Nonionic surfactant micellization, thermodynamic

In the present work, we have synthesized two betaines and three sulfobetaines in very pure form and have determined their surface and thermodynamic properties of micellization and adsorption. From these data on the two classes of zwitterionics, energetics of micellization and adsorption of the hydrophilic head groups have been estimated and compared to those of nonionic surfactants. 

The importance of entropic considerations in the formulation of a thermodynamic model for micelle formation in mixtures of ionic and nonionic surfactants has been demonstrated by the ability of the... 

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. 

There are several approaches to derive the Gibbs free energy of micellization. We only discuss one of them which is called the phase separation model. Even this approach only leads to approximate expressions for nonionic surfactants. More detailed discussions of the thermodynamics of micellization can be found in Refs. [3,528,529],... 

Others have studied the volumetric changes occurring in mixed micelles of anionic-anionic and nonionic-nonionic surfactants as a determinant of intermolecular interactions and a measure of the thermodynamic ideality of mixing. In particular, Funasaki et al. (1986) have studied the volumetric behavior of mixed micelles of ionic and nonionic surfactants and analyzed their results in terms of regular solution theory. They found that in water, anionic surfactants such as SDS bind to PEG,... 

An amine oxide surfactant solution can be modeled as a binary mixture of cationic and nonionic surfactants, the composition of which is varied by adjusting the pH. The cationic and nonionic moieties form thermodynamically nonideal mixed micelles, and a model has been developed which quantitatively describes the variation of monomer and micelle compositions and concentrations with pH and... 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

As the length of the hydrocarbon chain increases, it is thermodynamically more favorable, and the molecule forms a micelle. As the length of the hydrophobic chain increases by an additional CH2 group, the c.m.c. of the ionic surfactants is reduced to less than half. Nonionic surfactants are able to form micelles at much lower concentrations than ionic surfactants do because of the absence of the electrostatic repulsion between ionic head groups. Likewise, the addition of a simple electrolyte lowers the electrostatic repulsion between the ionic head groups, and micelle formation occurs at... 

IV. Detergents are surface active substances that have features of all three surfactant groups described above, and in addition they are able to spontaneously form thermodynamically stable colloidal systems (for micellization in surfactant solutions please refer to Chapter VI). The particles that are washed away may become incorporated into the nuclei or micelles, i.e. solubilization (See Chapter VI) takes place. Various anionic, cationic, and nonionic surfactants that are encountered further in this section are typically members of this surfactant group. 

In fact, the value of (ca. 10" g/dL) is exactly that found for a wide range of nonionic surfactants (9). Thus, the same thermodynamic forces that cause micellization of surfactants or phase separation of hydrophobic solvents appear to govern the solution properties of HMHECs (9). 

Adsorption of nonionic surfactants on porous solids has been studied by Huinink et al. in a series of p ers [ 149,150]. They elaborated a thermodynamic approach that accounts for the major features of experimental adsorption isotherms. It is a very well known fact that during the adsorption of nonionic surfactants there is a sharp step in the isotherm. This step is interpreted as a change from monomer adsorption to a regime where micelle adsorption takes place. Different surfactants produce the step in a different concentration range. The step is more or less vertical depending on the adsorbate. The thermodynamic analysis made by Huinink et al. is based on the assumption that the step could be treated as a pseudo first order transition. Their final equation is a Kelvin-like one, which shows that the change in chemical potential of the phase transition is proportional to the curvature constant (Helmholtz curvature energy of the surface). 

Anionic surfactants are the most commonly used type in the emulsion polymerization. These include sulfates (sodium lauryl sulfate), sulfonates (sodium dodecylbenzene sulfonate), fatty acid soaps (sodium or potassium stearate, laurate, palmitate), and the Aerosol series (sodium dialkyl sulphosuccinates) such as Aerosol OT (AOT, sodium bis(2-ethylhexyl) sulfosuccinate) and Aerosol MA (AMA, sodium dihexyl sulphosuccinates). The sulfates and sulfonates are useful for polymerization in acidic medium where fatty acid soaps are unstable or where the final product must be stable toward either acid or heavy-metal ions. The AOT is usually dissolved in organic solvents to form the thermodynamically stable reverse micelles. [22] Nonionic surfactants usually include the Brij type, Span-Tween 80 (a commercial mixture of sorbitol monooleate and polysorbate 80), TritonX-100[polyoxyethylene(9)4-(l,l,3,3-tetramethylbutyl)-phenyl... 

