# Temperature-sensitive Soft Microgels at Interfaces: Air-Water versus Oil-
Water

Steffen Bochenek  Institute of Physical Chemistry, RWTH Aachen University,
Landoltweg 2, 52056 Aachen, Germany  Andrea Scotti  Institute of Physical
Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany  Walter
Richtering<EMAIL_ADDRESS>Institute of Physical Chemistry, RWTH
Aachen University, Landoltweg 2, 52056 Aachen, Germany

###### Abstract

The formation of smart emulsions or foams whose stability can be controlled
on-demand by switching external parameters is of great interest for basic
research and applications. An emerging group of smart stabilizers are
microgels, which are nano- and micro-sized, three-dimensional polymer networks
that are swollen by a good solvent. In the last decades, the influence of
various external stimuli on the two-dimensional phase behavior of microgels at
air- and oil-water interfaces has been studied. However, the impact of the
top-phase itself has been barely considered. Here, we present data that
directly address the influence of the top-phase on the microgel properties at
interfaces. The dimensions of pNIPAM microgels are measured after deposition
from two interfaces, _i.e._ , air- and decane-water. While the total in-plane
size of the microgel increases with increasing interfacial tension, the
portions or fractions of the microgels situated in the aqueous phase are not
affected. We correlate the area microgels occupy to the surface tensions of
the interfaces, which allows to estimate an elastic modulus. In comparison to
nanoindentation measurements, we observe a larger elastic modulus for the
microgels. By combining compression, deposition, and visualization, we show
that the two-dimensional phase behavior of the microgel monolayers is not
altered, although the microgels have a larger total in-plane size at higher
interfacial tension.

##  1 Introduction

Two-phase systems formed by two liquids or a gas and a liquid, _e.g._ ,
emulsions and foams, are omnipresent in industrial processes, nature, and
everyday life. To prevent phase separation emulsifiers or foamers are used,
which adsorb to the interface and obstruct the interfaces from reaching
molecular close contact. Typical emulsifiers (or foamers) are amphiphilic
molecules, called surfactants, or hard colloids, 1, 2 such as silica or
polystyrene particles. More recently, soft colloids, for example synthetic,
non-amphiphilic macromolecules 3, polymer particles 4, 5, or proteins 6, have
been explored at liquid interfaces, too. One of the most successful soft
objects at interfaces are nano- and micro-sized gels, so-called microgels.

Microgels are three-dimensional cross-linked polymer networks, which are
swollen by a good solvent. Based on the specific nature of the polymer,
microgels are stimuli-responsive 7; for example, microgels composed of the
poly _N_ -isopropylacrylamide (pNIPAM) have a volume phase transition
temperature (VPTT) of $\approx$ 32∘C at which their polymer network de-swells.
8 pNIPAM microgels have a dense-core-fuzzy-shell structure with a decreasing
polymer content from the center to the periphery, 9 as a consequence of the
faster consumption of the cross-linker _N,N’_ -methylenebisacrylamide (BIS)
during the synthesis. 10 Starting from this, incorporation of other polymers
and more sophisticated synthesis routines lead to a broad variety of microgels
systems with different architectures 11, 12, 13 and responsivenesses to
several environmental stimuli. 14, 15, 16

Microgels are highly interfacial active and adsorb to liquid-gas 17, liquid-
liquid 5, and solid interfaces 18. Similar to rigid colloids, they
irreversibly adsorb to interfaces with desorption energies in the order of 106
$k_{B}T$. 19 But there are also some significant and advantageous distinctions
from hard particles. First, their stimuli-responsiveness in bulk is inherited
to their adsorbed state. Emulsions20, 5, 21, 22, 23 and foams24, 25, 26, 27
formed by microgels can be broken at will by mild changes in the environmental
conditions, for example by heating above the VPTT or changing the pH. Second,
the solvent swollen, porous, and soft polymer network of the microgels can
adapt and stretch upon adsorption. 20, 28, 29, 30, 31, 32, 33, 34, 35, 36 Thus
increasing the contact area with the interface to maximize the gain in
interfacial energy.

The combination of these characteristics leads to additional functions and
rich phase behavior. Microgels have been employed to explore manifold
interfacial phenomena and advanced applications. However, the coupling of
particle elasticity, molecular architecture, and interfacial effects is also a
source of complexity. Understanding the structure of single microgels, the
two-dimensional macroscopic phase behavior of microgel monolayers, and the
mechanism of demulsification or defoaming has been a hot topic in the last two
decades.

External stimuli, such as pH 28, 37, 33, 38, salt concentration 38, 33, 39,
and temperature 35, 34, 40, but also intrinsic proprieties, _e.g._ , different
cross-linker contents 32, 33, architectures 41, 42, and incorporation of
inorganic cores 43, 41, have been investigated at interfaces in the
literature. These studies were conducted on both air-water and oil-water
interfaces. Although a comparison of the different studies indicates that
there is no qualitative difference between the interfaces, _i.e._ , air-water
and alkane-water, there has been no quantitative and direct experimental
investigation of the very same microgel system.

