Characterization of Micelles of Small Triblock Copolymer by Small

Article pubs.acs.org/Macromolecules

Characterization of Micelles of Small Triblock Copolymer by SmallAngle Scattering Amandine Forny-Le Follotec,*,†,‡,⊥ Otto Glatter,§ Isabelle Pezron,† Loïc Barré,‡ Christine Noïk,‡ Christine Dalmazzone,‡ and Léa Metlas-Komunjer† †

Groupe Interfaces et Milieux Divisés, EA 4297 UTC/ESCOM Transformations Intégrées de la Matière Renouvelable, Université de Technologie de Compiègne, BP 20529, 60200 Compiègne Cedex, France ‡ IFP Energies Nouvelles, 1&4 avenue de Bois-Préau, 92852 Rueil-Malmaison, France § Department of Chemistry, University of Graz, A-8010 Graz, Austria ABSTRACT: We investigated the structures formed in water by small triblock copolymers consisting of two poly(ethylene oxide) (PEO) side blocks and a central poly(dimethylsiloxane) (PDMS) block. Our objective was to identify a possible relation between the spontaneous curvature of copolymers and their efficiency in the demulsification of water-in-crude oil emulsions observed in earlier studies. Spherical micelles containing about 100 molecules, i.e., with a highly curved structure, were detected using concentrations of up to 20 wt % of copolymer in water at room temperature. At higher concentrations or at higher temperature, micelles changed to an elongated shape. SAXS data and analysis of electron density profile variation inside the scattering object revealed the inner structure of the micelle. Finally, the role of the high spontaneous curvature of the demulsifier copolymer is discussed in relation with the mechanism of coalescence of water droplets involved in the breaking of water-in-crude oil emulsions.

I. INTRODUCTION During crude oil extraction, off-shore but also on-shore, water is present in abundance. The strong mechanical solicitations in pumps and valves often foster the formation of extremely stable emulsions of water-in-oil type. In order to diminish the transportation and refinery costs, water should be separated from crude oil. Classical physical treatments based on gravitational separation, optionally enhanced by electrocoalescence, or heating can be used to break these emulsions. However, these processes are time-consuming; therefore, chemical additives such as amines, copolymers of ethylene oxide, of propylene oxide, etc., in various solvents are used to accelerate the demulsification process.1−3 Unfortunately, the efficiency of such formulations is strongly dependent on the composition of the crude oil. In fact, some of the most important parameters are the content and ratio of surface-active constituents, namely asphaltenes and resins. To avoid expensive on-field tests, demulsifier molecules with high efficiency, and working independently of the origin of the crude oil, were actively searched for.4 Laboratory studies and field tests showed that small triblock copolymers of poly(ethylene oxide) (PEO) and poly(dimethylsiloxane) (PDMS) type act as efficient demulsifiers regardless of the crude oil composition. The interfacial properties of such copolymers have been studied in detail;5−7 however, no clear-cut relationship between chemical composition and demulsifying efficiency is established so far. The conformation of the demulsifier molecules at the oil−water interface was suggested to be of particular interest for © 2012 American Chemical Society

the understanding of the demulsification process. In particular, the spontaneous curvature of interfaces formed in presence of surface-active molecules was shown to be related to the stabilizing (and destabilizing) ability of these molecules in emulsification processes.8 A detailed examination of self-assembled structures of the aforementioned molecules should provide useful information about their spatial configuration at interfaces. Therefore, we propose small-angle scattering as a method to determine the size and shape of self-assembled structures of efficient crude oil demulsifier and to consequently deduce the structural requirements for these compounds to successfully separate water from crude oil. The interfacial behavior of triblock silicone copolymers has been studied by numerous authors. Kanellopopoulos and Owen measured the surface tension γ of aqueous solutions of a series of triblock silicone copolymers and determined the area occupied by each molecule via the Gibbs equation. 9 Surprisingly, they observed that the minimum area per molecule was 0.6 nm2 for a central dimethylsiloxane segment length DMS > 23, while being 0.8−1.3 nm2 for DMS < 23. As explanation, the authors proposed the looping of the central silicone segment when exceeding a certain length (DMS > 23). Measuring γ versus solution concentration also allowed to Received: December 1, 2011 Revised: March 1, 2012 Published: March 12, 2012 2874

