Modification of the Two-Point Scaling Theory for the Description of the Phase Transition in Solution. Analysis of Sodium Octanoate Aqueous Solutions

Journal of Solution Chemistry (Impact Factor: 1.18). 03/2012; 41(2):318-334. DOI: 10.1007/s10953-012-9795-6
Source: PubMed


On the basis of conventional scaling theory, the two-point scaling theory was modified in order to describe the influence of composition on the partial molar heat capacity and volume during the micellization process. To verify the theory, isobaric heat capacities and densities of aqueous sodium octanoate solutions were measured over wide composition and temperature ranges and the modified approach was used to analyze the calculated partial molar heat capacities and volumes of the surfactant in water. The results obtained indicate that the micellization process is subject to the scaling laws. The results were compared with those for other systems. Peculiar behavior of the critical indices was observed and correlated with the structure of the micelles.

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    ABSTRACT: The apparent molar heat capacities of two short-chain surfactants (C(6)TAB and C(8)TAB) aqueous solutions were determined at four temperatures. For the hexyltrimethylammonium bromide no structural transition was observed. For the octyltrimethylammonium bromide a maximum of the apparent molar heat capacity was observed, which corresponds to the monomer to micelle transition. The two-point scaling theory, that describes the thermodynamics of the system, which can appear in two phases in the vicinity of critical point, was applied to analyse this system. The critical molalities and critical indices were obtained. Comparison of these values with those previously reported for other similar systems allowed to relate them with structure changes in the solution.
    No preview · Article · Nov 2013 · Fluid Phase Equilibria