Voor meer informatie of aanvraag van een exemplaar. Summary
The recent development of new technology of micro-electronics and instrumentation
has given the opportunity to construct a laboratory-scale Real-Time Powder
Diffractometer (RTPD): an instrument to investigate processes of ordered
compounds on molecular scale under controllable conditions on a time-scale
of seconds.
In chapter 2 this RTPD is described in detail, with all its capacities and
limitations. Also the development of two methods to analyse the data obtained
by the RTPD is described in detail. One of these methods is strictly connected
to the data as delivered by the RTPD. The other method, the predictive power
method, can be used to establish relationships between observed data and
parameters describing the sample. This method is not limited to data obtained
by the RTPD.
In chapter 3 the non-standard diffraction geometry of the RTPD is described.
A mathematical analysis of the diffraction path shows which instrumental
parameters are effectively determining the focussing conditions for X-ray
diffraction. The non-standard geometry leads to line broadening in the observed
diffraction pattern. A formula expressing this line broadening in terms of
instrumental and experimental parameters is derived. This formula can be
used to select the optimal instrumental parameters for a specific experiment
and to correct for the line broadening. An experimental assessment of the
derived formula is presented.
In chapter 4 the investigation of the decomposition of CuCl2-intercalated
graphite by means of the RTPD is described. An introduction to the subject
is presented prior to the experiments which assess the air stability of CuCl2-intercalated
graphite. The larger part of the de-intercalation takes place within the
first 25 minutes of exposure to air, but it continues at a low rate. The
mechanism of the de-intercalation process is established by means of phase
analysis of the observed diffraction patterns.
The RTPD has also been used to investigate the polymorphic behaviour of cocoa
butter. Cocoa butter is a natural fat with a rather constant, but complex,
composition. Nevertheless, the differences in geographic and climatological
conditions under which the cocoa beans are grown, cause small but significant
deviations from the average composition. As a result the physical behaviour,
such as melting and solidification, is not the same for all cocoa butter
samples.
In chapter 5 a review of this long explored field of science, with many closely
related subjects, is presented. This survey leads to the conclusion that,
although much knowledge has been built up, many unanswered questions still
remain. It appears that the complexity of the polymorphic behaviour of fats
and similar compounds has lead to a large variety of, often inconsistent,
results.
In chapter 6 the experimental work is described, which has been carried out
with cocoa butter in order to answer some of the questions resulting from
chapter 5. In these experiments four different solid phases have been observed:
phase γ, which melts between 268 and 273 K; phase α, which melts between
290 and 295; phase β' with a melting range from 293 to 300 K and finally
phase β, melting from 302 to 307 K.
Via the predictive power method it has been found that the melting behaviour
of cocoa butter can be predicted on basis of the chemical composition of
the particular sample. The temperature at which the sample is completely
molten can be predicted, for example, by the expression:
melting point = 250 + 0.57 * % oleic acid + 0.62 * % saturated fatty acids
Also a relationship between cooling rate, solidification temperature and
the polymorphic form obtained under static conditions could be established.
All possible cooling rates (0.01 K min-1 to 6 K s-1) resulted in crystalline
phases. The least stable form (γ) was never observed pure. The most stable
phase (β) did not crystallize directly from the melt. Tempering, or aging,
resulted automatically in transitions from less to more stable forms at temperatures
above 270 K.
A very surprising phenomenon observed is the memory effect. If β-cocoa butter
is melted, but not heated too much above the melting point, the butter will
re-solidify as β-cocoa butter when cooled to room temperature. In contrast,
standard crystallization at room temperature, starting from a melt heated
to 333 K, results first in the formation of β-cocoa butter, which subsequently
transforms slowly to ,8. For each cocoa butter a memory point could be assessed,
being the lowest temperature to which the molten cocoa butter should be heated
in order to prevent resolidification in the β-modification within 45 minutes.
The analyses of these memory points via the predictive power method resulted
in establishing StOlSt as most relevant TAG, and stearic acid as the most
relevant fatty acid concerning the memory effect.
Finally, on basis of the literature and our experimental results, a new view
is presented on the polymorphic or, more correctly frased, multi-phase behaviour
of cocoa butter, as well as a proposal for a modified nomenclature of the
polymorphic forms of fat-like substances.
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