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critical micelle concentration and critical micelle temperature is crossed. P85 has been studied in scattering vector q range of 0.2− 5 nm−1 by both small-angle neutron and X-ray scattering (SANS and SAXS).17 Pair distance distribution functions (p(r)) and excess scattering length density distributions gave information about the shape and the size of micelles. Spheres with an inner PPO core radius of 3 nm and an outer shell of PEO with a thickness of 5 nm have been detected. Pedersen also elucidated the particular q−2 behavior at large q as corresponding to a hazy interface due to dissolved flexible PEO chains in water. Mortensen and Pedersen already suggested similar structures in 199318 and in 1994 Glatter et al.19 defined the P85 micelles as a star-shaped structure consisting of a compact PPO core and a soft PEO shell with starlike arms extending into the water phase. In such a model, the electron density of the shell, determined by deconvolution of the p(r) function, decreases monotonically with distance because the interface is not sharp. Mortensen et al. also showed that by increasing temperature, the shape of micelles changes from spheres to ellipsoids.18 Orthaber et al. also used SAXS and SANS to study micelles of a similar PEO−PPO−PEO copolymer P94 with composition EO17−PO42−EO17.20 The electron density distribution from deconvolution of the p(r) function revealed a lower electron density for the hydrophobic part, but a higher electron density of hydrophilic part with respect to the solvent. The zero crossing of the density can be used to estimate the radius of the hydrophobic part, which is about 2.5 nm, whereas the PEO shell thickness is 5.5 nm. In view of these results it seems reasonable to conclude that sufficient contrast between hydrophobic and hydrophilic parts makes small-angle scattering a powerful tool to study the structure of micelles of PEO−PPO− PEO triblock copolymers in detail. In this study, we propose to investigate the structure formed by triblock copolymer EO12−DMS13−EO12 with concentrations up to 30 wt % in water by means of SAXS and SANS. The efficiency of this polymer in demulsifying water in crude oil emulsions has been demonstrated in a previous study.21 Indeed, our objective is to relate its structural features with its demulsifying properties.

determine the critical micelle concentrations (CMC) of the copolymers. Thus, a copolymer containing on average 12.5 dimethylsiloxane monomers (i.e., 950 g/mol) as central block and two polyether side blocks of 2200 g/mol, with each containing an equal number of randomly distributed poly(ethylene oxide)−poly(propylene oxide) (PEO−PPO) monomers, they found a CMC value of 0.126 g/L (24 × 10−6 mol/L). However, the properties of the formed micelles are not discussed. Yang et al. established phase diagrams for PEO−PDMS− PEO polymers with different proportions of EO and DMS segments: 350−750−350, 550−750−550, 750−750−750, 750−1000−750, and 750−1300−750, which correspond to 48.3, 59.5, 66.7, 60, and 53.7 wt % of PEO.10 The concentrations and lengths of the hydrophobic and hydrophilic blocks of these polymers cause the formation of either hexagonal or lamellar phases. At very low and very high polymer concentrations, the solution is said to be isotropic. Micelles are obviously present but not described in detail: only strong dependence of their size on temperature is mentioned. Zhan et al. studied the behavior of aqueous solutions of small triblock copolymer EO9−DMS7−EO9 with TEM. On the basis of the obtained images, the authors suggested a sphere-to-rod transition of the assemblies as the concentration of copolymer in the solution increases.11 Yan et al. observed 1 wt % of EO15−DMS15−EO15 in water by cryo-TEM and observed small unilamellar and large multilamellar vesicles as well as threadlike micelles.12 Zou et al. published a phase diagram of the same copolymer in glycerol−water mixtures: lamellar phase and micelle formation depended on the polymer concentration.13 Hill et al. and Lin et al. observed vesicle formation, or lamellar phase dispersions, and micelles with EO7−DMS15− EO7 and EO12−DMS15−EO12.14,15 They concluded that the coexistence of vesicles and micelles was due to the polydispersity of the copolymer sample. They also suggested that micelles are of spherical shape for longer hydrophilic chain. Taken together, literature indicates that spherical micelles of PEO−PDMS−PEO triblock copolymers cannot be formed in water if the hydrophilic PEO chains are below a critical length. Comparing with classical surfactants, this corresponds to the situation where the area per polar headgroup is small and the critical packing parameter (cpp) exceeds 1/3; i.e., the structure is less curved. The cpp gives understanding about the spontaneous curvature a surface-active molecule will adopt. It takes into account the volume of the hydrophobic chain v, its fully stretched length lc, and the effective area occupied by the polar head a0: cpp = v/(a0lc).16 It is not straightforward to estimate the cpp value of a triblock copolymer since the volume v should also take into account the hindrance of the whole molecule, i.e., the volume between the two hydrophobic chains, which depends on the conditions (e.g., bulk, plane or curved interfaces, curvature of the molecule, etc.). To the best of our knowledge, the detailed quantitative study of the structures of PEO−PDMS−PEO block copolymer micelles in water is not proposed in the literature up to now. In contrast, micelles of several poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) copolymers have been fully described.17−20 One of the best-documented block copolymers of this type is P85 surfactant with formula EO25−PO40−EO25. It was extensively studied by small-angle scattering. The molecules form spherical micelles in water whenever a phase line of

