Zwitterionic and charge-balanced polyampholyte copolymer hydrogels

Zwitterionic copolymers were synthesised from N,N-dimethyl-N-(2- acryloylethyl)-N-(3-sulfopropyl) ammonium betaine (SPDA) and 2-hydroxyethyl methacrylate (HEMA) produce a series of polyzwitterion hydrogels. For the synthesis of the charge-balanced copolymer hydrogels, two cationic monomers were selected: 2-(diethylamino) ethyl methacrylate (DMAEMA) and 3-(dimethylamino) propyl methacrylamide (DMAPMA), and an anionic monomer; 2-acrylamido-2- methylpropane sulphonic acid (AMPS). Two series of charge-balanced copolymers were synthesized from stoichiometrically equivalent ratios of DMAEMA or DMAPMA and AMPS with HEMA as a termonomer. All synthesized copolymers produced clear and cohesive hydrogels. The zwitterionic and charge-balanced copolymers displayed similar equilibrium water contents together with similar mechanical and surface energy properties. The swelling of the zwitterionic and the charge-balanced copolymers shows some features of antipolyelectrolyte behavior.

Liposomes based on dimethyldioctadecylammonium promote a depot effect and enhance immunogenicity of soluble antigen

The mechanism behind the immunostimulatory effect obtained with the cationic liposomal vaccine adjuvant DDA:TDB remains unclear. One of the proposed hypotheses is the 'depot effect' in which the liposomal carrier helps to retain the antigen at the injection site thereby increasing the time of vaccine exposure to the immune cells. In the present study we devise a method to quantify the in vivo movement of liposomes and vaccine antigen using the radioisotopes H(3) and I(125) respectively. H(3)-labeled liposomes composed of dimethyldioctadecylammonium bromide (DDA) or an 8:1 molar ratio of DDA and trehalose 6,6-dibehenate (TDB) were administered in combination with I(125)-labeled Ag85B-ESAT-6 antigen, both via intramuscular and subcutaneous injection to mice. Furthermore characterisation of the liposomal system in simulated in vivo conditions was undertaken. Our results show that this dual-labeling technique is functional and reproducible. The administration of Ag85B-ESAT-6 without a liposomal carrier leads to rapid dissemination of the antigen from the site of injection. The administration of Ag85B-ESAT-6 together with either DDA or DDA:TDB liposomes however leads to deposition of the antigen at the injection site with detectable levels still being present 14 days post injection. Neither the incorporation of TDB nor the route of injection had any significant influence on the depot effect of DDA-based liposomes. The presence of TDB in DDA liposomes improves draining of liposomes to the lymph node in addition to increasing monocyte influx to the site of injection as highlighted by the intensive blue colouring of the injection site after pontamine blue staining of phagocytic cells in vivo. Our findings provide conclusive evidence for a cationic liposome-mediated deposition of antigen at the injection site with improved monocyte infiltration.

Effect of vesicle size on tissue localization and immunogenicity of liposomal DNA vaccines

The formulation of plasmid DNA (pDNA) in cationic liposomes is a promising strategy to improve the potency of DNA vaccines. In this respect, physicochemical parameters such as liposome size may be important for their efficacy. The aim of the current study was to investigate the effect of vesicle size on the in vivo performance of liposomal pDNA vaccines after subcutaneous vaccination in mice. The tissue distribution of cationic liposomes of two sizes, 500 nm (PDI 0.6) and 140 nm (PDI 0.15), composed of egg PC, DOPE and DOTAP, with encapsulated OVA-encoding pDNA, was studied by using dual radiolabeled pDNA-liposomes. Their potency to elicit cellular and humoral immune responses was investigated upon application in a homologous and heterologous vaccination schedule with 3 week intervals. It was shown that encapsulation of pDNA into cationic lipsomes resulted in deposition at the site of injection, and strongest retention was observed at large vesicle size. The vaccination studies demonstrated a more robust induction of OVA-specific, functional CD8+ T-cells and higher antibody levels upon vaccination with small monodisperse pDNA-liposomes, as compared to large heterodisperse liposomes or naked pDNA. The introduction of a PEG-coating on the small cationic liposomes resulted in enhanced lymphatic drainage, but immune responses were not improved when compared to non-PEGylated liposomes. In conclusion, it was shown that the physicochemical properties of the liposomes are of crucial importance for their performance as pDNA vaccine carrier, and cationic charge and small size are favorable properties for subcutaneous DNA vaccination.

