Mesoporous silica nanoparticles with controllable ...

Journal of Colloid and Interface Science 467 (2016) 253–260

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Mesoporous silica nanoparticles with controllable morphology prepared from oil-in-water emulsions Hanna Gustafsson a,b, Simon Isaksson a, Annika Altskär c, Krister Holmberg a,⇑ a

Applied Surface Chemistry, Dept. of Chemistry and Chemical Engineering, Chalmers University of Technology, Gothenburg SE-412 96, Sweden Biological Physics, Dept. of Applied Physics, Chalmers University of Technology, Gothenburg SE-412 96, Sweden c Structure and Material Design, SP – Food and Bioscience, Box 5401, Gothenburg SE-402 29, Sweden b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

A simple synthesis of 40–50 nm large

mesoporous silica particles is presented. The particle and the pore sizes are controlled by the type of surfactant used. A mechanism for the formation is proposed.

a r t i c l e

i n f o

Article history: Received 10 December 2015 Revised 13 January 2016 Accepted 13 January 2016 Available online 14 January 2016 Keywords: Mesoporous silica Nanoparticle Emulsion Microemulsion Surfactant

a b s t r a c t Mesoporous silica nanoparticles are an important class of materials with a wide range of applications. This paper presents a simple protocol for synthesis of particles as small as 40 nm and with a pore size that can be as large as 9 nm. Reaction conditions including type of surfactant, type of catalyst and presence of organic polymer were investigated in order to optimize the synthesis. An important aim of the work was to understand the mechanism behind the formation of these unusual structures and an explanation based on silica condensation in the small aqueous microemulsion droplets that are present inside the drops of an oil-in-water emulsion is put forward. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction Mesoporous silica is of interest in many applications such as catalysis, drug delivery, separation technology, chemical sensors,

⇑ Corresponding author. E-mail address: [email protected] (K. Holmberg). http://dx.doi.org/10.1016/j.jcis.2016.01.026 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.

nanoreactors, and photonics [1–8] because it has a large surface area and it is chemically inert, thermally stable, biocompatible and inexpensive. The mesoporous silica is most commonly prepared in the form of particles, typically ranging from a few hundred nm to a few lm. The pore diameter of mesoporous materials is by definition within the range of 2–50 nm [9,10] but the mesoporous silica particles that are attracting most interest have pore sizes in the lower range of this interval, typically below 15 nm.

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There is currently considerable interest in loading the pores of mesoporous material with active agents such as synthetic homogeneous catalysts, enzymes, drugs, and pesticides [11–14]. Mesoporous silica particles are well suited for this purpose because the pore dimension can be tailor-made to fit a specific molecule and altering the pH of the surrounding medium can vary the surface charge of the pore walls. The charge of the walls may influence the rate of immobilization of the guest molecule, as well as the activity of this molecule once entrapped in the pore [15,16]. In order to further optimize the environment for immobilized molecules, the silica pore walls can be easily functionalized with silanes terminated with various functional groups (e.g. thiol, carboxyl, amine, methyl) [17]. If the guest molecule is a catalyst, such as an enzyme [18–20] or a metal–organic compound [21–24], it will only exert its action when situated close to the pore openings where it can meet the substrate in the surrounding medium. A catalyst buried in the inner part of the pore will be less exposed to the substrate and will thus be less active. For such applications it is therefore advantageous to use as small particles as possible, with more shallow pores, but still with a pore diameter in the 2–15 nm range. However, the established methods of preparing ordered mesoporous silica particles do not give very small particles. It turns out to be difficult to prepare such particles with large pores and a spherical shape below 100 nm in diameter. In 2009 Nandiyanto et al. [25] presented a one-pot method to synthesize a new class of nano-sized mesoporous silica particles. The synthesis route was the following. First an oil-in-water emulsion was formed with the silica precursor tetraethylorthosilicate (TEOS), styrene and octane as oil component. A cationic surfactant was used as emulsifier, a basic amino acid was added as a catalyst for TEOS hydrolysis, which leads to formation of the silica polymer, and a free radical initiator was added to initiate the polymerization of styrene. The two polymerization processes were believed to occur simultaneously within the oil droplets. After completed polymerization, octane, which only served as a solvent for the polymerizing species, was removed to give a composite sphere consisting of interdigitated polystyrene and silica domains. Removal of the polystyrene by calcination gave the porous silica particles. By varying either the octane or styrene concentration Nandiyanto et al. indicates a particle size range of 20–80 nm and a pore size range of 4–15 nm. Such spherical silica particles are referred to as dendritic silica particles and typically have pores in the meso range; however, they are not ordered like the well-known materials in the SBA and MCM series [26,27]. We have synthesized and utilized mesoporous silica particles as hosts for enzymes for several years [12,16,18,19] and we have recently become interested in these very small silica nanoparticles with pores in the meso range. The reason for this interest is that small particles possess shallower pores compared to the larger SBA and MCM particles and could therefore result in more efficiently loaded pores and enzymes more accessible to the surrounding environment, thus increasing the efficiency in terms of enzymatic activity per gram of mesoporous silica. We have therefore looked into the synthesis of such particles in detail with the aim to understand the mechanism behind the formation of these unusual structures. Emphasis has also been put on simplifying the previously developed synthesis protocol. The mechanistic view that has emerged differs considerably from that published by Nandiyanto et al. [25]. In this paper we present studies from a series of reactions, using different protocols and different kinds of surfactants as structure directing agent, and the collected results constitute the basis for a completely new mechanism for formation of silica particles with a diameter of 40–90 nm and with pore diameters of 3–9 nm.

