Nonionic Surfactant Brij 35 in Water and in Various Simple Alcohols: Structural Investigations by Small-Angle X-ray Scattering and Dynamic Light Scattering

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Abstract

Small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) techniques have been used to study the structural properties of nonionic surfactant dodecyl-poly(ethylene oxide-23) ether (C12E23, commercial name Brij 35) in water and in various alcohols from ethanol to 1-decanol. All measurements were performed at 25 °C. SAXS experimental data were put on absolute scale using water as a secondary standard and evaluated using the generalized indirect Fourier transformation method (GIFT), which is based on the simultaneous determination of the intra- and interparticle scattering contributions. In this way, the size and shape of interacting scattering particles in real space could be deduced. In the Brij 35/water binary system (surfactant concentrations from 0.5 to 25 mass %) globular micelles with maximum dimensions ranging from 9 to 10.5 nm and radii of the hydrophobic core around 1.7 nm were found. At low surfactant concentrations the micelles were more or less spherical, whereas slight elongation of the micelles was observed at concentrations above 10 mass %. Dynamic light scattering (DLS) measurements confirmed SAXS results. SAXS spectra of Brij 35/alcohol binary systems showed strong alcohol interaction peaks at higher values of the scattering vector q that represented the structuration of the solvent itself and were therefore eliminated during the evaluation procedure. Two populations of scattering particles were found in these systems: globular particles with maximum dimension around 2.5 nm (solvated Brij 35 monomers) and large quite polydisperse Brij 35 objects with dimensions well above the resolution of SAXS measurements, thus determined by the DLS method. Accordingly, these samples were found to be very close to the limit of phase separation, which could be actually observed with the naked eye, if the samples were left resting for longer period of time. With substitution of only 2 % of the alcohol by water in the Brij 35/alcohol binary systems the samples became stable. The SAXS data of these ternary systems indicated the transformation of the large objects into smaller structures that resembled the smaller population found in binary systems. However, the structures in the ternary systems were slightly larger and swollen into a more spherical shape. The observed structuration of pure alcohols, which were used as solvents in the Brij 35/alcohol binary system samples, was also explored.

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