The Influence Of The Alkylammonium Chlorides Hydrocarbon Radical Length On Their Adsorption
THE INFLUENCE OF THE ALKYLAMMONIUM CHLORIDES HYDROCARBON RADICAL LENGTH ON THEIR ADSORPTION
BY FLOTATION CARRIERS
E. A. Streltsova, L. D. Skrylev, A. F. Tymchuk, E. A. Hromysheva
Odessa State University I.I. Mechnikov, Ukraine
The analysis of the experimental investigations devoted to the studying of the regularities of the surfactants adsorption by solid carriers (adsorbents) shows that in some cases with the increasing of the surfactant hydrocarbon radical length their adsorption on the carriers surface at first increases and then decreases. The reasons of such influence of the surfactant hydrocarbon radical length on their adsorption are not finally clear.
From our point of view the conception about adsorption-micelle energetic ratio of surfactant , that is the ratio of the free energy of adsorption (DG0ads) of the surfactant on the surface of the solid adsorbent to the free energy of the micelle formation (DG0mic) of the surfactant in the solution:
AMER = (DG0ads) / (DG0mic) (1)
can assist to explain this fact.
If the surfactant hydrocarbon radical length is smaller than their AMER is greater than one (because (DG0ads) > (DG0mic)), else - vice versa, it is less than one ((because (DG0ads)
The justice of the conclusion we made is confirmed by the results of our investigations.
The objects of the investigations were aqueous solutions of alkylammonium chlorides that contained from 10 to 18 carbon atoms in the hydrocarbon radical. We used silicon dioxide, potassium-manganese hexacyanoferrate(II) and n-octadecane as carriers.
The carried investigations showed that the maximal adsorption capacity on the solid carrier surface belonged to that member of the homologous series of alkylammonium chlorides that had AMER close to one (hexadecylammonium chloride). If AMER is
then the process of the surfactant micelle formation in the solution volume is thermodynamically profitable. With the increase of the micelle concentration in the solution the ions adsorption decreases and the formed micelles shielded by polar groups are badly adsorbed by carriers that have surface charge insufficient to adsorb great surfactant associates. If AMER is > 1 (in the case of the lower surfactant homologues) the surface activity decreases so their adsorption on the surface of the solid carrier is hindered (the more the greater is AMER).
This agrees with the results of the experiments on flotation isolation of alkylammonium chlorides with the carrier such as finely dispersed n-octadecane from which results that hexadecylammonium chloride is flotated better from the homologous series of alkylammonium chlorides.
The change of pH of the solutions changes AMER and therefore their adsorption capacity. The acidifying of alkylammonium chlorides solutions changes AMER to greater values so the value of the maximal adsorption (A¥) shifts to the alkylammonium chloride with longer hydrocarbon chain. The main cause of AMER change is the increase of the free energy of adsorption (DG0ads) as a result of the changing of the adsorption mechanism.
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