Investigation of microstructure of disordered colloidal systems by small-angle scattering

Abstract

Small-angle scattering (SAS) has been widely applied to study the microstructure of colloidal systems. Although colloids cover a wide range of materials, in general they can simply be viewed as basic building particles arranging themselves, according to their interaction, in a continuous medium. In this study, three seemingly very different systems were investigated under various conditions. They are the calcium-silicate-hydrate (C-S-H) gel, magnesium-silicate-hydrate (M-S-H) gel, and micellar solution formed by Pluronics triblock copolymers. C-S-H is the main binding phase of ordinary Portland cements. An elaborate analytical model for the form factor of C-S-H basic building particles was established for the first time. This model has ability to integrate two different models together by taking two different limits of the form factor formula. Essential structural parameters of C-S-H gels prepared at various conditions were extracted through model fitting. It was found in this study that microstructure of C-S-H gels changes from continuous planar pore structure to discrete colloidal structure when increasing water content or adding methylhydroxyethyl cellulose additive. Open microstructure or small globule size leads to higher flowability or facilitates the extrusion process as macroscopic properties. Much attention has been paid recently to the MgO-based green cements due to the little CO₂ generated during their production process compared with the ordinary cements. However, the poor mechanical properties prevent them from implementing widescale use. This current study on microstructure of both C-S-H and M-S-H gels indicates that the primary unit at the nanoscale level of C-S-H to be a multilayer disk-like globule, whereas for M-S-H it is a spherical globule. This prominent difference at the nanoscale also reflects in gel structure at micrometer lengthscale. The surface contact between the basic particles found in C-S-H gels leads to better mechanical properties than M-S-H gels which interact through point contact. This study therefore gives essential insight to design future robust and eco-friendly binders. Pluronics is a class of amphiphilic copolymers which aggregate to form micelle particles in water. Small-angle neutron scattering contrast variation measurements were conducted to extract the microstructure, especially the solvent distribution within the micelle particles, under several conditions. It is suggested in this study that high water content found in the micelles formed by short copolymer chains but same PO/EO ratio promotes composition fluctuation within the micelles and in turn stabilizes the liquid-like micelle phase. In addition, the dehydration of core region of the micelles due to increasing concentration or temperature leads to phase transition from liquid-like to crystalline micelle state. These results can deepen the current understanding of the complicated phase behaviors of amphiphilic copolymers. Although the three systems studied have very different features, this work demonstrates that they can all be tackled by similar SAS analysis. Furthermore, structure-property relationships and structure-phase behavior relationships are established based on the results.