EPR spin probe study of molecular ordering and dynamics in monolayers at oil/water interfaces

The spin-labeled surfactants: n(4,4-dimethyloxazolidine-N-oxyl)stearic acids where n = 5 (I) and n = 16 (II), and N,N-dimethyl, N′-octyl, N′-(4-Tempoyl) ammonium bromide (III) were used to investigate molecular dynamics in monolayers of sodium dodecylsulfate (SDS) in oil-in-water macroemulsions. The order parameters (S0) and rotational diffusion coefficients (R∥, R⊥) for rotation around the long and short molecular axes, respectively, were deduced from the EPR spectral simulations. Two main issues were addressed: (i) the influence of the oil phase nature and (ii) SDS concentration in the monolayer on the molecular dynamics and conformation at the interface. The organic liquids of different polarity, viscosity, and hydrocarbons of different chain length were used as oil phases. The motion in the polar part of the SDS monolayer, as probed by I, is rapid and anisotropic. The S0 and R⊥ values for different oil phases are in narrow ranges (S0 = 0.41 − 0.465, R⊥ = 1.25 − 4 × 108s−1 at 20 °C). Both parameters do not correlate with the oil-phase polarity and viscosity, and change monotonically with temperature upon the crystallization of the oil phase. Contrary to I, the conformation of the hydrophobic part of the molecule, as probed by II, depends significantly on the polarity of the oil-phase. For ester/water interfaces, II is mainly in an extended (trans) conformation whereas for the alkane/water interfaces it is in a bent conformation. Increasing SDS concentration in monolayer results in an increase in S0 and a decrease in R⊥ for the negatively charged I. The former changes correlate well with an increase in surface pressure in flat monolayers of the same composition. For the positively charged IIIS0 decreases with increasing SDS concentration, apparently because of a gradual change in orientation of the NO group of the probe on the SDS/water interface.