Mesoporous silica nanoparticles with controllable morphology prepared from oil-in-water emulsions

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This is the submitted version of a paper published in Journal of Colloid and Interface

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Citation for the original published paper (version of record):

Gustafsson, H., Isaksson, S., Altskär, A., Holmberg, K. (2016)

Mesoporous silica nanoparticles with controllable morphology prepared from

oil-in-water emulsions

Journal of Colloid and Interface Science, 467: 253-260

https://doi.org/10.1016/j.jcis.2016.01.026

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N.B. When citing this work, cite the original published paper.

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Mesoporous silica nanoparticles with controllable

mor-phology prepared from oil-in-water emulsions

Hanna Gustafsson

a

, Simon Isaksson

a

, Annika Altskär

b

and Krister Holmberg

a

*

a _{Applied Surface Chemistry, Dept. of Chemistry and Chemical Engineering, Chalmers University of Technology,} Gothen-burg SE-412 96, Sweden.

b _{Structure- and Material design, SP – Food and Bioscience, Box 5401, Gothenburg, SE-402 29, Sweden.}

ABSTRACT: 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 10 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.

INTRODUCTION

Mesoporous silica is of interest in many applications such as catalysis, drug delivery, separation technology, chemical sen-sors, nanoreactors, and photonics 1-8_{because it has a large} surface area and it is chemically inert, thermally stable, bio-compatible and inexpensive. The mesoporous silica is most commonly prepared in the form of particles, typically ranging from a few hundred nm to a few µm. The pore diameter of mesoporous materials is by definition within the range of 2 to 50 nm 9, 10_{but the mesoporous silica particles that are} attract-ing most interest have pore sizes in the lower range of this interval, typically below 15 nm.

There is currently considerable interest in loading the pores of mesoporous material with active agents such as synthetic homogeneous catalysts, enzymes, drugs, pesticides, etc. 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 en-trapped in the pore 15, 16. In order to further optimize the envi-ronment for immobilized molecules, the silica pore walls can be easily functionalized with silanes of 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 very small silica particles, typically 40-50 nm, with pores around 10 nm in diameter. The synthesis route was the following. First an oil-in-water (O/W) emulsion was formed with the silica precursor tetraethylorthosilicate (TEOS), styrene and octane as oil component. A cationic sur-factant 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 polysty-rene and silica domains. Removal of the polystypolysty-rene by calci-nation gave the porous silica particles. Such particles have pores in the meso range; however, they are not ordered like the well-known materials in the SBA and MCM series 26.

We have synthesized and utilized mesoporous silica parti-cles as hosts for enzymes for several years 12, 16, 18, 19_{and we} have recently become interested in these very small silica na-noparticles with pores in the meso range. Small particles pos-sess shallower pores compared to larger SBA and MCM parti-cles 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 under-stand the mechanism behind the formation of these unusual structures. Emphasis has also been put on simplifying the pre-viously 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 modified reactions, using different kinds of surfac-tants as structure directing agent, and the collected results

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con-stitute the basis for an alternative mechanism for formation of silica particles in the 40-90 nm in diameter size range and with pore diameters of 6-11 nm.

EXPERIMENTAL SECTION

Chemicals

Cetyltrimethylammonium bromide (CTAB, ≥99 %), tetrae-thylorthosilicate (TEOS, ≥99 %), L-lysine (≥98 %), 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AIBA, 97 %), ethanolamine (≥98 %), 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,N’-didodecyl-N,N,N’,N’-tetramethyl-N,N’-ethanediyl-di-ammonium dibro-mide (12-2-12) and N,N’-didodecyl-N,N,N’,N’-tetramethyl-N,N’-hexanediyl-di-ammonium dibromide (12-6-12) were synthesized as described in the literature 27.

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 L-lysine 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.

Variations of the synthesis conditions

In order to study the formation process of the particles sev-eral 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

synthe-sis 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.

CTAB was replaced with an equal molar amount of a cationic gemini surfactant with two C12 chains and with a C6 linker (N,N’-didodecyl-N,N,N’,N’-tetramethyl-N,N’-hexanediyl-di-ammonium dibromide).

CTAB was replaced with an equal molar amount of a cationic gemini surfactant with two C12 chains and with a C2 linker (N,N’-didodecyl-N,N,N’,N’-tetramethyl-N,N’-ethanediyl-di-ammonium dibromide).

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 Figure 1.

Figure 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.

Characterization

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 hours at 60°C. Ul-trathin 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. I m-ages 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 Mi-cromeritics 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 (Barrett–Joyner–Halenda) method based on the adsorp-tion isotherms 28_{and the surface area was determined using the} BET (Brunauer–Emmett–Teller) procedure 29.

RESULTS AND DISCUSSION

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 (Figure 2). A mesoporous structure

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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 interdigi-tated domains of polystyrene and silica is formed in the syn-thesis and subsequent removal of the organic matter results in porous silica particles. However, we here show that this ex-planation cannot be true. When styrene was excluded from the recipe, porous particles (MPS-1) with very similar characteris-tics were obtained (Figure 3). It is therefore highly unlikely that the two polymerization processes, i.e. of silica and of sty-rene, are occurring simultaneously. The initial silica condensa-tion 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 is well known that surfactants solubilize water into oil forming a W/O micro-emulsion, also sometimes called a reversed micellar system. This means that the droplets of an O/W emulsion usually do not contain only the oil component; they also contain a vary-ing amount of very small water droplets 30, 31. Also the contin-uous 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 micro-emulsion dispersed in an oil-in-water micro-emulsion. The overall system can therefore be described as a water-in-oil-in-water (W/O/W) emulsion, as is illustrated in Figure 4.

