A novel organoclay with antibacterial activity prepared from montmorillonite and Chlorhexidini Acetas

A series of novel organoclays with antibacterial activity were synthesized using Ca-montmorillonite and Chlorhexidini Acetas (CA) by ion-exchange. The resultant organoclays were characterized using X-ray diffraction (XRD), high-resolution thermogravimetric analysis (HRTG) and Fourier transform infrared spectroscopy (FTIR). Their antibacterial activity was assayed by so-called halo method. In the organoclays prepared at low CA concentration, CA ions within the clay interlayer adopt a lateral monolayer while a 'kink' state or a special state with partial overlapping of the intercalated CA in the organoclays prepared at 1.0-4.0 CEC. HRTG analysis demonstrates that CA located outside the clay interlayer exists in all synthesized organoclays, resulting from the complex molecular configuration of CA. The dramatic decrease of the surface adsorbed water and interlayer water is caused by the surface property transformation and the replacement of hydrated cations by cationic surfactant. These observations are supported by the results of FTIR. Antibacterial activity test against E. coli demonstrates that the antibacterial activity of the resultant organoclays strongly depends on the content of CA. Meanwhile, the resultant organoclay shows a long-term antibacterial activity that can last for at least one year. These novel organoclays are of potential use in synthesis of organoclay-based materials with antibacterial activity.     

Effects of organoclays on morphology and thermal and rheological properties of polystyrene and poly(methyl methacrylate) blends

Morphology, thermal and rheological properties of polymer‐organoclay composites prepared by melt‐blending of polystyrene (PS), poly(methyl methacrylate) (PMMA), and PS/PMMA blends with Cloisite® organoclays were examined by transmission electron microscopy, small‐angle X‐ray scattering, secondary ion mass spectroscopy, differential scanning calorimetry, and rheological techniques. Organoclay particles were finely dispersed and predominantly delaminated in PMMA‐clay composites, whereas organoclays formed micrometer‐sized aggregates in PS‐clay composites. In PS/PMMA blends, the majority of clay particles was concentrated in the PMMA phase and in the interfacial region between PS and PMMA. Although incompatible PS/PMMA blends remained phase‐separated after being melt‐blended with organoclays, the addition of organoclays resulted in a drastic reduction in the average microdomain sizes (from 1–1.5 μm to ca. 300–500 nm), indicating that organoclays partially compatibilized the immiscible PS/PMMA blends. The effect of surfactant (di‐methyl di‐octadecyl‐ammonia chloride), used in the preparation of organoclays, on the PS/PMMA miscibility was also investigated. The free surfactant was more compatible with PMMA than with PS; the surfactant was concentrated in PMMA and in the interfacial region of the blends. The microdomain size reduction resulting from the addition of organoclays was definitely more significant than that caused by adding the same amount of free surfactant without clay. The effect of organoclays on the rheological properties was insignificant in all tested systems, suggesting weak interactions between the clay particles and the polymer matrix. In the PS system, PMMA, and organoclay the extent of clay exfoliation and the resultant properties are controlled by the compatibility between the polymer matrix and the surfactant rather than by interactions between the polymer and the clay surface. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 44–54, 2003     

A X-ray photoelectron spectroscopy study of HDTMAB distribution within organoclays

X-ray photoelectron spectroscopy (XPS) in combination with X-ray diffraction (XRD) and high-resolution thermogravimetric analysis (HRTG) has been used to investigate the surfactant distribution within the organoclays prepared at different surfactant concentrations. This study demonstrates that the surfactant distribution within the organoclays depends strongly on the surfactant loadings. In the organoclays prepared at relative low surfactant concentrations, the surfactant cations mainly locate in the clay interlayer, whereas the surfactants occupy both the clay interlayer space and the interparticle pores in the organoclays prepared at high surfactant concentrations. This is in accordance with the dramatic pore volume decrease of organoclays compared to those of starting clays. XPS survey scans show that, at low surfactant concentration (<1.0 CEC), the ion exchange between Na⁺ and HDTMA⁺ is dominant, whereas both cations and ion pairs occur in the organoclays prepared at high concentrations (>1.0 CEC). High-resolution XPS spectra show that the modification of clay with surfactants has prominent influences on the binding energies of the atoms in both clays and surfactants, and nitrogen is the most sensitive to the surfactant distribution within the organoclays.     

