Kieran Trickett

Surfactant-based gels

This article reviews known approaches to generating viscoelastic and gel-like surfactant systems focusing on how the formation of these viscous phases are often sensitive to a variety of chemical and physio-chemical factors. An understanding of this sensitivity is essential for generating high viscosity surfactant phases in more challenging solvent environments. The initial focus is on the generation of worm-like and reverse worm-like micelles. In addition, other approaches for using surfactant self-assembly for viscosity enhancement have been examined, namely gelatin microemulsion based organogels and the addition of substituted phenols to AOT reverse micelles.

Universal Surfactant for Water, Oils, and CO2

A trichain anionic surfactant sodium 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate (TC14) is shown to aggregate in three different types of solvent: water, heptane, and liquid CO(2). Small-angle neutron scattering (SANS) has been used to characterize the surfactant aggregates in water, heptane, and dense CO(2). Surface tension measurements, and analyses, show that the addition of a third branched chain to the surfactant structural template is critical for sufficiently lowering the surface energy, tipping the balance between a CO(2)-incompatible surfactant (AOT) and CO(2)-philic compounds that will aggregate to form micelles in dense CO(2) (TC14). These results highlight TC14 as one of the most adaptable and useful surfactants discovered to date, being compatible with a wide range of solvent types from high dielectric polar solvent water to alkanes with low dielectrics and even being active in the uncooperative and challenging solvent environment of liquid CO(2).

Tri‐Chain Hydrocarbon Surfactants as Designed Micellar Modifiers for Supercritical CO2

Getting their feet wet: Low-cost hydrocarbon surfactants act as fluid modifiers for supercritical carbon dioxide (scCO(2)). Increased terminal branching of the surfactant chains aids micelle formation (see middle picture: CO(2) green), and more chains allows water to be incorporated (right, blue).

Effect of solvent quality on aggregate structures of common surfactants

Aggregate structures of two model surfactants, AOT and C12E5 are studied in pure solvents D2O, dioxane-d8 (d-diox) and cyclohexane-d12 (C6D12) as well as in formulated D2O/d-diox and d-diox/C6D12 mixtures. As such these solvents and mixtures span a wide and continuous range of polarities. Small-angle neutron scattering (SANS) has been employed to follow an evolution of the preferred aggregate curvature, from normal micelles in high polarity solvents, through to reversed micelles in low polarity media. SANS has also been used to elucidate the micellar size, shape as well as to highlight intermicellar interactions. The results shed new light on the nature of aggregation structures in intermediate polarity solvents, and point to a region of solvent quality (as characterized by Hildebrand Solubility Parameter, Snyder polarity parameter or dielectric constant) in which aggregation is not favored. Finally these observed trends in aggregation as a function of solvent quality are successfully used to predict the self-assembly behavior of C12E5 in a different solvent, hexane-d14 (C6D14).

Rod-like micelles thicken CO2

A new approach to thicken dense liquid CO(2) is described using the principles of self-assembly of custom-made CO(2) compatible fluorinated dichain surfactants. Solutions of surfactants in CO(2) have been investigated by high-pressure phase behavior, small-angle neutron scattering (HP-SANS) and falling cylinder viscosity experiments. The results show that it is possible to control surfactant aggregation to generate long, thin reversed micellar rods in dense CO(2), which at 10 wt % can lead to viscosity enhancements of up to 90% compared to pure CO(2). This represents the first example of CO(2) viscosity modifiers based on anisotropic reversed micelles.

Microemulsion-based organogels containing inorganic nanoparticles

Under certain conditions thermo-reversible transparent gels can be formed by addition of gelatin to water-in-oil (w/o) microemulsions stabilized by the anionic surfactant AOT. Here it is shown that a variety of different surfactant-coated nanosized inorganic particles can be immobilized in MBGs, thus combining beneficial nanoparticle and organogel properties to form unique inorganic–organic hybrid materials with potential applications. UV-visible absorption suggests there to be little particle aggregation in these gel matrices, whilst small-angle neutron scattering (SANS) indicates only minimal changes to the organogel structure after particle inclusion. Furthermore, UV-visible spectra show no leaching of the nanoparticle components when the organogels are pelleted and stirred in apolar solvents.

Hydrocarbon Metallosurfactants for CO2

Cobalt and nickel salts of the highly branched trichain anionic surfactant sodium 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate (TC14) are shown to be soluble in dense CO(2) at concentrations up to 6 wt % at 500 bar pressure. This is a remarkably high solubility for such hydrocarbon transition metal surfactants in CO(2). High-pressure small-angle neutron scattering (HP-SANS) has been used to study the surfactant aggregates in a normal organic solvent, cyclohexane, dense CO(2), and also mixtures of these two pure solvents. The results show that transition metal TC14 derivatives are viable compounds for incorporating reactive and functional metal ions into CO(2).

Stabilization of CeO(2) nanoparticles in a CO(2) rich solvent

Here it is shown that the chemical nature of outer organic surfactant layers, used to stabilize inorganic nanoparticles (NPs), is a key factor controlling solubility in a mixed liquid CO(2)-heptane (10% vol) solvent.

Turbulence and Cavitation Suppression by Quaternary Ammonium Salt Additives

We identify the physical mechanism through which newly developed quaternary ammonium salt (QAS) deposit control additives (DCAs) affect the rheological properties of cavitating turbulent flows, resulting in an increase in the volumetric efficiency of clean injectors fuelled with diesel or biodiesel fuels. Quaternary ammonium surfactants with appropriate counterions can be very effective in reducing the turbulent drag in aqueous solutions, however, less is known about the effect of such surfactants in oil-based solvents or in cavitating flow conditions. Small-angle neutron scattering (SANS) investigations show that in traditional DCA fuel compositions only reverse spherical micelles form, whereas reverse cylindrical micelles are detected by blending the fuel with the QAS additive. Moreover, experiments utilising X-ray micro computed tomography (micro-CT) in nozzle replicas, quantify that in cavitation regions the liquid fraction is increased in the presence of the QAS additive. Furthermore, high-flux X-ray phase contrast imaging (XPCI) measurements identify a flow stabilization effect in the region of vortex cavitation by the QAS additive. The effect of the formation of cylindrical micelles is reproduced with computational fluid dynamics (CFD) simulations by including viscoelastic characteristics for the flow. It is demonstrated that viscoelasticity can reduce turbulence and suppress cavitation, and subsequently increase the injector’s volumetric efficiency.

Illustrating the effect of viscoelastic additives on cavitation and turbulence with X-ray imaging

The effect of viscoelastic additives on the topology and dynamics of the two-phase flow arising within an axisymmetric orifice with a flow path constriction along its main axis has been investigated employing high-flux synchrotron radiation. X-ray Phase Contrast Imaging (XPCI) has been conducted to visualise the cavitating flow of different types of diesel fuel within the orifice. An additised blend containing Quaternary Ammonium Salt (QAS) additives with a concentration of 500 ppm has been comparatively examined against a pure (base) diesel compound. A high-flux, 12 keV X-ray beam has been utilised to obtain time resolved radiographs depicting the vapour extent within the orifice from two views (side and top) with reference to its main axis. Different test cases have been examined for both fuel types and for a range of flow conditions characterised by Reynolds number of 35500 and cavitation numbers (CN) lying in the range 3.0–7.7. It has been established that the behaviour of viscoelastic micelles in the regions of shear flow is not consistent depending on the cavitation regimes encountered. Namely, viscoelastic effects enhance vortical (string) cavitation, whereas hinder cloud cavitation. Furthermore, the use of additised fuel has been demonstrated to suppress the level of turbulence within the orifice.