Mixed surfactant micelle

The interactions between azo oil dye and mixed surfactant systems will be dependent on the difference in mixed surfactant micelle due to different molecular characteristic of nonionic surfactant. 

Another approach has been intensively developed recently by Puvvada and Blankschtein, who considered also the formation of mixed micelles in surfactant mixtures [20-22]. For example, the free energy of formation of binary mixed surfactant micelle, containing Ha molecules of the surfactant A, ns molecules of the surfactant B and, n water moleeules in thermodynamic equilibrium was expressed as... 

A recent study on electrokinetic removal of ethyl benzene from contaminated clay showed a promising use of anionic-nonionic mixed surfactants. Surfactant addition resulted in 1.6-2.4-fold more removal than afforded by EK alone, and a mixed surfactant system, including 0.5% SDS and 2.0% PANNOX 110 (nonyl phenol polyethylene glycol ether) permitted optimal ethyl benzene removal (98%). This indicated that, in the presence of mixed surfactant micelles, the zeta potential of the soil particles significantly increased compared with that seen when anionic surfactant micelles were formed, and electrolytic mobility was thus enhanced (Yuan and Weng, 2004). The use of anionic-nonionic mixed surfactants in EK will also improve the desorption and migration of PAHs. 

Small micelles in dilute solution close to the CMC are generally beheved to be spherical. Under other conditions, micellar materials can assume stmctures such as oblate and prolate spheroids, vesicles (double layers), rods, and lamellae (36,37). AH of these stmctures have been demonstrated under certain conditions, and a single surfactant can assume a number of stmctures, depending on surfactant, salt concentration, and temperature. In mixed surfactant solutions, micelles of each species may coexist, but usually mixed micelles are formed. Anionic-nonionic mixtures are of technical importance and their properties have been studied (38,39). 

An aqueous dispersion of a disperse dye contains an equilibrium distribution of solid dye particles of various sizes. Dyeing takes place from a saturated solution, which is maintained in this state by the presence of undissolved particles of dye. As dyeing proceeds, the smallest insoluble particles dissolve at a rate appropriate to maintain this saturated solution. Only the smallest moieties present, single molecules and dimers, are capable of becoming absorbed by cellulose acetate or polyester fibres. A recent study of three representative Cl Disperse dyes, namely the nitrodiphenylamine Yellow 42 (3.49), the monoazo Red 118 (3.50) and the anthraquinone Violet 26 (3.51), demonstrated that aggregation of dye molecules dissolved in aqueous surfactant solutions does not proceed beyond dimerisation. The proportion present as dimers reached a maximum at a surfactant dye molar ratio of 2 5 for all three dyes, implying the formation of mixed dye-surfactant micelles [52]. 

Among the purposes of this paper is to report the results of calorimetric measurements of the heats of micellar mixing in some nonideal surfactant systems. Here, attention is focused on interactions of alkyl ethoxylate nonionics with alkyl sulfate and alkyl ethoxylate sulfate surfactants. The use of calorimetry as an alternative technique for the determination of the cmc s of mixed surfactant systems is also demonstrated. Besides providing a direct measurement of the effect of the surfactant structure on the heats of micellar mixing, calorimetric results can also be compared with nonideal mixing theory. This allows the appropriateness of the regular solution approximation used in models of mixed micellization to be assessed. 

Calorimetric measurements can be used to obtain heats of mixing between different surfactant components in nonideal mixed micelles and assess the effects of surfactant structure on the thermodynamics of mixed micellization. Calorimetry can also be successfully applied in measuring the erne s of nonideal mixed surfactant systems. The results of such measurements show that alkyl ethoxylate sulfate surfactants exhibit smaller deviations from ideality and interact significantly less strongly with alkyl ethoxylate nonionics than alkyl sulfates. 

The mixed cmc behavior of these (and many other) mixed surfactant systems can be adequately described by a nonideal mixed micelle model based on the psuedo-phase separation approach and a regular solution approximation with a single net interaction parameter B. However, the heats of micellar mixing measured by calorimetry show that the assumptions of the regular solution approximation do not hold for the systems investigated in this paper. This suggests that in these cases the net interaction parameter in the nonideal mixed micelle model should be interpreted as an excess free energy parameter. 

