The freezing points, enthalpies of dilution, volumetric heat capacities, densities, and sound velocities of the homologous series R(CH3)2NO, for R = butyl, hexyl, octyl, and decyl, were measured in water at 25 °C and as a function of temperature in the case of octyl. The osmotic coefficients and the apparent and partial molar relative enthalpies, heat capacities, volumes, compressibilities, and expansibilities were calculated. Isochoric heat capacities and isothermal compressibilities can also be derived from these data. There is a gradual change in the trends of these functions when going from the lower homologue, which behaves like a medium-size alcohol, to the higher one, which is a typical nonionic surfactant. The osmotic coefficients are positive in the premicellar region at the freezing temperature but become negative at higher temperatures. The concentration dependence of the various functions can be accounted for quantitatively with a simple mass-action model. Aggregation numbers and thermodynamic functions of micellization can be derived with this model.

-Surfactants tend to aggregate above a certain concentration, called the critical micelle concentration, into large clusters or micelles. With modern calorimetric and other related techniques it is now possible to measure precisely and rapidly the thermodynamic properties of liquid systems over a wide range of concentration and temperature. An overview is given of data obtained in our laboratory for heat capacities, enthalpies, entropies and free energies of aqueous sodium decanoate, octylammonium hydrobromide, nonyl and decyltrimethylammonium bromide. The thermodynamic functions of micellization are derived and interpreted with a phase-separation model. The closely related microemulsions are also briefly discussed. INTRODUCTIONMicellar systems are very special types of macromolecular solutions. Arnphiphilic molecules or ions tend to aggregate in solution into units which are comparable in size to biocolloids, and their properties can be studied with many of the standard techniques that are used for other types of macromolecules (1-5). They differ however in their stability. They form only above concentrations that are called critical micelle concentrations (CMC), and the aggregates, known as micelles, are in dynamic equilibrium with the singly-dispersed solute in the system. The investigation of thermodynamic properties of surfactants are primarily concerned with the study of various factors affecting this equilibrium.The most common way of investigating the thermodynamics of micellar systems is to determine the changes in the CMC with temperature or in the presence of various additives or cosolvents. This is done through measurement of a property, such as conductance or surface tension, which shows a break in the micellization region. This approach is limited since the CMC is not always sharply defined, very high precision is required to extract.the changes in heats and in heat capacities during micellization, there is no easy way of knowing if the observed changes are due to modification in properties of the singly-dispersed molecules, micellar units or both, and oversimplified models for micellization have to be used.In the last ten years or so (6-11) we have undertaken very systematic studies of the direct measurement of thermodynamic properties of surfactants in water. Our aim was to measure all thermodynamic properties of well-characterized surfactants, over a wide range of concentration covering the pre and postmicellar regions, in order to predict the temperature and pressure dependence of various equilibrium properties and to test rigorously the main models and theories of micellization. EXPERIMENTALThis paper is concerned with the thermochemical properties of aqueous surfactants. It is sufficient to mention that the heats of dilution and heat capacities per unit volume were measured with commercial versions (SQDEV INC) of flow microcalorimeters that have been welldescribed in the literature (12)(13)(14). For heat capacities, simultaneous determination of densities are required, and these were...

No abstract

No abstract

No abstract