Spectroscopic studies of polyethers polyelectrolytes and corneae

Abstract

This study combines fundamental and applied topics in surfactant polymer chemistry and biological systems, and has spectroscopic techniques as a linking theme. Non-ionic polyether surfactant-salt systems, surfactant-mercury(II) halide complexes, eye-banked corneae and aqueous dextran sulfate solutions as model compounds have been investigated using macro-and micro-Raman, as well as FT-IR spectroscopic techniques. The structures of surfactant-mercury(II) halide complexes in the solid state were also characterized by X-ray crystallography and NMR spectroscopy, including magic angle spinning to acquire solid state NMR data. The aqueous surfactant-salt systems were investigated to elucidate the effect of the alkali-metal and alkaline-earth-metal cations on the polyether chain conformations of alkylphenoxy-and n-alkylpolyoxyethylene surfactants. Both polydisperse and monodisperse surfactant preparations were used. Conformational changes related to coordination effects were monitored by Raman difference spectroscopy and curve-fitting. Addition of some cations resulted in the appearance of a distinctive feature at ca. 868 cm-1. This has been associated with a surfactant-cation complex incorporating a polyether chain exhibiting the TGT-TĜT conformation sequence about the O-CH2-CH2-CH2-O-CH2-O segment (T, trans; G, gauche; Ĝ, gauche-minus). Complexation of the surfactant was found to be dependent on the size and charge of the cation with the coordination effects maximized for Ba2+. Concentration-dependence studies employing the surfactant-Ba2+ systems suggested that a ratio of three oxyethylene sub-units per cation optimizes the coordination of this cation. The length of the polyether chain also played a significant role in determining complex formation. Investigations of the alkylphenoxypolyoxyethylene surfactant-Ba2+ system indicate that the coil-conformer is the precursor to the complex. Counter anions such as halides appeared to have a negligible effect on the formation of formation of the surfactant-cation complex. Novel crystalline complexes of a monodisperse alkypolyoxyethylene surfactant with mercury(II) chloride and mercury(II) bromide have been isolated and their structures determined by X-ray crystallography. These are the first crystal structures of a solid phase incorporating a non-ionic surfactant. An unusual feature of these complexes is in the arrangement of the molecules in the crystal lattice, forming clearly separated hydrophillic (complexed polyether chain)and hydrophobic (alkyl) lamellar-like regions. The vibrational spectra of these complexes reported here constitute the first sets of spectroscopic fingerprints of the polyether chain conformation for these non-ionic surfactant phases. MAS NMR spectroscopy proved useful for documenting the structure and conformation of the surfactant alkyl chain. Vibrational spectroscopy, in particular Raman microscopy, is one of the few sensitive and non-destructive techniques which allows in vivo sampling of intact corneal tissue. In the present work, Raman microscopy was employed in the analysis of eye-banked corneae with the prospect of developing the technique for evaluating corneal viability for transplantation. The investigations focused on the state of hydration of the cornea and revealed that the water content of the tissue can be semi-quantitatively related to its vibrational spectrum. This was achieved using the O-H band in the region 3000-4000 cm-1 and the C-H stretching vibration of the collagen at 2945 cm-1. Such a correlation is significant as the state of hydration of the cornea is a direct measurement of the viability of the endothelial cells of the tissue which is one of the critical factors in determining transplantation success. In a related study, aqueous dextran sulfate systems were employed as a model to elucidate the interaction of water with polyelectrolytes in corneal tissue and related biological systems. The results indicated that a highly charged polyanion with both hydrophobic and hydrophillic sites on its surface, affects the hydrogen-bonding network of water in the system. Consequently, this cause "structuring" of the water molecules and is reflected in the spectral profile of the O-H stretching mode in the region 3000-4000 cm-1. Variations in the vibrational spectra were dependent upon the extent of the different populations of water present and can be explained by invoking the concepts of long-range intermolecular coupling. Analysis of the dextran skeletal bands in the region 750-1850 cm-1 confirmed that the –SO3- sites play an important role in the hydration of dextran sulfate. The presence of two symmetric –SO3- stretching modes in the Raman spectra of these systems, both of which are sensitive to the state of hydration of dextran sulfate, has been explained in terms of a model based on non-equivalent –SO3- sites. The effects of ion-pairing in aqueous dextran sulfate systems were also investigated by the addition of potassium chloride to aqueous sodium dextran sulfate. The greater affinity of dextran sulfate for K+ ions was explained in terms of differences in the hydration radii of these cations, Na+>K+. The studies above illustrate the versatility of vibrational spectroscopic techniques and contribute to a better understanding of the functioning of hydrophillic polymer-based materials in aqueous media.