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Solubilization of oil in system based on nonionic surfactants
Nagarajan and Ruckenstein [13] consider two general types of solubilization which are labeled as type I and type II at the beginning of the solubilization process (Figure I-6). For type I solubilization the soluble molecules are located between the surfactant tails in the micelles (penetration effect). For type II solubilization the soluble molecules form a core surrounded by the surfactant molecules as in microemulsions (swelling effect). For nonpolar solutes which would be expected to be located mainly in the hydrocarbon tails region of the micelle, the situation should resemble a mixture of two hydrocarbons.
However, it was noted that the location of the oil molecules can vary with their molecular structure and also with their quantities [15]. For example, at low concentration benzene is solubilized mainly between the chains of micelles, while that beyond a mole fraction of 0.7, a large quantity of these molecules are solubilized in the core of the micelles. It is important to note that the types I and II of solubilization describe ideals cases. Indeed, in practice, the penetration effect and the swelling effect occur often simultaneously.
In a nonionic surfactant/oil/water system, a surfactant can form direct micelles and at low temperature, whereas it forms reversed micelles at higher temperature [16]. The type of emulsion also inverts from O/W to W/O with the increase in temperature. At the transition temperature, there is a three-phase region consisting of water, oil and surfactant phases. Shinoda termed this temperature the Phase-Inversion Temperature (PIT) of the emulsion (HLB temperature). The PIT is useful index for evaluating the hydrophile-lipophile property of a nonionic surfactant for a given oil: the more hydrophilic the surfactant is, the higher the PIT.
To determine the phase inversion temperature, it is possible to use the method of Friberg [17-19] by drawing a “fish” diagram (so called because of its appearance). To obtain this type of diagram we have to determine the one phase domain labeled as surfactant phase as a function of temperature and concentration of surfactant for water/oil ratio equals to 1. The concentration of surfactant is plotted on the abscissa and the temperature is on the ordinate (Figure I-7). From this diagram we can determine the minimum concentration of surfactant, C* required to solubilize the equal mass of water and oil. The temperature associated with this concentration is the temperature of phase inversion (THLB). Based on the study of solubilization of hydrocarbon in the hydrogenated nonionic systems, empirical relationships which allow connecting the PIT of the ternary system with the HLB of the surfactant and the molecular structure of oil characterized by ECN (Equivalent Carbon Number) has been established [20, 21]. The ECN is identical to the actual number of carbon atoms only for a linear saturated chain. It is defined as the carbon numbers corresponding to the linear saturated oil, which would have the same phase behavior as the considered oil (cyclic, unsaturated or the others). Thus it could be evaluated by measuring experimentally the inversion phase temperature of the system and searching for linear saturated oil which leads to the same PIT. Shinoda also determined a relation for calculating the PIT of nonionic surfactant systems [22]. Comparing the values obtained by these two relations, they are in agreement with each other.
Material-Templated by Surfactant (MTS)
The first synthesis of an ordered mesoporous material was described in the patent literature in 1969 [97, 98]. However, due to a lack of analysis, the remarkable features of this product were not recognized [85]. In 1992, a similar material was obtained by scientist in Mobil Oil Corporation who discovered the remarkable features of this novel type of silica and leads to a whole field of research [99], meanwhile, the concept of “template” was first postulated in the synthesis of mesoporous silicate materials. In Mobil’s report, quaternary ammonium cationic surfactants such as cetyltrimethyl ammonium bromide (C16H33N(CH3)3-Br, CTAB) were first used as templates to prepare highly ordered mesoporous silicate molecular sieves under hydrothermal, basic conditions. These new family of mesoporous sieves with regular and constant pore diameters in the range of 1.5-10 nm are so called M41S [100, 101] which consist on three kinds of materials. MCM-41, which stands for Mobil Composition of Matter No. 41, shows a highly ordered hexagonal array of unidimensional pores with a very narrow pore size distribution [100, 101]. The walls, however, very much resemble amorphous silica. Other related phases such as MCM-48 and MCM-50, which have a cubic and lamellar mesostructure, respectively, were reported in these early publications as well (Figure I-10) [99-101]. At approximately the same time, an alternative, but less versatile approach to prepare mesoporous materials was described by Yanagisawa et al. [102]. Kanemite, a layered silicate, serves as a silica source and the pathway leading to the ordered mesoporous material is thought to proceed via surfactant intercalation into the silicate sheets, warping of the sheets and transformation to the hexagonally packed material. Modifying and optimizing the reaction conditions yields to highly ordered mesoporous silicates and aluminosilicates as well [103, 104]. The obtained materials are designated as FSM-n, Folded Sheet mesoporous Materials-n, here n is the number of carbon atoms in the surfactant alkylchain used to synthesize the material. Since these early discoveries a large research effort has been invested in the synthesis and characterization of a variety of different, although related materials.
