Search

The effects of a homologous series of sodium p-n-alkylbenzoate hydrotropes in water-in-supercritical CO2 (w/c) microemulsions have been investigated, by comparing the phase behaviour and droplet structures obtained using small-angle neutron scattering (SANS). The w/c microemulsions appeared to be generally stable upon addition of hydrotropes, however, on increasing the alkyl chain length of the hydrocarbon and fluorocarbon moieties of the surfactants, different effects on stability were observed. Using high-pressure SANS (HP-SANS), the effects of hydrotrope type on the structures of microemulsion droplets were studied. Interestingly, evidence was found for multiple shell structures with a hydrotrope rich layer between the water cores and the surfactant films. These findings are relevant to the understanding of self-assembly of co-adsorbed species in supercritical CO2 (scCO2), as the hydrotrope layers potentially have significant effects on surfactant packing, and can modify the physico-chemical properties of scCO2 through formation of worm-like micellar assemblies.

The effect of hydrotropes on the geometry of reverse water-in-oil AOT-microemulsions is investigated as a function of water content, and hydrotrope additive architecture. SANS reveals that hydrotropes induce cylindrical morphologies which transition to ellipsoidal and then spherical geometries with increasing water content (w). The length of the elongated particles appeared to show some dependence on the hydrotrope-AOT tail compatibility, which is also reflected in the phase behaviour of these systems. This is the first report of hydrotrope-induced axial elongation of water microemulsions in the oil phase.

The physicochemical properties of a homologous series of sodium p-n-alkylbenzoates have been investigated. The objective was to determine whether there is a clear transition point from hydrotropic to surfactant-like behavior with increasing alkyl chain length n, so as to shed clear light on the aggregation mechanism of so-called "hydrotropes". Electrical conductivity measurements were used for a first estimation of the critical aggregation concentrations (cac). As for classical surfactants, log(cac) depends on alkyl chain length n, but two branches of behavior were observed: one having a gradient typical of long chain fatty acid salts and the other with a more shallow dependence. Surface tension (γ) measurements of high purity aqueous solutions were used to generate limiting headgroup areas A(cac), which were in the range (40-50 Å(2)) being consistent with monolayer formation. Small-angle neutron scattering conclusively shows that the lower chain length homologues (classed as hydrotropes) exhibit sharp transitions in aggregation as a function of bulk concentration, typical of regular surfactants. As such, there is little to suggest from this study that hydrotropes differ in association behavior from regular surfactants.

The synthesis of amphoteric polystyrene latex particles, using a mixture of cationic (amidinium based) and anionic (carboxylic acid based) initiators in a surfactant-free emulsion polymerization reaction is investigated; this extends work described in an earlier paper by Bolt et al. Electrophoretic mobility measurements show the effect of the initiator concentration ratio on the isoelectric point (IEP) of the particles. A good correlation with theoretical predictions is obtained. Particle size and polydispersity are determined as a function of the lag time between the addition of each initiator. An increase in particle size and polydispersity is observed at short lag times. It is shown that this is due to the ratio of the cationic to anionic surface charge approaching unity during the reaction. At long lag times an increase in polydispersity may occur due to late-stage, secondary nucleation upon addition of the second initiator. Increasing the reaction pH to reduce the degree of ionization of the cationic initiator greatly reduces the polydispersity and has a significant effect on the IEP of the particles. This effect is ascribed to the burial of a fraction of the neutral amidine groups below the particle surface due to their increased solubility in the monomer. Slow addition of the second initiator was found to reduce the polydispersity of the particles, while maintaining an IEP value consistent with that expected for the ratio of initiators added.

The ability to induce morphological transitions in water-in-oil (w/o) and water-in-CO2 (w/c) microemulsions stabilized by a trichain anionic surfactant 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2- sulfonate (TC14) with simple hydrotrope additives has been investigated. High-pressure small-angle neutron scattering (SANS) has revealed the addition of a small mole fraction of hydrotrope can yield a significant elongation in the microemulsion water droplets. For w/o systems, the degree of droplet growth was shown to be dependent on the water content, the hydrotrope mole fraction, and chemical structure, whereas for w/c microemulsions a similar, but less significant, effect was seen. The expected CO2 viscosity increase from such systems has been calculated and compared to related literature using fluorocarbon chain surfactants. This represents the first report of hydrotrope-induced morphology changes in w/c microemulsions and is a significant step forward toward the formation of hydrocarbon worm-like micellar assemblies in this industrially relevant solvent.

