Surfactant solutions, thermodynamics thermodynamic properties

Physico-Chemical Properties of Surfactant Solutions. Thermodynamic and... 

The question may then be raised as to whether insoluble monolayers may really be treated in terms of equilibrium thermodynamics. In general, this problem has been approached by considering (i) the equilibrium spreading pressure of the monolayer in the presence of the bulk crystalline surfactant, and (ii) the stability of the monolayer film as spread from solution. These quantities are obtained experimentally and are necessary in any consideration of film thermodynamic properties. In both cases, time is clearly a practical variable. 

Tomasic V, Chittofrati A, Kallay N (1995) Thermodynamic properties of aqueous solutions of perfluorinated ionic surfactants. Colloids and Surfaces, Physicochemical and Engeneering Aspects 104 95-99... 

In spite of this wide applicability, a survey of the literature reveals that, compared to ionic and non ionic surfactants, there have been relatively few investigations of their surface and thermodynamic properties. Investigation has been hampered by the nonavailability of pure compounds and proper analytical techniques to determine their concentration in solution. 

Thermodynamic Properties of Surfactant Micellization in Aqueous Solutions at 300°K... 

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. 

Most of the studies on thermodynamics of mixed micellar systems are based on the variation of the critical micellar concentration (CMC) with the relative concentration of both components of the mixed micelles (1-4). Through this approach It Is possible to obtain the free energies of formation of mixed micelles. However, at best, the sign and magnitude of the enthalpies and entropies can be obtained from the temperature dependences of the CMC. An Investigation of the thermodynamic properties of transfer of one surfactant from water to a solution of another surfactant offers a promising alternative approach ( ), and, recently, mathematical models have been developed to Interpret such properties (6-9). 

Work Is presently under way to extend the above model so as to extract from the experimental data the relevant parameters from a least-squares analysis (13). This model should be applicable to non-lonlc and Ionic systems. In the latter case, an extra term Is required to account for the shift In the CMC of solute 2 due to the sal-tlng-out of the monomers of 2 by solute 3 (7 ). The model In Its present form can still be used to estimate the thermodynamic properties of solute 3 In the micelle of surfactant 2 by adjusting the parameters to get a good fit with the experimental data. 

The chemical equilibrium model of Roux et al (6) is a powerful tool for the study of the thermodynamics of mixed micellar solutions. It can estimate the distribution constant of the surfactant 3 between water and micelles of the surfactant 2 and the thermodynamic properties of the surfactant 3 in the mixed micelles. For this it is necessary to obtain reliable data over a large concentration range of solute 2. 

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. 

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. 

By using equation (18.72), equations can be obtained for relating the other thermodynamic properties to m. The total Gibbs free energy G of the surfactant solution is the sum of the contributions from the solvent, the monomer, and the micelle. That is,... 

The Mass Action Model The mass action model represents a very different approach to the interpretation of the thermodynamic properties of a surfactant solution than does the pseudo-phase model presented in the previous section. A chemical equilibrium is assumed to exist between the monomer and the micelle. For this reaction an equilibrium constant can be written to relate the activity (concentrations) of monomer and micelle present. The most comprehensive treatment of this process is due to Burchfield and Woolley.22 We will now describe the procedure followed, although we will not attempt to fill in all the steps of the derivation. The aggregation of an anionic surfactant MA is approximated by a simple equilibrium in which the monomeric anion and cation combine to form one aggregate species (micelle) having an aggregation number n, with a fraction of bound counterions, f3. The reaction isdd... 

The addition of solute will also influence the cmc of the surfactant, which in turn means that a correction is needed for the overall partial molar quantity, F This has been taken into account in the models proposed by Roux et al. and DeLisi et al. ° The models have been applied to different thermodynamic properties, mostly volumes and heat capacities, and for different surfactant-solute systems. 

Adsorption of organic solutes, control, 260 Adsorption of surfactants cation enhancements, 261 conceptual models, 261 Adsorption option in SOLMINEQ.88, description, 124—125 Adsorptive additivity, description, 272 Al, thermodynamic properties, 415 Aluminate ion... 

All the definitions of the CMC discussed above reflect a general feature of surfactant solutions, namely, a qualitative change in the concentration dependencies of their properties at the CMC. This means that the thermodynamic state of such systems must also differ from the state below the CMC and cannot be described by conventional theories proposed for non-micellar solutions. A brief review of the thermodynamics of micellisation is presented in the following section. 

Motomura et al. proposed a method of evaluation of various thermodynamic properties of micellar solutions from the surface tension data in the framework of the pseudophase treatment of micellisation [50, 52-55]. According to these authors, the micellar composition at the CMC can be found from the functional dependence of the CMC on the overall surfactant mole fraction using an analogy to the method proposed by Nguyen et al. [49], The approach of Motomura et al. [50] gives also a possibility to determine the relation between the composition of the surface layer and the micelles. Application of the Gibbs-Duhem equation to the whole... 

Thus, for block copolymers based on dimethylsiloxane with alkylene glycols as surface-active additives there is a relation between the structure of the epoxide system in the hquid state and that in the hardened state. This allows regulation of the properties of polyepoxides using the thermodynamic characteristic of these surfactant solutions in monofunctional epoxide compoimds. In addition, it is found that the introduction of surfactants of the epoxide series to polyepoxides affects the parameters of the pol5rmer structiue rather differently, depending on the arrangement (parallel or perpendicular) of lyophobic groups of KEP-2 in a monolayer with respect to the interface. 

Swelling of polyurethane networks. Proceeding from the general conception of the role of surfactants in the formation of the secondary structure of polyurethanes, studies of the influence of KEP-2 on the distribution of hydrogen bonds in polyurethane networks were required. The literature on the thermodynamic properties of polymer solutions and gels was searched and, in particular, the work of the authors of [117] is of direct interest in this context. This work... 

Anand, K Yadav, O. P. Singh, P. P. Studies on the Surface and Thermodynamic Properties of Some Surfactants in Aqueous and Water+l,4-Dioxane Solutions Colloids Surf. 1991,55,345-358. 

---