Rotational-translational decoupling, in which translational motion is apparently enhanced over rotational motion in violation of Stokes-Einstein (SE) and Debye-Stokes-Einstein (DSE) predictions, has been observed in materials near their glass transition temperatures (Tg). This has been posited to result from ensemble averaging in the context of dynamic heterogeneity. In this work, ensemble and single molecule experiments are performed in parallel on a fluorescent probe in high molecular weight polystyrene near its Tg. Ensemble results show decoupling onset at approximately 1.15Tg, increasing to over three orders of magnitude at Tg. Single molecule measurements also show a high degree of decoupling, with typical molecules at Tg showing translational diffusion coefficients nearly 400 times higher than expected from SE/DSE predictions. At the single molecule level, higher degree of breakdown is associated with particularly mobile molecules and anisotropic trajectories, providing support for anomalous diffusion as a critical driver of rotational-translational decoupling and SE/DSE breakdown.

Students often enter chemistry laboratory courses with a well-documented disconnect between their experiences outside the classroom and the procedures, concepts, and techniques that occur in our courses. Strong base, weak acid titration experiments are present in general chemistry laboratories but also relate to foods our students are familiar with. An experiment using commonly consumed acids offers an opportunity to decrease the distance students may feel between the laboratory and everyday chemistry. By investigating reagents that may be purchased from the grocery store (citric acid in citrus soda, acetic acid in vinegar, or acetyl salicylic acid in aspirin), students gain an opportunity to see the chemistry they are learning in their everyday foods and medicines. This experiment uses an automatic titrator such as LabQuest to obtain the titration curve of each food acid. Students then use the shapes of these curves to identify a given unknown solid and aqueous version of one of these acids. This was carried out in a university general chemistry lab course, and quantitative and qualitative analysis of how the class approached the graphical identification process was performed. This article lays out an approach to teaching acid−base titrations that holds the potential to link students' chemistry lives both in and outside of the classroom while offering instructors a low-stakes method of evaluating student understanding. The development of this food acid investigation and modifications possible for online or hybrid learning are discussed.

Rotational-translational decoupling in systems near T g, in which translational diffusion is apparently enhanced relative to rotation, has been observed in ensemble and single molecule experiments and has been linked to dynamic heterogeneity. Here, simulations of single molecules experiencing homogeneous diffusion and static and dynamic heterogeneous diffusion are performed to clarify the contributions of heterogeneity to such enhanced translational diffusion. Results show that time-limited trajectories broaden the distribution of diffusion coefficients in the presence of homogeneous diffusion but not when physically reasonable degrees of static heterogeneity are present. When dynamic heterogeneity is introduced, measured diffusion coefficients uniformly increase relative to input diffusion coefficients, and the widths of output distributions decrease, providing support for the idea that dynamic heterogeneity can drive apparent translational enhancement. Among simulations with dynamic heterogeneity, when frequency of dynamic exchange is correlated with initial diffusion coefficient, measured diffusion coefficient behavior as a function of observation time matches that seen experimentally, the only set of simulations explored in which this occurs. Taken together with experimental results, this suggests enhanced translational diffusion in glassy systems occurs through dynamic exchange consistent with wide underlying distributions of diffusion coefficients and exchange coupled to local spatiotemporal dynamics.