The Nucleation of Particulate FeS Observed with Small and Wide Angle X-Ray scattering
(SAXS/WAXS)
 
 

Liane G. Benning1, Sam Shaw2, Nick J. Terrill3
1School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK (liane@earth.leeds.ac.uk),
2 Department of Earth Sciences, University of Manchester, Manchester, M13 9PL, UK
3 Daresbury Laboratory, CCLRC, Warrington, WA4 4AD, UK

       SAXS/WAXS experiments can be used to monitor nucleation reactions at times scales of several 10 to 100's of milliseconds.  Preliminary test runs show that in the FeS system at low pH and low concentrations, nucleation occurs after about 30 milliseconds and the main precipitation is over within about 20 milliseconds.  However, in some cases, part of the precipitate seem to re-dissolve in the next 100 milliseconds before equilbrating as the final product.  This process is presumably attributable to localized changes in pH.   At higher pH the initial precipitation process seems to take longer (~ 60 msec.)  as  a plot of integrated intensities of 200 injection cycles shows.   However, note that due to oscillatory effects from the mica windows these results could be erroneously interpreted as changes in SAXS patterns.
       The reactions to precipitate colloidal iron sulphide particles is fast and aggregation over longer time scales needs to be accounted for.  Further work will concentrate on determining the kinetic reaction rates for these reactions and also onto trying to improve the various characteristics of the stopped-flow capabilities on station 8.2 of the Daresbury Laboratory and stations ID2 and BM26 at the ESRF.


Fig. 1.  Details of the stopped-flow cell system mounted on station 8.2.
Shown are the two solution containers (connected to a N2 back pressure), the remotely
triggered solenoids and the perspex cell where the in situ precipitation occurs.
The beam passes through the cell before being collected on the SAXS and
WAXS detectors (courtesy of NDC, SRS Daresbury).






Fig. 2. A  2 D plot of the change in integrated intensity of the  SAXS                      Fig. 3.  Stacked SAXS patterns for a run with a single injection/cycle
patterns for the first 100 msec (pH=7, dead time = 10 msec,                                   that was  left to develop over several hours.  The y-axis is intensity
accumulated for 200 cycles.  Note that the windows of the stopped-flow                   (arbitrary units). The observed relative increase in intensity with time
 cell are made of mica and vibrational effects can not be ruled out.                            can be interpreted as aggregation of fast precipitated  nano - particles
Therefore, particularly the data from the very fast kinetics                                       into larger clusters.
experiments have to be interpreted with caution.