|Title||Colloids in ultra-low dielectric media : surface forces and self-assembly|
|Source||Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Mieke Kleijn. - S.l. : s.n. - ISBN 9789461736857 - 257|
Physical Chemistry and Colloid Science
|Publication type||Dissertation, internally prepared|
|Keyword(s)||schoonmaken - was - kooldioxide - oppervlaktechemie - oppervlakte-interacties - oplosmiddelen - cleaning - laundry - carbon dioxide - surface chemistry - surface interactions - solvents|
|Categories||Colloid and Surface Chemistry|
This thesis aims at gaining fundamental insight on colloidal interactions in two types of apolar media, namely, liquid CO2 and (as a model for liquid CO2) n-hexane. The other components playing major roles are surfactants and water. The background of the work was to address the challenges met in the use of liquid CO2 as a dry-cleaning solvent, in particular the insufficient removal of particulate soil.
Since the dielectric constant of liquid CO2 is extremely low (1.6 at 60 bar and 10 °C), it has a low individual Hamaker constant. This in turn leads to a many orders of magnitude higher van der Waals force between interacting surfaces through liquid CO2 as opposed to traditionally employed solvents like perchloroethylene (PERC), which are toxic and environmentally unfriendly. Both the absence of charge on surfaces and the high Van der Waals force mean that a high solvodynamic force (high Reynolds number) is required to dislodge particles from fabrics.
The situation becomes worse in the presence of water (without a suitable surfactant), which is a minor component in any dry-cleaning formulation. Our atomic force microscopy results indicate that water-mediated capillary bridges can lead to higher adhesion forces between the interacting surfaces. The roughness and softness of the surfaces were found to affect the kinetics, magnitude and range of the interaction force.
Further, we have shown that using suitable surfactants these forces can be reduced. Following a systematic selection approach based on the hypothesis that a hydrocarbon surfactant for liquid CO2 should have a low molecular weight and a branched t-butyl tail in its alkyl part, Igepal CA520 was chosen. This surfactant has an ethylene oxide (EO) chain as a headgroup (CiEOj type surfactant). The surfactant solubility was tested first in the model solvent, followed by measuring its cloud point in the liquid CO2 system, which showed that the surfactant is soluble at ~ 50 bar and 5 - 10 °C (CO2 dry-cleaning conditions). Furthermore, we found that Igepal CA520 was surface active at the water - liquid CO2 interface. Igepal CA520 was further tested in a pilot scale dry-cleaning apparatus, where it showed marked improvement in detergency of particulate soil.
The interfacial behaviour of the surfactant - water - liquid CO2 system was also studied using the self-consistent field theory of Scheutjens and Fleer (SF-SCF). We showed that the interfacial tension of bare water - CO2 interface decreases with increasing pressure and becomes invariant of pressure beyond the saturated vapour pressure. The water contact angle on a hydrophilic surface in CO2 increases with increasing pressure. The first phenomenon was explained from the increasing Gibbs excess of CO2 at the water - vapour interface. The increase in contact angle was shown to result from the adsorption of CO2 on the -OH populated surfaces with increasing pressure. The model further predicted complete wetting of the water - vapour interface by a CO2 layer, in line with the fact that the system conditions were chosen not far from criticality.
The model was further extended to describe and predict the interfacial and bulk properties of the liquid CO2/surfactant/water system. The experimental water - CO2 interfacial tension data and the SF-SCF modeling of the Igepal/water/liquid CO2 system indicated that Igepal adsorbs at the water - liquid CO2 interface. The model also predicted the formation of reverse micelles both at the three-phase (water/liquid CO2/gaseous CO2) coexistence (at P/Psat = 1 ) and for P/Psat > 1. With increasing pressure the critical reverse micellar concentration (CRMC) increases and the aggregation number at the CRMC decreases. A higher pressure leads to a stronger stopping mechanism for reverse micellization due to the better solvation (better solvency power of liquid CO2) of the surfactant tails by CO2.
Apart from the bulk phase behaviour, the presence of the surfactant gives rise to interesting wetting phenomena at the water - vapour interface. Partial wetting by CO2 was noted, followed by a re-entrant wetting transition as the surfactant concentration in the bulk water phase was increased.
The theoretical phase behaviour was validated by small angle X-ray scattering experiments (SAXS) on Igepal/water/liquid CO2 ternary systems. The SAXS results indicated conclusively the presence of self-assembled structures. The role of water in driving the self-assembly has been mapped and it was concluded that water acts as a mesogen (promoter of liquid crystals) in CiEOj type surfactant - liquid CO2 systems. In the absence of water, at a particular range of surfactant concentrations, the system contains isotropic reverse micellar mesophases (often termed as L2) and with the addition of water L2 undergoes a phase transition to a lamellar phase, Lα. The lamellar repeat distance increases with increasing water content. Following these findings, a partial phase diagram in liquid CO2 has been generated. Comparing the phase diagrams of Igepal CA520/water in liquid CO2 and in n-hexane it is clear that water plays similar roles in the two systems. Based on this we also conclude that n-hexane is a good model for liquid CO2.
The model was used to arrive at design guidelines for surfactants for liquid CO2. It is interesting to note that the stability window for reverse micelles in liquid CO2 is rather narrow with respect to the two key Flory-Huggins interaction parameters, namely the χC3Dand χOW, characterizing the interaction between methyl groups in the surfactant tail and CO2, and between the head group oxygen and water, respectively. The first interaction parameter dominates the stopping mechanism for micellization, while the latter determines the driving force for this process. The fact that this window is narrow essentially points out the difficulties involved in designing amphiphiles for liquid CO2. The design criteria emerging from modeling are based on the numerical results that for smaller molecules (< C10), branching is important and for longer molecules (C10 and above), CH3 type interactions are more important over branching.
Apart from CO2 dry-cleaning, the knowledge gained in this thesis can be beneficial to many other environmentally friendly industrial processes involving liquid CO2, such as enhanced oil recovery and extraction of polar compounds. The outcome of this thesis can also be extended to alleviate the problems associated with the geological storage of CO2 at high pressure under the ocean floor (deep saline aquifers).