Flocculation and Micellization of Sodium Dodecyl Sulfate Solutions in the Presence of Aluminium Nitrate: Effect of Concentration and Temperature

Rui F. P. Pereira,1 Artur J. M. Valente,1 Hugh D. Burrows,1 M. Luísa Ramos,1,2 Ana C. F. Ribeiro1 and Victor M. M. Lobo1,*

1 Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal
2 Centre for Neuroscience and Cell Biology (CNC), University of Coimbra, Portugal
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The present work addresses the question of the effect of the presence of Al(III) on the aggregation and micellization properties of sodium dodecyl sulfate in water. At SDS pre-micellar concentrations, and in the presence of Al(III), aggregation between Al(III) and dodecylsulfate occurs when the ionic concentration reaches equimolar values. Under this condition, the turbidity of the solution increases and is accompanied by a decrease of free SDS in solution and an increase of the bulk pH. This latter result, coupled with data from 27Al NMR spectroscopy, suggests that the DS- anion exchanges the hydroxyl ions from the coordination sphere of the hydrolyzed aluminium, and the interfacial pH will decrease upon increasing the DS- in solution; the formation of aggregates is also supported by the analysis of electrical conductivity data. At concentration ratios greater than around 5, which corresponds to the binding ratio (SDS:Al(III)) value, the dissolution of aggregates occurs upon increasing the SDS concentration. The effect of Al(III) on the micelle concentration was analyzed using an electrical conductivity technique by calculating the micellization thermodynamic parameters at different Al(III) concentrations and temperatures. It is possible to conclude that in the presence of Al(III) the free energy of micellization (ΔGm0 ) increases; although the critical micelle concentrations of SDS in the presence of aluminium nitrate, for each temperature, are independent of the Al(III) concentration and are lower than the cmc, a drastic increase in the degree of micelle counter-ion dissociation is observed. By increasing the temperature, by a system with a constant Al(III) concentration, we have found a decrease in the ΔGm0 , and as a general trend the dependence of ΔGm0 on T decreases upon increasing the salt concentration.

Keywords: Aluminum (III), sodium dodecyl sulfate, flocculation, micellization parameters, aggregation.