Thermodynamics, static properties and transport behaviour of fluids with competing interactions

Competing interaction fluids have become ideal model systems to study a large number of phenomena, for example, the formation of intermediate range order structures, condensed phases not seen in fluids driven by purely attractive or repulsive forces, the onset of particle aggregation under in- and out-of-equilibrium conditions, which results in the birth of reversible and irreversible aggregates or clusters whose topology and morphology depend additionally on the thermodynamic constrictions, and a particle dynamics that has a strong influence on the transport behaviour and rheological properties of the fluid. In this contribution, we study a system of particles interacting through a potential composed by a continuous succession of a short-ranged square-well, an intermediate-ranged square-shoulder and a long-ranged square-well. This potential model is chosen to systematically analyse the contribution of every component of the interaction potential on the phase behaviour, the microstructure, the morphology of the resulting aggregates and the transport phenomena of fluids described by competing interactions. Our results indicate that the inclusion of a barrier and a second well leads to new and interesting effects, which in addition result in variations of the physical properties associated to the competition among interactions.

Quantifying Phase Behaviour in Model Food Composites Using 3D Confocal Laser Scanning Microscopy

There is an increasing demand for the design of complex bio-composites with customized structural characteristics for use in processed food products. Phase behaviour of these mixtures determines textural properties, encouraging the pursue of a rapid technique that can accurately quantify it. The present work tests the efficacy of confocal laser scanning microscopy (CLSM) coupled with image analysis software (Imaris), for the quantification of phase behaviour in complex tertiary systems. In doing so, it develops phase separated gels of agarose and gelatin supporting inclusions of canola oil. The polysaccharide was replaced with whey protein isolate (WPI) and the topology of the tertiary dispersion with gelatin and canola oil was also examined. Reproducible phase volume estimates were obtained, including those of the lipid phase, which were a close match to the actual concentrations added to the hydrocolloid gel. The approach could offer an alternative to the rheological estimation, via theoretical blending law analysis, of phase volumes in bio-composites. Graphical Abstract

Chamber investigation of the formation and transformation of secondary organic aerosol in mixtures of biogenic and anthropogenic volatile organic compounds

A comprehensive investigation of the photochemical secondary organic aerosol (SOA) formation and transformation in mixtures of anthropogenic (o-cresol) and biogenic (α-pinene and isoprene) volatile organic compound (VOC) precursors in the presence of NOx and inorganic seed particles was conducted. Initial iso-reactivity was used to enable direct comparison across systems, adjusting the initial reactivity of the systems towards the assumed dominant oxidant (OH). Comparing experiments conducted in single precursor systems at various initial reactivity levels (referenced to a nominal base case VOC reactivity) and their binary and ternary mixtures, we show that the molecular interactions from the mixing of the precursors can be investigated and discuss limitations in their interpretation. The observed average SOA yields in descending order were found for the α-pinene (32 ± 7 %), α-pinene/o-cresol (28 ± 9 %), α-pinene at ½ initial reactivity (21 ± 5 %), α-pinene/isoprene (16 ± 1 %), α-pinene at ⅓ initial reactivity (15 ± 4 %), o-cresol (13 ± 3 %), α-pinene/o-cresol/isoprene (11 ± 4%), o-cresol at ½ initial reactivity (11 ± 3 %), o-cresol/isoprene (6 ± 2 %) and isoprene systems (0 ± 0 %). We find a clear suppression of the SOA yield from α-pinene when it is mixed with isoprene, whilst the addition of isoprene to o-cresol may enhance the mixture’s SOA formation potential, however, the difference was too small to be unequivocal. The α-pinene/o-cresol system yield appeared to be increased compared to that calculated based on the additivity, whilst in the α-pinene/o-cresol/isoprene system the measured and predicted yield were comparable. However, in mixtures where more than one precursor contributes to the SOA mass it is unclear whether changes in the SOA formation potential are attributable to physical or chemical interactions, since the reference basis for the comparison is complex. Online and offline chemical composition and SOA particle volatility, water uptake and “phase” behaviour measurements that were used to interpret the SOA formation and behaviour are introduced and detailed elsewhere.

The phase behaviour of cetyltrimethylammonium chloride surfactant aqueous solutions at high concentrations: An all-atom molecular dynamics simulation study

We explore the phase behaviour of aqueous solutions of the cetyltrimethyl ammonium chloride (CTAC) surfactant and in particular the transition from the micellar phase (L1) to the hexagonal columnar phase...

