Unusual dependence of particle architecture on surfactant concentration in partially fluorinated decylpyridinium templated silica

A series of porous silica particles is prepared with different concentrations of the fluorinated cationic surfactant 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10)-heptadecafluorodecyl)pyridinium chloride (HFDePC) to trace the changes in pore structure and particle morphology as the surfactant concentration increases. At the lowest concentration studied (1.5 mmol/L), the product consists of small round particles with close-packed cylindrical mesopores. As the HFDePC concentration increases, macroporous voids are introduced to create multi-chambered hollow particles with mesoporous walls. With a still higher concentration of HFDePC the macropore volume decreases, and elongated, tactoid-like nanoparticles are formed with random mesh-phase pores oriented with silica layers perpendicular to the main axis of the particles. Further increasing the concentration of HFDePC eventually leads to the formation of round particles with disordered pores. These changes are consistent with increasing HFDePC concentration favoring increasingly oblate or disklike micelles. The process of forming the elongated particles with random mesh-phase structure is investigated by TEM of chilled and dried samples. The results indicate that the oriented tactoid-like structure forms spontaneously within 2 min by co-assembly of silica and HFDePC rather than by preferred growth perpendicular to the layers. The particle shape and layer orientation are consistent with what would be expected for a liquid-crystal particle with orientation-dependent surface tension. Finally, we compare samples prepared with a high HFDePC and with good or poor mixing. With inadequate mixing, a gel layer forms at the top of the sample which is composed of elongated mesoporous particles with a thick coating of microporous silica. The lower particulate phase contains small disordered particles similar to those obtained in a well-mixed sample. Presumably, the structure of the upper layer results from initial immiscibility of the precursor and slow diffusion of silicates out of the gel.     

Tailoring porous silica films through supercritical carbon dioxide processing of fluorinated surfactant templates

The tailoring of porous silica thin films synthesized using perfluoroalkylpyridinium chloride surfactants as templating agents is achieved as a function of carbon dioxide processing conditions and surfactant tail length and branching. Well-ordered films with 2D hexagonal close-packed pore structure are obtained from sol-gel synthesis using the following cationic fluorinated surfactants as templates: 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl)pyridinium chloride (HFOPC), 1-(3,3,4,4,5,5,6,6,7,8,8,8-dodecafluoro-7-trifluoromethyl -octyl)pyridinium chloride (HFDoMePC), and 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-decyl)pyridinium chloride (HFDePC). Processing the sol-gel film with CO2 (69-172 bar, 25 and 45 degrees C) immediately after coating results in significant increases in pore diameter relative to the unprocessed thin films (increasing from 20% to 80% depending on surfactant template and processing conditions). Pore expansion increases with CO2 processing pressure, surfactant tail length, and surfactant branching. The varying degree of CO2 induced expansion is attributed to the solvation of the "CO2-philic" fluorinated tail and is interpreted from interfacial behavior of HFOPC, HFDoMePC, and HFDePC at the CO2-water interface.     

Novel intermediate phases in an ionic fluorocarbon surfactant/water system

This small angle X-ray scattering study of the lyotropic phases in the binary tetramethylammonium perfluorodecanoate/water system shows that there are no classical lyotropic mesophases present. Much of the liquid crystal region is taken up with a random mesh intermediate phase, Mh₁(0) and a phase with rhombohedral symmetry which is probably a rhombohedral mesh intermediate phase, Mh₁(R3m). This behaviour is unusual since previously these mesh phases have been associated with hydrocarbon surfactants or diblock copolymer melts. All the mesophases found have non-uniform interfacial curvature and a sufficiently strong inter-layer interaction to ensure the long range correlation of structures in some phases.     

