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Phase behaviour
In order to better understand the initial preparation step of mesoporous material by nonionic surfactant templating method, we have focused on the properties of nonionic surfactants. The hydrophilic part of nonionic surfactant is often formed by several oxyethylene groups, EO. The CmH2m+1 (OC2H4)OH, [Cm(EO)n] compounds consist of an alkyl chain of m carbons linked to a polyoxyethylene with n oxyethylene groups. They are commercially available with many combinations of values of n and m, which leads to a variety of mesophases in water. The binary phase behaviours of several Cm(EO)n have been reviewed as a function of surfactant volume fraction and micelle curvature [14]. When the number of EO units increases, with the fixed C12-chain length, there are two phenomena. First, the importance of the stability range of mesophases follows the expected sequence: lamellar phase (L), direct bicontinuous cubic phase (I1), direct hexagonal phase (H1) and direct micellar phase (L1). Large head groups and low temperatures are in favour of I1 and H1 phases, while small head groups and high temperature rather lead to L and L2. Therefore, at low to medium temperature, increasing temperature suggests a decrease in surface area per molecule at the micelle surface. On heating, the EO groups are dehydrated and phase separation is observed at high temperature and low surfactant concentrations.
Mesoporous materials
Ordered porous materials are high-performance materials as catalysts or highly selective adsorbents; but in addition they provide excellent opportunities for the creation of materials with additional functionality. According to the standards set forth in the IUPAC definition [27], porous materials are classified by the following ranges of pore diameters [28]:
Microporous Materials: pore diameters of less than 2 nm.
Mesoporous Materials: pore diameters ranging from 2 nm to 50 nm.
Macroporous Materials: pore diameters of greater than 50 nm.
Among the porous materials, one of the oldest members is zeolite. Zeolites are hydrated crystalline aluminosilicates, which have a narrow and uniform microporous size distribution. Zeolites were discovered from the mineral ‘stilbit’ in 1756 by Cronsted [29]. They have extensively attracted the attention due to their structural characteristics and capabilities [23]. Approximately at the same time, the preparation of mesostructured silica was independently investigated by scientists from the Mobil Oil Corporation [1] and by Kuroda’s group [24] in the early 1990’s. However, a significant breakthrough in the mesoporous materials research has come when Mobil Oil Corporation’s scientists disclosed the M41S Type silica-based compounds and analogues in 1992. The discovery of the M41S family of mesoporous silica helped in the zeolite synthesis by providing methodologies to expand the pore size into the mesopore range.
The synthesis of mesoporous silica material was achieved by several synthetic pathways, such as self-assembly, templated self-assembly, sol-gel processing, dealumination of Al-rich aluminosilicates, spray drying method and etc. However, in our study, we will focus on the mesoporous material preparations via surfactant templating process.
Mesoporous silica materials made by the use of self-assembled surfactants as templates have attracted a lot of interest in recent years. Since, the formation of M41S mesostructured silica, this route of synthesis immediately attracted attention and this “surfactant-templated synthesis of mesoporous materials” has become a new field of research in materials science community. Combined with the sol-gel chemistry, the difference depends on the surfactant concentration. Two mechanisms can lead to the formation of the ordered material. The first one is the self-assembly mechanism by Cooperative Templating Mechanism (CTM); in this case the building blocks are the micelles, so the CTM occurs at low surfactant concentrations. The second approach to the preparation of ordered mesostructures utilizes the liquid crystal phase and is labeled as the direct liquid crystal templating (LCT) pathway.
For synthesis of M41S, they used the self-assembly mechanism involving the dissolution of a surfactant species in the pre-hydrolysed inorganic pre-cursor. The mechanism is strongly influenced by electrostatic and steric interactions between the solvent molecules, the inorganic species and the self assembled organic surfactants. The most important family member of M41S is a hexagonal MCM-41 (Mobil Composition of Matter No.41). This material showed a highly ordered hexagonal array of unidimensional pores. It has a relatively large diameter ranging from 2-10 nm with a very narrow pore size distribution. A typical feature is its extremely high surface areas in excess of 200 – 1000 m2/g, and a specific pore volume of up to 1.3 ml g−1 [28] [30] [31].
Cooperative Templating Mechanism (CTM)
This pathway is established on the basis of the interactions between silicates and surfactants to form inorganic-organic mesostructured composites. When the mesoporous material is prepared from a micellar solution, it is today well-accepted that the formation of these materials occurs through the cooperative templating mechanism [5] [38] [39]. It consists of 3 steps. In the initial step, the interactions between silica and isolated spherical or cylindrical micelles drive to the formation of an organic–inorganic mesophase. Then, the condensation of the inorganic precursor at the external surface of the micelles occurs (intramicellar condensation) [39] [41] [42]. Finally, the self-organization of micelles surrounded by silica is gradually occurred to form a hybrid hexagonal mesophase (intermicellar condensation). The hydrothermal treatment at higher temperature completes the assembly of micelles and the polymerization of the silica source.
