Spatial and temporal small angle x-ray scattering (SAXS) scans show that 8.6 and 11.8 nm iron oxide nanoparticles (NPs) in heptane drop-cast on top of a heptane layer atop a diethylene glycol (DEG) layer are trapped at the DEG/heptane interface to generally form a single ordered, hexagonal close packed monolayer (ML), and this occurs long before the heptane evaporates. Many NPs remain dispersed in the heptane after this NP assembly. Assembly occurs faster than expected from considering only the diffusion of NPs from the drop-cast site to this liquid/liquid interface. The formation of the ordered NP ML occurs within 100 s of drop-casting, as followed by using the (10) ordered NP SAXS peak, and on the same time scale there is a concomitant decrease in the SAXS form factor from disordered NPs that is apparently from disordered NPs at the meniscus. Usually, most of the ordered NPs are close packed, but there is evidence that some are ordered though not close packed. After the heptane evaporates, a close-packed ordered NP ML remains at the DEG/vapor interface, though with smaller NP--NP separation, as expected due to less van der Waals shielding caused by the upper medium in the interface. x-ray beam transmission at different vertical heights characterizes the heptane and DEG bulk and interfacial regions, while monitoring the time dependence of SAXS at and near the DEG/heptane interface gives a clear picture of the evolution of NP assembly at this liquid/liquid interface. These SAXS observations of self-limited NP ML formation at the DEG/heptane interface are consistent with those using the less direct method of real-time optical reflection monitoring of that interface.

Iron oxide NPs encapsulated by oleate ligands were synthesized using a previous method. NPs with core diameters of 8.6 nm and 11.8 nm sizes were used, with their sizes determined by fitting their form factors, obtained by the SAXS of free dispersed NPs. The sample cell used to assemble NP MLs on top of a DEG layer and perform in-situ SAXS was designed with a wall angle of 75° to suppress the meniscus of the DEG/vapor interface on the window material of fluorinated ethylene propylene (FEP); this results in a nearly flat interface forming a 90° contact angle relative to the vertical axis of the sample cell of DEG on FEP. Sample cells using this design have previously been shown to collect much larger signals and improve the degree of order visible in SAXS images of ordered iron oxide NP self-assembly on liquid/vapor interfaces. This geometry enables limited x-ray exposure and so better time-resolution and yields superior SAXS images of the final structure after drying volatile solvents atop the DEG evaporated. The cell was set on a stage that could move horizontally and vertically relative to the x-ray beam direction at the National Synchrotron Light Source II 11-BM Complex Materials Scattering (CMS) Beamline.

When studying the time dependence of assembly, 2 mL of DEG was added into the sample cell to form the lower liquid substrate, a ~3.85 mm thick DEG layer, with the top surface of 54.33 mm × 11.33 mm. This was followed by adding 1 mL of pure heptane in the main runs to form an upper liquid substrate that is called the heptane reservoir. The reservoir would be an ~1.64 mm thick layer, with top surface of 54.72 mm × 11.73 mm, if the menisci were flat, but here corresponds to a ~0.50 mm separation of the top of the DEG/heptane and bottom of the heptane/vapor menisci, as measured by x-ray transmission. Then this was followed by drop-casting a much smaller volume, 60 µL, of the 8.6 nm or 11.8 nm core diameter iron oxide NP dispersion in heptane, of selected concentration; this would be equivalent to a layer that is ~0.1 mm thick if flat, and so to a meniscus separation of ~0.6 mm. Then these two layers above the DEG mixed. A cap was placed atop the sample cell after dropping the pure heptane reservoir to minimize evaporation in the relevant time frames. This cell is well sealed, so upper solvent evaporation is slowed to better distinguish it from other time scales in assembly events, and is also slowed due to using heptane here rather than hexane. Several comparison experiments were also conducted by drop-casting the NP dispersion directly on the DEG layer.

One advantage of this two-step procedure in NP injection on an existing heptane reservoir atop the DEG is the spatial stability of the DEG/heptane interface after aligning the x-ray beam. After the DEG is first added to the cell, a flat DEG/vapor interface forms due to the design of the cell. (If the cell had vertical walls, this interface would have been concave up.) After heptane is added, and before it evaporates, the DEG interface moves, and the lower, DEG/heptane interface becomes concave down and the upper, heptane/vapor interface becomes concave up. The x-ray beam was then aligned relative to the DEG/heptane interface, before the NPs were drop-cast. In the current study, the speed of the x-ray beam alignment to the DEG/heptane after adding pure heptane, as described below, was not critical, and the shape and position of this interface, and so that of x-ray alignment, did not change, and assembly at the DEG/heptane interface could be followed immediately. Furthermore, the second step in this two-step injection of heptane provided a “delta function” NP concentration at t = 0 at the heptane/vapor interface, ~2 mm away above the center of the heptane surface, and so transport to the DEG/heptane interface and NP layer formation at this interface could in principle be resolved more cleanly. Turbulence near the DEG interface was probably lessened by using this smaller volume and vertically displaced heptane dispersion.       