The top-phase is a key factor to consider when the microscopic conformation of
the microgels at interfaces is inspected. Similar to bulk, the swelling
equilibrium of a neutral microgel at the interface is determined by the free
energy of mixing and elasticity but also the free energy of the surface,
_i.e._ , surface tension. 31, 19 Likewise, the solubility of the polymer in
the two immiscible phases plays a dominant role in determining the final shape
of confined microgels. 29, 44 When polar organic liquids are used as top-
phase, which are solvents for pNIPAM, the structural heterogeneity (core-
corona structure) of adsorbed microgels is mitigated. 44 Additionally, one has
to consider the low surface tension between water and polar organic liquids.
45

In this study, we directly address the in-plane dimensions of single microgels
and the two-dimensional phase behavior of microgel monolayers as a function of
the top-phase, i.e., at air-water and decane-water interfaces. For this, a
well-characterized model system of pNIPAM microgels is used. 42, 35 Both air
and decane are non-solvents for pNIPAM 46, 29 and, therefore, changes to the
adsorbed microgel properties can be predominately related to the difference in
surface tension. Combined compression and deposition experiment were
conducted, with subsequent visualization of the microgels. Isotherms show an
earlier first increase of the surface pressure at lower concentrations. These
experiments were also performed at temperatures above and below the VPTT of
the microgels in bulk. Independently of the top-phase, microgels monolayers
show the same temperature dependence. We measure the stretching of individual
microgels, connect the results to the different surface tensions of the two
interfaces, and estimate the elastic moduli of the microgels. The entirety of
the data unambiguously shows no alteration in the two-dimensional phase
behavior of the microgel monolayers, although the microgels have a larger
total in-plane size at higher interfacial tension.

##  2 Experimental

###  2.1 Materials

For all interface experiments, ultrapure water (Astacus2, membraPure GmbH,
Germany) with a resistivity of 18,2 MOhm$\cdot$cm was used as a sub-phase.
Decane (Merck KGaA, Germany) was used as oil-phase. The decane was filtered
three times over basic aluminum oxide (90 standardized, Merck KGaA, Germany).
The last filtration step was done just before the experiment. Pieces of an
ultra-flat silicon wafer ($\approx$ 1.1 cm x 6.0 cm, P{100}, NanoAndMore GmbH,
Germany) were used for depositions. Aqueous solutions of microgel were mixed
with propan-2-ol (Merck KGaA, Germany). Microgels were also dispersed in
chloroform (Merck KGaA, Germany).

###  2.2 Compression isotherms and Depositions

Gradient Langmuir-Blodgett type depositions30 were conducted according to Ref.
35. Compression of a microgel monolayer is combined with the deposition to a
solid substrate. While the monolayer is compressed the substrate is lifted
through the interface in between the barriers with an angle (25∘). This
produces a gradient of packing density of the deposited film on the substrate.
Subsequently, the depositions are imaged _ex situ_ with atomic force
microscopy (AFM). Because the compression and deposition is carried out at the
same time, and the angle of the substrate is know, the physical properties can
be connected to the microstructure of the monolayer.

Before each measurement, the Langmuir-Blodgett trough was carefully cleaned
and a fresh interface (air-water or decane-water) was created. The through is
temperature controlled by circulating thermostated water through its base
using an external water bath. Microgels were spread from solutions with a
concentration of 10 mg mL-1, mixed with 20 vol$\%$ of propan-2-ol (decane-
water measurements), 50 vol$\%$ of propan-2-ol (air-water measurements), or
from a solution with a concentration of 1 mg mL-1 in chloroform (air-water
measurements). Aqueous solutions were mixed with propan-2-ol to facilitate
spreading. After temperature equilibration, the microgels were added to the
interface using a syringe. Gradient Langmuir-Blodgett type depositions and
compression isotherms were conducted at $(20.0\pm 0.5)~{}^{\circ}$C and
$(40.0\pm 0.5)~{}^{\circ}$C.

###  2.3 Atomic force Microscopy

The microgel monolayers were imaged using a Dimension Icon atomic force
microscope with closed loop (Veeco Instruments Inc., USA, Software: Nanoscope
9.4, Bruker Co., USA) in tapping mode. As probe OTESPA tips with a resonance
frequency of 300 kHz, a nominal spring constant of 26 N m-1 of the cantilever
and a nominal tip radius of $<$ 7 nm (Opus by Micromasch, Germany) were used.
The programmed move function was employed to capture images of 7.5 $\mu$m x
7.5 $\mu$m (512 pixels x 512 pixels) in a straight line along the gradient
direction on the substrate every 250 or 500 $\mu$m. For each line, an image at
the position of highest compression was taken to obtain a reference position.

###  2.4 Image Analysis

For all images, the open-source analysis software Gwyddion 2.54 was used to
remove the tilt and fix zero height to the minimum z-value of the image. AFM
images of the deposited dried microgel monolayers were analyzed with a custom-
written Matlab script based on the image analysis routine of Ref. 30. The
script was already used in Ref. 35 and details can be found there. The number
of microgels per area, the mean nearest neighbor distances, and the hexagonal
ordering parameter were calculated for all images.

The analysis of microgel monolayers was furthered by height profiles of the
microgels in the dried state. Profiles were extracted through the apizes of
the microgels and at different angles with respect to the fast scan direction.
Multiple height profiles of microgels at the same interfacial concentration
are summarized and aligned to the microgels’ apizes to obtain averaged
microgel profiles and to not bias the results. Representative averaged height
profiles were calculated from at least 40 microgels ($\approx$ 120 profiles).
The averaged profiles are presented with the standard deviations as the error.
The height of the microgels at the apex, $H_{core}$, is determined from the
maximum value on the y-axis using the Matlab function _findpeaks_. $D_{core}$
is computed using the Matlab function _knnsearch_ with a threshold value of
1.5 nm above the background (zero height of the image). At the highest surface
pressures, microgel cores start to overlap and multiple microgel centers are
shown. Here, $D_{core}$ was calculated from the distance between the apices.