Added surfactant can change the phase behavior of aqueous polymer-particle mixtures

The phase behavior of aqueous mixtures of the "clouding" polymer ethyl(hydroxyethyl)cellulose (EHEC) mixed with colloidal particles and surfactants has been studied. These types of mixtures are important in many technical formulations. Two types of particles, polystyrene latex and silica, and two types of EHEC, nonmodified EHEC (N-EHEC) and hydrophobically modified EHEC (HM-EHEC), were studied. The EHECs adsorb to both kinds of particles. Both the amount and the type of added surfactant were seen to dramatically influence the partitioning of the particles between the EHEC-rich and EHEC-poor phases of phase-separated mixtures (above the cloud point temperature). Surfactants that are known not to associate with the EHEC backbone, that is, nonionic surfactants and short-chain cationic surfactants, changed the interaction between EHEC and the colloidal particles from attraction to repulsion above a specific surfactant concentration, resulting in a change in the partitioning of the particles from the EHEC-rich to the EHEC-poor phase. No such particle inversion was observed for ionic surfactants that bind to the EHEC backbone. An analysis considering both the binding of surfactant to EHEC and the competitive adsorption of surfactant to the particle surfaces could rationalize all observations, including the large variations observed, among the studied mixtures, in the surfactant concentration required for particle inversion.     

Spherical ordered mesoporous silicas and silica monoliths as stationary phases for liquid chromatography

Ordered mesoporous silicas such as micelle-templated silicas (MTS) feature unique textural properties in addition to their high surface area (approximately 1000 m2/g): narrow mesopore size distributions and controlled pore connectivity. These characteristics are highly relevant to chromatographic applications for resistance to mass transfer, which has never been studied in chromatography because of the absence of model materials such as MTS. Their synthesis is based on unique self-assembly processes between surfactants and silica. In order to take advantage of the perfectly adjustable texture of MTS in chromatographic applications, their particle morphology has to be tailored at the micrometer scale. We developed a synthesis strategy to control the particle morphology of MTS using the concept of pseudomorphic transformation. Pseudomorphism was recognized in the mineral world to gain a mineral that presents a morphology not related to its crystallographic symmetry group. Pseudomorphic transformations have been applied to amorphous spherical silica particles usually used in chromatography as stationary phases to produce MTS with the same morphology, using alkaline solution to dissolve progressively and locally silica and reprecipitate it around surfactant micelles into ordered MTS structures. Spherical beads of MTS with hexagonal and cubic symmetries have been synthesized and successfully used in HPLC in fast separation processes. MTS with a highly connected structure (cubic symmetry), uniform pores with a diameter larger than 6 nm in the form of particles of 5 microm could compete with monolithic silica columns. Monolithic columns are receiving strong interest and represent a milestone in the area of fast separation. Their synthesis is a sol-gel process based on phase separation between silica and water, which is assisted by the presence of polymers. The control of the synthesis of monolithic silica has been systematically explored. Because of unresolved yet cladding problems to evaluate the resulting macromonoliths in HPLC, micromonoliths were synthesized into fused-silica capillaries and evaluated by nano-LC and CEC. Only CEC allows to gain high column efficiencies in fast separation processes. Capillary silica monolithic columns represent attractive alternatives for miniaturization processes (lab-on-a chip) using CEC.     

High-temperature synthesis of stable ordered mesoporous silica materials by using fluorocarbon-hydrocarbon surfactant mixtures

Highly ordered hexagonal mesoporous silica materials (JLU-20) with uniform pore sizes have been successfully synthesized at high temperature (150-220 degrees C) by using fluorocarbon-hydrocarbon surfactant mixtures. The fluorocarbon-hydrocarbon surfactant mixtures combine the advantages of both stable fluorocarbon surfactants and ordered hydrocarbon surfactants, giving ordered and stable mixed micelles at high temperature (150-220 degrees C). Mesoporous JLU-20 shows extraordinary stability towards hydrothermal treatment (100 % steam at 800 degrees C for 2 h or boiling water for 80 h), thermal treatment (calcination at 1000 degrees C for 4 h), and toward mechanical treatment (compressed at 740 MPa). Transmission electron microscopy images of JLU-20 show well-ordered hexagonal arrays of mesopores with one-dimensional (1D) channels and further confirm that JLU-20 has a two-dimensional (2D) hexagonal (P6 mm) mesostructure. 29Si HR MAS NMR spectra of as-synthesized JLU-20 shows that JLU-20 is primarily made up of fully condensed Q4 silica units (delta=-112 ppm) with a small contribution from incompletely cross-linked Q3 (delta=-102 ppm) as deduced from the very high Q4/Q3 ratio of 6.5, indicating that the mesoporous walls of JLU-20 are fully condensed. Such unique structural features should be directly attributed to the high-temperature synthesis, which is responsible for the observed high thermal, hydrothermal, and mechanical stability of the mesoporous silica materials with well-ordered hexagonal symmetry. Furthermore, the concept of "high-temperature synthesis" is successfully extended to the preparation of three-dimensional (3D) cubic mesoporous silica materials by the assistance of a fluorocarbon surfactant as a co-template. The obtained material, designated JLU-21, has a well-ordered cubic Im3m mesostructure with fully condensed pore walls and shows unusually high hydrothermal stability, as compared with conventional cubic mesoporous silica materials such as SBA-16.     

