5772
J. Phys. Chem. 1985, 89, 5772-5718
The Martire-Boehm theory predicts selectivity relative to a “cubic solute”. Although benzene may be the best choice to approximate a cubic solute, it differs significantly in size from many of the solutes and is not significantly retained in organic-rich mobile phases. This could introduce error into the selectivity calculations, and therefore, a second reference solute was selected. Mesitylene, a symmetric molecule with chromatographic behavior similar to benzene, exhibits significantly greater retention under the same mobile-phase conditions. Table I also contains selectivity calculations using mesitylene as the reference solute (b,,,) for C#I = 0.7,0.8, and 0.9 mobile phase compositions. Although the values for percent change are slightly higher in some cases, the shape selectivity exhibited when C#I = 0.7 with benzene as the reference solute is unchanged. Interestingly, as the amount of organic modifier in the mobile phase increases, in going from I$ = 0.7 to q5 = 0.9 for instance,
the percentage difference in selectivity between the ODS and 55B phases for all rodlike and platelike solutes decreases. This suggests that, perhaps as the mobile phase becomes richer in organic modifier, the CBP chains of the ODS phases are better solvated and are more like the 55B chains in extension and order.
Acknowledgment. This work was supported, in part, by the National Science Foundation under Grant CHE-8500658. Fruitful discussions with Professor Martire at several stages of this work are gratefully acknowledged. Registry No. ODs, 18643-08-8;55B, 991 12-68-2;chloroplatinic acid, 16941-12-1; biphenyl, 92-52-4; p-terphenyl, 92-94-4; butylbenzene, 104-51-8;amylbenzene, 538-68-1; phenylheptane, 1078-71-3;phenylhexane, 1077-16-3;chrysene, 218-01-9; benzanthracene, 56-55-3; triphenylene, 217-59-4;pyrene, 129-00-0;anthracene, 120-12-7; phenanthrene, 85-01-8;naphthalene, 91-20-3.
Chemistry of Colloidal Manganese Dioxide. 1. Mechanism of Reduction by an Organic Radical (A Radiation Chemical Study) C. Lume-Pereira, S. Baral, A. Henglein,* and E. Janata Hahn-Meitner-Institut f u r Kernforschung Berlin, Bereich Strahlenchemie, 0-1000 Berlin 39, Federal Republic of Germany (Received: June 11, 1985; In Final Form: August 29, 1985)
Transparent sols of manganese(1V) oxide and manganese(II1) oxide were prepared by y-irradiating solutions of potassium permanganate. The reaction of radiolytically produced I-hydroxy-1-methylethylradicals, (CH,),COH, with the Mn02colloid was investigated by recording the accompanying changes in optical absorption and conductivity. The yield of reduction is 5.7 Mn02 molecules reduced per 100 eV. Electron transfer to the colloidal particles leads to the release of OH- ions, depending on the pH and the degree of reduction. Mn3+centers are formed in the reduction which accelerates the reaction of the radicals. A fast conproportionation reaction between Mn2+and Mn4+centers at the surface of the colloidal particles is postulated in alkaline sols. A strong accumulation of Mn3+centers thus is achieved. In acidic solutions, the Mn3+centers disproportionate. Mn3+centers also are formed when Mn2+ions are added to a Mn02 sol of pH >4. It is also pointed out that the absorption spectra of colloidal and macrocrystalline Mn02 are distinctly different.
Introduction Manganese dioxide is used as an oxidizing reagent in many reactions in inorganic and organic chemistry and also as a catalyst for redox processes such as the decomposition of peroxides.] When kinetic studies on these reactions are carried out, the manganese dioxide generally is present in the form of a fine powder. Intermediates formed at the solid/solution interface cannot optically be detected under these conditions, although their existence is inferred from the rate measurements. In the present work, transparent solutions of manganese dioxide are used which allow one to apply optical methods for the detection of chemical changes in the solid. The use of colloidal solutions has been quite successful during the last years to study fast interfacial processes.2 In part 1, we report on the preparation of colloidal manganese dioxide by radiation chemical reduction of permanganate in alkaline solution. Radiation recently has been used to prepare various metal colloids. The advantage of this method is a good reproducibility and the fact that conditions can often be found where very small particles are formed. The MnO, colloid made by y-irradiation is more stable, less opalescent, and chemically more active than a colloid prepared conventionally by mixing solutions of manganeous salt with potassium permanganate. In part 1, we also describe the degradation of colloidal manganese dioxide by y-irradiation under various conditions. y-Irradiation of aqueous solutions which contain organic additives allows one (1) ‘Gmelins Handbuch der Anorganischen Chemie”; Verlag Chemie: Weinheim, West Germany, 1973; 8. Auflage, Mangan 56, Teil C1. ( 2 ) Henglein, A. In “Modern Trends of Colloid Science in Chemistry and Biology”; Eicke, H.-F., Ed.; Birhauser-Verlag: Basel, Switzerland, 1985; pp 126-147.
