The First X-Ray Diffraction Peak of Water As a Function of Temperature in the Range 4 – 63 °C S.W

The First X-Ray Diffraction Peak of Water As a Function of Temperature in the Range 4 – 63 °C S.W

The first x-ray diffraction peak of water as a function of temperature in the range 4 – 63 °C S.W. Hughes, J. Barry Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia. X-ray and neutron diffraction are commonly used to investigate the structure of water under varying physical conditions [1-3]. Here we report a preliminary experiment to measure the position and amplitude of the first x-ray diffraction peak (Q0) of water as a function of temperature. The experiment was performed on the SAXS/WAXS beamline of the Australian Synchrotron with a Pilatus 1M SAXS (1,000 × 1,000 pixel) camera. The wavelength of the x-rays was 0.61992 ± 0.00003 Å, corresponding to an energy of 20.026 ± 0.005 keV. Distilled/deionised water flowed between a tempering beaker immersed in a temperature controlled water bath and a 1.5 ± 0.1 mm quartz capillary placed in the x-ray beam (figure 1). Thermocouples were placed in the tubing either side of the capillary to estimate the temperature at the point where the beam passed through the capillary. A sequence of 10 s integrations was obtained as the temperature of the water was raised from 4 – 63 °C. Figure 2 shows the structure function for the air-filled and water-filled capillary at 25 °C and the curve produced by a subtraction of the former from the latter. We compared our data with structure functions obtained on the Advanced Light Source (ALS) using a 11 keV beam glancing off a 0.5 mm thick sample of water in a temperature controlled chamber [3]. Figure 3 shows a plot of the first structural peak (Q0) as a function of temperature for the AS and ALS, and figure 4 shows a plot of the normalised intensity of Q0 as a function of temperature for the ALS and AS. Both sets of data show that the position of the peak increases with temperature whilst the amplitude decreases. Further experiments are required to ascertain if the subtle differences seen between the AS and ALS structure functions are instrumental or due to differences between the structure of water confined to a 0.5 mm thick flat sheet and flowing in a 1.5 mm diameter capillary. Front face of SAXS 0.2 camera Plastic 0.18 tubing 250 µm T/C 1 0.16 water + capillary 125 µm 0.14 T/C 2 Tempering beaker X-ray beam connected to water 0.12 bath Quartz capillary 0.1 water 0.08 Upper reservoir of 0.06 deionised/distilled units) (detector Intesnity water 0.04 Lower empty capillary reservoir 0.02 Return pump 0 0 0.5 1 1.5 2 2.5 3 3.5 Q (Å-1) Figure 1. Schematic diagram of the apparatus. The FWHM dimensions of the x-ray beam Figure 2. Structure functions of the air-filled capillary, water-filled capillary and are given. subtraction of the two, at 25 °C. 2.12 102 2.1 100 2.08 Qo 98 2.06 1) - 2.04 96 Qo (Å 2.02 94 2 92 AS of intensity Normalised 1.98 ALS 90 1.96 1.94 88 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 Temperature (oC) Temperature (°C) Figure 3. Q0 as a function of temperature for data obtained using the Advanced Light Source Figure 4. Plot of the normalised intensity of Q0 as a function of temperature for (ALS □) and the Australian Synchrotron (AS ○). A third order polynomial has been fitted to the ALS and AS. In both cases the error bars are about 0.5% of the amplitude. For both sets of data to guide the eye. The ALS error bars are ± 0.0036 Å-1. The majority of the both sets of data the intensity of the structure function at the lowest temperature AS error bars are ± 0.0002 Å-1 with some at ± 0.0003 Å-1. These have not been plotted as (1 °C for the ALS and 4 °C for the AS) was taken as 100%. they are only about one third the vertical height of the circle symbols and therefore too small to be seen. References. 1. Narten, A.H., Danford, M.D., Levy, H.A. X-ray diffraction study of liquid water in the temperature range 4-200 °C. Discuss. Faraday Soc. ,43, 7-107 (1967) DOI: 10.1039/DF9674300097 2. Soper, A.K. The radial distribution functions of water and ice from 220 to 673 K and at pressures up to 400 MPa. Chem. Phys. 258, 121-137 (2000). 3. Hura, G. Sorenson, J. M., Glaeser, R.M., Head-Gordon, T. A high-quality x-ray scattering experiment on liquid water at ambient conditions. J. Chem. Phys. 113, 9140-9148 (2000). Acknowledgments. The Australian Synchrotron is thanked for funding this work. .

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