Prediction of partition coefficients of organic compounds between SPME/PDMS and aqueous solution

Electromagnetically-vibrated solid-phase microextraction for analysis of aqueous-miscible organic compound transport in soil columns

Current methods of sampling pore water from soil columns to determine solute concentrations are slow and require relatively large volumes. Accordingly, an electromagnetically-vibrated (EMV) solid-phase microextraction (SPME) device was evaluated for determining temporal and spatial distributions of solute pore-water concentrations (solute concentration profiles) for four organic compounds, two polar (2-hexanone, 2,4-dimethyl phenol) and two nonpolar (toluene, 1,4-dichlorobenzene), in columns packed with simulated aquifer sands with different fractions of organic carbon. In batch equilibrium extraction tests, the equilibrium extraction time of the organic compounds in aqueous mixtures decreased from 30 to less than 10 min as the frequency of electromagnetic vibration increased from zero to 250 Hz. Mixture effects were not statistically significant (p > 0.05) in the extraction process using EMV SPME. Comparisons of the solute concentration profiles within the soil columns at different elapsed times measured by pore-water samples and in situ EMV SPME extractions revealed both methods were equally effective. However, EMV SPME extraction removed no solution volume and only 0.6-14% of the solute mass removed by the pore-water sample collections, substantially minimizing disturbances to solute transport and fate. Thus, the equilibrium extraction-based calibration method using EMV SPME offers an effective approach for rapidly and accurately determining solute concentration profiles in column tests with negligible solute mass loss and minimal solution flow disturbance.

Oligomeric models for estimation of polydimethylsiloxane-water partition ratios with COSMO-RS theory: impact of the combinatorial term on absolute error

A series of 12 oligomeric models for polydimethylsiloxane (PDMS) were evaluated for their effectiveness in estimating the PDMS-water partition ratio, K_PDMS-w. Models ranging in size and complexity from the -Si(CH₃)₂-O- model previously published by Goss in 2011 to octadeca-methyloctasiloxane (CH₃-(Si(CH₃)₂-O-)₈CH₃) were assessed based on their RMS error with 253 experimental measurements of log K_PDMS-w from six published works. The lowest RMS error for log K_PDMS-w (0.40 in log K) was obtained with the cyclic oligomer, decamethyl-cyclo-penta-siloxane (D5), (-Si(CH₃)₂-O-)₅, with the mixing-entropy associated combinatorial term included in the chemical potential calculation. The presence or absence of terminal methyl groups on linear oligomer models is shown to have significant impact only for oligomers containing 1 or 2 -Si(CH₃)₂-O- units. Removal of the combinatorial term resulted in a significant increase in the RMS error for most models, with the smallest increase associated with the largest oligomer studied. The importance of inclusion of the combinatorial term in the chemical potential for liquid oligomer models is discussed.