Particle-stabilised foams: an interfacial study

Transcription

1 PAPER Soft Matter Particle-stabilised foams: an interfacial study Antonio Stocco,* a Wiebke Drenckhan, a Emanuelle Rio, a Dominique Langevin a and Bernard P. Binks b Received 20th January 2009, Accepted 19th March 2009 First published as an Advance Article on the web 24th April 2009 DOI: /b901180c In an attempt to elucidate the remarkable stability of foams generated from dispersions of partially hydrophobic nanoparticles (fumed silica), we present investigations into the static and dilational properties of the gas liquid interfaces of such dispersions. By relating the dynamic surface tension g(t) and the dilational elasticity E measured using an oscillating bubble device, we confirm that the Gibbs stability criterion E > g/2 against foam coarsening is fulfilled. We complement these studies using ellipsometry and Brewster angle microscopy, which provide evidence for a pronounced adsorption barrier for the particles and a network-like structure in the interface at sufficiently high concentrations. We observe this structure also in freely suspended films drawn from the same particle dispersions. 1. Introduction It is well known that Pickering emulsions, 1,2 i.e. emulsions stabilized solely by partially hydrophobic particles of nano- or micrometre size, are remarkably stable. More recently, it has been demonstrated also that foams can be stabilized by particles. 3,4 Colloidal particles play, in particular, a key role in the stabilization of metallic foams 4 since the more classical foaming agents, such as surfactants or polymers, degrade at the temperatures required for the foaming process. Several research groups are now working on aqueous foams stabilized solely by particles 3,5 10 or on individual particle-coated bubbles. 11,12 All the existing studies confirm that the particle layer at the gas liquid interface forms a colloidal armour which inhibits the two main ageing mechanisms of the foams: bubble coalescence (film rupture) and coarsening (exchange of gas between bubbles due to differences in Laplace pressure) which are often completely stopped. This leads to super-stable foams (lifetimes of months!), provided that the foaming medium contains a sufficient amount of particles. 5,8 10 The shape and size of the stabilizing particles can vary significantly, ranging from micrometre sized particles 6 to nanometric aggregates 10 or even rods. 7 Recent investigations revealed a strong correlation between the hydrophobicity of the particles and foam stability. 8,13 For example, in the case of the silica particles used in this article, a clear maximum in foamability is found at a particle hydrophobicity of 34%, expressed as the percentage of unreacted SiOH groups after silanisation. 8,10 Previous research indicates that a key physical parameter underlying the origin of foam stabilization by particles is the dilational elasticity E of the particle-coated bubble surfaces. 8 A general argument, provided by Cervantes-Martinez et al. 8 and based on an analysis by Gibbs, 14 proceeds as follows: foam coarsening occurs because the derivative of the bubble capillary pressure P with respect to the bubble radius R is negative a Laboratoire de Physique des Solides, Universite Paris-Sud, F Orsay Cedex, France b Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull, UK, HU6 7RX (dp/dr ¼ 2g/R 2 < 0). This is generally the case as in most systems the surface tension g is independent of the bubble size. Solid particles at a gas liquid interface, however, have such high desorption energies 5 that their number can be considered fixed, meaning that their concentration (and therefore the surface tension) varies with the interfacial area A (and hence the bubble size). This provides an interfacial, dilational elasticity E ¼ dg/ dln(a), and the derivative of the bubble capillary pressure can now be written as dp/dr ¼ 2g/R 2 +4E/R 2. Hence, a bubble becomes stable against coarsening when E > g/2, which is called the Gibbs stability criterion. 14 Safouane et al. 13 and Zang et al. 15 measured g and E for particle monolayers obtained by spreading particles dispersed in alcohol on water surfaces and the measurements confirm the validity of this criterion. The behaviour of foams stabilized solely by silica particles was investigated based on two different aspects: the effect of the particle hydrophobicity and of the particle concentration on the foamability of the aqueous dispersions. 