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O + H
2
O
→
H
2
O
2
(thermally hot)
(20)
H
2
O
2
(thermally hot) in the gas phase
→
2
•
OH
(21)
H
2
O
2
(thermally hot) in water
→
H
2
O
2
(22)
Upon the collapse of the bubble, the radicals (mainly
•
OH, see above) diffuse
into the bulk solution, where they either recombine [reactions (23)−(25)] or re-
act with a solute [reaction (26)].
2
•
OH
→
H
2
O
2
(23)
H
•
+
•
OH
→
H
2
O
(24)
2 H
•
→
H
2
(25)
•
OH + S
→
S
•
(26)
The concentration of radicals in the short liquid layer surrounding the bubble is
very high (a value of approximately 10
−2
mol dm
−3
has been estimated; Gutierrez
et al. 1991; von Sonntag et al. 1999), and scavenging of
•
OH by a solute [reaction
(26)] is very inefficient at low scavenger concentrations due to the competing
reaction (23). Figure 2.1 exemplifies this point. Hydroxyl radicals react readily
with iodide ions [reaction (27),
k
=
10
9
dm
3
mol
−1
s
−1
; (Buxton et al. 1988)],
and as the iodide concentration is raised the yield of I
3
−
[reactions (28) and (29)]
increases while that of H
2
O
2
decreases accordingly. However, molar concentra-
tions of iodide are required to scavenge
•
OH fully.
×
(27)
•
OH + I
−
OH
−
+ I
•
→
I
•
+ I
−
I
2
•
−
→
(28)
2 I
2
•
−
I
3
−
+ I
−
→
(29)
Besides being degraded by
•
OH, volatile solutes are drawn into the gas bubble
and decomposed there. As a consequence, low-molecular-weight solutes that
are capable of protonation/deprotonation are degraded to a lesser extent when
charged (Tauber et al. 2000). In addition, hydrophobic solutes may accumulate
at the water/gas interface (Henglein and Kormann 1985; von Sonntag et al. 1999).
This may cause two effects: they scavenge
•
OH more readily than is accounted
for by their scavenging capacity in the bulk, but they can also undergo excessive
thermal degradation in this hot supercritical layer [experimental evidence for
that transient state has not yet been obtained (Tauber et al. 1999b)].
In accordance with the above discussion, most of the damage encountered in
DNA model systems (Mead et al. 1975; Kondo et al. 1988a,b, 1989, 1990) and in
DNA (Fuciarelli et al. 1995) is due to
•
OH reactions. Interestingly, ultrasound of
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