Tommy S. Horozov

Highly Permeable Macroporous Polymers Synthesized from Pickering Medium and High Internal Phase Emulsion Templates

Various applications require macroporous materials with high permeability and a signifi cant compressive strength. For instance, the oil servicing industry is interested in utilizing a liquid medium that can be placed within the annulus between the oil bearing natural formation and a screen wrapped perforated pipe, which turns into a macroporous permeable and mechanically stable solid during a curing step. [ 1 ] The minimum requirements for the solid macroporous material are a permeability of 1 D (10 − 12 m 2 ) and a compressive strength ≥ 3.5 MPa. This challenge could be addressed by employing high internal phase emulsions (HIPE), whose continuous phase consists of monomers, as a template to produce macroporous polymers, commonly known as poly(merized)HIPEs, [ 2 ] with a well defi ned controllable pore structure. However, conventional polyHIPEs synthesized from surfactant stabilized water-in-oil (w/o) HIPEs have poor mechanical properties [ 3 , 4 ] and low permeabilities [ 5 ]

Aspects of the stabilisation of emulsions by solid particles: Effects of line tension and monolayer curvature energy

It is well-documented that solid particles can be effective stabilisers of emulsions, and that the type of the emulsion formed is related, inter alia, to the particle wettability. In general it is to be expected that the free energy change accompanying the formation of a solid-stabilised emulsion will be positive and that emulsion stability will therefore be kinetic in nature. Recently work has appeared showing that very small solid particles, with radius around say 15 nm, can be excellent stabilisers of emulsions. This raises the possibility that line tension acting in the three-phase contact lines around adsorbed particles could have an effect on particle adsorption and hence on their effectiveness as emulsion stabilisers. We explore the effects of both positive and negative line tension on the free energy of adsorption of spherical particles at spherically curved liquid interfaces and on the free energy of drop formation. For small particles it is shown that physically realistic values of positive line tension can lead to exclusion of particles from drop interfaces, either by rendering the adsorption free energy positive or by creating energy barriers to adsorption. We also consider the effects of lateral interactions between adsorbed particles on particle adsorption, particularly strong Coulombic repulsion mediated through the oil phase. It is known that the preferred type of emulsions stabilised by surfactants can be accounted for by the existence of a curvature energy possessed by close-packed surfactant monolayers. We show here that close-packed monolayers of spherical particles at a liquid surface also possess curvature energies, and we calculate bending elastic moduli (κ) as a function of particle size, oil/water interfacial tension, line tension and contact angle. For a monolayer of (hypothetical) particles with radius 0.5 nm (similar to that of a low molar mass surfactant), the value of κ is expected to be of the order of kT, just as for surfactant monolayers. It has been assumed in our work that equal spherical particles are hexagonally close-packed around spherical drops. It is well known however that such packing is not possible, but we show in an Appendix that the assumption of hexagonal packing leads to only small errors (in the context) in calculated free energies of emulsion formation and in the calculated bending elastic moduli of particle monolayers.

Colloidal Particles at Liquid Interfaces: Colloidal Particles at Liquid Interfaces: An Introduction

Colloidal particles are an intrinsic part of systems in which finely divided matter (particles) is dispersed in a liquid or gas. Their size usually ranges from 1 nm to several tens of micrometres, thus covering a broad size domain.1–3 They are not necessarily solid and examples of “soft” colloidal particles (microgels and bacteria) will be briefly considered later in this chapter. Colloidal particles, similar to surfactant molecules, can spontaneously accumulate at the interface between two immiscible fluids (liquid–gas or liquid–liquid); they are therefore surface active.4 This fact was realised in the beginning of the last century by Ramsden5 and Pickering6 whose merit for instigating the field of particles at liquid interfaces will be discussed later. It is important to emphasise that the surface activity of these particles is not necessarily due to their amphiphilic nature. Solid particles with homogeneous chemical composition and properties everywhere on their surface (Figure 1.1(a)) can strongly attach to liquid interfaces and the reason for their surface activity is made clear below. There is, however, another class of particles with two distinct surface regions

Adsorption of Sterically Stabilized Latex Particles at Liquid Surfaces: Effects of Steric Stabilizer Surface Coverage, Particle Size, and Chain Length on Particle Wettability