8,10,13 It seems likely that particles stabilize foams due to the surface elasticity E of the particle-coated bubble surfaces. Cervantes-Martinez et al. showed that by increasing the particle concentration from 0.1 to 0.7 wt.%, foam coarsening could be entirely prevented over the duration of the experiment, which was attributed to the effect of the surface elasticity. The bubble size of polydisperse foam was monitored by a multiple light scattering technique, and no significant change was detected during 10 h for dispersion concentration $0.7 wt%. Also, photographic and optical images of foam samples left in sealed containers displayed no remarkable variations in volume and macroscopic structure over several months. 8 The properties of adsorbed particle layers have been investigated both at liquid liquid and gas liquid interfaces (ref. 16,17 and references therein). In addition to steric and electrostatic interactions, colloidal particles at interfaces commonly experience capillary-mediated interactions which are either induced by non-spherical colloid shapes, by chemical heterogeneities at the surface or by direct interactions. 18 A model using these capillary forces was shown recently to satisfactorily explain the rupture of particle clusters at the air water interface upon application of shear. 19 To complement those investigations we present here studies of the dynamic surface tension g(t) and of This journal is ª The Royal Society of Chemistry 2009 Soft Matter, 2009, 5,

2 the dilational elasticity E, both of which are measured directly on the gas liquid interfaces of aqueous particle dispersions. Thus, we emulate more realistic conditions occurring during foam generation. We used dispersions of fumed silica nanoparticles with intermediate hydrophobicity (34% SiOH). Among different % SiOH contents, those dispersions provide the maximum volume of foam after production, i.e. maximum foamability. 8,10 We present measurements of the dynamic surface tension g(t) (change of surface tension with time) and of the dilational elastic modulus E conducted using an oscillating bubble device, for a range of particle concentrations previously investigated in a foam stability study. 8,10 We complement these studies by investigations of the nature of the interfacial particle layer using ellipsometry, Brewster angle microscopy and observations of free-standing liquid films. 2. Materials and methods 2.1 Materials Particles. Fumed silica nanoparticles were kindly provided by Wacker-Chemie (Germany). The particles investigated in this work were chemically coated with a short-chain silane reagent (dichlorodimethylsilane) by the manufacturer. The hydrophobic character of the particles is expressed by the percentage of surface silanol groups SiOH. We used a 34% grade throughout our experiments which corresponds to the particle hydrophobicity which provides maximum foamability. 10 The primary particles are quasi-spherical of approximately 20 nm diameter, but aggregate into clusters over 200 nm in size. 20 Particle dispersions. We prepared aqueous dispersions of particles at 1 wt.% concentration by a stepwise procedure using doubly distilled and deionized Milli-Q water (ph z 5.8), silica particles and a small amount of ethanol (<2 wt.%) to facilitate particle wetting. Virtually all ethanol evaporates during the preparation of the final dispersions. We obtained particle dispersions with different concentrations by successive dilution of more concentrated dispersions. We sonicated the dispersions for one hour (20 khz, amplitude of 70%) using an ultrasonic processor, which turned out to be a fundamental step to ensure sufficient monodispersity of the particle aggregates and reproducibility of the measurements. The highest particle concentration which we could disperse in this way was 1 wt.%. The dispersions were generally stable for one month. After this time sedimentation or further aggregation took place. In order to determine the typical aggregate size after sonication, we conducted standard static and dynamic light scattering studies, which confirmed that by following accurately the preparation protocol described above, we reproducibly obtain fairly monodisperse aggregates. In light scattering, the scattered intensity due to refractive index (n) fluctuation is measured at a given angle q between the incident and the scattered radiation. The scattering vector q is defined as q ¼ 4pn l 1 sin(q/2). 