A multi-analytical study of physical behaviour in clay hydrogel media
Date
2023-10-10
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of leeds
Abstract
Hydrogels have many properties which make them suitable for biomedical applications. These properties can be improved by the integration of inorganic ordered systems, in particular clays, into the gels. Amongst the class of
layered clay minerals, synthetic materials such as the laponites have been found especially valuable as thixotropic gel additives and this project focuses on this class of gel materials. This project presents pa multi-analytical study of two grades of laponite (RD -XLG) clay and cation-modified laponite (RD & XLG) hydrogels has been performed to probe features of structural and visco-elastic behavior. The specific context to this work lies within the potential role of geologically relevant hydrogels, and laponite hydrogels in particular, to abiogenesis, the emergence of biological organisms on Earth. Wherever possible, the results have been related to this context.
Description
Chapter 1 offers an overview of the core principles that are relevant to the subject matter addressed in this study, and discusses hydrogels, clay hydrogels, their synthesis, properties and especially their potential role in abiogenesis. In addition, some other application fields of laponite hydrogels are considered where they are deemed relevant. The chapter thereafter finishes by providing a discussion of important analytical techniques employed in the project.
Chapter 2 describes the preparation of two grades of laponite (RD - XLG) and cation-modified laponite (the main hydrogels used in this work) along with surface analyses using scanning electron microscopy and associated techniques including surface area (BET) measurements, energy dispersive X ray and powder X-ray diffraction. The presented analyses demonstrate a direct correlation between the valency of introduced cations and the microstructures of modified hydrogels. As the valency of the introduced cation increases, the peeling degree of the modified laponite discs in water decreases, resulting in a reduction in the modified laponite's ability to form the "house of cards" structure.
Chapter 3 presents a thermal analysis study of the effect of replacing cations of different valences on laponite hydrogels. Thermogravimeyric analysis and differential scanning colorimetry studies have been perfoemed and these showed that increasing cation valences led to a decrease in thermal resistance and water retention along with a decrease in Tmax and VPTT.
Chapter 4 examines the use of two methods to monitor light scattering as a means to explore the temporal variations in scattering shown by gelling laponites. Turbidimetry measurements and borad-wavelength light scattering sing UV-VIS spectrophotometry have been used to investigate how the presence of metalions with different oxidation states influence light scattering and hence gelation. According to the light-scattering data using UV-VIS spectrophotometry, the introduction of cations has a significant effect on the gelation process, with the trend being monovalent > trivalent > divalent. While turbidity meter measurements showed limitations to measure the modified laponite because of the colouration of the cobalt and copper samples. So it was not possible to measure the turbidity.
Chapter 5 outlines two sets of analytical investigations. In the first of these, measurements of the viscosity of hydrogels of laponite and cation-modified laponite were made whilst the second involved a detailed rheological investigation of the visco-elastic behaviour of laponite hydrogels. These latter involved performing three rheological stress-tests (oscillatory, frequency sweep, and rotational tests) were used. Both kinds of measurement investigated the effects of substituting cations with different valences on the viscosity and rheological properties of hydrogels. The study of the viscosity of the laponite hydrogels showed large fluctuations in the data occurred producing no reliable trends. However, the study of the viscoelastic properties showed viscous and stiffness of laponite (RD &XLG) and cation-modified laponite can be arranged in the order RD-Na >RD>RD-Cu>RD-Co and for laponite XLG, XLG-Na >XLG >XLG>XLG-Cu>XLG-Co.
Chapter 6 outlines how the amphiphilic self-assembly (micelle formation) of a probe molecule, sodium dodecylsulphate (SDS), behaviour differs between the aqueous and hydrogel phases (laponite and cation modified laponite ) by measuring critical micelle concentrations(CMC). Two mechanisms of measurement were used, UV-VIS spectrophotometry with pinacyanol chloride dye and fluorescence spectroscopy with pyrene dye In the measurements of UV-Vis absorptions, the CMC value decreased in XLG laponite compared to the aqueous phase and remained almost constant in the modified XLG laponite phase. In the measurements of intensities using fluorescence spectroscopy, the CMC value of XLG laponite was close to the CMC value that was measured in water. This result does not reflect the CMC in clay hydrogel; rather, it reflects the partitioning of the fluorescent probe.
Chapter 7 presents all experimental procedures, preparations and analytical
detail the preparation of samples from chapters 2 to 6.
Keywords
Soft matter, hydrogels, laponite