Film Formation from Concentrated Reactive Silicone Emulsions. 1

Drying mechanisms of concentrated, reactive poly(dimethylsiloxane) in water emulsions, stabilized by a polyethoxylated fatty alcohol, were studied by ...
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Film Formation from Concentrated Reactive Silicone Emulsions. 1. Drying Mechanism D. Guigner,† C. Fischer,‡ and Y. Holl*,† Institut de Chimie des Surfaces et Interfaces (CNRS), BP 2488, 68057 Mulhouse, France, and Rhodia Silicones, BP 22, 69191 St Fons Cedex, France Received December 7, 2000. In Final Form: March 24, 2001 Drying mechanisms of concentrated, reactive poly(dimethylsiloxane) in water emulsions, stabilized by a polyethoxylated fatty alcohol, were studied by visual observation, gravimetry, attenuated total reflectance, and infrared microscopy. The effects of some parameters were investigated, namely, thickness of the cast layer, surfactant concentration, and presence of a CaCO3 filler. The drying process can be described as three successive fronts, parallel to the surface, moving in a vertical direction, from top to bottom. The first front transforms the color of the emulsion from white to transparent, the second is a coalescence front, and the third restores the initial white color. The main effect of all studied parameters is to alter the global drying rate and drying duration. The fate of the surfactant and the structure of the film after drying are examined in parts 2 and 3 of this series, respectively.

Introduction The process of film formation from polymer colloids has been extensively investigated since the early 1950s because of the enormous practical interest in this kind of film. Indeed, they are used in the fields of paints, papers, coatings, adhesives, textiles, and many other applications. The interest in film formation mechanisms was renewed in the last 15 years thanks to the introduction of new techniques such as fluorescent labeling, atomic force microscopy (AFM), environmental scanning electron microscopy, ellipsometry, magnetic resonance imaging, . . . (this list is not exhaustive), and theoretical approaches. This field has been reviewed recently.1-3 The mechanism of water loss from polymer colloids is surprisingly complex. It is this stage of film formation that is, at present, the least understood.2 This specific question of the drying mechanisms was also recently reviewed.4 Drying modes are very important for several reasons. They determine drying rates which are of technological importance. On the other hand, drying implies transport of water in the emulsion. The water fluxes are particularly interesting because they can transport polymer particles, pigment, and water-soluble species to different regions of the film. Transport of particles will influence the array of particles and thus the morphology of the film.5 Transport of watersoluble species such as surfactants or soluble oligomers will influence the final distribution of such species in the dry film.6 Both morphology and distribution of water* Corresponding author. † Institut de Chimie des Surfaces et Interfaces (CNRS). ‡ Rhodia Silicones. (1) Keddie, J. L. Mater. Sci. Eng. 1997, R21, 101. (2) Winnik, M. A. Curr. Opin. Colloid Interface Sci. 1997, 2, 192. Winnik, M. A. In Emulsion Polymerization and Emulsion Polymers; Lovell, P. A., El-Aasser, M. S., Eds.; John Wiley: New York, 1997; p 467. (3) Steward, P. A.; Hearn, J.; Wilkinson, M. C. Adv Colloid Interface Sci 2000, 86, 195. (4) Holl, Y.; Keddie, J. L.; McDonald, P. J.; Winnik, M. A. In Film Formation in Coatings: Mechanisms, Properties, and Morphology; Provder, T., Urban, M. W., Eds.; ACS Symposium Series 790, American Chemical Society: Washington, DC, 2001; Chapter 1, p 2. (5) Denkov, N. D.; Velev, O. D.; Kralczevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183; Nature 1993, 61, 26. (6) Holl, Y. Macromol Symp. 2000, 151, 473.

