Adsorption of microgels at an oil-water interface: correlation between packing and 2D elasticity.

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Adsorption of microgels at an oil–water interface: correlation between packing and 2D elasticity† Cite this: Soft Matter, 2014, 10, 6963

Florent Pinaud,a Karen Geisel,c Pascal Masse´,a Bogdan Catargi,e Lucio Isa,d Walter Richtering,c Vale´rie Ravaine*a and Ve´ronique Schmitt*b The aim of this paper is to determine how microgels adsorb at a model oil–water interface and how they adapt their conformation to compression, which gives rise to surface elasticity depending on the microgel packing. The structure of the film is determined by the Langmuir films approach (forced compression) and compared to spontaneous adsorption using the pendant drop method. The behaviour of microgels differs Received 13th March 2014 Accepted 24th April 2014

significantly from that of non-deformable particles but resembles that of linear polymers or proteins. We also correlate the properties of microgels spontaneously adsorbed at model interfaces to their forced

DOI: 10.1039/c4sm00562g

adsorption during emulsification. Finally we propose a route to easily control a posteriori the microgel

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packing at the surface of droplets and the flow properties of emulsions stabilised by the microgels.

I.

Introduction

Microgels are so colloidal particles made of poorly cross-linked polymers that can be swollen by a solvent. Among numerous applications, these particles have shown high potential as emulsion stabilisers.1–23 Depending on the nature of the polymer, they may also be stimuli-sensitive, bringing responsiveness to the emulsions. Temperature and pH are the main, but not the only, parameters used as triggers. Microgels composed, at least partially, of the well-known poly(N-isopropylacrylamide) pNIPAM polymer, which exhibits a lower critical solution temperature (LCST), are thermo-sensitive and they undergo a volume contraction when the temperature is raised above the volume phase transition temperature (VPTT). In order to get better insight into the understanding of the emulsion stabilisation mechanism, their adsorption at interfaces has attracted recent attention. Two main strategies may be considered; either the study of model planar air–water or oil–water interfaces or in situ study of the microgels packed at the surface of emulsion droplets.7,18–22 Recent advances using cryo-SEM observations showed unambiguously that emulsication leads to attening of the a

Université de Bordeaux, Institut des Sciences Moléculaires, ENSCBP, 16 Av. Pey Berland, 33607 Pessac Cedex, France. E-mail: [email protected]

b

Centre de Recherche Paul Pascal, CNRS, Université de Bordeaux, UPR 8641, 115 Avenue Dr Albert Schweitzer, 33600 Pessac, France. E-mail: schmitt@crpp-bordeaux. cnrs.fr

c Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, D-52056, Aachen, Germany d Laboratory for Interfaces, So Matter and Assembly, ETH Zurich, Vladimir Prelog Weg 5, 8093 Zurich, Switzerland e CBMN UMR 5248, Université de Bordeaux, Allée de Saint-Hilaire, 33600, Pessac, France

† Electronic supplementary 10.1039/c4sm00562g

information

(ESI)

This journal is © The Royal Society of Chemistry 2014

available.

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DOI:

initially spherical particles with a “fried egg-like” morphology attributed to their core–shell structure, inherent to their synthesis method.21 The deformed microgels pack densely and regularly at the oil–water interface. In some specic conditions such as low energy emulsication22 or, for thermally sensitive microgels, emulsication at a temperature above the VPTT followed by a temperature quench,19 they adopt a more compressed conformation with an even higher packing density. Their state of adsorption is of prime importance since emulsions' macroscopic properties such as stability and ow behaviour are determined by the way microgels adsorb at the oil–water interface. Adsorption studies at model uid interfaces have also shown the good affinity for microgels toward interfaces. FreSCa cryo-SEM observations have conrmed that microgel attening occurs also at model uid interfaces.12 The most widespread method to investigate interfacial microgel adsorption remains the measurement of interfacial tension: all the reported literature studies show a signicant interfacial tension decrease compared to the pristine interface.24–28 In most cases, authors were mainly interested in the temperature effect on the interfacial tension measurements.27,28 A minimum of the tension has been observed at a temperature corresponding to the VPTT. This result has been interpreted as follows: below the VPTT, the decrease likely originates from the adsorption of dense layers because of the decrease of the excluded volume interactions. Above the VPTT, the increase is thought to come from the adsorption of loosely packed microgels. Many authors attempted relating model interface surface tension values to emulsion stability.6,7,27 However the evidence was not conclusive and viscoelasticity seemed to be a more important parameter. Brugger et al.9 investigated the viscoelastic properties of the interfacial layer and nicely evidenced a good correlation between dilational interfacial elastic moduli

