Roles of the Hydrophobic Tail Groups on the Properties of CO2-Switchable Surfactants

Abstract

Switchable surfactants can be reversibly converted between surface active and inactive forms by induced triggers including pH, ozone, ultraviolet light, CO2, N2 and heat. Examples of the CO2 triggered switchable surfactants are guanidines, imidazoles and amidines. In a typical process using CO2 triggered switchable surfactants, an emulsion originating from two immiscible phases is stabilized when CO2 is introduced. Afterwards, the emulsion is flushed by N2 or air, resulting in the destabilization and phase separation. These distinctive properties of the switchable surfactants make them appealing chemicals in the transportation and recovery of the crude oil. N'-alkyl-N, N- dimethylacetamidine bicarbonates, as a CO2-triggered switchable surfactant, has been reported in stabilizing the light crude oil (Lu 2014). However, the influence of hydrophobic tail groups on the properties of CO2-switchable surfactants in the protonation and emulsification has not yet been well elucidated. In this work, a series of acetamidines with differing hydrophobic tail group were synthesized, and the synthesis conditions were optimized. The effect of the hydrophobic tail group on the conductivity and emulsion stability were also investigated in details. All the acetamidines presented an excellent switchable property in the conductivity test. Compared to the surfactant with shorter hydrophobic tail group, the acetamidine bicarbonate with longer hydrophobic tail group presented a faster deprotonation rate during bubbling N2. Shorter hydrophobic tail group was beneficial to the protonation of the acetamidines in the presence of CO2, resulting in the formation of hydrophilic bicarbonates compound. However, these bicarbonates with shorter hydrophobic groups were more difficult in deprotonation during the bubbling N2 stage. To examine the emulsifying ability of N'-alkyl-N,N-dimethylacetamidines with different hydrophobic tail groups, emulsifying experiments were conducted at 30°C using a mixture of oil-water containing as synthesized acetamidines (0.1%wt.). The mixture of dodecane-water formed an emulsion after bubbling CO2. The variation in phase separation should be ascribed to the different length of hydrophobic groups of these acetamidines. This revealed the correlation between tail group carbon numbers and hydrophobicity.