Asoka Weerawardena

Protic Ionic Liquids: Physicochemical Properties and Behavior as Amphiphile Self-Assembly Solvents

The physicochemical properties of 22 protic ionic liquids (PILs) and 6 protic molten salts, and the self-assembly behavior of 3 amphiphiles in the PILs, are reported. Structure-property relationships have been explored for the PILs, including the effect of increasing the substitution of ammonium cations and the presence of methoxy and hydroxyl moieties in the cation. Anion choices included the formate, pivalate, trifluoroacetate, nitrate, and hydrogen sulfate anions. This series of PILs had a diverse range of physicochemical properties, with ionic conductivities up to 51.10 mS/cm, viscosities down to 5.4 mPa.s, surface tensions between 38.3 and 82.1 mN/m, and densities between 0.990 and 1.558 g/cm3. PILs were designed with various levels of solvent cohesiveness, as quantified by the Gordon parameter. Fourteen PILs were found to promote the self-assembly of amphiphiles. High-throughput polarized optical microscopy was used to identify lamellar, hexagonal, and bicontinuous cubic amphiphile self-assembly phases. The presence and extent of amphiphile self-assembly have been discussed in terms of the Gordon parameter.

Nanostructured Protic Ionic Liquids Retain Nanoscale Features in Aqueous Solution While Precursor Brønsted Acids and Bases Exhibit Different Behavior

Small- and wide-angle X-ray scattering (SWAXS) has been used to investigate the effect that water has on the nanoscale structure of protic ionic liquids (PILs) along with their precursor Brønsted acids and bases. The series of PILs consisted of primary, secondary, and tertiary alkylammonium cations in conjunction with formate, nitrate, or glycolate anions. Significant differences were observed for these systems. The nanoscale aggregates present in neat protic ionic liquids were shown to be stable in size on dilution to high concentrations of water, indicating that the water is localized in the ionic region and has little effect on the nonpolar domains. The Brønsted acid-water solutions did not display nanostructure at any water concentration. Primary amine Brønsted bases formed aggregates in water, which generally displayed characteristics of poorly structured microemulsions or a form of bicontinuous phase. Exceptions were butyl- and pentylamine with high water concentrations, for which the SWAXS patterns fitted well to the Teubner-Strey model for microemulsions. Brønsted base amines containing multiple alkyl chains or hydroxyl groups did not display nanostructure at any water concentration. IR spectroscopy was used to investigate the nature of water in the various solutions. For low PIL concentrations, the water was predominately present as bulk water for PIL molar fractions less than 0.4-0.5. At high PIL concentrations, in addition to the bulk water, there was a significant proportion of perturbed water, which is water influenced in some way by the cations and anions. The molecular state of the water in the studied amines was predominately present as bulk water, with smaller contributions from perturbed water than was seen in the PILs.

Protic Ionic Liquids and Ionicity

Protic ionic liquids (PILs) are a subset of ionic liquids formed by the equimolar mixing of a Bronsted acid and a Bronsted base. PILs have been categorized as poor ionic liquids. However, the issue of assessing the ionicity of PILs is still a matter of debate. In this work we studied some physicochemical properties of three chosen PILs, namely, ethanolammonium acetate (EOAA), 2-methylbutylammonium formate (2MBAF), and pentylammonium formate (PeAF), at the initial equimolar (stoichiometric) acid/base ratio and in the presence of excess acid and base. DSC phase-transition studies along with NMR, IR, and Raman spectroscopy were performed on the chosen PILs. The results are discussed in terms of the degree of ionization (extent of proton transfer from the Bronsted acid to Bronsted base), and the possibility of the formation of polar 1:1 complexes and larger aggregates in the neat stoichiometric PILs.

Protic ionic liquids with fluorous anions: physicochemical properties and self-assembly nanostructure

A series of 11 new protic ionic liquids with fluorous anions (FPILs) have been identified and their self-assembled nanostructure, thermal phase transitions and physicochemical properties were investigated. To the best of our knowledge this is the first time that fluorocarbon domains have been reported in PILs. The FPILs were prepared from a range of hydrocarbon alkyl and heterocyclic amine cations in combination with the perfluorinated anions heptafluorobutyrate and pentadecafluorooctanoate. The nanostructure of the FPILs was established by using small- and wide-angle X-ray scattering (SAXS and WAXS). In the liquid state many of the FPILs showed an intermediate range order, or self-assembled nanostructure, resulting from segregation of the polar and nonpolar hydrocarbon and fluorocarbon domains of the ionic liquid. In addition, the physicochemical properties of the FPILs were determined including the melting point (T(m)), glass transition (T(g)), devitrification temperature (T(c)), thermal stability and the density ρ, viscosity η, air/liquid surface tension γ(LV), refractive index n(D), and ionic conductivity κ. The FPILs were mostly solids at room temperature, however two examples 2-pyrrolidinonium heptafluorobutyrate (PyrroBF) and pyrrolidinium heptafluorobutyrate (PyrrBF) were liquids at room temperature and all of the FPILs melted below 80 °C. Four of the FPILs exhibited a glass transition. The two liquids at room temperature, PyrroBF and PyrrBF, had a similar density, surface tension and refractive index but their viscosity and ionic conductivity were very different due to dissimilar self-assembled nanostructure.