##  3 Results and Discussion

For the investigation of microgels at air-water and oil-water interfaces a
suitable model system is required. We decided to use pNIPAM-based microgels,
which have already been study in previous publications. 42, 35 The synthesis
protocol and characterization of the physical properties of these microgels in
solution can be found in Ref. 35. Briefly, the pNIPAM microgels have been
synthesized with approximately 5 mol% cross-linker (BIS) content. They have a
hydrodynamic radius, $R_{h}$, of (153 $\pm$ 3) nm at T = 20∘ C and of (84
$\pm$ 2) nm at T = 40∘ C; their VPTT is around 33∘ C. The microgels were
synthesized with $\approx$ 2 mol% of _N_ -(3-aminopropyl) methacrylamide
hydrochloride (APMH). The primary amine moities of the incorporated APMH allow
post-modification of the microgels, _e.g._ , covalent labeling with
fluorescent dyes. 47 Consequently the microgels are positively charged in pure
water. Small-angle neutron scattering (SANS) data show a decay of the polymer
volume fraction from the center to the periphery of the microgels at 20∘ C and
a box-like profile in the collapsed state (T = 40∘ C). The microgels have a
narrow size distribution and a size polydispersity of (7 $\pm$ 1) % as
obtained from the fit to the SANS data. 35

###  3.1 Stretching of Individual Microgels and Estimation of the Elastic
moduli.

We first evaluate the effect of the top-phase on the microgels by measuring
the dimensions of individual, well-separated microgels after deposition with
_ex situ_ AFM in the dried state. 36, 35 The depositions were performed at
interfacial concentrations far below the contact of the microgels. A clear
contrast between the microgels and the silica wafer, but also between the core
and corona can be seen in the phase contrast images of the AFM measurements.
34, 35 Examples of the phase images are presented in Figure 1A-D. Circles
representing the cores and the overall microgels are added for clarity. From
the images we estimate the size distributions for the overall and core
diameters (Figure 1E and F, black lines). The averaged overall and core
diameters, $D_{2D}$ and $D_{core}$, respectively, are determined by fitting
Gaussians to the data. The fits are plotted with the respective colors of
Figure 1A-D.

The corresponding AFM height images are used to extract topographic
information of the microgels. We averaged the height profiles of multiple
microgels at the same state (interface and temperature) to determine the
height, $H_{core}$, and core diameter, $D_{core}$, of the dried microgels. The
experiments were conducted with the very same pNIPAM microgels, Langmuir-
Blodgett setup, and the same type of sub-phase and substrates. Thus,
differences in the profiles must stem from conformational differences of the
microgels at the air-water or oil-water interface, similar to the case of
microgels deposited from bulk and air-water interfaces. 34 $D_{core}$ was
determined twice, from respective height and phase images. Both methods yield
the same $D_{core}$-values within the errors. The results from the phase
images and height profiles are summarized in Table LABEL:tab:tab_NND.

Figure 1: Interfacial dimensions of dried microgels at different temperatures
and interfaces. (A-D) AFM phase images with circles indicating the area of the
core and the whole microgel. (A) T = 20 ∘C, (B) T = 20 ∘C, (C) T = 40 ∘C, and
(D) T = 40 ∘C. (E and F) Black lines are the size frequency distributions of
pNIPAM microgels. Normal distributions are given in the corresponding colors
of the circles in A-D, the labels give the mode and the variance. The diameter
was calculated from the area with $D=2\cdot\sqrt{A/\pi}$. (G) Averaged height
profiles of microgels at T = 20. The standard deviations are given as shaded
areas in the complimentary colors. For each state (A-D) at least 250 microgels
were analyzed.

Figure 1 shows that at the same temperatures, $D_{2D}$ of the microgels
increases from the decane-water to the air-water interface by $\approx$ 60 nm.
At 20 ∘C, $D_{2D}$ increases from 627 $\pm$ 38 to 688 $\pm$ 33 nm and at 40 ∘C
from 592 $\pm$ 45 to 651 $\pm$ 37 nm. In contrast, $D_{core}$ stays constant
(within the errors) from oil-water to air-water interfaces below (357 $\pm$ 25
to 360 $\pm$ 22 nm) and above the VPTT (274 $\pm$ 22 to 289 $\pm$ 15 nm). A
decrease of $H_{core}$ is observed for microgels below (from 21 $\pm$ 2 to 20
$\pm$ 2 nm) and above (from 27 $\pm$ 3 to 23 $\pm$ 2 nm) the VPTT from the
decane- to the air-water interface (Tab. LABEL:tab:tab_NND).