Liquid Crystal Templating Mechanism (LCT)
The second pathway to the preparation of ordered mesostructures utilizes the liquid crystal phase and it is labeled as the direct liquid crystal templating (LCT) mechanism [134, 135]. The inorganic precursors grow around the liquid crystal. After the polymerization and the condensation, the template can be removed, leaving a mesoporous material whose structure, pore size and symmetry are determined by the liquid crystal scaffold. In addition, the high surfactant concentration templating method often leads to monolithic materials rather than powders which are associated with mesostructured silica prepared from micellar solution [136]. Attard and co-workers synthesized mesoporous silicas using high concentrations of nonionic surfactants as templates and TMOS was added as the silica source [134]. Hydrolysis of the alkoxide generated methanol, which would destroy the liquid crystalline phase. Removal of the methanol through gentle vacuum distillation recreated the phase. The condensation of inorganic precursors is improved owing to the confined growth around the surfactants and thus ceramic-like frameworks are formed. After the condensation, the organic templates can be removed by calcination or extraction. The mechanism involved may seem straight forward and the technique is sometimes referred to as nanocasting [80]. The inorganic materials “cast” the mesostructures, pore sizes and symmetries from the liquid-crystal scaffolds. It has mainly been used in combination with nonionic surfactants as templating agents. One interesting aspect of the direct templating method is that it does not require a specific surfactant–silicate interaction, as does the method of synthesis in a micellar solution.
Mesoporous materials via nonionic fluorinated surfactant templating approach
As we noted in Part 1 of this chapter, fluorinated surfactants constitute a class of amphiphiles, different from that of hydrogenated surfactants, because of their specific properties [25, 146, 147]. These surfactants have been studied from fundamental standpoint, in terms of physico-chemical and emulsifying, but have been rarely exploited in the field of mesoporous materials until recently. In this context, interest focused on these surfactants is their high thermal stability compared to that of hydrogenated surfactants. Indeed, this property is expected to make hydrothermal treatment at higher temperatures, leading to a better condensation of silica and result in a material with improved hydrothermal stability.
The first mesoporous materials prepared from a fluorinated surfactant were obtained in the laboratory using a nonionic surfactant, C8F17C2H4(OC2H4)9OH [RF8(EO)9] provided by DuPont [148]. Mesoporous materials with a hexagonal channel array are prepared at 80 oC in a wide range of surfactant contents (5-20 wt.%). The structural and textural analysis shows that these materials have a higher degree of structure compared to those prepared via the hydrogen homologous C16H33(EO)10 according to the difference in hydrophobicity of hydrogenated and fluorinated chains (1CF2 = 1.7 CH2) [149, 150]. This difference may be attributed to the higher volume of fluorocarbon chains. Subsequently, the effect of solubilization of perfluorodecalin (PFD C10F18) on the phase behavior of the system RF8(EO)9-water and on characteristics of materials has been studied [151, 152].
Kunieda et al. [153] also succeeded in preparing mesoporous materials from fluorinated nonionic surfactant C8F17SO2(C3H7)N(C2H4O)nH (surfactants provided by Mitsubishi in Japan but not in Europe). By varying the length of the oxyethylene chain (n = 6, 10, 20), the authors followed the degree of structuring of materials prepared at various pH and surfactant concentrations. The results show that there is an optimal length of the hydrophilic chain (n = 10) for which the hexagonal structures are formed. For values below or above 10, only wormlike materials are obtained. Furthermore, the results show that this fluorinated nonionic surfactants family is suitable for the preparation of materials mesostructures with pore sizes relatively small (between 3.3 and 3.5 nm) and walls thick (greater than 2 nm). Xiao et al. used fluorinated nonionic surfactants with formula C5F11C2H4(EO)10 [154] and C6F13C2H4(EO)14 [122] to show that it is possible to obtain materials with both, small pore diameter (1.6-4.0 nm) and thick walls (2.5-2.9 nm). A higher degree of structuring material was obtained after the addition of trimethylbenzene without increasing the size of pores. In 2004, Antonietti et al. [155] used mixtures of fluorinated and hydrogenated surfactants. The idea developed from the fact that these two types of surfactants did not mix and form two types of micelles. Thus, condensation of the silica had place on these two types of templates giving rise to mesoporous materials with a bimodal distribution of pore size. Afterward, mixed fluorinated– hydrogenated surfactant-based system [C8F17C2H4(OC2H4)9–C12H25(OC2H4)8] was investigated in the laboratory. Mesostructured silicas with a well hexagonal array of their channels were prepared via a cooperative templating mechanism (CTM) if the loading of fluorinated surfactant is more than 50%. Decreasing the proportion of the fluorinated amphiphile in the mixture leads to the formation of mesoporous silica with a disordered structure. In addition, a study [156] dealing with mixtures of fluorinated [RmF(EO)n] and hydrogenated [RmH(EO)n] surfactants have been done in the laboratory as well. These mixed systems were used as templates for the preparation of mesoporous silica materials via the self-assembly mechanism. Hexagonal mesostructures had been obtained for the ratio between the volumes of the hydrophilic headgroup (VA) and the hydrophobic part (VB) in the range of 0.95- 1.78. The main properties of the surfactant which influence the pore ordering in a hierarchical way were also established.
