Peter J. Dowding

Suspension polymerisation to form polymer beads

Abstract The conventional method employed for the production of large beaded particles is suspension polymerisation. A review of suspension polymerisation is presented, with particular reference to variations in chemical composition of the component phases. Both oil-in-water and water-in-oil systems are considered, as are the relevant methods of droplet stabilisation for such systems. Factors governing droplet stability and particle size and morphology are discussed. New developments including the use of continuous type reactors are also included.

A two-step model for surfactant adsorption at solid surfaces

A theoretical model is presented which accounts for two stages of surfactant adsorption onto a solid surface. The model incorporates both mass transfer (diffusion) and attachment terms, making it applicable to systems where mass transport and adsorption may occur on similar timescales. The model is tested against example systems which appear to show two-step adsorption processes, consisting of cationic ammonium bromide surfactants adsorbing onto silica from water and organic solvents. Kinetic parameters suggest that adsorption may occur in a broadly similar fashion from both water and low-dielectric solvents, and that the fast adsorption step appears to be transport-limited for organic solvents but may experience an adsorption barrier in aqueous systems.

Production of porous suspension polymer beads with a narrow size distribution using a cross-flow membrane and a continuous tubular reactor

Abstract The work described focuses on a two-stage process for the production of large porous suspension polymer beads of defined particle size and narrow size distribution. Emulsification has been performed using purpose built cross-flow membrane equipment, which allows controlled production of large emulsion droplets with a much narrower size distribution. The work described compares the production of large emulsion droplets of monomer prepared both by agitation and using a cross-flow membrane. The effects of variations in the pore size of the membrane and flow-rates on the size of the emulsion droplets produced are also investigated. The second stage of the process is polymerisation of performed monomer emulsion droplets using a continuous tubular reactor. Samples polymerised using such a method show a narrower size distribution than similar systems polymerised as a batch.

Action of hydrotropes and alkyl-hydrotropes

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.

Factors governing emulsion droplet and solid particle size measurements performed using the focused beam reflectance technique

Abstract The focused beam reflectance (FBRM) method is a relatively new method, developed to perform particle size measurements in the range 0.8–1000 μm. The particle size determined is given in terms of a chord length. The relationship between chord length and particle size obtained using other methods (image analysis and Malvern Mastersizer) has been investigated. The advantage of the FBRM method is the ability to perform in-situ particle size analysis in real time, without the need for sampling or dilution. In this work, we study the effects of various experimental factors (e.g. agitation rate and particle concentration) on FBRM measurements, and compare the size distributions with those obtained by other sizing methods. FBRM has also been used for studying the formation of (oil-in-water) emulsion droplets (with a droplet size >100 μm) with time under shear, where both sampling and dilution may affect the droplet size distribution.

Adsorption and Desorption of Nonionic Surfactants on Silica from Toluene Studied by ATR-FTIR

The adsorption and desorption kinetics and equilibrium surface coverages for a range of poly(ethylene glycol) monoalkyl ether nonionic surfactants (C(n)E(m)s) from toluene onto silica were studied using attentuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) coupled with a constant-flow liquid cell. The initial adsorption rate appears to be transport controlled, and broadly independent of molecular structure, whereas equilibrium adsorbed amounts are dependent on surfactant head group size (which controls both surface interactions and the surface-bulk partitioning). Desorption kinetic data are fitted to a simple two-step model based upon Langmuir-type kinetics. The results are interpreted in terms of a simple model comprising two modes of surfactant adsorption, each with a characteristic adsorbed amount and desorption rate constant; a possible physical basis for this observation is presented. Trace water levels appear to have a strong impact on the equilibrium surface coverages in these systems, possibly by influencing both surfactant interactions and solubility.

The characterization of porous styrene–glycidyl methacrylate copolymer beads prepared by suspension polymerization

Abstract Large, cross-linked polymer beads have been produced by suspension polymerization. The surface area of the beads can be increased by making the structure porous by the addition of an inert solvent to the droplet phase. Kinetic measurements have been performed with systematic changes in monomer ratio (styrene–glycidyl methacrylate), with an increase in glycidyl methacrylate concentration leading to an increase in reaction rate. The effects of changing the initiator type have also been studied, the use of azobisisobutyronitrile, as opposed to benzoyl peroxide, results in higher reaction rates and higher yields. The effects of these variations on both the emulsion and bead droplet size distributions have also been studied. An increase in styrene concentration has no significant effect on the average size of the emulsion droplets, but increases the final bead size. The average pore size and specific surface areas of the polymer beads (determined from BET isotherms) have also been investigated.

Cylinder to sphere transition in reverse microemulsions: the effect of hydrotropes.

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.

Formation of surfactant-stabilized silica organosols.

Organosols comprising silica nanoparticles, stabilized by adsorbed surfactant layers in low dielectric organic solvents were formulated, and their properties studied. A range of different methods for organosol formation starting from aqueous sols were evaluated and compared, in order to determine the most reliable and reproducible approach. To understand the influence of surfactant type and solvent on stability, samples were prepared with a range of surfactants and in different solvents and solvent blends. Structural properties and interparticle interactions were probed using dynamic light scattering (DLS), zeta potentials were determined, and the surfactant layers were investigated with contrast-variation SANS. SANS data suggest that for systems stabilized by ionic surfactants, the nanoparticles are in equilibrium with a population of reverse micelles, but this is apparently not the case for those stabilized by nonionic surfactants. Low zeta potentials show evidence of a small amount of surface charging in these nonaqueous systems, although it is unlikely to have any significant effect on their overall stability.

Effects of small ionic amphiphilic additives on reverse microemulsion morphology

HYPOTHESIS Initial 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. EXPERIMENTS Water-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]=0.10M 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. FINDINGS Under 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.