Effect of Organoclays on Silicate Layer Structures and Thermal Stabilities of in-situ Polymerized Poly(D-lactide)/Clay Nanocomposites

Six kinds of organoclays were prepared through three kinds of polyols (PTMG, PEA and PCL) to investigate the effects of molecular weight and the chemical structure of organifiers. PTMG based organoclays showed higher ion-exchanged fraction than other organoclays and long chain organifier showed better efficiency in ion-exchanged fraction in the case of PTMG based organifiers. From WAXD and TEM analysis, it was confirmed that PTMG based organoclays formed partially exfoliated or fully exfoliated silicate layer structures. PDLA/clay nanocomposites were prepared by in-situ ring-opening polymerization of D-lactide with PTMG based organoclays as macro-initiators in the presence of equimolar Sn(Oct)₂/PPh₃ complex catalysts. The molecular weight of PDLA/clay nanocomposite decreased as increasing the feeding amount of organoclay because organoclay had hydroxyl terminal groups which can initiate the ring-opening polymerization of D-lactide. From TGA analysis, thermal stabilities of PDLA/clay nanocomposites improved with increasing organoclay content. From WAXD and TEM analysis, organoclay which was prepared by high molecular weight of PTMG based organifier was effective on the exfoliation of silicate layers in the in-situ polymerized PDLA/clay nanocomposite.     

Stabilization of hydroxyl-group-terminated SERS-marker molecules on microAg particles by silanization

Surfactant modified montmorillonitic clays synthesized by ion exchange using the hydrothermal reaction method have been compared using XRD and thermal analysis. X-ray diffraction (XRD) shows the changes in the surface properties of organoclays through expansion with surfactant loading. A polynomial relationship exists between the basal spacing and the CEC loading described by the equation y=0.3232x(2) + 0.2052x+1.2834 with R(2)=0.9955. Different arrangements of the surfactant molecules in the organoclays are inferred from the changes in basal spacings. para-Nitrophenol also causes the expansion of the montmorillonite clay and affects the arrangements of the surfactant molecules within the clay layers. Changes in the surfactant molecular arrangements were analyzed by thermogravimetry. Additional thermal decomposition steps were observed when para-nitrophenol is adsorbed on the organoclay.     

Adsorption of p-nitrophenol on organoclays

In this study, organoclays were prepared through ion exchange of a single cationic surfactant, hexadecyltrimethylammonium bromide and characterised by a range of methods including X-ray diffraction (XRD) and thermogravimetric analysis. Changes in the surface properties of montmorillonite and the organoclays were observed and the basal spacings of organoclays with and without p-nitrophenol were determined using XRD. The thermal stability of both organoclays were measured using thermogravimetry. As the surfactant loading increased, the expanded basal spacings were observed, and different molecular configurations of surfactant were identified. When the surfactant loading exceeded 1.0 CEC, surfactant molecules tend to adsorb strongly on the clay surface and this resulted in increased affinity to organic compounds. The adsorbed p-nitrophenol and the surfactant decomposed simultaneously. Hence, the surfactant molecules and adsorbed p-nitrophenol are important in determining the thermal stabilities of organoclays. This study enhances the understanding of the structure and adsorption properties of organoclays and has further implications for the application of organoclays as filter materials for the removal of organic pollutants in aqueous solutions.     

Effect of surfactant and solvent properties on the stacking behavior of non-aqueous suspensions of organically modified clays

Montmorillonite clay was treated with quaternary ammonium surfactants with 1-3 long chains of 10-18 carbons to form organoclays which can be suspended in non-aqueous solvents. The effects of surfactant chain length, number of long chains, and the properties of the solvent on the colloidal behavior of the surfactant coated clay plates were studied using small-angle X-ray scattering. The scattering data were modeled using a one-dimensional aggregation theory to describe the stacking of the clay plates. The plates self-organize into stacks with a reproducible basal spacing in the range of 30-50 A, and for each surfactant, the basal spacing falls into one of two preferred distances. We interpret this by considering that the surfactant layer on the clay has two strata, one being the polar near-clay headgroup region and the other the nonpolar alkane chain region. Polar solvents will swell the polar stratum preferentially while nonpolar solvents will swell the nonpolar stratum of the surfactant. As the nonpolar stratum is larger than the polar one, the nonpolar solvents increase the basal spacing between the clay plates more than the polar solvents. The number of long chains on the surfactant does not have an effect on the basal spacing, as the density of surfactant molecules on the surface is low enough to allow the unimpeded swelling of the chains. The one-dimensional aggregation theory can be used to determine the number of plates in a stack, but the effect of changing clay particle size or concentration is not as great as would be expected from this theory. This may be due to the formation of large-scale structures in the suspensions which prevent a true equilibrium stack size being attained.     

