Generation of Gaseous ClO₂ from Thin Films of Solid NaClO₂ by Sequential Exposure to Ultraviolet Light and Moisture

Abstract

We report that thin films of solid sodium chlorite (NaClO) can be photochemically activated by irradiation with ultraviolet (UV) light to generate gaseous chlorine dioxide (ClO) upon subsequent exposure to moisture. The limiting role of water in the reaction is evidenced by an increase in yield of ClO with relative humidity of the gas stream passed over the UV-activated salt. The UV-activated state of the NaClO was found to possess a half-life of 48 h, revealing the presence of long-lived UV activated species that subsequently react with water to produce gaseous ClO. The yield of ClO was determined to be proportional to the surface area of NaClO particles projected to the incident illumination, consistent with activation of a ∼10 nm-thick layer of NaClO at the surface of the micrometer-sized salt crystals (for an activation wavelength of 254 nm). We also found that the quantity of ClO released can be tuned ∼10-fold by varying wavelength of UV irradiation and relative humidity of the gas stream passed over the UV-activated NaClO. The UV-activated species were not detectable by electron paramagnetic resonance spectroscopy, indicating that the activated intermediate is not an excited triplet state of ClO. Additionally, neither X-ray photoelectron spectroscopy, nor Raman spectroscopy, nor attenuated total reflection infrared spectroscopy revealed the identity of the activated intermediate species. The ability to preactivate solid phase chlorite salt for subsequent generation of ClO upon exposure to moisture suggests the basis of new materials and methods that permit triggered release of ClO in contexts that use its disinfectant properties.

Polymeric Films Containing Sodium Chlorite That Release Disinfectant Gas upon Activation with UV Light

“…Both X‐rays and electron beams rapidly decompose NaClO 2 and thus we avoided their use when characterizing the polymer‐NaClO 2 films. Instead, we used UV–vis absorption spectrophotometry and AFM to estimate the thickness of polymer‐NaClO 2 films, and we used optical microscopy, Raman chemical imaging, and TOF‐SIMS to characterize the state of dispersion of the NaClO 2 within polymer‐NaClO 2 films.…”

“…First, motivated by our prior conclusion that ClO 2 is generated from an ≈10 nm thick layer at the surface of UV‐activated NaClO 2 crystals, we hypothesized that finer dispersions of NaClO 2 , prepared by dispersing NaClO 2 in polymeric matrices, would enable yields of ClO 2 that are higher than those achieved in our past study. To test this hypothesis, we report the preparation of NaClO 2 in polymeric films comprised of polyvinyl alcohol (PVA) or polyolefins (polyethylene copolymerized with octene; POD), and characterization of the films by atomic force microscopy (AFM), optical microscopy, Raman microscopy, and time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS).…”

“…To test this hypothesis, we report the preparation of NaClO 2 in polymeric films comprised of polyvinyl alcohol (PVA) or polyolefins (polyethylene copolymerized with octene; POD), and characterization of the films by atomic force microscopy (AFM), optical microscopy, Raman microscopy, and time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). Second, as noted above, our previous experiments with powdered NaClO 2 indicated that UV‐activated NaClO 2 crystals possess half‐lives of up to 48 h . Because the interfacial regions of UV‐activated NaClO 2 crystals, if dispersed in polymeric films, will be proximal to and able to potentially react with the polymers in the films, we sought to test for a correlation between the lifetime of the UV‐activated states of the NaClO 2 and the reactivity/inertness of the polymeric matrices.…”

“…The approach described in this paper builds from our recent discovery that powdered NaClO 2 , when activated by UV‐irradiation and subsequently exposed to moisture, generates ClO 2 gas . Specifically, we discovered that UV‐activated states of the NaClO 2 powder can be generated with half‐lives as long as 48 h, and that subsequent exposure to moisture within 48 h of activation liberates ClO 2 .…”

“…Because the interfacial regions of UV‐activated NaClO 2 crystals, if dispersed in polymeric films, will be proximal to and able to potentially react with the polymers in the films, we sought to test for a correlation between the lifetime of the UV‐activated states of the NaClO 2 and the reactivity/inertness of the polymeric matrices. Third, because our past study revealed that water played a central role in ClO 2 generation from UV‐activated NaClO 2 , we hypothesized that polymeric matrices impact ClO 2 generation through recruitment of water to the imbedded NaClO 2 crystals. To test this hypothesis, we dispersed NaClO 2 in polymeric matrices that differed in their hydrophilicity, and measured whether the generation of ClO 2 was a strong function of the polarity of the polymer.…”