To a first approximation, the rate at which soiling occurs via partitioning is independent of surface orientation. Vertical surfaces soil at rates similar to horizontal surfaces. The surface accumulation resulting from partitioning is commonly referred to as a “film.” We estimate that a pollutant film of 5 to 10 monolayers is thick enough to alter the interface with which airborne organic compounds interact. For a film composed of primarily indoor SVOCs, this is equivalent to a thickness on the order of 10 nm or less. Rates at which films accumulate on initially clean impermeable indoor surfaces have been measured in different indoor environments and are summarized in Table 2 of Weschler and Nazaroff. Film growth rates in the range 0.1-0.2 nm/d have been reported. At such rates, films with thicknesses of 5-10 nm would accumulate on indoor surfaces in 1-3 months. In a composite sample of five homes in urban Toronto, a film thickness of 5 nm was measured 3-5 months after window cleaning.Toward the higher end of organic soiling conditions, Wu et al. exposed aluminum, polished glass and ground glass disks for 2-3 weeks in a kitchen where cooking occurred daily. Prior to exposure, the clean surfaces exhibited significantly different sorptive partitioning of DEHP. The 2-3 weeks of exposure was sufficient for the disks to acquire an organic film that resulted in similar sorptive partitioning of DEHP across diverse substrate materials. Taken together, these measurements suggest that within a period of a few months, impermeable indoor surfaces are covered by a film, acquired via partitioning, that is thick enough to alter the chemical surface that it presents to indoor air. We note that the deposition velocities for lower molecular weight organic acids are comparable to those for SVOCs. However, the indoor gas-phase concentrations of species such as formic and acetic acid are orders of magnitude larger than those of indoor SVOCs. Hence,vertical rack the flux of lower molecular weight organic acids to an aqueous surface is potentially orders of magnitude larger than the flux of SVOCs to an incipient surface film.
While the rate at which SVOC partitioning contributes to surface soiling is independent of the surface’s orientation, the rate at which particles deposit on a surface varies substantially with orientation. Because particles settle under gravity’s influence, upward facing horizontal surfaces soil much faster than vertical or downward-facing surfaces. Based simply on geometry, upward horizontal surfaces account for roughly 20% of exposed indoor surfaces. Table 3 in Weschler and Nazaroff presents estimated rates for particle accumulation on vertical and upward-facing horizontal surfaces. The rate at which vertical surfaces soil via particle deposition is predicted to be much slower than the rate at which they soil via partitioning. Even in the case of upward-facing horizontal surfaces, soiling by fine particle deposition is relatively slow. Since water soluble salts and associated water comprise larger mass fractions of fine particles than of coarse particles, this is an important consideration.In summary, on impermeable indoor surfaces, semivolatile organic compounds accumulate much faster than particle-associated water-soluble salts. Only in the instance of an upward facing horizontal surfaces does particle deposition become important, and, in this case, gravitational settling of coarse particles dominates the deposition process. Upward-facing horizontal surfaces can accumulate particles to a level of 1 µg/cm2 on a time scale as short as a few days, whereas surfaces of all orientation can have a 1 µg/cm2 accumulation of SVOCs in a few months. As expected from theoretical considerations, the specific SVOCs that comprise indoor organic films are similar to those found in indoor airborne particles. In a recent intensive field study conducted in a test house, the HOMEChem project, such similarity was observed experimentally: “the signal intensities of the mass spectra for the indoor aerosol filter and surface extracts have high overlap, with a dot product of 0.98.” While we know something about the formation and composition of films on impermeable indoor surfaces, we have little information about the soiling of semipermeable or porous surfaces such as paint films, vinyl flooring, brick, concrete, carpets and upholstery. Among other factors, morphology, as well as orientation, are expected to influence the soiling of such materials.
The chemicals that comprise surface films evolve. They might initially be dominated by SVOCs; however, over time, they acquire particles and the water-soluble salts associated with these particles. The initial SVOCs in the film oxidize, which should tend to increase the oxygen to carbon ratio. This process may make the surface film more hygroscopic. The acquisition of inorganic salts and increases in the O/C ratio should lead to increased water content of surface films, especially during periods with higher indoor humidity, and the viscosity of the film may decrease as a consequence. Such changes could influence heterogeneous acid/base reactions, making them more likely in aged films under high RH conditions. Oxidation of surface films may also lead to phase separations that might further influence acid/base reactions. Evidence from nicotine The pH of skin’s surface typically is in the range 4.5-6.The pKa of monoprotonated nicotine is 8.0.Hence, at skin’s pH, one would anticipate that most would be ionized . Based on measurements from excised pig skin, ionized nicotine passes through skin about fifty times slower than neutral nicotine. If there is a homogeneous film on the surface of skin that is a mix of water, salt and skin lipids with pH of 4.5-6, then the capacity of this film for acquiring a combination of ionized nicotine and neutral nicotine would be very large. Since only the much less abundant neutral fraction is rapidly absorbed by the skin, transport from air through skin to blood should be relatively slow. However, actual measurements of the dermal uptake of nicotine from the gas phase indicate a fast transport rate.A possible explanation is that skin surface lipids and aqueous salt solutions coexist on the skin isolated from one another rather than being homogeneously mixed.As a crude analogy, they may exist more in the fashion of oil and vinegar rather than as mayonnaise. According to this conceptualization, nicotine that partitions from the gas-phase into islands of skin lipids would remain neutral, subsequently passing through the stratum corneum and viable epidermis to the dermal capillaries. Something similar may occur on impervious indoor surfaces; there may exist regions with aqueous surface films isolated from regions with organic rich films. Within porous surfaces, there may exist pockets of aqueous solutions and pockets of hydrophobic organics. If such is the case, then organic acids and bases could partition to both aqueous and organic substrates, and the relative amounts in each phase could influence the resultant surface chemistry.
