In situ vs ex situ: Comparing the structure of PNIPAM microgels at the air/water and air/solid interfaces

TL;DR

This work uses in situ specular and off-specular X-ray reflectivity to map the vertical and lateral structure of PNIPAM microgels at the air/water interface and compares it with ex situ LB-deposited monolayers on solid substrates. The authors show that microgels form 2D hexagonal domains at low to intermediate surface pressures, with domain-specific lattice constants that shrink under compression; LB transfer, however, contracts interparticle distances and can induce bilayer formation ordomain rearrangements, revealing transfer-induced artefacts. The combination of XRR and OSR provides a non-invasive, lab-based approach to resolve both vertical density profiles and in-plane ordering, highlighting how compression and substrate interactions shape interfacial microgel structure without doping. These findings refine the understanding of soft interfacial self-assembly and emphasize caution when interpreting LB-transferred structures as representatives of in situ interfaces, with implications for designing and characterising interfacial soft matter systems.

Abstract

For studying the structure of microgel particles at the air/water interface, specular and off-specular X-ray reflectivity (OSR/XRR) allows in situ measurements without any labelling techniques. Herein we investigate the vertical and lateral structure of poly(N-isopropylacrylamide) (PNIPAM) microgels (MGs) at the air/water interface and the effect of Langmuir-Blodgett (LB) transfer onto solid substrates. The initial ex situ atomic force microscopy (AFM) scans of LB-transferred MGs at the air/solid interface reveal strong lateral 2D hexagonal ordering across a broad range of lateral surface pressures at the air/water interface before LB-transfer. Notably, for the first time, these results were confirmed by OSR, demonstrating the existence of the long-range hexagonal ordering at low and intermediate surface pressures. For in situ conditions and upon uniaxial compression at the air/water interface, the MG lattice constant decreases non-monotonically. This indicates the formation of domains at low pressures that approach each other and only compress when the surface isotherm reaches a plateau. Comparing the results of in situ and ex situ measurements, our study demonstrates a clear transfer effect during the LB-deposition on the lateral ordering of the MGs: the distance between the particles decreased during LB-transfer, and at high pressures ($Π\,>\,17\,\mathrm{mNm^{-1}}$) a second distance occurs indicating small domains with hexagonal internal ordering. The novel surface characterisation approaches debuted here highlight the use of both XRR and OSR to probe the vertical and lateral structure of adsorbed MGs, offering in situ, non-invasive insights without the need for doping or transfer-induced artefacts.

Figures (7)

• Figure 1: Artistic representation for a 2D-hexagonal lattice in (left) real and (right) reciprocal space, together with the used convention for lattice vectors and indicated resolution in reciprocal space.
• Figure 2: Langmuir compression isotherm of the investigated microgel monolayers with representative AFM topologies of LB-transferred samples at the surface pressures $\Pi$ = 0.5, 10, 22, and 27 $\mathrm{mNm^{-1}}$. The AFM images are also shown in Figure \ref{['pic:DIP_AFM_OSR']}C.
• Figure 3: Ex situ off-specular reflectometry (OSR) (columns A and B) and atomic force microscopy (AFM) (column C and D) data of PNIPAM microgels after Langmuir-Blodgett deposition onto a silicon substrate at surface pressures of $\Pi\,=\,0.5,\,10,\,22$ and $27mNm^{-1}$ (see Figure \ref{['Dip positions']}, from bottom to top). In column A, the orientation ($\phi$) resolved OSR (upper) and ($\phi$) integrated (lower) spectra are displayed in Cartesian coordinates. Corresponding polar coordinate representation with the same colour coding are presented in column B. Column C shows the AFM scans for the respective pressures. All topographies have the same colour-map and scan size. Corresponding histograms of the nearest neighbour distances $D_\mathrm{cc}$ shows column D, from which the mean interparticle distances are extracted by fitting the histograms using 1 (for $\Pi\,\leq\,10\,\mathrm{mNm^{-1}}$) or 2 ($\Pi\,>\,10\,\mathrm{mNm^{-1}}$) Gaussians.
• Figure 4: A: $q_\mathrm{x}^{1.8}$ weighted in situ off-specular X-ray scattering of MG particles at the air-water interfaces at the pressures highlighted on the isotherm in panel B, starting from the bottom with the lowest lateral pressure. Black symbols are the measured values, and red solid lines are optimised models as described in the text. For improved visibility, the curves are shifted by $0.05$ against each other. B: $\Pi-A$ isotherm of the PNIPAM MG particles at the air/water interface (black solid line). Red symbols indicate the pressures at which the OSR were recorded. C: Centre positions of the fitted Gaussian representing the (01) peak.
• Figure 5: (A) Fresnel normalised specular X-ray reflectometry of MG particles at the air/water interface and corresponding (B) SLD profiles at the pressures highlighted on the isotherm in the inset in A, starting from the bottom with the lowest lateral pressure. Black symbols are the measured values and red solid lines are the optimised models corresponding to the SLD profiles shown in (B). For improved readability, reflection profiles in (A) are offset by a factor of $\sqrt[3]{10}$ and SLD profiles in (B) are offset by $5 \cdot 10^{-6}~\hbox{\normalfont\AA}^{-2}$. The inset displays the sum of the length of the two slabs ($l_1$, $l_2$), including the adjacent transition region ($2\sigma_1$) to parameterise the layer present above the apparent air/water interface, as function of $\Pi$.