Henry A. Ellis

The thermal behaviour of divalent and higher valent metal soaps: a review

The thermal behaviour of di-, tri- and tetravalent metal soaps (carboxylates) is reviewed, with reference to both the phase behaviour and thermal decomposition. General methods of preparation and purification are considered, and results on the structures of the solid phase obtained by techniques such as X-ray diffraction, NMR and vibrational spectroscopy are discussed. The general phase behaviour of these systems is reviewed, and the effects of metal ion, unsaturation and chain branching on this is considered. Transport and other physical properties of molten metal carboxylates, both as pure systems and mixtures, are discussed. Both the products of thermal decomposition and the proposed reaction mechanisms for the degradation of these compounds are considered, and the literature on the kinetics of these reactions is reviewed. Although this literature survey demonstrates that a considerable amount is already known about the thermal behaviour of these systems, it is clear that there is still much to be learnt.

Thermotropic Phase Transitions in Some Lead(II) Carboxylates

Abstract Differential scanning calorimetry (DSC) is used to determine the phase trantion temperatures and heats of phase changes for teh even-chain-length lead(II) carboxylates from hexanoate to octadeanoate inclusive. Phase texture studies by polarising light microscopy and x-ray diffraction suggest the phase sequence:

The thermal behaviour and spectral properties of some long chain copper(II) carboxylates

Abstract Results are presented on the temperatures, enthalpies, and entropies of the phase transitions of the even chain length copper(II) carboxylates from decanoate to octadecanoate. The dodecanoate, tetradecanoate and hexadecanoate melt directly from crystal to isotropic liquid. However, with the decanoate and octadecanoate, an intermediate phase is observed between solid and isotropic liquid. Tentative assignments are made on the intermediate phases. Electronic and infrared spectra are reported for the solid copper(II) carboxylates. In both cases, the spectra provide clear evidence for there being at least two copper ions in the head-group region of each copper carboxylate system.

Adsorbed metal ions as stabilizers for the thermal degradation of polyacrylamide

Abstract The thermal degradation of polyacrylamide has been studied in the presence of various metal ions using thermogravimetry. The thermal decomposition of the polymer in general occurs at higher temperatures in the presence of metal ions than with the pure polymer. Empirical measures of the stabilizing effect of the metal ions have been made by determining the temperatures of 12% and 50% decomposition, and the integral procedural decomposition temperature (IPDT). For main group metal ions, the stabilizing effect, as measured by the difference in ipdt between polymer-metal complex and copolymer, was inversely proportional to the radius of the metal ion, suggesting that the strength of the complex between the ion and polymer is important in determining stability. Transition metal ions, in contrast, showed no simple dependence of stabilizing effect on ionic radius, but showed, instead, a dependence upon the ligand field stabilization energy.

X-ray Diffraction Studies on Some Even-Chain-Length Lead(II) Carboxylates

Abstract X-ray diffraction is used to study structural changes associated with thermotropic phase transitions in a homologous series of even-chain-length lead(II) carboxylates from hexanoate to octadecanoate. For homologues above lead(II) dodecanoate, a lamellar structure with a monolayer molecular organization is indicated at all temperatures up to the melting point. For shorter-chain compounds, the lamellar structure appears to be a bilayer one. In these compounds, the layer spacing decreases markedly with temperature at the crystal-to-mesophase transition point and thereafter remains constant in the temperature range of the mesophase. It is suggested that the layer planes tilt with respect to an axis defined by the hydrocarbon chains on forming a smectic mesophase.

Kinetics and reaction mechanism for the thermal decomposition of some even chain lead(II) carboxylates

Abstract The kinetics of the thermal decomposition of lead(II) dodecanoate, tetradecanoate and octadecanoate have been studied thermogravimetrically in the temperature range 298–1073 K by the modified method of Freeman and Carroll. The thermogravimetric curves show that while the octadecanoate decomposes in one single step to lead oxide, the dodecanoate and tetradecanoate decompose in two stages to lead oxide. The kinetic results show all the reactions, except the second step breakdown of dodecanoate (which is nearly unity), to be of zero order. Activation energy values are presented for the desorption of carbon dioxide and the diffusion controlled rate of formation of a degradative intermediate identified as a ketone. Tentative mechanisms are advanced to explain the degradative route of these compounds.

An X-Ray and Microscopic Investigation of the Thermotropic Phases of Lead(II)Decanoate

Abstract The phase textures observed on heating lead(II)decanoate, (CH3(CH2)8COO)2Pb, from the room temperature crystalline solid to isotropic liquid melt are characterised by polarising light microscopy and X-ray diffraction. The results suggest a phase sequence: Though the phase assignments are consistent with conoscopic observations, molecular modelling and other calculations, they disagree somewhat with the assignments previously proposed by other workers. However, unlike previous work, the effects of thermal expansion. and orientational disordering in the long axes of carboxylate chains are quantified as a function of temperature, and the phase sequence assigned in the light of these considerations in conjunction with the microscopic and X-ray data.

Thermal behaviour of mercury(II) carboxylates

Abstract Data are presented on thermotropic phase transition temperatures and heats of phase changes for even-chain-length mercury(II) carboxylates from octanoate to octadecanoate inclusive. Phase structures are characterised by hot stage polarising light microscopy and x-ray diffractometry. Mesophases are not observed in the phase sequences, for octanoate to tetradecanoate, as suggested by Adeosun. Rather, these compounds exhibit simple melting and melt recrystallisation only. In contrast, a pre-melting transition is observed in each of mercury(II) hexadecanoate and octadecanoate, probably as a consequence of some modifications to the usual bilayer arrangement of molecules within the layers. For these, crystalline polymorphism is observed. The data are interpreted in terms of the disordering of hydrocarbon chains as the major process occurring up to and during melting.

Binary phase diagrams of some bivalent metal carboxylate systems

Abstract Binary phase diagrams are presented for mixtures of lead(II), zinc(II), cadmium (II), manganese(II) and mercury(II) octadecanoates. Evidence is presented to show that addition of one metal carboxylate to another, of the same chain length, does not alter the degree of order of the hydrocarbon chains either in the solid or liquid phase.

Polymorphism, crystal–crystal and liquid crystalline thermotropic phase transition behaviour of even chain length zinc(II) n-alkanoates

The thermotropic phase behaviour of a homologous series of even chain length polymeric zinc(II) n-alkanoates, Zn(CnH2n-1O2)2 (ZnC4–20) has been studied by means of differential scanning calorimetry (DSC) and temperature variation polarising light microscopy. The compounds exhibit both enantiotropic and monotropic phase behaviour on heating to the isotropic liquid and on cooling back to the room temperature solid, as seen from the following phase sequences: ZnC4–12, Lamellar Crystal ↔ Crystal I ↔ Crystal II ↔ Isotropic Melt; ZnC14–16, Lamellar Crystal ↔ Crystal I ↔ Isotropic Liquid; ZnC18–20, Lamellar Crystal → Isotropic Liquid, Lamellar Crystal ← Smectic C ← Isotropic Liquid. The total enthalpy and entropy change for melting increase linearly with chain length and are higher than expected for complete fusion of alkyl chains suggesting that the step-wise melting process involves fusion of alkyl chains and changes in electrostatic interactions in the polar region of the molecule. The phase diagrams show that phase sequences for first and second heating cycles are almost indistinguishable, where the lamellar crystal is most stable for the long chain length homologues. In addition, there is a stable mesomorphic region over a narrow temperature range, at high temperatures for the long chain length compounds. Polymorphism is exhibited and is dependent on the method of preparation.