soluble species have a profound influence on properties of latex films. In this work, we have studied concentrated oil-in-water emulsions of poly(dimethylsiloxane) (PDMS) containing a cross-linking agent. These emulsions were allowed to dry, leaving a polymeric, cross-linked film. Little information can be found on this kind of waterborne PDMS film in the open literature. The whole study can be divided into three main parts, namely, drying mechanisms (presented in this paper), distribution of the surfactant in the drying emulsion and in the dry film,7 and structuring of the film.8 Drying mechanisms were studied by visual observation when the sample was thick enough, gravimetric measurements of global water loss, infrared microscopy (in transmission), and reflection infrared spectroscopy (attenuated total reflection, ATR). The role of some parameters was investigated, namely, the thickness of the deposited emulsion layer, the surfactant concentration, and the presence of a filler. 2. Experimental Section The oil was a R,ω-difunctional poly(dimethylsiloxane) (PDMS) polymer (Mw ) 120 000 g/mol, polydispersity index ) 2) from Rhodia, France. It contained a cross-linking agent, totally soluble in oil, able to react with the functional groups of PDMS at room temperature with the help of a catalyst. The surfactant was a commercial polyethoxylated fatty alcohol, denoted C13E8, used as received. The hydrophobic part was a linear, saturated alkyl group with 13 carbon atoms. The hydrophilic part contained an average of eight ethoxy groups. Some details in this section (commercial source of the surfactant, nature of cross-linking agent, and catalyst) were omitted because they are proprietary. The emulsion was formed by adding surfactant containing water to oil plus cross-linking agent under fast stirring in a metallic reactor. After a few minutes, the water-to-oil (w/o) emulsion inverted into an oil-to-water (o/w) emulsion. Stirring was carried on until the mean droplet size reached 0.45 µm (measured by standard dynamic light scattering). The droplet size distribution was unimodal and Gaussian with a full width at half-maximum of 0.3 µm. The emulsion was then diluted to a water content of 16.5 wt %. The final step was to add the catalyst in the form of an emulsion and degas the product. The composition (7) Guigner, D.; Fischer, C.; Holl, Y. This series, part 2, to be published. (8) Guigner, D.; Fischer, C.; Holl, Y. This series, part 3, to be published.

10.1021/la001711d CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001

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Figure 1. Experimental setup used for infrared microscopy analysis of water distribution in the 10 mm thick drying emulsion. Table 1. Composition of the Reference Emulsion product

percentages (wt %)

oil (PDMS) cross-linking agent surfactant

73.7 5.2 4.4

product

percentages (wt %)

water catalyst

16.5 0.2

of the most extensively studied emulsion (reference system) is given in Table 1. One hour after the catalyst was added, the emulsion was cast on a glass substrate with polypropylene sides and allowed to dry. The sizes of the wet sample were 15 cm length and 6 cm width. Drying was performed at fixed and constant conditions: 23 °C and 50% relative humidity (RH). After the sample was dried, the residual thickness was around 84% of the initial one. The effects of the following parameters on the drying mechanisms were investigated in this study: thickness of deposited emulsion layer, surfactant concentration, and presence of a filler. Thickness effects were studied using samples of thicknesses 0.19, 2, 5, and 10 mm. Usually, the surfactant concentration was 4.4 wt %. A lower concentration of 2.6 wt % was used in some experiments. The emulsion containing 2.6 wt % of surfactant was prepared in such a way that its particle size distribution was the same as the emulsion containing 4.4 wt % of surfactant (longer stirring time). As a filler, precipitated calcium carbonate was used, either untreated (hydrophilic) or pretreated with 2.9 wt % of stearic acid to render its surface hydrophobic. The filler specific area was 20 m2/g. Its concentration was 25 wt %. Periodically, a small sample (1 cm2) was cut out of the 10 mm thick drying emulsion in order to observe its macroscopic aspect: appearance of layers, color (white, translucent, or transparent). Kinetics of water evaporation were followed in three ways, namely, a global, gravimetric study, a more local study by infrared microscopy, and a measurement of water loss at the interface between the emulsion and the substrate by reflection infrared spectroscopy. Water losses in the whole emulsion during drying were followed by recording the weight of the sample versus time. Reproducibility was checked by repeating the measurements with a new emulsion prepared several weeks after the first one. It was excellent.9 Using infrared microscopy (IRM), it was possible to draw concentration profiles of water along the film thickness at various times during drying, for the 10 mm thick samples. The sample for infrared analysis was prepared as follows (Figure 1). A U-shaped poly(tretrafluoroethylene) (PTFE) spacer of thickness 25 µm was deposited on an IR transparent CaF2 crystal. The area inside the U was (1 × 1) cm2. A small amount of emulsion was put inside the U and covered with a second CaF2 crystal. (9) Guigner, D. Ph.D. Thesis, University of Mulhouse, France, 2000.

Figure 2. Comparison between dry content versus time results from gravimetry on the full size sample (full circles) and from infrared microscopy on the small sample (open circles): surfactant concentration, 4.4 wt %; no filler. The two crystals were held together with screws. To ensure proofness of this cell, silicone grease was added outside, all around the U-shaped spacer. Only one side was open to allow evaporation of water. This small sample was equivalent to a sample of same sizes in the normal drying emulsion. IR analysis was performed in transmission mode by moving the spot along the vertical direction corresponding to the normal sample thickness. Spot sizes of 50 or 100 µm were used. The complete analysis of all points was repeated several times during the total drying time of 16 days. Unfortunately, it was not possible to ensure that an analysis spot was rigorously at the same vertical position in two different sets of measurements. This gives rise to apparent slight displacements of heterogeneities with time. As only qualitative conclusions are drawn at that point, this is not too much of a problem. The main absorption bands of water in our samples were located at 3597 and 3230 cm-1 (hydrogen bonds), 3450 cm-1 (OH valence band), and 1645 cm-1 (OH deformation band). The last band at 1645 cm-1 was used for quantitative analysis. The area of the corresponding peak was recorded initially (A0) and at times t (At). The fraction of water (compared to the initial amount of water) remaining in the sample at a given time and at a given position along the thickness was calculated by