Lipoplexes formulation and optimisation:in vitro transfection studies reveal no correlation with in vivo vaccination studies

The aim of these studies was to compare the effect of liposome composition on physico-chemical characteristics and transfection efficacy of cationic liposomes both in vitro and in vivo. Comparison between 4 popularly used cationic lipids, showed 3b-N-(dimethylaminoethyl)carbamate (DC-Chol) to promote the highest transfect levels in cells in vitro with levels being at least 6 times higher than those of 1,2-di-O-octadecenyl-3-trimethylammonium propane (DOTMA). 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), and dimethyldioctadecylammonium (DDA) and approximately twice as efficient as dipalmitoyl-trimethylammonium-propane (DPTAP). To establish the role of the helper lipid, DC-Chol liposomes were formulated in combination with either 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) or cholesterol (Chol) (1:1 molar ratio) with and without the addition of phosphatidyl choline. The choice of helper lipid incorporated within the bilayer was found to influence the formation of complexes, their resultant structure and their transfection efficiency in vitro, with SUV-DNA complexes containing optimum levels of DOPE giving higher transfection than those containing cholesterol. The inclusion of PC within the formulation also reduced transfection efficiency in vitro. However, when administered in vivo, SUV-DNA complexes composed of PC:Chol:DC-Chol at a molar ratio of 16:8:4 micromole/ml were the most effective at inducing splenocyte proliferation upon exposure to antigen in comparison to control spleens. These results demonstrate that there is no in vitro/in vivo correlation between the transfection efficacy of these liposome formulations and in vitro transfection in the above cell model cannot be taken as a reliable indicator for in vivo efficacy of DNA vaccines.

Pegylation of DDA:TDB liposomal adjuvants reduces the vaccine depot effect and alters the Th1/Th2 immune responses

The adjuvant efficacy of cationic liposomes composed of dimethyldioctadecylammonium bromide and trehalose dibehenate (DDA:TDB) is well established. Whilst the mechanism behind its immunostimulatory action is not fully understood, the ability of the formulation to promote a 'depot effect' is a consideration. The depot effect has been suggested to be primarily due to their cationic nature which results in electrostatic adsorption of the antigen and aggregation of the vesicles at the site of injection. The aim of the study was to further test this hypothesis by investigating whether sterically stabilising DDA:TDB with polyethylene glycol (PEG) reduces aggregation, and subsequently influences the formation of a depot at the site of injection. Results reported demonstrate that high (25%) levels of PEG was able to significantly inhibit the formation of a liposome depot and also severely limit the retention of antigen at the site, resulting in a faster drainage of the liposomes from the site of injection. This change in biodistribution profile was reflected in the immunisation response, where lower levels of IgG2b antibody and IFN-? and higher level of IL-5 cytokine were found. Furthermore entrapping antigen within DDA:TDB liposomes did not improve antigen retention at the injection site compared surface adsorbed antigen. © 2011 Elsevier B.V. All rights reserved.

Vaccine adjuvant systems: enhancing the efficacy of sub-unit protein antigens

Vaccination remains a key tool in the protection and eradication of diseases. However, the development of new safe and effective vaccines is not easy. Various live organism based vaccines currently licensed, exhibit high efficacy; however, this benefit is associated with risk, due to the adverse reactions found with these vaccines. Therefore, in the development of vaccines, the associated risk-benefit issues need to be addressed. Sub-unit proteins offer a much safer alternative; however, their efficacy is low. The use of adjuvanted systems have proven to enhance the immunogenicity of these sub-unit vaccines through protection (i.e. preventing degradation of the antigen in vivo) and enhanced targeting of these antigens to professional antigen-presenting cells. Understanding of the immunological implications of the related disease will enable validation for the design and development of potential adjuvant systems. Novel adjuvant research involves the combination of both pharmaceutical analysis accompanied by detailed immunological investigations, whereby, pharmaceutically designed adjuvants are driven by an increased understanding of mechanisms of adjuvant activity, largely facilitated by description of highly specific innate immune recognition of components usually associated with the presence of invading bacteria or virus. The majority of pharmaceutical based adjuvants currently being investigated are particulate based delivery systems, such as liposome formulations. As an adjuvant, liposomes have been shown to enhance immunity against the associated disease particularly when a cationic lipid is used within the formulation. In addition, the inclusion of components such as immunomodulators, further enhance immunity. Within this review, the use and application of effective adjuvants is investigated, with particular emphasis on liposomal-based systems. The mechanisms of adjuvant activity, analysis of complex immunological characteristics and formulation and delivery of these vaccines are considered.