Fig.1 Formation of reverse micelles in a self-assembled mixed surfactant system. The addition of water tends to link these droplets to form a highly viscous bi-continuous microemulsion with aqueous and isooctane nanochannels separated by the surfactants...

The adsorption of mixed surfactants at the air—water interface (monolayer formation) is mechanistically very similar to mixed micelle formation. The mixed monolayer is oriented so that the surfactant hydrophilic groups are adjacent to each other. The hydrophobic groups are removed from the aqueous environment and are in contact with other hydrophobic groups or air. Therefore, the forces tending to cause monolayers to form are similar to those causing micelles to form and the thermodynamics and interactions between surfactants are similar in the two aggregation processes. 

Equation 1 has proved to be a better predictor of the equilibrium which exists between monomer and micelles for mixed surfactant systems than is the regular solution theory model. It also predicts well the mixture CMC and shows the heat of mixing to be smaller than that predicted by the regular solution theory in agreement with the experiment (13). The purpose of this paper is to further explore... 

Mixed surfactant systems are of importance from a fundamental and practical point of view. Therefore, many recent papers have reported on the micellar properties of mixed surfactant solutions. For example, Tokiwa et al. have measured the NMF spectra W Ingram has measured surface tension ( 5). Previously, we have reported the solution properties of anionic-nonlonlc surfactant mixed systems from the point of view of electrical (., 7) and surface tension measurements (8-10), and investigated the mixed micelle formation. 

In this paper, we report the solution properties of sodium dodecyl sulfate (SDS)-alkyl poly(oxyethylene) ether (CjjPOEjj) mixed systems with addition of azo oil dyes (4-NH2, 4-OH). The 4-NH2 dye interacts with anionic surfactants such as SDS (11,12), while 4-OH dye Interacts with nonionic surfactants such as C jPOEn (13). However, 4-NH2 is dependent on the molecular characteristics of the nonionic surfactant in the anlonlc-nonlonic mixed surfactant systems, while in the case of 4-OH, the fading phenomena of the dye is observed in the solubilized solution. This fading rate is dependent on the molecular characteristics of nonionic surfactant as well as mixed micelle formation. We discuss the differences in solution properles of azo oil dyes in the different mixed surfactant systems. 

In mixed surfactant systems, physical properties such as the critical micelle concentration (cmc) and interfacial tensions are often substantially lower than would be expected based on the properties of the pure components. Such nonideal behavior is of both theoretical interest and industrial importance. For example, mixtures of different classes of surfactants often exhibit synergism (1-3) and this behavior can be utilized in practical applications ( ).In addition, commercial surfactant preparations usually contain mixtures of various species (e.g. different isomers and chain lengths) and often include surface active impurities which affect the critical micelle concentration and other properties. 

The purpose of this paper will be to develop a generalized treatment extending the earlier mixed micelle model (I4) to nonideal mixed surfactant monolayers in micellar systems. In this work, a thermodynamic model for nonionic surfactant mixtures is developed which can also be applied empirically to mixtures containing ionic surfactants. The form of the model is designed to allow for future generalization to multiple components, other interfaces and the treatment of contact angles. The use of the pseudo-phase separation approach and regular solution approximation are dictated by the requirement that the model be sufficiently tractable to be applied in realistic situations of interest. 

The pseudo-phase separation approach has been successfully applied in developing a generalized nonideal multicomponent mixed micelle model (see I4) and it is Interesting to consider whether this same approach can be used to develop a generalized treatment for adsorbed nonideal mixed surfactant monolayers. The preferred form for suoh a model is that it be suitable (at least in principle) for treating multiple components and be extendable to other interfaoes and properties of interest suoh as oontaot angles. Earlier models (5, 18, 27) based on the pseudo-phase separation approach and... 

In mixed surfactant systems above the cmc the chemical potential of the ith component (in the mixed micelle) is given by... 

One interesting feature of the functional form derived here is the direct relationship of the activity coefficients and composition between the micellar and surface psuedo-phases. This allows a comparison of nonideal interactions in the micelle and monolayer as modeled by their respective net interaction parameters. In principle, this form may also allow extension to more complicated situations such as the treatment of contact angles in nonideal mixed surfactant systems. Here, the functional form derived above depends on differences in surface pressures and these may be directly obtained from experimentally measured parameters under the proper conditions (30). 

---