*2.2.3 Semi-fluorinated alkanes*

*Enhanced Oil Recovery Processes - New Technologies*

*Association mechanism of tributyltin fluoride [54].*

**Figure 4.**

atom of neighbouring molecules. In fact, the tin atom is slightly electropositive which interacts with the electronegative fluorine atom to form an intermolecular Sn-F association, as can be seen in **Figure 4**, while the hydrocarbon arms branching from the tin atom enhance the free volume which facilitates the solubility in CO2 [55]. Apparently, these molecular structures form linear and associating structures in which the alkyl arms stabilise the aggregation, while the tin atoms in each molecule associate with the fluorine atoms in adjacent neighbour molecules [55]. Although there was some success with tributyltin fluoride or other trialkyltin fluorides in thickening light alkane components, these compounds were insoluble in CO2 and ineffective as thickeners, even with the addition of pentane as a cosolvent [56, 57]. Later on, Shi et al. [55] synthesised semi-fluorinated trialkyltin fluorides and fluorinated telechelic ionomers to prepare a solution containing both CO2-philic fluorinated groups to enhance the solubility and CO2-phobic associating group to promote intramolecular association for viscosity enhancement. Both ionomers were soluble in CO2 at 2–4 wt% without requiring the addition of a cosolvent. Their results indicated that both ionomers were capable of increasing the viscosity of CO2 by 2–3-fold over a concentration range of 2–4 wt%. For example, at 4 wt% of tri(2-perfluorobutyl ethyl) tin fluoride in CO2, the viscosity increased three times at 298 K and 16.5 MPa. This viscosity increase was found to be much less than expected because the side chain fluorine atoms on the Sn-F associations were disrupted. This is attributed to the fluorine atom at the end alky arms competing with the fluorine atom attached to the tin atom caused by the electronegativity differences between these chain-end fluorines and those adjacent to the tin. Hence, the disruption of the fluorinated alkyl chains is responsible for the viscosity increase [55]. Overall, given the necessary high concentrations of the ionomers required and their high costs, these fluorinate oligomers are not considered viable thickeners for

**74**

field applications [2, 6, 55].

*2.2.2 Fluorinated and non-fluorous hydroxyaluminum disoaps*

Hydroxyaluminum disoaps were developed to thicken gasoline which was used to make napalm, the infamous weapon type used in World War II [58–60]. These molecules are an aluminium-based soap with two carboxylic acid groups linked to the aluminium atom [61]. A small amount of hydroxyaluminum disoap added to low-viscosity gasoline transforms it to a thick and extremely viscous fluid referred as napalm. In an analogous manner, these compounds were studied to determine their solubility in CO2 and quantify their ability to thicken CO2. Enick and co-workers synthesised a series of hydroxyaluminum disoaps [62]. Unfortunately, none of the hydroxyaluminum disoaps were soluble in CO2. Similar to the results with trialkyl tin compounds summarised above, unpublished results by Enick showed that the solubility of some of these compounds in CO2 could be enhanced either by fluorinating the alkyl arms or using highly branched alkyl chains [2]. However, this trial has not been successful in fully dissolving the hydroxyaluminum disoaps in CO2 [2]. Another attempt to thicken CO2 was done by heating a mixture of CO2 and

Iezzi and co-workers [64] made an early attempt to thickened CO2 by using semi-fluorinated alkanes. They designed a series of linear diblock alkane compounds (F(CF2)n (CH2)m H), which contained two immiscible segments forced to interact via a covalent carbon-carbon bond. It is found that this compound can gel organic liquid (e.g. decane and octane) through the formation of microfibrillar network if the solution is heated and then left to cool down. After the solution (CO2 and semi-fluorinated alkane) cools, the semi-fluorinated compounds form a covalent cross-link between molecules and high-porosity and micro-fibrillar networks that can gel the dense CO2. The fluorinated segments stack with other adjacent fluorinated segments (analogous to hydrocarbon segments) to form the fibre network [7, 56]. However, the gel solution is not suitable for gas mobility control due to its phase behaviour where the viscous solution could not flow through a porous medium and retained at the surface of the rock. This solution may be applicable for conformance control to block fractures or high permeable zones.