Environmental scanning electron microscopy offers real-time morphological analysis of liposomes and niosomes

Liposomes have been imaged using a plethora of techniques. However, few of these methods offer the ability to study these systems in their natural hydrated state without the requirement of drying, staining, and fixation of the vesicles. However, the ability to image a liposome in its hydrated state is the ideal scenario for visualization of these dynamic lipid structures and environmental scanning electron microscopy (ESEM), with its ability to image wet systems without prior sample preparation, offers potential advantages to the above methods. In our studies, we have used ESEM to not only investigate the morphology of liposomes and niosomes but also to dynamically follow the changes in structure of lipid films and liposome suspensions as water condenses on to or evaporates from the sample. In particular, changes in liposome morphology were studied using ESEM in real time to investigate the resistance of liposomes to coalescence during dehydration thereby providing an alternative assay of liposome formulation and stability. Based on this protocol, we have also studied niosome-based systems and cationic liposome/DNA complexes. Copyright © Informa Healthcare.

Liposome-mediated DNA immunisation via the subcutaneous route

Compared to naked DNA immunisation, entrapment of plasmid-based DNA vaccines into liposomes by the dehydration-rehydration method has shown to enhance both humoural and cell-mediated immune responses to encoded antigens administered by a variety of routes. In this paper, we have investigated the application of liposome-entrapped DNA and their cationic lipid composition on such potency after subcutaneous immunisation. Plasmid pI.18Sfi/NP containing the nucleoprotein (NP) gene of A/Sichuan/2/87 (H3N2) influenza virus in the pI.18 expression vector was incorporated by the dehydration-rehydration method into liposomes composed of 16 μmol egg phosphatidylcholine (PC), 8 μmoles dioleoyl phosphatidylethanolamine (DOPE) or cholesterol (Chol) and either the cationic lipid 1,2-diodeoyl-3-(trimethylammonium) propane (DOTAP) or cholesteryl 3-N-(dimethyl amino ethyl) carbamate (DC-Chol). This method, entailing mixing of small unilamellar vesicles (SUV) with DNA, followed by dehydration and rehydration, yielded incorporation values of 90-94% of the DNA used. Mixing or rehydration of preformed cationic liposomes with 100 μg plasmid DNA also led to similarly high complexation values (92-94%). In an attempt to establish differences in the nature of DNA association with these various liposome preparations their physico-chemical characteristics were investigated. Studies on vesicle size, zeta potential and gel electrophoresis in the presence of the anion sodium dodecyl sulphate (SDS) indicate that, under the conditions employed, formulation of liposomal DNA by the dehydration-rehydration generated submicron size liposomes incorporating most of the DNA in a manner that prevents DNA displacement through anion competition. The bilayer composition of these dehydration-rehydration vesicles (DRV(DNA)) can also further influence these physicochemical characteristics with the presence of DOPE within the liposome bilayer resulting in a reduced vesicle zeta potential. Subcutaneous liposome-mediated DNA immunisation employing two DRV(DNA) formulations as well as naked DNA revealed that humoural responses (immunoglobulin total IgG, and subclasses IgG1 and 1gG2a) engendered by the plasmid encoded NP were substantially higher after dosing twice, 28 days apart with 10 μg liposome-entrapped DNA compared to naked DNA. At all time points measured, mice immunised with naked DNA showed no greater immune response compared to the control, non-immunised group. In contrast, as early as day 49, responses were significantly higher in mice injected with DNA entrapped in DRV liposomes containing DOTAP compared to the control group and mice immunised with naked DNA. By day 56, all total IgG responses from mice immunised with both DRV formulations were significantly higher. Comparison between the DRV formulations revealed no significant difference in immune responses elicited except at day 114, where the humoural responses of the group injected with liposomal formulation containing DC-Chol dropped to significantly lower levels that those measured in mice which received the DOTAP formulation. Similar results were found when the IgG1 and IgG2a subclass responses were determined. These results suggest that, not only can DNA be effectively entrapped within liposomes using the DRV method but that such DRV liposomes containing DNA may be a useful system for subcutaneous delivery of DNA vaccines. © 2003 Taylor & Francis Ltd.