2. Materials and methods 2.1. Chemicals Cetyltrimethylammonium bromide (CTAB, P99%), tetraethylorthosilicate (TEOS, P99%), L-lysine (P98%), 2,20 -azobis(2-methyl propionamidine) dihydrochloride (AIBA, 97%), ethanolamine (P98%), styrene (99%) and n-octane (98%) were all purchased from Sigma–Aldrich. Ethylan 1008 (octa(ethylene glycol)monodecyl ether, C10E8) was received as a gift from AkzoNobel Surface Chemistry (Stenungsund, Sweden). The cationic gemini surfactants N,N0 didodecyl-N,N,N0 ,N0 -tetramethyl-N,N0 -ethanediyl-di-ammonium dibromide (12-2-12) and N,N0 -didodecyl-N,N,N0 ,N0 -tetramethyl-N, N0 -hexanediyl-di-ammonium dibromide (12-6-12) were synthesized as described in the literature [28].

2.2. Particle synthesis The reference material was synthesized using a protocol adapted from Nandiyanto et al. [25], where CTAB is used as the structure directing agent, TEOS as silica source, octane and styrene as hydrophobic components and lysine as a catalyst. In the synthesis an oil-in-water emulsion was first formed by vigorously stirring 200 mg CTAB, 62 g Milli-Q water, 19.9 g n-octane and 45 mg Llysine for 1 h at 70 °C in a three-necked reactor. Thereafter 2.77 g styrene, 2.0 g TEOS and 77.6 mg AIBA (used as polymerization initiator) were added and the mixture was stirred and kept under N2 atmosphere at 70 °C for 20 h. Prior to use the styrene was prewashed with 2.5 M NaOH in order to remove the stabilizer. The suspension was decanted into a funnel and cooled to room temperature. The mesoporous particles where collected and freeze dried. Finally the residual organic material was removed through calcination, by increasing the temperature from room temperature to 650 °C during 8 h and holding for 6 h at 650 °C.

2.3. Variations of the synthesis conditions In order to study the formation process of the particles several different synthesis experiments were performed. Except for the particular component to be varied all other parameters were kept the same to avoid artifacts from compositional or conditional variations. The following components were varied: MPS-1: Styrene and AIBA were removed from the synthesis in absence of N2 gas flow. MPS-2: The same conditions as for MPS-1 were used and the catalyst L-lysine was replaced with an equal molar amount of ethanolamine. MPS-3: The same conditions as for MPS-2 were used and CTAB was replaced with an equal molar amount of a cationic gemini surfactant with two C12 chains and with a C6 linker (N,N0 -didode cyl-N,N,N0 ,N0 -tetramethyl-N,N0 -hexanediyl-di-ammonium dibromide). MPS-4: The same conditions as for MPS-2 were used and CTAB was replaced with an equal molar amount of a cationic gemini surfactant with two C12 chains and with a C2 linker (N,N0 -didode cyl-N,N,N0 ,N0 -tetramethyl-N,N0 -ethanediyl-di-ammonium dibromide). MPS-5: The same conditions as for MPS-2 were used and CTAB was replaced with an equal molar amount of the nonionic surfactant Ethylan 1008 (C10E8). The chemical structures of the four surfactants are shown in Fig. 1.