P ) At/A0 The concentration of water remaining in the emulsion was given by C ) (100 × P × 16.5)/(16.5P + 83.5) and the dry content by DC ) 100 - C. These data allowed us to draw concentration profiles of water at various times and drying kinetics through dry content versus time plots. Reproducibility of initial water content measurements was assessed by analyzing three points in the same sample at the same height and one central point in three different samples. Results can be found in ref 9. They are reproducible within 2%. The confined nature of the small IRM sample could induce special effects, such as development of stresses and/or preferential drying along the sides of the cell. However, it could be shown that water loss was identical in the normal sample and in the small, confined one. Water content data from infrared microscopy at a given time were cumulated over the whole sample height (corresponding to the thickness of the normal sample) and compared with gravimetric data from the full size sample at the same time. Results are shown in Figure 2. The agreement between the two sets of data seems to indicate that the small sample represents the large one with a good approximation. Another argument supporting the equivalence between the small and real size samples will be presented elsewhere.8 On the other hand, some peculiar results (negative peaks and absence of shrinkage upon drying, see below) might be an effect of confinement. Studies of phenomena occurring inside of a drying system are difficult. One is still in the situation where imperfec-

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tions in the experimental procedures, leading to results with a certain uncertainty, have to be accepted. Finally, phenomena taking place at the interface between the drying emulsion and its substrate were studied by ATR in Fourier transform infrared spectroscopy (FTIR). The wet emulsion was put in contact with a ZnSe ATR crystal and allowed to dry. Evaporation by the sides was avoided by the use of a small PTFE mold containing the drying sample. When the intensity of a water band was recorded, the water content of the around 1 µm thick layer in contact with the substrate versus time could be determined in a similar way as in the IR microscopy study described above. The only difference was that the band at 1645 cm-1 became very small when water content was low. The sum of the intensities of the three bands centered at 3400 cm-1 was thus chosen as the reference band despite of the fact that it was partially overlapping with a band from the silicone. The silicone contribution was subtracted after decomposition of this large peak. Two experiments with newly prepared emulsions gave exactly the same results.9

3. Results Results will be presented in the following way. First, data concerning the most extensively studied system will be shown. This reference system is an emulsion containing 4.4 wt % of surfactant and no filler, cast with a thickness of 10 mm. Then, the effects of parameters will be investigated, namely, thickness of the cast layer, surfactant concentration, and presence of a filler. 3.1. Reference System. 3.1.1. Macroscopic Observation, Gravimetry. Under standard drying conditions (23 °C, 50% relative humidity), the 10 mm thick emulsion layer dried in 16 days (except for traces of water). This period could be divided in two main steps, the first 6 days and the remaining 10 days. In the first step, a transparent layer appeared at the top and the thickness of the transparent zone progressively increased toward the substrate. There was a front separating a transparent domain at the top and a white domain underneath which progressed toward the bottom. At 6 days, the emulsion was totally transparent; it had an average dry content of 96 wt %. Totally transparent means that one could easily read through a 8-9 mm thick layer. However, in this apparently homogeneous layer, two very different domains coexisted: a thin skin at the top whose thickness had progressively increased during the 6 days to reach 200 µm, and the rest of the layer which was redispersible in water. Directly after the transparency appeared in a region of the layer, the emulsion in this region had exactly the same particle size distribution as the initial one. The formation of the skin started at the very beginning of the process; it was already detectable a few hours after the emulsion had been cast on the substrate. It will be shown in a subsequent paper8 that the particles in the skin were coalesced. In the second step, under the 200 µm thick transparent skin, a white layer appeared whose thickness progressively increased. Similar to the first step but with the layers in the opposite order, a front progressed toward the substrate separating a white layer at the top from a transparent layer underneath. The transparent part was redispersible in water. The second front reached the glass substrate 16 days after drying had started, when all water was evaporated. A scheme of this two-step drying mechanism is shown in Figure 3. It has to be mentioned here that, laterally, drying was homogeneous. That means that no difference in the drying behavior could be noted between the edges and the center of the sample. Gravimetric measurements of water loss as a function of time are shown in Figure 2. It can be seen that, as

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Figure 3. Schematic representation of the two-step drying mechanism of the 10 mm thick emulsion layer under standard conditions (23 °C, 50% RH): surfactant concentration, 4.4 wt %; no filler. In the first step (0-6 days), a front separating a top transparent layer from a bottom white layer progressed toward the substrate. At 6 days, the whole system was transparent and consisted in a thin (200 µm), coalesced skin, the rest being formed of the initial particles. In the second step (6-16 days), a second front moved toward the glass, separating a white domain (plus the thin transparent skin) (top) from a transparent domain underneath.

