Surfactants system thermodynamics

In the following, the focus will be on the ability of two imidazolium-based ILs in modifying the polar-apolar curvature of the anionic, double-tailed surfactant sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT). At first, the reader will be introduced to the NMR technique used to investigate these systems. Then, the microstructure of water/IL solutions will be discussed. The basic of surfactant systems thermodynamics will be subsequently recalled and the NaAOT behavior in water reviewed. Finally, the nanostructure of the micellar phases originated by loading aqueous solutions of imidazolium-based ILs with NaAOT will be discussed. 

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 mixture CMC is plotted as a function of monomer composition in Figure 1 for an ideal system. Equation 1 can be seen to provide an excellent description of the mixture CMC (equal to Cm for this case). Ideal solution theory as described here has been widely used for ideal surfactant systems (4.6—18). Equation 2 can be used to predict the micellar surfactant composition at any monomer surfactant composition, as illustrated in Figure 2. This relation has been experimentally confirmed (ISIS) As seen in Figure 2, for an ideal system, if the ratio XA/yA < 1 at any composition, it will be so over the entire composition range. In classical phase equilibrium thermodynamic terms, the distribution coefficient between the micellar and monomer phases is independent of composition. 

Medium-chain alcohols such as 2-butoxyethanol (BE) exist as microaggregates in water which in many respects resemble micellar systems. Mixed micelles can be formed between such alcohols and surfactants. The thermodynamics of the system BE-sodlum decanoate (Na-Dec)-water was studied through direct measurements of volumes (flow denslmetry), enthalpies and heat capacities (flow microcalorimetry). Data are reported as transfer functions. The observed trends are analyzed with a recently published chemical equilibrium model (J. Solution Chem. 13,1,1984). By adjusting the distribution constant and the thermodynamic property of the solute In the mixed micelle. It Is possible to fit nearly quantitatively the transfer of BE from water to aqueous NaDec. The model Is not as successful for the transfert of NaDec from water to aqueous BE at low BE concentrations Indicating self-association of NaDec Induced by BE. The model can be used to evaluate the thermodynamic properties of both components of the mixed micelle. 

Scamehorn et. al. (20) also presented a simple, semi—empirical method based on ideal solution theory and the concept of reduced adsorption isotherms to predict the mixed adsorption isotherm and admicellar composition from the pure component isotherms. In this work, we present a more general theory, based only on ideal solution theory, and present detailed mixed system data for a binary mixed surfactant system (two members of a homologous series) and use it to test this model. The thermodynamics of admicelle formation is also compared to that of micelle formation for this same system. 

In this paper we apply basic solution thermodynamics to both the adsorption of single surfactants and the competitive adsorption of two surfactants on a latex surface. The surfactant system chosen in this model study is sodium dodecyl sulfate (SDS) and nonylphenol deca (oxyethylene glycol) monoether (NP-EO o) These two surfactants have very different erne s, i.e. the balance between their hydrophobic and hydrophilic properties are very different while both are still highly soluble in water. 

Surfactant Activity in Micellar Systems. The activities or concentrations of individual surfactant monomers in equilibrium with mixed micelles are the most important quantities predicted by micellar thermodynamic models. These variables often dictate practical performance of surfactant solutions. The monomer concentrations in mixed micellar systems have been measured by ultraf i Itration (I.), dialysis (2), a combination of conductivity and specific ion electrode measurements (3), a method using surface tension of mixtures at and above the CMC <4), gel filtration (5), conductivity (6), specific ion electrode measurements (7), NMR <8), chromatograph c separation of surfactants with a hydrophilic substrate (9> and by application of the Bibbs-Duhem equation to CMC data (iO). Surfactant specific electrodes have been used to measure anionic surfactant activities in single surfactant systems (11.12) and might be useful in mixed systems. ... 

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. 

The micellization of surfactants has been described as a single kinetic equilibrium (10) or as a phase separation (11). A general statistical mechanical treatment (12) showed the similarities of the two approaches. Multiple kinetic equilibria (13) or the small system thermodynamics by Hill (14) have been frequently applied in the thermodynamics of micellization (15, 16, 17). Even the experimental determination of the factors governing the aggregation conditions of micellization in water is still a matter of considerable interest (18, 19) and dispute (20). 

