Biomedical Engineering Reference
In-Depth Information
the decomposition of any acyl ascorbate at 75% relative humidity was nearly equal to or
slightly less than unity, while the
n
values at 12% and 44% relative humidities were higher
than unity. In particular, the
n
value at 44% relative humidity strongly depended on the acyl
chain length of the ascorbate, and the
n
value for the decomposition of an ascorbate having a
long acyl chain was high. This indicates that the decomposition of the ascorbate with a longer
acyl chain progressed via a longer induction period. It is presumed that sorption of water onto
the ascorbate with a longer acyl chain was weak at 44% relative humidity due to its high
hydrophobicity, and that the induction period was consequently prolonged.
The decomposition processes of the acyl ascorbates at 37, 50 and 65
o
C and at 75%
relative humidity were shown in Figure 12. All acyl ascorbates decomposed fast as
temperature increased. The acyl chain length also affected the decomposition of the
ascorbates, and ascorbates with shorter acyl chains decomposed faster. This tendency was
significant at 65
o
C. The decomposition of all acyl ascorbates at all temperatures was analyzed
based on the Weibull equation in order to estimate the kinetic parameters. The curves in the
figure were drawn using the estimated
k
and
n
values. Figure 10(b) shows the temperature
dependency of the
k
values for the decomposition of the acyl ascorbates at 75% relative
humidity. Although the
k
value increased with increasing temperature, there was no strong
temperature dependency for the
n
value and the values were almost 1.0 ± 0.4. The
k
values for
the decomposition of ascorbates increased as acyl chains decreased. The Arrhenius plots in
Figure 10(b), in each case, produced a straight line to evaluate the activation energy
E
and the
frequency factor
A
from the slope and the intercept of the line, respectively. Both the
E
and
A
values for the ascorbate increased as acyl chain length decreased. The
E
values are plotted
versus the natural logarithms of the
A
values in the inset of Figure 11. The plots lie on a
straight line (R
2
= 0.997). The linear relationship indicated that the compensation held during
the decomposition and that the decomposition of every acyl ascorbate essentially proceeded
by the same mechanism. The
T
β
value was estimated to be 101
o
C from the slope. At this
temperature, the rate constant
k
for the decomposition of all acyl ascorbates in air would be
the same.
Figure 12. The decomposition processes of (
{
) octanoyl, (
U
) decanoyl, (
) lauroyl, (
V
) myristoyl
and (
) palmitoyl ascorbate at 75% relative humidity and (a) 37ºC, (b) 50ºC and (c) 65ºC. The solid
curves were calculated using the estimated kinetic parameters of the Weibull model.