II. EXPERIMENTAL SECTION AND DATA TREATMENT A. Sample. The demulsifier molecule purchased from Momentive Performance Materials, Meyrin, Switzerland, was used as received. It is a triblock copolymer consisting of two equal side blocks of PEO and a central block of PDMS. The stated composition is PEO12−PDMS13− PEO12 with a molecular weight of about 2100 Da: 550 Da for each of the PEO parts and 1000 Da for the PDMS part and a density of 1.049 g/cm3. The critical micelle concentration determined in ultrapure water (18 MΩ cm−1) at 20 °C is 0.0017 wt % or 8.1 × 10−6 mol/L. The area per molecule was 0.45 nm2 as calculated from the slope of γ vs ln C curve.5 B. Dynamic Light Scattering (DLS). Dynamic light scattering measurements were performed at a scattering angle of 90° with a Zetasizer 3000HSA (Malvern Instruments) on three different samples of 1 wt % copolymer in water at room temperature (about 20 °C). The method consists in determining the hydrodynamic radius from the diffusion coefficient and the Brownian movement of nanosized colloids. A DLS experiment has been performed to check if this hydrodynamic radius was in accordance with SAXS and SANS results. Only low concentration (i.e., 1%) was used to avoid interaction between micelles. The angle of the beam was kept constant (90°), and the autocorrelation function was analyzed with the assumption of Brownian motion of noninteracting micelles. 2875

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C. Small-Angle Neutron Scattering (SANS). SANS experiments were carried out on a small-angle PAXE spectrometer at the Laboratoire Léon Brillouin at CEA/Saclay, France. Three wavelength/sample-to-detector distance configurations, 10 Å/5 m, 6 Å/2 m and 5 Å/1.1 m, were used corresponding to a range of 0.08−3.9 nm−1 scattering vectors q, respectively. The polydispersity wavelength Δλ/λ is close to 5%.22 Solutions of 1, 2, 5, 10, 20, and 30 wt % of copolymer in deuterated water were studied in quartz cells (2 mm, Hellma) at room temperature (about 22 °C). The detected intensities were divided by the transmission and sample thickness, corrected for emptycell scattering, and normalized to scattering from hydrogenated water. Absolute units (cm−1) were obtained by measuring the incident flux. D. Small-Angle X-ray Scattering (SAXS). SAXS measurements were performed at IFPEN, Rueil-Malmaison (France) on an in-house experimental setup. A copper rotating anode generator (Rigaku Micromax 007), operating at 0.8 kW, provides an X-ray beam, which is reflected on a parabolic multilayer mirror from Xenocs. The reflected monochromatic beam (λ = 0.154 nm) is collimated by two pairs of crossed slits whose parasitic scattering is removed by another pair of crossed guard slits located just before the sample. These measurements will be referred to as “pointlike beam” experiments. The 5 wt % solution of copolymer in distilled water was loaded in a cell closed by two thin-glued mica windows with 1 mm optical path. A 2D multiwire proportional position-sensitive detector from Rigaku collected the scattering intensity, which was found isotropic. The position of each pixel, relatively to the position of the direct beam, is converted into the magnitude of the scattering vector q (defined as q = (4π/λ) sin(θ/2) for an angle θ) thanks to external standard (Ag behenate). The covered q range is 0.1−2.66 nm−1. After normalization with respect to thickness, transmission, and measuring time, the water signal was subtracted from the sample signal and the raw intensities were converted to the scattering cross section I(q) in absolute scale (cm−1) thanks to measurement of toluene as secondary standard. The present experiments were conducted at various temperatures from 20 to 60 °C. Another set of SAXS experiments were performed at the University of Graz, Austria at 25 °C (referred to as “linear beam” experiments). The SAXS equipment comprises a SAXSess camera (Anton-Paar, Graz, Austria), connected to an X-ray generator (Philips, PW1730/10) operating at 40 kV and 50 mA with a sealed-tube Cu anode. A Göbel mirror was used to convert the divergent polychromatic X-ray beam into a focused line-shaped beam of Cu Kα radiation (λ = 0.154 nm). The 2D scattering pattern was recorded by a PI-SCX fused fiber-optic taper charged-couple device (CCD) camera from Princeton Instruments, which is a division of Roper Scientific, Inc. (Trenton, NJ). Background subtraction was performed on the 2D image before integration into the one-dimensional scattering function I(q). The temperature of the capillary and the metallic sample holder was controlled by a Peltier element. Samples were exposed to X-rays for 10 min, and measurements were performed in triplicate. After averaging, scattering curves were put on absolute scale using water as a secondary standard,20 and the smeared data were corrected for instrumental broadening. Ideally, a scattering experiment uses a pointlike primary beam, sample, and detector resolution and implies monochromatic radiation. However, most experiments do not meet all of the requirements, which leads to smearing effects. Moreover, instrumental broadening effects result in broader and shorter peaks in the intensity curve. Figure 1 illustrates the smearing effects for the linear beam experiment in comparison to the pointlike beam experiment. The data shows that comparison between the two beams is only possible after desmearing data from the linear experiment. The scattering length densities used for SAXS (ρ) and SANS (b) are presented in Table 1. Smeared linear beam data (crosses) result in a curve with broader and less pronounced peaks. In contrast, peaks of the desmeared linear beam curve are similar to those of the pointlike beam curve and statistical noise is lower. Data were treated and modeled with the PCG software, GIFT, from the Department of Chemistry, University of Graz, Austria. This

