9412
J. Phys. Chem. C 2008, 112, 9412–9417
O2(1∆) Quenching Mechanism in Cl2/Basic Hydrogen Peroxide(Basic Deuterium Peroxide) Gas/Liquid Reaction and the Determination of O2(1∆)/BHP(BDP) Interface Free Energy Wenbo Shi, Liezheng Deng, Shuyan Du, Rongrong Cui, Heping Yang, Guohe Sha,* and Cunhao Zhang State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, P. R. China ReceiVed: January 19, 2008; ReVised Manuscript ReceiVed: March 14, 2008; In Final Form: April 7, 2008
In a jet-type singlet oxygen generator, we have studied the generation of O2(1∆) via the reaction of chlorine gas with aqueous basic hydrogen peroxide (BHP) and with basic deuterium peroxide (BDP). The O2(1∆) detachment yield with BDP is measured to be 72.5 ( 1.5%, merely 2.5% higher than that with BHP, despite a 10 times longer O2(1∆) lifetime in BDP than in BHP. By a careful kinetic analysis of the Cl2 + BHP(BDP) reactions, we found that the main resistance that prevents the nascent O2(1∆) from escaping off the solution into the bulk gas flow does not lie in the liquid or gas phase, but in the gas/liquid interface. Thus, the seemingly weird experimental result can be justified by postulating a higher energy barrier at the O2(1∆)/BDP interface than that of the O2(1∆)/BHP so as to detain the O2(1∆) for a longer time in BDP. In fact, this postulation has been proved as follows: according to the physical model of Copeland and Zagidullin, the O2(1∆) mass accommodation coefficient is calculated from our experimental solvation detachment yield to be ∼4.0 × 10-6 on the BHP surface and ∼1.7 × 10-6 on the BDP surface. Then, based on the thermodynamics of phase equilibrium in dilute solutions, the corresponding Gibbs energy of O2(1∆) at the BHP surface is computed as ∼2.65 × 104 J/mol, and the value of O2(1∆) at the BDP surface is ∼2.86 × 104 J/mol. This higher O2(1∆)/ BDP interface energy barrier may result from both larger D2O molecular mass and stronger hydrogen bonding between D2O molecules in BDP solution. The present methodology can be further improved by using microwave-discharge production of O2(1∆) so as to make direct measurements of its quenching probability. Hence, more key thermodynamic and kinetic information about the O2/aqueous electrolyte solution interfaces will be made available. Work along this line is now under way. 1. Introduction O2(1∆) is the lowest electronically excited state of the O2 molecule. The study on its physical and chemical properties has been extending for several decades because of the important role it plays in life and biological processes, in atmospheric chemistry, as well as in applications to industry.1–3 Although O2(1∆) can be generated by optical or electric excitation, the most efficient way is to react gaseous chlorine Cl2 with aqueous basic hydrogen peroxide (BHP). This is a complex consecutive reaction.4–6
H2O2+OH- f HO2-+H2O
(1)
Cl2+HO2- f HOOCl2-
(2)
HOOCl2- f HOOCl+Cl-
(3)
HOOCl+HO2- f H2O2+OOCl-
(4)
-
-
OOCl f Cl +O2( ∆) 1
(5)
The nascent O2 yield, defined as + O2(X3Σ)], from eqs 2-5 is well-established to be ∼100%,4,7 and the O2(1∆) radiative lifetime is as long as ∼45 min. However, the O2(1∆) collisional lifetime in the condensed phases is much shorter. For example, in water and heavy water, its lifetime is 3.9 µs and 56 µs, respectively; the longer O2(1∆) lifetime in D2O is due to the larger energy mismatch for (1∆)
O2(1∆)/[O2(1∆)
* Corresponding author. E-mail:
[email protected].
