Increasing correlation length in bulk supercooled H2O, D2O, and NaCl solution determined from small angle x-ray scattering

Using small angle x-ray scattering, we find that the correlation length of bulk liquid water shows a steep increase as temperature decreases at subzero temperatures (supercooling) and that it can, similar to the thermodynamic response functions, be fitted to a power law. This indicates that the anomalous properties of water are attributable to fluctuations between low-and high-density regions with rapidly growing average size upon supercooling. The substitution of H2O with D2O, as well as the addition of NaCl salt, leads to substantial changes of the power law behavior of the correlation length. Our results are consistent with the proposed existence of a liquid-liquid critical point in the deeply supercooled region but do not exclude a singularity-free model. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3495974]