HypothesisInitial studies (Hopkins Hatzopoulos et al. (2013)) have shown that ionic hydrotropic additives can drive a sphere-to-cylinder (ellipsoid) transition in water-in-oil (w/o) microemulsions stabilized by the anionic surfactant Aerosol-OT; however the origins of this behaviour remained unclear. Here systematic effects of chemical structure are explored with a new set of hydrotropes, in terms of an aromatic versus a saturated cyclic hydrophobic group, and linear chain length of alkyl carboxylates. It is proposed that hydrotrope-induced microemulsion sphere-to-cylinder (ellipsoid) transitions are linked to additive hydrophobicity, and so a correlation between the bulk aqueous phase critical aggregation concentration (cac) and perturbation of microemulsion structure is expected.ExperimentsWater-in-oil microemulsions were formulated as a function of water content w (= [water]/[AOT]) and concentration of different hydrotropes, being either cyclic (sodium benzoate or sodium cyclohexanoate), or linear chain systems (sodium hexanoate, sodium heptanoate and sodium octanoate). Phase behaviour studies were performed as a function of w, additive type and temperature at total surfactant concentration [ST] = 0.10 M and constant mole fraction x = 0.10 (x = [hydrotrope]/[ST]). Microemulsion domain structures were investigated by small-angle neutron scattering (SANS), and these data were fitted by structural models to yield information on the shapes (spheres, ellipsoids or cylinders) and sizes of the nanodroplets.FindingsUnder the conditions of study hydrotrope chemical structure has a significant effect on microemulsion structure: sodium cyclohexanoate does not induce the formation of cylindrical/ellipsoidal nanodroplets, whereas the aromatic analogue sodium benzoate does. Furthermore, the short chain sodium hexanoate does not cause anisotropic microemulsions, but the more hydrophobic longer chain heptanoate and octanoate analogues do induce sphere-to-ellipsoid transitions. This study shows that underlying microemulsion structures can be tuned by hydrotropes, and that the strength of the effect can be identified with hydrotrope hydrophobicity in terms of the bulk aqueous phase cac.

The physicochemical behavior of the phenyl-n-alkanoate (PhenCx) and cyclohexyl-n-alkanoate (CyclohexCx) series has been investigated, supporting previous work on the understanding of hydrotropes (Hopkins Hatzopoulos, M.; Eastoe, J.; Dowding, P.J.; Rogers, S. E.; Heenan, R.; Dyer, R. Langmuir2011, 27, 12346-12353). Electrical conductivity, surface tension, 1H NMR, and small-angle neutron scattering (SANS) were used to study adsorption and aggregation in terms of critical aggregation concentration (cac). The PhenCx series exhibited very similar d log(cac)/dn to n-alkylbenzoates (CnBenz), exhibiting two branches of behavior, with a common inflection point at four linear carbons, whereas the CyclohexCx series showed no break point. Electrical conductivity and 1H NMR concentration scans indicate a difference in physicochemical behavior between higher and lower homologues in both the PhenCx and CyclohexCx series. Surface tension measurements with compounds belonging to either group gave typical Gibbs adsorption profiles, having d log(cac)/dn curves consistent with limiting headgroup areas in the region of (35-55 Å 2) indicating monolayer formation. SANS profiles showed no evidence for aggregates below the electrical conductivity determined cac values, inferring an "on-off" mode of aggregation. Analyses of SANS profiles was consistent with charged ellipsoidal aggregates, persisting from lower through to higher homologues in both the PhenCx and CyclohexCx series.

Hydrotropes are small molecule amphiphiles, having considerable industrial importance as agents for solubilization of hydrophobic substances in aqueous systems. The physico-chemical origin and mechanism of hydrotrope action has been a subject of academic debate and controversy for many years. One important issue is how close the solution physical chemistry of hydrotropes resembles that of common surfactants. This article seeks to improve the appreciation of this field by comparing thermodynamic, phase, spectroscopic and scattering studies of hydrotrope aqueous solutions. In addition, alkyl-hydrotropes are discussed, which represent a structural evolution from classic hydrotropes towards common surfactants, having solution properties more reminiscent of surfactants.