混合阳离子-非离子表面活性剂为模板剂合成含沸石次级结构单元的介孔分子筛

利用混合阳离子-非离子表面活性剂为模板剂采用两步晶化法合成了孔壁具有沸石次级结构单元的介孔分子筛，通过XRD、N2吸附-脱附、FT-IR以及异丙苯裂解探针反应等手段对样品进行了表征。结果表明，合成的介孔材料在结构上与相应的M41S材料类似，无微孔沸石相的存在。立方介孔材料具有较高的热稳定性和水热稳定性，在催化异丙苯裂解反应中，六方介孔材料的催化活性明显高于相似条件下用单一表面活性剂为模板剂合成的含沸石次级结构单元的六方介孔材料。     

Syntheses of complex mesoporous silicas using mixtures of nonionic block copolymer surfactants: understanding formation of different structures using solubility parameters

The use of block copolymer (BCP) nonionic surfactant mixtures (including Pluronic, Brij and Tetronic types) as templates for synthesizing porous silica materials of mixed pore sizes is explored here. These systems have important applications because combinations of pore sizes can allow rapid access of reactants (via large pores) whilst providing the very high surface area of small pores for higher reaction rates or size selectivity. Examples of the materials prepared here include pore size bimodal hexagonal p6mm channel structures and cubic Im3m cage structures. It is shown here that the chemical similarity, as indicated by the solubility parameter, of the surfactants is an important factor in determining the pore structure and size distribution (PSD) of the pores. Monomodal pore structures are usually obtained when the solubility parameters of the surfactants are similar and bimodal pore structures when the solubility parameters are reasonably different. When the interaction parameter is very high disordered porous systems are formed. Ternary co-surfactant systems, e.g. P123-25R4-P65, can also yield highly ordered bimodal mesoporous silica with a hexagonal structure.     

Synthesis of highly-ordered mesoporous silica particles using mixed cationic and anionic surfactants as templates

We applied a molecular assembly formed in an aqueous surfactant mixture of cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium octylsulfate (SOS) as templates of mesoporous silica materials. The hexagonal pore size can be controlled between 3.22 and 3.66 nm with the mixed surfactant system. In addition, we could observe the lamellar structure of the mixed surfactants with precursor molecules, which strongly shows the possibility of precise control of both the pore size and the structure of pores by changing the mixing ratio of surfactants. Moreover, use of the cationic surfactant having longer hydrophobic chain like stearyltrimethylammonium bromide (STAB) caused the increase in d(100) space and shifted the point of phase transition from hexagonal phase to lamellar phase to lower concentration of SOS.     

Understanding on the Surfactants Engineered Morphology Evolution of Block Copolymer Particles and Their Precise Mesoporous Silica Replicas