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2.4. Characterization

(a) Br

-

+

-

+

(b) Br

N

N

Br

+

N

(c) Br

-

+

N

Br

+

N

(d)

O OH 8

Fig. 1. Chemical structure of (a) the cationic surfactant CTAB, (b) the cationic gemini surfactant 12-6-12, the cationic gemini surfactant 12-2-12 and the nonionic surfactant Ethylan 1008.

(a)

Transmission electron microscopy (TEM) was performed with a FEI Tecnai T20 LaB6 transmission electron microscope operated at 200 kV. The samples were prepared by grinding and dispersing the particles in ethanol, put in an ultrasonic bath and deposited onto a hollow carbon-coated copper grid. The ethanol was subsequently evaporated. Scanning electron microscopy (SEM) was performed with a Leo Ultra 55 FEG SEM scanning electron microscope operated a 2 kV and a working distance of 1.7–2.2 mm. To confirm a porous interior, the particles were infiltrated and embedded in epoxy resin TLV from TAAB Laboratories Equipment Ltd and polymerized for 20 h at 60 °C. Ultrathin sections around 60 nm were cut with a diamond knife using an ultramicrotome Powertome XL, RMC products. The ultrathin sections were placed on 400 mesh cupper grids. images of the cross sectioned mesoporous silica particles were taken with a transmission electron microscope (TEM) LEO 706E at 80 kV accelerating voltage. Nitrogen sorption isotherms were measured using a Micromeritics ASAP 2010 instrument. Prior to the measurements the calcined MPS samples were degassed in vacuum at 225 °C for 4 h. The pore size distributions were determined using the BJH (Bar rett–Joyner–Halenda) method based on the adsorption isotherms [29] and the surface area was determined using the BET (Bru nauer–Emmett–Teller) procedure [30].

(b)

100 nm

50 nm

Fig. 2. (a) TEM and (b) SEM micrographs of particles synthesized following the reference protocol.

(a)

(b)

100 nm

50 nm

Fig. 3. (a) TEM and (b) SEM micrographs of MPS-1 particles.

256


oil-swollen micelle oil droplet

water water droplet Fig. 4. Proposed emulsion system prior to addition of silica precursor and with an excess of surfactant. An O/W emulsion is formed and stabilized by the surfactant, with the oil droplets constituting a W/O microemulsion, resulting in a W/O/W system. The droplets are surrounded by oil-swollen micelles; thus, the continuous water phase is in reality an O/W microemulsion.

3. Results and discussion 3.1. Suggested particle formation mechanism The particles obtained with the reference synthesis, which is a repeat of the procedure published by Nandiyanto et al. [25], were spherical and had a narrow particle size distribution with a mean diameter of 46 nm (Fig. 2). A mesoporous structure was observed with a non-ordered pore structure where the pore walls are built up by a silica network. According to Nandiyanto et al. a composite particle consisting of interdigitated domains of polystyrene and silica is formed in the synthesis and subsequent removal of the organic matter results in porous silica particles. However, we here show that this explanation cannot be true. When styrene was excluded from the recipe, porous particles (MPS-1) with very similar characteristics were obtained (Fig. 3). It is therefore highly unlikely that the two polymerization processes, i.e. of silica and

