Motion and interaction of aspirin crystals at aqueous-air interfaces.

Small-molecule amphiphiles such as aspirin have unique properties arising from a combination of an aromatic hydrophobic part and a hydrophilic part. We show that crystals of aspirin are capable of generating convective flows at the air-aqueous interface from both Marangoni effects (through weak surface activity) and capillarity (surface deformations). The flow-driven motion of millimeter-sized crystals was found to depend on the presence of other ions in solution as well as the distance and orientation of the crystals. The interactions lead to the formation of groups of two or more crystals that also underwent motion. The convective flows created by small amphiphile crystals might be exploited in the dynamic self-organization of particles at interfaces.

pH wave-front propagation in the urea-urease reaction.

The urease-catalyzed hydrolysis of urea displays feedback that results in a switch from acid (pH ~3) to base (pH ~9) after a controllable period of time (from 10 to >5000 s). Here we show that the spatially distributed reaction can support pH wave fronts propagating with a speed of the order of 0.1-1 mm min(-1). The experimental results were reproduced qualitatively in reaction-diffusion simulations including a Michaelis-Menten expression for the urease reaction with a bell-shaped rate-pH dependence. However, this model fails to predict that at lower enzyme concentrations, the unstirred reaction does not always support fronts when the well-stirred reaction still rapidly switches to high pH.

Base-catalyzed feedback in the urea-urease reaction.

The bell-shaped rate-pH curve coupled to production of base in the urea-urease reaction was utilized to give feedback-driven behavior: an acid-to-base pH clock (a kinetic switch), bistability and hysteresis between an acid/base state when the initial pH was adjusted by a strong acid, and aperiodic pH oscillations when the initial pH was adjusted by a weak acid in an open reactor. A simple model of the reaction reproduced most of the experimental results and provided insight into the role of self-buffering in the dynamics. This reaction suggests new possibilities in the development of biocompatible feedback to couple to pH-sensitive processes for bioinspired applications in medicine, engineering, or materials science.