Molecular Dynamics Simulations of the Interactions of Organic Compounds at Indoor Relevant Surfaces.

Can bioinformed design promote healthy indoor ecosystems?

Higher prevalence of breathlessness in elderly exposed to indoor aldehydes and VOCs in a representative sample of French dwellings

The migration of volatile organic compounds (VOCs) from recycled polyethylene (rPE) matrices poses significant safety concerns for food packaging applications primarily due to potential consumer exposure risks. In this work, united-atom (UA) models with VOCs (1-octene, myrcene, linalool, octanal, and tetradecane) of rPE by treating multiple atoms as a single bead were developed. We performed UA molecular dynamics (MD) simulations to reveal the mechanism of VOCs diffusion within rPE under the influence of temperature, crystallinity, and the deformation of fluid-induced crystallization (FIC). The results show that the diffusion capability of VOCs is stronger in the amorphous regions of rPE, and weaker in the crystalline regions. The crystallinity of the rPE model exhibited a negative correlation with the mean squared displacement (MSD) of VOCs, attributable to the reduction of amorphous regions and free volume at higher segment orientation. With increasing temperatures, a significant enhancement in the diffusion of VOCs was observed, exemplified by the diffusion coefficient of 1-octene, which rose from 1.69 ± 0.60 × 10-6 to 6.49 ± 0.94 × 10-6 cm2·s-1 at 293-353 K. Concurrently, the elevated temperatures intensify the free volume expansion in rPE. The significantly increased orientation of rPE induced by fluid flow correlates well with the restricted diffusion behavior of VOCs, further confirming the decisive role of the crystallinity of rPE in the retention and removal of VOCs. Finally, the degree of VOCs diffusion trajectories in rPE space can be indirectly predicted from their MSD values and potential energy values obtained through molecular simulations.

TS1-20 Abstract: A model has recently been proposed to predict the emission rate of semivolatile organic compounds (SVOCs) from polymer materials. The model was based on data collected in an experimental study that measured emissions of di-2-ethylhexyl phthalate (DEHP) from vinyl flooring. The analysis revealed that although emissions of volatile organic compounds (VOCs) are generally subject to “internal” control (the material-phase diffusion coefficient), emissions of the very low volatility SVOCs (such as DEHP) are subject to “external” control (partitioning into the gas phase, the convective mass-transfer coefficient, and adsorption onto interior surfaces). The analysis showed that adsorption to interior surfaces strongly influences the emission rate. This inference is supported by the recent experimental finding that the presence of indoor dust increases the emission rate of DEHP from vinyl flooring in a test chamber. Compared with the 2 simple chamber systems studied, a regular indoor environment has many other types of surface that will adsorb SVOCs to different extents. The emission rate measured in the test chambers may therefore be vastly different from the actual emission rate from the exact same material into a typical indoor environment. The new model provides a convenient means to estimate the true emission rate and evolving gas phase and adsorbed surface concentrations likely to occur in real indoor environments. The missing information is the adsorption isotherm for DEHP on the different types of indoor surfaces. Although very little information is available on adsorption of DEHP and other SVOCs to indoor surfaces, the recent EPA-sponsored Children's Total Exposure to Persistent Pesticides and Other Persistent Organic Pollutants (CTEPP) study has measured concentrations of di-n-butylpthalate (DBP) in indoor air and on several interior surfaces in a large number of residences and day-care centers. The DBP surface sorption data is used together with estimates of the available surface area of the different classes of materials found indoors to estimate the emission rate and evolving gas-phase concentration after the installation of vinyl flooring in a typical room. Because it is based on fundamental mechanisms and the properties of the specific SVOCs, simple variants of the model should ultimately be able to predict exposure to phthalate plasticizers, brominated flame retardants, biocides, and other SVOCs emitted from a wide range of consumer products and building materials. Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official agency policy.

Roles of pollution in the prevalence and exacerbations of allergic diseases in Asia

Organic compounds in indoor air—their relevance for perceived indoor air quality?

Strong temperature influence and indiscernible ventilation effect on dynamics of some semivolatile organic compounds in the indoor air of an office.

Semivolatile organic compounds in indoor environments

The effectiveness of various tracers for measurements of exposure to environmental tobacco smoke (ETS) as a complex chemical mixture is based on the physicochemical properties of four major organic components and their dynamic behavior in indoor environments. For the particulate matter (PM) component and the very volatile organic compounds, emission and ventilation rates are generally the most important processes controlling indoor concentrations and exposures of nonsmokers. For the volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs), sorption on and desorption from indoor surfaces are additional processes that influence exposures. Laboratory and modeling studies of the dynamic behavior of nicotine, an SVOC, and PM indicate that nicotine can be used to estimate PM exposures from ETS in indoor environments when certain criteria are met: (italic>a(/italic>) smoking occurs regularly in the environment, (italic>b(/italic>) the system is near quasi-steady state, and (italic>c(/italic>) sampling time is longer than the characteristic times for removal processes. Measurements in residential and workplace buildings also support the use of nicotine as a tracer for PM in ETS. Recent laboratory and field data indicate that the VOCs from ETS can be traced using compounds with similar physicochemical properties, such as 3-ethenylpyridine, pyrrole, or pyridine. The effectiveness of nicotine for estimating exposures to the VOCs and SVOCs has not been determined, although these constitute major mass fractions of ETS.

