The role of colloidal precipitates in the interfacial behavior of alkyl amines at gas–liquid and gas–liquid–solid interfaces

•Amine true solutions lacked any kinetic effect.•Kinetic effects existed whenever amine precipitates were involved.•The size of single bubbles was smaller in the presence of amine precipitates.•Amine species migrate from the air–water interface to the air–quartz interface.•The migration of amine species resulted in an increase in the contact angle.

Alkyl amines are widely used as cationic collectors in froth flotation. They are generally referred to as weak electrolyte collectors and frequently form solid insoluble precipitates. Much work has been done to measure the interfacial tension and contact angle regarding alkyl amine collectors. However, the role of these colloidal precipitates in adsorption phenomena and the kinetics involved in the adsorption have not been systematically studied and decoupled from those of soluble species in flotation. Using dodecyl amine (DDA) as the model surfactant, four sets of intimately linked measurements were made on the dynamic surface tension of amine true solutions and solutions containing precipitates to demonstrate the difference in the surface activity kinetics of colloidal precipitates and soluble species. The effects of the interfacial kinetics on the bubble size in a two-phase system and on the contact angle in a three-phase system were also studied to understand the migration of precipitates between different interfaces. The experimental data showed that kinetic effects existed whenever precipitates were involved, but were absent from true amine solutions. A three-stage mechanism (migration–adsorption–aggregation) was proposed and tested to explain the kinetic behavior associated with precipitates. The size of air bubbles generated in the true solutions was not affected by the surface age of bubbles, while bubble size was affected by the surface age in the presence of precipitates. The observed kinetics of contact angle evolution suggested that the migration of DDA species between air–water–interfaces occurred.

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