Self-Assembly of Alkylammonium Chains on Montmorillonite: Effect of Interlayer Cations,CEC, and Chain Length

Recently, polymeric materials have been filled with synthetic or natural inorganic compounds in order to improve their properties. Especially, polymer clay nanocomposites have attracted both academic and industrial attention. Currently, the structure and physical phenomena of organoclays at molecular level are difficultly explained by existing experimental techniques. In this work, molecular dynamics (MD) simulation was executed using the CLAYFF and CHARMM force fields to evaluate the structural properties of organoclay such as basal spacing, interlayer density, energy and the arrangement of alkyl chains in the interlayer spacing. Our results are in good agreement with available experimental or other simulation data. The effects of interlayer cations (Na⁺, K⁺, Ca²⁺), the cation exchange capacity, and the alkyl chain length on the basal spacing and the structural properties are estimated. These simulations are expected to presage the microstructure of organo-montmorillonite and lead relevant engineering applications.     

Degradation of poly(ethylene terephthalate)/clay nanocomposites during melt extrusion: Effect of clay catalysis and chain extension

Organoclays with various contents of hydroxyl groups and absorbed ammonium were prepared and compounded with poly(ethylene terephthalate) (PET), forming PET/clay nanocomposites via melt extrusion. Dilute solution viscosity techniques were used to evaluate the level of molecular weight of PET/clay nanocomposites. Actually, a significant reduction in PET molecular weight was observed. The level of degradation depended on both the clay structure and surfactant chemistry in organoclays. The composites, based on clay with larger amount of hydroxyl groups on the edge of clay platelets, experienced much more degradation, because the hydroxyl groups acted as Brønsted acidic sites to accelerate polymer degradation. Furthermore, organoclays with different amounts of absorbed ammonium led to different extents of polymer degradation, depending upon the acidic sites produced by the Hofmann elimination reaction of ammonium. In addition, the composite with better clay dispersion state, which was considered as an increasing amount of clay surface and ammonium exposed to the PET matrix, experienced polymer degradation more seriously. To compensate for polymer degradation during melt extrusion, pyromellitic dianhydride (PMDA) was used as chain extender to increase the intrinsic viscosity of polymer matrix; more importantly, the addition of PMDA had little influence on the clay exfoliation state in PET/clay nanocomposites.     

Encapsulation of non-chemically modified montmorillonite clay platelets via emulsion polymerization

This paper demonstrates that a covalent bonding between clay platelets and polymer chains is not necessary for a successful encapsulation of the inorganic compounds through emulsion polymerization. In the work described in this paper, the chemical modification of clay was performed using two kinds of titanate coupling agents: titanium IV, (2-propanolato)tris(2-propanoata-O), 2-(2-methoxyethoxy) ethanol and titanium IV, 2-propanolato,tris(isooctadecanoato-O), where the former is saturated and the second has unsaturated alkyl groups. The hydrolytic stability of the organoclays thus synthesized was thereafter investigated. It was found that the titanate modifiers were highly sensitive towards hydrolysis as evidenced by Fourier transform infrared and thermal gravimetry analyses. The effect of a chemical modification step on clay encapsulation was studied by comparing the results obtained between reactions using the synthesized organoclays and the ones using native clay platelets. It was found that the incorporation of polymerizable double bonds on clay platelets was unnecessary to achieve successful encapsulation and that, surprisingly, the chemical modification step could be omitted for the synthesis of clay-containing latex particles. The effects of monomer feed composition, i.e., monomer mixtures consisting of different weight ratios methyl methacrylate/butyl acrylate, and process types on clay encapsulation were also investigated. Both parameters were found to have a strong influence on clay encapsulation. Finally, the surfactant concentration and the surfactant type were statistically found to have a significant effect on the clay/polymer interaction as evidenced by the results of glass transition temperatures of dried latex/clay nanocomposites powders.     