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and emulsion stability. Indeed for emulsions stabilised by pNIPAM-co-MAA microgels, a decrease of pH and additional heating well-above the VPTT led to emulsion breakage and separation of the oil from water while, at low temperature and high pH, the interface was highly elastic and the emulsion was stable. The relevance of visco-elasticity was reinforced by combining shear, dilational interfacial rheology as well as compression isotherms. The pH-dependent emulsion stability was strongly correlated with the visco-elastic properties of the pNIPAM-co-MAA microgel covered oil–water interface: at high pH when the microgels were charged, the interface exhibited a so gel-like behaviour, while at low pH, the interface covered by uncharged microgels was rather brittle. The investigation of compression isotherms of charged and uncharged microgels29 also revealed unexpected inuence of charges on the compression behaviour of microgel-covered oil–water interfaces. This highlighted the importance of microgel swelling on the packing density. This phenomenon could not be explained in detail. In the present study, we chose to work on simpler, neutral microgels to avoid the effects of electrostatics. In the present paper, we aim at relating interfacial properties such as surface tension, pressure and elasticity of neutral pNIPAM microgels to their conformation and packing at a model oil–water interface. The Langmuir compression isotherms allow determining the packing of the microgels under compression, whereas pendant drop experiments give information about their spontaneous adsorption. We conrm our view by direct visualisation through FreSCa cryo-SEM and we compare the results obtained on microgels to adsorbed solid particles, linear polymer and proteins. Finally, we attempt to correlate the morphology and packing of microgels spontaneously adsorbed at model interfaces to the ones of the same microgels forced to adsorb at emulsion interfaces through energy input during mechanical emulsication.

radical precipitation polymerisation classically employed for the synthesis of thermo-responsive microgels and especially pNIPAM microgels.30–32 Polymerisation was performed in a 500 mL three-neck round-bottom ask, equipped with a magnetic stir bar, a reux condenser, thermometer, and argon inlet. The initial total monomer concentration was held constant at 62 mM. NIPAM and BIS were dissolved in 98 mL of water. The solutions were puried through a 0.2 mm membrane lter to remove residual particulate matter. The solutions were then heated up to 70 C with argon thoroughly bubbling during at least 1 h prior to initiation. Free radical polymerisation was then initiated with KPS (2.5 mM) dissolved in 2 mL of water. The initially transparent solutions became progressively turbid as a consequence of the polymerisation process. The solutions were allowed to react for a period of 6 h in the presence of argon under stirring. To eliminate possible chemical residues, the microgels were puried by centrifugation–redispersion cycles at least ve times (21 000g for 1 hour, where g is the gravity constant). For each cycle, the supernatant was removed and its surface tension was measured by the pendant drop method. The purication was repeated until the surface tension of the supernatant reached a value close to the one of pure water, i.e. above 70 mN m1, showing that the microgel dispersions were free of surface active impurities.

II.

4. Langmuir lm

Experimental section

1. Chemicals All reagents were purchased from Sigma-Aldrich. N-Isopropylacrylamide (NIPAM) was recrystallized from hexane (ICS) and dried under vacuum overnight prior to use. N,N0 -Methylenebis(acrylamide) (BIS), potassium persulfate (KPS), n-decane (purity >99%), n-dodecane (purity >99%) for interfacial tension measurements and isopropanol were used as received. nDecane for isotherms (Merck, purity >94%) was ltered over basic Al2O3 three times prior to use. Milli-Q water was used for all synthesis reactions, purication, and solution preparation. 2. Particles synthesis and purication We synthesised uncharged pNIPAM microgels at different cross-linking densities (2.5 and 5 mol%, respectively) keeping their diameter at 25 C approximately constant at 650 nm (632 nm for 2.5% BIS and 680 nm for 5% BIS). This size was chosen because their morphology and packing at the surface of emulsion droplets has been previously well studied and is now well documented. The microgels were obtained by an aqueous free-

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3. Particle characterisation Particle sizes and polydispersity index were determined by dynamic light scattering (DLS) with a Zetasizer Nano S90 Malvern Instruments equipped with a HeNe laser at 90 . Hydrodynamic diameters were calculated from diffusion coefficient using the Stokes–Einstein equation. All correlogram analyses were performed with soware supplied by the manufacturer. The polydispersity index is given by the cumulant analysis method.

Compression isotherms were recorded using a Langmuirtrough (KSV NIMA trough: dimensions 54 mm 736.5 mm, 398 cm2 compressible area) that was modied for oil–water systems. Both trough and barriers were made of polyoxymethylene (Delrin). The surface pressure was recorded by a balance equipped with a platinum Wilhelmy-plate. Note that a paper plate was also tested and that no signicant differences were detected. The microgels were dispersed in a mixture of water and isopropanol (5 : 1). Prior to deposition, it was veried by DLS that the dispersion did not contain any aggregates. The suspension was then deposited using a Hamilton syringe. Aer a stabilisation time of 30 minutes, the barriers were displaced symmetrically at a constant speed of 10 mm mn1. The surface pressure was recorded as a function of surface area. The whole pressure-surface curve was obtained varying the concentration of microgels in the dispersion, owing to the nite size of the balance and limited displacement of barriers. This allowed reaching concentrated states. We checked that the two parts of the isotherm curves overlapped. All the experiments were carried out at 25 C.


