Adsorption of non-ionic surface-active agents at the Graphon/solution interface

Abstract

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

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“…Whereas in the case of ionic surfactants, the adsorption decreases with increasing temperature, according to the laws of physical adsorption, an increase is observed in the case of nonionic surfactants [7]. The explanation of this atypical behavior is the dehydration of the polyglycol ether chains at higher temperatures.…”

Critical Phenomena in the Clouding Behavior of Nonionic Surfactants Induced by Additives

“…Equivalently, it is the temperature at which a cloud or haze disappears when a warm cloudy surfactant solution is slowly cooled (ASTM 2024), and, more scientifically, it may be described as the lower consolute temperature in the temperatureconcentration phase diagram of a non-ionic surfactant I water system (1,2). This is a useful property in applications such as detergency since (i) adsorption of non-ionic surfactants on substrates has been found to increase significantly near their cloud points (3), and (ii) oily soil removal from substrates is optimized at the phase inversion temperature (PIT) of non-ionic surfactants, which is a closely related phenomenon (4). It is, therefore, advisable to operate in the vicinity of the cloud point for such applications.…”