(121g) Can Small Molecules Self Assemble in Aqueous Solutions ?

It is observed that small molecules of certain alcohols and amines, namely tertiary butyl alcohol (3-methyl-2-propanol) and 3-picoline (3-methyl pyridine), exhibit some kind of mesoscale inhomogeneities (around 100's of nanometers in size) in aqueous solutions. However, the exact nature of these inhomogeneities is still unclear. Various hypotheses attribute this phenomenon to occurrence of a structural ?phase transition?[1], or formation of clathrate hydrates[2,3], or formation of supramolecular clusters induced by impurities[4], or some alleged artifacts, such as air nanobubbles[5]. In this work, a detailed experimental study of aqueous solutions of tertiary butyl alcohol is carried out to investigate the nature of these inhomogeneities. Results from static light scattering indicate a strong, reversible and reproducible effect of temperature on the observed light scattering intensity, with a sharp increase in intensity at lower temperatures (below 10-15°C), which quite remarkably, stay stable. A characteristic length scale of around 200 nm emerges from analyzing the dynamic correlation function. However, on filtering this system with 200 nm nylon filter under cold conditions (below 10°C), followed by another filtration at room temperature by using a 20 nm Anopore® filter; these inhomogeneities disappear, indicating that the source of these inhomogeneities may be eliminated in a controlled fashion.

References:

1. M. F. Vuks and L. V. Shurupova, ?The scattering of light and phase transition in solutions of tertiary butyl alcohol in water?, Opt. Commun. 5, 277-278 (1972).

2. K. Iwasaki and T. Fujiyama, "Light scattering study of clathrate hydrate formation in binary mixtures of tert-butyl alcohol and water", J. Phys. Chem. 81, 1908-1912 (1977).

3. K. Iwasaki and T. Fujiyama, "Light-scattering study of clathrate hydrate formation in binary mixtures of tert-butyl alcohol and water: 2. Temperature effect", J. Chem. Phys. 83, 463-468 (1979).

4. M. Sedlak, ?Large-scale supramolecular structure in solutions of low molar mass compounds and mixtures of liquids: I. Light scattering characterization?, J. Phys. Chem. B 110, 4329-4338 (2006).

5. F. Jin, J. Ye, L. Hong, H. Lam, and C. Wu, "Slow relaxation mode in mixtures of water and organic molecules: Supramolecular structures or nanobubbles?", J. Phys. Chem. B 111, 2255-2261 (2007).