In an effort to go beyond regular solution theory and better understand the molecular basis of mixed surfactant system behavior, several molecular-thermodynamic theories for surfactant mixtures have been developed. The molecular-thermodynamic theory will be brieLy introduced and discussed below. Readers are referred to a recent summary (Shiloach and Blankschtein, 1998) o can obtain more detailed description and discussion in literature (Nagarajan, 1985, 1986 Puwada and Blankschtein, 1990,1992 Nargarajan and Ruckenstein, 1991 Bergstroem and Eriksson, 1992 Sarmoria et al., 1992 Zoeller and Blankschtein, 1995, 1998 Almgren et al., 1996 Barzykin and Almgren, 1996 Bergstroem, 1996 Zoeller et al., 1996 Blankschtein et al., 1997 Shiloach and Blankschtein, 1997,1998 Thomas et al., 1997 Shiloach et al., 1998). 

Finally, it should be mentioned that a combination of COSMO-RS with tools such as MESODYN [127] or DPD [128] (dissipative particle dynamics) may lead to further progress in the area of the mesoscale modeling of inhomogeneous systems. Such tools are used in academia and industry in order to explore the complexity of the phase behavior of surfactant systems and amphiphilic block-co-polymers. In their coarse-grained 3D description of the long-chain molecules the tools require a thermodynamic kernel... 

Microemulsions are transparent or translucent, thermodynamically stable emulsion systems (Griffin 1949). Forming a middle phase microemulsion (MPM) requires matching the surfactant system s hydrophobicity with that of the oil. The HLB (hydrophilic-lipophilic balance) number reflects the surfactant s partitioning between water and oil phases higher HLB values indicate water soluble surfactants while lower values indicate oil soluble surfactants (Kunieda et. al. 1980, Abe et. al. 1986). While a balanced surfactant system produces middle phase microemulsions, an underoptimum surfactant system is too water soluble (high HLB) while an over-optimunTSystem is too oil soluble (low HLB). 

The thermodynamic models discussed in the proceeding paragraph provide no insight into the underlying mechanism and molecular interactions leading to aggregational phenomena. The particular value, however, of such models is emphasized by the fact that they apply equally well to both, aqueous and nonpolar surfactant systems. 

The phase diagrams of aqueous surfactant systems provide information on the physical science of these systems which is both useful industrially and interesting academically (1). Phase information is thermodynamic in nature. It describes the range of system variables (composition, temperature, and pressure) wherein smooth variations occur in the thermodynamic density variables (enthalpy, free energy, etc.), for macroscopic systems at equilibrium. The boundaries in phase diagrams signify the loci of system variables where discontinuities in these thermodynamic variables exist (2). 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

Similar attempts were made by Likhtman et al. [13] and Reiss [14]. Reference 13 employed the ideal mixture expression for the entropy and Ref. 14 an expression derived previously by Reiss in his nucleation theory These authors added the interfacial free energy contribution to the entropic contribution. However, the free energy expressions of Refs. 13 and 14 do not provide a radius for which the free energy is minimum. An improved thermodynamic treatment was developed by Ruckenstein [15,16] and Overbeek [17] that included the chemical potentials in the expression of the free energy, since those potentials depend on the distribution of the surfactant and cosurfactant among the continuous, dispersed, and interfacial regions of the microemulsion. Ruckenstein and Krishnan [18] could explain, on the basis of the treatment in Refs. 15 and 16, the phase behavior of a three-component oil-water-nonionic surfactant system reported by Shinoda and Saito [19],... 

For the encapsulation of pigments by miniemulsification, two different approaches can be used. In both cases, the pigment/polymer interface as well as the polymer/water interface have to be carefully chemically adjusted in order to obtain encapsulation as a thermodynamically favored system. The design of the interfaces is mainly dictated by the use of two surfactant systems, which govern the interfacial tensions, as well as by employment of appropriate functional comonomers, initiators, or termination agents. The sum of all the interface energies has to be minimized. 

In our laboratories, extensive use has been made of vapor pressure (14-18) and membrane methods ( 2, 3, 19, 20) to Infer thermodynamic results for ternary aqueous systems containing an ionic or a nonionic surfactant and an organic solute. The most precise solubilization measurements ever reported have been obtained with an automated vapor pressure apparatus for volatile hydrocarbon solutes such as cyclohexane and benzene, dissolved In aqueous solutions of sodium octylsulfate and other Ionic surfactants (15, 16). A manual vapor pressure apparatus has been employed to obtain somewhat less precise results for solutes of lower volatility (17, 18). Recently, semi-equilibrium dialysis (19, 20) and MEUF (2) methods have been used to investigate solute-surfactant systems in which the organic solubilizates are too involatile to study by ordinary vapor pressure methods. 

