Measurement of the rate of transfer of a probe molecule through a surfactant layer at the water:oil interface using TF-MuSR

Lead Research Organisation: University of East Anglia
Department Name: Chemistry


Molecular transfer across interfacial layers is a rapid process that cannot generally be measured by laboratory-based fast reaction techniques (stopped flow, temperature-jump). To date experimental techniques have largely been based around slow mass transfer measurements using diffusion cells. The approach here does not exclusively quantify the interfacial transfer but measures rates that include transfer through diffusion layers close to the interface [Steytler, D. C.; Towey, T. F.; Robinson, B. H.; Atay, N. Z., Mechanisms of solute interfacial transfer in Winsor-II systems. Langmuir 2001, 17, (2), 417-426.]. With judicial choice of system and probe molecules , TF-MuSR offers a unique opportunity to directly probe interfacial transfer dynamics and energetics. By choosing a suitable probe (muon capture) molecule that exhibits balanced partitioning between oil and water, a dispersed oil:water system can be used with a spin-quencher in one of the liquid phases. The muon-electron hyperfine interaction can then be monitored for which the width of the signal gives the probe lifetime for crossing the interface. Combined with temperature variation, energetic parameters can also be extracted using this approach. To provide the large interfacial area, we propose to employ a well characterised water-in-oil (w/o) micro-emulsion stabilised by bis(2-ethyl hexyl) sodium sulphosuccinate (Aerosol OT) [2]. The system comprises thermodynamically stable nanodroplets of water-in-oil for which the droplet size can be controlled through the water-to-surfactant molar ratio (w)


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Description On our request, beamtime at PSI was divided into two separate sessions with enough time in between for the analysis of the results from the first part of the experiment and the planning of the second part.

Microemulsions of water in heptane were made with allyl alcohol (AA) as the probe molecule, as planned. Both TF-MuSR and ALC-MuSR techniques were used. These two techniques are complementary in the case of studies in liquid state because moderately high field TF-MuSR measures the muon-electron hyperfine interaction, AMu , while the ALC-MuSR measures the coupling of the unpaired electron to the neighbouring protons, Ap's. Both muoniated radicals of AA were observed. All observed resonances showed significant and different solvent dependancies, with the resonances from the microemulsion appearing at the weighted average positions of those in water and heptane. This shows the fast exchange limit with respect to the frequency windows provided by the differences in the resonance frequencies between water and heptane. Temperature dependence measurements showed significant band broadening of ALC resonances of the microemulsion with the largest broadening observed for the biggest frequency window. Simulation of the observed band broadening showed the rate of transfer of the AA between water and heptane separated by a surfactant (AOT) barrier to be in the 10^8 s time scale. Therefore we have shown that our initial premise that MuSR is an appropriate technique to estimate the rates of transfer of small molecules across interfacial molecular barriers to be correct by making the first ever direct measurement of such a property. A report has been drafted for publication, and its title and the authors i are given below.
"Rates of Molecular Transfer Across Interfacial Layers Using ?SR
U.A. Jayasooriya1, D.C. Steytler1, N.J. Clayden1, J. Peck1, V.S. Oganesyan1, Robert Scheuermann2, Alexey Stoykov2 and Rustem Khasanov2
1School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, UK
2Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institut, WBBA/120, CH-5232 Villigen PSI, Switzerland."
Exploitation Route This is the first ever successful measurement of the rate of transfer of a small molecule across a liquid/liquid interface separated by a surfactant layer. This finding is of importance to fields of endeavour from drug delivery to industrial use of emulsions. This measurement has opened the way for detailed investigation of these rates and their dependence on a variety of properties of the system, such as the size and character of the probe molecule and the character and thickness of the interfacial barrier.
Sectors Agriculture, Food and Drink,Chemicals,Construction,Education,Environment,Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology,Other

Description Too early for any impact to be observed.
Description ISIS 
Organisation Science and Technologies Facilities Council (STFC)
Department ISIS Neutron and Muon Source
Country United Kingdom 
Sector Academic/University 
PI Contribution Original idea, sample and experimental preparation and completion.
Collaborator Contribution Assist in the experiment with the muon spectroscopic work.
Impact Over the years, this collaboration has resulted in more than fifty publications in high impact journals.
Description PSI 
Organisation Paul Scherrer Institute
Department Laboratory for Muon Spin Spectroscopy
Country Switzerland 
Sector Charity/Non Profit 
PI Contribution The initial idea for the experiment, design of the experiment and its completion.
Collaborator Contribution PSI scientists provide the Muon Spectroscopic facilities needed to carry out the experimental measurements.
Impact An article has been drafted for publication of the experimental outcomes. This work involves the disciplines of nanoscience and muonspectroscopy.
Start Year 2009