Mixtures of surfactants

Practical systems will often have mixtures of surfactants. A useful rule is that the CMC of the mixture. Cm, is given by [196,197]... 

As the example indicates, these liquids contained a complex mixture of surfactant actives. The LAS used in these products was of higher molecular weight (C,2 5-i3 average carbon chain length) than previous laundry liquids. Typical LAS active levels were 12% or less, presumably due to solubility constraints. 

To minimize the quantitative input of surfactants in consumer products, synergistic properties of suitably composed mixtures of surfactants can be used [54]. Therefore, knowledge of the structure-performance behavior of the single surfactants and their mixtures in the bulk and at the interfaces is important. 

Dispersant compositions for the treatment of oil spills at the surface of the water consist of a mixture of water, a hydrocarbon solvent, and a mixture of surfactants consisting of 55% to 65% by weight of emulsifiers and 35% to 45% by weight of dioctyl sodium sulfosuccinate. The emulsifying agents consist of a mixture of various sorbitan oleates [351-354]. 

Monomer/Micelle Equilibrium Mixtures of surfactants, like any surfactant species in an aqueous solution, give rise to monomer or micelle aggregates provided that the concentration reaches a minimum value, called the critical micellar concentration (CMC). The micelles thus formed are mixed, i.e. made up of the different surfactant species in solution. 

Wade, W.H., et al. "Low Interfacial Tensions Involving Mixtures of Surfactants," SPE paper... 

Wade, W.H., Schechter, R.S. and Jacobson, J.K. "Low Interfacial Tensions Involving Mixtures of Surfactants," SPE paper 6002, 1976 SPE Annual Technical Conference and Exhibition, New Orleans, October 3-6. 

The effect of mixtures of surfactants and polyelectrolytes on spontaneous, water-catalysed hydrolysis (Fadnavis and Engberts, 1982) was mentioned in Section 4, but mixtures of functionalized polyelectrolytes and cationic surfacants are effective deacylating agents (Visser et al., 1983). Polymerized isocyanides were functionalized with an imidazole group and the deacylation of 2,4-dinitrophenyl acetate in the polyelectrolyte was speeded by addition of single or twin chain quaternary ammonium ion surfactants, up to a plateau value. Anionic surfactants had essentially no effect. It is probable that the cationic surfactants accelerate the reaction by increasing the deprotonation of the imidazole groups. 

These microdroplets can act as a reaction medium, as do micelles or vesicles. They affect indicator equilibria and can change overall rates of chemical reactions, and the cosurfactant may react nucleophilically with substrate in a microemulsion droplet. Mixtures of surfactants and cosurfactants, e.g. medium chain length alcohols or amines, are similar to o/w microemulsions in that they have ionic head groups and cosurfactant at their surface in contact with water. They are probably best described as swollen micelles, but it is convenient to consider their effects upon reaction rates as being similar to those of microemulsions (Athanassakis et al., 1982). 

Commercial mixtures of surfactants comprise several tens to hundreds of homologues, oligomers and isomers of anionic, nonionic, cationic and amphoteric compounds. Therefore, their identification and quantification in the environment is complicated and cumbersome. The requirement of more specific analytical methods has prompted a replacement of many of the separate steps in traditional methods of analysis, usually non-chromatographic, by chromatographic tools. 

Gas chromatography (GC) has developed into the most powerful and versatile analytical separation method for organic compounds nowadays. A large number of applications for the analysis of surfactants have emerged since the early 1960s when the first GC papers on separation of non-ionics were published. The only major drawback for application of GC to surfactants is their lack of volatility. This can be easily overcome by chemical modification (derivatisation), examples of which will be discussed extensively in the following paragraphs. This chapter focuses on surfactant types, and in addition discusses some structural aspects of alkylphenol ethoxylates (APEOs) that are important for, as well as illustrative of, aspects of separation and identification that are linked to the complexity of the mixtures of surfactants that are involved. 

Commercial mixtures of surfactants consist of several tens to hundreds of homologues oligomers and isomers. Their separation and quantification is complicated and a cumbersome task. Detection, identification and quantification of these compounds in aqueous solutions, even in the form of matrix-free standards, present the analyst with considerable problems. The low volatility and high polarity of some surfactants and their metabolites hamper the application of gas-chromatographic (GC) methods. GC is directly applicable only for surfactants with a low number of ethylene oxide groups and to some relatively volatile metabolic products, while the analysis of higher-molecular-mass oligomers is severely limited and requires adequate derivatisation. 