To understand these changes in the dimensions ($D_{2D}$, $D_{core}$, and
$H_{core}$) of the dried microgels we have to consider the adsorption process
to and the structure of microgels at interfaces before deposition. When a
microgel adsorbs to an interface the polymer network is able to adapt and
deform to increase their contact area and maximize the gain in interfacial
energy. Thereby, fractions of its polymer network are compressed and stretched
(Fig. 2A and B, outlined in red) while other fractions do not experience
significant deformation (Fig. 2A and B, outlined in blue). Due to this and to
their inhomogeneous cross-linker distribution, microgels display a core-corona
(or “fried-egg” like) structure at interfaces. 28 In direct contact and in
close proximity to the interface the polymer network becomes denser and
collapses (Fig. 2B, outlined in red). 36, 48, 49, 50 The fractions or portions
of the microgels which are positioned further away from the interface (Fig.
2B, outlined in blue) keep similar properties to microgels in the bulk aqueous
phase, _e.g._ , they can still de-swell as a function of temperature. 51, 35
The sketch in Fig. 2B illustrates another crucial fact: The polymer layer in
the interfacial plane consists of both dangling chains and cross-linked
microgels’ fractions. In other words, a thin cross-section of the microgel
along interface (Fig 2C) displays a decrease of cross-linking and polymer
distribution from the center to the periphery.

Figure 2: Sketch of the conformation of a pNIPAM microgel with inhomogeneous
cross-linker distribution at the oil-water interface. (A) Side view during
adsorption, (B) Side view after adsorption, and (C) 2D top view of a cross-
section along the interfacial plane. For further explanation see text.

The larger $D_{2D}$ at air-water interfaces is not surprising as the microgels
stretch further with increasing surface tension 44 from the decane-water
interface (52.3 mN m-1 at 20 ∘C) 52 to the air-water interface (72.3 mN m-1 at
20 ∘C) 53. In contrast, we observe that the size of the core ($D_{core}$) is
independent of the top-phase. Considering the sketches in Fig. 2 and the
literature, 36, 41, 50, 54, 34 this might be explained by: (i) the fact that
the polymer network in the center of the microgel gives more resistance to
deformation due to the higher cross-linker content, and experiences less
stress due to the higher polymer content (Fig. 2C). Consequently, the
stretching of the core could be already “saturated” 36 or so small that it is
not registered by our analysis.

(ii) Attenuation of the stretching for the microgels’ portions that are in the
aqueous phase due to the flexibility of the polymer network. In close contact
with the interface, portions of the microgels are stretched due to the tensile
stress (Fig. 2), but the stress can only be passed on through the cross-
linking points to portions of the network further away from the interface.
With increasing distance, the deformation is mitigated by the flexible polymer
segments between the interconnection points. As a result, the portions
situated in the aqueous phase are not (further) deformed.

(iii) Adsorption of more polymer sub-chains from the third dimension, _i.e._ ,
out-of the interfacial plane, resulting in a stronger compression normal to
the interface (Fig. 2A). In other words, instead of stretching the polymer
network more in-plane, the higher surface tension is compensated by adsorbing
more of the polymer network situated in the aqueous phase (Fig. 2B, outlined
in blue) leading to a thinner microgel center. The height profiles in Figure
1G show a decrease in $H_{core}$ from the oil-water to the air-water
interfaces, however, the changes are within the experimental uncertainties,
and a clear statement cannot be made.

Regardless of which explanation or combination of explanations apply, we
clearly observe an increase in $D_{2D}$, while the lateral expansion of the
core remains constant. A similar comparison by Camerin  _et al._ between
microgels at benzene- and hexane-water interfaces was conducted using both
numerical and experimental results. 36 In their work they observe no
significant variation of the in-plane size of the microgels. It is proposed
that the stretching of the microgels is already “saturated” between the
surface tension of benzene- (35.0 mN m-1 at 20 ∘C) 45 and hexane-water (51.1
mN m-1 at 20 ∘C) 45, contradicting the results presented here. However, in the
experiments, Camerin  _et al._ used larger microgels. Recently, for the pH
response of adsorbed microgels, the size of the system has been shown to play
a pivotal role on the in-plane dimensions 39. Consequently, further
experiments have to be conducted, using different oils and microgels of
different size and softness to clarify this discrepancy.

$D_{2D}$ as a function of the surface tension can be used to estimate the mean
surface elasticity and its counterpart in bulk, the elastic modulus, of the
whole microgel. Since in this experiment the spreading of individual microgels
after adsorption is compared, we start with the equation of the hydrostatic
compression in two dimensions, defined as 55:

$K\equiv-\frac{\Delta p}{\Delta s/s}\,,$ (1)

where $K$ is the two-dimensional modulus of compression, $p$ is the two-
dimensional analog of pressure (in force per unit length), and $s$ area of the
interface. Hydrostatic compression states that the film is compressed
uniformly in all directions. 55 For our case, we can rewrite this equation
into:

$K\equiv\frac{\Delta\gamma}{\Delta A_{2D}/A_{2D}}\,,$ (2)

where $\gamma$ is the surface tension and $A_{2D}$ is the total area
calculated from $D_{2D}$ at the air- and decane-water interface. We omit the
negative sign as the force is in the opposite direction, _i.e._ , an expansion
is performed. With Equation 2, a two-dimensional modulus of compression of $K$
= (96 $\pm$ 15)$\,\cdot\,10^{-3}$ N m-1 is calculated for the microgels at the
air- and decane-water interface.

