*2.2.5 Fluorinated and non-fluorinated bis-ureas*

A group of researchers at Yale University and the University of Pittsburgh developed a series of small-molecule compounds containing either one or two urea groups [66]. The urea groups in these compounds induce self-assembly interactions via a hydrogen bond; thereby, these interactions form macromolecular associations that can enhance the viscosity of the CO2-rich solution. Out of the 12 compounds tested, 4 fluorinated bis-urea compounds were highly soluble in CO2 without needing heat and capable of improving the CO2 viscosity by 3–5-fold at 5 wt% of bisurea at 298 K and 31 MPa. In the hope of obtaining a non-fluorous bis-urea, Paik et al. [67] attempted to incorporate the CO2-philic groups (hydrocarbon, carbonyl, and ether groups) into the molecular structure of the bis-urea as illustrated in **Figure 5**. However, their results revealed that after forming a transparent solution, microfibres began to form slowly due to the molecules undergoing self-assembly

and precipitating out of solution. Therefore, these compounds cannot be considered for EOR applications.

### *2.2.6 Surfactants with twin and divalent metal cations*

Eastoe and co-workers designed semi-fluorinated surfactants based on a previous study that used aerosol-OT (AOT)-based water-in-oil microemulsions in cyclohexane solvent [49, 68, 69]. The molecular structure of these surfactants is illustrated in **Figure 6**. These surfactants are soluble in CO2 and form rodlike micelles that enhance CO2 viscosity with the addition of a small amount of water. The purpose of adding water into the solution is to form a stable microemulsion in the presence of AOT surfactant and to promote an aggregate shape that changes the self-association of the surfactant from a spheroid (non-viscous) configuration to a rod shape (viscous) that significantly enhances the viscosity of the solution. Two di-chain perfluorinated sulphosuccinate surfactants (nickel bis-nonofluoropentane sulphosuccinate (Ni-diHCF4) and sodium pentadecfluoro-5-dodecyl sulphate (NaF7H4)) yielded the greatest viscosity increase among the compounds tested. These surfactants have been modified in such a way that can form rodlike micelles to promote solubility in CO2 and viscosity enhancement. This was achieved by the exchange of Na<sup>+</sup> ions with Ni2+ or Co2+ to drive a sphere-to-rod

**Figure 5.** *Molecular structure of non-fluorous bis-ureas [67].*

**Figure 6.** *Molecular structure of a fluorinated twin-tailed surfactant as a CO2 thickener [49].*

### *Direct Gas Thickener DOI: http://dx.doi.org/10.5772/intechopen.88083*

*Enhanced Oil Recovery Processes - New Technologies*

*2.2.6 Surfactants with twin and divalent metal cations*

ered for EOR applications.

achieved by the exchange of Na<sup>+</sup>

*Molecular structure of non-fluorous bis-ureas [67].*

and precipitating out of solution. Therefore, these compounds cannot be consid-

Eastoe and co-workers designed semi-fluorinated surfactants based on a previous study that used aerosol-OT (AOT)-based water-in-oil microemulsions in cyclohexane solvent [49, 68, 69]. The molecular structure of these surfactants is illustrated in **Figure 6**. These surfactants are soluble in CO2 and form rodlike micelles that enhance CO2 viscosity with the addition of a small amount of water. The purpose of adding water into the solution is to form a stable microemulsion in the presence of AOT surfactant and to promote an aggregate shape that changes the self-association of the surfactant from a spheroid (non-viscous) configuration to a rod shape (viscous) that significantly enhances the viscosity of the solution. Two di-chain perfluorinated sulphosuccinate surfactants (nickel bis-nonofluoropentane sulphosuccinate (Ni-diHCF4) and sodium pentadecfluoro-5-dodecyl sulphate (NaF7H4)) yielded the greatest viscosity increase among the compounds tested. These surfactants have been modified in such a way that can form rodlike micelles to promote solubility in CO2 and viscosity enhancement. This was

ions with Ni2+ or Co2+ to drive a sphere-to-rod

**76**

**Figure 6.**

**Figure 5.**

*Molecular structure of a fluorinated twin-tailed surfactant as a CO2 thickener [49].*

transition as shown in **Figure 6** [49, 70]. Furthermore, a di-chain perfluorinated AOT analogue is known to stabilise microemulsions of water in CO2 [71–74]. A high-pressure small-angle neutron scattering (SANS) confirmed the solubility of both surfactant in CO2 and formation of rodlike micelles. At 298 K and 40 MPa, both surfactants (0.05 mol dm<sup>−</sup><sup>3</sup> ) with 10–12.5 moles of water per mole of surfactant achieved a transparent solution in CO2 [49, 70]. At 298 K, 35 Mpa, 6 wt% of Ni-diHCF4, and 10 moles of water per mole of surfactant added into CO2, viscosity enhancements of up to 1.5-fold have resulted [49], and 4.4 wt% of NaF7H4 with 12.5 moles of water per mole of surfactant caused a 2-fold increase in viscosity at 313 K and 40 MPa [70]. However, these surfactants required a very high pressure to attain a single phase, and high concentrations of 5–7 wt% were necessary to achieve a significant viscosity increase. Therefore, these thickeners would not be suitable for field applications as both need a relatively high concentration of these expansive surfactants.

## *2.2.7 Cyclic and aromatic amide and urea based*

Most of the successful associating small-molecule compounds as CO2 thickeners that have been described above are fluorinated or semi-fluorinated materials. These fluorinated materials are both expensive and environmentally persistent due to the fluorine content and high concentrations (3–5 wt%) required for use as a CO2-EOR thickener [75]. Therefore, in a recent publication, Doherty et al. [75] synthesised and examined a series of cyclic and aromatic amide and urea compounds as non-fluorous small-molecule thickeners for dense CO2 and organic liquids. They designed the molecular structure of the compounds as shown in **Figure 7**. These compounds contain cyclic or aromatic core molecules (e.g. cyclohexane or benzene) which are mildly CO2-phobic to promote intermolecular interactions. These core ring groups are combined with associating or linking groups (labelled as 'X') which are typically either amide, urea, or ester groups to establish the intermolecular interaction for viscosity enhancement. In addition, these linking groups also aid the connection of CO2-philic segments (siloxane or heavily acetylated) to cyclic or aromatic core molecules to improve dissolution in CO2. It has been found that after heating and cooling the mixture, these compounds were capable of thickening organic liquids such as hexane and toluene. Researchers have found branched benzene trisurea (propyltris(trimethylsiloxy)silane-functionalised benzene trisurea and trisurea compounds functionalised with varying proportions of propyltris(trimethylsiloxy)silane and propyl poly(dimethylsiloxane)-butyl groups), which are soluble in dense CO2. These compounds are capable of thickening CO2 (3–300-fold) at remarkably low concentrations (0.5–2 wt%) in the presence of hexane as a co-solvent at high concentrations (18–48 wt%) [2, 8]. A 300-fold viscosity increase is too large and definitely not suitable for EOR applications. The high concentration of the required co-solvent at low concentration of the additive severely limits the applicability of this approach due to the high manufacturing costs and environmental concerns (**Table 1**).

**Figure 7.** *General molecular structure of small-molecule cyclic amide and urea based [75].*