21 From static measurements, extrapolating to the scattering vector q ¼ 0, we found a radius of gyration of the aggregates of R g ¼ 71 4 nm; and by extrapolating to the concentration c ¼ 0, a positive second virial coefficient B 2 ¼ m 3 mol kg 2, corresponding to repulsive particle interactions (see the Zimm plots 22 in Fig. 1(a)). From dynamic measurements, we analyzed the field correlation functions f m (q,s) by using the method of cumulants and an expansion up to the second order: f m (q,s) ¼ exp( <D>q 2 s)(1 + 1/2s( <D>q 2 s) 2 ), where D is the diffusion coefficient and s is the correlation time. From this analysis, we find a hydrodynamic radius R h ¼ kt/6ph<d> ¼ nm (where kt is the thermal energy and h is the viscosity of the solvent, i.e. water) and a polydispersity index s ¼ (<D 2 > <D> 2 )/ <D> (see Fig. 1(b)). 22 Thus, the particle aggregates are fairly monodisperse and have approximately spherical shapes, the ratio R h /R g ¼ 1.2 being close to (5/3) Methods Tensiometry. We carried out static and dynamic (change of the surface tension g with time) surface tension measurements for a range of particle concentrations using the Tracker pendant drop/rising bubble tensiometer apparatus (Teclis, France). 23 In this method, either a pendant drop of the dispersion is generated in air or a rising air bubble (both of volumes between 5 and 10 ml) is generated in the dispersion using a syringe. Both, drop and bubble remain attached to the tip of the syringe needle. The interfacial tension is then calculated from the shape of the drop/ bubble, more specifically from its deformation under gravity. We monitor the change of surface tension during particle adsorption for up to t ¼ 10 4 s. Above these times, evaporation of the solvent may introduce artefacts. At high concentrations, the results obtained with the rising bubble were more reproducible since depletion of particles from Fig. 1 (a) Zimm plot obtained using static light scattering measurements for aqueous dispersions of silica nanoparticles at different concentrations (given). K is an optical constant, R q is the Rayleigh ratio and a is an arbitrary constant. (b) Field autocorrelation function f m (q,s) measured at the scattering angle q ¼ 90 for systems in (a): 0.1 (,), 0.3 (B), 0.5 (O), 0.7 (P) wt.%. Solid lines are cumulant fits Soft Matter, 2009, 5, This journal is ª The Royal Society of Chemistry 2009

3 the dispersion was negligible. Using the same apparatus, we also performed oscillating bubble experiments in order to obtain the dilational elastic modulus E ¼ dg/dlna. We imposed sinusoidal variations of the bubble volume DV ¼ ml and we measured the corresponding changes of surface tension by image analysis. The frequency of the oscillation was swept from 0.1 to 1 Hz. Ellipsometry and Brewster angle microscopy (BAM). We used an imaging ellipsometer (Nanofilm, Germany) working with green laser light (l ¼ 532 nm) in order to gain information on particle organization in the interfacial region. The aqueous dispersions were placed in a large dish (diameter ¼ 8 cm, depth ¼ 3 cm) and the laser beam was directed at the surface in the center of the cell where the meniscus effect is negligible. A multiple angle of incidence (MAI), fixed compensator (¼ 45 ) and 4- zone averaging nulling scheme was adopted. 24 The ellipsometric parameters (J,D), which are related to the ratio r of the complex reflection coefficients by r ¼ r p /r s ¼ tanj exp(id), deviate significantly from the values of the bare water surface since the larger refractive index of silica particles (n Si ¼ 1.475) provides a good optical contrast. 25 The ellipsometric parameters J and D were measured around the Brewster angle, scanning the incident angle by steps of 0.1. From the fits of the latter quantities, the refractive index profile along the interfacial region could be resolved and information on layer thickness and particle concentration at the water surface could be extracted. Brewster angle microscopy was also implemented in the same setup. By this method, we could gain information on the texture and organization of particles in the interfacial plane, which served to complement our ellipsometric data with visual information. To check the accuracy of our measurement protocols, we always compared our data with the bare air water interface. Individual films. We observed qualitatively the behaviour of freely suspended horizontal films formed from the same dispersions using a home-built apparatus in which we placed 5 ml drops between two metal barriers of 2 cm width. One barrier was moved horizontally by a micrometric screw; hence a film could be formed between the barriers and videos of light reflected from the film were recorded by means of a camera placed below the film. The system was closed to the atmosphere and saturated in vapour, to minimize evaporation effects. All measurements in this article were conducted at room temperature, being 20 2 C. as no significant particle adsorption to the interface takes place. 