Preparation, characterisation and entrapment of a non-glycosidic threitol ceramide into liposomes for presentation to invariant natural killer T cells

Dendritic cells (DCs) are able to present glycolipids to invariant natural killer T (iNKT) cells in vivo. Very few compounds have been found to stimulate iNKT cells, and of these, the best characterised is the glycolipid a-galactosylceramide, which stimulates the production of large quantities of interferon-gamma (IFN-?) and interleukin-4 (IL-4). However, aGalCer leads to overstimulation of iNKT cells. It has been demonstrated that the aGalCer analogue, threitol ceramide (ThrCer 2), successfully activates iNKT cells and overcomes the problematic iNKT cell activation-induced anergy. In this study, ThrCer 2 has been inserted into the bilayers of liposomes composed of a neutral lipid, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), or dimethyldioctadecylammonium bromide (DDA), a cationic lipid. Incorporation efficiencies of ThrCer within the liposomes was 96% for DSPC liposomes and 80% for DDA liposomes, with the vesicle size (large multilamellar vs. small unilamellar vesicles) making no significant difference. Langmuir-Blodgett studies suggest that both DSPC and DDA stack within the monolayer co-operatively with the ThrCer molecules with no condensing effect. In terms of cellular responses, IFN-? secretion was higher for cells treated with small DDA liposomes compared with the other liposome formulations, suggesting that ThrCer encapsulation in this liposome formulation resulted in a higher uptake by DCs.

Liposomes:Formulation and characterisation as contrast agents and as vaccine delivery systems

Liposome systems are well reported for their activity as vaccine adjuvants; however novel lipid-based microbubbles have also been reported to enhance the targeting of antigens into dendritic cells (DCs) in cancer immunotherapy (Suzuki et al 2009). This research initially focused on the formulation of gas-filled lipid coated microbubbles and their potential activation of macrophages using in vitro models. Further studies in the thesis concentrated on aqueous-filled liposomes as vaccine delivery systems. Initial work involved formulating and characterising four different methods of producing lipid-coated microbubbles (sometimes referred to as gas-filled liposomes), by homogenisation, sonication, a gas-releasing chemical reaction and agitation/pressurisation in terms of stability and physico-chemical characteristics. Two of the preparations were tested as pressure probes in MRI studies. The first preparation composed of a standard phospholipid (DSPC) filled with air or nitrogen (N2), whilst in the second method the microbubbles were composed of a fluorinated phospholipid (F-GPC) filled with a fluorocarbon saturated gas. The studies showed that whilst maintaining high sensitivity, a novel contrast agent which allows stable MRI measurements of fluid pressure over time, could be produced using lipid-coated microbubbles. The F-GPC microbubbles were found to withstand pressures up to 2.6 bar with minimal damage as opposed to the DSPC microbubbles, which were damaged at above 1.3 bar. However, it was also found that DSPC-filled with N2 microbubbles were also extremely robust to pressure and their performance was similar to that of F-GPC based microbubbles. Following on from the MRI studies, the DSPC-air and N2 filled lipid-based microbubbles were assessed for their potential activation of macrophages using in vitro models and compared to equivalent aqueous-filled liposomes. The microbubble formulations did not stimulate macrophage uptake, so studies thereafter focused on aqueous-filled liposomes. Further studies concentrated on formulating and characterising, both physico-chemically and immunologically, cationic liposomes based on the potent adjuvant dimethyldioctadecylammonium (DDA) and immunomodulatory trehalose dibehenate (TDB) with the addition of polyethylene glycol (PEG). One of the proposed hypotheses for the mechanism behind the immunostimulatory effect obtained with DDA:TDB is the ‘depot effect’ in which the liposomal carrier helps to retain the antigen at the injection site thereby increasing the time of vaccine exposure to the immune cells. The depot effect has been suggested to be primarily due to their cationic nature. Results reported within this thesis demonstrate that higher levels of PEG i.e. 25 % were able to significantly inhibit the formation of a liposome depot at the injection site and also severely limit the retention of antigen at the site. This therefore resulted in a faster drainage of the liposomes from the site of injection. The versatility of cationic liposomes based on DDA:TDB in combination with different immunostimulatory ligands including, polyinosinic-polycytidylic acid (poly (I:C), TLR 3 ligand), and CpG (TLR 9 ligand) either entrapped within the vesicles or adsorbed onto the liposome surface was investigated for immunogenic capacity as vaccine adjuvants. Small unilamellar (SUV) DDA:TDB vesicles (20-100 nm native size) with protein antigen adsorbed to the vesicle surface were the most potent in inducing both T cell (7-fold increase) and antibody (up to 2 log increase) antigen specific responses. The addition of TLR agonists poly(I:C) and CpG to SUV liposomes had small or no effect on their adjuvanticity. Finally, threitol ceramide (ThrCer), a new mmunostimulatory agent, was incorporated into the bilayers of liposomes composed of DDA or DSPC to investigate the uptake of ThrCer, by dendritic cells (DCs), and presentation on CD1d molecules to invariant natural killer T cells. These systems were prepared both as multilamellar vesicles (MLV) and Small unilamellar (SUV). It was demonstrated that the IFN-g secretion was higher for DDA SUV liposome formulation (p<0.05), suggesting that ThrCer encapsulation in this liposome formulation resulted in a higher uptake by DCs.