O2(1∆)-D2O radiationless E-V transfer.8 Notably, the O2(1∆) lifetime in BHP is only ∼1.8 µs (in BDP it is ∼20 µs),9 and for the Cl2 + BHP reaction, the O2(1∆) yield is usually only ∼70%, with around 30% quenching loss.10–12 Nevertheless, the detailed kinetic mechanism of the O2(1∆) quenching is as yet unclear because of the complexity of the overall process, including the gas-liquid reaction, diffusion, and the molecular dynamic performance relevant to the gas/liquid interface. Considering the longer lifetime of O2(1∆) in BDP, Vetrovec et al.13 proposed to use BDP instead of BHP to generate O2(1∆) and envisaged that an appreciably higher yield of O2(1∆) (∼88%) should be obtained as a result of this replacement. In our laboratory, experiments were run to compare the O2(1∆) yield of the Cl2 + BHP with that of the Cl2+ BDP reaction on a jet-type singlet oxygen generator (SOG). Contrary to our expectation, O2(1∆) detachment yields (Yd) for the two reaction systems do not differ much from each other, being ∼70% for the former and ∼73% for the latter. By numerical analysis of the relevant kinetic model of the Cl2 + BHP(BDP) reactions,14–16 we found that the main resistance that prevents the nascent O2(1∆) from escaping off the solution into the bulk gas flow does not lie in the liquid phase or the gas boundary layer, but in the gas/liquid interface. The transport of molecules across the gas/liquid interface is a fundamental process in the areas of atmosphere, biology, and industrial chemistry.17–20 Served as the bridge between the experiment and theory, interface Gibbs free energy is one of the most important parameters in the research of gas/liquid
10.1021/jp8005348 CCC: $40.75 2008 American Chemical Society Published on Web 06/03/2008
O2(1∆) Quenching Mechanism and Interface Free Energy
Figure 1. Experimental setup of the jet-type SOG. JI: jet injector; CL: reaction cylinder; RZ: reaction zone; W: quartz window; DC: diagnostic cell; YAG: 532 nm frequency-doubled YAG laser; CCD: CCD spectrograph; OF: optical fiber; CUDS: Cl2 utilization measure system; PV, PW: manometer; T: thermocouple; Tank1: pressurized BHP tank; Tank2: BHP collection tank; V1-V5: valves; EX: exit for the O2(1∆) gas flow.
interface and is usually obtained through the mass accommodation coefficient measured from experiments. However, due to the low solubility, the O2(1∆) mass accommodation coefficients are difficult to measure using the normal way; thus, the interface Gibbs free energy of O2(1∆) cannot be obtained. In this paper, while analyzing the O2(1∆) quenching mechanism in the Cl2/ BHP(BDP) gas/liquid reaction, the mass accommodation coefficients of O2(1∆) on BHP and BDP were also simultaneously obtained; thus, the Gibbs free energies of O2(1∆) gas molecules on the BHP and BDP interface were then obtained for the first time. Referring to the meaning represented by the interface Gibbs free energy, our seemingly weird experimental result was also rationalized by assuming a higher energy barrier at the O2(1∆)/BDP interface that detains the nascent O2(1∆) in BDP for a longer time, thereby offsetting the effect of lower collision quenching rate of O2(1∆) in BDP. 2. Experimental Section The schematic of the experimental setup is shown in Figure 1. BHP is prepared with 7 L of 50 wt % H2O2 and 10 kg of 50 wt % KOH aqueous solution (the mole ratio of H2O/H2O2/KOH ) 1:0.24:0.17). BHP was kept in tank 1 at about -14 °C. Under the N2 pressure, the BHP flows through 19 0.8 mm diameter nozzles (JI) to form the jet streams that eject the sample with a velocity of 6.5-12.5 m/s to the reaction cylinder, which is 16 cm in length with a 1 cm inner diameter. Both the injector and cylinder are made of Plexiglas. The gaseous Cl2 enters from the bottom, flowing up in counter-flow to the BHP streams with a velocity of 3-21 m/s. The gaseous product including O2(1∆), O2(X3Σ), the residual Cl2, and a small amount of O2(1Σ) and water vapor, flows out through the top exit (EX), enters the diagnostic cell (DC), where the O2(1∆) yield (Y∆) is measured by the Raman spectroscopy of O2(1∆) and O2(X3Σ) excited by the 532 nm light of a frequency-doubled YAG laser.21 A typical spectrum is shown in Figure 2. A CCD spectrograph is also mounted on the front of the DC’s window to record the spectra of 2O2(1∆) f 2O2(X3Σ) cooperative emission together with the O2(1Σ) f O2(X3Σ) spontaneous emission. A typical spectrum
J. Phys. Chem. C, Vol. 112, No. 25, 2008 9413
Figure 2. A typical Raman spectrum of O2(1∆) and O2(3Σ) excited by the 532 nm light of a frequency-doubled YAG laser, the O2(1∆) yield (Y∆) can be calculated by the strength ratio of the two peaks.