Thermal Behaviour of Microgels Composed of Interpenetrating Polymer Networks of Poly(N-isopropylacrylamide) and Poly(Acrylic Acid): A Calorimetric Study

Stimuli-responsive microgels have recently attracted great attention in fundamental research as their soft particles can be deformed and compressed at high packing fractions resulting in singular phase behaviours. Moreover, they are also well suited for a wide variety of applications such as drug delivery, tissue engineering, organ-on-chip devices, microlenses fabrication and cultural heritage. Here, thermoresponsive and pH-sensitive cross-linked microgels, composed of interpenetrating polymer networks of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc), are synthesized by a precipitation polymerization method in water and investigated through differential scanning calorimetry in a temperature range across the volume phase transition temperature of PNIPAM microgels. The phase behaviour is studied as a function of heating/cooling rate, concentration, pH and PAAc content. At low concentrations and PAAc contents, the network interpenetration does not affect the transition temperature typical of PNIPAM microgel in agreement with previous studies; on the contrary, we show that it induces a marked decrease at higher concentrations. DSC analysis also reveals an increase of the overall calorimetric enthalpy with increasing concentration and a decrease with increasing PAAc content. These findings are discussed and explained as related to emerging aggregation processes that can be finely controlled by properly changing concentration, PAAc content an pH. A deep analysis of the thermodynamic parameters allows to draw a temperature– concentration state diagram in the investigated concentration range.

Recovering local structure information from high-pressure total scattering experiments

High pressure is a powerful thermodynamic tool for exploring the structure and the phase behaviour of the crystalline state, and is now widely used in conventional crystallographic measurements. High-pressure local structure measurements using neutron diffraction have, thus far, been limited by the presence of a strongly scattering, perdeuterated, pressure-transmitting medium (PTM), the signal from which contaminates the resulting pair distribution functions (PDFs). Here, a method is reported for subtracting the pairwise correlations of the commonly used 4:1 methanol:ethanol PTM from neutron PDFs obtained under hydrostatic compression. The method applies a molecular-dynamics-informed empirical correction and a non-negative matrix factorization algorithm to recover the PDF of the pure sample. Proof of principle is demonstrated, producing corrected high-pressure PDFs of simple crystalline materials, Ni and MgO, and benchmarking these against simulated data from the average structure. Finally, the first local structure determination of α-quartz under hydrostatic pressure is presented, extracting compression behaviour of the real-space structure.

Controlling the Phase Behaviour and Photophysical Properties of Metallomesogens Through Structural Modification

<p>Metallomesogens (metal-containing liquid crystals) have been of interest to chemists since the early 1980s. Since this period, many of the studies published on metallomesogens have focussed on the synthesis of novel metallomesogens, and studies of their phase behaviour. As a result there is a substantial body of knowledge of their synthesis and phase behaviour, however many of these studies have overlooked the interesting physical properties that transition metals or lanthanides may impart to the mesophase (liquid crystal state). The studies that have been carried out suggest that the optical and photophysical properties resulting from their self assembly are very different to those observed in the crystalline or isotropic liquid phases, and are highly dependent on the structure of the mesophase. A series of salicylaldimine-based ligands and copper(II) complexes with a variety of structural modifications were synthesised and characterised. The structure, phase behaviour and phase relaxation kinetics of these compounds in the crystalline state were studied using differential scanning calorimetry (DSC), single crystal X-ray crystallography and variable temperature powder X-ray diffraction. The mesomorphism of the compounds was studied using small angle X-ray scattering (SAXS) and polarised optical microscopy (POM). The photophysical properties of the complexes and ligands were studied in the solution phase using ultraviolet-visible (UV-vis) spectroscopy. It was found that the smallest complexes (copper(II) N-alkyl,4-alkoxysalicylaldimine complexes) were not metallomesogens, but did exhibit multiple crystalline phases that formed as a result of changes in the conformation of the N-alkyl chains. The transition temperatures of these crystalline phase changes were strongly dependent on the length of the alkyl chains due to kinetic phenomena. The extension of the rigid core of the complex via synthesis of an N-(4-butylphenyl) derivative was successful in inducing mesomorphism in both the complex and the ligand. The ligand formed an enantiotropic nematic mesophase, while the complex formed a monotropic smectic A mesophase. The structural differences between the non-mesomorphic complexes, the mesomorphic ligand and the mesomorphic complex indicate that the determining factor in the formation of mesophases is the magnitude of lateral interactions between the molecules, which is governed by the size and shape of the rigid core. Further attempts at inducing mesomorphism by formation of bimetallic copper complexes were unsuccessful due to chemical instability. The photophysical properties of the compounds showed that the salicylaldimine ligands exist in solution in a tautomeric equilibrium, which can be influenced by the hydrogen-bonding character of the solvent. The ligands also show evidence of photochromism, while the complexes exhibit LMCT bands, both features which could affect and be affected by self assembly. It was also determined from their UV-vis spectra and DFT studies that the ligands bind to the metal centre in a manner which is intermediate to the two tautomeric forms, but close to the higher energy keto-amine tautomer. These results demonstrate that structural modification can be used to control both the phase behaviour and physical properties of salicylaldimine complexes. The compounds studied here also show potential to exhibit a variety of self assembly-dependent photophysical properties in the mesophase and would be good candidates for future research in this area.</p>

