Exploring the Gel State: Optical Determination of Gelation Times and Transport Properties of Gels with an Inexpensive 3D-Printed Spectrophotometer

The study of gels and their properties is a compelling topic both technologically and scientifically, and should therefore be emphasized in chemistry and material science syllabuses. In the present laboratory experiment, we propose two experiences, aimed at introducing gelation and gel transport properties using silica gels made from sodium silicate and sodium citrate buffer. In particular, gelation times are used to teach kinetics and optical properties to first and second year General Chemistry students. Gelation times are determined by measuring the increment in scattering intensity with a 3D-printed spectrophotometer. The gel transport properties are presented to further exemplify Fickian and non-Fickian behavior through ionic dyes diffusing in gels. A qualitative description is obtained from charge interactions, and a quantitative description utilizing the diffusion coefficient is achieved by analyzing absorbance profiles. These tasks were designed to encourage students to work with unusual topics in a holistic way, approaching new materials, properties, and DIY equipment. Blueprints for the spectrophotometer, resources for instructors, and a detailed students' guide are provided together with a short report model to promote critical discussion of the observations.