Masters defense by Unique Gyanu Yogi

Title: Analysis of Quasi-Elastic Neutron Scattering Data on Confined Water at the Nanosecond Time Scale

Abstract: Water confined within the interlayer spaces of clay minerals plays a critical role in deter- mining their physical, chemical, and transport properties. Among the phyllosilicates, smectites such as Montmorillonite (Mt) and Hectorite (Ht) are of particular importance due to their high swelling capacity, ion-exchange ability, and extensive use in environ- mental and industrial applications.

Despite decades of study, a detailed microscopic understanding of how structural heterogeneity and layer charge influence the dynamics of interlayer water remains incomplete. The challenge arises from the coexistence of multiple water populations, ranging from tightly bound to freely diffusing, each contributing differently to the overall relaxation processes.

This thesis addresses this problem through quasi-elastic neutron scattering (QENS), a technique uniquely suited for probing atomic scale dynamics on ps–ns timescales and Å–nm length scales. QENS data obtained from two complementary instruments at J-PARC: the time-of-flight spectrometer AMATERAS and the backscattering spectrometer DNA. These instruments provide overlapping but distinct temporal windows, enabling a comprehensive multi- scale view of the dynamics.

The data were analyzed using a novel time-domain framework, here referred to as the Minimal Model approach, which avoids the traditional multi-Lorentzian fitting of S(Q, w) by focusing instead on the intermediate scattering function F(Q, t). The relaxation of F(Q, t) was modeled using the Mittag–Leffler function, providing access to the elastic incoherent structure factor EISF(Q), the characteristic relaxation time t(Q), and the stretching exponent a(Q), which together describe the extent of confinement and dynamical heterogeneity. 

The results reveal differences between Mt and Ht. Mt exhibits a higher EISF(Q) and shorter relaxation times, indicative of stronger confinement and faster localized reorientations, whereas Ht displays longer relaxation times and a more mobile interlayer environment. Combining the AMATERAS and DNA datasets produced a F(Q, t) span- ning from picosecond to nanosecond timescales, providing a unified description of both localized and diffusive processes.

Supervisors: Heloisa Nunes Bordallo & Martin Hoffmann Petersen 

Censor: Kristine Niss, Roskilde University