The compression modulus in two dimensions and the surface elasticity are
connected by the following relation: $E^{(2D)}=2K(1-\nu^{(2D)})$ 56. Here,
$\nu^{(2D)}$ is the two-dimensional Poisson’s ratio and $E^{(2D)}$ is the
surface elasticity or two-dimensional elastic modulus. The two-dimensional
Poisson’s ratio can be obtained from the three-dimensional one assuming plane
stress and plane strain conditions. 56 As aforementioned, it is observed that
(i) $D_{2D}$ changes with $\gamma$, but $D_{core}$ does not, and (ii) in
literature it has been shown that the compressibility at larger microgel-to-
microgel distances is independent of the polymer network portions in the
aqueous phase. 35 Thus, we assume that for the stretching of the individual
microgels only plane stress conditions are valid, where the polymer at the
interface dominates the elasticity. This assumption is in agreement with
recent literature 57 and, therefore, $\nu^{(2D)}=\nu^{(3D)}$ 56 is used. This
relation leads to the equation:

$\frac{E^{(2D)}}{2-2\nu^{(2D)}}=K\equiv\frac{\Delta\gamma}{\Delta
A_{2D}/A_{2D}}~{}.$ (3)

In the literature, $\nu^{(3D)}$ values are reported in the range between
$\approx$ 0.25 to 0.5 for pNIPAM gels. 58, 59, 60, 61 For a perfectly
incompressible isotropic material ($\nu^{(2D)}$ = $\nu^{(3D)}$ = 0.5),
Equation 3 becomes the same as Hooke’s law. Using the reported Poisson’s
ratios, we compute surface elasticities in the range of 96$\,\cdot\,$10-3 N
m-1 $<$ $E^{(2D)}$ $<$ 140$\,\cdot\,$10-3 N m-1. For further calculations we
use $E^{(2D)}$ of (115 $\pm$ 18)$\,\cdot\,10^{-3}$ N m-1 calculated with
$\nu^{(3D)}$ = 0.4 60.

In the literature, 58, 62, 63 typically the Young’s Moduli of microgels are
presented. For comparison, we convert the surface elasticity in 2D into
$E^{(3D)}$. As we assumed that the stress normal to the interfacial plane is
zero (plane stress), the relation between $E^{(2D)}$ and $E^{(3D)}$ is given
by 57: $E^{(3D)}=E^{(2D)}/d$, where $d$ is the thickness perpendicular to the
interface. Although the maximal extension of the polymer network into the
aqueous phase can be measured, for example, with ellipsometry 51, 35, we
follow the suggestion of Camerin _et al._ 57 and consider $d$ to be
significantly smaller, in the order of a few nm. This is also in good
agreement with the assumption that the polymer at the interface dominates the
elasticity and the discussion about the structure of adsorbed microgels (Fig.
2B). We calculate $E^{(3D)}$ using $d$ = 5 $\cdot$ 10-9 m according to the
film thickness of linear pNIPAM polymer measured with ellipsometry and neutron
reflectometry at the air-water interface, 64, 65, 66, 48 and obtain a Young’s
modulus of $E^{(3D)}$ = (23 $\pm$ 5) MPa.

Finally, we can compare the value for $E^{(3D)}$ to the E-moduli measured by
AFM normal to the interface. Thereby, one has to consider that the adsorbed
layer is de-swollen and collapsed, containing a low amount of solvent. In
comparison to the values in the literature for E-moduli above the VPTT (100
kPa to 13 MPa), 58, 62, 63 our result is one to two orders of magnitude
larger, showing that adsorbed microgels are stiffer within the interfacial
plane. Recently, Camerin  _et al._ addressed the microgel-to-microgel
interactions at interfaces. 57 They calculate larger E-moduli for the lateral
compression of adsorbed microgels compared to the bulk. Their data depicted
that the in-plane stiffening of the microgels is attributable to the presence
of the interface and the resulting restrain of the confined polymer chains.

###  3.2 Two-dimensional Phase Behavior of Microgel Monolayers.

The two-dimensional phase behavior of microgel monolayers at air- and decane-
water interfaces was captured by gradient Langmuir-Blodgett type depositions,
which combine compression with simultaneous deposition and subsequently
imaging in dried state. 30 The positions the images on the substrate are than
encode with the barrier positions, and thus, to the surface pressure, $\Pi$.
The images are analyzed with a custom-written Matlab script to determine the
number concentration, $N_{area}$, the center-to-center distances, _NND_ , and
hexagonal order parameter, $\Psi_{6}$ (see Ref.35 for detailed description of
the Matlab routine).

Figure 3: $\Pi$-$N_{area}$ compression isotherms of pNIPAM microgels at the
air-water and decane-water interface. (left) Isotherms at T = 20 ∘C. (right)
Isotherms at T = 40 ∘C. The errors are presented as shaded areas. The
different regimes visible in the isotherms are labeled in roman numbers.
Dashed lines in the respective colors indicate the transitions.
$\Pi$-$area/mass$ compression isotherms are presented in the Supporting
Information (Fig. S1).

