Overcoming Angular Dependency When Teaching Light Scattering Using a Spectrofluorometer: The Molecular Weight of Latex Beads

When using a spectrofluorometer in light-scattering experiments, it is impossible to perform angle-dependent studies. At first glance, this restriction seems to students to be an insurmountable obstacle. However, a spectrofluorometer offers some potentialities not available with classical laser light scattering spectroscopy, namely, the availability of broader wavelength ranges. Changing the wavelength changes the magnitude of the scattering vector. This has the same effect as a change in the measurement angle. Thus, it is possible to overcome angle dependency by accounting for wavelength dependency. This "shift" in dependencies is easily accessed by students because it results directly from the usual formalisms used in light scattering. Moreover, the students are encouraged, in practice, to elaborate innovative solutions, even from classical formalisms. This methodology is applied to the calculation of a series of molecular weights of polystyrene latex spheres. This system has several advantages: (i) water miscibility, (ii) definite shape, (iii) monodispersity (both size and shape), (iv) low cost, and (v) availability of latex spheres in a wide variety of sizes and composition. Although very well characterized with respect to size and shape (mainly by dynamic light scattering and electronic microscopy), latex spheres have been poorly studied in terms of molecular weight. Consequently, working in an innovative area of research proves stimulating for the students.