The thermodynamics of micelle formation of n-alkyl trioxyethylene, hexaoxyethylene and nonaoxyethylene glycol monoethers has been examined using both the kinetic and phase models. The two models of micelle formation give the same expression for the standard free energy of micellization for systems in which the aggregation number is not too low. The contribution to the total standard free energy of the methylene groups in the alkyl chain is of opposite sign and a factor of ten larger than that of the ethylene oxide groups in the hydrophilic head-group. Despite the uncertainty in calculating an accurate value for the standard enthalpy of micelle formation the standard entropy of micellization is always positive showing that micellization is governed primarily by the gain in entropy associated with the removal of the monomer from an aqueous environment to the micelIe.

A study has been made of the adsorption of homogeneous non-ionic detergents from aqueous solution on Graphon. At a fixed temperature maximum adsorption increases with increasing alkyl chain length and diminishes with increasing size of the hydrophilic head group. In contrast to the behaviour normally observed for the adsorption of a single component, the adsorption of these compounds from solution increases with increasing temperature. This effect has been ascribed to the temperature-dependent hydration of the molecules and, in general, their adsorption behaviour is influenced not only by the conventional adsorbate + adsorbent interactions, but also by solute + solvent interactions, the effect of the latter becoming progressively more pronounced as the lower consolute phase boundary is approached.Previous work with non-ionic surface-active agents at the hydrophobic solidsolution interface has been confined to the adsorption of commercial polyoxyethylated nonyl phenol derivatives on carbon black.1 The work described here is concerned with the adsorption of a series of homogeneous n-alkyl polyoxyethylene glycol monoethers (C,E,) from aqueous solution on to Graphon. Isotherms have been determined at different temperatures and the heats of adsorption have been measured calorimetrically. EXPERIMENTAL M A T E R I A L SADSORBENT.-Graphon, a graphitized carbon black, was used as the adsorbent. This material was supplied by Cabot Carbon Ltd. and had been prepared by heating a carbon black in an induction furnace to between 2700 and 3000°C. The specific area determined by nitrogen adsorption was found to be 91 m2fg and the ash content 0.05 %. A water adsorption isotherm confirmed that at low relative pressures water adsorption by the solid was extremely small, indicating an extremely hydrophobic surface2 Electron microscopy showed that the particles were reasonably uniform in size with average diameters of 200-300 A.ADSORBATE.--The n-alkyl polyoxyethylene glycol monoethers were prepared and purified by techniques described previously.3 TECHNIQUES ADSORPTION ISOTHERMS.-AliqUOtS (30 d) Of the aqueous solutions Of the surfaceactive agents were run into vessels containing weighed samples of the adsorbent. Initial outgassing of the Graphon made no difference to the results. Stoppered tubes were used for the 25 and 40°C isotherms, the tubes being rotated end over end inside an air thermostat until equilibrium had been reached. Some of the supernatant solution was then removed and, where necessary, centrifuged until clear. The concentration of the solution before and after adsorption was measured in a 5-cm cell using a Rayleigh interferometer (Hilger type M154). Experiments at 45°C were conducted in a constant low-temperature room. cALommmY.-The calorimeter has been previously described.4 Heats of immersion were determined using ca. 1 g samples of the adsorbent in sealed glass bulbs. The samples 979 980 ADSORPTION OF N O N -I O N I C SURFACE-ACTIVE AGENTS

A light-scattering study has been made of the effect of concentration and temperature on the micellar size in a series of homogeneous non-ionic detergents in aqueous solution. It has been found that above the critical micelle concentration, as determined by surface tension measurements, an increase in micellar size takes place until a critical concentration is reached. Above this value, constant micellar molecular weights are observed for a finite concentration increase.All the compounds examined show an approximately exponential dependence of micellar molecular weight on temperature. The logarithmic temperature coefficients are similar for the higher homologues examined.

A study has been made of the effect of temperature on the size of xnicelles formed by a pure non-ionic detergent in aqueous solution. It has been shown by light-scattering measurements that the micelle molecular weight increases exponentially with increasing temperature, reaching a value of nearly two million close to the lower consolute temperature. Increase in size of the micelles has little influence on their solubilization properties, but viscosity studies indicate that the particle asymmetry increases as the molecular weight increases.

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