Surfactant-directed block copolymer(BCP) particles have gained intensive attention owing to their attractive morphologies and ordered domains. However, their controllable fabrication suffers several limitations including complex design and synthesis of multiple surfactant systems, limited choices of block copolymers, and time-consuming post-processes, etc. Herein, a surfactant size-dependent phase separation route is proposed to precisely manipulate the architectures of the anionic block copolymer particles in the binary co-assembly system of BCP and surfactants. In the system of polystyrene block polyacrylic acid (PS-b-PAA) and quaternary ammonium surfactants, it is verified that facile control on the ordered phase separation structures and morphologies of BCP particles can be achieved via simply varying the alkyl lengths of the surfactants. The cationic surfactants are demonstrated participating in the fabrication of the internal structures of BCP particles. Especially, it is found that the cationic surfactants are integrate into the anionic polyacrylic acid(PAA) domain of BCP particles of PS-b-PAA to influence the volume fraction of PAA blocks, so that varied architectures of BCP particles are constructed. Based on these understandings, spherical or ellipsoidal BCP particles are obtained as expected, as well as their precisely inorganic mesoporous silica replicas through the block copolymer nanoparticle replicating route. More interestingly, the ellipsoidal mesoporous silica exhibits higher cellular internalization capability due to its lower energy expenditure during the internalization process, which presents promising potentials in biomedical applications, especially for high-efficient drug delivery systems. These findings may provide valuable insights into the confinement assembly of anionic block copolymers and the creation of special nanocarriers for high-efficiency biomacromolecule delivery in the biomedical community.     

Use of the ternary phase diagram of a mixed cationic/glucopyranoside surfactant system to predict mesostructured silica synthesis

Mixed surfactant systems have the potential to impart controlled combinations of functionality and pore structure to mesoporous metal oxides. Here, we combine a functional glucopyranoside surfactant with a cationic surfactant that readily forms liquid crystalline mesophases. The phase diagram for the ternary system CTAB/H(2)O/n-octyl-beta-D-glucopyranoside (C(8)G(1)) at 50 degrees C is measured using polarized optical microscopy. At this temperature, the binary C(8)G(1)/H(2)O system forms disordered micellar solutions up to 72 wt% C(8)G(1), and there is no hexagonal phase. With the addition of CTAB, we identify a large area of hexagonal phase, as well as cubic, lamellar and solid surfactant phases. The ternary phase diagram is used to predict the synthesis of thick mesoporous silica films via a direct liquid crystal templating technique. By changing the relative concentration of mixed surfactants as well as inorganic precursor species, surfactant/silica mesostructured thick films can be synthesized with variable glucopyranoside content, and with 2D hexagonal, cubic and lamellar structures. The domains over which different mesophases are prepared correspond well with those of the ternary phase diagram if the hydrophilic inorganic species is assumed to act as an equivalent volume of water.     

Competitive adsorption of surfactants and hydrophilic silica particles at the oil-water interface: interfacial tension and contact angle studies

The effect of surfactants' type and concentration on the interfacial tension and contact angle in the presence of hydrophilic silica particles was investigated. Silica particles have been shown to have an antagonistic effect on interfacial tension and contact angle in the presence of both W/O and O/W surfactants. Silica particles, combined with W/O surfactant, have no effect on interfacial tension, which is only dictated by the surfactant concentration, while they strongly affect interfacial tension when combined with O/W surfactants. At low O/W surfactant, both particles and surfactant are adsorbed at the interface, modifying the interface structure. At higher concentration, interfacial tension is only dictated by the surfactant. By increasing the surfactant concentration, the contact angle that a drop of aqueous phase assumes on a glass substrate placed in oil media decreases or increases depending on whether the surfactant is of W/O or O/W type, respectively. This is due to the modification of the wettability of the glass by the oil or water induced by the surfactants. Regardless of the surfactant's type, the contact angle profile was dictated by both particles and surfactant at low surfactant concentration, whereas it is dictated by the surfactant only at high concentration.     

Self-organization of double-chained and pseudodouble-chained surfactants: counterion and geometry effects

Self-organization in aqueous systems based on ionic surfactants, and their mixtures, can be broadly understood by a balance between the packing properties of the surfactants and double-layer electrostatic interactions. While the equilibrium properties of micellar systems have been extensively studied and are understood, those of bilayer systems are less well characterized. Double-chained and pseudodouble-chained (or catanionic) surfactants are among the amphiphiles which typically form bilayer structures, such as lamellar liquid–crystalline phases and vesicles. In the past 10–15 years, an experimental effort has been made to get deeper insight into their aggregation patterns. With the double-chained amphiphiles, by changing counterion, adding salt or adding anionic surfactant, there are possibilities to depart from the bilayer aggregate in a controlled manner. This is demonstrated by several studies on the didodecyldimethylammonium bromide surfactant. Mixtures of cationic and anionic surfactants yield the catanionics, surfactants of the swelling type, and also show a rich phase behavior per se. A variety of liquid–crystalline phases and, in dilute regimes, equilibrium vesicles and different micellar shapes are often encountered. Phase diagrams and detailed structural studies, based on several techniques (NMR, microscopy and scattering methods), have been reported, as well as theoretical studies. The main features and conclusions emerging from such investigations are presented.     