Facile synthesis of well-defined hydrophilic methacrylic macromonomers using ATRP and click chemistry

A range of well-defined hydrophilic methacrylic macromonomers has been synthesized by the judicious combination of atom transfer radical polymerization (ATRP) and copper-catalyzed 1,3-dipolar cycloaddition (azide-alkyne click chemistry). An azido a-functionalized ATRP initiator was used to produce well-defined homopolymers with terminal azide functionality via ATRP in protic media at 20 °C, with generally good control being achieved over both target molecular weight and final polydispersity (Mw/Mn = 1.10-1.35). Suitable methacrylic monomers include 2-aminoethyl methacrylate hydrochloride, 2-(diethylamino)ethyl methacrylate, 2-(dimethylamino)ethyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2-(methacryloyloxy)ethyl phosphorylcholine, glycerol monomethacrylate, potassium 3-sulfopropyl methacrylate, and quaternized 2-(dimethylamino)ethyl methacrylate. These homopolymer precursors were then efficiently clicked using either propargyl methacrylate or propargyl acrylate to yield near-monodisperse (meth)acrylate-capped macromonomers with either cationic, anionic, nonionic, or zwitterionic character. Moreover, this generic route to well-defined hydrophilic macromonomers is also suitable for “one-pot” syntheses, as exemplified for 2-hydroxyethyl methacrylate and glycerol monomethacrylate-based macromonomers.

The vesicle size of DDA:TDB liposomal adjuvants plays a role in the cell-mediated immune response but has no significant effect on antibody production

The use of cationic liposomes as experimental adjuvants for subunit peptide of protein vaccines is well documented. Recently the cationic liposome CAF01, composed of dimethyldioctadecylammonium (DDA) and trehalose dibehenate (TDB), has entered Phase I clinical trials for use in a tuberculosis (TB) vaccine. CAF01 liposomes are a heterogeneous population with a mean vesicle size of 500 nm; a strong retention of antigen at the injection site and a Th1-biassed immune response are noted. The purpose of this study was to investigate whether CAF01 liposomes of significantly different vesicle sizes exhibited altered pharmacokinetics in vivo and cellular uptake with activation in vitro. Furthermore, the immune response against the TB antigen Ag85B-ESAT-6 was followed when various sized CAF01 liposomes were used as vaccine adjuvants. The results showed no differences in vaccine (liposome or antigen) draining from the injection site, however, significant differences in the movement of liposomes to the popliteal lymph node were noted. Liposome uptake by THP-1 vitamin D3 stimulated macrophage-like cells did not show a liposome size-dependent pattern of uptake. Finally, whilst there were no significant differences in the IgG1/2 regardless of the liposome size used as a delivery vehicle for Ag85B-ESAT-6, vesicle size has a size dependent effect on cell proliferation and IL-10 production with larger liposomes (in excess of 2 µm) promoting the highest proliferation and lowest IL-10 responses, yet vesicles of ~500 nm promoting higher IFN-? cytokine production from splenocytes and higher IL-1ß at the site of injection.

