Thickening supercritical CO₂ at high temperatures with rod-like reverse micelles

Earlier studies demonstrated the ability of some fluorinated surfactants to form rod-like reverse micelles with the ability to thicken water/supercritical CO2 (scCO(2)) mixtures at temperatures below 45 degree celsius [Langmuir 26 (2010) 83-88. Soft Matter 8 (2012) 7044-7055. Colloids and Surfaces B, 168 (2018), 201-210.]. Such viscosity enhancement of scCO(2) is known to increase sweep efficiency for oil recovery with CO2 flooding. However, temperatures of up to similar to 100 degree celsius in conventional reservoirs are much higher than those employed in laboratory studies, and tend to weaken inter- and intra-molecular interactions between surfactant molecules, discouraging rod-like reverse micelle formation. With the aim of designing surfactants which form rod-like reverse micelles and thicken CO2 at high temperatures, this study examined phase behavior, nanostructures of reverse micelles and thickening ability of double omega-hydroperfluorocarbon-tail anionic surfactants in W/scCO(2) mixtures at temperatures of 35 - 75 degree celsius and pressure of 80 - 400 bar with different water-to-surfactant molar ratios (W-0). The measured CO2 viscosity increased by 1.9-2.2 x for double-chain surfactants M(di-HCF6)(x) (counterion Mx+ = Ni2+ and Co2+) at 40 mM, over the experimental temperature range. On the other hand, the shorter chain H(CF2)(4)CH2 twin-tail surfactants M(di-HCF4)(x) and Na(di-HCF6) gave only 1.1-1.5 x viscosity enhancements. The maximum thickening ability of M(di-HCF6)(2) was at W-0 = 10 in the W-0 range of 5-20 75 degree celsius and 350 bar. High pressure and high temperature small-angle neutron scattering (SANS) was used determine the micellar structure in these systems, and rod micelles of aspect ratios of 4.5-6.5 were found. The results clearly suggest that omega-hydroperfluorohexyl-tails and divalent counterions induce the formation of rod-like reverse micelles in W/CO2 mixtures, even at high temperatures commensurate with in-reservoir conditions.