for comparison. 

Extensive studies have been reported by Kunieda s group regarding the formation of worm-like micelles and micellar transient networks in water-surfactant-cosurfactant systems. However, for applications, it is also relevant to know the effect of additives on systems containing worm-hke micelles. It is reported that oils induce a rod-sphere transition in surfactant micellar solutions, leading to a reduction in viscosity [32]. Kunieda s group studied the solubilization of different oils in wormlike micellar solutions [19, 33]. The amount of solubilized oil, its location within the micelle, and its effect on micellar shape and size demonstrated to strongly depend on the nature of the oil and its interactions with the surfactants. 

Li and L2 can be described by considering the phase diagram of a ternary system of water-surfactant-cosurfactant system, as shown in Figure 13.20 for water-ionic sulphate-long-chain alcohol system. 

In most instances the cosurfactant was a medium-chain length alcohol. The topic has been recently reviewed, and only the main features are presented here. Note that studies of the dynamics of ternary water/surfactant/cosurfactant systems... 

Microemulsions are composed of two mutually immiscible liquid phases, one spontaneously dispersed in the other with the assistance of one or more surfactants and cosurfactants. While microemulsions of two nonaqueous liquids are theoretically possible (e.g., fluorocarbon/hydrocarbon systems), almost all the reported work is concerned with at least one aqueous phase. The systems may be water-continuous (OAV) or oil-continuous (W/0), as illustrated in Figure 5.11 the result is determined by variables such as the surfactant/cosurfactant system employed, temperature, electrolyte levels, the chemical nature of the oil phase, and the relative ratios of the components. 

Electrical double-layer effects favor O/W systems. As a result, the addition of electrolyte will tend to push the same surfactant/cosurfactant system toward the formation of W/O microemulsions. 

Eigure 6 illustrates how the three tensions among the top, middle, and bottom phases depend on temperature for a system of nonionic surfactant—oil—water (38), or on salinity for a representative system of anionic surfactant—cosurfactant—oil—water and electrolyte (39). As T approaches from lower temperatures, the composition of M approaches the composition of T, and the iaterfacial teasioa betweea them, goes to 2ero at T =. ... 

A microemulsion droplet is a multicomponent system containing oil, surfactant, cosurfactant, and probably water therefore there may be considerable variation in size and shape depending upon the overall composition. The packing constraints which dictate size and shape of normal micelles (Section 1) should be relaxed in microemulsions because of the presence of cosurfactant and oil. However, it is possible to draw analogies between the behavior of micelles and microemulsion droplets, at least in the more aqueous media. 

A microemulsion is defined as a thermodynamically stable and clear isotropic mixture of water-oil-surfactant-cosurfactant (in most systems, it is a mixture of short-chain alcohols). The cosurfactant is the fourth component, which effects the formation of very small aggregates or drops that make the microemulsion almost clear. 

Microemulsions are thermodynamically stable mixtures. The interfacial tension is almost zero. The size of drops is very small, and this makes the microemulsions look clear. It has been suggested that microemulsion may consists of bicontinuous structures, which sounds more plausible in these four-component microemulsion systems. It has also been suggested that microemulsion may be compared to swollen micelles (i.e., if one solubilizes oil in micelles). In such isotropic mixtures, short-range order exists between droplets. As found from extensive experiments, not all mixtures of water-oil-surfactant-cosurfactant produce a microemulsion. This has led to studies that have attempted to predict the molecular relationship. 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

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... 

One major concern regarding the safety profile of ME systems intended for oral administration is the comparatively high amphiphile content. Both o/w and w/o ME systems are amphiphile-rich systems compared to conventional emulsions and would contain in the most conservative case up to 15-20% w/w surfactant-cosurfactant. This is further complicated by the limited models available to evaluate chronic toxicology in comparison to conventional oral dosage forms such as tablets [91]. 

In particular, contacting studies where two phases of varied constituents (surfactant, cosurfactant, oil, water, electrolyte) are in contact and results interpreted on the basis of mass transfer and phase diagrams have become the standard method for studying transport in such systems (10-18). 

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. 

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