Tracking the heat-triggered phase change of polydopamine-shelled, perfluorocarbon emulsion droplets into microbubbles using neutron scattering.

Perfluorocarbon emulsion droplets are hybrid colloidal materials with vast applications, ranging from imaging to drug delivery, due to their controllable phase transition into microbubbles via heat application or acoustic droplet vapourisation. The current work highlights the application of small- and ultra-small-angle neutron scattering (SANS and USANS), in combination with contrast variation techniques, in observing the in situ phase transition of polydopamine-shelled, perfluorocarbon (PDA/PFC) emulsion droplets with controlled polydispersity into microbubbles upon heating. We correlate these measurements with optical and transmission electron microscopy imaging, dynamic light scattering, and thermogravimetric analysis to characterise these emulsions, and observe their phase transition into microbubbles. Results show that the phase transition of PDA/PFC droplets with perfluorohexane (PFH), perfluoropentane (PFP), and PFH-PFP mixtures occur at temperatures that are around 30-40 °C higher than the boiling points of pure liquid PFCs, and this is influenced by the specific PFC compositions (perfluorohexane, perfluoropentane, and mixtures of these PFCs). Analysis and model fitting of neutron scattering data allowed us to monitor droplet size distributions at different temperatures, giving valuable insights into the transformation of these polydisperse, emulsion droplet systems.

Rich liquid crystal phase behavior of novel alkyl-tri(ethylene glycol)-glucoside carbohydrate surfactants.

Carbohydrates are appealing non-ionic surfactant head-groups as they are naturally abundant, generally biocompatible and biodegradable, and readily functionalized. Herein, we explore the phase behavior of seven novel carbohydrate-based surfactants (CBS) containing a tri-ethylene glycol (TEG) linker between a glucose head-group and alkyl tail-group, with linear saturated (C8-18) and cis-unsaturated (C18:1) alkyl chains. At high aqueous concentrations, these glycolipid-like surfactants transition into a variety of lyotropic liquid crystalline phases following an expected concentration phase sequence: hexagonal (H1) → bicontinuous cubic (V1) → lamellar (Lα). Using polarizing light microscopy (PLM), a binary (surfactant-water) phase diagram for each surfactant was constructed across a temperature range (25-80 °C) revealing thermotropic behavior and a broadening of liquid crystal phase regions with increasing alkyl chain length. There was also a significant difference between saturated and unsaturated alkyl chains, due to the cis-unsaturated 'statistical bend' lowering the melting point. Small-angle X-ray scattering (SAXS) measurements were performed to characterize the liquid crystal phases, identifying highly-ordered p6m,Ia3d, and Lα crystallographic space-groups with up to 7 resolved Bragg peaks, likely due to the highly anisometric nature of the TEG-linked surfactants. The phases were shown to be more numerous and exhibited greater thermal-stability compared to well-characterized alkyl glucoside surfactants lacking an oligoethylene spacer in the literature. Finally, the characteristic dimensions of each phase were determined to enable visualization of the internal microstructures, providing insight into the impact of molecular shape and the distribution of hydro-philicity/phobicity on the formation and stability of liquid crystalline mesophases.

Structural and rheological changes of lamellar liquid crystals as a result of compositional changes and added silica nanoparticles.

Lamellar liquid crystals comprising oil, water and surfactant(s) were formulated and analysed in order to examine how these materials responded to the inclusion of inorganic nanoparticles, in terms of their structural and rheological characteristics. Lamellar phases were formed from mixtures of water, para-xylene and Triton X-100, and analysis was performed via small-angle neutron scattering (SANS), polarising light microscopy (PLM), and amplitude and viscosity sweeps. The partial replacement of Triton X-100 with oleic acid appeared to cause an increase in bilayer thickness, attributed to less efficient packing of the different molecules. Addition of oleic acid also appeared to cause both a loss in lamellar repeat ordering, attributed to heterogeneity of the bilayers, and a rise in long range order, potentially caused by the stiffer bilayers. Adding silica nanoparticles of different size and surface chemistry caused a stiffening of the samples at the expense of a longer-range lamellar repeat order. This strengthening is attributed to aggregation at the domain boundaries, and it was found that hydrophobic particles tended to form stronger aggregates while for larger particles (20 nm as opposed to 10 nm) aggregation was apparently reversible. These results give a more comprehensive understanding of how to reliably control the structural and rheological properties of lamellar liquid crystals, and emphasise the importance of the size and surface chemistry of any inclusions, for applications in cosmetics, drug delivery, and microfluidics.

Photoswitchable carbohydrate-based fluorosurfactants as tuneable ice recrystallization inhibitors.

Cryopreservation is an important technique employed for the storage and preservation of biological tissues and cells. The limited effectiveness and significant toxicity of conventionally-used cryoprotectants, such as DMSO, have prompted efforts toward the rational design of less toxic alternatives, including carbohydrate-based surfactants. In this paper, we report the modular synthesis and ice recrystallization inhibition (IRI) activity of a library of variably substituted, carbohydrate-based fluorosurfactants. Carbohydrate-based fluorosurfactants possessed a variable mono- or disaccharide head group appended to a hydrophobic fluoroalkyl-substituted azobenzene tail group. Light-addressable fluorosurfactants displayed weak-to-moderate IRI activity that could be tuned through selection of carbohydrate head group, position of the trifluoroalkyl group on the azobenzene ring, and isomeric state of the azobenzene tail fragment.