0022-3654/85/2089-5772$01.50/0
to produce organic free radicals at a known rate. In the presence of colloidal MnO,, the radicals undergo electron transfer with the colloidal particles. These reactions can be studied by measuring the accompanying changes in optical absorption and conductivity of the solutions. For fast kinetic measurements, the method of pulse radiolysis is applied. These studies led to a mechanism of the reduction of M n 0 2 in which reactions of lower valence states of manganese play an important role. In the succeeding parts 2 and 3, the findings about the mechanism of reduction of M n 0 2 are applied to understand the course of some reactions in which M n 0 2 is involved either as an oxidant, a catalyst, or a product, such as the destruction of superoxide anion^,^ the decomposition of hydrogen peroxide,, and the oxidation of manganous ion.,
Experimental Section 2.1. Colloidal Mn02 by y-Irradiation. A 4 X IO4 M KMnO, solution at pH 10 was y-irradiated at 20 OC under air. The dose rate was 7 X lo4 rad/h. The absorption of the Mn04- ion around 530 nm disappeared during irradiation and a new broad band with maximum at 336 nm appeared. After a dose of 4.0 X lo5 rad, the reduction was complete as the absorption of MnO, could no longer be observed. An absorption coefficient of 1.O X IO4 M-’ cm-’ was calculated at 336 nm. Further irradiation caused the absorption to decrease. The colloid was stable for months, particularly when C 0 2 from the air was prevented from diffusing into (3) Baral, S.; Lume-Pereira, C.; Henglein, A.; Janata, E. J . Phys. Chem., following paper in this issue. (4) Baral, S.; Lume-Pereira, C ; Janata, E.; Henglein, A. J . Phys. Chem., to be submitted for publication.
0 1985 American Chemical Society
Chemistry of Colloidal Manganese Dioxide the solution. Boiling of the colloidal solution under reflux conditions for 8 h did not lead to flocculation or any changes in the absorption spectrum. However, slightly different absorption spectra were observed when the y-irradiation was carried out at lower or higher temperatures: at a temperature of preparation of 3 OC, the wavelength of maximum absorption was 324 nm; at 20 "C, 334 nm; and at 70 OC, 356 nm. The dose needed for complete reduction of K M n 0 4 did not change with temperature. The increase in wavelength of the maximum with increasing temperature is possibly an indication that larger particles were formed. The colloid was found to react with ferrous and iodide ions stoichiometrically (to form Mn2+). No thermal reaction with added 2-propanol was observed (in many of the following experiments, 2-propanol was used as radical scavenger). None of these colloidal solutions showed opalescence which indicates that the size of the particles was very small. In fact, the solutions could be filtered through a IO-nm nucleopore filter without loss in absorbance. The exact size of the particles is not yet known. Electron microscopic studies made at the Fritz-Haber-Institut of the Max Planck-Gesellschaft in Berlin led to results which we regard as very preliminary. The pictures were taken after evaporation of a drop of solution on a thin carbon carrier. Particles of 3-5-nm diameter with an ordered fine structure seemed to be present in a background of the irregularities of the carbon film. Further electron microscopic experiments are being carried out. On the other hand, an effective kinetic size of 4.2 nm of the particles was calculated from the kinetic data (see section 4.1). This effective size is an upper limit of the true particle size. The colloidal particles were negatively charged as they migrated to the anode in an electrical field. The pH of the solution could be changed up to 11.5 by adding NaOH or down to 3.0 by adding perchloric acid without flocculation of the colloid. In all these solutions, the excess charge of the particles was negative. 2.2. Conventional MnO, Colloid. A 75-mL aqueous solution of 4 X M Mn(C104), was added to 50 mL of an aqueous 4 X lo4 M KMn04 solution. The colloid formed instantaneously in the strongly stirred mixture. No Mn2+ ions could be detected by polarography after the reaction. N o excess Mn0; ions were detected by optical absorption measurements. The colloid could not be filtered through a 10-nm nucleopore filter which shows that the particles were much larger than in the radiolytically produced colloid. The sol opalesced and became more and more turbid upon standing for several weeks. Its absorption spectrum contained a broad band with maximum at 380 nm (e = 8.4 X lo3 M-' cm-I ). 