Hypothesis: The reactivity of small molecules in emulsions is believed to depend on their partitioning between phases, yet this is hard to verify experimentally in situ. In the present work, we use electron paramagnetic resonance (EPR) spectroscopy to simultaneously measure the distribution and reactivity of a homologous series of lipophilized spin probes in an emulsion. Experiments: 4-Hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) was derivatized with saturated fatty acids to create a series of spin probes with increasing lipophilicity (C4-, C8-, C12-, and C16-TEMPO). The probes were added to a 10 wt.% tetradecane-in water emulsions (d32 _ 190 nm) stabilized with sodium caseinate (1 wt.% in the aqueous phase, pH 7). The distribution of the probes between phases was measured by electron paramagnetic resonance (EPR) spectroscopy. Findings: TEMPOL partitioned into the aqueous phase, C4-TEMPO distributed between the lipid and aqueous phases (69% and 31% respectively) while the more lipophilic probes dissolved exclusively within the lipid droplets. Interestingly, the more lipophilic probes initially precipitated upon their addition to the emulsion, and only slowly redistributed to the droplets over hours or days, the rate of which was dependent on their carbon chain length. The reactivity of the probes with aqueous an aqueous phase reductant (ascorbate) generally depended on the proportion in the aqueous phase (i.e., TEMPOL > C4-TEMPO > C8-TEMPO _ C12-TEMPO _ C16-TEMPO). (Résumé d'auteur)

The reactivity of small molecules in emulsions is believed to depend on their partitioning between phases, yet this is hard to verify experimentally in situ. In the present work, we use electron paramagnetic resonance (EPR) spectroscopy to simultaneously measure the distribution and reactivity of a homologous series of lipophilized spin probes in an emulsion.4-Hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) was derivatized with saturated fatty acids to create a series of spin probes with increasing lipophilicity (C4-, C8-, C12-, and C16-TEMPO). The probes were added to a 10wt.% tetradecane-in water emulsions (d32∼190nm) stabilized with sodium caseinate (1wt.% in the aqueous phase, pH 7). The distribution of the probes between phases was measured by electron paramagnetic resonance (EPR) spectroscopy.TEMPOL partitioned into the aqueous phase, C4-TEMPO distributed between the lipid and aqueous phases (69% and 31% respectively) while the more lipophilic probes dissolved exclusively within the lipid droplets. Interestingly, the more lipophilic probes initially precipitated upon their addition to the emulsion, and only slowly redistributed to the droplets over hours or days, the rate of which was dependent on their carbon chain length. The reactivity of the probes with aqueous an aqueous phase reductant (ascorbate) generally depended on the proportion in the aqueous phase (i.e., TEMPOL>C4-TEMPO>C8-TEMPO∼C12-TEMPO∼C16-TEMPO).

The adsorption of hydrophobically modified polyacrylic acid (HM-PAAc) has been compared to the adsorption of unmodified polymers by means of reflectometry. The polymers were adsorbed onto a noncharged hydrophobic polystyrene surface. The adsorption kinetics of both types of polymer is the same until a certain surface coverage. Then the unmodified sample shows a saturation while the hydrophobically modified polyacrylic acid continues to adsorb. The adsorption behavior of the polyelectrolyte can be controlled by the pH and the ionic strength of the solution. For ionic strengths of 0.001 M NaCl the hydrophobically modified polymer shows a larger adsorbed amount at pH 3 to 4 compared to the unmodified polymer. At pH higher than 4 the differences are less significant. At higher ionic strength the amount of adsorbed material increases for both polymers. While doing adsorption-desorption cycles a hysteresis-effect was detected. At the same pH the hydrophobically modified polymer sticks to the surface while the unmodified polymer is already desorbing completely. The hysteresis vanishes when the ionic strength of the solution is increased.

The adsorption of hydrophobically modified polyacrylic acid (HM-PAAc) has been compared to the adsorption of unmodified polymers by means of reflectometry. The polymers were adsorbed onto a noncharged hydrophobic polystyrene surface. The adsorption kinetics of both types of polymer is the same until a certain surface coverage. Then the unmodified sample shows a saturation while the hydrophobically modified polyacrylic acid continues to adsorb. The adsorption behavior of the polyelectrolyte can be controlled by the pH and the ionic strength of the solution. For ionic strengths of 0.001 M NaCl the hydrophobically modified polymer shows a larger adsorbed amount at pH 3 to 4 compared to the unmodified polymer. At pH higher than 4 the differences are less significant. At higher ionic strength the amount of adsorbed material increases for both polymers. While doing adsorption-desorption cycles a hysteresis-effect was detected. At the same pH the hydrophobically modified polymer sticks to the surface while the unmodified polymer is already desorbing completely. The hysteresis vanishes when the ionic strength of the solution is increased.

International audience | Adsorption of purified apo-ovotransferrin at the air-water interface was studied by ellipsometry, surface tension, polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and shear elastic constant measurements. No significant difference was observed between pH 6.5 and 8.0 as regards the final value of surface concentration and surface pressure. However at low concentration, a weak barrier to adsorption is evidenced at pH 6.5 and confirmed by PM-IRRAS measurements. At a pH where the protein net charge is negative (pH 8.0), the behavior of ovotransferrin at the air-water interface is more influenced by charge effects rather than bulk concentration effects. At this pH, the interface exhibits a low shear elastic constant and a spectral signature not usual for globular proteins.

Adsorption of purified apo-ovotransferrin at the air–water interface was studied by ellipsometry, surface tension, polarization–modulation infrared reflection–absorption spectroscopy (PM-IRRAS), and shear elastic constant measurements. No significant difference was observed between pH 6.5 and 8.0 as regards the final value of surface concentration and surface pressure. However at low concentration, a weak barrier to adsorption is evidenced at pH 6.5 and confirmed by PM-IRRAS measurements. At a pH where the protein net charge is negative (pH 8.0), the behavior of ovotransferrin at the air–water interface is more influenced by charge effects rather than bulk concentration effects. At this pH, the interface exhibits a low shear elastic constant and a spectral signature not usual for globular proteins.