Lanthanide Phytanates: Liquid-Crystalline Phase Behavior, Colloidal Particle Dispersions, and Potential as Medical Imaging Agents

Lanthanide salts of phytanic acid, an isoprenoid-type amphiphile, have been synthesized and characterized. Elemental analysis and FTIR spectroscopy were used to confirm the formed product and showed that three phytanate anions are complexed with one lanthanide cation. The physicochemical properties of the lanthanide phytanates were investigated using DSC, XRD, SAXS, and cross-polarized optical microscopy. Several of the hydrated salts form a liquid-crystalline hexagonal columnar mesophase at room temperature, and samarium(III) phytanate forms this phase even in the absence of water. Select lanthanide phytanates were dispersed in water, and cryo-TEM images indicate that some structure has been retained in the dispersed phase. NMR relaxivity measurements were conducted on these systems. It has been shown that a particulate dispersion of gadolinium(III) phytanate displays proton relaxivity values comparable to those of a commercial contrast agent for magnetic resonance imaging and a colloidal dispersion of europium(III) phytanate exhibits the characteristics of a fluorescence imaging agent.

Monodisperse nonionic phytanyl ethylene oxide surfactants: high throughput lyotropic liquid crystalline phase determination and the formation of liposomes, hexosomes and cubosomes

The phase behaviour (both neat and lyotropic) and toxicity of eight new ethylene oxide amphiphiles (EO = 1 to 8) with a single phytanyl chain (3,7,11,15 tetramethylhexadecyl) is reported. There is a discontinuity at EO > 4 where the neat and lyotropic behaviour exhibit a tipping point which is qualitatively rationalised in terms of the molecular geometry of the surfactant. Below four EO units the behaviour of the neat surfactants show only a glass transition, Tg ∼ −90 °C. Above four EO units crystallisation (Tcrys) and crystal-isotropic liquid (Tm) transitions are also observed. These increase monotonically with the hydrophilicity of the surfactant; consistent with the greater cohesiveness of the molecules due to van der Waals interactions. The increase in hydrophilicity corresponds to a decrease in curvature of the surfactant layer towards water. However, the exaggerated splay of the phytanyl chain is effective in promoting various self-assembled structures with inverse cubic and hexagonal phases preferred below ambient temperatures for EO < 4, and these are stable to dilution. Variation of the EO head group length promotes an interesting diversity of cubic phases, with an inverse micellar cubic phase (Fd3m) present for EO = 2 and the bicontinuous gyroid cubic (Ia3d) and double diamond cubic (Pn3m) phases present at higher ethoxylation. DIT-NIR microspectroscopy provided a high throughput, low volume, fast equilibrating method for obtaining the approximate partial temperature-composition phase diagrams of the binary systems with water. The toxicity of colloidal dispersions of these amphiphiles was assayed against normal breast epithelial (HMEpiC) and breast cancer (MCF7) cell lines. The IC50 of the EO amphiphiles was similar in both cell lines with moderate toxicity ranging from ∼80–110 μM in an in vitro cell viability assay.

Amino Acid-derived Protic Ionic Liquids: Physicochemical Properties and Behaviour as Amphiphile Self-assembly Media

The thermal phase transitions and physicochemical properties of a series of 21 amino acid-derived protic ionic liquids and four protic molten salts have been investigated. Structure–property comparisons for this series were investigated for alkyl- and cyclic amino acid cations, and ethoxy and methoxy groups on the cation, combined with nitrate or various carboxylate-containing anions. All the protic fused salts were found to be ‘fragile’. Most of the protic fused salts exhibited a glass transition, with the transition temperatures ranging from –90° to –42°C. Viscosities and conductivities ranged from 0.03 to 15.46 Pa s and 0.02 to 2.20 mS cm–1 at 25°C respectively. The protic ionic liquids alanine methyl ester glycolate, proline methyl ester nitrate, and proline methyl ester glycolate were found to be capable of supporting amphiphile self-assembly. Lamellar or hexagonal liquid crystalline phases were observed with the cationic surfactant hexadecyltrimethylammonium bromide and the non-ionic surfactant Myverol 18–99K.

