Thermodynamic analysis of nonelectrolyte sorption in plant cuticles: The effects of concentration and temperature on sorption of 4-nitrophenol

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Abstract
The sorption of 4-nitrophenol (4-NP) in enzymatically isolated cuticles ofLycopersicon esculentum fruits andFicus elastica leaves was studied as a function of temperature and solute concentration. Plots of the concentrations of 4-NP sorbed in the cuticle versus the equilibrium concentrations in the aqueous phase gave linear isotherms at low concentrations that tended to approach plateaus at higher sorbate concentrations (≥ 10 mmol·kg-1). At low concentrations of sorbed 4-NP, cuticles have sorptive properties similar to those of organic solvents which are able to form intermolecular hydrogen bonds, while at higher concentrations their solid nature becomes apparent. During sorption of 4-NP the cutin matrix swells and new sorption sites are successively formed. The partition coefficients of 4-NP in the system cuticle/buffer are functions of temperature and concentration. At high sorbate concentrations (approx. 1 mol·kg-1) they approach a value of 1. Different sorptive properties were observed for the cutin regions normally encrusted with soluble cuticular lipids (SCL) and those without SCL. Increasing temperature augmented the number of sorption sites in the cutin ofLycopersicon while no effect was observed withFicus. The changes of partial molar free energy (ΔG o tr), enthalpy (ΔH o tr), and entropy (ΔS o tr) for the phase transfer of 4-NP also depended on sorbate concentration: ΔH o tr and ΔS o tr were negative and steeply decreased at high sorbate concentrations. This is due to solute-solute interactions replacing solute-cutin interactions at high concentrations resulting in solid precipitates of solute within the cutin matrix. This formation of ordered solid domaines starting from a small number of nonelectrolyte molecules interacting with the cutin is proposed as a model for the intracuticular deposition of SCL.