Aggregation and adsorption of reactive dyes in the presence of an anionic surfactant on mesoporous aminopropyl silica

A surface tension technique was used to determine the critical aggregation concentration (cac) of a yellow and a red dye in relation to the presence of the anionic surfactant sodium dodecylbenzene sulfonate (DBS) and to temperature changes in buffered aqueous solutions. The cac values of the yellow dye increase from 25 to 45 °C (from 41.37 to 50.32 mg L−1) and decrease from 45 to 55 °C (from 50.32 to 38.72 mg L−1). The cac values for the red dye/DBS aggregates decrease (from 124.52 to 88.50 mg L−1) from 25 to 55 °C. Adsorption of the two dyes onto a mesoporous aminopropyl silica (Sil–NH2) was also studied. The adsorption of the yellow dye increases with an increase in temperature from 25 to 55 °C. In the presence of DBS the adsorption on Sil–NH2 for the yellow dye decreases, and for the red dye increases from 25 to 55 °C. Adsorptions occurred below and above the cac of the anionic dyes/DBS aggregates. Adsorption of the dyes onto Sil–NH2 fitted well to the Langmuir, Freundlich, and Redlich–Peterson adsorption models. However, in the presence of DBS, only the Freundlich model fit the experimental adsorption data at low dye concentrations (less than 400 mg L−1). In this case, the Redlich–Peterson model was only fitted to the red dye adsorption data. The magnitude of the Dubinin–Radushkevich energetic parameters (E, from 7.00 to 15.00 kJ mol−1) indicates that the adsorption of the dyes onto Sil–NH2, in the absence and in the presence of DBS, is controlled by water adsorbed/dye in solution ion-exchange interactions. It is observed that the values of ΔadsHΔadsH are positive for both dyes and the values are quite similar to each other. The exception is the adsorption of the yellow dye in the presence of DBS, which is slightly exothermic. The ΔadsGΔadsG values are all negative. However, the interactions of the dyes with Sil–NH2 silica are more spontaneous in the presence of the surfactant. The positive adsorption entropy values (ΔadsSΔadsS) for the interaction of the dyes suggest that entropy is a driving force of the dye adsorptions. However, the entropic contribution is higher for the adsorptions in the presence of DBS. It was suggested that the chemical structures of the dyes play an important role in the formation of the dye/DBS aggregates and in dye adsorption onto the aminopropyl silica.

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