Effects of Antioxidant Hydrophobicity, Surfactant Concentration, Edible Oil and Oil to Water Ratio on the Antioxidant Efficiency of Phenolipids in O/W Emulsions 
F. Paiva-Martins1, M. Costa1, M. Meireles1, I. Ferreira1, R. Silva1, S. Losada-Barreiro1, C. Bravo-Díaz2 , L. S. Romsted3 
1 REQUIMTE-LAQV, Departamento de Química e Bioquímica. Faculdade de Ciências. Universidade do Porto, Porto, Portugal.
2Universidad de Vigo, Fac. Química, Dpto Química Física, 36200, Vigo, Spain.
 3Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854

Antioxidant (AO) efficiency in emulsions depends on their concentration in the interfacial region, where oxidation is believed to be initiated. Increasing the number of C atoms in the alkyl chain of antioxidants not only change their hydrophilic-lipophilic balance but also affect their relative solubility in the oil, water, and interfacial regions of the emulsion or AO oxidation/reduction potentials. Recent investigations on alkyl chain length effects for several series of homologous AOS shown that their efficiency does not increase linearly with increasing the number of C atoms in their alkyl chain but is parabolic-like, with a maxima at an intermediate (C4 – C12) chain length. This parabolic dependence of AO efficiency upon chain length for both chemical and biological activities of series of homologous AOs up to a critical point after which their activity decreases is known as the “cut-off” effect. Reported for the first time more than a century ago, it is now commonly observed for various antioxidant, biological, and cytotoxic activities of almost every amphiphilic homologous series tested.
We employed the pseudophase kinetic model [1] to determine distributions and interfacial molarities of series of antioxidants, AOs, with the same reactive group but of different hydrophobicity in intact oil/water/tween 20 emulsions with different surfactant concentration, edible oil or oil to water ratios. We also determined the effects of the AOs on the oxidative stability of the emulsions by the Schall Oven test.
Results provide physical evidence that the variations in the efficiency of homologous series of antioxidants in emulsions are due to differences in their interfacial concentrations, confirming that, other things being equal, there is a direct relationship between the percentage of AO in the interfacial region of the emulsions and their efficiency, providing a natural explanation, based on molecular properties, to the cut-off effect. Finally, we have demonstrated that the distribution and interfacial molarities obtained in these experiments show that the assumptions of the pseudophase model are fully consistent with the “cut-off” effect and that the “cut-off” effect is simply a natural consequence of the differential solubility of the AOs in the aqueous, interfacial and oil regions of the emulsion or rate determining transport of reactants between regions in the emulsion. The results demonstrate that the pseudophase kinetic model provides sensible interpretation of the distributions of AOs in emulsions and their reactivity with arenediazonium ions in terms of the properties of stirred, fluid emulsions.
[1] Losada-Barreiro, S., Sánchez Paz, V., Bravo Díaz, C., Paiva Martins, F., Romsted, L. S. (2012). J. Colloid. Interface Sci., 370, 73-79.