26 Even if adsorption of particles takes place, no significant change of g is reported when particle interactions are of purely steric nature. 27 This is different to the kind of particles investigated here, which adsorb to the interface due to their partial hydrophobicity and whose interactions are governed by repulsive electrostatic forces (see also light scattering results, Section 2.1) and additionally by attractive capillary forces. We expect the latter to be generated by surface chemical inhomogeneities or by the irregular shape of the particle aggregates on whose surfaces the wetting angle condition must be fulfilled locally, leading to a curvature of the liquid surface around the particle and hence to particle interactions. 18 Here we have measured changes of the air water surface tension g with time t ( dynamic surface tension ) for a range of particle concentrations. Some representative examples are displayed in Fig. 2. We note that the measurement is started (t ¼ t 0 ¼ 0) as soon as the droplet/bubble is formed in the measurement device (refer to Section 2.2), which takes about 1 s. It is evident from the variation of the observed surface tension values at t 0 that during this period a significant, concentrationdependent amount of particles is already adsorbed at the gas liquid interface. This is probably due to the additional energy input resulting from the droplet/bubble generation. At concentrations lower than 0.1 wt.%, no appreciable changes of g were observed during a time of up to 10 4 s. Between 0.1 and 1 wt.%, however, we measured a slow relaxation of the surface tension with an overall decrease of approximately 5mNm 1. At the highest concentration studied (1 wt.%), the surface tension value decreases to around 50 mn m 1, which is close to the predicted value given by Binks and Clint. 28 Thus, the adsorption of particles can be detected by the change of g with time, where slow kinetics is usually observed. In fact, as we shall illustrate in Section 3.2, nanoparticle adsorption is not only diffusion-controlled but also displays kinetics with an energy barrier arising from steric and/or electrostatic interactions, as was demonstrated by Kutuzov et al. for nanoparticles with a size <10 nm at the oil water interface. 29 Furthermore, due to the high density of silica particles, gravity driven motion is expected to play a role, which is discussed in more detail in Section 3.2. In Fig. 3, we plot the surface tension g measured at times close to 10 4 s as a function of the particle concentration c. We expect these values of g to be close to the ones which would be obtained 3. Results and discussion We performed a range of interfacial studies at the air water interface of particle dispersions of different concentrations. In the first part, we combine measurements of the dilational elasticity E and of the dynamic surface tension g(t) and correlate these results to foam stability. In the second part, we describe the adsorption and organization mechanisms of the particles at the water surface. 3.1 Purely elastic stabilization against foam coarsening Non-chemically modified silica particles have commonly a hydrophilic character; hence the surface tension of their dispersions is generally independent of the particle concentration Fig. 2 Dynamic air water surface tension measured in the rising bubble experiments for different silica nanoparticle concentrations: 0.1 (,), 0.3 (B), 0.5 (O), 1 (P) wt.%. This journal is ª The Royal Society of Chemistry 2009 Soft Matter, 2009, 5,

