Laser pulse-induced optoacoustic calorimetry (LIOAC) was applied to determine enthalpy changes in aeratedaqueous suspensions of TiO
2 nanoparticles (2.4 nm mean diameter) at pH = 1 in the absence and presenceof the halide ions X
- = Br
- or Cl
-. LIOAC demonstrates that in the system TiO
2![](/images/entities/verbar.gif)
H
2O, O
2 10% of theabsorbed optical energy are stored for at least a few
![](/images/entities/mgr.gif)
s, which can be assigned to the decomposition of water.In the systems containing the X
-, the overall photoprocess was exothermic although the oxidation of X
- to
![](/isubscribe/journals/jpccc<font color=)
k/111/i11/eqn/jp067297te10001.gif">
- and the concurrent decomposition of water are endothermic. As an explanation we suggest a model inwhich the photodeaggregation of TiO
2 nanoparticle aggregates produces additional photocatalyst surface area.The subsequent establishment of the thermodynamic equilibria on the additional surface comprises protonationof the TiO
2 particles, halide adsorption, and formation of the electrochemical double layer. All these processesare exothermic. Adsorption of X
- is best described by a Tem
kin-isotherm with
![](/isubscribe/journals/jpccc<font color=)
k/111/i11/eqn/jp067297te10002.gif"> in the range -5 to -20
kJ mol
-1 for the transfer of X
- from the outer to the inner Helmholtz plane on the TiO
2 particles. The enthalpyof deaggregation is ca
. + 30
kJ mol
-1 as was calculated on the basis of energy conservation considerations.