Francesca Caboi

Addition of hydrophilic and lipophilic compounds of biological relevance to the monoolein/water system. I. Phase behavior.

The solubilization of hydrophilic and lipophilic molecules, with biological relevance, in the monoolein/water (MO/W) system has been investigated for phase behavior. Small angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR) and optical microscopy (OM) have been used to characterize the microstructure of the liquid crystalline phases. Partial phase diagrams of the MO/W system in the presence of sodium decanoate, 1-adamantanamine hydrochloride, decanoic and dodecanoic acids, acetyl salicilic acid and retinol have been determined. The stability of the various phases has been followed for at least eight months. The polarity and the molecular structure of the additive determine whether it is located at the polar interface or in the apolar region of the lipid layer. Therefore, the additive affects the interfacial curvature of the lipid layer differently, which in turn will trigger transition to disparate phases. A cubic-to-reverse hexagonal phase transition has been observed with time for most of the ternary systems, with the exception of 1-adamantanamine hydrochloride and retinol. The release of free glycerol and oleic acid due to MO hydrolysis has been clearly demonstrated by 13C NMR. This would account for the changes in phase behavior observed with time. The released oleic acid, located in the MO acyl chain region, favors the inverse interfacial curvature. The average lipid dimensions in the cubic and in the reverse hexagonal phases have been calculated from SAXS data.

Acyl migration and hydrolysis in monoolein-based systems

The extent of acyl migration and hydrolysis phenomena in glycerol monooleate/water based systems and how they are affected by acetic acid and oleic acid has been examined. To facilitate analysis and interpretation of the data, samples with a low water content, 2–5 wt%, which form a liquid reverse micellar, L2, phase, were chosen. Acyl migration and hydrolysis were monitored versus time by 13C NMR analysis. In addition, the acid value to monitor hydrolysis has been also determined. The formation of small amounts of glycerol-2-monooleate as well as hydrolysis products was observed. These phenomena depend on the water content and storage conditions. The implications of these findings for the observed phase behaviour of the glycerol monooleate/water system are discussed.

Counterion effect on the phase behavior of perfluoropolyether carboxylates: micelles and liquid crystals in water

Perfluoropolyether carboxylates having sodium, potassium or ammonium counterions, and hydrophobic chains, mostly ClC3F6O terminated, have been investigated in aqueous binary systems, with the focus laid upon the counterion effect on the major self-assembly features. Micellization and liquid crystal formation have been detected via surface tension, conductivity, optical microscopy and NMR.

Addition of hydrophilic and lipophilic compounds of biological relevance to the monoolein/water system II - 13C NMR relaxation study.

The addition of hydrophilic and hydrophobic molecules to the 1-monooleoyl glycerol (MO)/water (W) system has been investigated at a molecular level by 13C nuclear magnetic resonance (NMR) relaxation. Depending on the nature of the additive, the liquid crystalline phases of the MO/W binary system are modified. The 13C NMR spin lattice relaxation rates of the various MO carbons were determined in the presence of the additives for different types of L(2) and liquid crystalline phases. Data revealed that local dynamics are independent of type and amount of additive (within 5 wt.%), and also of the type of the structural arrangement. The curvature of the interface does not affect the local mobility of MO carbons, with the exception of the glycerol G3 and the carboxylic C1 carbons. Moreover, the presence of the double bond in the mid part of the hydrocarbon chain induces a levelling in the relaxation rates on the neighboring carbons. The 13C NMR spin lattice relaxation rates at two magnetic field strengths and the Overhauser enhancement were measured in the L(2) phase of the MO/W/sodium decanoate system. The use of a two-step model of relaxation allowed to estimate order parameters, and slow and fast motions of MO in the structured aggregate.

Solid lipid nanoparticle preparation by a warm microemulsion based process: influence of microemulsion microstructure.

Warm microemulsions (WME) containing lipids are used as starting systems to obtain solid lipid nanoparticles (SLN) in alternative processes to those based on high pressure homogenization technique. SLN characteristics can be influenced by the microemulsion composition and the specific conditions adopted in the quenching process related to the transformation of WME into nanoparticles. To establish optimized conditions for the production of SLN starting from WME, in a first step of this work we have defined the microstructure of warm microemulsions highlighted in the lecithin (LCT)/water (W)/tripalmitin (TP)/1-butanol (B)/taurocholate sodium salt (ST) phase behavior at 70°C. Moreover, we have further studied the LCT/W/TP/B system by evaluating the effect on the microemulsion area due to the LCT/B weight ratio, the replacement of 1-butanol with different alcohols (ROH), and the addition of taurocholate sodium salt (ST) at different LCT/ST weight ratios. The microstructure of the isotropic phase region obtained in the presence of ST has been characterized by both (1)H NMR PGSE measurements and electrical conductivity. The characteristics of final nanoparticles are discussed taking into account both the microstructure of the parent WME and the conditions of the quenching process leading to SLN. The present results highlight the relevance of the microstructural characteristic of WME to assure the obtainment of SLN with average diameter in the order of 100-2000 nm and narrow size distribution.

On the bicontinuous microstructure induced by a guest protein in a typical AOT microemulsion

Abstract The ternary microemulsion AOT/water/isoctane was investigated in the presence of the human serum albumin (HSA) protein as solubilized guest molecule by NMR relaxation and self-diffusion measurements. The analysis of the 23 Na NMR relaxation rates of the counterions and the interpretation of the 1 H relaxation and self-diffusion data of water in terms of the usual spherical water droplet model gave a strong and direct evidence for important modifications of the water-in-oil droplet microstructure in the presence of HSA. With decreasing the water/surfactant ( W S ) molar ratio, which implies a linear decrease of the water core, the system containing the protein shows a transition to a bicontinuous microstructure. Some protein residues are likely to contribute to the total interfacial area to retain the favored curvature of the AOT molecules located at the polar-apolar interface. At W S larger than 25, which corresponds to an average radius of the water core larger than 2.8 nm, the relaxation parameters and the self-diffusion coefficients are almost unaffected by the presence of HSA. Consequently, it can be suggested that HSA molecules are hosted in closed water domains, which are likely to assume an average spherical shape as well as the unfilled, AOT stabilised, water droplets.