Kathryn M. McGrath

Switching between kinetic and thermodynamic control: calcium carbonate growth in the presence of a simple alcohol

Calcium carbonate was grown by diffusion of gaseous carbon dioxide into aqueous calcium chloride–alcohol solutions and the relationship between the alcohol additive and the growing crystal was investigated. There was no evidence that crystal growth was controlled through any specific molecular interaction between the growing calcium carbonate facets and the alcohol. Under control conditions, both calcite and vaterite were grown in a ratio of 1∶3, respectively. However, the overall trend for all the alcohols investigated was preferential precipitation of calcite, as compared to the control. In addition to this primary alcohol effect, a morphological variant of perfect calcitic rhombohedra, a hopper crystal, was observed. This morphological form was present in all growth solutions, but made up an increasing proportion of the calcite fraction for solutions with viscosities above approximately 1.6 mPa s, irrespective of the absolute alcohol concentration. The different morphological and polymorphic forms of the calcium carbonate crystals were examined by scanning electron microscopy and powder X-ray diffraction, respectively.

Controlling the kinetic versus thermodynamic crystallisation of calcium carbonate

Calcium carbonate was grown by diffusion of gaseous carbon dioxide into an aqueous solution of calcium chloride. The interplay between kinetic and thermodynamic control of the growth was investigated. It was found that the calcium ion is the dominant species in producing thermodynamic growth of rhombohedral calcite, while kinetic growth is preferentially controlled by carbon dioxide. This was evidenced by the formation of vaterite as the partial pressure of carbon dioxide increased for a given calcium ion concentration. The different morphological and polymorphic forms grown were examined by scanning electron microscopy and powder X-ray diffraction, respectively.

Quantitative determination of binary and tertiary calcium carbonate mixtures using powder X-ray diffraction

The ability to determine the calcium carbonate polymorphic ratio of calcite, aragonite and vaterite in a mixture is important for a variety of applications, particularly the fields of biomineralisation and crystal engineering. Raman spectroscopy and powder X-ray diffraction were used to quantitatively determine the polymorphic composition of both binary and tertiary mixtures of calcium carbonate. It was found that the quantitative detection limits of powder X-ray diffraction were superior to both Raman and infrared spectroscopy.

Size and charge characterisation of a submicrometre oil-in-water emulsion using resistive pulse sensing with tunable pores.

Resistive pulse sensing (RPS) with tunable pores (TPs) has been used to investigate an oil-in-water emulsion stabilised with β-lactoglobulin (BLG). The mode of the droplet size distribution steadily increased over four months, from less than 150 nm to more than 200 nm. Results suggest that the dominant growth mechanism was migration of oil to relatively large droplets, as in Ostwald ripening. In contrast, the growth dynamics for salt-induced aggregation suggest flocculation and coalescence of droplets coming into contact. The charge measurement method recently developed by Vogel et al. was also applied to the emulsion. The two data analysis methods used yielded average droplet ζ-potentials of -18.9 mV and -21.8 mV, compared with -27.6 mV obtained using light scattering. Methods for measuring emulsion droplet deformation and the charge on individual droplets are under development. Tunable pores are a useful tool for improved characterisation of submicrometre emulsions, as well as other synthetic and biological particles, as they provide better precision than light scattering for particle number distributions.

Polymerisation of liquid crystalline phases in binary surfactant/water systems

The polymerisation of a polymerisable fatty acid surfactant (sodium 10-undecenoate) has been studied in both its self-assembled and non self-assembled forms. Polymerisation in non self-assembled solution was achieved to near completion. The polymerisation produces a surface active polymer. The self-assembling behaviour of this pre-polymerised form differs markedly from that observed for the monomeric surfactant [1]. A lamellar phase only is formed in the polymeric phase diagram with no hexagonal or lamellar gel phases being observed. Polymerisation in the different self-assembled forms of sodium 10-undecenoate reached a limit of approximately 30% only, i.e., the surfactant aggregates act to inhibit the polymerisation. The nature of the hydrocarbon chain was found to play a critical role in determining the effect that polymerisation had on the underlying geometry of the surfactant molecules. When the chains are in a fluid-like state (as for the micellar and hexagonal phases) the original monomeric matrix remains largely unchanged. Whereas partial polymerisation of the lamellar gel phase results in a phase transformation.In addition the hydrolysis of the fatty acid soap at low concentrations (close to the critical micelle concentration) has been investigated. Hydrolysis was shown to produce both the parent fatty acid and an acid soap dimer. The presence of these species greatly affects the solution behaviour in this region of the phase diagram shifting the critical micelle concentration to very high concentrations of sodium 10-undecenoate (ca. 0.4 M).

