Investigation of gas/liquid interface of small bubbles formed in solutions of different alkylammonium chlorides

The electrophoretic mobility of nitrogen gas bubbles was measured in solutions of different n-alkylammonium chlorides in a cylindrical cell, 30 mm in diameter and 70 mm in length, rotating in order to keep the bubbles on a line of the rotation axis of the cylinder. An electric charge field was applied to the solutions through two electrodes located at both ends of the cylinder. The mobility was measured as a function of the size of bubbles. The maximum mobility U*(=v/E  ) and the corresponding diameter of the bubble, db* were used to determine the apparent surface charge density, σ at the gas/liquid interface. The surface charge density of bubbles was found (i) to be negative even in solutions of all n-alkylammonium chlorides carbon atoms, the number n of which was selected to be n = 1, 2, 3, and 10, and (ii) to decrease in magnitude with increasing number of carbon atoms. Decylammonium chloride (DAC; n = 10) is a typical cationic surfactant, however, the surface was revealed to still be negative, which strongly suggests that DAC molecules do not always come out to the gas/liquid interface. The change in surface charge density with the added salt concentration was analyzed, and as a result, it was found that only DAC follows the Langmuir adsorption isotherm.