Figure 4. Water loss versus square root of time (in hours at the bottom, in days on top) for the 10 mm thick layer of emulsion dried at 23 °C and 50% RH: surfactant concentration, 4.4 wt %; no filler.

mentioned above, at the end of the first drying step (6 days ) 144 h), the dry content was around 96%. It is also interesting to plot these data versus the square root of time (Figure 4). Three domains can be distinguished in the curve: domain I, at the beginning, nonlinear, and over around 6 h; domain II, linear from a water loss of 2.5% to 70%, ends after 6 days. The departure from linearity corresponds to the end of the first step in the macroscopic drying mechanism. The remaining water is lost in domain III, nonlinear again, where the drying rate strongly decreases, especially after 95%. According to Crank,10 the linear part of the curve (domain II) can correspond to a mechanism of water loss by diffusion through a top layer of increasing thickness with a constant diffusion coefficient. The diffusion coefficient, D (in cm2/ s), can be calculated through (valid at short t)

Mt 2(Dt)1/2 ) M∞ Lπ1/2 with Mt ) mass of water evaporated at time t, M∞ ) mass of water evaporated at equilibrium, t ) time, and L ) initial thickness of the layer in cm. (10) Crank, J. The Mathematics of Diffusion; Clarendon: Oxford, 1975.

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Figure 5. Concentration profiles of water in the drying emulsion at various times: water evaporates from the top; surfactant concentration, 4.4 wt %; no filler.

From this expression, D was found to be 8.7 × 10-7 cm2/s. 3.1.2. Infrared Microscopy (IRM). In Figure 5, concentration profiles of water are shown at various times. The difference between the two steps previously described (0-6 days/7-16 days, see Figure 3) can be seen in these curves. In the first days, a strong concentration gradient is apparent in the middle part of the drying emulsion. After 6 days, the gradient becomes much less pronounced. On the other hand, in the two first millimeters on top of the emulsion, water-rich heterogeneities appear very early in the drying process (after 20 h). A heterogeneity becomes also apparent at a depth of 6 mm at the end of the drying time. A surprising “negative” peak appears at a depth of around 9 mm during the second drying step. As the confined sample is not free to retract upon drying, it could correspond to an artifact associated with confinement. 3.1.3. Emulsion-Substrate Interface Studied by

ATR. Drying kinetics can be calculated from ATR spectra and compared to the overall kinetics obtained from gravimetric measurements (Figure 6). The interfacial area shows a special feature: 70% of the initial amount of water is lost monotonically during 6 days leading to a film with a dry content of around 95%. Then, the water content strongly increases again to reach a maximum at 10 days followed by a decrease to the dry state. 3.2. Effects of Various Parameters. 3.2.1. Thickness. When the thickness of the deposited emulsion layer was decreased from 10 mm to 5, 2, and 0.19 mm, the total drying times were reduced from 16 days to 4 days, 15 h, and 30 min, respectively. However, in all cases, the same curve shape was observed with a linear central part leading to the same value of the diffusion coefficient (Figure 7). Lateral homogeneity of the drying process, already mentioned for the 10 mm thick sample, was also observed for thinner samples, whatever the thickness.

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Figure 6. Water loss versus time for the 10 mm thick drying emulsion: open squares, whole sample (gravimetric data); full circles, emulsion-substrate interfacial area (ATR data). Surfactant concentration: 4.4 wt %. No filler.

Figure 7. Thickness effects: water loss versus square root of time for various thicknesses; standard drying conditions, 23 °C, 50% RH; surfactant concentration, 4.4 wt %; no filler.