Table of contents :
Chapter I. Introduction
1. Surfactants based system
1.1 Properties of surfactant molecules
1.2 Phase behavior
1.3 Highly concentrated emulsions
2. Porous materials
2.1 Sol-gel process
2.2 Material-Templated by Surfactant (MTS)
2.3 Hierarchical porous materials
3. Context of study
Chapter II. Experimental part
1. Materials
1.1 Surfactants
1.2 Hydrogenated compounds
1.3 Fluorocarbons
2. Phase diagram determination
3. Preparation of porous materials
3.1 Source of silica
3.2 Preparation of the mesoporous materials
4. Techniques of characterization
4.1 Polarized light microscopy
4.2 Small angle X-ray scattering (SAXS)
4.3 Nitrogen adsorption-desorption analysis
4.4 Dynamic light scattering (DLS)
4.5 Scanning electron microscopy (SEM)
Chapter III. Preparation and characterization of porous silica templated by RH 12A(EO)9 based-system
1. RH
12A(EO)9-water system
1.1 Characterization of the RH
12A(EO)9-water systemError! Bookmark not defined.
1.2 Characterization of the liquid crystal phases
1.3 Characterization of the mesoporous materials prepared from the RH 12A(EO)9-water system
2. Solubilization of hydrocarbons in the RH 12A(EO)9-water system
2.1 Phase behavior
2.2 Structural parameters of the hexagonal and the lamellar phases
2.3 Influence of the solubilization of oils on the characteristics of materials
3. Discussion
4. Conclusion
Chapter IV. Preparation and characterization of porous silica templated by nonionic fluorinated systems
1. The C8F17C2H4(OC2H4)9OH based systems
1.1 Phase behavior
1.2 Porous silica prepared from RF 8(EO)9/ PFD/ water RF 8(EO)9/ PFD/ water systems.
2. C7F15C2H4(OC2H4)8OH based systems
2.1 Phase behavior
2.2 Porous materials prepared from with microemulsions and emulsions
3. Discussion
3.1 Macroporous silica
3.2 Expansion of the mesopores
3.3 Pore ordering in the presence of PFOBr
4. Conclusion
Chapter V. The effect of the alcohols addition in the RF 8(EO)9-based system on the characteristics of mesoporous silica
1. Solubilization of methanol and iso-propanol in the RF 8(EO)9-water system
1.1 Ternary diagram
1.2 Structural parameters of hexagonal crystal liquid phase
1.3 Mesoporous materials prepared from system RF 8(EO)9-water with methanol or isopropanol
1.4 Discussion
2. Solubilization of butanol in the RF 8(EO)9-water system
2.1 Ternary diagram
2.2 Structural parameters of the hexagonal crystal liquid phase
2.3 Mesoporous materials prepared from the system RF 8(EO)9/water in the presence of butanol
3. Solubilization of octanol in the RF 8(EO)9-water system
3.1 Ternary diagram
3.2 Structural parameters of hexagonal crystal liquid phase
3.3 Mesoporous materials prepared from system RF 8(EO)9-water with octanol.
3.4 Discussion
4. Solubilization of fluorinated octanol in the RF 8(EO)9- water system
4.1 Ternary diagram
4.2 Structural parameters of hexagonal crystal liquid phase
4.3 Mesoporous materials prepared from the system RF 8(EO)9-water in the presence of fluorinated octanol
4.4 Discussion
5. Conclusion
Conclusions
Outlooks
Appendix
References