4 for infinite waiting times. The systematic deviation between surface tensions measured with pendant drops and rising bubbles (Fig. 3) may be due to two effects. Firstly, particle depletion may come into play due to the finite amount of particles available in the small pendant drop (between 5 and 10 ml). Secondly, gravitational effects may lead to a more efficient transport of particles to the interface in the case of the rising bubble. Nonetheless, measurements obtained by both methods confirm a significant decrease of g between 0.1 and 1 wt.%. In order to evaluate the surface elastic properties of the particleladen interfaces, we carried out oscillating droplet and bubble experiments. To obtain quantitative information we preferred the oscillating bubble over the pendant drop configuration, because in the latter, after adsorption times of 10 4 s, the shape of the drop tended to become non-laplacian with a corrugated surface. This phenomenon was seldom observed in the rising bubble configuration. A similar phenomenon was reported for irreversibly adsorbed mixed polyelectrolyte-surfactant layers, where the surface can be deformed by thickness gradients that can relax towards uniformity, at extremely long times. 30 The dilational elastic modulus E ¼ dg/dlna was measured (after t z 10 4 s) with oscillating bubble experiments performed at the frequency n ¼ 1 Hz, as a function of the particle concentration c. The results are presented in Fig. 4(a). The absolute value of E increases markedly with increasing particle concentration c, reaching a value of approximately 40 mn m 1 at c ¼ 0.7 wt.%. At lower frequencies, we observed non-elastic phenomena in the form of non-smooth profiles of surface tension and surface area with time. It seemed that at low frequencies particles had enough time to re-arrange during the slow sinusoidal change of the bubble area. 30 Combining the results from Fig. 3 and Fig. 4(a), we show in Fig. 4(b) the dilational elastic modulus E as a function of the surface tension g in the range of particle concentrations c of interest. For comparison, Fig. 4(b) also displays the line E ¼ g/2 corresponding to the Gibbs stability criterion. From this, it follows that only dispersions with particle concentrations above 0.5 wt.% can generate foams with significantly reduced coarsening. This result is in perfect agreement with the foam stability investigations reported by Cervantes-Martinez et al. 8 for the same particle dispersions and now allows us to predict when foams will be stable or not against coarsening. Indeed, the original work by Cervantes-Martinez et al. 8 was based on estimations of the particle concentration at bubble surfaces and Fig. 4 (a) Dilational elastic modulus E as a function of particle concentration c. (b) Variation of E with the surface tension g measured at t ¼ 10 4 s (reported in Fig. 2) for four different particle concentrations (given). The solid line shows E ¼ g/2. comparison with independent measurements made with spread layers. Moreover, the oscillating bubble experiments indicate the existence of an adsorption barrier: the surface tension values obtained for a static bubble after long waiting times can be reached much more rapidly by oscillating the bubble and hence by providing an additional energy input facilitating particle adsorption (see Fig. 5). Such effects are known from similar systems. For example, anionic latex particles do not adsorb spontaneously at the air water interface because the interface itself appears anionic and hence repulsive. 31 Similarly, the silica particles under investigation, being decorated on the surface by SiO groups, are also anionic (see also Section 2.1). Thus, an energy barrier resulting from electrostatic interactions between the particles and the air water surface, and between the particles at the surface and in bulk, may hinder adsorption to the interface. Additionally, steric interaction between Fig. 3 Surface tension values g obtained at t z 10 4 s as a function of silica nanoparticle concentration c measured using the pendant drop (-) and the rising bubble (B) technique. Fig. 5 In dynamic surface tension measurements in the rising bubble configuration, the surface tension g decreases more rapidly when the bubble is oscillated (here n ¼ 0.1 Hz and c ¼ 0.1 wt.%) Soft Matter, 2009, 5, This journal is ª The Royal Society of Chemistry 2009