2.3. Analytical Procedures. MnZ+ions in the irradiated sols were detected by polarography. If free ions in solution were to be determined, the colloid was removed by repeated freeze-thaw cycles and centrifuging. A 2-mL sample of the supernatant fluid was added to 1 mL of phosphate buffer (pH 7). One milliliter of 2 M KSCN solution was added, and the mixture diluted as desired and argon bubbled for 4 min before the polarogram was taken (wave of Mn2+at -1.65 V vs. Ag/AgCl). In the cases where the total Mn2+content was to be measured (free plus chemisorbed Mn2+),the irradiated alkaline sample was acidified under exclusion of air and then analyzed in the polarograph. The limit of detection was M Mn2+. The colloids of both manganese(1V) and manganese(II1) reacted with ferrous ions in 0.4 M sulfuric acid. The reactions were followed quantitatively by adding an excess of ferrous ion and determining the ferric ion formed spectrophotometrically at 304 nm ( 6 = 2201 M-' cm-I). 2.4. Irradiation Techniques. y-Irradiations were carried out in the field of a 6oCosource. The pulse radiolysis equipment consisted of a 4-MeV Van de Graaff generator delivering 0.5-ps electron pulses, a detection system based on the measurement of the changes in conductivity and optical absorption of the solutions, and a data processing system.s Both ac and dc conductivity measurements were applied.6 In many experiments the dose in the pulse was kept very (5) Henglein, A,; Lilie, J. J . Am. Chem. SOC.1981, 103, 1059.
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5773
100
100
50
0
tlrnin]
Figure 1. Conductivity of a 4 X lo4 M MnOz sol of pH 10.6 as a function of the time of irradiation. Dose rate: 8.4 X lo4 rad/h. The solution contained 0.1 M 2-propanol and was saturated with nitrous oxide.
low to produce radicals in concentrations of only M. Several pulses then were given and the signals averaged. The base line was recorded between the pulses and subtracted from the recorded curves. The detection system had a back-off circuit for automatic base-line compensation and measurement of the background c~rrent.~ Experiments were normally done on flowing solutions, so that every time fresh solutions were exposed to radiation pulses. However, when preirradiation of the sample was desired, a small volume of the standing solution was used which was irradiated uniformly over the entire volume by the electrons in successive pulses.
3. Results 3.1. Conductivity Change during the Formation of the Colloid.
-
The overall reaction in the radiolytic reduction of K M n 0 4 is
K+ + Mn04- + '/,H20
Mn0,
+ 3/402 + K+ + OH-
(1)
The reaction, which was carried out in the presence of 1.0 X M NaOH, should be accompanied by an increase in molar conductivity of 120 Q-' cm2, which is the difference in the molar conductivities of OH- and Mn04-. At an initial concentration of Mn0,- of 4 X M, the pH should increase from 10.0 to 10.7. However, a smaller increase in pH was found. The conductivity before irradiation was 781 pS and after irradiation 95.5 pS. Knowing the concentrations and molar conductivities of all species involved one calculates that only half of the OH- ions formed were mobile. It is concluded that 50% of the OH- ions were either adsorbed at the colloidal particles or reacted with surface hydroxyl groups. In both cases, excess negative charges were produced on the colloidal particles which helped to stabilize them against flocculation. 3.2. Conductivity Changes during the Reduction of the Colloid. As is described in more detail in section 3.3, the MnO, colloid is reduced by 1-hydroxy- 1-methylethyl radicals. Regardless of whether divalent or trivalent manganese is produced in this reduction, according to the overall equations MnO2 + 2(CH3),COH 2Mn0,
+ 2(CH3),COH
-
-
Mn(OH),
Mn,03
+ 2(CH3),C0
(2)
+ H 2 0 + 2(CH3),C0
(3) one would not expect any conductivity changes to occur. However, an increase in conductivity was found which is shown by Figure 1. In these experiments, the MnO, sol was y-irradiated in the presence of 0.1 M 2-propanol and N 2 0 . It is known from radiation chemistry that 1-hydroxy-1-methylethyl radicals are produced under these conditions as all the primary radicals from the radiolysis. of water undergo reactions which finally lead to (CH,),COH radicals: H20 eaq- + N 2 0 OH(H)
--
+ (CH3)2CHOH
ea
