Study of time-dependent structural changes of laponite colloidal system by means of near-infrared spectroscopy and hybrid hard- and soft-modelling multivariate curve resolution–alternating least squares

In this work, we study the gel formation dynamics after shear rejuvenation of a laponite colloidal model system using near-infrared (NIR) spectroscopy. The aim of this study is to investigate the ability of NIR spectroscopy combined with hybrid hard- and soft-modelling based on multivariate curve resolution–alternating least squares (MCR-ALS) analysis to monitor and detect changes in structural properties of colloidal systems during dynamic processes. To enhance the fine spectroscopic changes occurring during the process and to characterize the effects of slight perturbations on the NIR spectra, difference spectra were used. The hybrid hard- and soft-modelling version of MCR-ALS (HS-MCR), which allows the introduction of kinetic models to describe the process behavior, is used in this case to separate the contributions found to be present during laponite ageing (“gel formation”) dynamics: one related to a chemical effect (loss of solvated water in the network structure of the laponite system, following a kinetic model, i.e. hard-modelled) and one related to a physical effect (changes in scattering phenomenon due to the presence of laponite particles, i.e. soft-modelled).The presented methodology is proposed as a general modus operandi applicable to time-dependent kinetic processes in colloidal systems, where the nature of the colloid can produce chemical and physical contributions to the NIR measured signal and transformations may have to be followed through the subtle changes of the water environment surrounding the colloid particles.