5 particles at the surface and in bulk tend to slow the adsorption at high surface concentrations Organization of nanoparticles at the air water interface In this part we discuss and provide experimental evidence of the adsorption mechanism and of the arrangement of partially hydrophobic fumed silica particles at the air water interface of particle dispersions. We performed ellipsometric measurements in order to gain information on the kinetics of the adsorption, the layer thickness and the surface concentration G of particles. We investigate the influence of the method of preparation of the dispersions on the organization of particles at the interface. At the outset, for comparison with previous findings, 13,15 we compare the case of adsorbed particle layers with those of spread layers Spread layers. We performed ellipsometric scans of high accuracy around the Brewster angle (¼ arctan(n water /n air ) ¼ ) on a spread layer of particles. We obtained such a monolayer by spreading a known amount of particles dispersed in isopropyl alcohol of c ¼ 0.1 wt.% onto a pure water surface of fixed area. We studied two spread layers with a surface concentration of G ¼ 20 mg m 2 and G ¼ 40 mg m 2 which served as reference concentrations to be compared with the adsorbed particle layers. From the angular dependence of D and J, 24 as displayed in Fig. 6, we can simultaneously extract information on the thickness d and on the refractive index n l of the particle layer (see inset in Fig. 6). 32 From data fitting we obtain d ¼ nm and n l ¼ for G ¼ 20 mg m 2. For twice this surface concentration (G ¼ 40 mg m 2 ), we find a similar layer thickness (d ¼ nm), whilst the refractive index increases to n l ¼ From the latter data we could evaluate the surface concentration seen by ellipsometry using the relation G elli ¼ (n l n water )/(vn/vc)$d, where vn/vc is the differential refractive index increment. Calculating for the two reference surface concentrations, G elli changed from 17.6 mg m 2 to 56.9 mg m 2. Those values are in a reasonable agreement with the actual surface concentrations of 20 and 40 mg m 2. Thus, for spread layers, in the range of concentration studied, it seems that the particle aggregates remain trapped at the water surface. It is worth noting that for n water ¼ < n l < n Si ¼ 1.475, no satisfying fit can be obtained assuming the presence of nonaggregated particles of 20 nm diameter. In fact, if d ¼ 20 nm, around the Brewster angle, D would change from 180 to 360 instead of from 180 to Adsorbed layers. We performed the same ellipsometric scans on the surface of aqueous particle dispersions of concentrations between 0.1 and 0.7 wt.%. The dispersions were prepared and sonicated one day before use. We cleaned the surface of the dispersions (and of water) several times by means of a pump before performing the first ellipsometric scan. Then we waited at least 1 h to allow the interface to equilibrate before performing further measurements. Fig. 7 shows J and D as a function of the incident angle 4 for the bare air water interface and for a particle dispersion at Fig. 6 Results of ellipsometric scans performed around the Brewster angle on a spread layer of silica nanoparticles at the air water surface with surface concentrations G ¼ 20 mg m 2 (,) and G ¼ 40 mg m 2 (B). The solid lines represent the fits. Inset: sketch of the stratified layer model. Fig. 7 Ellipsometric scans performed around the Brewster angle at the bare air water interface (x) compared to those conducted at the surface of an aqueous silica dispersion with a particle concentration of c ¼ 0.1 wt.% after adsorption for one day (,). > corresponds to measurements obtained with the same dispersion after 10 min of strong sonication. The dotted line is the simulated step-like profile of the air water surface and the solid line is a film model fit giving a layer thickness of d ¼ 149 nm with a refractive index n l ¼ This journal is ª The Royal Society of Chemistry 2009 Soft Matter, 2009, 5,

6 c ¼ 0.1 wt.% after 1 day. Both surfaces give nearly identical signals. The same holds for measurements in the whole concentration range between 0.1 and 0.7 wt.