Control of organoclay structure in hydrocarbon polymers

Two different kinds of organoclays were prepared by mixing a pristine montmorillonite and a double‐chain ammonium salt in many different thermoplastic or elastomeric polymers. Independently of the chemical nature of the considered polymers, the obtained organoclays presented a basal spacing of 4 or 6 nm, when the mixing occurred in the absence or in the presence of a small amount of stearic acid (SA), respectively. X‐ray diffraction and Fourier transform infrared measurements support the hypothesis that these two kinds of organoclays correspond to paraffin‐type tilted and perpendicular bi‐layer intercalates, respectively. The co‐intercalation of SA molecules with the double‐chain amphiphile is suggested, to explain the observed expansion of the clay interlayer distance. The obtained results suggest an easy way to control the organoclay structure in polymer composites. Moreover, the authors on the basis of these results propose a criticism to the extensive literature that systematically explains most d basal spacing increase observed for clays in polymer with the penetration of apolar polymer chains in the clay interlayer space. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of cationic surfactant removal on the thermal stability of organoclays

The microstructure, thermal stability, surface energy, and swelling characteristics of two kinds of commercial organoclays, before and after washing treatment with a mixture of H2O/ethanol, were investigated using X-ray diffraction (XRD), thermogravimetric analysis (TG/DTG), wettability measurement, and swelling measurement. This study demonstrates that the external-surface physically adsorbed surfactant can be removed after washing treatment, resulting in an increase in thermal stability and a decrease in surface energy of the resultant organoclays. Organoclays are difficult to be introduced into a solvent when their surface energies are lower than that of the solvent. On the other hand, the organoclay with gamma(organoclay) < gamma(solvent) is easier to be swollen and expandable by the solvent. The swelling and basal spacing measurements of the organoclays introduced into organic media indicate that the swelling factor and the interlayer swelling are two independent parameters. Both the polar character of the solvent and the swelling capacity of clay have a prominent effect on the interlayer swelling of the organoclays.     

Organoclay sorption of benzene as a function of total organic carbon content

The sorption of benzene to bentonite, activated carbon, and two organo-clays synthesized with the quaternary ammonium organic cations hexadecyltrimethylammonium (HDTMA) and benzyltriethylammonium (BTEA) was quantified as a function of total organic carbon content. The unmodified bentonite sorbed no benzene, while the activated carbon exhibited the strongest benzene uptake. For the organoclays, organic cations were exchanged onto Wyoming bentonite at four different percentages of the clay's cation exchange capacity. For HDTMA clay, in which partitioning is the dominant sorptive medium, it was determined that benzene sorption increased as the total organic carbon content increased (as the clay became more hydrophobic). In contrast, the sorption of benzene to BTEA clay, an adsorptive clay, decreased as the total organic carbon content of the clay was increased. It is believed that the sorptive capacity of BTEA clay decreases due to the formation of positively charged dimers on the clay's surface that block access to the sorptive sites. The organoclays sorbed more benzene than the unmodified bentonite, but less than the activated carbon.     

Surface modification of clay and its effect on the intercalation behavior of the polymer/clay nanocomposites

Melt intercalation of the methylsilylated organoclays with polar polymers such as SAN was examined to verify the adhesive role of guest polymeric chains between hydrophilic clay layers, so-called “glue effect” on intercalation behavior. Once methylsilylated organoclay was melt-blended with SAN, it was found that the mixture presented significant retardation of increase of interlayer spacing, d₀₀₁ with heating time, and a noticeable decrease of d₀₀₁ after the methylsilylation of organoclay, implying that the diffusion of SAN was highly suppressed by the decrease of polar interaction force caused by conversion of OH to methylsiloxyl groups. However, when applying shear force for the methylsilylated organoclay/SAN nanocomposites during melt intercalation, a noticeable increase of d₀₀₁ was observed, expressing that intercalation of clay by SAN occurred much more effectively because of the reduction of gluing force between host clay and guest polymers, which was well supported by dramatic improvements of mechanical properties after methylsilylation of organoclays. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2367–2372, 2004     

The effect of fluorination: a crystallographic and computational study of mesogenic alkyl 4-[2-(perfluorooctyl)ethoxy]benzoates

The series of alkyl 4-[2-(perfluorooctyl)ethoxy]benzoates (F8-n) shows a systematic change of crystal structures depending on the length of the alkyl chain: separate packing of perfluorooctyl (Rf) and alkyl (Rh) chains from each other for shorter (n=2) and longer (n=11) members, alternate packing of Rf and Rh chains for middle (n=6,7) members, and an intermediate type of packing for n=4. Semiempirical MO calculations show slightly repulsive interactions between the Rf chains, and attractive ones between Rf and Rh chains and between Rh and the core of a molecular pair. It is concluded that fluorination determines the molecular shape of the crystal structures by making the chain rigid. It is confirmed that the interactions between Rf chains are small compared with those between other moieties and that they are forced to aggregate owing to the exclusion from other moieties. Thus, the effect is dependent on the geometries and intermolecular interactions of the other moieties.     

