Influence of different electrolytes on bubble motion in ionic surfactants solutions

Abstract

We determined the influence of concentration and type of added inorganic electrolytes on local and terminal velocities of bubbles in solutions of anionic, sodium n-hexadecylsulfate (SHS) and cationic, n-cetyltrimethylammonium bromide (CTAB) surfactants at concentrations below 3×10−6M. We found that addition of inorganic electrolyte strongly affected the bubble motion. Bubble local velocity passed through the maximum and reached terminal value, which for the electrolyte concentration around 10−3M, corresponded to the immobilized surface. Up to that concentration the width of the maximum of the local velocity profile decreased with electrolyte contents. Surprisingly, addition of higher concentration of inorganic electrolyte caused the reverse effect, the increase and broadening of the maximum, which depended on a type of counterions. For CTAB the effect followed the Hofmeister series of ion activity. That reverse effect may be connected with the modified structure of the dynamic adsorption layer in the presence of high excess of various anions or with the micelization enhanced surfactant desorption rate at the rear part of the rising bubble, reducing the Marangoni stress that immobilizes bubble surface.