A series of five near-monodisperse sterically stabilized polystyrene (PS) latexes were synthesized using three well-defined poly(glycerol monomethacrylate) (PGMA) macromonomers with mean degrees of polymerization (DP) of 30, 50, or 70. The surface coverage and grafting density of the PGMA chains on the particle surface were determined using XPS and (1)H NMR spectroscopy, respectively. The wettability of individual latex particles adsorbed at the air-water and n-dodecane-water interfaces was studied using both the gel trapping technique and the film calliper method. The particle equilibrium contact angle at both interfaces is relatively insensitive to the mean DP of the PGMA stabilizer chains. For a fixed stabilizer DP of 30, particle contact angles were only weakly dependent on the particle size. The results are consistent with a model of compact hydrated layers of PGMA stabilizer chains at the particle surface over a wide range of grafting densities. Our approach could be utilized for studying the adsorption behavior of a broader range of sterically stabilized inorganic and polymeric particles of practical importance.

Colloid-stabilized emulsions: behaviour as the interfacial tension is reduced

We present confocal microscopy studies of novel particle-stabilized emulsions. The novelty arises because the immiscible fluids have an accessible upper critical solution temperature. The emulsions have been created by beginning with particles dispersed in the single-fluid phase. On cooling, regions of the minority phase nucleate. While coarsening, these nuclei become coated with particles due to the associated reduction in interfacial energy. The resulting emulsion is arrested, and the particle-coated interfaces have intriguing properties. Having made use of the binary-fluid phase diagram to create the emulsion we then make use of it to study the properties of the interfaces. As the emulsion is re-heated toward the single-fluid phase the interfacial tension falls and the volume of the dispersed phase drops. Crumpling, fracture or coalescence can follow. The results show that the elasticity of the interfaces has a controlling influence over the emulsion behaviour.

Adsorption of shape-anisotropic and porous particles at the air–water and the decane–water interface studied by the gel trapping technique

We have studied the attachment and orientation of anisotropic and porous microparticles at liquid surfaces by using the gel trapping technique (GTT). This technique involves spreading of the microparticles of interest at the liquid interface, subsequent setting of the aqueous phase to a hydrogel thus “arresting” the particle positions at the liquid surface, and further replication of the hydrogel surface with curable polydymethilsiloxane (PDMS). The advantage of the GTT comes from the possibility to look at the PDMS replica with scanning electron microscopy (SEM) or atomic force microscopy (AFM), which allows even sub-micrometer particles to be studied at the air–water and the oil–water interface. Here we report our results on the adsorption of non-spherical anisotropic particles at liquid surfaces using the GTT. Although the GTT was originally designed to measure three-phase contact angles of spherical colloid particles, here we used this technique to reveal the orientation of a variety of shape-anisotropic and porous microparticles of practical interest at both the air–water and decane–water interfaces. We show results on typical attachment and orientation of needle-like (aragonite), rhombohedra-like (calcite) microcrystals, ethyl cellulose micro-rods, as well as highly porous hydrophilic and hydrophobic silica microparticles at these liquid interfaces. The results are important for understanding the adsorption behaviour of shape-anisotropic particles as well as porous microparticles which are used in industrial formulations as fillers, foam stabilisers and emulsifiers.

Solid particles as emulsion stabilisers

It has long been known that solid particles can act as very effective stabilisers of emulsions in the absence of surfactant. The particles are usually small in relation to the emulsion drop size (say, particle radius 0.1 times the drop radius or less). There are apparent similarities in the ways in which particles and low-molar-mass surfactants act as emulsion stabilisers. For systems with roughly equal volumes of oil and water, hydrophobic particles tend to stabilise water-in-oil emulsions whereas hydrophilic particles favour the formation of oil-in-water emulsions. Particle wettability therefore appears to parallel the hydrophile/lipophile balance of surfactants, with, for example, high hydrophile/lipophile surfactants corresponding to hydrophilic particles. In the cases of both surfactants and particles, it is probable that for significant emulsion stability, close-packed layers coating the droplets are required. Here, we give a preliminary account of work directed towards understanding more clearly the origins of the stability of solid-stabilised emulsions. Surfactant-stabilised emulsions are kinetically rather than thermodynamically stable, and we ask if the same is also true for solid-stabilised emulsions. Aspects of the stability of surfactant-stabilised emulsions can be understood in terms of the curvature properties of surfactant monolayers, and we probe the possibility that close-packed monolayers of spherical particles have curvature properties similar to those of surfactant monolayers.

