Dissolution behaviour of binary mixtures in capillary tubes

Dissolution behaviour of binary mixtures in capillary tubes.
Experimental study
Mihaela Stevar1and Anatoliy Vorobev1
1University of Southampton, University Road, SO17 1BJ, Southampton,
United Kingdom
Using a CCD camera we examined the dissolution behaviour of two binary
mixtures (glycerol/water and isobutyric acid (IBA)/water) within capillary tubes
of circular and square cross-sections. Tubes with different diameters (0.2 mm
- 0.8 mm) and lengths were used in the experiments. A solute-filled tube was
immersed into a thermostatic solvent-filled bath with no pressure gradient
applied between the ends of the tube. The shape and the position of the
interface were tracked and analysed. For the glycerol/water mixture the
dissolution behaviour for different temperatures (20◦C - 50◦C) was studied. We
observed two clear interfaces with finger-like shapes affected by gravity
penetrating from both ends of the tube. During the displacement process, the
interfaces retained their shape with no oscillations being observed. Towards
their fusion point minor modifications in the interfaces shapes were noticed. At
higher temperatures the interfaces became diffuse and propagated faster.
The dissolution behaviour of IBA/water mixture studied for temperatures
below and above its consolute point (26◦C) was completely different. The
dissolution scenario below the critical point was characterized by the one-side
penetration of the solvent into the tube. The shape of the interface was almost
flat with a small inclination in the larger diameter tubes due to gravity effects.
Under certain conditions the interface shape experienced quite complex
modifications at the time of entrance, but afterwards it stabilized. The
influence of the capillary forces resulted in oscillations of the interface during
its propagation. Above the critical point the solvent penetrated from both sides
and under-rode the solute. At higher temperature the interfaces became
diffuse and unnoticeable very quickly. The dissolution behaviours were similar
in the tubes with different cross-sections. For both mixtures studied, the rate
of the interface propagation did not follow the predictions of the diffusion