Dipole moments and Kerr effect of poly(n-vinylcarbazole) and its complexes with trinitrofluorenone

N-vinylcarbazole was polymerised using the free radical catalyst (azo-bisisobutyronitrile) and cationic catalysts (boron-trifluoride etherate and aluminium chloride). The polymers produced were characterised by molecular weight measurements and powder x-ray diffraction. The tacticity of the polymer samples was determined using proton and carbon-13 nuclear magnetic resonance spectroscopy. Measurements of their static dielectric permittivity and electro-optical birefringence (Kerr effect) in solution in 1,4-dioxane were carried out over a range of temperatures. The magnitudes of the dipole moments and Kerr constants were found to vary with changes in the tacticity of poly(N-vinylcarbazole). The results of these measurements support the view that the stereostructure of poly(N-vinylcarbazole) is sensitive to the mechanism of polymerisation. These results, together with proton and carbon-13 N.M.R. data, are discussed in terms of the possible conformations of the polymer chains and the relative orientation of the bulky carbazole side groups. The dielectric and molecular Kerr effect studies have also been carried out on complexes formed between 2,4,7-trinitro-9-fluorenone (TNF) and different stereoregular forms of poly(N-vinylcarbazole) in solution in 1,4-dioxane. The differences in the molar Kerr constants between pure (uncomplexed) and complexed poly(N-vinylcarbazole) samples were attributed to changes in optical anisotropy and dipole moments. A molecular modelling computer program Desktop Molecular Modeller was used to examine the 3/1 helical isotactic and 2/1 helical syndiotactic forms of poly(N-vinylcarbazole). These models were used to calculate the pitch distances of helices and the results were interpreted in terms of van der Waal's radii on TNF. This study indicated that the pitch distance in 3/1 isotactic helices was large enough to accommodate the bulky TNF molecules to form sandwich type charge transfer complexes whereas the pitch distance in syndiotactic poly(N-vinylcarbazole) was smaller and would not allow a similar type of complex formation.

Studies in clay chemistry

A study of clay chemistry has been approached with three aims: - to modify the conducting properties by intercalation of tetrathiafulvalene, - to study the electrochemistry of redox-active coordination compounds immobilised on clay coated electrodes, and - to study the role of clays as reagents in inorganic glass forming reactions using mainly solid-state magic-angle-spinning NMR. TTF was intercalated by smectites containing different interlayer and lattice cations. Evidence from ESR and 57Fe Mossbauer indicated charge-transfer from TTF to structural iron in natural montmorillonite, and to interlayer Cu2+ in Cu2+ exchanged laponite. No charge transfer was observed for laponite (Na+ form) itself. Ion exchange of TTF3(BF4)2 with laponite was found to proceed quantitatively. The intercalated species were believed to be (TTF)2+ dimers. Conductivity data showed an order of magnitude increase for the intercalated clays. The mechanism is thought to be ionic rather than CT as Na+ laponite showed a similar enhancement in conductivity. Mechanically robust colloidal clay films were prepared on platinum electrodes. After immersion in solutions containing redox active complexes [Co(bpy)3]3+ and [Cr(bpy)3]3+, the films became electroactive when a potential was applied. Cyclic voltammograms obtained for both complexes were found to be of the diffusion controlled type. For [Co(bpy)3]3+ immobilised on clay coated electrodes, a one-step oxidation and four-step reduction wave was observed corresponding to a one electron stepwise reversible reduction of Co(III), through Co(II), Co(I), Co(O) to Co(I) oxidation state. For [Cr(bpy)3]3+ the electrochemistry was complicated by the presence of additional waves corresponding to the dissociation of [Cr(bpy)3]3+ into the diaquo complex. ESR and diffuse reflectance data supported such a mechanism. 29Si, 27Al and 23Na MAS NMR spectroscopy, supported by powder XRD and FTIR, was used to probe the role of clays as reagents in glass forming reactions. 29Si MAS NMR was found to be a very sensitive technique for identifying the presence and relative abundance of crystalline and non-crystalline phases. In thermal reactions of laponite formation of new mineral phases such as forsterite, akermanite, sillimanite and diopside were detected. The relative abundance of each phase was dependent on thermal history, chemical nature and concentration of the modifier oxide present. In continuing work, the effect of selected oxides on the glass forming reactions of a model feldspar composition was investigated using solid state NMR alone. Addition of network modifying oxides generally produced less negative 29Si chemical shifts and larger linewidths corresponding to a wider distribution of Si-O-Si bond angles and lengths, and a dominant aluminosilicate phase with a less polymerised structure than the starting material. 29Si linewidths and 27Al chemical shifts were respectively correlated with cationic potential and Lewis acidity of the oxide cations. Anomalous Al(4) chemical shifts were thought to be due to precipitation of aluminate phases rather than a breakdown in Lowenstein's aluminium avoidance principle.