Controlling the Phase Behaviour and Photophysical Properties of Metallomesogens Through Structural Modification

<p>Metallomesogens (metal-containing liquid crystals) have been of interest to chemists since the early 1980s. Since this period, many of the studies published on metallomesogens have focussed on the synthesis of novel metallomesogens, and studies of their phase behaviour. As a result there is a substantial body of knowledge of their synthesis and phase behaviour, however many of these studies have overlooked the interesting physical properties that transition metals or lanthanides may impart to the mesophase (liquid crystal state). The studies that have been carried out suggest that the optical and photophysical properties resulting from their self assembly are very different to those observed in the crystalline or isotropic liquid phases, and are highly dependent on the structure of the mesophase. A series of salicylaldimine-based ligands and copper(II) complexes with a variety of structural modifications were synthesised and characterised. The structure, phase behaviour and phase relaxation kinetics of these compounds in the crystalline state were studied using differential scanning calorimetry (DSC), single crystal X-ray crystallography and variable temperature powder X-ray diffraction. The mesomorphism of the compounds was studied using small angle X-ray scattering (SAXS) and polarised optical microscopy (POM). The photophysical properties of the complexes and ligands were studied in the solution phase using ultraviolet-visible (UV-vis) spectroscopy. It was found that the smallest complexes (copper(II) N-alkyl,4-alkoxysalicylaldimine complexes) were not metallomesogens, but did exhibit multiple crystalline phases that formed as a result of changes in the conformation of the N-alkyl chains. The transition temperatures of these crystalline phase changes were strongly dependent on the length of the alkyl chains due to kinetic phenomena. The extension of the rigid core of the complex via synthesis of an N-(4-butylphenyl) derivative was successful in inducing mesomorphism in both the complex and the ligand. The ligand formed an enantiotropic nematic mesophase, while the complex formed a monotropic smectic A mesophase. The structural differences between the non-mesomorphic complexes, the mesomorphic ligand and the mesomorphic complex indicate that the determining factor in the formation of mesophases is the magnitude of lateral interactions between the molecules, which is governed by the size and shape of the rigid core. Further attempts at inducing mesomorphism by formation of bimetallic copper complexes were unsuccessful due to chemical instability. The photophysical properties of the compounds showed that the salicylaldimine ligands exist in solution in a tautomeric equilibrium, which can be influenced by the hydrogen-bonding character of the solvent. The ligands also show evidence of photochromism, while the complexes exhibit LMCT bands, both features which could affect and be affected by self assembly. It was also determined from their UV-vis spectra and DFT studies that the ligands bind to the metal centre in a manner which is intermediate to the two tautomeric forms, but close to the higher energy keto-amine tautomer. These results demonstrate that structural modification can be used to control both the phase behaviour and physical properties of salicylaldimine complexes. The compounds studied here also show potential to exhibit a variety of self assembly-dependent photophysical properties in the mesophase and would be good candidates for future research in this area.</p>

The Microstructure and Rheology of Complex Fluids Containing Na-caseinate

<p>Emulsions are widely utilised in commercial environments, such as in the food and cosmetic industries. In their simplest form, emulsions are a system consisting of two immiscible liquids in the presence of emulsifiers. To form an emulsion, an input of energy is required. In this thesis, Na-caseinate was used as the emulsifier and three systems were studied: soybean oil/Na-caseinate/water, palm oil/Na-caseinate/water and tetradecane/Nacaseinate/ water. Four main techniques were used to characterise the stabilised emulsions: laser diffraction particle sizing, PGSTE-NMR, rheology and cryo-SEM. Emulsion systems are extremely complex making control and predictability over their phase behaviour practically difficult. This is because the required overall characteristics of these colloids are strongly dependent on both the energy of formulation and the choice of an appropriate combination of emulsifier, dispersed phase and continuous phase. A full understanding of the microstructure, stability and physicochemical properties of caseinatestabilised emulsions has as yet not been achieved. For example, how does caseinate selfassembly control emulsion stability? How do concentrated caseinate-based emulsions differ from dilute ones and how do the different oils (food grade oils vs. straight chain hydrocarbon) affect the formation of emulsions? The aim of this PhD programme was to obtain data to allow a better fundamental understanding of the underlying parameters defining emulsion behaviour to be obtained ...</p>