$\Pi$-$N_{area}$ compression isotherms of the pNIPAM microgels measured at
both interfaces and at temperatures below and above their VPTT in solutions
are presented in Figure 3. All compression isotherms show distinct regimes, in
agreement with the literature 31, 30, 35, 32. The transitions between the
regimes are indicated by dashed lines in the corresponding colors of the
isotherms. Below the VPTT (Fig. 3 left) microgels display five regimes labeled
in roman numbers from I to V, namely a diluted state (I), the corona-to-corona
contact (II), the isostructural phase transition region (III), the core-to-
core contact (IV), and the failure of the monolayer (V). The microgel
monolayers at temperature above the VPTT (Fig. 3 right) display only a single
increase and regime IV, where microgels in core-to-core contact are
compressed, is missing. A detailed discussion of the effect of temperature on
microgel monolayers can be found in our previous work. 35 The regimes of the
monolayers are further highlighted by AFM images below (Fig. 4A-F, M-R) and
above (Fig. 4G-L, S-X) the VPTT. In general, the monolayers at the air-water
interface display the same two-dimensional phase behavior as their temperature
equivalents at the decane-water interface.

Figure 4: Atomic force micrographs of the microgel monolayers deposited at T =
20 ∘C from the air-water interface (A-F, green box) and the decane-water
interface (G-J, blue box), and deposited at 40 ∘C from the air-water interface
(M-R, golden box) and decane-water interface (S-X, red box) at different
compressions. The scale bars are 2 $\mu$m.

However, at all temperatures, the microgels monolayers at air-water interfaces
have a transition at lower $N_{area}$-values from separated (regime I) to
closed monolayers (regime II). AFM images of the first observed closed layers
are presented in the first column of Figure 4. At 20 ∘C, the closed film is
formed at 2.8 $\mu$m-2 and at 3.5 $\mu$m-2 at the air- and decane-water
interfaces, respectively. For the monolayers above the VPTT (Fig 4G and S),
the same difference of $N_{area}$ is observed between the two top-phases. The
microgels at the air-water interfaces occupy a larger area compared to the
decane-water interfaces, confirming the above presented size difference
measured for the individual microgels.

Table 1: Interfacial dimensions of microgels at different compressions
deposited from air- and decane-water interfaces at T = 20 and 40 ∘C. Center-
to-center distance for the first phase, _NND_ 1st, the second phase, _NND_
2nd, the in-plane diameter, $D_{2D}$, the core diameter, $D_{core}$, from
phase and height images, and height of the dried microgels, $H_{core}$.

Interface | $\Pi$ | $N_{Area}$ |  _NND_ 1st |  _NND_ 2nd | $D_{2D}$ |  $D_{core}$a |  $D_{core}$b | $H_{core}$
---|---|---|---|---|---|---|---|---
- | (mN m-1) | ($\mu$m-2) | (nm) | (nm) | (nm) | (nm) | (nm) | (nm)
T = 20 ∘C
| 0 |  $<$ 2.8 | - | - | (688 $\pm$ 33) | (360 $\pm$ 22) | (369 $\pm$ 35) | (20 $\pm$ 2)
| 0.2-1 | 2.8 | (620 $\pm$ 20) | - | - | - | (328 $\pm$ 34) | (19 $\pm$ 2)
| 10 | 4.2 | (500 $\pm$ 25) | - | - | - | (304 $\pm$ 30) | (22 $\pm$ 2)
| 20 | 5.3 | (450 $\pm$ 30) | - | - | - | (292 $\pm$ 30) | (26 $\pm$ 3)
air-water | 26 | 7.6 | (390 $\pm$ 40) | - | - | - | (286 $\pm$ 27) | (31 $\pm$ 3)
| 26.5 | 8.1 | (380 $\pm$ 35) | (220 $\pm$ 20) | - | - | - | -
| 30 | 23.2 | (335 $\pm$ 55) | (185 $\pm$ 20) | - | - | (163 $\pm$ 43) | (33 $\pm$ 5)
| 31.2 | 29.0 | - | (191 $\pm$ 12) | - | - | - | -
| 32.2 | 43.9 | - | (160 $\pm$ 12) | - | - | - | -
| 0 |  $<$ 3.5 | - | - | (627 $\pm$ 38) | (357 $\pm$ 25) | (358 $\pm$ 25) | (21 $\pm$ 2)
| 0.2-1 | 3.5 | (575 $\pm$ 30) | - | - | - | (363 $\pm$ 36) | (20 $\pm$ 2)
| 10 | 4.8 | (495 $\pm$ 20) | - | - | - | (294 $\pm$ 30) | (28 $\pm$ 2)
| 20 | 5.8 | (450 $\pm$ 30) | - | - | - | (272 $\pm$ 25) | (35 $\pm$ 3)
decane-water | 26 | 7.0 | (385 $\pm$ 25) | - | - | - | (266 $\pm$ 26) | (41 $\pm$ 3)
| 27.5 | 8.3 | (365 $\pm$ 35) | (220 $\pm$ 15) | - | - | - | -
| 30 | 15.3 | (330 $\pm$ 35) | (230 $\pm$ 15) | - | - | (181 $\pm$ 19) | (43 $\pm$ 2)
| 32.1 | 25.8 | - | (182 $\pm$ 22) | - | - | - | -
| 34.7 | 48.1 | - | (146 $\pm$ 12) | - | - | - | -
T = 40 ∘C
| 0 |  $<$ 3.2 | - | - | (651 $\pm$ 37) | (289 $\pm$ 15) | (277 $\pm$ 23) | (23 $\pm$ 2)
| 0.2-1 | 3.2 | (615 $\pm$ 20) | - | - | - | (289 $\pm$ 31) | (22 $\pm$ 2)
| 20 | 5.6 | (445 $\pm$ 20) | - | - | - | (253 $\pm$ 25) | (29 $\pm$ 3)
| 30 | 12.5 | (300 $\pm$ 20) | - | - | - | (245 $\pm$ 26) | (34 $\pm$ 3)
air-water | 31.3 | 18.0 | (255 $\pm$ 20) | (185 $\pm$ 20) | - | - | - | -
| 32 | 23.0 | (250 $\pm$ 35) | (190 $\pm$ 20) | - | - | (249 $\pm$ 23) | (35 $\pm$ 3)
| 32.2 | 24.5 | (235 $\pm$ 35) | (185 $\pm$ 20) | - | - | (167 $\pm$ 23) | (35 $\pm$ 3)
| 33.2 | 39.5 | (185 $\pm$ 15) | (159 $\pm$ 15) | - | - | - | -
| 33.2 | 40.1 | - | (162 $\pm$ 14) | - | - | - | -
| 0 |  $<$ 3.9 | - | - | (592 $\pm$ 45) | (274 $\pm$ 22) | (280 $\pm$ 30) | (27 $\pm$ 3)
| 0.2-1 | 3.9 | (540 $\pm$ 25) | - | - | - | (265 $\pm$ 25) | (26 $\pm$ 3)
| 20 | 5.2 | (470 $\pm$ 25) | - | - | - | (245 $\pm$ 24) | (36 $\pm$ 4)
decane-water | 30 | 10.3 | (320 $\pm$ 25) | - | - | - | (239 $\pm$ 20) | (44 $\pm$ 3)
| 32 | 17.0 | (270 $\pm$ 30) | (168 $\pm$ 20) | - | - | (230 $\pm$ 30) | (45 $\pm$ 5)
| 33.2 | 23.7 | (250 $\pm$ 35) | (158 $\pm$ 20) | - | - | (156 $\pm$ 30) | (49 $\pm$ 3)
| 34.2 | 40.5 | (187 $\pm$ 15) | (152 $\pm$ 15) | - | - | - | -
| 34.8 | 47.8 | - | (146 $\pm$ 15) | - | - | - | -
a values determined from phase images
b values determined from height profiles

