An Examination of the Thermodynamics of Fusion, Vaporization, and Sublimation of Several Parabens by Correlation Gas Chromatography

Experimental and computational thermochemistry: how strong is the intramolecular hydrogen bond in alkyl 2-hydroxybenzoates (salicylates)

Hydrogen bonding (HB) is a fascinating phenomenon that exhibits unusual properties in organic and biomolecules. The qualitative manifestation of hydrogen bonds is known in numerous chemical processes. However, quantifying HB strength is a challenging task, especially in the case of intra-molecular hydrogen bonds. It is qualitatively well established that the alkyl 2-hydroxybenzoates have strong intra-HB. The thermochemical methods suitable for the determination of intra-HB strength were the focus of this study. The experimental gas phase formation enthalpies for alkyl 2-hydroxybenzoates (including methyl, ethyl, n-propyl and n-butyl) at 298.15 K were derived from a combination of vapour pressure measurements and high-precision combustion calorimetry and validated by the quantum chemical methods G3MP2 and G4. The intra-HB strength in methyl 2-hydroxybenzoate was determined from the evaluated gas-phase enthalpies of formation by comparing the energies of cis- and trans- conformers, by well-balanced reactions, the "para-ortho" method and the "HB and Out" method. All these methods give a common level of intra-molecular hydrogen bond strength of -43 kJ mol-1. The intra-HB strength was found to be independent of the chain length of the alkyl 2-hydroxybenzoates.

Predicting Transdermal Uptake of Phthalates and a Paraben from Cosmetic Cream Using the Measured Fugacity

Transdermal uptake models compliment in vitro and in vivo experiments in assessing risk of environmental exposures to semivolatile organic compounds (SVOCs). A key parameter for mechanistic models is the chemical driving force for mass transfer from environmental media to human skin. In this research, we measure this driving force in the form of fugacity for chemicals in cosmetic cream and use it to model uptake from cosmetics as a surrogate for condensed environmental media. A simple cosmetic cream, containing no target analytes, was mixed with diethyl phthalate (DEP), di-n-butyl phthalate (DnBP), and butyl paraben (BP) and diluted to make creams with concentrations ranging from 0.025% to 6%. The fugacity, relative to the pure compound, was measured using solid-phase micro extraction (SPME). We found that the relationship between the concentration and fugacity is highly nonlinear. The relative fugacity of the chemicals for a 2% w/w formulation was used in a diffusion-based model to predict transdermal uptake of each chemical and was compared with excretion data from a prior human subject study with the same formulation. Dynamic simulations of excretion are generally consistent with the results of the human subject experiment but sensitive to the input parameters, especially the time between cream application and showering.