Smart use of tertiary amine to design CO2-triggered viscoelastic fluids

Abstract

The construction of CO 2 -responsive surfactantbased viscoelastic fluids is an area of great endeavor, hitherto achieved mainly through the aid of synthetic surfactant with CO 2 -sensitive group. Here, classic cetyltrimethylammonium bromide (CTAB)-sodium salicylate (NaSal) wormlike micellar system is endowed with CO 2 response by simply introducing CO 2 -responsive triethylamine (TEA) without needing specialized organic synthesis. Such a system can be reversibly switched between water-like solution (∼3 mPa s) and viscoelastic fluid (∼40,000 mPa⋅s) with alternately bubbling CO 2 or N 2 , reflecting microstructures evolution from spheres to worms, and this cycle can be repeated more than 15 times without any deterioration. Whereas HCl-induced viscoelastic fluid exhibits obvious weaken after 15 cycles. Combined with pH, conductivity, rheology, UV-Vis, cryo-TEM, and surface activity parameters, it was demonstrated that the CO 2 response of CTAB-NaSal-TEA system originated from the effect of TEA on the binding ability of NaSal to CTAB as species vary, which generally results in a viscosity gap. This simple route to design CO 2 -triggered viscoelastic fluids can be extended to other tertiary amines, even other types of surfactants, and these CO 2 -switchable worms are of great interests for scientific community.

CO2-induced smart viscoelastic fluids based on mixtures of sodium erucate and triethylamine

“…Compared to other CO 2 -responsive worm-based VEF [15,[21][22][23][24][25], the NaOEr-TEA system shows some superiority. First, in contrast with those CO 2 -switchable VEFs formed by other ultra-long-chain amine-based surfactants [21,22], the present worm possesses stronger biodegradability and less toxicity [27][28][29][30], but is more cost-effective and less time-consuming, because NaOEr used in the current system could be simply prepared by neutralizing erucic acid, which can be extracted from rapeseed plants.…”

“…In other words, with bubbling CO 2 , the g 0 of NaOEr-TEA first maintains basically unchanged when pH > 11.10; then drastically rises with a small pH change, see, 10.30-11.10, followed by another viscosity plateau with a gel-like behavior in the range of 9.50-10.30; after that the g 0 quickly jumps down in the pH range of 8.90-9.50, and finally reaches a third plateau with another low viscosity region when pH < 8.90. From the variation of g 0 with CO 2 bubbling described above, one can easily deduce that NaOEr-TEA undergoes a WLM formation -disruption process with monotonously bubbling CO 2 other than alternately streaming CO 2 and inert gas as described in other CO 2 -sensitive VEF [15,[21][22][23][24][25].…”

“…As established above, the initial g 0 is only $1.1 mPa s (NaOEr-TEA-12); after bubbling CO 2 (pH = 10.00), g 0 jumps up to 1,000,000 mPa s (NaOEr-TEA-CO 2 -10), six orders of magnitude higher than that of NaOEr-TEA-12; and then quickly returns back to the initial value if displacing CO 2 with N 2 under heating. Although the reversible switchability in viscoelasticity can also be realized by adjusting the pH value with HCl/NaOH, the alternating addition of HCl and NaOH over and over again usually leads to the various compositions of solutions due to the accumulation of by-product [15], but gas does not. Thus, such a reversible switchability can be easily repeated more than 3 times without any deterioration to its response.…”

“…Among which the pH-sensitive VEF is more easily prepared in both laboratory and industrial scales because of its simple designation and fast response. However, the traditional pH-switchable SVEF generally has some disadvantages from the alternate addition of acid/base, for example, the contamination of the system and the accumulation of the by-product [13][14][15], deteriorating the viscoelasticity of SVEF. Hence, developing novel stimuli-responsive VEF utilizing simpler and environmentally-benign handle instead of pH is highly desirable.…”

“…In the last ten years, CO 2 , an alternative of pH, has attracted considerable attentions [14][15][16][17][18][19][20][21][22][23][24][25] owing to its renewable, cost effective, pollution-free, and energy-efficient characteristics. Although CO 2 essentially acts as pH, it offers additional advantages over the latter as a trigger in avoiding contamination and accumulation in the system, and has been investigated to tune the properties of polymers [16,17], emulsifiers [14], solvents [18], as well as solutes [19], but just a few for switching surfactant-based VEF.…”