Nonionic fluorinated-hydrogenated surfactants for the design of mesoporous silica materials

We have investigated the influence of the ratio between the volume of the hydrophilic head ( V A) and the volume of the hydrophobic part ( V B) of the surfactant on the mesopore ordering. To understand the difference of behavior we have performed a complete study dealing with fluorinated [R m (F)(EO) n ] and hydrogenated [R m (H)(EO) n ] surfactants. Their mixtures have also been taken into account. Here only the phase diagrams and the structural parameters of the liquid crystal phases of the mixed systems are reported. We have shown that the mutual or partial miscibility of the fluorinated and the hydrogenated surfactants depends on the number of oxyethylene units of each surfactant. To follow, various systems were used for the preparation of silica mesoporous materials via a cooperative templating mechanism (CTM). Results clearly reveal that V A/ V B ratios in the range between 0.95 and 1.78 lead to the formation of well-ordered mesostructures. Wormhole-like structures are obtained for higher or lower values. Moreover, results show that from the V A/ V B point of view, polyoxyethylene fluoroalkyl ether surfactants behave like their hydrogenated analogues.     

Relation between the lower consolute boundary and the structure of mesoporous silica materials

In this study, we have shed some light on the relation between the position of the lower consolute boundary of various nonionic surfactants in water and the structure of the mesoporous silica materials synthesized from these surfactants-based systems. In the first part, the lower consolute boundary was shifted by adding salts. Depending on the features of the phase diagram, we have looked for either a salting out or a salting in effect. Mesoporous materials were prepared from a micellar solution of the investigated surfactants. Results clearly evidenced that the cooperative self-assembly mechanism is not favored if the lower consolute boundary is not shifted toward high temperatures. Moreover, the higher the difference between the phase separation temperature and the temperature at which the silica precursor is added to the surfactant solution, the better the mesopore ordering is. In the second part, this tendency has been confirmed by using a hydrogenated surfactant as additive.     

Structure of mesh phases in a cationic surfactant system with strongly bound counterions

We report the observation of an intermediate mesh phase with rhombohedral symmetry, corresponding to the space group Rm, in a mixed surfactant system formed by the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the organic salt 3-sodium-2-hydroxy naphthoate (SHN). It occurs between a random mesh phase (L(alpha)(D)) and a lamellar phase (L(alpha)) at low temperatures; at higher temperatures, the (L(alpha)(D)) phase transforms continuously into the (L(alpha)) phase with an increasing surfactant concentration (phi(s)). To separate the effects of salt and phi(s) on the phase behavior, the ternary system consisting of cetyltrimethylammonium 3-hydroxy-naphthalene-2-carboxylate (CTAHN), sodium bromide (NaBr), and water was studied. The intermediate mesh phase is found in this system at high NaBr concentrations. The micellar aggregates, both in the intermediate and random mesh phases, are found to be made up of a two-dimensional network of rod-like segments, with three rods meeting at each node. The average mesh size increases with phi(s), and the transition from the random mesh phase to the intermediate phase is found to occur when it is approximately 1.5 times the lamellar periodicity. The intermediate mesh phase is absent in the equimolar dodecyltrimethylammonium bromide (DTAB)-SHN system, indicating the role of the surfactant chain length in the formation of this phase. This system exhibits a random mesh phase over a very wide range of water content, with the average mesh size decreasing upon an increasing phi(s), contrary to the trend seen in the CTAB-SHN system.     

