Critical surfactant mixtures

A. Ciach, J. S. Hoye, G. Stell. Microscopic model for microemulsion. II. Behavior at low temperatures and critical point. J Chem Phys 90 1222-1228, 1989. A. Ciach. Phase diagram and structure of the bicontinuous phase in a three dimensional lattice model for oil-water-surfactant mixtures. J Chem Phys 95 1399-1408, 1992. 

The critical micellar concentrations of anionic/nonionic surfactant mixtures examined are low in a saline medium, so that, at the concentrations injected in practice, the chromatographic effects resulting from the respective adsorption of monomers are masked. Such surfactants propagate simultaneously in the medium in the form of mixed micelles. 

The deviations from the Szyszkowski-Langmuir adsorption theory have led to the proposal of a munber of models for the equihbrium adsorption of surfactants at the gas-Uquid interface. The aim of this paper is to critically analyze the theories and assess their applicabihty to the adsorption of both ionic and nonionic surfactants at the gas-hquid interface. The thermodynamic approach of Butler [14] and the Lucassen-Reynders dividing surface [15] will be used to describe the adsorption layer state and adsorption isotherm as a function of partial molecular area for adsorbed nonionic surfactants. The traditional approach with the Gibbs dividing surface and Gibbs adsorption isotherm, and the Gouy-Chapman electrical double layer electrostatics will be used to describe the adsorption of ionic surfactants and ionic-nonionic surfactant mixtures. The fimdamental modeling of the adsorption processes and the molecular interactions in the adsorption layers will be developed to predict the parameters of the proposed models and improve the adsorption models for ionic surfactants. Finally, experimental data for surface tension will be used to validate the proposed adsorption models. 

Shiloach, A., and D. Blankschtein. 1997. Prediction of critical micelle concentrations and synergism of binary surfactant mixtures containing zwitterionic surfactahtengmuirl3 3968-3981. 

One of the medical problems already raised in sec. I.l, is that premature babies may not have produced enough of the surfactant mixtures, leading to so-called respiratory distress syndrome (RDS), This disease is nowadays readily overcome by letting the baby inhale a spray containing the mixture. The remedy is facilitated by the non-specificity of the mixture the composition is not as critical as it would have been in say, implants. 

The solubility of n-decanol in the LI phase is also important (up to 12% at the end of the LI phase). The LI phase is accountable for the observation of oil-inwater (o/w) microemulsions. The La domain, generally located in the middle of the diagram, points toward the water side for a critical surfactant-to-cosurfactant ratio. (A 1 2 sodium octanoate to n-decanol ratio leads to a lamellar phase with as little as 17% surfactant-cosurfactant mixture.) In some cases, such as for octyl trimethylammonium bromide (OTAB)-hexanol-water, the lamellar phase already exists for 3% hexanol + 3% OTAB ... 

When surfactants dissolve in water at low concentrations they exist as monomers (ionic surfactants are dissociated). As the concentration is increased aggregates termed micelles are formed. These appear at a well-defined concentration known as the critical micelle concentration (CMC). This is not a critical point in the sense of modern physics since micelle formation occurs over a very narrow range of concentrations. This range is so small that for almost all practical purposes it can be represented by a specific value, the CMC. For pure single surfactants below the CMC, all of the dissolved surfactant exists as monomers, while above the CMC all added surfactant forms micelles. With surfactant mixtures the phenomenon is more complex because the components usually have different individual CMCs, but the same considerations apply [2]. 

The thermodynamic modeling of microemulsions has taken various lines and gave conflicting results in the period before the thermodynamic stability and microstructure were established. It was early realized that a maximal solubilization of oil and water simultaneously could be discussed in terms of a balance between hydrophilic and lipophilic interactions the surfactant (surfactant mixture) must be balanced. This can be expressed in terms of the HLB balance of Shinoda,Winsor s R value, and a critical packing parameter (or surfactant number), as introduced to microemulsions by Israelachvili et al. [37], Mitchell and Ninham [38], and others. The last has become very popular and useful for an understanding of surfactant aggregate structures in general. 

Between these systems and the so-called microenudshns a clear differentiation must be made. The microemulsions are often wrongly described as parenteral fat emulsions (142) and represent—from the physical point of view—a completely different. system. Microemuisions are thermodynamically. stable, single phase, transparent or opalescent, liquid, and in unsheared state arc optically isotropic systems consisting of several liquids that cannot be mixed together and are produced with the aid of surfactants or surfactant mixtures (88). The physical structure of these systems is the object of many investigations (5U6.64. 84,103,152), in which they are described either as swollen micelles or as critical solutions (47). 

Since alkyl betaines may exist as either zwitte-rionic or cationic surfactants, depending on the pH, their critical micelle concentration shows a complicated behaviour. The CMCs of alkyl betaines are significantly higher in dilute acid solutions than in dilute alkaline solutions because at low pH the surfactant is at least partly in the cationic form (43-45). As the concentration of HCl increases, the amount of the cationic form in solution increases and the resulting CMC of the zwitte-rionic/cationic surfactant mixture increases because the value of the cationic form is higher than that of the... 

Anionic-cationic surfactant mixtures make the potential of the bubble close to zero over the wide mid-range composition, where the critical micelle concentration corresponds to the catanionic species. 

The critical association concentration between a polymer and a surfactant is defined as the free surfactant concentration at which the cooperative adsorption is initiated [16]. This concentration, that depends on ionic strength [3, 27] and polyelectrolyte concentration [28] can be determined from e.g. the adsorption isotherm. For the polyelectrolyte-surfactant mixtures studied here we have not yet determined the adsorption isotherm but instead we estimate an upper limit of the critical association concentration in bulk solution (caCb) as the total (i.e. bound -t- free) surfactant concentration needed to give a significant increase in turbidity due to formation of large floes in the polyelectrolyte-surfactant solution. The values obtained for the 100%, 30% and 10% charged polyelectrolyte (20 ppm polyelectrolyte solution, 0.1 mMKBr as background electrolyte) were about 0.005, 0.01 and 0.01 cmc. The free surfactant concentration at caCb is thus lower than these values. 

There have been developments in understanding how the type of emulsion is related to the phase diagram of mixtures of oil + water + surfactant at equilibrium. These have come from the studies with microemulsions and as an example we take the case of a nonionic surfactant of the polyoxyethylene glycol ether type, C.E,.. Let us consider equilibrium systems of heptane and water (equal volumes) containing C12E5. At low [surfactant], monomer distributes between oil and water but heavily in favour of the oil. The partition coefficient defined as (molar concentration in heptane)/(molar concentration in water) increases from 130 at 10 C to —1500 at 50 C. Above a critical surfactant concentration, reached in both phases and designated (typically 5 X 10 m) and c Coji (typically... 

As already mentioned, it is not easy to find systems where the mixture of surfactants has the proper HLB to promote the change in curvature by the sole addition of water. Sajjadi [19] proposes a PIC method where the water containing the hydrophilic surfactant is added to the oil containing the lipophilic surfactant. Then, the change in curvature is more easily reached, as the HLB of the surfactant mixture increases continuously by the addition of the aqueous solution containing the hydrophilic surfactant, and the formulation of the system is not so critical. However, they obtain droplet sizes of the order of 0.5-1 im, which are quite big droplets compared with the other works already discussed. 

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