In the 1 µm thick layer in contact with the substrate, ATR analysis (Figure 8) showed that the effect mentioned above for the 10 mm thick samples (the dry content first increasing, then strongly decreasing before increasing again to 100%) was also observed with thinner samples but slightly less marked when the thickness was 2 mm and only faintly developing when it was 0.19 mm. 3.3.2. Surfactant Concentration. When the surfactant concentration was decreased from 4.4 to 2.6 wt %, the emulsion dried faster. This is shown by gravimetric measurements for 10 mm thick samples (Figure 9) and by ATR data for 2 mm thick ones (Figure 10). 3.3.3. Filler. The effect of the filler on the global drying kinetics is shown in Figure 11. The filler slightly decreased the drying rate. Treating the filler with stearic acid to make it hydrophobic had almost no effect. Interestingly, in the 1 µm thick layer in contact with the substrate, the special effect observed previously (dry content decrease during the drying process) disappeared in the presence of the filler (Figure 12). All filled emulsions remained white during and after drying. 4. Discussion Normal versus Lateral Drying. The system described in this paper exclusively dries in the mode normal to the surface whatever the thickness, between 0.19 and 10 mm,

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and the drying conditions.8 Let us briefly discuss the possible reasons why drying occurs in a pure normal mode with no contribution at all of the lateral mode. The question of the parameters controlling the mechanisms of drying was extensively treated in ref 4. The main ones are thickness and geometric effects, structure and rheology of the emulsion, and global rate of water loss. Differences in thickness of the cast emulsion are crucial, whatever the origin of the difference. In thinner parts of the layer, the water concentration decreases faster and this, in turn, provokes a lateral water flux in order to compensate for concentration gradients. On the other hand, for lateral drying to occur, fluxes within the wet portion of the drying film transport particles to the dry-wet boundary layer. The rate of this flux is sensitive to the viscosity of the wet domain. When the emulsion is fluid, the system is expected to be very sensitive to thickness effects. On the contrary, if the emulsion is able to form a colloidal crystal or is highly concentrated or the continuous phase in highly viscous (containing water-soluble polymers, for instance), the particles no longer have the possibility to move freely in the liquid. The flux will be suppressed, thickness effects will be much less effective, and drying is more likely to occur as a front moving normal to the surface. Qualitatively, it can be argued that a slow rate of evaporation (as caused by low temperature, high relative humidity, or static air) favors uniform drying (because the water concentration has more time to equilibrate within the emulsion) or lateral drying. Conversely, a fast rate of evaporation will lead to a rapid elimination of water in the top region of the drying layer all over the surface and lead to the normal drying mode. This is neatly illustrated in work reported by Ciampi et al.11 In our case, the dominant parameter seems to be the mobility of the droplets in the emulsion. The high concentration of the oil phase suppresses long-range motion of the drops, and drying only takes place in the normal mode even if the thickness is probably not very constant and whatever the global drying rate. The relative importance of the three mentioned parameters (thickness differences, mobility of the drops, global rate of evaporation) should be further investigated. In classical models of normal drying,4,12,13 there is always a first step of constant drying rate. This is not observed in our case: the drying rate decreases continuously from the beginning. A constant drying rate is observed in dilute systems where water is eliminated through a continuous water-air interface. Our emulsion being concentrated from the beginning, the drying rate is never constant. Two-Step Drying Mechanism. In its initial state, our emulsion is already concentrated above the volume fraction of close-packed spheres. Thus, droplets present 12 flat areas per drop. A geometrical description of such a system14 involves two parameters: the thickness (δ) and diameter (∆) of the flat regions (Figure 13). Instead of ∆, often used is the angle between the direction defined by the center of the droplet and the center of the flat area and the direction defined by the center of the droplet and the edge of the flat area. A different but equivalent way (11) Ciampi, E.; Goerke, U.; Keddie, J. L.; McDonald P. J. Langmuir 2000, 16, 1057. (12) Vanderhoff, J. W.; Bradford E. B.; Carrington, W. K. J. Polym. Sci.: Polym. Symp. 1973, 41, 155-174. (13) Croll, S. G. J. Coat. Technol. 1986, 58 (734), 41. Croll, S. G. J. Coat. Technol. 1987, 59 (751), 81. (14) Princen; H. M. J. Colloid Interface Sci. 1979, 71, 55. Princen; H. M.; Aronson, M. P.; Moser; J. C. J. Colloid Interface Sci. 1980, 75, 246. Aronson, M. P.; Princen, H. M. Nature 1980, 286, 370.

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Figure 8. Thickness effects: water loss or dry content versus time in the 1 µm thick layer of the emulsion in contact with the substrate (ATR analysis) for various thicknesses; standard drying conditions, 23 °C, 50% RH; surfactant concentration, 4.4 wt %; no filler.

Figure 9. Effect of surfactant concentration: water loss versus time for 10 mm thick samples; standard drying conditions, 23 °C, 50% RH; no filler.