Molecular simulation studies of the structure of phosphorylcholine self-assembled monolayers

We report a study of the structure of phosphorylcholine self-assembled monolayers (PC-SAMs) on Au(111) surfaces using both molecular mechanics (MM) and molecular dynamics (MD) simulation techniques. The lattice structure (i.e., packing densities and patterns) of the PC chains was determined first, by examining the packing energies of different structures by MM simulations in an implicit solvent. The chain orientation (i.e., antiparallel and parallel arrangements of the PC head groups) was then evaluated. The initial azimuthal angles of the PC chains were also adjusted to ensure that the optimal lattice structure was found. Finally, the two most probable lattice structures were solvated with explicit water molecules and their energies were compared after 1.5 ns of MD simulations to verify the optimal structures obtained from MM. We found that the optimal lattice structure of the PC-SAM corresponds to a radical7 x radical7 R19degree lattice structure (i.e., surface coverage of 50.4 A(2)molecule) with a parallel arrangement of the head groups. The corresponding thickness of the optimal PC-SAM is 13.4 A which is in agreement with that from experiments. The head groups of the PC chains are aligned on the surface in such a way that their dipole components are minimized. The P-->N vector of the head groups forms an angle of 82 degrees with respect to the surface normal. The tilt direction of molecular chains was observed to be towards their next nearest neighbor.     

Relation between packing density and thermal transitions of alkyl chains on layered silicate and metal surfaces

Self-assembled layers of alkyl chains grafted onto the surfaces of layered silicates, metal, and oxidic nanoparticles are utilized to control interactions with external media by tuning the packing density of the chains on the surface, head group functionality, and chain length. Characterization through experiment and simulation shows that the orientation of the alkyl layers and reversible phase transitions on heating are a function of the cross-sectional area of the alkyl chains in relation to the available surface area per alkyl chain. On even surfaces, a packing density less than 0.2 leads to nearly parallel orientation of the alkyl chains on the surface, a high degree of conformational disorder, and no reversible melting transitions. A packing density between 0.2 and 0.75 leads to intermediate inclination angles, semicrystalline order, and reversible melting transitions on heating. A packing density above 0.75 results in nearly vertical alignment of the surfactants on the surface, a high degree of crystalline character, and absence of reversible melting transitions. Curved surfaces can be understood by the same principle, taking into account a local radius of curvature and a distance-dependent packing density on the surface. In good approximation, this simple model is independent from the length of the alkyl chains (a minimum length of C10 is required to form sufficiently distinctive patterns), the chemical nature of the surface, and of the surfactant head group. These structural details primarily determine the functionality of alkyl modified surfaces and the temperature of thermal transitions.     

Molecular simulation of the interlayer structure and the mobility of alkyl chains in HDTMA+/montmorillonite hybrids

Molecular simulation techniques are used to find the basal spacing of organoclay on the basis of the energy minimum, using the canonical NVT ensemble. Then, the interlayer structure and mobility of alkyl chains are explored based on the interlayer atomic density profiles. Besides the basic lateral-monolayer arrangement, lateral-bilayer accompanied by partial a pseudo-trilayer and a transition structure between the two basic lateral models are observed. The later provides an excellent explanation about the reflection at 16 angstroms on XRD patterns in the literature. The atomic density profiles reveal that nitrogen atoms show stronger layering behavior than carbon atoms do. Our simulation demonstrates that the molecular mobility of the confined alkyl chains decreases from lateral-monolayer to lateral-bilayer with the increase of the intercalated surfactant. This is in accordance with the suggestion deduced from experiments. Furthermore, our simulation indicates that the mobility of the alkyl chains strongly depends on the surfactant arrangement rather than the surfactant packing density.     

Polymer Layered Silicate/Carbon Nanotube Nanocomposites: Morphological and Rheological Properties

Morphological and rheological properties of new ternary nanocomposites based on ethylene vinyl acetate copolymers (EVA), commercial organo-modified clays (organoclays) and purified multi-walled carbon nanotubes (MWNTs), prepared via direct melt blending, have been evaluated. For sake of comparison, the corresponding binary compositions, i.e., EVA filled with either organoclays or MWNTs, have been investigated as well. While extensive exfoliation can be observed for binary EVA/clay nanocomposites, the addition of MWNTs appears to limit clay exfoliation. Rheological properties show that both clay and MWNTs increase the elastic modulus of the nanocomposites, reflecting the high degree of nanoparticle interconnectivity that can be found in these materials.