Figure 3. A typical emission spectrum of the O2(1∆) and O2(1Σ) measured at the diagnostic cell using a CCD spectrograph, the peak at 634 nm is from the cooperative transition of 2O2(1∆) to their groundstate O2(X3Σ), the peak at 762 nm is from the O2(1Σ) transition to O2(X3Σ). The water vapor content in the gases can be calculated from the light intensity ratio of the peaks at 634 and 762 nm.
is shown in Figure 3. Then, the water vapor content in the gas phase can be calculated from the light intensity ratio of the peak at 634 nm versus that at 762 nm.22 A chlorine utilization measure instrument (CUDS) is located downstream to the diagnostic cell. By measuring the UV (334 nm) absorption rate of Cl2 before and after the reaction with BHP, the chlorine utilization can be calculated using the Beer-Lambert law.23 To carry out the reaction of Cl2 with BDP, the BDP solution is prepared by D2O2 (purity > 99%) and KOD heavy water solution with the similar molar ratio as that of BHP (D2O2/ KOD/D2O ) 1:0.22:0.13). D2O2 was synthesized in our laboratory via the reaction of D2O with K2S2O8 (eq 6). D2SO4
K2S2O8 + 2D2O 98 2KDSO4 + D2O2
(6)
The Cl2 + BDP reaction is conducted in the same device and under essentially the same operating conditions as that for BHP. 3. Results and Discussion 3.1. The O2(1∆) Detachment Yield. The O2(1∆) detachment yield (Yd) is defined as the yield of O2(1∆) when it has just
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Figure 4. The reciprocal of O2(1∆) yield 1/Y∆ as a function of Pτ value. In the gas phase, O2(1∆) yield Y∆ is related to the O2(1∆) detachment yield Yd and Pτ value as follows: 1/Yd ) 1/Y∆ - 2Pτkp/ KBT. Thus, from the intercepts of the fitted lines at Pτ f 0, the O2(1∆) detachment yields in the BHP + Cl2 and BDP + Cl2 reactions are obtained as 70.0 and 72.5% respectively.
escaped off the BHP solution into the bulk gas flow. In our experiments, the O2(1∆) molecule flowing from the reaction zone to the diagnostic cell needs a time (τ). In this period, the O2(1∆) will suffer from the loss of gas phase quenching mainly via the O2(1∆) energy pooling reaction (eq 7). kp
O2(1∆) + O2(1∆) 98 O2(1Σ) + O2(X3Σ)
(7)
Thus, the O2(1∆) yield (Y∆) measured at the diagnostic cell will be lower than the detachment yield (Yd). The relation between Yd and Y∆ can be expressed as follows,14
2kp 1 1 ) Pτ Yd Y∆ KBT
(8)
where kp is the O2(1∆) energy pooling reaction rate constant, KB is the Boltzmann constant, T is the gas temperature, and P is the O2 total pressure. It is evident from eq 8 that Yd approaches Y∆ as the value of Pτ approaches to zero. Figure 4 shows our experimental 1/Y∆ as a function of Pτ, where the solid line is for the Cl2 + BHP reaction, which gives Yd ) 70.0 ( 1.5%, and the dash line is for Cl2 + BDP, gives Yd ) 72.5 ( 1.5%. Contrary to our expectation, the O2(1∆) detachment yield in Cl2 + BDP is only 2.5% higher than that of Cl2 + BHP, considerably smaller than the prediction by Vetrovec et al.13 3.2. O2(1∆) Quenching Mechanism and Its Accommodation Coefficient on the Gas/Liquid Interface. The kinetic model used by Vetrovec et al.13 to estimate the O2(1∆) detachment yield is as follows,
1 Yd ) l 1 + (DCl2τR ⁄ (DOl 2τOl 2))0.5 l DCl 2
DOl 2
Figure 5. The kinetic scheme of O2(1∆) generation and quenching in Cl2 + BHP reaction. GBL: gas boundary layer above the BHP surface; NCL: nascent O2(1∆) generation layer. As the first step, Cl2 is absorbed by BHP and O2(1∆) is generated within the NCL via the reaction of Cl2 with HO2-, then O2(1∆) escapes from the liquid phase into the gas phase by overcoming the resistance of the liquid phase, the liquid/gas interface, and the gas phase. In these processes, some O2(1∆) molecules will be deactivated by the solvent molecules.