Figure 10. Effect of surfactant concentration: water loss versus time in the 1 µm thick layer of the emulsion in contact with the substrate (ATR analysis); standard drying conditions, 23 °C, 50% RH; layer thickness, 2 mm; no filler.

is to describe the system through δ and rPB, the radius of curvature of the adjacent Plateau border. It is possible to explain the two-step drying mechanism observed with our emulsion. The first step corresponds to the development of transparency (between 0 and 6 days of drying, for the 10 mm thick sample). The behavior of the skin is specific. In the skin, transparency is due to

Figure 11. Filler effect: water loss versus time for 10 mm thick samples; standard drying conditions, 23 °C, 50% RH; surfactant concentration, 4.4 wt %; filler concentration, 25 wt %.

coalescence: a homogeneous and thus transparent layer results.8 Under the skin, the transparent state has a different structure because, directly after transparency appears, the emulsion remains redispersable and leads to the same particle size distribution as the initial emulsion. Our interpretation is that this transparent state corresponds to noncoalesced but highly deformed drops. The flat areas have reached their minimum thickness and maximum extension (∆ maximal). The Plateau borders contain all remaining water plus all the nonadsorbed surfactant. The flat areas do not scatter visible light because their thickness is too small. They are structurally identical to Newton black films observed in dry foams and highly concentrated emulsions.15,16 The Plateau borders do not scatter light either because they are also too small or because there is a refractive index matching between the oil in the droplets and the surfactant solution they contain. Both hypotheses are plausible, but the size effect is more likely because transparency was also (15) Sonneville-Aubrun, O.; Bergeron, V.; Gulik-Krzywicki, T.; Jo¨nsson, B.; Wennerstro¨m, H.; Lindner, P.; Cabane, B. Langmuir 2000, 16, 1566. (16) Poulin, P.; Nallet, F.; Cabane, B.; Bibette, J. Phys. Rev. Lett. 1996, 77, 3248.

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Figure 12. Filler effect: water loss or dry content versus time in the 1 µm thick layer of the emulsion in contact with the substrate (ATR analysis); standard drying conditions, 23 °C, 50% RH; layer thickness, 2 mm; surfactant concentration, 4.4 wt %; filler concentration, 25 wt %.

Figure 13. Geometrical degree of freedom of the system at a given water volume fraction: more water plus surfactant in the Plateau border (left) leading to small δ and ∆ and large rPB or more water plus surfactant in the flat region (right) leading to large δ and ∆ and small rPB.

observed in the system containing less surfactant. It is unlikely that systems with different surfactant concentrations can both lead to index matching. Transparency was also observed by Ruckenstein17 or by Sonneville et al.15 in concentrated (centrifugated) hydrocarbons in water emulsions stabilized by the nonionic Triton X-100 or SDS, respectively. This kind of transparent state is only possible because of the high resistance of the emulsion to coalescence (except in the skin). It appears that the surfactant was properly selected for that purpose. After the transparent state, as water further evaporates, the system whitens again. It will be shown below that coalescence occurs between the transparent and the white state, in fact, shortly after the appearance of transparency. The white color is probably due to domains of segregated surfactant. It is also possible that, due to the fact that cross-linking occurs in the drops during drying, coalescence and particle deformation are incomplete and this leaves light scattering voids in the film. As will be discussed in a forthcoming paper,8 voids can also originate from microcracks developing during film formation.18,19 Thus, at the end of drying, except for a very thin skin on top, the film is totally white again. The first step in the drying mechanism corresponds to a linear portion in the curve giving the global water loss versus the square root of time (Figure 4). This result is (17) Ruckenstein, E. Adv. Polym. Sci. 1997, 127, 1. (18) Allain, C.; Limat, L. Phys. Rev. Lett. 1995, 74, 2981. (19) Pekurovsky, L. A.; Scriven, L. E. Submitted for publication in J. Colloid Interface Sci.