Figure 2. (a) TEM and (b) SEM micrographs of particles

synthe-sized following the reference protocol.

Figure 3. (a) TEM and (b) SEM micrographs of MPS-1 particles. The amount of water taken up by the oil droplets of the emulsion will depend on the choice of surfactant used. Tech-nical surfactants are always mixtures of molecules with differ-ent 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 32, 33_{. It is therefore likely that the more} hy-drophilic fraction of the surfactant stabilizes the oil droplets and that the more hydrophobic fraction solubilizes water in-side these droplets, resulting in a W/O/W emulsion. There are many examples of different populations of surfactants stabiliz-ing different interfaces in oil-water-surfactant systems. Double emulsions are one such well-known example 34.

Figure 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 con-stituting a W/O microemulsion, resulting in a W/O/W emulsion. The droplets are surrounded by oil-swollen micelles; thus, the continuous water phase is in reality an O/W microemulsion.

The proposed formation process is illustrated in Figure 5. The hydrolysis of TEOS, which is solubilized in the oil drop-lets, starts at the interface between oil and water and that inter-face is very 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 subse-quent condensation into a silica network. Attempts to replace this basic amino acid with a more common amine, ethanola-mine, 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.

Figure 5. Proposed formation process of spherical, porous silica

nanoparticles in a W/O/W emulsion with water solubilized in the oil droplets.

To verify that the particles were mesoporous also in the in-terior, 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 (Figure 6).

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Figure 6. TEM micrographs of MPS-1 particles embedded in

epoxy plastic and cut into thin (~ 60 nm) slices.

Particle morphology control by using different su r-factants

The role of the surfactant for the particle size and the pore dimension was investigated. CTAB (Figure 1a), the surfactant used in MPS-1 (Figure 7a and Figure 8a) and MPS-2, is cati-onic 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, Figure 1b) particles with a similar parti-cle size but with smaller pores (MPS-3) were obtained, as was determined using TEM (Figure 7b and Table 1). The pores were slightly too small to be visualized properly with SEM (Figure 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, Figure 1c) resulted in particles of roughly twice the size but with a similar pore structure as for CTAB (Figure 7c, Figure 8c and Table 1). A nonionic surfac-tant of fatty alcohol ethoxylate type, a branched C10 alcohol with 8 moles of ethylene oxide added (Ethylan 1008, Figure 1d), was also used as surfactant. This resulted in a similar par-ticle size and pore structure as for the 12-6-12 gemini surfac-tant (Figure 7d, Figure 8d and Table 1).

Figure 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).

Figure 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). N2-sorption isotherms of the calcined materials, shown in Figure 9a, confirm the pore structure observed in the TEM micrographs. All particles display a type IV sorption isotherm, representative for a mesoporous material, with H3 hysteresis, which is characteristic for slit-shaped pores 35_{. The pore size} distributions (Figure 9b) are wide for MPS-1 and MPS-4, whereas the pore size distributions for MPS-3 and MPS-5 are narrow. According to the BJH method the pore size is around 11 nm for MPS-1 and MPS-4 whereas MPS-3 and MPS-5 have smaller pores, around 7 nm and 6 nm, respectively (see Table 2). These observations confirm the results obtained through the TEM and SEM analyses.

Table 1. Mean particle diameter.

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 con-centration were approximately the same, around 1 mM 36, 37. CTAB is a straight chain, single head group surfactant, 12-2-12 is a gemini surfactant with the shortest possible linker be-tween the two head groups and 12-6-12 is a gemini surfactant

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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.

Figure 9. Nitrogen sorption measurements. (a) Nitrogen

adsorp-tion-desorption isotherms and (b) the pore size distribution of the mesoporous nanoparticles.

All three surfactants form spherical micelles at low concen-trations. At higher concentration the micelles become elongat-ed, 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 38, 39_{. The reason for the particular behavior} of geminis with a short linker is the following. When the link-er is vlink-ery short, the distance between the charged head groups becomes shorter than the inter-head group distance in conven-tional micelles. This means that such micelles will have two different distances between the head groups, one distance dic-tated by the length of the linker and the other distance gov-erned 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 three carbon at-oms. (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 hydro-carbon 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 surfac-tant 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 sur-factants that can form W/O/W emulsions can be used for for-mation 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 will have to be explored before one can come up with a correlation between surfactant characteristics and parti-cle morphology.

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

CONCLUSION

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 precur-sor, TEOS, together with inert n-octane and a surfactant is present both around the oil drops and around the microemul-sion droplets present within the oil drops. The oil-water inter-face in such a system is very large and most of the interinter-face is within the oil drops. When exposed to water TEOS will hydro-lyze 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 com-monly known as a transition from a water-in-oil microemul-sion into a bicontinuous microemulmicroemul-sion. 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 cati-onic surfactant, a caticati-onic gemini surfactant with either a short or a long spacer and a regular nonionic surfactant all gave porous particles ranging in size from 46-91 nm in diameter. However, in order to clarify the correlation between surfactant characteristics and particle morphology additional surfactants will have to be explored.

AUTHOR INFORMATION

Corresponding Author

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Present Addresses

† Biological Physics, Dept. of Applied Physics, Chalmers Univer-sity of Technology, Gothenburg SE-412 96, Sweden.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manu-script.

ACKNOWLEDGMENT

We would like to thank Dr. Ali Tehrani, now at The American University in Beirut, for providing us with the cationic gemini surfactants.

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