We follow the evolution of the in-plane diameter of the microgels within the
monolayers at both interfaces more closely by plotting _NND_ as a function of
$N_{area}$ (Figure 5). Starting from regime II a confluent monolayer is
observed. The lateral compression with the barriers leads to a compression of
the microgel coronae until in regime III the isostructural phase transition
takes place (Figure 5A and B). Here, microgels in two phases coexist and the
number of microgels in the second phase constantly increases with compression.
Below the VPTT, regime IV is reached when all microgels have passed to the
second phase. This layer can be compressed further. The monolayers above the
VPTT do not display the regime IV and a direct transition from the third to
the fifth regime is observed where the monolayers fail. _NND_ -values at
characteristic interfacial concentrations are summarized in Table
LABEL:tab:tab_NND.

Figure 5: Mean nearest neighbor distances, _NND_ , versus number of microgels
per area, $N_{area}$, of pNIPAM microgels at the air-water and decane-water
interface. (A) T = 20 ∘C, air-water interface. (B) T = 40 ∘C, air-water
interface. (C) T = 20 ∘C, decane-water interface. (D) T = 40 ∘C, decane-water
interface. Shaded areas in the respective color represent the errors. Dashed
lines indicate the transitions between different regimes. The regimes are
labeled in roman numbers.

Although the transition from regime I to regime II takes places at larger
center-to-center distances, the isostructural phase transitions take place at
nearly the same _NND_ , which is in good agreement with the size measurements
of the single microgels (Fig. 1). For monolayers below the VPTT, the _NND_ at
the transition from regime II to III is between 360-380 nm and above the VPTT
between 250-270 nm. A different _NND_ for the air- and decane-water interfaces
is also observed in regime V, when the monolayer is fully compressed and
fails. Independent of temperature, the failure at the air-water interface is
observed at a _NND_ of 160 nm, in contrast to 145 nm at the decane-water
interface. This indicates that the amount of confined polymer per microgel
increases with increasing surface tension of the interface.

We also extract height profiles of microgels within monolayers to obtain a
more detailed picture of their conformation under compression. The profiles of
microgels at different compressions deposited from the air-water and oil-water
and below and above the VPTT are presented in Figure 6A-D. At the highest
surface pressures, the microgel cores overlap and multiple microgels are
shown. Here, $D_{core}$ was measured from the distance between the apices. The
resulting values for $D_{core}$ and $H_{core}$ are summarized in Table
LABEL:tab:tab_NND.

Figure 6: Averaged height profiles of microgels in dried state after
deposition to a solid substrate at different temperatures, interfaces, and
compressions. (A) T = 20 ∘C, air-water interface. (B) T = 40 ∘C, air-water
interface. (C) T = 20 ∘C, decane-water interface. (D) T = 40 ∘C, decane-water
interface. Different colors represent different compressions. The standard
deviation in height and length is given as shaded areas in the corresponding
color. For each profile at least 40 microgels were analyzed.

