## 3 Sample Environments
### Stop Flow/Chemistry On-Line
Pressurized CO\({}_{2}\) gas can be used as a neutron scattering contrast agent in porous materials like lignite and shale [79], and SANS/USANS experiments are part of the toolset  
Figure 9: Schematic of an experimental set-up that allows structure formation due to chemical reactions to be followed in real time by SANS. The system consists of a reaction vessel where the reactions can be initiated using remotely controlled syringe pumps. A peristaltic pump is used to bring the mixture to the measuring cell. The system has the option to be used in open or closed loop configuration. In a closed loop system, one can, for instance, observe the effects of a gradual change in pH. Here, ‘Nomad’ is the name of the central control computer (reproduced with permission from [78], Springer Nature, 2018). that can be used to determine pore distribution size and morphology for pores that are accessible to the CO\({}_{2}\). However, when introduced in its supercritical state (S\(\mathrm{cCO_{2}}\)), it is a good solvent, primarily for nonpolar low-molecular-weight compounds. For polar higher molecular weight compounds, it is a rather poor solvent. Thanks to the fairly benign conditions at which criticality occurs (7.39 MPa, 31.1 \({}^{\circ}\)C), \(\mathrm{SCO_{2}}\) is a fairly attractive option for performing chemical synthesis of surfactants, polymers, and biomaterials [80]. On-line neutron scattering investigations of poly(dimethylsiloxane) polymer-S\(\mathrm{cCO_{2}}\) solvent phase diagrams were performed and fundamental applications demonstrated [81]. Time-resolved scattering experiments where the \(\mathrm{S\mathrm{cCO_{2}}}\) to \(\mathrm{CO_{2}}\) transition was used as a foaming agent to create micro and nano foams allowed the kinetics of foam formation to be studied as a function of pressure and pressure modulation [82]. The use of fluorinated surfactants in \(\mathrm{CO_{2}}\) microemulsions is widespread but not desired for environmental reasons. In a systematic pressure and contrast variation study, it was shown that partial substitution of \(\mathrm{S\mathrm{cCO_{2}}}\) by cyclohexane reduced the required amount of fluorinated surfactants considerably [83]. Contrast variation through the use of \(\mathrm{D_{2}O}\) exchange was crucial in this case and the only way this kind of structural information could be obtained. Performance of on-line chemical synthesis experiments has so far not been widely explored, although it appears that neutron scattering could play an important role, as the above example has shown. When used for structure forming on-line chemical processing, temperature control and homogenization of the reaction mixture is of the utmost importance. The designs for such cells used in on-line experiments should take this into consideration, as well in allowing for the possibility of siphoning off small amounts of the reaction mixture (for off-line chemical analysis) during the course of the experiments without perturbing the pressure/temperature conditions [84]. The use of on-line size-exclusion chromatography to deliver well-defined and non-aged samples for scattering experiments has become feasible, although here one has to keep in mind the restrictions on the amount of available material. This method requires beamlines that can deliver high intensity and small beam sizes. However, when these conditions are met, the experimental accuracy can be considerably improved. For example, proteins that have a tendency to aggregate in solution can still be studied in their non-aggregated state [85].