Water/Silica Nanoparticle Interfacial Kinetics of Sulfate, Hydrogen Phosphate, and Dithiocyanate Radicals

Abstract

The values of the rate constants for the reactions of the sulfate (2.5 x 10(9) M(-1) s(-1)) and hydrogen phosphate (2.2 x 10(8) M(-1) s(-1)) radicals with silica nanoparticles are obtained by flash photolysis experiments with silica suspensions containing S(2)O(8)(2-) or P(2)O(8)(4-), respectively. The interaction of these radicals with the silica nanoparticles leads to formation of transients, probably adsorbed sulfate and hydrogen phosphate radicals, with absorption maxima at around 320 and 350 nm, respectively. A different mechanism takes place for the interaction of the less oxidizing dithiocyanate radicals with the silica nanoparticles. These radicals selectively react with the dissociated silanol groups of the nanoparticles with a rate constant at 298.2K of 7 x 10(7) M(-1) s(-1) (per mol of SiO(-) groups), and there is no evidence for their adsorption at the surface. All the results are discussed in terms of the Smoluchowski equation and redox potential of the inorganic radicals.

Reactions of Cl•/Cl2•− Radicals with the Nanoparticle Silica Surface and with Humic Acids: Model Reactions for the Aqueous Phase Chemistry of the Atmosphere

“…The previous results clearly indicate that Cl • and Cl 2 •− radicals interact with the NP surface. Moreover, in aqueous suspensions of pH < 5, silica NS have almost all its surface silanols undissociated (pK ag ≈4.5 and pK as ≈8.5 for the deprotonation of geminal and single silanols, respectively [19,20]). The lack of reaction between the Cl • /Cl 2 •− radicals and the NP surface at pH < 5, and the higher rates observed with increasing pH, clearly indicate that the deprotonated silanols are the reactive surface species, as shown in .…”

“…The absorbance of SiO • surface radicals strongly depends on the pH of the suspensions and despite that a broader absorption band was observed for suspensions of higher pH, it was not possible to differentiate the absorption of SiO • radicals formed from geminal or single silanols (19,20). Therefore, average absorption coefficients for SiO • radicals were estimated for each pH of the suspensions relative to the reported average value at 600 nm and pH 8.3, < ε 600 > pH 8 = 2100 M −1 cm −1 (20). The average absorption coefficients at the different wavelengths λ and suspension pH, < ε λ > pH , were calculated according to Eq.…”

“…The higher reduction potential of Cl • atoms compared with that of Cl 2 •− radicals, vide supra , is in line with the observation that SiO • radicals are only formed from the one electron oxidation of deprotonated silanols by chlorine atoms, . As SiO • surface defects are also formed from the reaction with sulfate radicals (E(SO 4 •− /SO 4 2− )≈2.43 V [7,8]) and not with dithiocyanate radicals (E((SCN) 2 •− /2 SCN − )≈1.31 V [20]), the SiO • reduction potential is expected to be 2.0 < E(SiO/SiO − ) ≤ 2.4 V.…”

Tropospheric Aqueous‐Phase Free‐Radical Chemistry: Radical Sources, Spectra, Reaction Kinetics and Prediction Tools

“…Even if the spectra agree in general well, slight differences can be seen which could be of importance for spectroscopic studies. With the exception of Adams et al [74] and Caregnato et al [75] the maximum absorption is found around l % 478 nm. In the study of Caregnato and coworkers [75] the whole absorption band is blue-shifted by approximately 15 nm.…”

“…With the exception of Adams et al [74] and Caregnato et al [75] the maximum absorption is found around l % 478 nm. In the study of Caregnato and coworkers [75] the whole absorption band is blue-shifted by approximately 15 nm. The spectra published by Czapski et al [76] shows a higher absorption at the peak absorbance.…”