For all compressions, $H_{core}$ decreases from the decane-water to the air-
water interfaces at the same temperature and $H_{core}$ increases from
temperatures below to temperatures above the VPTT at the same top-phase. The
later is related to the de-swelling of the microgel portions situated in the
aqueous phase, although counter-intuitive. At the liquid interface, the
portions situated in the aqueous phase still undergo a volume phase
transition. At temperatures above the VPTT, they are de-swollen and display a
more compact conformation with a smaller lateral extension and thickness.
Since the amount of polymer within a microgel does not change during drying,
the resulting profiles are flatter and more laterally extended below the VPTT,
and higher and less laterally extended above the VPTT.

The decrease in $H_{core}$ from the decane-water to the air-water interface
suggests a more compact conformation of the microgel cores with a higher
polymer density at and near the air-water interface. The height profiles only
display the _ex situ_ structures and to fully understand the conformational
changes experiments which resolve the _in situ_ structure are needed.
Nevertheless, in addition with the aforementioned larger _NND_ in regime IV
(Fig. 5 and Tab. LABEL:tab:tab_NND), the smaller $H_{core}$ at air-water
interfaces gives a strong indication that the larger surface tension is
compensated by adsorption of more polymer segments from the core of the
microgels.

Lastly, the ordering of the interfaces shows no substantial difference and the
trend of $\Psi_{6}$ at the air-water interface (Figure S2A-B) and the decane-
water interface (Figure S2C-D) with compression is the same. Based on
$\Psi_{6}$ and the characterization presented above, there is no evidence that
a change in the top-phase influences the short- and long-range order of the
pNIPAM microgel monolayers. A more detailed discussion of the evolution of
both the hexagonal order parameter and the pair correlation functions, $G(r)$,
with compression can be found elsewhere. 30, 35

##  4 Conclusion

In this study, we compared of pNIPAM microgels at two different interfaces:
air-water and decane-water. Although, the two-dimensional phase behavior of
microgels at air-water or decane-water is well known, so far, only qualitative
comparison with the existing literature can be made. Here, we present a
systematic study of the impact of the top-phase and elucidate quantitative
differences between the confined microgels. The investigation is completed by
measurements of the microgels, not only below but also above their VPTT.

We show that the polymer network of microgels experience a different
stretching at the two interfaces. Because both air and decane are non-solvents
for pNIPAM, we attribute changes in their contact area to the differences in
surface tension. The total diameter ($D_{2D}$) of the microgels increases from
the decane- to the air-water interfaces, but the size of the microgel cores
($D_{core}$) is constant. We suggest that this is a consequence of the core-
corona structure of microgels at the interface and associated with one or with
a combination of three explanations: (i) in the center of a microgel the
polymer network gives more resistance to deformation due to the higher cross-
linker content, and experiences less stress due to the higher polymer content;
(ii) Attenuation of the stretching for the microgels’ fractions that are in
the aqueous phase due to the flexibility of the polymer network; and (iii)
Adsorption of more of the polymer network situated in the aqueous, _i.e._ ,
out-of the interfacial plane, instead of stretching the polymer network in-
plane.

We obtain strong evidences that more polymer of an individual microgels is
adsorbed to the air-water interface considering the height profiles (Fig. 6)
and the inter-particle distance (_NND_) close to the microgel monolayer
failure (Fig. 5, regime IV). For the decane-water interface, it has been
reported that the failure of the monolayer is independent of the temperature
and that there is a maximum number of polymer segments that can be confined at
or in close proximity of the interface. 35 When comparing the decane- and air-
water interface, this limit is reached at larger _NND_ for the air-water
interface. Additionally, height profiles of dried, deposited microgels are
flatter at air-water interfaces than at oil-water interfaces; suggesting that
the increase of interfacial tension is compensated by adsorption of more
polymer segments of a single microgel to the interface.

Furthermore, we use the obtained size of the individual microgels to estimate
their elastic modulus between oil-water and air-water interfaces. In
comparison to nanoindentation experiments normal to the interface, we
determine a significantly larger modulus of the microgels in-plane ($\approx$
23 MPa). This increase is attributed to both, the reduced amount of solvent in
the polymer network close to the interface and the restrain of the polymer
segments at the interface, in agreement with the literature. 57

$\Pi$-$N_{area}$ Isotherms at the air-water interfaces at temperatures below
and above the VPTT of the microgels show the same temperature-dependent
changes as at the decane-water interface. At both interfaces, the temperature-
dependent swelling perpendicular to the interface affects the compressibility
parallel to the interface resulting in an isotherm with a single increase.
Compared to the impact of temperature, the effect of the top phase (at the
same temperatures) is negligible.

In conclusion, our results unambiguously demonstrate that the two-dimensional
phase behavior of microgels, in terms of their microstructure and their
ordering, is not affected by having oil or air as top-phase. In contrast to a
recent study at benzene- and hexane-water interfaces, 36 a dominant impact of
the top-phase is observed for the in-plane stretching of the microgels corona,
while their more cross-linked core and the microgels’ portions situated in the
aqueous phase are unaffected.

##  5 Acknowledgement

The authors acknowledge financial support from the SFB 985 ”Functional
Microgels and Microgel Systems” of Deutsche Forschungsgemeinschaft within
projects A3 and B8. This work is based upon experiments performed at the KWS-2
instrument operated by Jülich Centre for Neutron Science (JCNS) at the Heinz
Maier-Leibnitz Zentrum (MLZ), Garching, Germany. We thank Marie Friederike
Schulte and Maximilian Schmidt for fruitful discussions.

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