Interlamellar modification of smectite clays

Clay minerals, both natural and synthetic, have a wide range of applications. Smectite clays are not true insulators, their slight conductivity has been utilized by the paper industry in the development of mildly conducting paper. In particular, the synthetic hectorite clay, laponite, is employed to produce paper which is used in automated drawing offices where electro graphic printing is common. The primary objective of this thesis was to modify smectite clays, particularly laponite, to achieve enhanced conductivity. The primary objective was more readily achieved if the subsidiary objective of understanding the mechanism of conductivity was defined. The cyclic voltammograms of some cobalt complexes were studied in free solution and as clay modified electrodes to investigate the origin of electroactivity in clay modified electrodes. The electroactivity of clay modified electrodes prepared using our method can be attributed to ion pairs sorbed to the surface of the electrode, in excess of the cationic exchange capacity. However, some new observations were made concerning the co-ordination chemistry of the tri-2-pyridylamine complexes used which needed clarification. The a.c. conductivity of pressed discs of laponite RD was studied over the frequency range 12Hz- 100kHz using three electrode systems namely silver-loaded epoxy resin (paste), stainless-steel and aluminium. The a. c. conductivity of laponite consists of two components, reactive (minor) and ionic (major) which can be observed almost independently by utilizing the different electrode systems. When the temperature is increased the conductivity of laponite increases and the activation energy for conductivity can be calculated. Measurement of the conductivity of thin films of laponite RD in two crystal planes shows a degree of anisotropy in the a.c. conductivity. Powder X-ray diffraction and 119Sn Mossbauer spectroscopy studies have shown that attempts to intercalate some phenyltin compounds into laponite RD under ambient conditions result in the formation of tin(IV) oxide pillars. 119Sn Mossbauer data indicate that the order of effectiveness of conversion to pillars is in the order: Ph3SnCl > (Ph3Sn)2O, Ph2SnCl2 The organic product of the pillaring process was identified by 13C m.a.s.n.m.r. spectroscopy as trapped in the pillared lattice. This pillaring reaction is much more rapid when carried out in Teflon containers in a simple domestic microwave oven. These pillared clays are novel materials since the pillaring is achieved via neutral precursors rather than sacrificial reaction of the exchangeable cation. The pillaring reaction depends on electrophilic attack on the aryl tin bond by Brønsted acid sites within the clay. Two methods of interlamellar modification were identified which lead to enhanced conductivity of laponite, namely ion exchange and tin(IV) oxide pillaring. A monoionic potassium exchanged laponite shows a four fold increase in a.c. conductivity compared to sodium exchanged laponite RD. The increased conductivity is due to the appearence of an ionic component. The conductivity is independent of relative humidity and increases with temperature. Tin(IV) oxide pillared laponite RD samples show a significant increase in conductivity. Samples prepared from Ph2SnCl2 show an increase in excess of an order of magnitude. The conductivity of tin(IV) oxide pillared laponite samples is dominated by an ionic component.