based on a more advanced model of Copeland,14–16 as we will see in the following calculation. A schema of the O2(1∆) generation and quenching is presented in Figure 5. The process starts from the diffusion of Cl2 from the gas phase into BHP. Then the Cl2 reacts with HO2(eqs 1–5), and the generated O2(1∆) escapes from the liquid phase into the gas phase by successively overcoming the resistance of liquid phase, gas/liquid interface, and the gas boundary layer on the liquid surface. In these processes, a fraction of O2(1∆) will be quenched by collision E-V energy transfer. According to this physical mode, Copeland14,15 and Zagidullin16 separately wrote a mathematical model, leading to an exact solution of the O2(1∆) detachment yield in the wellmixed limit, under the conditions that the surface concentration of HO2- is constant and the heterogeneous deactivation of O2(1∆) is small. Thus, the detachment yield of O2(1∆) can be expressed as the product of three survival probabilities (eq 10),
Yd ) fliq · fint · fgas
where fliq, fint, and fgas represent, respectively, the survival probability of nascent O2(1∆) molecules that diffuse successively through the liquid reaction zone, the gas/liquid interface, and the gaseous boundary layer on the liquid surface. The survival probability fliq, in eq 10 can be expressed as eq 11,14–16
1
fliq ) (9)
where and are, respectively, the diffusion coefficients of chlorine and oxygen in BHP (or BDP), τOl 2 is the lifetime of O2(1∆) in BHP (or BDP), and τR the characteristic time for the reaction of Cl2 with HO2- (or DO2-). This model only considers the resistance of diffusion in the liquid phase and omits the critical resistance in the gas/liquid interface that impedes the O2(1∆) from escaping off the liquid. This neglect will lead to a fairly large error of Yd, in comparison with the exact solution
(10)
1+
(11) l DCl 2
k[HO2-]DOl 2τO2
where DlCl2 and DlO2 are the Cl2 and O2(1∆) diffusion coefficients in BHP, k is the rate controlling reaction rate constant of Cl2 with HO2-, [HO2-] is the HO2- concentration, and τO2 is the lifetime of O2(1∆) in BHP. By introducing the values of these parameters (listed in Table 1), we get fliq ≈ 0.988 for Cl2 + BHP reaction. This fairly large survival probability can be justified by the very fast Cl2 + HO2- reaction; it turns out that
O2(1∆) Quenching Mechanism and Interface Free Energy
J. Phys. Chem. C, Vol. 112, No. 25, 2008 9415
the depth of the reaction zone is only a few Å, and the nascent O2(1∆) can easily diffuse out of it. As for the probability (fgas in eq 10) of O2(1∆) molecules diffusing through the gaseous boundary layer, Copeland14,15 and Zagidullin16 gave eq 12,
fgas )
)
10, we get the survival probability for O2(1∆) diffusing through the gas/liquid interface fint ≈ 0.708. From these data we distinctly see that the main loss of O2(1∆) originates from the O2(1∆)/ BHP interface. However, it was overlooked by Vetrovec et al.13 fint, in eq 10 is expressed as eq 13.14–16
ROint2 + ROliq2 ROgas2 + ROint2 + ROliq2
4 + HO2 VO2γO2 8PV σDOg 2Sh
+
1
fint ) 1+
τO2 DOl 2
4 + HO2 VO2γO2
(12) τO2 DOl 2
int liq where Rgas O2 , RO2, RO2 represent, respectively, the O2 mass transport resistances for the gas phase, the gas/liquid interface, and the liquid phase; VO2 ) √(8RT/πµO2) is the O2 mean molecular velocity in gas phase, R is the gas constant, µO2 is the O2 molecular weight, γO2 is the mass accommodation coefficient of O2(1∆) on the liquid surface, HO2 is the Henry coefficient of O2/BHP phase equilibrium system in the temperature T, PV is the gas pressure in reaction cavity, σ is the specific reaction surface (cm-1) of SOG, defined as the jet surface area (cm2) in one cubic centimeter cavity volume, DgO2 is the O2(1∆) diffusion coefficient in 1 Pa gas pressure, and Sh is the Sherwood number. By introducing the value of the relevant parameters for Cl2 + BHP reaction (see Tables 1 and 2) in eq 12, we get ROgas2 ) 2.1 × 10-5 to 3.5 × 10-4 s/cm, ROint2 ) 7.43 s/cm, ROliq2 ) 17.0 s/cm, and fgas > 0.999. This means that there is almost no quenching loss of O2(1∆) when it is diffusing through the gaseous boundary layer, because DOg 2 .DOl 2 and ROgas2 , ROliq2. By introducing the values of fliq ≈ 0.988, fgas ≈ 0.999, and our experimental Yd ≈ 0.70 for the Cl2 + BHP reaction into eq
