Effect of the number of hydroxyl groups of CO2-triggered surfactants on capability and performance in CO2-stimulated response

This paper reports the synthesis and characterization of three CO2-triggered surfactants (S-0, S-2, S-4) containing different number of hydroxyl groups. The impact of the hydroxyl groups on S-0, S-2, and S-4 was investigated by observing the morphological appearance, conductivity, and pH after CO2 bubbling. The surface tension of the three CO2-triggered surfactants decreased from S-0 to S-2 then to S-4, meaning surface tension decreasing with increasing the number of hydroxyl groups. The interfacial tension results showed that surfactants with more hydroxyl groups can better reduce the interfacial tension at a given pH. The pKa and micromorphology results revealed different degree of protonation of the CO2-triggered surfactants by CO2 in water, causing variations in their aggregation patterns. Molecular simulations revealed that increasing the number of hydroxyl groups increases the thickness of the hydration layer. And S-4 with a thicker hydration layer will more likely form a complex network structure than S-0 and S-2 with a thinner hydration layer. The new surfactants utilize a simple and mild approach to synthesis; solving problems such as insolubility and phase separation through the introduction of hydroxyl groups. The study serves as a guide to the application of CO2-trigger surfactant solutions.