The structure and melting transition of two-dimensional colloidal alloys

We study theoretically the structure and melting transition of two-dimensional (2D) binary mixtures of colloidal particles interacting via a dipole–dipole potential. Using a lattice sum method, we find that at zero temperature (T = 0) the system forms a rich variety of stable crystalline phases whose structure depends on the composition and dipole moment ratio. Using Monte Carlo (MC) simulations, we also find that the melting temperature of the different T = 0 structures is a very strong and non-monotonic function of composition. For example, from a direct analysis of the radial distribution function vs.temperature, we find that the melting temperature of hexagonal AB2 and AB6 phases is three orders of magnitude higher than that of hexagonal AB5. Finally the melting transition for our binary colloidal system is found to proceed via at least two stages for hexagonal AB2 and AB6 and at least three stages for hexagonal AB5 and is thus much richer compared to the melting transition of 2D one component colloidal systems.

An ultra melt-resistant hydrogel from food grade carbohydrates

We report a binary hydrogel system made from two food grade biopolymers, agar and methylcellulose (agar–MC), which does not require addition of salt for gelation to occur and has very unusual rheological and thermal properties. It is found that the storage modulus of the agar–MC hydrogel far exceeds those of hydrogels from the individual components. In addition, the agar–MC hydrogel has enhanced mechanical properties over the temperature range 25–85 °C and a maximum storage modulus at 55 °C when the concentration of methylcellulose was 0.75% w/v or higher. This is explained by a sol–gel phase transition of the methylcellulose upon heating as supported by differential scanning calorimetry (DSC) measurements. Above the melting point of agar, the storage modulus of agar–MC hydrogel decreases but is still an elastic hydrogel with mechanical properties dominated by the MC gelation. By varying the mixing ratio of the two polymers, agar and MC, it was possible to engineer a food grade hydrogel of controlled mechanical properties and thermal response. SEM imaging of flash-frozen and freeze-dried samples revealed that the agar–MC hydrogel contains two different types of heterogeneous regions of distinct microstructures. The latter was also tested for its stability towards heat treatment which showed that upon heating to temperatures above 120 °C its structure was retained without melting. The produced highly thermally stable hydrogel shows melt resistance which may find application in high temperature food processing and materials templating.

Sound transmission loss of hierarchically porous composites produced by hydrogel templating and viscous trapping techniques

We have developed two different methods for fabrication of hierarchically porous composites which are environmentally friendly, inexpensive and give a large amount of control over the composite microstructure. The hydrogel bead templating method involved introducing a slurry of hydrogel beads as templates into a gypsum slurry that, upon drying, left pores reflecting their size. The overall porosity reflected the volume percentage of hydrogel bead slurry used. Using mixtures of large and small hydrogel beads in controlled volume ratios as templates, we produced hierarchically porous gypsum composites that had tailorable microstructures at the same overall porosity. The viscous trapping method involved utilisation of an aqueous solution of a thickening agent, methylcellulose, during the setting process of an aqueous gypsum slurry. The methylcellulose solution traps the hydrated gypsum particles in solution and stops their sedimentation as the continuous gypsum network forms, allowing formation of an expanded microstructure. This method allows a good degree of control over the porosity which is directly controlled by the volume percentage of methylcellulose solution used. The mechanical strength of the porous composites decreased as the porosity increased. The composites with smaller pores had increased compressional strength and Youngs modulus compared to the ones produced with large pores, at constant porosity. The hierarchically porous gypsum composites showed an intermediate Youngs modulus and an increased compressional strength. We also studied the sound transmission loss of these hierarchically porous composites. We found that the ones produced by the viscous trapping method had a lower sound transmission loss over the frequency range investigated as the overall porosity was increased. We demonstrated the effect of the composite pore size at a constant porosity on the sound transmission loss. Our experiments showed that porous composites with large pores showed increased sound transmission loss at lower sound frequencies compared to those with small pores. As the sound frequency increased, the difference between their STL spectra decreased and at the higher frequency range (>2420 Hz) the composites with smaller pores began to perform better. The hierarchically porous composite had an intermediate STL spectrum, suggesting a way of tailoring the hierarchically porous structure at constant porosity to achieve desired sound insulating properties at certain frequencies.