Structure and composition of highly viscoelastic (α-CD)/anionic surfactant films investigated by neutron reflectivity

Why?

Understanding the surface rheological behavior of adsorbed films at the aqueous/air interface composed of sugars, surfactants, or complexes between them, is of great interest due to their relevance in different application areas ranging from food, medical diagnostics and oil recovery. Recently, we have found that α-cyclodextrin (α-CD)/anionic surfactant mixtures form films at the solution/air interface that show a remarkable viscoelasticity, the magnitude of which has not been observed in similar systems. This viscoelasticity can be tuned through changes in bulk composition, temperature and surfactant alkyl chain length and head group type (see Figure 1). In addition, knowledge of the structure and composition of the films is of great importance for a bottom-up strategy, i.e, design of devices with specific macroscopic properties based on the control of molecular level structures. The use of neutron reflectivity has given us insight into these properties for the most viscoelastic films as a monolayer of (α-CD)2:surfactant inclusion complexes with a minimal amount of any other free component forms. There are also indications into the origin of the viscoelasticity from suggestions that the complexes align with the surfactant chains parallel to the interface.

Figure 1: a) Remarkable viscoelastic behavior of a α-CD- anionic surfactant solution as a function of increasing the injection volume. b) Sketch of the α-CD- anionic surfactant complex. c) Viscoelastic modulus as a function of the surfactant alkyl length chain.

How?

An international research group has teamed up to perform experiments on the time-of-flight reflectometer FIGARO at the Institut Laue-Langevin (ILL). Profiles of the neutron reflectivity, R, at the aqueous/air interface as a function of the momentum transfer, q, were recorded at 283.15 K using neutron pulses in the wavelength range λ = 2−30 Å and at fixed incident angles of θ = 0.62° and 3.8°. Special troughs to work at the aqueous/air interface at this low temperature were designed to avoid condensation on the windows. The low-q interfacial composition analysis method was used to resolve the surface excesses of α-CD and C12SO4 for equilibrated samples for a broad range of studied bulk compositions. Also, for structural analysis measurements, solutions of {h-C12SO4 + α-CD} and {d-C12SO4 + α-CD} were prepared in two different isotopic contrasts of the solvent: ACMW and D2O. The data were analyzed using SANGRA (Script Automated Neutron Grazing Reflectivity Analysis), a home-made program based on the implementation of the Parratt method for the modeling of homogeneous stratified layers by using the Fresnel equations.

Who?

This work resulted from an international collaboration between a fellow of the Lund Institute of Advanced Neutron and X Ray Science (LINXS), two Mexican research groups (Universidad Autónoma Metropolitana-Cuajimalpa and Universidad Nacional Autónoma de México), the University of Santiago de Compostela (Spain), the University of Manchester (United Kingdom) and the University of Chemistry and Technology (Czech Republic). Funding was obtained by these universities, CONACyT (MEX), MINECO (ES), ILL and LINXS.

This project is funded by the Swedish Research Council, the Craaford foundation and the Interreg funded ESS & MaxIV: Cross border science and society project. Experiments are performed at Institute Laue Langevin, the ISIS neutron and muon source and Research Neutron Source Heinz Maier-Leibnitz (FRM II).

What´s next?

Now we aim to understand better the origin of the viscoelasticity, so we have recorded various lab data on systems with 3 different surfactant head groups. Also, we aim to control better the magnitude of the viscoelasticity for which more neutron beam time is planned on FIGARO as only NR can resolve the interfacial composition. Indeed for the mixtures we will use selective deuteration to resolve the interfacial stoichiometry of inclusion complexes resulting from different surfactant mixtures for the first time.

Contact:

Prof. José Campos-Terán, Fellow-Guest Researcher
Lund Institute of Advanced Neutron and X Ray Science (LINXS)
jose.campos_teran@linxs.lu.se