of styrene, are occurring simultaneously. The initial silica condensation is a rather fast process, while it can be assumed that the polymerization rate of styrene under the synthesis conditions used is slow. We believe that the reason why the emulsion-based synthesis process results in porous and not solid silica particles is that the dispersed phase is not a plain oil phase but a water-in-oil (W/O) microemulsion. It has been observed before that the water phase of an oil-in-water emulsion may contain small water drops. This has, for instance, been determined by inspection of the echo decays of water in NMR diffusometry experiments related to emulsion of alkyl ketene dimer (AKD) in water. Two very different decay curves were obtained, one (giving fast decay) corresponding to diffusion of free water in the continuous medium and one (giving slow decay) corresponding to water confined in nano-sized droplets [31]. The methodology for using NMR diffusometry to elucidate the structure of droplets inside emulsion drops was worked out by Söderman and coworkers [32]. Also the continuous water phase of an O/W emulsion is not pure water. The water phase is a micellar solution of the surfactant and some oil is usually solubilized into the micelles. Thus, strictly speaking an O/W emulsion is usually a water-in-oil microemulsion dispersed in an oil-in-water emulsion. The overall system can therefore be described as a water-in-oil-in-water (W/O/W) emulsion, as is illustrated in Fig. 4. The amount of water taken up by the oil droplets of the emulsion will depend on the choice of surfactant used. Technical surfactants are always mixtures of molecules with different critical packing parameters (CPPs). Surfactants with CPP values just below 1 are useful for giving O/W emulsions and surfactants with CPP values above 1 are best suited for W/O microemulsions [33,34]. It is therefore likely that the more hydrophilic fraction of the surfactant stabilizes the oil droplets and that the more hydrophobic fraction solubilizes water inside these droplets, resulting in a W/ O/W emulsion. There are many examples of different populations of surfactants stabilizing different interfaces in oil–watersurfactant systems. Double emulsions are one such well-known example [35]. The proposed formation process is illustrated in Fig. 5. The hydrolysis of TEOS, which is solubilized in the oil droplets, starts at the interface between oil and water and that interface is very

Addition of silica precursor (TEOS)

Hydrolysis and coalescence

W/O/W microemulsion

Calcination

Water Surfactant Octane TEOS Silica Porous particle

Fig. 5. Proposed formation process of spherical, porous silica nanoparticles in an emulsion with water solubilized in the oil droplets.


257

Fig. 6. TEM micrographs of MPS-1 particles embedded in epoxy plastic and cut into thin (60 nm) slices.

50 nm

50 nm

50 nm

50 nm

Fig. 7. TEM micrographs of (a) MPS-1 (CTAB), (b) MPS-3 (12-6-12), (c) MPS-4 (12-2-12), and (d) MPS-5 (Ethylan 1008).

large due to the many small water droplets present in the oil. The water droplets (swollen reversed micelles) start to coalesce, eventually forming a silica network, which gives rise to the pore structure after removal of the organic material. In the synthesis reported by Nandiyanto et al. lysine was added in order to catalyze hydrolysis of TEOS and the subsequent condensation into a silica network. Attempts to replace this basic amino acid with a more common amine, ethanolamine, were successful. The particles obtained (MPS-2) with ethanolamine as catalyst were identical to those with lysine as catalyst (not shown). Thus, a basic amino acid is not needed in the formulation. To verify that the particles were mesoporous also in the interior, the MPS-1 particles were embedded in epoxy plastic and cut into

ultrathin (60 nm) slices. The thin slices were analyzed with TEM. Cross-sections clearly showed that the pores protrude through the whole particle (Fig. 6). 3.2. Particle morphology control by using different surfactants The role of the surfactant for the particle size and the pore dimension was investigated. CTAB (Fig. 1a), the surfactant used in MPS-1 (Figs. 7a and 8a) and MPS-2, is cationic and has a single C16 hydrocarbon tail. By replacing CTAB with a cationic gemini surfactant with two C12 tails and a C6 linker (12-6-12, Fig. 1b) particles with a similar particle size but with smaller pores (MPS-3) were obtained, as was determined using TEM (Fig. 7b and Table 1).

258


(b)

(a)

20 nm

20 nm

(d)

(c)

20 nm

20 nm

Fig. 8. SEM micrographs of (a) MPS-1 (CTAB), (b) MPS-3 (12-6-12), (c) MPS-4 (12-2-12), and (d) MPS-5 (Ethylan 1008).

