NMR Diffusion Studies in Heterogeneous Systems. Surfactant solutions, Polymer solutions and Gels

University dissertation from Physical Chemistry 1, Lund University

Abstract: The aim of this study has been to investigate the self-diffusion behaviour in polymer solutions and in surfactant solutions using the pulsed field gradient (PFG) NMR technique. The NMR self-diffusion experiment was shown to provide detailed information in complex surfactant mixtures. In particular, the microemulsion phase and the hexagonal phase of Didodecyldimethylammoniumsulphate (DDAS) formed with water and oil was investigated. It is argued that the microemulsion phase contains spherical micelles of normal curvature close to the emulsification failure line. When decreasing the oil content the micelles grow, presumably in one dimension, to form a bicontinuous micellar phase. In mixtures of a microemulsion of normal curvature (DDAS) with a microemulsion of reversed curvature (DDAB) it was found that a one-phase solution was formed. In this phase the diffusion coefficients of all three components revealed detailed information about the structure in the system. In particular, the lateral diffusion coefficient of the surfactant could be directly measured. When applied to polymer solutions, the PFG NMR experiment is argued to convay detailed information about both structure and dynamics in many different polymer solutions. In particular, the echo decay in a PFG NMR experiment displays features that are closely connected with both the structure and the dynamics in these systems. However, separation of the different diffusion mechanisms that give rise to the observed features, is sometimes difficult and great care must be taken when evaluating the PFG NMR data. The diffusion work in the polymer part of this thesis is focused on extracting information about structure and dynamics in self-associating aqueous polymer systems. More specifically, ethyl(hydroxyethyl)cellulose (EHEC) and its hydrophobically modified analogue, HMEHEC, has been investigated. In these systems the polymers display a varying degree of association depending on the chemical structure, concentration and temperature. A novel result is the conclusion that a static network is formed in the polymer solution, the presence of which was seen as a very slowly diffusing component in comparison to the rest of the matrix that diffused much faster. It is argued that the presence of this slowly diffusing component could to some extent explain the anomalous scaling behaviour that is found in semi-dilute polymer solutions. It is further suggested that in order to obtain more detailed information and to elucidate some of the still remaining questions in the field of polymer dynamics, the slow component should be further investigated using the PFG NMR technique.

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