Tracking Water Transport with Short-Wave Infrared: Kinetic Phase Diagrams, Dissolution, and Drying

Abstract

Short-wave infrared (SWIR) imaging has been extensively used in defense applications but remains underutilized in the study of soft materials and the broader consumer product industry. Water molecules absorb around ∼1450 nm, making moisture-rich objects appear black, whereas surfactants and other common molecules in consumer products do not absorb and provide a good contrast. This experimental study showcases the varied capabilities of SWIR imaging in tracking water transport in soft material systems by analyzing dissolution dynamics, tracking phase transitions (when combined with cross-polarized optical imaging), and monitoring drying kinetics in the surfactant and polymer solutions. The dynamic phase evolution to equilibria of a binary aqueous solution of a nonionic surfactant hexaethylene glycol monododecyl ether (C₁₂E₆) is presented. The influence of confined hydration in dynamic-diffusive interfacial transport capillaries was investigated by tracking the micellar to hexagonal phase transition concentration (C*). The effects of varying concentrations of an industrially relevant additive─monovalent common salt (NaCl) on the radial (2D) dissolution of lamellar-structured concentrated sodium lauryl ether sulfate (70 wt % SLE₁S) pastes was studied. An equation was developed to estimate the radial dissolution coefficients based on total dissolution time and surfactant concentrations in the sample and solvent. Water loss was investigated by tracking the drying of aqueous poly(vinyl) alcohol films. In situ monitoring of drying kinetics is used to draw correlations between the solution viscosity and drying time. SWIR imaging has already revealed previously inaccessible insights into surfactant hydration and holds the potential to become a turnkey method in tracking water transport, enabling better quality control and product stability analysis.