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5. Dynamic surface tension


We determined dynamic interfacial tension of the n-decane– water interface using the pendant-drop technique (Teclis). An aqueous drop containing a known concentration of microgels is immersed in the oil. The tension is deduced from the axisymmetric drop shape by tting with the Laplace equation. A constant drop volume is maintained over 10 000 s and the tension is recorded as a function of time. In this method, microgels adsorb spontaneously at the interface. 6. Surface dilational rheology The mechanical response to compression–dilation of the interface is studied by the oscillatory drop method using a modied pendant drop apparatus (Tracker apparatus from Teclis). The aqueous drop containing the microgels is submitted to a sinusoidal variation in the drop surface area A ¼ A0 sin(2p) and the shape, and hence the interfacial tension g, are recorded. Using the complex notation, the Gibbs interfacial dilational elasticity is dened as: E* ¼ dg*/d ln A* where the complex modulus can be separated into an elastic modulus and a loss modulus corresponding to the real and imaginary parts of E*, respectively. As long as the amplitude of the area variation A0 is small enough, the elastic and loss moduli are constant (linear domain) and independent of A0. An aqueous solution of microgels at a given concentration is injected via an airtight syringe (SGE Analytical Science) in 3 mL of n-decane or n-dodecane, contained in a quartz cell. The volume of the created drop is 6 mL. The dilational moduli were measured by performing sinusoidal oscillations of the drop volume at low frequencies (0.2 Hz). We always checked that the applied area variation belongs to the linear domain (about 12% of relative area variation). We also veried that the surface pressure and the elasticity were independent of the alkane chain length (decane or dodecane). 7. Freeze-fracture shadow-casting cryo-SEM Freeze-fracture shadow-casting (FreSCa) cryo-SEM is a recently developed inspection method that allows resolving the threedimensional position of colloidal particles trapped at a millimetre-sized planar oil–water interface, and thus extracting information of their conformation and wetting at the nanoscale. The details of the method have been described elsewhere.33,34 Briey, the technique involves the vitrication (ashfreezing) of a model particle-laden oil–water interface using liquid propane jets (Bal-Tec-Leica JFD 030, Balzers-Vienna), followed by fracture, unidirectional metal coating (Bal-TecLeica BAF060) and imaging in an SEM in ultra-high vacuum cryo conditions (Zeiss Gemini 1530, Oberkochen), without the use of a replica. The samples were prepared using aqueous dispersions of the microgels at a controlled particle concentration injected in a custom-made copper sample holder and covered by n-decane. In this technique, the microgels adsorb


spontaneously at the interface and no spreading solvent is necessary. 8. Imaging of the microgels at the interface of emulsion drops Typical emulsion batches were composed of 14 g of aqueous phase containing microgels and 6 g of dodecane. This mixture was then stirred by an Ultra-Turrax T25 mixer, at constant speed (9500 rpm) for 30 s (shear rate of 18 103 s1). Emulsication was carried out at 25 C. We checked that stirring in such conditions did not induce a temperature raise above the microgel VPTT. Emulsions were usually prepared exploiting the limited coalescence process. The system was emulsied with a low amount of particles (0.03 wt%) so that the newly created droplets are insufficiently protected by the particles. In this case, the drops coalesce aer emulsication and their nal diameter is determined by the amount of microgels that provide sufficient protection against further coalescence (all the microgels are adsorbed at the oil–water interface). Immediately aer emulsication, the centre-to-centre distance between the microgels at the oil–water interface was determined by the method described below. Then, the emulsion was divided into two batches. In one of the batches, the aqueous sub-phase was replaced by a suspension containing a large amount of microgels (Cmicrogels ¼ 1 wt%). The two batches were aged for 5 days before further observation. Cryo-SEM observations were carried out with a JEOL 6700FEG electron microscope equipped with liquid nitrogen cooled sample preparation and transfer units. A small amount of emulsion was rst deposited on the aluminum specimen holder. The sample was frozen in the slushing station with boiling liquid nitrogen. The specimen was transferred under vacuum from the slushing station to the preparation chamber. The latter was held at T ¼ 150 C and P ¼ 105 Pa and is equipped with a blade used to fracture the sample. Once fractured, the sample was coated by a layer of Au–Pd and was then inserted into the observation chamber equipped with a SEM stage cold module held at 150 C. Dodecane was used for cryoSEM observations because of its low melting temperature (9.6 C) that avoided oil crystallization during the freezing step. Moreover dodecane was amorphous in the solid state, so the droplet interfaces remained spherical and smooth.

III.

Results and discussion

1. Surface pressure In a rst experiment, a 10 mL drop of a dispersion containing 1 wt% of microgels (containing isopropanol) with 2.5 mol% of cross-linker was deposited on the decane–water interface of the Langmuir trough. Aer stabilisation, the surface area was decreased by barrier compression and the surface pressure increased up to 30–35 mN m1 where it seemed to reach a plateau. The experiment was repeated with increasing volumes or/and increasing concentration of microgel dispersion, up to a 60 mL drop of a 2 wt% solution of microgels. By increasing the amount of microgels at the interface, the pressure exhibited a

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second transition and reached a second plateau at 37–38 mN m1. The various isotherms collapsed into a single one that is plotted in Fig. 1a. This isotherm differs signicantly from isotherms obtained with hard non-deformable particles. In this latter case, the pressure is zero at high area per particle and diverges upon compression due to steric or electrostatic repulsions between particles.35,36 Assuming that all microgels are adsorbed at the interface whatever the initial concentration, the data from Fig. 1 may also be expressed in area per particle (secondary abscises axis in Fig. 1a). For that purpose, the number of microgels in the dispersion should rst be calculated. Following the work published by Lele et al.,37 we considered that a microgel particle is composed of 71 wt% of polymer and 29 wt% of bound water at 50 C. From the hydrodynamic particle diameter, dHT¼50 C,

