Spectral Reflectance Measurement of Evaporating Chemical Films: Initial Results and Application to Skin Permeation

The present study has 2 aims. First, the method of spectral reflectance was used to measure evaporation rates of thin (∼25-300 μm) films of neat liquid volatile organic chemicals exposed to a well-regulated wind speed u. Gas-phase evaporation mass transfer coefficient (k_evap) measurements of 10 chemicals, 9 of which were measured at similar u, are predicted (slope of log-log data = 1.01; intercept = 0.08; R² = 0.996) by a previously proposed mass transfer correlation. For one chemical, isoamyl alcohol, the dependence of k_evap on u^0.52 was measured, in support of the predicted exponent value of ½. Second, measured k_evap of nicotine was used as an input in analytical models based on diffusion theory to estimate the absorbed fraction (F_abs) of a small dose (5 μL/cm²) applied to human epidermis in vitro. The measured F_abs was 0.062 ± 0.023. Model-estimated values are 0.066 and 0.115. Spectral reflectance is a precise method of measuring k_evap of liquid chemicals, and the data are well described by a simple gas-phase mass transfer coefficient. For nicotine under the single exposure condition measured herein, F_abs is well-predicted from a theoretical model that requires knowledge of k_evap, maximal dermal flux, and membrane lag time.