Sedimentation in Micellar Solutions of Tetradecyltrimethylammonium Bromide
Advancing the chemical engineering fundamentals
Interfacial & Colloidal Phenomena (T2-6P)
Keywords: sedimentation, surfactant solution, tetradecyltrimethylammonium bromide
In dilute solution the molecules of surfactant cover the interface with the hydrophilic part uniaxially oriented to water. Above the value of CMC the surfactant builds up the spherical micellar structures. The increase of concentration of surface-active agent in a solution causes the transformation of micelles from spherical form to rodlike structure. The transformation takes place when the solution concentration is equal to so-called second critical micellization concentration (CMC2). The sphere-rod transition of the alkyltrimethylammonium halides micelles can be caused by addition of salt too. At a certain threshold micelle concentration, rodlike micelles interact, overlap and entangle together to form a network. The longest micellar associates (called wormlike micelles) in alkyltrimethylammonium halides solutions with added sodium salicylate, have been formed.
In this paper the results of experimental studies on the sedimentation process of barium carbonate (of mean particles diameter of 25 m) in aqueous solutions of tetradecyltrimethylammonium bromide, have been presented. As the additives assisting in micellar association the sodium bromide and sodium salicylate, have been used. The sedimentation tests were carried out in the transparent cylinder of 1 dm3 volume and height of 34 cm. It has been shown that in tetradecyltrimethylammonium bromide solutions of concentrations about 1 CMC and 2 CMC, the sedimentation process proceeds faster than in clear water. The reason of that observation can be a fact that the cationic surfactants are adsorbed on the barium carbonate molecule surface. As the result it causes the both, reduced surface charge and decreased repulsion between particles. Aggregation is induced by the hydrophobic interactions between particles. It will facilitate the approach of molecules to each other and the greater agglomerates are formed. For the sake of bigger mass, the molecules fall faster. Growth of the TTAB concentration to 10 CMC level and addition of sodium bromide into a solution causes slowing down the sedimentation process. It is the result of the growth in fluid density and viscosity. The sedimentation velocity for barium carbonate falling in TTAB/NaSal system was lower than this one in the clear solvent observed. Density of this solution was comparable with water density, whereas the growth of the viscosity of solution have been observed. It can be the result of the decrease of sedimentation velocity.
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Presented Thursday 20, 13:30 to 15:00, in session Interfacial & Colloidal Phenomena (T2-6P).