Table 1 Mean particle diameter. Material

Mean diameter (nm)

Number of analyzed particles

Standard deviation (% of mean value)

Reference MPS-1 MPS-2 MPS-3 MPS-4 MPS-5

46 53 41 56 91 60

25 25 22 25 30 25

10 16 9 12 14 11

The pores were slightly too small to be visualized properly with SEM (Fig. 8b). Using a cationic gemini surfactant with the same length of the hydrocarbon tails as the 12-6-12 surfactant but with a C2 linker (12-2-12, Fig. 1c) resulted in particles of roughly twice the size but with a similar pore structure as for CTAB (Figs. 7c, 8c and Table 1). A nonionic surfactant of fatty alcohol ethoxylate type, a branched C10 alcohol with 8 mol of ethylene oxide added (Ethylan 1008, Fig. 1d), was also used as surfactant. This resulted in a similar particle size and pore structure as for the 12-6-12 gemini surfactant (Figs. 7d, 8d and Table 1). N2-sorption isotherms of the calcined materials, shown in Fig. 9a, confirm the pore structure observed in the TEM micrographs. All particles display a type IV sorption isotherm, representative for a mesoporous material [36]. The pore size distributions (Fig. 9b) are wide for MPS-1 and MPS-4 with a pore size around 9 nm for MPS-1 and 6 nm for MPS-4, whereas the pore size distributions for MPS-3 and MPS-5 are narrow with a smaller pore size around 3 nm. These observations confirm the results obtained through the TEM and SEM analyses (see Table 2).

From this limited study of the role of the surfactant for the particle size and the pore dimension it can be concluded that the geometry of the surfactant was found to affect both the particle diameter and the pore size, which is not surprising considering that surfactant packing at the oil–water interface should be of significant importance for the resulting particle morphology. However, the choice of surfactant head group is not decisive since small porous silica particles were obtained both with a cationic and with a nonionic surfactant. The three cationic surfactants, with very different structures, were chosen such that their values for the critical micelle concentration were approximately the same, around 1 mM [37,38]. CTAB is a straight chain, single head group surfactant, 12-2-12 is a gemini surfactant with the shortest possible linker between the two head groups and 12-6-12 is a gemini surfactant with a relatively long linker. The reason for choosing two gemini surfactants, one with a short and one with a long spacer unit is that their packing into micelles (and at surfaces) differs. All three surfactants form spherical micelles at low concentrations. At higher concentration the micelles become elongated, which is the normal behavior for hydrophobic amphiphiles. For gemini surfactants the micelle growth as a function of concentration is particularly pronounced for species with very short spacer units [39,40]. The reason for the particular behavior of geminis with a short linker is the following. When the linker is very short, the distance between the charged head groups becomes shorter than the inter-head group distance in conventional micelles. This means that such micelles will have two different distances between the head groups, one distance dictated by the length of the linker and the other distance governed by the physical interactions involved in the self-assembly process. This particular behavior is only seen for the surfactants with very short spacers, two and


259

will have to be explored before one can come up with a correlation between surfactant characteristics and particle morphology.

a

4. Conclusion

b

Fig. 9. Nitrogen sorption measurements. (a) Nitrogen adsorption–desorption isotherms and (b) the pore size distribution of the mesoporous nanoparticles.

Table 2 Material properties of mesoporous silica particles analyzed by N2 sorption. Material

BET surface area (m2/g)

Total pore volume (cm3/g)

MPS-1 MPS-3 MPS-4 MPS-5

493 769 650 737

1.42 1.38 2.27 1.18

three carbon atoms (one cannot make gemini surfactants with a one carbon linker). The distance between the head groups for cationic geminis with a four carbon spacer is approximately the same as the distance between the head groups for normal cationic surfactants, such as CTAB, in a micelle. As a consequence of the short distance between the head groups gemini surfactants with very short spacer units have severe packing constraints in micelles. It is difficult to accommodate two long chain hydrocarbon tails within a closed structure when the polar head groups are so close. In this study we used one cationic gemini surfactant with the shortest possible linker, two carbon atoms, and one with six carbon atoms in the spacer unit to see if there was a noticeable difference in the morphology of the porous particles formed, i.e. if the difference in the packing pattern was of importance. The results seem to indicate that this is not the case. 12-2-12 gave approximately the same pore size as the regular surfactant CTAB, but the particle size was larger. 12-6-12, which from a packing constraint point of view resembles CTAB, gave much smaller pores, but a similar particle size. Taken together, the results show that both cationic and nonionic surfactants that can form W/O/W emulsions can be used for formation of very small mesoporous particles. The results also indicate that the particle size and the pore dimension can be tailored by the choice of amphiphile. Obviously, many more surfactants

