Surfactant aggregation model

The first meaningfnl stndies on the kinetics of dynamic processes within micellar solutions, which looked at dissociation rates via temperature-jump (T-jump) and related techniqnes, were carried out in the 1960s. The first notable attempt at a complete theory of micellization kinetics was the work of Kresheck et al. [77], who proposed a stepwise surfactant aggregation model based on a monodisperse system, where all micelles have the same aggregation nnmber, n. However, this model found... 

The fonnation of surface aggregates of surfactants and adsorbed micelles is a challenging area of experimental research. A relatively recent summary has been edited by Shanna [51]. The details of how surfactants pack when aggregated on surfaces, with respect to the atomic level and with respect to mesoscale stmcture (geometry, shape etc.), are less well understood than for micelles free in solution. Various models have been considered for surface surfactant aggregates, but most of these models have been adopted without finn experimental support. 

Equations 27 and 28 present the extension of the Szyszkowski-Langmuir model to the adsorption of one-surfactant systems with aggregation at the interface. For the formation of dimmers on the surface, n = 2 and Eqs. 27 and 28 can be expanded to obtain the Frumkin equation of adsorption state. In general, the surface aggregation model described by Eqs. 27 and 28 contains four free parameters, including coi, n, b and Fc, which can be obtained by regression analysis of the data for surface tension versus surfactant concentration in the solution. 

It is noted that the investigation of a mixed adsorption layer of CioEs and TPeAB (tetrapentyl ammoniiun bromide) [35] shows evidence for attractive forces / > 0), which suggests that the presence of the ionic surfactant can prevent aggregation in the extended S-L adsorption layer. Therefore, the main question of interest concerns how the Frumkin model and the aggregation model are related. One can find from Eq. 29 that the size of the elementary adsorption cell increases with the aggregation munber resulting in a reduction in the munber of cells. Negative has the same effect of de-... 

Micelles and monolayers composed of homologous mixtures of anionic surfactants can be approximately described by ideal solution theory to model the mixed surfactant aggregate (35). Therefore, surprising that mixed admicelles composed surfactants also obey ideal solution theory, important to note that this is true at all levels within Region II, as seen by the... 

The interaction between SDS and a more hydrophobic polymer, polyvinyl pyrrolidone, has been studied by 13C NMR spectroscopy. The results suggest a model similar to the polyethylene oxide-SDS system, in that the polymer molecules are wrapped around the surfactant aggregate.276 All the carbon atoms of the surfactant except the one closest to the head group undergo l3C chemical-shift changes that are accounted for on the basis of a change in the distribution of... 

Surfactant aggregates (microemulsions, micelles, monolayers, vesicles, and liquid crystals) are recently the subject of extensive basic and applied research, because of their inherently interesting chemistry, as well as their diverse technical applications in such fields as petroleum, agriculture, pharmaceuticals, and detergents. Some of the important systems which these aggregates may model are enzyme catalysis, membrane transport, and drug delivery. More practical uses for them are enhanced tertiary oil recovery, emulsion polymerization, and solubilization and detoxification of pesticides and other toxic organic chemicals. 

The synergisms of mixtures of anionic-cationic surfactant systems can be used to form middle-phase micro emulsions without adding short-chain alcohols [109, 110]. The surfactants studied were sodium dihexyl sulphosuccinate and benzethonium chloride. The amount of sodium chloride required for the middle-phase microemulsion decreased dramatically as an equimolar anionic-cationic surfactant mixture was approached. Under optimum middle-phase microemulsion conditions, mixed anionic-cationic surfactant systems solubilised more oil than the anionic surfactant alone. Upadhyaya et al. [109] proposed a model for the interaction of branched-tail surfactants (Fig. 8.16). According to this model the anionic-cationic pair allows oil to penetrate between surfactant tails and increases the oil solubilisation capacity of the surfactant aggregate. Detergency studies were conducted to test the capacity of these mixed surfactant systems to remove oil from... 

Table II displays the PLMA MW in the model system as a function of the alkyl chain length of the anionic surfactant, all other concentrations remaining constant. When the prepolymerization solution contains only C12EO5, the solution is opaque and the PLMA MW is large this result implies the existence of large micelles. Addition of sodium 2-ethylhexyl sulfate to the C12EO5-LMA prepolymerization solution at the concentration used for SDS (0.035 M) dramatically increases the MW. This result is presumably due to an electrolyte effect that increases nonionic surfactant aggregation numbers (3i), because the sodium 2-ethylhexyl sulfate concentration is far below the surfactant s CMC. However, doubling the sodium 2-ethylhexyl sulfate con-...

As one can see the Frumkin model does not reflect all the details of the adsorption process. In some cases, the reorientation and aggregation models lead to better results. The scope of the data available is still insufficient to formulate a criterion for the best choice of the adsorption model. However, it follows from the results summarised in this chapter, that the reorientation model can be successfully applied to oxyethylated surfactants, and also for surfactants with relatively large molar area (o > 2.5T0 m /mol). At the same time, the surfactant molecules with relatively high values of the Frumkin constant and low molar area (m < 2.5-10 m /mol) are more capable for aggregation in the surface layer. 

In some cases, a better agreement with the experimental surface tension isotherms and other data (dynamic surface tension, optical methods) is provided by the reorientation or aggregation model, respectively. It follows from the presented results that the reorientation model is more appropriate for oxyethylated surfactants and for surfactants which possess relatively high molar area, ro > 2.5-lO m /mol. At the same time, the aggregation and cluster models describe better the behaviour of surfactants with a relatively large Frumkin constant and low molar area, (o< 2.5-10 m /mol. 

Another possibility discussed in Chapter 2 is the formation of small clusters or aggregates within the adsorption layer. It was shown that all those surfactants, which had been discussed as strongly interacting in the adsorption layer adsorb according to the surface aggregation model of Eqs. (2.110) - (2.112). The process of aggregation formation/dissolution as... 

Figure 7.8, Solution of l-dodecanol in water at 5 C surfactant concentration 1,2-10 mol/1. Points, experimental data [8] results calculated from the aggregation model dashed line, n = 2, r = 1-I0 ° molW solid line,n= 100,r = mol/ml...

Thus, the situation is much more complex if the characteristics of the reverse micellar system (e.g., aggregation, micellar concentration, and dynamic behavior) are functions of time. In light of the available results, no definitive explanation can be offered at this time to account for the remarkable monodispersity obtained at intermediate R values, at which maximum number of nuclei is formed. Quantitative evaluation of the main features of the proposed model must await further experimental data. Of particular interest in this connection are the development of a quantitative description of the nuclei-aggregation process and the determination of surfactant aggregation numbers with water-ethanol mixtures as aqueous solubilizate. 

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. 

An alternative approach is to use a coarse-grained structural model for the surfactant and the other components so that the self-assembly of the surfactants can be monitored in the simulations [11,12]. The early studies of Karaborni, Smit, and coworkers [13-15] are such an example. These studies are mostly qualitative and primarily demonstrate that the basic features of surfactant self-assembly can be modeled by simple MD simulations. However, the more recent work of Esselink et al. [16] takes advantage of parallel processing techniques and faster computers to provide some insights into the forces of importance for surfactant aggregation and the effects of surfactant structure on the size and shape of the micelle. Moreover, Esselink et al. [16] also present a preliminary study of the mechanism of oil solubilization inside micelles. We restrict ourselves here to some of the details of the simulation and the results presented in their paper. 

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