Partitioning, solubility and solubilization of limonene into water or short‐chain phosphatidylcholine solutions
2021
Karman, Andrew P. | Ebeler, Susan E. | Nitin, Nitin | Dungan, Stephanie R.
Using headspace solid‐phase microextraction (HS–SPME), equilibrium distributions could be determined for hydrophobic solutes in closed systems containing vapor and aqueous solution, either in the absence or presence of two short‐chain phospholipids, dihexanoyl‐phosphatidylcholine (diC₆PC) and diheptanoyl‐phosphatidylcholine (diC₇PC). Without phospholipid, HS–SPME with short extraction times was used to measure water–vapor partition coefficients Kvw for d‐limonene at five temperatures within 15–40°C, with results in good agreement with existing literature. The temperature dependence of Kvw yielded the enthalpy of volatilization ΔvolH¯= 34.5 kJ/mol for limonene. At 25°C, solubility values for d‐limonene, 1‐octanol, and n‐decane were obtained using similar measurements above aqueous solutions of various solute concentrations. Short‐time HS–SPME extraction of limonene in closed vials containing diC₆PC or diC₇PC micelles was also used to evaluate distributions of solute between vapor, aqueous dissolution, and micelles, for various surfactant concentrations at 15–40°C. Resulting vapor phase concentrations were analyzed using a mass balance and measured Kvw values, to obtain micelle–water partition coefficients Kmw and critical micelle concentrations. Kmw in diC₆PC solutions (1–2 mM⁻¹) weakly increased with temperature, but decreased significantly with increased temperature for diC₇PC micelles (2–4 mM⁻¹). Solubilization in short‐chain PC micelles has previously received little attention, and our results show that the extent of partitioning into these diacyl (i.e., two‐tailed) lipids is comparable to that for single‐tailed anionic or nonionic surfactants.
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