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measured by dynamic light scattering at 50 C, the particle number nparticles was calculated as: nparticles ¼

6mpolymer

pðdHT¼50 C Þ3 rpolymer

dCC ¼

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þ

0:29 0:71rwater

(1)

where mpolymer is the mass of dried microgels, rpolymer ¼ 1.269 g cm3, and rwater ¼ 0.988 g cm3 are the polymer and water densities at 50 C, respectively. For the 2.5 mol% cross-linked microgels dHT¼50 C ¼ 234 nm. From Fig. 1a, several regimes labelled I to V may be distinguished. In the rst regime (I) for large surface areas, microgels behave as a gas of non-interacting particles leading to the absence of interfacial pressure. The second regime (II) corresponds to a continuous increase of the interfacial pressure classically denoted as expanded liquid. In a third regime (III) the pressure becomes constant while decreasing the interfacial area. Classically for surfactants, this plateau has been described as the coexistence between expanded and condensed liquids. Then, a second increase (IV) followed by a second plateau (V) are observed. In order to get better insight into the microgel conformations in these various regimes, the isotherm could also be plotted as a function of the inter-particle distance. The area per particle can be transformed into a centre-to-centre distance dCC assuming a 2D hexagonal packing using:

Fig. 1 Evolution of the surface pressure as a function of (a) normalised surface area and (b) centre-to-centre distance between microgels at the interface (assumption of hexagonal lattice). Microgels contain 2.5 mol% BIS. I–V domains refer to different packing states which are commented in the text. The arrows in Fig. 1b show the microgel diameters in solution (swollen and collapsed state) and the characteristic distances at the oil–water interface, as measured on emulsion drops prepared in the limited coalescence regime.

1

1=2 2a pffiffiffi 3

(2)

where a is the area per microgel. The pressure can hence be plotted as a function of dCC (Fig. 1b). The regimes are now delimited by various centre-to-centre distances noted dCCI/II, dCCII/III, dCCIII/IV, dCCIV/V in the following. On the same curve, the known characteristic distances can be positioned: the hydrodynamic diameter at 25 C dHT¼25 C, at 50 C dHT¼50 C, the centre-to-centre distance of attened microgels at the surface of emulsion drops dCC (emulsion) as well as their core diameter obtained from cryo-SEM observation dCore (emulsion).21 Analysing this new representation of the adsorption isotherm allows for an interpretation of microgel deformation and packing at the interface during compression. The various stages of the proposed evolution are sketched in Fig. 2. Fig. 1b shows that the pressure grows from zero at inter-particle distances much larger than the diameter of the swollen microgels in bulk dHT¼25 . By a simple comparison between the regimes' boundaries and the size of attened microgels at the interface, obtained from cryo-SEM of emulsion droplets surface, the pressure appears to increase when peripheral shells of the attened microgels come into contact, as schematically represented in Fig. 2b. The increase of pressure corresponds then to interpenetration of the microgel shells (Fig. 2c and d). Regime II corresponds to distances of the order of the diameter of the attened cores; in this regime the pressure keeps increasing until the size of non-deformed microgels is reached (dccII/III z dHT¼25 C) and then it reaches a plateau. A constant pressure in the plateau means either, that the adsorbed surface density of anchored polymeric chain is constant, or that two packing states coexist. In the rst hypothesis, as the area decreases, the number of


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Fig. 2 Scheme describing the various packing states in the Langmuir film, showing top view and side view of the interface. The microgels are represented as core–shell objects (the microgels are in blue, the cores in black and the shells in grey). (a) Dilute state, the microgels are randomly distributed on the surface, without any lateral interaction; the surface pressure is zero; the cores of the microgels are flattened, the shells are extended on the surface; (b) at the transition between domain I and II, the microgels are packed in a hexagonal array made of flattened microgels, they start interacting through the shells, the surface pressure starts to increase; (c and d) the shells start interpenetrating, the surface pressure increases; the density of adsorbed segments increases (the number of adsorbed segments is constant and the surface area decreases); (e) the cores start interpenetrating; (f) the cores are compressed and the film thickness increases; the surface pressure increase is less pronounced because the number of adsorbed segments decreases. For the side view, despite the lack of experimental evidence we chose to represent microgels with a non symmetric morphology with respect to the water–oil interface.

adsorbed segments decreases progressively and proportionally to the area reduction. In the second hypothesis, the two coexisting states correspond to highly compressed microgels with an inter-particle distance equal to the hydrodynamic diameter of dehydrated microgels at 50 C (dccIII/IV z dHT¼50 C) and to less compressed microgels with an inter-particle distance equal to the hydrodynamic diameter at 25 C (dccII/III z dHT¼25 C). In any case, as particles are deformable, in the plateau, microgels deform perpendicularly to the interface, i.e. if the thickness of the adsorbed layer increases as sketched in the transition from Fig. 2e and f. A second increase of pressure arises for dccIII/IV z dHT¼50 C, showing that in this regime, the microgels are in a contracted state. At this stage, it becomes very difficult to compress and therefore dehydrate the microgels further, either by a temperature increase or by mechanical


compression. The maximal compressibility of adsorbed microgels is therefore reached and it is more favourable for the microgels either to desorb into the aqueous sub-phase or to form multilayers. The second plateau likely corresponds to a strong desorption, i.e. monolayer collapse, formation of multilayers or irreversible buckling of the surface. Indeed, aer reaching this pressure, the compression–expansion curve of the surface pressure shows a hysteresis (Fig. S1†). In order to conrm the generality of the proposed concepts, we repeated the surface pressure isotherms also for the 5 mol% cross-linked microgels (Fig. 3). Both isotherms are superimposed in the regime of low pressures (large inter-particle distances) but differ signicantly upon compression. It is worth emphasizing that by changing the cross-linking density, the mechanical properties of the shell are not changing