Biomimetic approach to forming chitin/aragonite composites

Biomimetic materials which display the complexity of biominerals like nacre are synthetically difficult to prepare. The formation of chitin/calcium carbonate composites, where CaCO(3) is present as aragonite, was achieved via reacetylation of preformed chitosan scaffolds followed by the combination of presoaking of chitin templates with mineral solutions in the presence of poly(acrylic acid). The as-synthesised composites are comprised of well-ordered ribbons of aragonite crystals held within an organic matrix, mimicking the structure of nacre.

Superimposed effect of kinetics and echinoderm glycoproteins on hierarchical growth of calcium carbonate

We present here the synthesis of hierarchical calcite, grown in the presence of acidic glycoproteins isolated from adult spines of the sea urchin Evechinus chloroticus. Hierarchical growth is only achieved when kinetic growth control (attained for high concentrations of calcium ion) is superimposed on the thermodynamic constraints imposed by crystal growth under limiting carbon dioxide in the presence of glycoproteins. Cleaved oligosaccharides produce analogous hierarchical structures, suggesting that they provide a similar growth environment for the calcium carbonate as the parent glycoproteins. Their effect is achieved at lower concentration as compared with the glycoproteins. Deglycosylated protein does not produce this effect and at high concentrations completely inhibits calcium carbonate growth.

Moisture adsorption effects in biaxial and chiral optical thin film coatings

Thin film form birefringence depends on the shape or bunching of nanostructural columns and on the difference in the refractive indices of the columns and the surrounding voids. When moisture from the atmosphere enters the nanostructure the linear form birefringence may decrease by a large amount, of the order of 50%. However, current data refers to tilted-columnar films. We report here on moisture penetration effects in films fabricated by serial bideposition and engineered for large linear form birefringence or large circular form birefringence. As well, we consider post-deposition processes that may retard the uptake of moisture. Vacuum deposition results in oxygen deficient surfaces which rapidly absorb oxygen from the atmosphere either in the form of O2 or H2O. In order to retard water uptake this deficiency must be reduced, possibly by annealing samples in an oxygen rich atmosphere subsequent to deposition. Alternatively, surface oxide and hydroxyl groups can be rendered hydrophobic via reaction with silane derivatives. Both methods have been tested for their effectiveness in retarding water uptake.

In situ continuous growth formation of synthetic biominerals

Continuous self-assembled growth of both the organic and inorganic components of materials with nacre-like structure is achieved upon mineralisation of chitin and chitosan scaffolds using a combined soaking method and the inclusion of poly(acrylic acid) and chitosan oligomers as additives.

Polymerisation of liquid crystalline phases in binary surfactant/water systems: Part 2. ?-undecenyltrimethylammonium bromide

The self-assembly behaviour of the polymerisable surfactant ω-undecenyltrimethylammonium bromide (ω-UTAB) both before and after polymerisation has been investigated. In addition polymerisation of the liquid crystalline phases formed by this surfactant in aqueous solution has been studied. Introduction of the carbon-carbon double bond at the end of the hydrocarbon chain increases the rigidity of the paraffinic chains such that the Krafft curve is shifted to higher temperatures compared with that of dodecyltrimethylammonium bromide, a nonpolymerisable analogue. Both the polymerised and non-polymerised forms have been observed to have the same phase progression, with the polymer being more soluble in water such that the liquid crystalline phases formed at high surfactant concentration are accessible at room temperature. Polymerisation of the liquid crystalline phases of ω-UTAB indicate that polymerisation proceeds to approximately 40% (in comparison) with 80% in a non-aggregated form) and that the original monomeric matrix is undisturbed upon partial polymerisation.