Good cleaning power Use a mixture of surfactants to better match the required HLB0p... 

The surfactant selected for CPE technique should not have too high a cloud point temperature. In practice, it is possible to obtain almost any desired temperature by choosing an appropriate mixture of surfactants, as cloud point temperatures of mixtures of surfactants are intermediate between those of the two pure surfactants, or by the choice of an appropriate additive (i.e., salts, alcohols, organic compounds) [105]. 

The property of interest to characterize a surfactant or a mixture of surfactants is its hydrophilic-lipophilic tendency, which has been expressed in many different ways through a variety of concepts such as the hydrophiUc-lipophilic balance (HLB), the phase inversion temperature (PIT), the cohesive energy ratio (CER), the surfactant affinity difference (SAD) or the hydrophilic-lipophilic deviation (HLD) [1], which were found to be more or less satisfactory depending on the case. In the next section, the quantification of the effects of the different compounds involved in the formulation of surfactant-oil-water systems will be discussed in details to extract the concept of characteristic parameter of the surfactant, as a way to quantify its hydrophilic-lipophilic property independently of the nature of the physicochemical environment. 

The same principle may be extended to characterize mixtures of surfactants. If the case of anionic surfactants is still taken as an example, and if... 

Wade WH, Morgan JC, Jacobson JK, Schechter RS (1977) Low interfacial tension involving mixtures of surfactants. Soc Petrol Eng J 17 122... 

An alternative to the injection method for importing enzymes into a microemulsion is the phase transfer method. In this method, a layer of an aqueous enzyme solution is located under a mixture of surfactant and oil. Upon gentle shaking, the enzyme is transferred into the reverse micelles of the hydrocarbon phase. Finally, the excess of water is removed and the hydrophobic substrates can be added. The main advantage of this method is that it ensures thermodynamically stable micro emulsions with maximum water concentrations. However, the method is very time consuming. The method is often applied in order to purify, concentrate or renaturate enzymes in the reverse micellar extraction process [54-58]. 

The effect of using mixtures of surfactants on micelle formation, monolayer formation, solubilization, adsorption, precipitation, and cloud point phenomena is discussed. Mechanisms of surfactant interaction and some models useful in describing these phenomena are outlined. The use of surfactant mixtures to solve technological problems is also considered. 

In a previous publication ( ), results were presented on the micellar properties of binary mixtures of surfactant solutions consisting of alkyldimethylamine oxide (C12 to Cig alkyl chains) and sodium dodecyl sulfate. It was reported that upon mixing, striking alteration in physical properties was observed, most notably in the viscosity, surface tension, and bulk pH values. These changes were attributed to 1) formation of elongated structures, 2) protonation of amine oxide molecules, and 3) adsorption of hydronium ions on the mixed micelle surface. In addition, possible solubilisation of a less soluble 1 1 complex, form between the protonated amine oxide and the long chain sulfate was also considered. 

Molecular Interaction and Synergism in Binary Mixtures of Surfactants... 

During the past few years, the determination of the interfacial properties of binary mixtures of surfactants has been an area in which there has been considerable activity on the part of a number of investigators, both in industry and in academia. The Interest in this area stems from the fact that mixtures of two different types of surfactants often have interfacial properties that are better than those of the individual surfactants by themselves. For example, mixtures of two different surface-active components sometimes reduce the interfacial tension at the hydrocarbon/water interface to values far lower than that obtained with the individual surfactants, and certain mixtures of surfactants are better foaming agents than the individual components. For the purpose of this discussion we define synergism as existing in a system when a given property of the mixture can reach a more desirable value than that attainable by either surface-active component of the mixture by itself. 

Synergism in surface tension reduction efficiency. The efficiency of surface tension reduction by a surfactant is defined (9) as the solution phase concentration required to produce a given surface tension (reduction). Synergism in this respect is present in a binary mixture of surfactants when a given surface tension (reduction) can be attained at a total mixed surfactant concentration lower than that required of either surfactant by itself. This is illustrated in Figure 2. 

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