Figure 1. Scattering intensity for 5 wt % data (SAXS) with pointlike beam (20 °C) or linear beam (25 °C) setup. Pointlike beam data should be compared to desmeared linear beam data.

Table 1. Electron Densities and Scattering Length Densities23 SAXS ρ (e− Å−3)

SANS b (1010 cm−2)

0.316 0.35 0.334

0.06 0.6 −0.56 6.38

DMS EO H2O D2O

method considers the intensity I(q) on absolute scale [cm−1] as the product of the particle density n [cm−3], the form factor P(q) being the scattering cross section per particle [cm2] (not normalized) and the structure factor S(q).

I(q) = nP(q)S(q) Methods for the comprehensive analysis of small-angle scattering data, including least-squares methods, indirect Fourier transformation, square-root deconvolution, size distribution determinations, and modeling with corrections for instrumental smearing effects have been reviewed in detail by Pedersen.17 He also described various models for form factors and structure factors for different geometries. The desmeared scattering curve is the Fourier transform of the pair distance distribution function (PDDF or p(r)): I(q) = 4π

∫0

∞ p(r ) sin(qr )

qr

dr

If one wants to take into account the slit length and width effects and the wavelength effect, this desmeared curve has to be smeared. This smeared curve is called an “approximated curve” as it is calculated by a weighted least-squares fit to experimental data, the indirect Fourier transformation (IFT). The method works for dilute systems, where interparticle effects are negligible and, hence, the structure factor is constant (S(q) = 1). The intensity is only related to the form factor containing the intraparticle effects. The generalized indirect Fourier transformation (GIFT) is the advanced version for interacting systems, taking into account the interparticle effects (S(q) ≠ 1). The convolution square root technique is then used to deconvolute the p(r) function of micelles into the radial density profile ρ(r) assuming spherical shape p(r ) = r 2⟨ρ̃2(r )⟩ = ⟨

∫ ρ(x)ρ(1 − x) dx⟩

where ⟨ ⟩ stands for the spherical average and ρ̃2(r) is the convolution square. For more details concerning the treatment the reader is referred to refs 24 and 25. 2876

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III. RESULTS Solutions containing 1 wt % of copolymer in water were analyzed by DLS. Figure 2 presents an example of the results

Figure 2. Intensity scattered by 1 wt % of copolymer in water from DLS measurement at 20 °C.

