Development of steady state adsorption distribution over interface of a bubble rising in solutions of n-alkanols (C 5, C 8) and n-alkyltrimethylammonium bromides (C 8, C 12, C 16)

Abstract

Initial accelerations, local and terminal velocities, dimensions and shape variations of the bubbles rising in n-pentanol, n-octanol, n-octyltrimethylammonium (OTAB), n-dodecyltrimethylammonium (DDTAB) and n-cetyltrimethylammonium (CTAB) bromides solutions were determined. Single bubbles were formed at the capillary orifice in a so controlled way that the time of the bubble surface rapid expansion size was always 1.6 ±0.2 s (adsorption time). Motion of the rising bubbles was monitored and recorded using the CCD camera coupled to the JVC professional video recorder and Motic Images Plus 2.0 digital recording system. Stroboscopic illumination of frequency 100 and 200 Hz was applied. It was found that initial bubble acceleration was decreasing with solution concentration from ca. 930 in distilled water to 500 cm/s 2 in 0.005 mol/dm 3 n-pentanol solution. Profiles of the local velocity variations with distance from the capillary depended on concentration of the solutions studied. Generally, two types of the velocity profiles were observed. At low concentrations a maximum value of velocity was achieved after initial acceleration, next bubble decelerated prior a plateau value of the terminal velocity was attained. Heights and widths of the maximum were diminishing with increasing solution concentration. There was no maximum in distilled water and high concentrations of the solutions. The bubbles terminal velocity was 34.7 ± 0.3 cm/s in distilled water and decreased to ca. 15 cm/s at high concentration of the solution. Magnitudes of the adsorption coverage at surface of the detaching bubble were calculated and the minimum degrees of adsorption coverage's needed for full immobilization of the bubble interface were determined. It was also evaluated that the time needed for development of dynamic steady state structure of the adsorption layer over surface of the rising bubbles was varying from ca. 0.1 up to over 1 s, depending on the solution concentration.