%. Even after 1 day, ellipsometric scans did not show any change of the interfacial profile, thus implying that no significant amount of particles had been adsorbed at the interface. Using Brewster angle microscopy, we observed islands of particles floating at the water surface, but their area fraction was not high enough to contribute to the ellipsometric signal. These results are in apparent contradiction with those obtained using spread particle monolayers. The main difference between the two cases is that in the latter all particles are placed directly at the surface where they remain due to large desorption energies. In the case of aqueous dispersions, however, the particles need to overcome two obstacles: firstly, they need to overcome energy barriers present at the interface, as discussed at the end of Section 3.1. Secondly, they need to diffuse against gravity from the bulk to the surface (silica particles are significantly heavier than water with a density of r ¼ 2.2 g cm 3 ). The latter effect may also explain why we see spontaneous particle adsorption in the case of bubbles or drops (Section 3.1), where particle sedimentation helps adsorption due to the different geometry of the air water interface. To see this, let us evaluate the relationship between particle diffusion and sedimentation for our system. The mean square displacement for a diffusing particle is given by <DR 2 (t)> ¼ 6Dt, with <D> ¼ m 2 s 1, as evaluated by dynamic light scattering (Section 2.1. and ref. 22). The displacement of a particle due to gravity can be written as L ¼ vt, where the Stokes velocity is given by v ¼ mg/(6phr h ) ¼ ms 1 (mg is the gravitational force and R h ¼ nm, the hydrodynamic radius (Section 2.1)). The two different lengths are equal (<DR 2 (t)> ¼ L 2 ) for t ¼ s, meaning that gravitational effects become important at these time scales, which corresponds well to what we see in Fig. 3. To test this hypothesis, we strongly sonicated (10 min, Section 3.2.3) the dispersions after one day (at which no significant particle adsorption had taken place). Performing ellipsometric scans immediately after sonication reveals significant changes of the interfacial profile, i.e. significant particle adsorption, which can be seen thanks to the changes of J and D displayed in Fig. 7 (>) for the case of a dispersion concentration c ¼ 0.1 wt.%. From fitting this profile we obtain a layer thickness of d ¼ nm with a refractive index of n l ¼ Particle networks at air water surfaces. Comparing Fig. 6 and 7, we noticed that the profile of the interface generated by sonication-induced adsorption for c ¼ 0.1 wt.% (d ¼ nm, n l ¼ ) differs significantly from the one of the spread layer for G ¼ 40 mg m 2 (d ¼ nm, n l ¼ ). This difference is even more evident from Brewster angle microscopy images. Whilst spread monolayers tend to be very homogeneous and show compact packing at a surface concentration of G ¼ 60 mg m 2, 13,15 we find very inhomogeneous particle distributions at the interface for adsorbed layers. Examples of typical Brewster angle microscopy images for dispersion concentrations of c ¼ 0.1 and 0.7 wt.% are displayed in Fig. 8, clearly indicating pronounced interfacial textures after sonication for adsorbed monolayers. At low concentrations (Fig. 8(a)), sonication leads to the formation of some solid Fig. 8 Brewster angle microscopy images of the air water surface for silica nanoparticle concentrations c of (a) 0.1 wt.% and (b) 0.7 wt.% taken after one day of aging and 10 min of sonication. The bright areas contain significantly more particles. At low concentrations (a), one observes isolated structures which slowly and freely move within a homogeneous layer of particles. At sufficiently high particle concentrations (b), a network-like structure of particles appears. The scale bar is 20 mm. structures which slowly and freely move within a homogeneous layer of particles. This observation may also explain the scattered ellipsometric data obtained after sonication (Fig. 7). At sufficiently high particle concentrations (Fig. 8(b)), a completely different scenario is observed: a network-like structure of particles appears. We did not observe changes of these textures within 24 h by Brewster angle microscopy. Between the two concentrations depicted in Fig. 8, i.e. 0.1 and 0.7 wt.%, foams dramatically change their stability (Section 3.1 and Fig. 4(b)). 8,10 Thus, the observation of the different kinds of structures shown in Fig. 8 could confirm the suggestion made by Kostakis et al., 33 who related foam stabilization to the formation of weak gel networks of particles between gas liquid interfaces separating the bubbles. At this stage, however, it is not clear to us what is the origin of the attractive forces between particles enabling the formation of the observed networks. They could be capillary-mediated interactions induced by the complex aggregate geometry. 18 Additionally, SiOH groups on adjacent particles can interact to form siloxane bonds (Si O Si) which allow particles to aggregate. Being partially hydrophobic, 2220 Soft Matter, 2009, 5, This journal is ª The Royal Society of Chemistry 2009