The NPs were drop-cast after the heptane dispersion was loaded into PTFE tubing with inner diameter 0.79 mm by remote injection via an auto-injector (Digital Controlled Infusion Syringe Pump EQ-300SP-H-LD). The end of this tubing was set very near, h ~ 2 mm above the heptane/vapor interface to lessen the impact of the falling drop on NP transport in the heptane, an attempt at “gentle” drop-casting. The tubing went through the small hole on the cap, and so was set near the center of the cell horizontally. Drop-cast NP concentrations are presented in terms of “ML equivalents,” defined as the number of MLs that would be expected to form on the DEG layer, after drop-casting and subsequent evaporation of heptane to form a flat DEG/vapor interface, in the Petri dish measurement vessel with dimensions similar to the SAXS sample cell; these are approximately, but not exactly, the number of MLs that could form for each experimental condition. For 8.6 nm NPs, this measured, calibration 1.0 ML-equivalent corresponded to ~1.45 MLs of close-packed ordered NPs on the curved, concave down DEG/heptane interface and ~1.09 MLs on the flat DEG/vapor interface in the SAXS sample cell, and for 11.8 nm it corresponded to ~1.25 MLs and ~1.09 MLs respectively. This means that drop-casting 0.69 (= 1.0/1.45) ML-equivalents of 8.6 nm NPs and 0.80 (= 1.0/1.25) ML-equivalents of 11.8 nm NPs would be expected to produce 1.0 ML of hexagonal close packed NPs at the DEG/heptane meniscus in the SAXS sample cell. For the 8.6 nm iron oxide NP studies, 0.2 to 12 ML-equivalents were drop-cast and for 11.8 nm NPs, 0.2-18 ML-equivalents were drop-cast. Even during injection, the sample cell was sealed off from the environment to slow down heptane evaporation. 

The x-ray wavelength was 0.9184 Å (13.5 keV), unless otherwise stated, and the cross-section was 50 μm (vertical, x axis) × 200 μm (horizontal, y axis), with the sample being in the y-z plane. SAXS measurements used an x-ray detector array (DECTRIS Pilatus 2M) placed 3 m away, with 1,475 × 1,679 pixels, each with 3.90 × 10-3 nm-1 width. The lateral dimensions and curvatures of the DEG/heptane interface shift and broaden the SAXS signals by less than a pixel in the x and y directions. The finite length of the scattering region along the flat DEG/vapor interface also affects the signals by less than a pixel.

Before NP injection, x-ray beam transmission through the sample was measured as a function of sample cell height. This enabled precise determination of the position of the DEG/heptane interface, so that time-resolved measurements could later be made at that interface nearly immediately after NP dispersion injection. This included x-ray transmission over a range of heights at and near each interface. Such a series of sequences of x-ray transmission scans took between 2 and 5 min, depending on the number of data points taken.    

Runs with 8.6 nm iron oxide NPs with 0.5, 1, 3, 6, 9 and 12 ML-equivalents drop-cast and with 11.8 nm NPs with 6 ML-equivalents drop-cast, were conducted with a strip beam stop for blocking to minimize noise and a 5 s exposure time for each beam position. In addition, survey runs during a previous visit to NSLS-II were conducted with 11.8 nm NPs and 1, 9, and 18 ML-equivalents drop-cast with 1 s exposure time and a very small circular beam stop, both which led to results with increased noise; and there was alignment only at the DEG/heptane meniscus.