However, some disagreement concerning the first step of the CTM mechanism appears and numerous investigations were carried out in order to better understand how the surfactant and the inorganic precursor could affect the formation of the hybrid mesophase. Even if the debate concerning the initial step of the CTM mechanism is still opened, it is admitted that the interactions between surfactant and inorganic precursor are responsible for the mesoporous materials formation. It appears that this step is strongly affected by the behaviour of surfactant in the synthesis solvent.
Nonionic fluorinated surfactant for preparation of mesoporous materials.
As their hydrogenated analogues, fluorinated surfactants are used in various domains, CmF2m+1C2H4(OC2H4)nOH. Indeed, due to their high stability, they can support strong temperature and severe pH conditions. The substitution of hydrogen atoms by fluorine ones enhances the chemical and thermal stability of the surfactant. For example the energy of the C-F bond is 552 kJ mol-1 instead of 338 kJ mol-1 for the C-H bond [53]. In 1972, Shinoda and coworkers suggested [54] that the presence of fluorine atoms also strongly affects the properties of the surfactant and particularly its hydrophobicity and its critical micellar concentration (cmc). Then, the comparative studies of nonionic hydrogenated and fluorinated surfactant properties have been reported by the Stébé team since 1988. For nonionic surfactant, they state that one CF2 group is equivalent to 1.7 CH2 groups [26], which triggers lower cmc’s. As an instance if the headgroup is not changed, a fluorinated surfactant with 7 carbon atoms has the same cmc as a hydrogenated one with 11 carbon atoms. As a consequence of the high hydrophobicity, fluorinated surfactants can decrease the surface tension of water from 30-40 to 15-20 mN.m-1. The phase behaviour of fluorinated surfactants is quite similar to the hydrogenated ones [26] [55]. However, some differences can be found, and a comparative study [26] of fluorinated and non-fluorinated based systems has emphasized that lamellar liquid crystals and larger aggregates are preferentially formed with fluorinated surfactants. This difference in behaviour has been ascribed to the fluorinated chain rigidity and to the higher value of the headgroup area (CF group) of fluorinated surfactants.
The aggregate structures in fluorinated surfactants have been described by Israelachvili and coworkers [20], as in hydrogenated surfactants, in terms of the value of the critical packing parameter (CPP). The fluorocarbon chains are bulkier than the hydrocarbon chain with the volume V of -CF2 and terminal -CF3 being 0.041 and 0.084 nm3, respectively compared with 0.027 and 0.037 nm3, for hydrocarbon analogues [21]. Moreover, the self-assembly in fluorinated surfactant systems were reviewed by Maura Monduzzi in 1998 [56]. The effect of a F chain lead to an increase of the hydrophobic interactions and a decrease in surface tension and cmc values. F chains are generally responsible for an increase of the chain volume, because of a higher cross-sectional area of the CF group and weaker van der Waals interactions among the F chains. These results in an increase of the critical packing parameter (CPP) value, so that interfacial curvature increases, thus allowing important variations of the phase behaviour to occur in comparison with the corresponding hydrogenated surfactants. As a general comment, it is worth noting that H/F mixed systems always give interesting microstructural features. Fluorinated surfactants allow cosolubilization of water and perfluoroalkanes [57] [58], and the specific property of fluorocarbon to dissolve high quantities of oxygenandcarbon dioxide make them very attractive for biomedical applications such as oxygen vectorization for instance [59] [60].
Rotational rheometer and shearing cells
The test fluid is sheared between two surfaces, one is usually fixed while the second is rotated under stress or strain controlled conditions. The velocity gradient in the sample is related to the dimensions of the cell and to the angular velocity of the moving plate. The three principal and mostly used shearing cells are sketched in figure II.2: (a) the Couette cell built with coaxial cylinders, (b) cone-plate, (c) parallel plates (See Figure II.2).
Due to its large surface in contact with fluid, the Couette cell is the most widely used system when working with low viscosity samples.
Zero shear viscosity measurement
The zero shear viscosities are measured as a function of the mass fraction of surfactant mass for the temperatures of 10 and 20°C. The values can be extrapolated from the non linear rheological measurements, or from the real part of the complex viscosities = G/ at = 0 in the linear measurements.
To complete the measurements at low concentration, we measured the viscosity of the sample for mass fractions mass less than 5 wt% with capillary viscometers. The capillary viscometers of Ubbeohlde type of internal diameters 0.36, 0.58 and 0.77 mm are used to cover the range of explored viscosities. The viscometer is immersed in a water bath stabilised at the desired temperature (±0.2°C). The viscosity of the surfactant solution is then given by measuring the flow time of pure water (tw) and the flow time of the solution (ts) between two points of the tube at the same temperature. ts and tw were determined with an uncertainty of the order of 0.5 s. The flow time ts is proportional to the kinematic viscosity of the solution = / (where is the density of the fluid) with a constant of proportionality intrinsic to the viscometer, which is determined by measuring the flow time tw of pure water. Assuming that the surfactant solution has the same density as pure water (a reasonable assumption since we work at low concentration) we have tw / ts = w / s which gives s = w. ts / tw, where s and w are the viscosities of the surfactant solution and water, respectively.