obtained from light scattering measurements. The micelle size distribution is monomodal and quite sharp, with a mean diameter of 15.5 nm. Measurements were performed on three different samples to investigate the reproducibility. All three samples gave very similar results, and only one of them is reported for clarity of the illustration. To assess the structural details of the formed micelles, we performed SAXS and SANS measurements on samples containing between 1 and 30 wt % of copolymer in D2O for SANS and H2O for SAXS. Characteristic scattered intensities I versus scattering vector q (scattering functions) are shown in Figure 3. The intensity is divided by the solution concentration: correction for monomer concentration was not necessary as the CMC is very low compared to the used concentrations. The intensity curves at high q-values (q > 0.5 nm−1) are superimposed for all examined concentrations, signifying that scattering curves only depend on the form factor P(q) of the micelles. The shape of the curves in this high q-region indicates that micelles have a similar shape. The position of the minimum and the first side maximum in SAXS data (between 0.4 and 0.7 nm−1) remained unchanged for each concentration; the size of the micelle can therefore be considered as constant. For low q-values (q < 0.5 nm−1), the scattering intensity decreases as the concentration increases, indicating beginning interactions between the micelles. This effect will be addressed below by means of variation of structure factor with the scattering vector. The GIFT method is used to determine the PDDF or p(r) function corresponding to the approximated curve with the best fit to the experimental data.24,25 Figure 4 shows the example of the treatment of the 2 wt % solution data. The p(r) functions, corresponding to the approximated curves for all examined concentrations, are presented in Figure 5. The SANS curves present the bell shape ending at 15 nm for all concentrations. This indicates that the copolymer forms globular micelles with a maximum dimension of ∼15 nm, in agreement with DLS data of Figure 2. For 30 wt % copolymer, the shape of the curve changes at large distances, indicating that micelles become elongated or that the polydispersity in the

Figure 3. (a) SANS and (b) linear beam SAXS data of 1, 2, 5, 10, 20, and 30 wt % copolymer in D2O and H2O, respectively. Inset: SANS 2D representations of intensity obtained with 1 and 30% of copolymer in D2O show isotropic scattering.

Figure 4. Scattering intensity (linear beam) for 2 wt % data (SAXS).

Figure 5. Pair distance distribution function at room temperature for (a) SANS and (b) SAXS data of 1, 2, 5, 10, 20, and 30 wt % of copolymer in D2O and H2O, respectively.

system increases. The shape of these curves confirms that the neutrons “see” a global homogeneous sphere because of the low 2877

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contrast between the PDMS core and the PEO shell (0.06 × 1010 and 0.6 × 1010 cm−2) compared with the deuterated water (6.38 × 1010 cm−2). According to the SAXS data, the difference in electron density induces negative contributions to the PDDF: the contrast of the silicone chain to water is negative (Δρ = −0.018 e− Å−3) while it is positive for ethylene oxide chain to water (Δρ = 0.016 e− Å−3). Thus, hydrophobic and hydrophilic parts of the assembly are distinguishable with X-rays. Moreover, SAXS data confirm the particle maximum dimension of about 15 nm, including the slight elongation (or the higher polydispersity) for the micelles in 30% solutions (quasi-linear part of the curve between 11 and 15 nm). The scattering length and the electron density distribution of spheres can be obtained by deconvolution of the p(r). The distribution from SANS data shown in Figure 6a confirms

compact PDMS core and a soft PEO shell with starlike arms extending into the water phase similarly to the micelles of PEO−PPO−PEO copolymers.25 Therefore, the micelle structure of triblock PEO−PDMS− PEO copolymer can be summarized as follows: a dense PDMS core, composed of hydrophobic chains, with a radius of about 2 nm, a transition region from 2 to 4.5 nm, where EO and DMS segments are mixed, and a shell with a thickness of ∼1 nm composed of hydrophilic PEO chains and water. The density profile presented in Figure 6b indicates that zero contrast; i.e., transition between DMS core and EO shell is situated at about 3.5 nm. Therefrom, one can estimate the aggregation number of micelle, i.e., the number of triblock copolymer molecules in the micelle, as Nagg =

