Soft X-ray stereo microscopy for investigation of dynamics and elemental distribution of colloidal systems from the environment

Abstract

Soil colloids are classified to be in a size range of 1nm to 1μm. Their huge specific surface determines the interactions of the soil colloids, which differ from the behaviour of the same material in a bulk system. For a long time, this led for example to an underestimation of matter transport through the soil matrix, which is of major impact when it comes for example to toxicants reaching the ground water. Despite their relevance, the interactions of soil colloids are not understood yet.For the investigation of the dynamics in the colloidal systems, both high spatial resolution and the allowance of a complete and natural colloidal system, thus an aqueous system of at least several micrometre thickness, are necessary. These requirements are uniquely matched by soft X-ray microscopy. The spatial resolution of 20nm to 50nm matches with the colloidal size range, and due to the advantageous absorption features in the soft X-ray energy range, water layers of up to 10 μm thickness can be penetrated while a natural contrast is given for soil colloids. Thus, high resolution micrographs of soil samples in aqueous media can be obtained. Furthermore, by taking images at two different X-ray energies, elemental mapping can be applied to identify e.g. iron containing particles within the heterogeneous soil sample unambiguously.However, for the investigation of colloidal interactions, three-dimensional distances have to be revealed. This can be done by soft X-ray tomography, but cryo fixation is necessary to avoid detectable radiation damage . Therefore, the combination of high resolution chemical-sensitive soft X-ray microscopy with stereo imaging and processing techniques presented here forms a novel tool for the investigation of aqueous colloidal systems. Information about the spatial distribution within the sample is provided with small calculation effort processing just a pair of stereo micrographs. Thus, the extension towards investigation of dynamical behaviour is possible on the part of the experiment as well as of the processing.The aim of this work was the application of stereo X-ray microscopy to aqueous samples from the environment and the extension to the investigation of dynamical behaviour of the samples. The results can be divided into three parts, an instrumental, a methodical, and a soil scientific one. On the instrumental part, the performance of stereo experiments at various X-ray microscopy stations are described. On the methodical part, the enhanced version of the self-written programme xstereo is introduced, including a discussion for the requirements for the alignment of two stereo X-ray micrographs. Furthermore, two types of sample holders, capillaries and fluidic cells based on coated Si3N4 membranes, are tested for possible sample manipulation during the stereo experiment. With regard to the soil science, experiments are initially performed with model systems of clay and iron oxides. Subsequently, more complex, clay containing soil samples are investigated similarly, as well as the interaction of mixed systems of clay or soil particles with additive haematite. The spatial arrangement of these mixed samples is revealed by xstereo processing, as the distances between clay edges and haematite particles enable conclusions to the charges, which correlate to the pH value of the particular systems. Thus, the determination of the point of zero charge of the investigated clays could be verified or specified. Therefore, the demonstrated capacity of stereo soft X-ray microscopy in revealing information about spatial arrangements of aqueous colloidal particles at the nanometre scale in combination with (induced) dynamical behaviour and with unambiguous particle identification or detection by elemental contrast enables an new sight into the interactions of soil colloids.