Different methods have been used to characterize surface acidity.The more common methods have been reviewed by Sun and Berg. These include colloid titration, indicator dye adsorption, X-ray photo electron spectroscopy, and calorimetry. Most metal surfaces acquire a charge and, consequently,microgreen flood table an electrical double layer at an aqueous interface. The isoelectric point is defined as the pH value at which the potential of the double layer at the interface is zero. The isoelectric point is frequently used to characterize the acidity of solid surfaces. The IEP is measured by electrokinetic titration, which is a type of colloid titration. Another approach that has been used to evaluate the acidity and basicity of metal oxides is microcalorimetry. Using NH3 and CO2 as probe molecules, Auroux and Gervasini determined the number and character of basic and acidic sites on twenty metal oxides. Some metal oxides are basic , some are acidic , and some are amphoteric , reacting with both acidic and basic gases. Recently, Rindelaub et al. have made direct measurements of pH in individual particles using a Raman micro-spectrometer coupled with a confocal optical microscope. Wei et al. have applied a related method using 2D and 3D confocal Raman microscopy to determine the pH of suspended aerosol droplets smaller than 50 µm diameter. Such methods might be adapted to probe surface acidity. The acid-base properties of glass have received considerable attention. 500 Silicon dioxide, forming the chemical framework of glass, is acidic in a Lewis-acid sense . Glass is relatively inert to acids, but is attacked by bases, especially when an aqueous layer in contact with glass has pH > 9. The acid base properties of a polymer can often be described as those of its repeating unit. A substantial proportion of the SVOCs found in surface films are organic acids. Some of these can have human origins. Liu et al. measured the major organic constituents in films on impermeable indoor surfaces from five sites in greater Toronto.Monocarboxylic acids with 11-31 carbons accounted for between 76% and 81% by mass of the total organic fraction. Together, monocarboxylic acids, dicarboxylic acids with 6-14 carbon atoms, nine aromatic polycarboxylic acids, and five terpenoid acids accounted for 81- 95% by mass of the total organic fraction, which included n-alkanes, PAHs, PCBs, and pesticides. These study results demonstrate an acidic character for organic surface films. Surfaces that are basic and porous can be large sinks for acidic gases.
Concrete, especially if it is improperly cured, and brick are examples of surfaces with basic properties. In China it is common to whitewash walls in apartments and other buildings. , prior to the introduction of modern paints and sealants. Among some enthusiasts in the US and Europe, whitewashed wood walls, brick walls and furniture are making a comeback . Whitewash is made by mixing hydrated lime with water, producing a white sealant. A whitewashed/limed wall is chemically basic and would have a high capacity for sorptive uptake of gas-phase acids. To what extent is the pH of a surface determined by the surface itself versus gases dissolved in water associated with the surface? In the case of a hydrophobic surface, the amount of sorbed water is small and the nature of the surface itself should determine its acidity. In contrast, the acidity of a hydrophilic surface may be largely influenced by gases that have dissolved in the water associated with the material. Such may be the case for cotton fabrics or untreated nylon carpeting.Some surfaces attract water ; others repel water . Surfaces are somewhat arbitrarily categorized as hydrophilic or hydrophobic based on the contact angle between a water droplet and the surface. If the contact angle is < 90° , the surface is considered hydrophilic . Common hydrophilic surfaces indoors include nylon, glass, stainless steel, gypsum in wallboard, and cotton fabrics. Hydrophobic examples include untarnished silver, chromium, candle wax, and polypropylene. Surfaces that are extremely hydrophobic, such as Teflon or those treated with perfluorocarbon stain repellants, repel water and other highly polar compounds. Ionization of an acidic or basic species is limited on a hydrophobic surface with very little water available, and so, acid-base chemistry would be deterred on such a surface. The degree of hydrophilicity also influences processes such as the disproportionation of NO2 onto indoor surfaces to form HONO and HNO3. An example of the latter process has been reported for a series of experiments in which NO2 was injected into a 2.5-m3 chamber and the airborne concentrations of both NO2 and HONO monitored. The chamber surfaces were either all Teflon or all vinyl wallpaper ; in a subset of experiments, the floor of the Teflon or vinyl chamber was covered with a hydrophilic synthetic carpet. The measured NO2 surface removal rate was more than an order of magnitude larger with carpet on the floor of the chamber than when all the surfaces were Teflon or vinyl . Additionally, the peak gas-phase concentration of HONO, generated from the NO2 injection, was larger with carpet in the chamber than when all the surfaces were Teflonor vinyl . These results were likely due to a combination of increased surface area and the larger moisture content of the carpet compared to either Teflon or vinyl. When the chamber surfaces were vinyl wallpaper and the RH was either 50% or 70% RH, HONO decayed significantly more slowly than the air-exchange rate indicating prolonged release of HONO from the surface. However, when the chamber surfaces were Teflon, HONO decayed significantly slower than the air-exchange rate only at 70% RH; it decayed at a rate similar to the air exchange rate at 50% RH.