4πr PDMS3 total volume of PDMS = volume of one PDMS chain 3v1PDMS

With a radius r of 3.5 nm per PDMS part and a volume v of 1.77 nm3 per PDMS chain (deduced from the molecular weight and the density of bulk silicone chain, which is different from the density of a hydrated polymer), the aggregation number is ∼100 molecules per micelle. Fritz et al. showed that the density inside the core of a micelle is smaller than in the bulk.26 In our calculation, variation in density was not taken into account, and the aggregation number of 100 is given as rough estimation only. As the total volume for one molecule of triblock copolymer is 3.31 nm3, 100 molecules should occupy about 330 nm3. The total volume of the micelle, determined from the external radius of 5.5 nm, is around 700 nm3. Therefore, the excess of water is about 370 nm3, and the number of water molecules per copolymer is estimated to be ∼120, i.e., 5 molecules of water for 1 ethylene oxide group. The hydration number correlates with the high affinity of ethylene oxide chains for water.27 The GIFT treatment also provides information about the structure factor of the assemblies (Figure 7). As pointed out earlier, structure factor is close to 1 (strictly speaking, equal to 1 for infinite dilution) for low copolymer concentration. This is almost the case for the structure factor calculated from 1% SAXS data (Figure 7b), which remains nearly constant and equal to 1 for all values of scattering vector q. As the structure factor describes the interference of interparticle scattering, interparticle interactions increase with the concentration and the distance between the micelles decreases. This effect is visible on the shape of structure factor calculated from both SANS and SAXS data: the S(0) value decreases dramatically with concentration, while the interaction peak becomes sharper and moves to higher q-values, reflecting the increasing interparticle interactions due to the decrease of the mean distance between neighboring particles. According to our observations, SAXS and SANS structure factors are comparable. This indicates that the only changing parameter between X-rays and neutrons is the form factor of the assemblies, corresponding to the different p(r) functions. The volume fractions corresponding to the structure factors are presented in Table 2. The estimated values from SAXS and SANS model are higher than the experimental values due to the presence of water in the micelle, similarly to what was reported by Lindner et al. for P94 copolymer.28 Another set of SAXS experiments was performed with the aim to examine the influence of temperature on the size and

Figure 6. Density profile difference for 1, 2, 5, 10, and 20 wt % of copolymer in (a) D2O and in (b) H2O deduced from the SANS and SAXS data, respectively.

that the contrast between deuterated water and silicone or ethoxylated chain is of same sign, indicating that the core and the shell of the micelle are not distinguishable with this technique. In contrast, the constant negative contribution of the PDMS part for distances up to 2 nm from the center of the micelle is visible by SAXS (Figure 6b); it approaches zero at about 3.5 nm. The density difference equal to zero can be used as an estimation of the radius of the core (interfacial layer) equal to 3.5 nm, whereas the positive density difference corresponds to the hydrophilic shell. The electron density difference increases monotonically; i.e., the interface between the core and the shell is not sharp: the mobility of the polymer chains causes a gradual transition between the two parts. The decrease in the electron density from 4.5 to 5.5−6 nm indicates that the copolymer forms star-shaped micelles in water, with a 2878

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0.47 to 0.35 nm−1, already indicating an increase of the micelle size. The increase of the size of the micelle is confirmed by the p(r) function presented in Figure 9: the maximum dimension is

Figure 9. Pair distance distribution function p(r) for SAXS of 5 wt % copolymer in H2O at temperatures between 20 and 60 °C.

13 nm at 20 and 30 °C and increases to 17, 19, and 21 nm respectively for 40, 50, and 60 °C. In addition, the bell-like shape of the p(r) curves for temperatures of 20 and 30 °C indicates that micelles are of spherical shape. For higher temperatures, the shape of the p(r) curve changes: at 60 °C it becomes almost linear at large distances, indicating that with increasing temperature the objects become rodlike and more elongated up to 21 nm. Similar effects have also been observed with PEO−PPO−PEO copolymers.18

Figure 7. Structure factor deduced from (a) SANS and (b) SAXS data for 1, 2, 5, 10, 20, and 30 wt % of copolymer in D2O and H2O solutions, respectively.