In this study, nanosized mesoporous silica particles have been synthesized. Emphasis has been put on simplifying the previously developed synthesis protocol and on understanding the mechanism behind the formation process. We propose that each oil drop in the O/W emulsion formed is in reality a water-in-oil microemulsion. The oil phase contains the silica precursor, TEOS, together with inert n-octane and a surfactant is present both around the oil drops and around the microemulsion droplets present within the oil drops. The oil–water interface in such a system is very large and most of the interface is within the oil drops. When exposed to water TEOS will hydrolyze and start to condense to form a gradually more three-dimensional silica network. This process will initially occur at all available oil–water interfaces and will lead to a gradual transition of the droplet structure within the diminishing oil drops into a structure composed of small, elongated water channels. This is in principle the same transition as is commonly known as a transition from a water-in-oil microemulsion into a bicontinuous microemulsion. The end result will be long narrow silica threads protruding through what remains of the oil drops when all the TEOS has been consumed. After removing the organic material, i.e. n-octane, the surfactant and the organic base, the mesoporous structure is obtained. The particle and pore size have been shown to be controlled by the type of surfactant used. It was found that a regular cationic surfactant, a cationic gemini surfactant with either a short or a long spacer and a regular nonionic surfactant all gave porous particles ranging in size from 46 to 91 nm in diameter. However, in order to clarify the correlation between surfactant characteristics and particle morphology additional surfactants will have to be explored. Acknowledgement We would like to thank Dr. Ali Tehrani, now at The American University in Beirut, for providing us with the cationic gemini surfactants. References [1] M. Vallet-Regi, A. Ramila, R.P. del Real, J. Perez-Pariente, Chem. Mater. 13 (2001) 308–311. [2] Y.J. Han, G.D. Stucky, A. Butler, J. Am. Chem. Soc. 121 (1999) 9897–9898. [3] I.I. Slowing, B.G. Trewyn, V.S.Y. Lin, J. Am. Chem. Soc. 129 (2007) 8845–8849. [4] A. Sayari, Chem. Mater. 8 (1996) 1840–1852. [5] M. Hartmann, Chem. Mater. 17 (2005) 4577–4593. [6] H.H.P. Yiu, P.A. Wright, J. Mater. Chem. 15 (2005) 3690–3700. [7] J. Deere, E. Magner, J.G. Wall, B.K. Hodnett, J. Phys. Chem. B 106 (2002) 7340– 7347. [8] J.F. Diaz, K.J. Balkus, J. Mol. Catal. B: Enzym. 2 (1996) 115–126. [9] G.J.D. Soler-illia, C. Sanchez, B. Lebeau, J. Patarin, Chem. Rev. 102 (2002) 4093– 4138. [10] Y. Wan, D.Y. Zhao, Chem. Rev. 107 (2007) 2821–2860. [11] J.A. Melero, G. Vicente, G. Morales, M. Paniagua, J.M. Moreno, R. Roldan, A. Ezquerro, C. Perez, Appl. Catal., A 346 (2008) 44–51. [12] C. Thorn, H. Gustafsson, L. Olsson, J. Mol. Catal. B: Enzym. 72 (2011) 57–64. [13] I.I. Slowing, B.G. Trewyn, S. Giri, V.S.Y. Lin, Adv. Funct. Mater. 17 (2007) 1225– 1236. [14] M. Brigante, M. Avena, Microporous Mesoporous Mater. 191 (2014) 1–9. [15] L.C. Sang, M.O. Coppens, PCCP 13 (2011) 6689–6698. [16] H. Gustafsson, E.M. Johansson, A. Barrabino, M. Oden, K. Holmberg, Colloids Surf., B 100 (2012) 22–30. [17] X. Feng, G.E. Fryxell, L.-Q. Wang, A.Y. Kim, J. Liu, K.M. Kemner, Science 276 (1997) 923–926. [18] H. Gustafsson, C. Thorn, K. Holmberg, Colloids Surf., B 87 (2011) 464–471. [19] C. Thorn, H. Gustafsson, L. Olsson, Microporous Mesoporous Mater. 176 (2013) 71–77.

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