Dynamic viscoelastic measurement
The experiments performed in dynamical mode by subjecting the sample to sinusoidal oscillations allow the study of the linear viscoelastic behaviour of the sample (if the sinusoidal strain is small) and to characterize its intrinsic properties i.e. its moduli G , G and , R relaxation time. A sinusoidal time dependent strain, , is applied to the sample: sint 0 (equation II.6) In the linear domain, the response of the solution to this strain is also a sinusoidal function of time with the same frequency but out of phase by the angle
Table of contents :
Chapter I. Literature reviews
1.1 Surfactant based system
1.1.1 Organized molecular system (OMS)
1.1.2 Phase behaviour
1.1.3 Fluorinated surfactants
1.2 Mesoporous material
1.2.1 Mechanisms of formation
1.2.2 Cooperative Templating Mechanism (CTM)
1.2.3 Nonionic surfactant for mesoporous material preparation
1.2.4 Nonionic fluorinated surfactant for preparation of mesoporous materials
1.2.5 Various fluorocarbon solubilization and oil incorporation in systems
1.2.6 Effect of salts and ethanol additions on the cloud point of nonionic, hydrogenated or fluorinated surfactant systems
1.3 Rheology of micelles
1.4 Context of study
1.4.1 Study of 8 F 7 R (EO) -based system
1.4.2 Study of 9 F 8 R (EO) -based system
1.4.3 Objectives
Chapter II. Materials and Experimental Methods
2.1 Materials: Surfactant solutions
2.2 Rheometry
2.2.1 Rotational rheometer and shearing cells
2.2.2 Flow experiments
2.2.3 Zero shear viscosity measurement
2.2.4 Dynamic viscoelastic measurement
2.3 Polarized light microscopy
2.4 Flow birefringence
2.4.1 The polarimetric bench
2.4.2 The measuring cell
2.4.3 The optical measurements. Principle of flow birefringence measurement
2.4.4 Measurement protocol
2.4.4.1 Determination of the angle of extinction
2.4.4.2 Determination of the birefringence intensity
2.5 Small Angle Neutron scattering
2.5.1 Devices and experiments
2.5.2 Definition of the measured quantities in a SANS experiment
2.5.3 Extraction of the spectra
References
Chapter III. Phase diagram of 8 F 7 R (EO) and 9 F R (EO) surfactant-based systems
3.1.1 Phase diagram of 8 7 R (EO) in water
3.1.2 Phase diagram of 8 F 7 R (EO) in sodium iodine (NaI)
3.2 9 F 8 R (EO) surfactant based system
3.2.1 Phase diagram of 9 F 8 R (EO) in water
3.2.2 Phase diagram of 9 F 8 R (EO) in sodium cloride (NaCl)
References
Chapter IV. Rheophysics of nonionic fluorinated surfactant based system 4.1 System I – 8 F 7 R (EO) in water
4.1.1 Rheological properties
4.1.1.1 Steady state rheological behaviour
4.1.1.2 Dynamic rheological behaviour
4.1.1.3 Stress cycle
4.1.1.4 Evolution of the zero shear viscosity with the concentration of surfactant (wt%)
4.1.2 Flow birefringence
4.1.3 Small angle scattering experiments
4.1.3.1 SANS at rest
4.1.3.2 SANS under flow
4.2 System II – 8 F R7 (EO) in NaI
4.2.1 Rheological properties
4.2.1.1 Steady state rheological behaviour
4.2.1.2 Dynamic rheological behaviour
4.2.2 Flow birefringence
4.3 System III – 9 8 R (EO) in water
4.3.1 Steady state rheological behaviour
4.3.2 Dynamic rheological behaviour
4.3.3 Flow birefringence
4.4 System IV – 9 F 8 R (EO) in 3M NaCl
4.4.1 Steady state rheological behaviour
4.4.2 Dynamic rheological behaviour
4.4.3 Flow birefringence
References
Chapter V. Discussion
5.1 Molecular organization of 8 F 7 R (EO) system at equilibrium
5.1.1 8 F 7 R (EO) in water
5.1.2 8 F R (EO) in 1M NaI solution
5.2 Formation of hybrid hexagonal mesophase: link to the rheological behaviours F 7 R (EO) in water
5.2.2 8 F R (EO) in 1M NaI solution
5.3 8 F 7 R (EO) -water vs 8 F 7 R (EO) -1M NaI systems
5.4 Comparison between flow birefringence, rheology and SANS
5.4.1 8 F 7 R (EO) in water
5.4.2 8 F 7 R (EO) in 1M NaI solution
5.5 Molecular orientation of 9 F R8 (EO) system
5.5.1 9 F 8 R (EO) in water
5.5.2 9 F 8 R (EO) in 3M NaCl solution
5.6 Comparison between two nonionic fluorinated surfactant systems
References
