UC Davis

In order to advance overall understanding of solubilization phenomena within micelles, this work examines the equilibrium partitioning behavior of hydrophobic aroma compounds using headspace solid-phase microextraction (HS-SPME) techniques. The first focus of this research was to develop a robust analytical method to characterize d-limonene’s distribution between water and air over a 15–40˚C temperature interval. Vapor-water partition coefficients (Henry’s coefficients) were evaluated for limonene in well-sealed vials representing a closed system. Through these experimental measurements, we were also able to quantify the enthalpy of volatilization of limonene. In addition, we determined the solubility concentrations of decane, limonene, and 1-octanol, which collectively spanned four orders in magnitude. Second, HS-SPME was used to quantify micellar partitioning of limonene within aqueous mixtures containing diacyl, short-chain phosphatidylcholine with acyl chains of 6–8 carbons, as a function of temperature, solute concentration, and tail length. The three phospholipids studied were 1,2 dihexanoyl-sn-glycero-3-phosphocholine (diC6PC), 1,2 diheptanoyl-sn-glycero-3-phosphocholine (diC7PC), and 1,2 dioctanoyl-sn-glycero-3-phosphocholine (diC8PC). By varying phospholipid concentration above the appropriate critical micelle concentration, at fixed dilute limonene concentration, the micelle partition coefficient K_mw=x_(i,m)/c_(i,w) was evaluated using the pseudo-phase model, with x_(i,m) the mole fraction of solute i within micelles and c_(i,w) the solute concentration within the water continuum. Additional studies were conducted at fixed phospholipid concentration but varying limonene concentrations. For all lecithins studied, our findings show enhanced micellar partitioning occurred with increases in tail length, but reduced with an increase in temperature. In diC6PC or diC7PC micellar solutions, the partition coefficient was constant at lower limonene concentrations, indicating negligible effects of non-ideal solute interactions or solute induced micellar rearrangements. However, at higher limonene concentrations, K_mw increased with increasing solute. The effect of limonene on the phase behavior of the longest studied short-chain lecithin, diC8PC, was studied and it was observed that the addition of limonene resulted in the depression of diC8PC’s upper consulate temperature. In the last focus of this dissertation, the partitioning behavior of several aroma compounds within aqueous nonionic surfactant solutions was examined. These measurements were related to a solubilization framework grounded in fundamental thermodynamic principles. It was determined that solute molecular volume alone was a strong predictor of the maximum amount of solute that could be dissolved for a given concentration of surfactant. Comparing limonene partition coefficients for various types of micelles as measured by us or reported in literature, we find that K_mw falls in the range of 1–6 M–1, regardless of the surfactant structure. Measurements of solute vapor concentration were also used to determine the chemical potential of solute i within the micelle solution. Based on the thermodynamics of mixing, the chemical potential of studied mixtures could be related to the mole ratio of solute-to-surfactant and partition coefficient K_mw. For solutes limonene and octanol, the data indicated that K_mw was nearly constant at all molar ratios, consistent with a dilute pseudo-phase model. However, at higher mole ratios in which decane was the solute, we observed micelles lowered the chemical potential of solute below predictions of dilute theory. Thus, enhanced micellar partitioning occurred as the decane concentration was varied up to its solubility limit. Finally, through these measurements, the molar Gibbs energy of dissolution for 1-octanol, decane, and limonene was determined.