Removal of a solid organic soil from a hard surface by glucose-derived surfactants: effect of surfactant chain length, headgroup polymerisation and anomeric configuration

Abstract The ability of sugar-derived surfactants to remove a solid organic soil from a hard surface has been investigated using a quartz crystal microbalance (QCM) technique. A matrix of anomerically pure alkyl glucosides and alkyl maltosides, as well as two polydisperse commercial alkylpolyglucoside (APG) surfactant mixtures, were investigated to study the way in which surfactant structure influences hard soil detergency behaviour. As expected, the removal of the hard soil, tristearin, from the gold surface was poor at surfactant concentrations below the critical micelle concentration (CMC) but rose dramatically at higher concentrations. At high concentrations above the CMC, the amount of hard soil ultimately removed was not dependent on the surfactant structure. In contrast, the kinetics of soil removal was dependent on the headgroup degree of polymerisation (DP), alkyl chain length, and anomeric configuration for the matrix of surfactants studied. Alkyl maltoside surfactants removed the hard soil faster than the corresponding alkyl glucoside; an alkyl chain length of ten carbon atoms provided faster soil removal than the octyl or dodecyl counterpart; and at short chain lengths, the α-anomers remove tristearin faster than the corresponding β-anomer.

Monodisperse nonionic isoprenoid-type hexahydrofarnesyl ethylene oxide surfactants: High throughput lyotropic liquid crystalline phase determination

The neat and lyotropic phase behavior of eight new ethylene oxide amphiphiles (EO = 1-8) with a hexahydrofarnesyl chain (3,7,11-trimethyldodecyl) and narrow polydispersity (>98.5% purity) is reported. Below five EO units the behavior of the neat surfactants show only a glass transition, Tg ∼ -90 °C. Above four EO units, crystallization (Tcrys) and crystal-isotropic liquid (Tm) transitions are also observed that increase with degree of ethoxylation of the surfactant headgroup. The lyotropic liquid crystalline phase behavior spans a complex spectrum of surfactant-water interfacial curvatures. Specifically, inverse phases are present below ambient temperatures for EO < 4, with HFarn(EO)2 exhibiting an inverse hexagonal (H(II)) phase stable to dilution. The phase diagram of HFarn(EO)3 displays both the gyroid (Ia3d) and double diamond (Pn3m) inverse bicontinuous cubic phases, with the latter being thermodynamically stable in excess water within the physiological regime. There is a strong preference for planar bilayer structures at intermediate headgroup ethoxylation, with the crossover to normal phases occurring at HFarn(EO)(7-8) which exhibits normal hexagonal (H(I)) and cubic (Q(I)) phases at ambient temperatures. The toxicity of colloidal dispersions of these EO amphiphiles was assayed against normal breast epithelial (HMEpiC) and breast cancer (MCF7) cell lines. The IC50 of the EO amphiphiles was similar in both cell lines with moderate toxicity ranging from ca. <5 to 140 μM in an in vitro cell viability assay. Observations are qualitatively rationalized in terms of the molecular geometry of the surfactant. The physicochemical behavior of the HFarnesyl ethylene oxide amphiphiles is compared to other ethylene oxide surfactants.

A quartz crystal microbalance study of the removal of solid organic soils from a hard surface in aqueous surfactant solution

Abstract A quartz crystal microbalance (QCM) has been employed to monitor the removal of two model solid organic soils, dotriacontane and tripalmitin, from the hard surface of the QCM crystal in aqueous surfactant solutions of octa-ethyleneglycol mono n -dodecyl ether (C 12 E 8 ). We have investigated the effect of varying the thickness of the soil coating on soil removal and the effect of soaking the soil in high-purity water for an extended period of time before adding surfactant. The QCM results support the view that net soil removal is preceded by a stage of water and surfactant penetration into the soil. The rate of penetration and rate of removal depends on the soil type. Water and surfactant take longer to penetrate dotriacontane compared to tripalmitin coatings. The removal process also occurs over a longer period of time in the case of dotriacontane coatings. The percentage of material removed is less for dotriacontane, compared to tripalmitin coatings. The initial coating thickness on the hard surface does not appear to govern the final percentage of soil removed, at least in the thickness range accessible to the QCM (approximately ≤800 nm). Immersing the soil coated surfaces in water for a relatively long time, hastens the onset of the removal stage after surfactant is added but does not significantly influence the rate and extent of removal from the hard surface.