Static and dynamic adsorption of anionic and amphoteric surfactants with and without the presence of alkali

The adsorption of anionic surfactants (alkyl benzene sulfonate and dodecyl benzene sulfonate) and amphoteric surfactants (betaine) onto the kaolinite has been studied by static and dynamic methods. The studies were conducted at 30 °C and several systems (sodium chloride, sodium carbonate, sodium metaborate and sodium tetraborate) with the same salinity (10 g/l) were considered. The static adsorption results showed that amphoteric surfactants adsorb onto the kaolinite at high rate than anionic surfactants, which is attributed to the strong electrostatic interaction between the negative charge on the surface of kaolinite and the cationic charge in the head of an amphoteric surfactant molecule. Alkali has been demonstrated to significantly decrease adsorption of both anionic and amphoteric surfactants onto the kaolinite. A novel kind of alkali, sodium tetraborate is introduced that has advantages of high salinity tolerance and low pH value, and better effect on decreasing adsorption onto the kaolinite compared with the conventional alkali sodium carbonate. The reason is assumed to the lower ionic strength of tetraborate at the same mass concentration as that of sodium carbonate. Thus it could be a potential alternative alkali under some conditions. The dynamic adsorption experiments were performed by injecting the chemical solution through kaolinite-containing sand pack until an equilibrium state is reached. Adsorption was determined by the decrease of adsorbed surfactant as determined from area integration during the stabilization process. The dynamic method better describes the adsorption process and the result is more reliable. The trends of the dynamic method results correspond to those obtained from the static method.

Research highlights
► Static and dynamic adsorptions of anionic and amphoteric surfactant were studied.
► Amphoteric surfactant adsorption is greater than anionic surfactant.
► The adsorption of betaine will decrease more significantly with alkali present for it will exist as an anionic surfactant in a caustic solution.
► Sodium tetraborate has better effect on decreasing adsorption compared to other alkalis at the same mass concentration due to its lower ionic strength.
► TOC detection method was used for betaine concentration determination.