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classically attributed to the existence of a slow step in the mechanism, corresponding to the diffusion of water through a layer of increasing thickness with a constant diffusion coefficient.10 That raises the question of the nature of this layer. It could be the coalesced skin on top of the emulsion. Its thickness increases indeed monotonically during the first step. However, if this were true, the concentration of water underneath would have time to equilibrate. This is obviously not the case: a strong concentration gradient builds up under the skin during step 1, as can be clearly seen in Figure 5. On the other hand, if the drying rate is markedly decreased, no skin is formed but there is still a linear portion in the curve.8 Thus, crossing the skin cannot be the rate-limiting step. The layer of increasing thickness is then heterogeneous and must comprise the coalesced skin plus the transparent part of the film. The measured diffusion coefficient is only an apparent one. Temporary Dilution of the Emulsion near the Interface. The most intriguing result in this study is the fact that, in the micrometer size region near the emulsionsubstrate interface, water is not lost monotonically, but the water content first decreases and then increases before decreasing again (Figure 6). Dilution of the zone starts soon after it has become transparent (6 days in the reference system). The maximum of the peak corresponds to a water content almost equal to the one of the initial emulsion. This observation was reproduced several times, it cannot be an artifact. The most likely interpretation is that this phenomenon is due to coalescence, following the transparent state described in the previous section. In a drying emulsion, it is logical to attribute the starting of the coalescence event to the increasing depression in the Plateau border (itself due to the increasing curvature) which destabilizes the surfactant layers confined between two particles. Film rupture is assumed to occur when the film thickness falls below a certain critical thickness.20 When the starting emulsion is very monodisperse, it has been shown that coalescence proceeds through the growth of droplets which remain all of similar size.21 On the other hand, if the starting emulsion is polydisperse, coarsening can occur through the growth of a few randomly distributed droplets that eat up the small ones, as observed by Bibette et al.22 in a silicone oil-in-water emulsion stabilized by sodium dodecyl sulfate above its critical micelle concentration (cmc).23 This later case is likely to occur in our system. Coalescence in a certain region of the emulsion drastically redistributes water (and surfactant) in this zone. As discussed above, there is a clear vertical main direction for water motions in this system. Water “freed” by coalescence goes upward to follow the drying flux, but it can also be rejected downward. In fact, the amount of water going down is superior to the amount going up because, above the region considered, coalescence has already started and the permeability of the upper layers is probably much lower than that of the lower layer. When this phenomenon takes place near the interface, the down stream of water which redilutes the interfacial zone is detected by the ATR measurements. (20) Bhakta, A.; Ruckenstein, E. Adv. Colloid Interface Sci. 1997, 70, 1. (21) Deminiere, B.; Collin, A.; Leal-Calderon, F.; Bibette, J. Phys. Rev. Lett. 1999, 82, 229. (22) Bibette, J.; Morse, D. C.; Witten, T. A.; Weitz, D. A. Phys. Rev. Lett. 1992, 69, 2439. (23) Leal-Calderon, F.; Poulin, P. Curr. Opin. Colloid Interface Sci. 1999, 4, 223.

Film Formation from Silicone Emulsions

The above discussion indicates that, in our mind, what happens near the interface, also happens earlier, higher in the film. This is confirmed by the observation of water peaks in the infrared microscopy data (Figure 5), similar to the water peak visible near the interface (see, for example, Figure 5-2 or Figure 5-5). However, the interfacial zone may have some specificity. Water could be attracted by the interface due to the existence of capillary and/or osmotic pressure gradients. This speculation, partly confirmed by the behavior of the filled system discussed below, needs to be further investigated. During drying of our system, a third front (a coalescence front), moving in a vertical direction, parallel to the film surface, from top to bottom, can be added to the two already defined earlier in this paper (Figure 3). This front closely follows the one above where transparency appears and separates the top layer where coalescence has started from the bottom zone where particles are still separated by surfactant layers. Interestingly, the presence of a CaCO3 filler suppresses the water peak in the interfacial zone (Figure 12). This could be a confirmation of the attractivity of the emulsionsubstrate interface toward water, postulated above. The water rejected from the zones where coalescence has taken place could be trapped by the numerous emulsion-filler interfaces existing in the filled system, and when the coalescence front would approach the emulsion-substrate interface, not enough water would be left to dilute the area. The considerations developed so far about drying mechanisms will be complemented by the results describing the fate of the surfactant in the next paper of this series.7 Effects of Parameters. The drying mechanisms described in the preceding sections seem almost independent of the deposited layer thickness. The total drying duration for thinner layers is obviously shorter, and this seems to be the main thickness effect. Indeed, thinner films also dry in a normal (vertical) mode; a thinner film has the same structure as the top layer of a thicker one (these structural aspects are developed in ref 8); the linear parts in the curves relating the water loss to the square root of time lead to exactly the same apparent diffusion coefficient (Figure 7); temporary dilution of the interfacial zone also occurs (Figure 8). However, Figure 8 indicates that the extent of the dilution decreases when thickness decreases. This can be directly attributed to the global rate of water loss. As discussed above, coalescence rejects water upward and downward. The higher the global rate of drying, the higher the upward drying fluxes and the less pronounced the dilution effect of the layer beneath the coalescence front. This is further confirmed by the effect of the surfactant content in the emulsion (Figure 9 and Figure 10). Again, when the global rate of water loss is lower, the water content near the interface is shifted upward (Figure 10). The question here is why does a decrease in the surfactant content from 4.4 to 2.6 wt % increase the drying rate (Figure 9)? The effect of the surfactant on drying rates is not straightforward. The simplest way is to consider a hydrophilic solute effect: an aqueous solution dries slower than pure water. We shall simply invoke this effect to explain our result. However, in other circumstances, the surfactant can also increase the drying rate, depending on its concentration, as observed by Winnik and Feng in drying kinetics studies of latex blends.24 These authors reasoned that the presence of the hydrophilic surfactant (24) Winnik, M. A.; Feng, J. J Coat. Technol. 1996, 68 (852), 39.