The full extent and variety of the phase behavior for water-isopropanol-C02 mixtures observed experimentally and calculated with the Peng-Robinson equation of state was not anticipated based on known phase behavior for the constituent binary mixtures or similar ternary mixtures. These results suggest that multiphase behavior for related model surfactant systems could also be complex. Measurements of all the critical endpoint curves, the tricritical points, and secondary critical endpoint for such systems would be tedious and are extremely difficult. However, by coupling limited experimental data with a thermodynamic model based on this cubic equation of state, complex multiphase behavior can be comprehensively described. 

A detailed derivation of Eq. (3) may be found elsewhere. In the presence of excess inorganic electrolyte in the univalent ionic surfactant system, the factor 2 in Eq. (3) can be reduced to 1 by thermodynamic modification. 

The diffusion path method has been used to interpret nonequilibrium phenomena in metallurgical and ceramic systems (10-11) and to explain diffusion-related spontaneous emulsification in simple ternary fluid systems having no surfactants (12). It has recently been applied to surfactant systems such as those studied here including the necessary extension to incorporate initial mixtures which are stable dispersions instead of single thermodynamic phases (13). The details of these calculations will be reported elsewhere. Here we simply present a series of phase diagrams to show that the observed number and type of intermediate phases formed and the occurrence of spontaneous emulsification in these systems can be predicted by the use of diffusion paths. 

We have studied the phase and micellization behavior of a series of model surfactant systems using Monte Carlo simulations on cubic lattices of coordination number z = 26. The phase behavior and thermodynamic properties were studied through the use of histogram reweighting methods, and the nanostructure formation was studied through examination ofthe behavior ofthe osmotic pressure as a function of composition and through analysis of configurations. 

Water/oil (W/0) emulsions are thermodynamically unstable aggregates. Similar to the reversed surfactant systems, the electrolyte solutions are encapsulated but in this case merely mechanically. However, the stability can usually be controlled from seconds (destabilization) to months (stabilization). The size of the emulsion droplets and the stability are sensitively dependent on the physicochemical processing conditions. The emulsion systems are best characterized by phase diagrams. 

Formulation essentially relates to the content of the systems and generally not to the way it is attained if thermodynamically stable systems are considered. The simplest microemulsion system would contain an organic oil phase (O), an aqueous phase generally referred to as water (W), and a surfactant (S) at a given temperature (T) and pressure (p). This means that at least five variables are required to describe the system. In practice, the situation is much more complicated. Water always contains electrolytes. Moreover, oils as well as nearly all commercial surfactants are mixtures. In most cases, particularly with ionic surfactant systems, a co-surfactant (e.g. an alcohol (A)) is added, among other functions, to reduce the rigidity of the surfactant layer and thus to prevent the formation of gel-like mesophases. 

It was a century ago that researchers started to study the factors affecting the behaviour of water-oil-surfactant systems but it is only with the introduction of Winsor s R theory (1954) that the formulation effects could be interpreted. Winsor s R theory was the first qualitative description of the formulation, paving the way to an understanding of how intermolecular interactions among the different chemical species present in a system are related to its behaviour. Throughout the following decades, several empirical experimental correlations such as the phase inversion temperature (PIT), semiempirical ones such as the cohesive energy ratio (CER), and models based on thermodynamics such as the surfactant affinity difference (SAD) or the hydrophilic-lipophilic deviation (HLD) [15, 143, 144] led... 

For all systems we characterise as physico-chemists, the fundamental issue we deal with is that of whether we have a thermodynamically stable system or not. However, in the case of microemulsions, looking back we can see that it were the spectacular properties of microemulsions that called attention, while issues of whether the system was kinetically or thermodynamically stable were not in focus. Therefore, in the early work, a phase diagram approach, already established for surfactant systems in general, was not applied. 

Our own involvement in microemulsion research was very much influenced by the contacts with the Swedish masters in the field of phase behaviour, Ekwall and Friberg, and at a later stage Shinoda, as well as by our previous experience of studying molecular interactions and association phenomena for other types of surfactant systems. Regarding the stability issue, we found it useful to suggest a definition [32] of a microemulsion as a system of water, oil and amphiphile which is a single isotropic and thermodynamically stable liquid solution . While this definition certainly provided nothing new, we felt it contributed to eliminate some confusion. 

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