4 VO2γO2HO2
(13) DOl 2 τO2
By introduce the value of fint obtained above and the relevant physical parameters into eq 13, the mass accommodation coefficient of O2(1∆) on the O2(1∆)/BHP interface γO2 is calculated to be ∼4.6 × 10-6. Now, let us turn to the Cl2 + BDP reaction. Although the experimental values for the diffusion constants DlCl2 and DlO2 in BDP are not available in the literature; they can, nevertheless, be calculated from the diffusion coefficients in BHP and the viscosity ratio between BHP and BDP using the Nernst-Einstein equation (eq 14),27
Dlµl ) constant Tl
(14)
where Dl is the diffusion coefficient of the solute in liquid, µl is the liquid viscosity, and Tl is the liquid temperature. We measured the viscosity of BHP and BDP using a rheometer (Broolfield DV-III) and found them to be almost the same at the same temperature. Thus, the diffusion coefficients of Cl2 and O2(1∆) in BHP and BDP should be essentially the same. The rate constant k in BDP is not available in the literature either. Considering that the reaction takes place between O-Oand Cl2, with H (or D) only as a spectator,4 we may assume the k value for Cl2 + DO2- is not much different from that of Cl2 + HO2-. In fact, even if k for BDP and for BHP had differed by 10 times, fliq would have changed only by 0.009, a negligible amount. Thus, by using the same procedure as that for Cl2+
TABLE 1: The Physical Constants Relevant to BHP and BDP value symbol
parameter
unit
BHP
BDP
reference
τO2 k DOl 2 l DCl 2 HO 2 γO2 ∆Gobs
Lifetime of O2(1∆) in liquid Cl2 + HO2- reaction rate constant O2 diffusion coefficient in liquid Cl2 diffusion coefficient in liquid O2-BHP(BDP) Henry constant in -14 °C O2(1∆) mass accommodation coefficient Gibbs free energy between the gas phase O2(1∆) and the interface Gibbs free energy of solvation of O2 Gibbs free energy between the solvated O2(1∆) molecules and the interface
µs cm3/mol · s cm2/s cm2/s J/mol
1.8 4.0 × 1011 2.7 × 10-6 2.0 × 10-6 59 4.0 × 10-6 ∼2.65 × 104
20 ∼4.0 × 1011a ∼2.7 × 10-6a ∼2.0 × 10-6a 56 1.7 × 10-6 ∼2.86 × 104
9 24 25 25 This work This work This work
J/mol J/mol
∼8.8 × 103 ∼1.77 × 104
∼8.7 × 103 ∼1.99 × 104
This work This work
∆Gsol ∆Gdes a
This work (please see the last paragraph of Section 3.2)
TABLE 2: The Nomenclature and Value of the Parameters Relevant to This Work symbol
parameter
PV T P τ Sh kp DOg 2 Y∆ Yd Yd
Pressure in reaction zone Temperature of BHP (BDP) O2 total pressure Overall residence time of O2(1∆) in SOG Sherwood number O2(1∆) energy pooling rate constant O2(1∆) diffusion coefficient in 1 Pa gas pressure O2(1∆) yield measured at the diagnostic cell O2(1∆) detachment yield for Cl2 + BHP O2(1∆) detachment yield for Cl2 + BDP
unit torr K torr ms cm3/mol · s cm2 · Pa/s % % %
value
reference
3∼50 259 2∼47 4∼21 16 2.7 × 10-17 1.2 × 104 30∼69.5 70 ( 1.5 72.5 ( 1.5
This work This work This work This work 26 26 27 This work This work This work
9416 J. Phys. Chem. C, Vol. 112, No. 25, 2008