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in the pore structure in which water moves toward the film surface increases the rate of transport. The filler decreases the global drying rate (Figure 11). In the drying kinetics of latex blends, it has been shown that hard particles added to a soft latex decrease the drying rate.24 The reason given for that phenomenon was that the hard particles increase the tortuosity of the path taken by water leaving the film.25 A similar interpretation seems valid in our case. However, the most spectacular effect of the filler is to alter the drying behavior near the interface (Figure 12): the water peak is suppressed. As already mentioned above, this result may indicate that an interface could have an attractive effect on the water and surfactant rejected by the emulsion upon coalescence. This should be confirmed and the exact mechanism of this attraction investigated. 5. Conclusion This emulsion, and it is likely a general situation for all concentrated ones, dries in a pure normal mode, whatever the thickness of the deposited layer, normal in the sense that the main motions of water occur essentially in a direction normal to the surface (vertical in this case). Furthermore, heterogeneities appear in the distribution of water in this normal direction: there is rapidly less water on top than on bottom of the system. Starting the drying process with a more dilute emulsion would possibly lead to a very different mechanism in which lateral water motions (in the plane of the layer) could play an important role. The initial concentration of the latex is most probably a crucial parameter as far as drying is concerned. The drying process can be described as three successive fronts, roughly parallel to the surface, moving in a vertical direction, from top to bottom. Two fronts are observed macroscopically in thick samples. The first front separates a transparent region on top from a white part on bottom. The transparent state is a metastable, highly concentrated (around 96% dry content), noncoalesced emulsion. It is transparent because the size of the interstitial medium is too small to scatter visible light. The existence of this metastable state is a direct consequence of the efficiency of the surfactant. A less stabilizing surfactant would not allow the passage through such a well-defined intermediate state. The second macroscopically detected front switches the colors in the opposite way: white again on top and transparent on bottom. The whitening of the film is attributed to large surfactant domains segregated from the cross-linked polymeric matrix. Voids due to incomplete particle deformation or to microcracks appearing during drying could also contribute to scattering of visible light. Using attenuated total reflectance (ATR) and infrared microscopy, it was possible to obtain evidence of a third front, closely following the first one, a coalescence front. It also moves from top to bottom: coalescence occurs first on top and progressively develops toward the bottom. Coalescence destabilizes the metastable transparent state and rejects water (and surfactant) mainly downward. Underneath this coalescence front the emulsion is thus rediluted. This dilution is particularly clearly observed near the emulsion-substrate interface by ATR. Infrared microscopy indicates, less clearly though, that the same phenomenon also takes place earlier, higher in the film. Here, a very interesting question is raised: does the interface with the substrate attract, to a certain extent, the water plus surfactant mixture? When an inorganic (25) Mackie, J. S.; Meares, P. Proc. R. Soc. London, Ser. A 1955, 232, 498.

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filler is introduced in the emulsion, the dilution of the interfacial region near the substrate is no longer observed. This result could indicate that the filler-emulsion interface, the total area of which is very high, effectively attracts water and surfactant, preventing the dilution of the interface with the substrate. Subtle capillary or osmotic pressure gradients could be imagined near an interface, influenced by the nature and shape of the surface in contact with the emulsion. This point really deserves further investigations. Some parameters were investigated, namely, thickness of the film, surfactant concentration, and presence of a CaCO3 filler. The main effect of these parameters is to alter the global and, therefore, the local drying rate and drying duration. Decreasing the total thickness decreases the total drying duration. The surfactant concentration alters the drying rate through a hydrophilic additive effect.

Guigner et al.

And finally, the filler decreases the drying rate because it increases the tortuosity of the escape routes for water. The distribution of the surfactant in the final film strongly depends on the drying mechanisms discussed in this paper. The corresponding results will be presented in part 2 of this series.7 The structure of the film resulting from drying (and cross-linking) of the emulsion was not detailed in this publication, it deserves a separate paper8 in which more considerations about drying will appear. Acknowledgment. We thank Rhodia for financial support. It is also a pleasure to thank Drs P. Branlard, Y. Giraud, and M. Feder (Rhodia, St Fons and Lyon) for helpful discussions. LA001711D