Dissolution of concentrated surfactant solutions: from microscopy imaging to rheological measurements through numerical simulations

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

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.

Graph Theoretical Description of Phase Transitions in Complex Multiscale Phases with Supramolecular Assemblies

Phase transitions are typically quantified using order parameters, such as crystal lattice distances and radial distribution functions, which can identify subtle changes in crystalline materials or high-contrast phases with large structural differences. However, the identification of phases with high complexity, multiscale organization and of complex patterns during the structural fluctuations preceding phase transitions, which are essential for understanding the system pathways between phases, is challenging for those traditional analyses. Here, it is shown that for two model systems- thermotropic liquid crystals and a lyotropic water/surfactant mixtures-graph theoretical (GT) descriptors can successfully identify complex phases combining molecular and nanoscale levels of organization that are hard to characterize with traditional methodologies. Furthermore, the GT descriptors also reveal the pathways between the different phases. Specifically, centrality parameters and node-based fractal dimension quantify the system behavior preceding the transitions, capturing fluctuation-induced breakup of aggregates and their long-range cooperative interactions. GT parameterization can be generalized for a wide range of chemical systems and be instrumental for the growth mechanisms of complex nanostructures.

Dynamic diffusive interfacial transport (D-DIT): A novel quantitative swelling technique for developing binary phase diagrams of aqueous surfactant systems

Current methods to develop surfactant phase diagrams are time-intensive and fail to capture the kinetics of phase evolution. Here, the design and performance of a quantitative swelling technique to study the dynamic phase behavior of surfactants are described. The instrument combines cross-polarized optical and short-wave infrared imaging to enable high-resolution, high-throughput, and in situ identification of phases and water compositions. Data across the entire composition spectrum for the dynamics and phase evolution of a binary aqueous non-ionic surfactant solution at two isotherms are presented. This instrument provides pathways to develop non-equilibrium phase diagrams of surfactant systems-critical to predicting the outcomes of formulation and processing. It can be applied to study time-dependent material relationships across a diverse range of materials and processes, including the dissolution of surfactant droplets and the drying of aqueous polymer films.