For each run, after initial beam alignment, a series of SAXS measurements was conducted with the cell cyclically repositioned near the top of the DEG/heptane interface, which is the boundary of Sections III and IV, to ensure that the top of the DEG/heptane meniscus was being probed each time in this series, usually along with regions immediately above and below it. Each run consisted of repeated cycles, each with measurements at three heights separated by 50 µm, so if the alignment happened to be a bit off after the DEG/heptane interface was determined, the top of the meniscus was at least being probed in one of these three positions. In the runs during which 0.5, 1, 3, 9, and 12 ML-equivalents of 8.6 nm NPs and 6 ML-equivalents of 11.8 NPs were drop-cast, the positions at and right above the top of DEG/heptane meniscus were both probed; in the runs in which 6 ML-equivalents of 8.6 nm NPs and 1, 9, and 18 ML-equivalents of 11.8 nm NPs were drop-cast, only the top of meniscus was probed. The three targeted positions in a cycle were at the top of the meniscus and either (a) 50 µm above and 50 µm below it, (b) 100 µm and 50 µm above it, or (c) 100 µm and 50 µm below it. For example, in the 8.6 nm NPs runs with 1, 3, and 12 ML-equivalents drop-cast, the first SAXS measurement was with the center of the beam 50 µm above the meniscus, and so the beam did not probe the interface at all, and showed no sharp SAXS peaks due to ordering; then the cell was raised by 50 µm, so the 50-µm tall beam was relatively lowered by this amount and was then “at” the top of the DEG/heptane meniscus and so approximately half of the beam was in the heptane and half in the DEG, and after NP ordering there were sharp SAXS peaks; then the cell was raised by 50 µm so the center of the beam was 50 μm below the interface, and so the beam did not probe the top of the meniscus at all, but still showed sharp SAXS peaks from regions probed away from the center, because the hexagonally close-packed NP ML formed over the entire DEG/heptane interface; and then the cell was lowered by 100 µm and the above/at/below the meniscus sequence was repeated again and again.

Each SAXS measurement took 8 s for the 8.6 nm NP runs and 6 ML-equivalents run for 11.8 nm NP (5 s exposure time and 3 s processing time at each position), and so the measurements at the top of the DEG/heptane meniscus in each cycle are separated by 24 s. Similarly, SAXS measurements took 4 s for the preliminary runs with 1, 9, and 18 ML-equivalents of 11.8 nm NPs (1 s exposure time and 3 s processing time at each position), and the measurements at the top of the DEG/heptane meniscus in each cycle are separated by 12 s. The series of cycles for each condition lasted for several hundred s. Monitoring the sharp SAXS peaks in these cyclic measurements helped keep the beam aligned at the meniscus and minimize variations in intensity of the ordered SAXS peak due to changes in the numbers of NPs probed and changes due to varying path lengths in heptane and DEG with differing x-ray absorption in the liquids. The SAXS collections at three positions in each cycle ensured that the time-resolved changes of how NPs self-assembled at the DEG/heptane interface were seen and offered space-resolved pictures for comparing SAXS signals at and above the top of the DEG/heptane meniscus. This x-ray position cycling and data collection began before NP dispersion auto-injection, which is at t = 0, so data collection began “immediately.” Detailed data sets are presented with the beam aligned above and at the top of the DEG/heptane meniscus.

SAXS NP form factor signals with the beam aligned above and at the top of the DEG/heptane meniscus were integrated along qx from -0.0780 nm-1 (-20 pixel) to +0.0780 nm-1 (+20 pixel) and then fit from qy = 1.057 nm-1 to 1.598 nm-1 for the 8.6 nm NPs and qy = 0.745 nm-1 to 1.150 nm-1 for the 11.8 nm NPs. (Though the Porod invariant accessible via integrating over dqxdqydqz entails integrating in the third dimension, the qz wave vector was almost the same throughout the qx, qy range analyzed in this small angle measurement; so, it is reasonable to analyze SAXS intensities as a function of qy after integration over a range of qx.) Gaussian functions were used to fit the sharp (10) peaks after the background was removed; first a single gaussian function was used and then the sum of two gaussian lineshapes was used for this fit. To account for the longer integration times for the 6 ML-equivalents of 11.8 nm NPs, SAXS intensities were divided by 5 when analyzed. The generally shorter integration times (and larger x-ray backgrounds) in the preliminary runs for the larger NPs led to smaller signal/noise ratios. In most cases, background SAXS images were taken near the top of the DEG/heptane meniscus before the NPs were drop-cast and then this background was removed during peak fitting after normalization. In some cases, SAXS collection before the NPs were drop-cast was used as the background that was subtracted.

After a given series of measurements, and when no more significant changes in the SAXS pattern were seen (~12 min), another vertical scan of lateral x-ray transmission traces was conducted at the center of the sample cell from Section II until the beam hit Section V to determine if and by how much the meniscus at the DEG/heptane interface had changed during the course of the experiment and to what degree the heptane layer had already evaporated. In most experiments the cell components and windows were sealed well and the heptane evaporated very little. An experiment was repeated when this was not so. Approximately 20 min after NP auto-injection and hours before the heptane evaporated when the images, and so the NP structure at the DEG/heptane interface, had reached steady state, a series of SAXS measurements at different heights were taken for additional space-resolved information and to examine NP assemblies away from the top of the DEG/heptane meniscus, at lower heights on the interface.