Structural Evolution of Wormlike Micellar Fluids Formed by Erucyl Amidopropyl Betaine with Oil, Salts, and Surfactants.

Solutions of extended, flexible cylindrical micelles, often known as wormlike micelles, have great potential as the base for viscoelastic complex fluids in oil recovery, drilling, and lubrication. Here, we study the morphology and nanostructural characteristics of a model wormlike micellar fluid formed from erucyl amidopropyl betaine (EAPB) in water as a function of a diverse range of additives relevant to complex fluid formulation. The wormlike micellar dispersions are extremely oleo-responsive, with even as little as 0.1% hydrocarbon oil causing a significant disruption of the network and a decrease in zero-shear viscosity of around 100-fold. Simple salts have little effect on the local structure of the wormlike micelles but result in the formation of fractal networks at larger length scales, whereas even tiny amounts of small organic species such as phenol can cause unexpected phase transitions. When forming mixtures with other surfactants, a vast array of self-assembled structures are formed, from spheres to ellipsoids, lamellae, and vesicles, offering the ultimate sensitivity in designing formulations with specific nanostructural characteristics.

The effects of alkylammonium counterions on the aggregation of fluorinated surfactants and surfactant ionic liquids.

The effects of organic counterions with varying carbon number on surfactant aggregation have been analysed by coupling perfluorooctanoate surfactant anions with various alkylammonium counterions. Both the degree of substitution (primary to tertiary) and alkyl chain length (0-3 carbons) of the counterions were varied to provide a comprehensive matrix of geometries and lipophilicities. Surface activity was measured using pendant drop tensiometry, while temperature-controlled small-angle neutron scattering was used to probe changes in aggregation morphology. It was found that the use of such alkylammonium counterions resulted in a strong preference for bilayer formation even at low surfactant concentration (<2wt%), when compared to simple inorganic counterions such as sodium which favour near-spherical micelles. At increased temperatures, some counterions led to unique phase behaviour wherein a transition between two structurally different lamellar phases is seen, rationalised as a transition into a microscopic phase separation wherein a surfactant-rich lamellar phase coexists with a dilute micellar phase. The results indicate that aggregation is controlled by a delicate balance of counterion size, hydrophilicity and diffuseness of charge, providing new methods for the subtle control of surfactant solutions.

Light-induced structural evolution of photoswitchable carbohydrate-based surfactant micelles.

We report the light-induced structural evolution of photoswitchable carbohydrate-based surfactant micelles using time-resolved small-angle neutron scattering (TR-SANS), monitoring the structural changes in micellisation in situ over time and demonstrating for the first time the course and implications of this process.

Controlling the characteristics of lamellar liquid crystals using counterion choice, fluorination and temperature.

The characteristics of robust and highly ordered fluorinated lamellar phases were explored as a function of temperature, counterion identity and fluorination of the surfactant and co-surfactant. Structural and composition effects were probed using a combination of small-angle scattering of X-rays and neutrons, polarising microscopy and calorimetry. It was found that in general, the phases remained remarkably stable with increasing temperature, showing only moderate loss of order and increased membrane flexibility. By changing the surfactant's cationic counterion, it was possible to exert influence on both the shape of micelles formed and the inter-layer spacing of the lamellar phases obtained. Ordering and crystallinity of the lamellar membranes could be controlled by the level of fluorination of both the surfactant and co-surfactant. These results suggest that subtle manipulations of selected control parameters including co-surfactant selection and counterion choice can provide a high level of control over membrane spacing and local order within lamellar phases, providing guidance where these materials are used as templates.

Reversible pH- and photocontrollable carbohydrate-based surfactants.

The parallel synthesis and properties of a library of photoswitchable surfactants comprising a hydrophobic butylazobenzene tail-group and a hydrophilic carbohydrate head-group, including the first surfactants to exhibit dual photo- and pH-responsive behavior, is reported. This new generation of surfactants shows varying micelle morphologies, photocontrollable surface tension, and pH-induced aggregation and adsorption.

Fluorinated lamellar phases: structural characterisation and use as templates for highly ordered silica materials.

Highly ordered silica was synthesised by using a lamellar phase comprising the anionic fluorinated surfactant sodium perfluorooctanoate and the partially-fluorinated co-surfactant/oil 1H,1H,2H,2H-perfluorooctan-1-ol in water. The phase behaviour of this system was thoroughly analysed, and it was found that even low levels of the alcohol (<0.5 mol%) were sufficient to induce a phase change from normal micelles to a lamellar phase, rationalised as a result of geometric and electrostatic effects. The properties of these phases were compared to their hydrocarbon analogues, demonstrating the unique and valuable properties exhibited by fluorocarbons, directly related with the observed nanostructure. Small-angle neutron scattering was used to analyse the internal structure of the systems, providing information on the inter-lamellar spacing, bilayer thickness and membrane elasticity. The potential for these phases to act as shear-thinning lubricants was assessed using oscillatory rheology, obtaining shear-dependent viscosity along with storage and loss moduli.