Table 2. Volume Fractions: Experimental Data and Estimation from the Fitting Curves of S(q) for SAXS and SANS Data Φv(exp), %

Φv(SAXS), %

Φv(SANS), %

0.95 1.9 4.8 9.5 19.1 28.6

0.6 2.0 11.9 16.4 26.2 38.6

1.9 3.8 6.4 16 26.5 35.3

IV. DISCUSSION AND CONCLUSION The small-angle scattering data presented here indicate that in the presence of water EO12−DMS13−EO12 molecules preferentially self-assemble to form spherical micelles at concentrations up to 20 wt %. The micelles have a core of low electronic density and an outer shell with a high electronic density when compared to water. The core contains the silicone segments of polymer followed by a mix of hydrophobic and hydrophilic chains. The ethoxylated chains spread into surrounding water. The radius of the micelle is about 5.5 nm. The transition between the hydrophobic core and hydrophilic chain is situated at about 3.5 nm. Similarly to P85 micelles studied by Mortensen and Pedersen,18 the micelles increase in size with increasing temperature (up to 21 nm). For temperatures over 40 °C, significant elongation can be observed. The spherical shape of self-assembled structures corresponds to a low value of critical packing parameter of the molecule (cpp ≤1/3) and to a high spontaneous curvature.16 This finding is is in agreement with our previous detailed study of five triblock PEO−PDMS−PEO copolymers with different proportions of hydrophilic PEO and hydrophobic PDMS parts,21 where we show that efficient demulsifier copolymers stabilize oil-in-water emulsions while inefficient copolymers stabilize water-in-oil emulsions. This observation seems to be in line with the Bancroft rule and Langmuir Harkins “truncated” wedge theory predicting that surfactants with a dominant hydrophilic part favor water as continuous phase while surfactants with a prevailing hydrophobic part favor oil as continuous phase.8 The effect of the surfactant is the opposite

shape of the micellar structure. A pointlike beam was used for the experimental setup. Figure 8 shows the scattering intensity I versus the scattering vector q for the 5 wt % copolymer solution in H2O at

Figure 8. Scattering intensity for 5 wt % data (SAXS) with pointlike beam for different temperatures from 20 to 60 °C.

temperatures between 20 and 60 °C. As the temperature increases, the position of the first minimum changes from q = 2879

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the energy barrier. However, the necessary condition for such action is the orientation of copolymer molecules at the oil− water interface. In the case of water drops dispersed in an oil phase, molecules with high curvature (or small ccp values) such as the one studied here may be appropriate. Good field performance of this copolymer agrees well with such analysis, as it has also been proven that molecules not forming spherical micelles in water, i.e., having higher cpp value, are less efficient demulsifiers.29

when considering the demulsification efficiency: a hydrophilic surfactant capable of stabilizing an oil-in-water emulsion favors breaking the water-in-oil emulsion. To test this hypothesis, the series of copolymers described above was studied at high concentration where liquid crystals form. Indeed, a poorly demulsifying copolymer formed inversed liquid crystal phases while highly efficient copolymers formed direct structures.29 However, due to the presence of two bulky hydrophilic “heads”, triblock copolymers should not be considered as classical surfactant molecules as one cannot calculate the so-called critical packing parameter (cpp) which provides an insight into the spontaneous curvature and hence the shape of the selfassembled structures. To the best of our knowledge, there is no analogous parameter adapted to amphiphilic triblock copolymers in the literature. Coming back to the role of the demulsifier molecule in the process of separation of water from water-in-crude oil emulsions, a necessary condition is the coalescence of small water droplets. As shown in this study, the spontaneous curvature of the self-assembled copolymer molecules in water corresponds to the curvature of an oil droplet in water, with small hydrophobic parts oriented inside the oil drop. It is likely that the molecule adopts the same spontaneous curvature at the oil/water interface. A schematic representation of two water droplets approaching each other is given in Figure 10a.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ⊥

University of Geneva, Pharmaceutical Sciences, CH-1211 Geneva 4, Switzerland.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank J. JESTIN from the Laboratoire Léon Brillouin for all the SANS experiments performed in CEA Saclay, J. M’HAMDI for his help with SAXS measurements carried out in IFPEN, and the members of the Department of Chemistry for helping in SAXS measurements in the University of Graz as well as data interpretation.

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REFERENCES

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Figure 10. Schematic representation of the nucleation of a hole in the oil lamella between two water droplets: efficient polymers spontaneous curvature fits the oil/water curvature of the hole.

In order to break the oil lamella separating the two water drops, a hole must nucleate in the film (Figure 10b). The molecule fits to a highly positively curved interface, created by the channel formation when two droplets approach each other. As suggested by Kalbanov and Wennerstrom, the energy barrier associated with the nucleation process can be lowered and nucleation of a hole favored if the curvature of the molecules at the oil/water interface corresponds to the curvature of the nucleating hole.8 As suggested in Figure 10b, the high local curvature of the nucleating hole may provoke regional accumulation of demulsifier molecules, thereby diminishing 2880

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