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From the measurement of g(t), the spontaneous surface pressure at time t can be deduced using:


pspontaneous(t) ¼ g0 g(t)

Influence of the cross-linking density on the packing in the Langmuir film. Evolution of the surface pressure as a function of centre-to-centre distance between microgels at the interface (assumption of hexagonal lattice). Microgels contain 2.5 mol% (black) and 5 mol% BIS (red).

Fig. 3

signicantly in agreement with previous observations made on microgels adsorbed at the interface of emulsion drops. Conversely, the transitions from III to IV and from IV to V domains are shied toward larger centre-to-centre distances for microgels with the higher cross-linking density. The compressibility of the more cross-linked microgels is thus lower than the compressibility of the microgels with lower crosslinking density.

(3)

and it is plotted in Fig. 4b. At equilibrium (long times), the spontaneous pressure is equal to pequilibrium ¼ 35 mN m1. A similar behaviour is obtained for 5 mol% cross-linker in a different concentration range since the adsorption kinetics is slower13 (Fig. S2†). An estimation of the pressure exerted by a 2D gas made of non deformable 650 nm-sized particles leads to a non measurable value of the order of 103 mN m1, very different from the real lm pressure. It can therefore be deduced that, spontaneously, at equilibrium, irrespective of the bulk concentration, microgels are compressed at the interface. From comparison with the packing in a Langmuir lm (Fig. 1), it can be deduced that microgels are also deformed perpendicularly to the interface. At this stage, the pressure is equivalent to the rst plateau (domain III) in Fig. 1. In order to determine whether the adsorption is reversible or not, we performed the following experiment: an oil drop is formed in an aqueous phase comprising microgels that adsorb as evidenced by a strong decrease of the surface tension. Once the stationary state is reached, the aqueous phase is replaced several times with pure water using extreme care to avoid the detachment of

2. Dynamic surface tension In order to determine the deformation and packing of spontaneously adsorbed microgels, we performed interfacial tension measurements as a function of time for suspensions with different microgel bulk concentrations (Fig. 4a). Without microgels, the dodecane–water pristine interface showed a constant interfacial tension g0 of 53 mN m1, compatible with tabulated values. When microgels were present in the aqueous phase at a sufficient concentration (Cmicrogel > 0.01 wt%), the interfacial tension g(t) decreased continuously with time until reaching a nite value gequilibrium of the order of 17 mN m1 irrespective of the bulk concentration Cmicrogel. Such a kinetic evolution demonstrates the spontaneous particle adsorption, this latter being faster as the concentration increases. Even if the nal interfacial tension is not as low as for surfactants (of the order of several mN m1) the tension reduction is signicant. It is worth noticing that the equilibrium value is independent of the concentration, only the kinetics is affected. This behaviour is unusual for particles since the equilibrium value is usually a decreasing value of the particles concentration for air– water38 and oil–water39 interfaces as well as for an oil–oil interface.40 Conversely, this behaviour is typical of pNIPAM polymer25,26,41 and has also been observed for a peptide.42 It reveals a cooperative behaviour of microgels and the existence of attractive interactions at the interface.

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Influence of the microgel concentration on the dynamic surface tension (a) and calculated spontaneous surface pressure (b) for a given cross-linking density (2.5 mol% BIS).

Fig. 4


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the oil drop. No increase of the surface tension could be detected, showing that microgels do not spontaneously desorb.


3. Surface elasticity We seek determining the surface elasticity conferred by the microgels in their different adsorption states i.e. at various surface pressures. For that purpose, we can take advantage of the spontaneous variation of g or equivalently the kinetic evolution of pspontaneous (Fig. 4a and b). Measuring the viscoelasticity at different times therefore corresponds to measuring it at different pressures. The same pressure range is accessible for different microgel concentrations with different adsorption kinetics. For concentrated microgel suspensions, the adsorption kinetics is so fast that measuring the elasticity in the various compression states is not possible. For the 0.01 wt% microgel suspension, the time scale is very favourable. We measured dilational visco-elasticity measurements by applying area oscillations to the pendant drop aer a variable delay. The obtained results are reported in Fig. 5 for the elastic and loss moduli. The interfacial layer exhibits mainly a solid behaviour as the elastic modulus is about ten times larger than the loss modulus. As we have measured the spontaneous surface pressure as a function of time, it becomes possible to plot (time parametric curve) the surface elasticity as a function of the pressure up to about 32 mN m1 (Fig. 6a). We veried that surface elasticity measurements performed with different microgel concentrations over different time scales collapsed into a single curve when converted into surface pressure (Fig. S3†). The same results were obtained for more cross-linked microgels (Fig. 6). The surface elasticity exhibits a marked maximum as a function of the pressure, independent of the degree of crosslinking. The maximal value of 55 mN m1 occurs at a pressure of approximately 10–20 mN m1. The surface pressure can be converted into packing parameter, as calculated from Fig. 1b. Thus, it can be seen that the maximal elasticity occurs at distances comprised between 800 and 1000 nm (Fig. 6b). This corresponds to the attened conformation of the microgels.

