[Show abstract][Hide abstract] ABSTRACT: Aqueous solutions of tetrahydrofuran, ethanol, urea, and alpha-cyclodextrin were studied by a combination of static and dynamic laser light scattering (LLS). In textbooks, these small organic molecules are soluble in water so that there should be no observable large structures or density fluctuation in either static or dynamic LLS. However, a slow mode has been consistently observed in these aqueous solutions in dynamic LLS. Such a slow mode was previously attributed to some large complexes or supramolecular structures formed between water and these small organic molecules. Our current study reveals that it is actually due to the existence of small bubbles ( approximately 100 nm in diameter) formed inside these solutions. Our direct evidence comes from the fact that it can be removed by repeated filtration and regenerated by air injection. Our results also indicate that the formation of such nanobubbles in small organic molecule aqueous solutions is a universal phenomenon. Such formed nanobubbles are rather stable. The measurement of isothermal compressibility confirms the existence of a low density microphase, presumably nanobubbles, in these aqueous solutions. Using a proposed structural model, that is, each bubble is stabilized by small organic molecules adsorbed at the gas/water interface, we have, for the first time, estimated the pressure inside these nanobubbles.
The Journal of Physical Chemistry B 04/2007; 111(9):2255-61. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A poly(ethylene oxide)-b-poly(epsilon-caprolactone) (PEO-b-PCL) diblock copolymer was micronized into small micelle-like particles (approximately 80 nm) via dialysis-induced microphase inversion. The enzymatic biodegradation of the PCL portion of these particles in water was in situ investigated inside a recently developed novel differential refractometer. Using this refractometry method, we were able to monitor the real-time biodegradation via the refractive index change (Deltan) of the dispersion because Deltan is directly proportional to the particle mass concentration. We found that the degradation rate is proportional to either the polymer or enzyme concentration. Our results directly support previous speculation on the basis of the light-scattering data that the biodegradation follows the first-order kinetics for a given enzyme concentration. This study not only leads to a better understanding of the enzymatic biodegradation of PCL, but also demonstrates a novel, rapid, noninvasive, and convenient way to test the degradability of polymers.
The Journal of Physical Chemistry B 03/2007; 111(7):1531-5. · 3.61 Impact Factor