A Small-Angle Neutron Scattering Environment for In-Situ Observation of Chemical Processes

Abstract

A new sample environment for the observation of ongoing chemical reactions is introduced for small-angle neutron scattering (SANS) experiments which enables structural changes to be followed continuously across a wide Q-range in response to changes in the chemical environment. The approach is demonstrated and validated by performing single and multiple potentiometric titrations on an aqueous anionic surfactant solution (oligo-oxyethylene alkylether carboxylic acid in D2O) with addition times varying from 1 s to 2 h. It is shown that the continuous flow set-up offers considerable advantages over classical ‘static’ measurements with regards to sample throughput, compositional precision and the ability to observe fast structural transitions. Finally, the capabilities and ongoing optimisation of the sample environment are discussed with reference to potential applications in the fields of biology, colloidal systems and complex soft matter.

Neutralisation rate controls the self-assembly of pH-sensitive surfactants

“…Curves from the same sample at different SD were then combined and rebinned (the protocol for the rebinning algorithm is given in the ESI †). Finally, the data were fitted, in absolute units, to a two-component model 18,21 consisting of a population of ellipsoidal core-shell micelles (interacting via a charged hard sphere structure factor) and a population of non-interacting, unilamellar, core-shell vesicles. A mass balance constraint ensured that all of the surfactant material known to be present in the sample is in either one or the other of these aforementioned aggregation states.…”

“…A mass balance constraint ensured that all of the surfactant material known to be present in the sample is in either one or the other of these aforementioned aggregation states. Details of the model, are given elsewhere 21 and the parameters used are given in the ESI. † At this point, it should be noted that, due to the existence of two polydisperse populations and the small weight fraction of the larger aggregates, the exact morphology of these aggregates could not be conclusively determined.…”

“…† At this point, it should be noted that, due to the existence of two polydisperse populations and the small weight fraction of the larger aggregates, the exact morphology of these aggregates could not be conclusively determined. However, as vesicles are known to exist at low pH 16,18 and the small-angle scattering has previously been observed to vary continuously with increasing pH, 21 consistent with a growing population of micelles and a shrinking population of vesicles, the description of the larger aggregates as vesicles has been adopted in this work.…”

“…As a result of this relatively well-known pH-responsive behaviour, this surfactant was previously used to test the concept of an in situ chemical reactor sample environment for small-angle neutron scattering (SANS) experiments. 21 Here, in order to maximise the signal-to-noise ratio, particularly for the low-Q measurements, the pH of the aqueous surfactant solution was adjusted very slowly compared to the previous experiments (10 mL of 0.1 M NaOH added over 120 minutes vs. 0.1 mL of 10 M NaOH added over 1 s). In this case, a small fraction (B1-2 wt%) of larger structures persisted in solution, even at high pH (B10).…”

“…The results observed in ref. 21 are notable for two reasons: firstly, it is rather unusual to have two distinct, coexisting populations with different morphologies arising from a single type of surfactant molecule in solution. Secondly, pathwaydependent self-assembly is rare in simple surfactant systems.…”