Fig. 6 Elastic modulus as a function of surface pressure (a) and center-to-center distance (b): 2.5 mol% BIS (Cmicrogels ¼ 0.01 wt%, black diamond); 5 mol% BIS (Cmicrogels ¼ 1 wt% or 0.5 wt%, red square).

Note that an estimate of the surface elasticity can also be obtained from compression isotherms (see Fig. S4†) through EG ¼ dp/d Ln A. The curves look very similar to the ones obtained from dynamic surface tension showing the consistency of both methods. This also highlights the fact that the two monolayers are very similar in the regime below 30 mN m1 (low compression). 4. Discussion

Elastic (full black squares) and loss (hollow blue squares) moduli as a function of time for microgels (2.5 mol% BIS) at 0.01 wt%.

Fig. 5


(a) Microgel morphology and packing from spontaneous adsorption. We can attempt correlating the surface elasticity to the microgel morphology and packing at the interface by comparing the results obtained by the various techniques. Microgels adsorb spontaneously and adopt a compressed morphology as can be deduced from the high interfacial tension decrease. Therefore the surface pressures measured aer compression or by dynamic interfacial tension are comparable up to regime III dened in Fig. 1. For low surface pressure, elasticity is very low because microgels pack as a gas of noninteracting particles. The elasticity increases when the peripheral chains are in contact and interpenetrate forming a 2D gel

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(regime II). It reaches a maximum when the attened less deformable cores come into contact and begin to compress (regime II). Then, for larger pressures, the 2D elasticity decreases when cores deform perpendicularly to the interface compensating for the area reduction (regime III). In order to check that microgels adsorb in a quite compressed state at a model interface, we performed FreSCa cryo-SEM experiments as detailed in Section II 7. It is worth emphasizing that, in this experiment, microgels initially in the aqueous sub-phase spontaneously adsorb at the interface. The centre-to-centre distance that can be deduced from the image as the one reported in Fig. 7a is equal to 540 nm and thus smaller than the hydrodynamic diameter (632 nm) showing that the microgels are compressed and densely packed at the interface. Other FreSCa images show that, during spontaneous adsorption, different surface coverage may coexist at a given time (See Fig. 7b). In the bottom right corner of the image, we see that at low local microgel concentration, the shells do not overlap and the particles are attened. In the top le corner of the image,

Fig. 7 Spontaneous adsorption at the flat water–decane interface as visualised by FreSCa cryo-SEM (sample 5 mol% BIS, Cmicrogels ¼ 0.3 wt%). The top image (a) shows that high density regular packing is achieved. The bottom image (b) shows the coexistence of two concentrations at the interface during the adsorption. On the bottom right side, dCC is 1100 nm, the microgels are flattened and the shells do not interact; on the upper left side, dCC is 860 nm and the shells are partially less flattened (the hydrodynamic diameter in solution is 632 nm). Scale bar is 2 mm.

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the particles are in closer packing and the shells are compressed, corresponding to a local inter-particle distance of 860 nm. (b) Comparison of microgels with other adsorbed species. At this stage, it becomes interesting to discuss the adsorption of microgels with regard to other species like surfactants, hard particles, linear polymers or proteins. Unlike surfactants, microgel adsorption does not result from an equilibrium between surface and bulk. Even if microgels adsorb spontaneously, a large barrier has to be overcome for desorption to occur. Microgel covered interfaces also behave very differently from hard particle layers in that they may fracture and then heal.38,43,44 Conversely, many similarities are observed with linear polymers or proteins. In particular, literature about linear pNIPAM polymers offers many data for comparison. It is well-known that the partial hydrophobicity of pNIPAM, responsible for its solution behaviour, is also the driving parameter for its adsorption at a liquid interface (either air–water or oil–water interface). Many studies have already reported the behaviour at the air–water interface,24 including the dynamics of its adsorption,26,45,46 the thickness of the adsorbed lm,41,47 the surface dilational rheology48,49 or surface shear rheology.50 Most of these studies focused on the effect of temperature at the phase transition. At the air–water interface, pNIPAM adsorbs to form a layer which reduces the interfacial tension to around 40 mN m1, meaning a surface pressure of 32 mN m1, which is not very different from that obtained previously at the oil–water interface5,27 and in the present work. Thus, a parallel can be drawn between the behaviour of the particles of cross-linked pNIPAM and that of the linear homolog. The similarities of their behaviour are particularly striking regarding the dilational elasticity measurements.48 Indeed, the authors have measured the dilational elasticity of aqueous pNIPAM solutions, as a function of time and concentration. In a dilute concentration range (109 to 104 wt%), they could show that the kinetic dependency was non-monotonic and presented a maximum at a given time of the adsorption. A simultaneous measurement of the lm thickness, by ellipsometry, provided information about the organisation of the lm. At short time scales, the chains adsorb individually, lying at at the interface. Over time, more and more chains adsorb. Their packing density increases until they lack of space and form loops protruding in the solution. The maximum of elasticity occurs before loops formation. Indeed, when the loops are formed in the sub-phase, the response of the interface to compression–dilation may be lower due to the possibility to exchange polymer segments between the surface (proximal zone) and the underlying zone (distal zone), thereby easily relaxing surface stresses. This behaviour is not specic about pNIPAM polymers but is general of amphiphilic ones, like PVP or PEG,51 although pNIPAM presents the highest elasticity among all. Therefore, the surface elastic behaviour of the microgels is strongly correlated to that of the constituting polymer since the maximum of elasticity is in the same range than that of the linear pNIPAM (55 mN m1 compared to 60 mN m1). Thanks to the microgel structure, we have the opportunity to calculate the packing parameter and indirectly measure the evolution of lm thickness. We nd that the elasticity decreases


