Determination of the contrast mechanism in ultra-small-angle X-ray scattering (USAXS) imaging

Abstract

LCST type phase separation, is known to have a large salt effect. In this study, we selected sodium tetraphenylborate (NaBPh4), whose affinities of anion and cation with water are largely different, as a salt because the solvation effect should be intensive and the formation of large clusters is expected theoretically. In cases of the lower amount of NaBPh4 and water-rich concentration, the mixture becomes colored in blue, and changes to orange with approaching critical temperature. This result suggests that a periodic structure is formed, and its repeat distance increases with increasing temperature. In order to verify the structural formation, we performed SANS measurement at SANS-U in JAEA, Tokai, for the mixtures with 100 mM NaBPh4. The results showed that the profiles have a peak around Q =0.1 A-1 , and the peak position shifts to lower-Q with increasing temperature. (K. Sadakane et al., J. Phys. Soc. Jpn. 76, 113602 (2007).) Adding ionic surfactant, for example AOT or SDS, also shows the structural formation as the case of NaBPh4.