Morphological transitions in model membrane systems by the addition of anesthetics

The mechanism of anesthetic action on membranes is still an open question, regardless of their extensive use in medical practice. It has been proposed that anesthetics may have the effect of promoting pore formation across membranes or at least switching transmembrane channels. In both cases this may be the result of changes in the interfacial curvature of the membrane due to the presence of anesthetic molecules. Aqueous solutions of surfactants display phases that mimic, in a simplified manner, real biological membranes. Therefore, in this study, two nonionic surfactant systems C16E6/H2O in concentrated solution and C10E3/H2O in dilute solution have been used as model membranes for the investigation of the effects of six common anesthetics (halothane, sodium thiopental, lidocaine base form and hydrochloride, prilocaine hydrochloride, and ketamine hydrochloride). Both binary surfactant-water systems exhibit phase transitions from the lamellar phase, Lalpha, that has zero spontaneous curvature and zero monolayer curvature to phases with more local interfacial curvature. These are the random mesh phase, Mh1(0), which consists of lamellae pierced by water-filled pores with local areas of positive interfacial curvature and the sponge phase, L3, that consists of the lamellar phase with interlamellae attachments, often referred to as a "melted" cubic phase, possessing negative monolayer curvature. Small-angle X-ray scattering and 2H NMR experiments upon the C16E6/2H2O system and optical observations of the C10E3/H2O system showed that all anesthetics employed in this study cause a shift in the Mh1(0) to Lalpha phase transition temperature and in the Lalpha to L3 transition temperature, respectively. All of the anesthetics studied bind to the interfacial region of the surfactant systems. Two types of behavior were observed on anesthetic addition: type I anesthetics, which decreased interfacial curvature, and type II, which increased it. However, at physiological pH both types of anesthetics decreased interfacial curvature.     

Molecularly ordered inorganic frameworks in layered silicate surfactant mesophases.

Self-assembled lamellar silica-surfactant mesophase composites have been prepared with crystal-like ordering in the silica frameworks using a variety of cationic surfactant species under hydrothermal conditions. These materials represent the first mesoscopically ordered composites that have been directly synthesized with structure-directing surfactants yielding highly ordered inorganic frameworks. One-dimensional solid-state 29Si NMR spectra, X-ray diffraction patterns, and infrared spectra show the progression of molecular organization in the self-assembled mesophases from structures with initially amorphous silica networks into sheets with very high degrees of molecular order. The silicate sheets appear to be two-dimensional crystals, whose structures and rates of formation depend strongly on the charge density of the cationic surfactant headgroups. Two-dimensional solid-state heteronuclear and homonuclear NMR measurements show the molecular proximities of the silica framework sites to the structure-directing surfactant molecules and establish local Si-O-Si bonding connectivities in these materials.     

阳离子与非离子混合表面活性剂模板合成介孔SiO2

利用各种两亲分子有序组合体构成超分子模板,合成从介观到宏观尺度不同形态的无机材料成为材料科学新崛起的研究方向[1].介孔SiO2在催化、吸附、分离介质及化学传感器等方面有广阔的应用前景.     

Examining the role of surfactant packing in phase transformations of periodic templated silica/surfactant composites

In this work, we examine the role of curvature and surfactant packing in controlling the structure of periodic silica/surfactant composites by driving such materials through a transformation from a hexagonal to a lamellar phase. We focus on how the interplay of desired packing and volume constraints dictates the resulting structures. In general, surfactants expand in a complex way upon heating, and this can cause a change in the optimal packing geometry. However, the presence of a rigid silica framework may prevent surfactants from reaching this preferred volume and/or curvature. Real-time in situ X-ray diffraction is used to monitor the structural evolution of these materials heated under hydrothermal treatments. Because the thermal-driven disorder of the surfactant tails drives the phase transition, we examine four types of composites with varying tail density. Ordinarily, composites consist of surfactants with one 20-carbon tail and one positively charged ammonium headgroup. Tail density is varied by replacing a small amount (0-16%) of these single-tail, single-head surfactants with single-tail, double-head 'gemini' surfactants. A greater head--tail ratio indeed produces different results, causing the phase transition to occur at higher temperatures. Using simple geometric models to gain better understanding of our experimental results, we find that, while both unfavorable curvature and limited volume may exist for the surfactants in these composites, the constrained curvature appears to be the dominant effect in driving structural rearrangement.