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for distances below 800 nm (Fig. 6b). At this distance, the cores start interpenetrating. On the Langmuir isotherm (Fig. 1b), the surface pressure increases less aer this value, implying desorption of polymer segments. Thus, the maximum of elasticity is also strongly correlated to the increase of the lm thickness by expansion of the polymers in the sub-phase, as depicted in Fig. 2f. In conclusion, microgels behave very similarly to linear pNIPAM in their spontaneous adsorption behaviour. The presence of cross-linking points does not seem to affect strongly the mechanical properties of the monolayer, at least under moderate dilation–compression disturbances (until p z 32 mN m1 i.e. end of regime III). We expect that the crosslinking density will affect the layer mechanical properties only at higher surface pressure (regimes IV and V). Moreover shear rheology is likely more adapted to detect the inuence of crosslinking.52,53 A similar behaviour has also been reported for some proteins. For example, Freer and co-workers54,55 studied extensively the hexadecane–water surface elasticity in presence of both a globular (lyzozyme) and a exible disordered protein (b-casein). In both cases, the elastic modulus exhibited a maximum as a function of the surface pressure. For the exible protein, the authors proposed that the maximum at about 40 mN m1 occurred through the protein conformation transition from mostly train to mostly loops and tails also in agreement with the observation of Noskov et al. for b-casein.56 Beyond a pressure of 29 mN m1, the monolayer collapsed and protein molecules were squeezed into the sub-phase. In the case of lysozyme, the protein was initially adsorbed in a compact globular state, resembling the native bulk conformation, the elasticity increased with surface concentration and reached a maximum value close to 78 mN m1. This large magnitude of the elasticity was attributed to the rigidity of the adsorbed protein molecule, stabilized by strong hydrophobic interactions and disulde bridges. The structural transitions included a change from globular to partially unfolded and to partially unfolded aggregated conformations. The elasticity decrease at higher surface pressures is believed to occur through partial unfolding and subsequent conformation changes that result in loss of intrinsic rigidity of the protein molecule.57,58 Unlike b-casein, a collapse pressure was not observed, lysozyme being more strongly bound to the interface through the unfavourable interactions of the exposed hydrophobic groups with the bulk solution and to inter-protein aggregation. More recent observations by Noskov et al.59 on bovine serum albumin (BSA), a globular protein, reinforce the role of protein interfacial denaturation on the interface elasticity. Only in presence of a large amount of guanidine hydrochloride, inducing fast adsorption and unfolding, a maximum in the surface elasticity can be observed. It can therefore be concluded that so particles such as microgels present strong common features with exible proteins or globular proteins that are able to adapt their conformation at the interface (denaturation). This shows that the exibility of the microgels allows them making loops and tails, and deforming at the interface. When packing is not too high, the network structure does not play a major role.


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Conversely, at high surface pressures, i.e. the domains IV and V of the Langmuir lm, the centre-to-centre distances strongly depend on the cross-linking density. This second phase transition, reached only aer strong compression by the barriers, occurs at lower surface area when the cross-linking density increases. The higher the cross-linking density, the lower the layer compressibility. No such transition has been reported for Langmuir lms made of linear pNIPAM chains.60,61 (c) Comparison with microgels adsorbed in emulsions. We can also compare the morphology of microgels at a model interface and at the surface of drops in emulsions. It is very surprising that microgels spontaneously adsorb in a compressed state while they are highly attened at the same dodecane–water interface of an emulsion. However, in the case of emulsions, the adsorption follows a different pathway, as they required mechanical energy to be produced. Moreover, imaging of the adsorbed microgels at drop interface was most oen obtained taking advantage of the limited coalescence process, that is to say, in a particle poor regime compared to the initial interfacial area created by emulsication. For such low amounts of particles, the drops were not protected enough and hence coalesced reducing the interfacial area while keeping the same amount of adsorbed particles. Indeed limited coalescence requires the existence of strong particle anchoring at the interface. Coalescence stopped when drops were sufficiently protected. Therefore, in this situation, the interfacial area adapted to the amount of particles. From the microgel conguration, it seems that drops are sufficiently protected as soon as covered by a thin layer of polymer i.e. as soon as shells come into contact, the monolayer exhibiting a low but non zero surface elasticity. In this particle-poor regime, no excess of microgel is present in the aqueous continuous phase. Conversely, in the pendant drop experiment, a large excess of microgels is present compared to the small interfacial area and may adsorb at long times: microgels compress themselves in order to increase the amount of adsorbed particles to lower the overall free energy. Adsorption stops when the energy gain per new adsorbed particle is surpassed by the energy penalty paid to compress the monolayer, as recently proven for monolayers of core–shell nanoparticles.62 To allow comparison, we therefore chose to add, a posteriori, microgels in the aqueous phase of an emulsion obtained by limited coalescence. In other words, we chose to ll in the “reservoir” to determine if the attened microgels conformation resulting from stirring could spontaneously evolve towards a more compressed morphology of microgels with an increasing number of adsorbed particles. The results are reported in Fig. 8. The centre-to-centre distance observed by cryo-SEM just aer stirring or aer 5 days without excess of microgels is much larger than the hydrodynamic diameter in suspension (dCC/dHT¼25 C ¼ 1.58) while, aer 5 days incubation in presence of 1 wt% excess microgels in the continuous aqueous phase, the ratio has decreased to 0.96. It is also worth noticing that the ow property of the emulsion has evolved concomitantly since the emulsion, just aer preparation, was highly occulated while it ew more easily aer incubation. This is in agreement with our previous observations about the emulsion properties and the density of microgels