Molecular simulation on competitive adsorption of ethanol, acetaldehyde and acetic acid on zeolites in humid conditions

Since children spend a considerable part of the day at school, classroom indoor air quality (IAQ) is a major contributor to their personal exposure. The geographical location of the school, the proximity of outdoor sources (industry or traffic), construction characteristics (including ventilation/heating system), as well as decorations and consumer products, all contribute to classroom IAQ. Considering the potential health impact of a poor IAQ on this susceptible population, suitable measures to assess and mitigate indoor air pollutants (IAP) in school buildings are taken. The last 4 years, several studies on classroom IAQ and source control were organized in Belgium. The use of new, innovative sampling techniques, designed for indoor air monitoring (Lazarov et al. 2013), led to novel insights into classroom environments. In 90 non-mechanically ventilated classrooms, indoor and outdoor levels of traffic-related volatile organic compounds (VOCs) were closely associated. Resuspension by room occupancy caused increased indoor PM2.5 during teaching periods. Indoor CO2 was elevated (reaching 5000ppm) and significantly correlated to indoor VOCs, formaldehyde and PM2.5. Mechanical ventilation in 26 newly built classrooms in low-energy and certified passive buildings (annual energy demand 15l/s.pp) was inversely associated with indoor toluene, formaldehyde, PM2.5 and CO2. Most abundant phthalates were di-ethylphthalate, di-n-butylphthtalate, and to lesser extent benzylbutylphthtalate. Concentrations up to 8μg/m3 were quantified, resulting from synthetic classroom decorations and products. Following the IAQ assessments, three strategies to optimize/enhance classroom environments were explored (www.vito.be/indoor_air; www.sinphonie.eu): (1) identification and quantification of classroom IAP sources, (2) validation of building materials that enhance IAQ, and (3) improved filter efficiency in the air supply of ventilation systems. The first measure was explored by quantifying emissions of classroom products in test chambers, respecting ISO 16000-9 whilst simulating representative classroom climates. A wooden kindergarten chair emitted 16 different VOCs and aldehydes, including formaldehyde at an emission rate of 4.5μg/h.chair, 6 days after installation. Dry-erase markers emitted 16 VOCs and aldehydes, including benzene, and couch textile emitted tri(2-chloroethyl)phosphate at a rate of 3.5μg/h.m2 60 days after installation. The second measure was explored by exposing a plaster board with IAQ enhancing characteristics to a controlled atmosphere of formaldehyde, toluene, benzene and limonene in a test chamber. The treated board selectively reduced formaldehyde with an efficiency of 79% (loading factor 0.38m2/m3). To explore the third measure, long-term experiments are organised in 4 classrooms to quantify the impact of filter efficiency upgrades on the occurrence of outdoor air pollutants indoors (PMx and soot). The use of innovative indoor sampling methods leads to the identification of critical aspects of school environments. This research illustrates that to create better IAQ at school, risks of IAQ can be tackled by dedicated source control and reduction. B. Lazarov, R. Swinnen, M. Spruyt, E. Goelen, M. Stranger, G. Desmet, E. Wauters. Optimisation steps of an innovative air sampling method for semi-volatile organic compounds. Atmospheric Environment 79(2013); 780-786.

The mobility of simple ions such as alkali–metal and halide ions at room temperature shows two anomalies. Firstly, there are maxima in mobilities as a function of ion size for both positive and negative ions and, secondly, the maximum for negative ions occurs at a larger ionic radius than the maximum for positive ions. Theoretical treatments of this problem are reviewed and it is concluded that a molecular treatment of the system is needed to understand the results. Computer simulation using the simple point charge model (SPC/E) for water reproduced the observations and is used to discuss the application of theories. In particular, the nature of the first solvation shell is correlated with ion mobility. Simulation reveals a further anomaly, namely that if the charge is removed from a large ion, then it moves more slowly. This is interpreted as the result of formation of a solvent cage around the hydrophobic solute. The changes in local structure resulting from changes in charge and size also affect the solvation thermodynamics. Simulations show that the solvation entropy has a double maximum when viewed as a function of charge. The local minimum near zero charge is interpreted as being due to hydrophobic order, and the maxima as the result of structure breaking. This double maximum in the entropy of solvation is a signature of the hydrophobic cage effect. Comparisons are made between ion mobilities in liquid water at ambient and supercritical conditions.