The enthalpies of micellization, AH, , can be calculated indirectly by use of the van t Hoff treatment or directly by isothermal titration calorimetry (ITC). Except for few cases (e.g., some nonionic surfactants), the results of these methods do not agree [38]. The main reason is that there is no provision in van t Hoff equation for factors that are important for micelle formation of ionic surfactants, in particular, the dependence of micellar geometry, surface-charge density, and extent of hydration on temperature T) [38]. On the other hand, the effects of (T) on the aforementioned micellar parameters are included in the direct (i.e., calorimetric) determination of AH, . From Gibbs free energy relationship, any uncertainty introduced in the calculation of Ai7, j will be carried over to so that A5 rK > AS, Where available, therefore, we compare the thermodynamic quantities of micellization, based on experimental data of the same technique. 

Microemulsions are thermodynamically stable systems. Oil-in-water (0/W) microemulsions are mixtures of monomer(s), water, surfactant, and, in some cases, cosurfactant. The cosurfactant is a surface-active compound that, in combination with the surfactant, reduces the interfacial tension between the monomer and the aqueous phase to very low values, ensuring the thermodynamic stability of the microemulsion. Alcohols are often used as cosurfactants. The low interfacial tension results in a frequent fluctuation in size and shape of the microemulsion droplets. In water-in-oil (W/0) microemulsions, a mixture of water-soluble monomers and water are dispersed in an organic solvent with the help of a surfactant. The use of a cosurfactant is not needed often because the monomers are surface active. The amount of surfactant required in microemulsion polymerization (>10wt%) is substantially higher than that used in emulsion polymerization. The droplet (swollen micelle) size of the both 0/W and W/0 microemulsions is in the range of 5-20 nm in diameter. Since these small droplets only weakly scatter light, the microemulsions are transparent. Bicontinuous microemulsions are sometimes formed using blends of nonionic surfactants [100]. Microemulsion polymerization has been reviewed [101]. 

An essentially equivalent approach to that of small-systems thermodynamics has been formulated by Corkill and co-workers and applied to systems of nonionic surfactants [94,176]. As with the small-systems approach, this multiple-equilibrium model considers equilibria between all micellar species present in solution rather than a single micellar species, as was considered by the mass-action theory. The intrinsic properties of the individual micellar species are then removed from the relationships by a suitable averaging procedure. The standard free energy and enthalpy of micellization are given by equations of similar form to Equations 3.44 and 3.45 and are shown to approximate satisfactorily to the appropriate mass-action equations for systems in which the mean aggregation number exceeds 20. 

The thermodynamic equilibria of amphiphilic molecules in solution involve four fundamental processes (1) dissolution of amphiphiles into solution (2) aggregation of dissolved amphiphiles (3) adsorption of dissolved amphiphiles at an interface and (4) spreading of amphiphiles from their bulk phase directly to the interface (Fig. 1.1). All but the last of these processes are presented and discussed throughout this book from the thermodynamic standpoint (especially from that of Gibbs s phase rule), and the type of thermodynamic treatment that should be adopted for each is clarified. These discussions are conducted from a theoretical point of view centered on dilute aqueous solutions the solutions dealt with are mostly those of the ionic surfactants with which the author s studies have been concerned. The theoretical treatment of ionic surfactants can easily be adapted to nonionic surfactants. The author has also concentrated on recent applications of micelles, such as solubilization into micelles, mixed micelle formation, micellar catalysis, the protochemical mechanisms of the micellar systems, and the interaction between amphiphiles and polymers. Fortunately, almost all of these subjects have been his primary research interests, and therefore this book covers, in many respects, the fundamental treatment of colloidal systems. 

Now that the mass-action model has been supported by a number of observations, we move to the thermodynamics of micelle formation based on this model. As would be predicted from the above discussion, micelle formation can be well expressed by a single association constant, even though the process strictly involves multiple association equilibria. The error is less than 5%, for example, for micelles having an aggregation number more than 50. For nonionic surfactants, the standard free energy change AG° per mole of surfactant molecules follows directly from the equilibrium constant and is given from (4.21) and [S] = Q by... 

The thermodynamic formulation of micellization is based on the assumption that micelles exist in equilibrium with micelle-forming surfactant monomers as expressed by Equation 1.4 for nonionic surfactants. ... 

Table IX. Thermodynamic Data for Surfactants Micellization at 25°C of Nonionic...

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. 

As the temperature of a mixed surfactant system is increased above its cloud point, the coacervate (concentrated) phase may go from a concentrated micellar solution mixed ionic/nonionic systems, it would be of interest to measure thermodynamic properties of mixing in this coacervate as this temperature increased to see if the changes from micelle to concentrated coacervate were continuous or if discontinuities occurred at certain temperatures/compositions. The similarities and differences between the micelle and coacervate could be made clearer by such an experiment. 

One important point to recognize about the Stern layer in ionic micelles is that the bound counterions help overcome the electrostatic repulsion between the charged heads of the surfactant molecules. For nonionics no such repulsion exists. It is incorrect to think that ionic micelles form and then adsorb counterions. The Stern layer is part of the micelle, and the energetics of its formation are part of the thermodynamics of micellization. 

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