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Fig. 8 Influence of the incubation of emulsion in the presence of excess microgels. The emulsion was prepared in the limited coalescence regime (7 mL of dodecane and 13 mL of aqueous solution containing 0.03 wt% microgels). The aqueous phase was replaced by an aqueous solution containing 1 wt% of microgels and incubated for 5 days. (a) CryoSEM images of the oil–water interface after partial sublimation of the oil and water phase for better visualisation; the microgels are smaller than their initial size because they are partially dehydrated, however, their inter-distance is the same as in non-sublimated samples; the inset shows the emulsion without excess microgels after 5 days; the magnification is the same for both images (scale bar is 2 mm); (b) and (c) are macroscopic views of the emulsion without additional microgels (b) and with excess microgels (c), showing the evolution of the flow properties.

adsorbed at the interface.18–22 Indeed we suggested that bridging is inhibited when microgels are densely packed at the interface while it is favoured for more attened microgels.23 In this latter case, some microgels, deformed perpendicularly to the interface, bridge the two monolayers of neighbouring drops.19 The observed evolution of microgels packing requires spontaneous compression and adsorption of microgels, as evidenced throughout the paper. As pointed out above, this happens owing to attractive interactions between pNIPAM chains in bad-

solvent conditions at the interface. This feature is shared by model surfaces and drop surface of emulsions. The spontaneous de-occulation of emulsions in the presence of an excess of microgels additionally needs some bridging microgels to desorb from one of the two interfaces. This likely occurs through changes of polymer chains of the microgels similarly to denaturation of adsorbed proteins with no mechanical energy supply. Emulsication can also be performed with an initial excess of microgels (1 wt%). In this case, emulsions are polydisperse and the drop surface observed by cryo-SEM experiments is highly covered by compressed microgels (see Fig. 9). These observations show that emulsication leads to similar trends to model interfaces.

IV. Conclusion

Fig. 9 Cryo-SEM image of an emulsion obtained in a microgel-rich regime (1 wt%).

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Microgels adsorb spontaneously and quasi irreversibly at model interfaces and in case of an excess of particles, they reach more dense packings even if it means that they are compressed. They behave very differently from non-deformable particles but resemble linear polymers or protein that may adapt their conformation at the interface. In the spontaneous adsorption regime i.e. for moderate compressions, we could not observe any signicant inuence of the cross-linking density. Conversely, the cross-linking density affects the concentrated regime that can only be reached by forced mechanical compression. A more systematic study of the effect of crosslinking on the packing and monolayer visco-elasticity is under current investigation. Indeed shear rheology should provide


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more appropriate information to distinguish the respective contributions of the entanglements originating from the shell interpenetration and of chemical cross-linking. In case of emulsions, obtained through energy input in a particle-poor regime, microgels adsorb in a attened morphology and prevent drops from coalescence. High interpenetration of shells or microgel compressions are not required to inhibit coalescence and low surface coverages are sufficient to ensure emulsion stability. When microgels are added in the continuous phase, the spontaneous adsorption coverage is recovered and emulsion plug ow may be reduced. This study sheds some new insights on the interplay between adsorption, microstructure and mechanical properties of interfaces stabilised by so objects. Our conclusion have the potential to help experimentalists to control independently the drop size through limited coalescence and the surface density of adsorbed microgels leading to less occulated emulsions.

Acknowledgements The authors thank the Aquitaine Region Concil and the Institut Polytechnique de Bordeaux for nancing Florent Pinaud's thesis. They also thank Mathieu Destribats for fruitful discussion and V´ eronique Lapeyre, Eric Laurichesse and Elisabeth Sellier for technical assistance, in microgel synthesis, pendent drop experiments, cryo-SEM imaging respectively. Lucio Isa acknowledges nancial support from the Swiss National Science Foundation grants PP00P2_144646-1. The FreSCa cryoSEM work was carried out at the ETH Electron Microscopy Center (ScopeM). KG, FP and WR thank the Deutsche Forschungsgemeinscha for support within the collaborative research center SFB 985 “Functional microgels and microgel systems”.

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