The virial stress is the most commonly used definition of stress in discrete particle systems. This quantity includes two parts. The first part depends on the mass and velocity (or, in some versions, the fluctuation part of the velocity) of atomic particles, reflecting an assertion that mass transfer causes mechanical stress to be applied on stationary spatial surfaces external to an atomic‐particle system. The second part depends on interatomic forces and atomic positions, providing a continuum measure for the internal mechanical interactions between particles. Historic derivations of the virial stress include generalization from the virial theorem of Clausius (1870) for gas pressure and solution of the spatial equation of balance of momentum. The virial stress is stress‐like a measure for momentum change in space. This paper shows that, contrary to the generally accepted view, the virial stress is not a measure for mechanical force between material points and cannot be regarded as a measure for mechanical stress in any sense. The lack of physical significance is both at the individual atom level in a time‐resolved sense and at the system level in a statistical sense. It is demonstrated that the interatomic force term alone is a valid stress measure and can be identified with the Cauchy stress. The proof in this paper consists of two parts. First, for the simple conditions of rigid translation, uniform tension and tension with thermal oscillations, the virial stress yields clearly erroneous interpretations of stress. Second, the conceptual flaw in the generalization from the virial theorem for gas pressure to stress and the confusion over spatial and material equations of balance of momentum in theoretical derivations of the virial stress that led to its erroneous acceptance as the Cauchy stress are pointed out. Interpretation of the virial stress as a measure for mechanical force violates balance of momentum and is inconsistent with the basic definition of stress. The versions of the virial‐stress formula that involve total particle velocity and the thermal fluctuation part of the velocity are demonstrated to be measures of spatial momentum flow relative to, respectively, a fixed reference frame and a moving frame with a velocity equal to the part of particle velocity not included in the virial formula. To further illustrate the irrelevance of mass transfer to the evaluation of stress, an equivalent continuum (EC) for dynamically deforming atomistic particle systems is defined. The equivalence of the continuum to discrete atomic systems includes (i) preservation of linear and angular momenta, (ii) conservation of internal, external and inertial work rates, and (iii) conservation of mass. This equivalence allows fields of work‐ and momentum‐preserving Cauchy stress, surface traction, body force and deformation to be determined. The resulting stress field depends only on interatomic forces, providing an independent proof that as a measure for internal material interaction stress is independent of kinetic energy or mass transfer.

PurposeThis study aims to evaluate the effects of volatile organic compounds (VOCs) on the indoor and outdoor air quality in Kuwait due to vehicular traffic.Design/methodology/approachAbout 700 VOCs samples were collected from randomly selected residences within Kuwait. For simplicity, the study was divided into three areas: area A between the first and third ring roads, area B between the third and fifth ring roads and area C between the fifth and sixth ring roads. Hazardous Air Pollutants on Site (HAPSITE), a portable Gas Chromatograph/Mass Spectrometer (GC/MS), was used to gather air samples inside and outside of the residences selected in the study area for a period of three months during 2008.FindingsMedian indoor air quality levels in the study area were similar to the outdoor levels. Indoor/outdoor ratios varied from 0.5 to 8 for most compounds, suggesting that the indoor air quality was less than the outdoor air quality. It was found that none of the indoor VOC concentrations measured exceeded the upper limits of the indoor air quality set by the Kuwait Environmental Protection Agency (KEPA), with the exception of only one residence where the benzene concentration was observed to be in excess of 17 per cent of the KEPA limit. Moreover, the indoor air quality for the study areas was found to be in accordance with level 1 set by KEPA, indicative of very good air quality.Originality/valueThis is the first study conducted in Kuwait to collect VOCs samples and to explore the air quality inside and outside of residential buildings.

Indoor surfaces are often coated with organic compounds yet a molecular understanding of what drives these interactions is poorly understood. Herein, the adsorption and desorption of limonene, an organic compound found in indoor environments, on hydroxylated silica (SiO2) surfaces, used to mimic indoor glass surfaces, is investigated by combining vibrational spectroscopy, atomistic computer simulations and kinetic modeling. Infrared spectroscopy shows the interaction involves hydrogen-bonding between limonene and surface O-H groups. Atomistic molecular dynamics (MD) simulations confirm the existence of π-hydrogen bonding interactions, with one or two hydrogen bonds between the silica O-H groups and the carbon-carbon double bonds, roughly one third of the time. The concentration and temperature dependent adsorption/desorption kinetics as measured by infrared spectroscopy were reproduced with a kinetic model, yielding the adsorption enthalpy of ∼55 kJ mol-1, which is consistent with the value derived from the MD simulations. Importantly, this integrated experimental, theoretical and kinetic modeling study constitutes a conceptual framework for understanding the interaction of organic compounds with indoor relevant surfaces and thus provides important insights into our understanding of indoor air chemistry and indoor air quality.