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BHP and our experimental value of Yd ≈ 0.725, we obtained fliq ≈ 0.996, fgas > 0.999, fint ≈ 0.73, and the mass accommodation coefficient of O2(1∆) at the O2(1∆)/BDP interface γO2 ≈ 1.7 × 10-6; this value is equivalent about 40% of γO2 value at the O2(1∆)/BHP interface. 3.3. Gibbs Free Energy of the O2(1∆)/BHP (or O2(1∆)/ BDP) Interface. It is very interesting to note that the Gibbs free energy ∆Gobs for a gas molecule to stick on the liquid surface can be deduced from the corresponding mass accommodation coefficients (γ). Davidovits et al.28 postulate the following schema to describe the performance of molecules on the gas/liquid interface: kads
ksol
ng {\} ns 98 nl
(15)
kdesorb
where ng, ns, and nl is, respectively, the number density of molecules in the gas phase, on the surface (interface), and in the liquid phase; kads, kdesorb, and ksol, respectively, are the kinetic rate constant of adsorption, desorption, and solvation processes. On the basis of the principle of thermodynamics for gas/liquid phase equilibrium in a diluted solution, Davidovits et al.28 deduced the following equations (eqs 16 and 17),
γ)
ksol ksol + kdesorb
(
-∆Gobs ksol γ ) exp ) 1 - γ kdesorb RT
(16)
)
(17)
where ∆Gobs is the Gibbs free energy difference between the gas molecule and the surface molecule. From eq 17, ∆Gobs can be calculated by eq 18.
( 1 -γ γ )
∆Gobs ) -RT ln
(18)
Using a similar approach, Ben-Naim et al.29 had deduced the expression for Gibbs free energy of gas solvation in liquid as eq 19,
∆Gsol ) RT ln H
Figure 6. The Gibbs free energy profile for O2(1∆) at the O2(1∆)/ BHP and O2(1∆)/BDP interface. ∆Gobs is the free energy difference between the gas O2(1∆) molecules and the O2(1∆) at the interface, ∆Gdes is the free energy for a solvated O2(1∆) molecule in the liquid phase moving to the interface, and ∆Gsol is the solvation free energy of O2(1∆) in BHP (or BDP). The higher ∆Gobs in the O2(1∆)/BDP system indicates that it is more difficult for gas O2(1∆) molecules to enter into BDP than into BHP, whereas the higher ∆Gdes in O2(1∆)/BDP system indicates that it is more difficult for O2(1∆) molecules solvated in BDP to escape from the liquid phase than those solvated in BHP.
(19)
where H is the Henry coefficient for a gas/liquid phase equilibrium system. Thus, using eq 18 and our experimental mass accommodation coefficient γO2 of O2(1∆) at the O2(1∆)/BHP(or BDP) interface, we have ∆Gobs ≈ 2.65 × 104 J/mol for O2(1∆)/BHP and ≈ 2.86 × 104 J/mol for O2(1∆)/BDP. From eq 19 and the correspond Henry coefficient, ∆Gsol values for O2(1∆)/BHP and O2(1∆)/BDP are calculated, respectively, to be ∼8.8 × 103J/ mol and ∼8.7 × 103 J/mol. In recent years, numerous molecular dynamics experiments and calculations have been implemented, giving hints on the structure and interface energy of the gas/liquid interface.17 Specifically, Jungwirth et al.18 have carried out a model calculation of the Gibbs free energies for O2/water interface by using a potential of mean force (PMF) model. We may now incorporate our experimental Gibbs energy data for the O2(1∆)/ BHP(BDP) interface with their calculated energy profile, as shown in Figure 6. The Gibbs free energy ∆Gdes is defined as the difference between ∆Gobs and ∆Gsol, which is calculated to be ∼1.77 × 104 J/mol for O2(1∆)/BHP and ∼1.99 × 104 J/mol for O2(1∆)/BDP. In fact, ∆Gdes represents an energy barrier that impedes O2(1∆) to get off the solution. Now, we can give an explanation as to why the survival probability of O2(1∆) molecules that diffuse through O2(1∆)/BDP is almost the same