The NPs at the DEG surface were also examined later after the purposeful loss of heptane. This was accelerated by removing the sample SAXS cell from the beamline, and then flowing nitrogen laterally over the uncovered top of the sample cell until no heptane was visible. This typically took ~1-2 h. Then, before any significant amount of the much-less volatile DEG had evaporated, the sample cell was reintroduced to the beamline stage, and x-ray transmission was measured in a vertical scan to determine the shape of the meniscus, for the 1, 6, 9, and 12 ML-equivalent runs with the 8.6 nm NPs and the 1 and 9 ML-equivalent runs for the 11.8 nm NPs. The sample was then probed at the DEG/vapor interface using SAXS to identify regions with NPs present and the degree of order present in those regions. All was cleaned before the next series of runs were started. 

The data in this folder is for the research article (J. Phys. Chem. C 2020, 124, 23949−23963. DOI: https://doi.org/10.1021/acs.jpcc.0c07024) and its supplementary information.

The original X-ray image data are in the format of tiff. It may not be readable in some systems. So, each tiff image has a corresponding fig (Matlab figure format) and jpg documents.

Each folder is a run. A file name has several parameters. For example, in a typical file name “FeO_8nm_1ML_60uL_hep_1000uL_resin_kinetics__403.5s_x-28.000_y7.575_10.00s_245130_saxs”, “FeO_8nm_1ML_60uL_hep_1000uL_resin_kinetics” is the folder name and gives the basic information of that run. 403.5s is the time mark since the run starts. Notice that there is a delay between the clock starts and the nanoparticle dispersion is drop-cast. x-28.000 and y7.575 are position parameters. 10.00s is the duration of X-ray exposure. 245130 is the SAXS ID, and it is unique.

Figures 1(c), 2, and 3 are plotted from data in the folder “FeO_8nm_1ML_60uL_hep_1000uL_resin_kinetics”. Figure 1(c) has A—E 5 SAXS images (D and E also in Figure S10) which have SAXS ID 245250, 245255, 245266, 245284, 245303.

Data shown in Figures 4–8, S11, and S12 include 8.6 nm 0.5, 1, 3, 6, 9, and 12 ML-equivalents runs and 11.8 nm 1, 6, 9, and 18 ML-equivalents runs. They are in folders “FeO_8nm_0.5ML_60uL_hep_1000uL_resin_kinetics”, “FeO_8nm_1ML_60uL_hep_1000uL_resin_kinetics”, “FeO_8nm_3ML_60uL_hep_1000uL_resin_kinetics_2”, “FeO_8nm_6ML_60uL_hep_1000uL_resin_kinetics”, “FeO_8nm_9ML_60uL_hep_1000uL_resin_kinetics”, “FeO_8nm_12ML_60uL_hep_1000uL_resin_kinetics”, “12nm_60uL_1ML_1000hep_auto_close_JH”, “FeO_12nm_6ML_60uL_hep_1000uL_resin_kinetics”, “12nm_60uL_halfstock_1000hep_auto_close”, and “12nm_60uL_18ML_1000hep_auto_close”, respectively.

Form factor raw data in Figure S9 can be found in the folder “form factor fitting”.

Data in Figure S13 and S14 are from “FeO_8nm_0.5ML_60uL_hep_1000uL_resin_kinetics__p3_122.0s_x-28.000_y7.475_5.00s_245688_saxs” and “FeO_8nm_9ML_60uL_hep_1000uL_resin_kinetics__p3_122.0s_x-28.000_y8.050_5.00s_244615_saxs”.

Data in Figures S16—S18 of different heptane reservoir volume (0.5 mL, 1 mL, 2 mL, and 4 mL) are in folders “FeO_12nm_6ML_60uL_hep_500uL_DEG_2ml”, “Test”, “FeO_12nm_6ML_60uL_hep_2000uL_DEG_2ml”, and “FeO_12nm_6ML_60uL_hep_4000uL_DEG_2ml”, respectively.

Data in Figure S21 are “20uL_12nm_1ML_baseline__3799.8s_x-24.300_y7.250_120.00s_190384_saxs” and “FeO_12nm_1ML_60uL_hep_1000uL_resin_dry_ex_situ__1900.8s_x-28.000_y8.650_60.00s_247145_saxs” in the folder “comparison study”.

The raw data of the vertical scan in Figure 1(b) is in the folder “fit scan”, and the scan ID is 190495.

The raw data of the vertical scans in Figure S19 are in the folder “fit scan”. Their scan ID are 345095 (0.5 mL), 343350 (1 mL), 343784 (2 mL), and 344584 (4 mL).

The main MATLAB codes can be found in the folder “codes”.