7 Fig. 9 Images of a freely suspended, horizontal film drawn from silica nanoparticle dispersions with (a) low c ¼ 0.05 wt.% and (b) high c ¼ 0.5 wt.% particle concentration. Time between images is 30 s. The scale bar is 1 mm. their charge is reduced compared with the equivalent fully hydrophilic silica Particle networks in thin films. The formation of network-like particle structures, as described in the previous section, can also be observed in freely suspended individual thin films (Section 2.2). Fig. 9 shows the development of two examples of films drawn from dispersions of two different concentrations (Fig. 9(a): c ¼ 0.05 wt.% and Fig. 9(b): c ¼ 0.5 wt.%). In each picture, the grey top and bottom parts correspond to the metal barriers. In the central part, a meniscus (black coloured) surrounds the thin film region (see sketch in Fig. 9(a)). Colours and light intensity result from light interference at the two gas liquid interfaces and therefore provide information on film thickness and thickness changes. At low concentrations (Fig. 9(a)), fairly homogenous thin films are formed whose thickness fluctuates in time accompanied by rapid colour changes, and decreases overall with time to cause film rupture after a few minutes. At sufficiently high particle concentrations (Fig. 9(b)), the scenario is quite different: as observed in the previous Section (Fig. 8(b)), the particles form network-like structures and lead to the formation of very thick films which are significantly more stable with lifetimes of up to 1 h. 4. Conclusions We have presented investigations into the static and dynamic properties of air water interfaces of aqueous dispersions of partially hydrophobic silica nanoparticles (34% SiOH coverage). Our measurements strongly support the argument that the stability of particle-stabilised, aqueous foams can be predicted by the Gibbs elasticity criterion E > g/2, which relates the surface tension g and the dilational elastic modulus E of the particle covered interfaces. Both g and E depend on the bulk particle concentration of the dispersion, making this therefore a key parameter for the control of foam stability. Our predictions are in perfect agreement with the results of Cervantes-Martinez et al., which were based on estimations of the particle concentration at the surface of bubbles. We find that particle adsorption is inhibited by a pronounced energy barrier 29 which can be overcome using strong sonication of the particle dispersions. The precise nature of this adsorption barrier is not yet clear to us, but it might explain why the generation of stable particle foams requires high energy techniques, such as turbulent mixing. Finally, we show that at sufficiently high particle concentrations and after sufficient energy input, particles form networklike structures at the surface of aqueous dispersions and in thick horizontal free-standing films. Similar structures were seen recently in electron microscopy experiments. 34 The formation of such structures requires the presence of attractive particle interactions in the interface, whereas we find repulsive particle interactions in the bulk from our light scattering data (B 2 > 0). The nature of these interfacial attractive interactions is still unclear. 18 Acknowledgements The authors would like to thank Pawel Pieranski for discussions and for kindly sharing his freely suspended film equipment. We thank Liliane Leger for lending us her ellipsometer, Eric Raspaud for providing the light scattering equipment and Wacker- Chemie (Burghausen) for providing the silica particles. A. Stocco was financed by SOCON (European contract RTN ). References 1 S. U. Pickering, J. Chem. Soc, 1907, 91, C. W. Ramsden, Proc. Roy. Soc. A, 1903, 72, B. P. Binks and R. Murakami, Nat. Mater., 2006, 5, J. Banhart, Adv. Eng. Mater., 2006, 8, Z. Du, M. P. Bilbao-Montoya, B. P. Binks, E. Dickinson, R. Ettelaie and B. S. Murray, Langmuir, 2003, 19, S. Fujii, A. J. Ryan and S. P. Armes, J. Am. Chem. Soc., 2006, 128, R. G. Alargova, D. S. Warhadpande, V. N. Paunov and O. D. Velev, Langmuir, 2004, 20, A. Cervantes-Martinez, E. Rio, G. Delon, A. Saint-Jalmes, D. Langevin and B. P. Binks, Soft Matter, 2008, 4, This journal is ª The Royal Society of Chemistry 2009 Soft Matter, 2009, 5,

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