as that through the O2(1∆)/BHP interface, despite the fact that the O2(1∆) lifetime (τO2) is an order-of-magnitude longer in BDP than that in BHP. A higher energy barrier at the O2(1∆)/BDP interface detains the nascent O2(1∆) in BDP for a longer time, setting off the effect of lower collisional quenching rate of O2(1∆) in BDP than in BHP. By comparing the physical properties of D2O and H2O, we may envisage a number of clues as to why the energy barrier of the O2(1∆)/BDP interface is higher than that of O2(1∆)/BHP. For a gas (or solvated) molecule to stick on the gas/liquid interface, it must first overcome the repulsive force against the surrounding molecules on the interface so as to burst through them. The work that the gas (or solvated) molecule does in this process is represented by ∆Gobs (or ∆Gdes). Because the mass of D2O is heavier than H2O and because the hydrogen bond length in liquid D2O (0.2760 nm) is shorter than that in H2O (0.2765 nm),30 more work must be done to burst through the O2(1∆)/BDP interface, calling for a higher ∆Gobs (or ∆Gdes). 4. Conclusion The detachment yields of O2(1∆) are separately measured to be ∼70 and ∼73% by the gaseous Cl2 reaction with aqueous basic hydrogen peroxide (BHP) and its deuterium-replaced counterpart (BDP) in a jet-type singlet oxygen generator. The yield of ∼73% for Cl2 + BDP reaction is much smaller than the predicted 88% by Vetrovec et al., in spite of the 10 times longer lifetime of O2(1∆) in BDP solution than that in BHP. Our analysis of the kinetic model for Cl2 + BHP(BDP) reactions shows that because the reaction of Cl2 + HO2- is exceedingly fast, reaching completion in a layer as thin as only a few Ångstro¨ms at the liquid surface, the gas/liquid interface sets the main resistance in the path of the nascent O2(1∆) diffusing from the solution into the bulk gas flow. So, the seemingly weird experimental result can be rationalized by assuming a higher energy barrier at the O2(1∆)/BDP interface that detains the nascent O2(1∆) in BDP for a longer time, offsetting the effect
O2(1∆) Quenching Mechanism and Interface Free Energy of lower collision quenching rate of O2(1∆) in BDP. This postulation is evidenced in this experiment. Using our experimental O2(1∆) yield and the relevant physical and kinetic parameters of O2(1∆) in the processes, the O2(1∆) mass accommodation coefficient and Gibbs free energy on O2(1∆)/ BHP surface are calculated, respectively, to be ∼4 × 10-6 and ∼2.65 × 104 J/mol; the corresponding values for O2(1∆)/BDP are ∼1.7 × 10-6 and ∼2.86 × 104 J/mol. The energy barrier from the bulk liquid to the interface (∆Gdes) is ∼1.99 × 104 J/mol for BDP, which is significantly higher than that for BHP, ∼1.77 × 104 J/mol. The higher O2(1∆)/BDP interface energy barrier may be explained by comparing the physical and thermodynamic properties of heavy and light water, for example, by the larger molecular mass of D2O and stronger hydrogen bonding between D2O molecules, so that the O2(1∆) molecule must do more work to burst through the liquid D2O interface. Further, a more general methodology may be developed to obtain critical physical and dynamical information of O2/aqueous electrolytic solution interface by directly measuring the quenching probability of O2(1∆) on the gas/solution interface. For this purpose, O2(1∆) generated by microwave discharge in O2 may be used. In our laboratory, work is now under way along these lines. Acknowledgment. This work was supported by the 973 program under Grant No. 2007CB815202. The authors thank Daorong Yu and Zongjuan Wang for their help in preparation of D2O2 and KOD solutions, Jianyong Liu for his help in construction of the JSOG, and Hongquan Wang and Qingwei Li for their help in measuring the viscosities of BHP and BDP. References and Notes (1) Frimer, A. A. Singlet O2; CRC Press: Boca Raton, Florida, 1985; Vol. I. (2) Lissi, E. A.; Encinas, M. V.; Lemp, E.; Rubio, M. A. Chem. ReV. 1993, 93, 699. (3) Schweitzer, C.; Schmidt, R. Chem. ReV. 2003, 103, 1685. (4) Storch, D. M.; Dymek, C. J. Jr.; Davis, L. P. J. Am. Chem. Soc. 1983, 105, 1765.
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