An approach to enhanced redispersibility of cellulose nanocrystals via freeze-drying their Pickering emulsions

To enhance the redispersibility of dried nanocellulose, cellulose nanocrystal (CNC) cryogels were produced by freeze-drying CNC-stabilized cyclohexane-in-water Pickering emulsions. The CNC cryogels were easily redispersed in water and organic solvents; thus, the approach proposed made it possible to significantly improve CNC redispersibility in aqueous and nonaqueous media.     

The preparation and characterization graphene-cross-linked phenol–formaldehyde hybrid carbon xerogels

A new synthesis route based on polycondensation of phenol and formaldehyde cross-linked by graphene oxide (GO) was developed. Wet gel after gelation was converted into an organic xerogel by ambient pressure drying to obtain GO-cross-linked phenol–formaldehyde (PF) organic xerogels (GOCPFOX). Graphene-cross-linked PF carbon xerogels (GCPFCX) were produced by carbonization. The morphology and chemical structure of GOCPFOX and GCPFCX were analyzed. The electrochemical behavior of GCPFCX as an electrode material in electric double-layer capacitors (EDLCs) was investigated. Results show that the high mechanical strength of GO increased the gel skeleton strength; thus, organic xerogels exhibit very low drying shrinkage. Scanning electron micrographs indicated that addition of GO altered the gel structure. Thus, when GO was added into the PF solution, the PF molecular chains were anchored on the surface of GO by chemical and physical interaction. The GCPFCX-10 sample achieved the highest specific surface area, mesoporous volume, and specific capacity with 378 m²/g, 0.56 cm³/g, and 116 F/g, respectively. Hence, GCPFCX is a potential material for EDLCs owing to its low production cost and ability to avoid supercritical drying.     

TEOS–colloidal silica–PDMS‐OH hybrid formulation used for stone consolidation

The consolidation of materials concept, which consists of introducing a chemical substance (consolidant) into degraded stone, has been applied to architectural conservation. Silicon compounds such as tetraethoxysilane (TEOS) are frequently used as a base for commercial consolidant formulations due to their ability to form a siloxane polymer such as SiO₂. However, the silica xerogels deposited into the stone show poor performance and the gels obtained are non‐porous and tend to crack during the drying stage. In order to avoid the fractures and to improve gel properties, we propose the synthesis of a hybrid consolidant based on TEOS and fillers such as colloidal silica (200 nm in diameter) and hydroxy‐terminated polydimethylsiloxane (PDMS‐OH). Both additives enhance gel properties such as porosity and elasticity, leading to the formation of non‐fractured and permeable gels. Characterization of the hybrid xerogel was carried out by nitrogen adsorption and ²⁹Si MAS‐NMR. The properties of the hybrid xerogels were compared with those prepared from a formulation based on TEOS (T‐ME) with a composition similar to a commercial product. In order to evaluate the effectiveness of the hybrid consolidant, it was applied to tuff‐stone of historical monuments in the city of Guanajuato, Mexico. The tuff‐stone was also treated with the formulation T‐ME. Both treatments were studied by determining the percentage of consolidant deposited, evaluating changes in porosity and hardness of the treated stone. The applicability of the hybrid consolidant for the decayed tuff‐stone is under study. Copyright © 2010 John Wiley & Sons, Ltd.     

Xerogels Modified with 4-(2-Pyridylazo)-Resorcinol and Cetylpyridinium Chloride: Determination of Zinc in Solutions by Solid-Phase Spectrophotometry

Silicon dioxide-based xerogels modified with 4-(2-pyridylazo)-resorcinol (PAR) and cetylpyridinium chloride (CP) were synthesized. The concentration of PAR and CP in the hydrolyzed mixture in the ranges (0.6–3.10) × 10^–3 and (5–12) × 10^–3 M, respectively, does not affect the rate of gel formation as well as the specific surface area and average pore diameter of xerogels obtained upon drying in a microwave oven with a power of 600 Wt. It was found that the retention of PAR can be significantly improved in the presence of CP. Reactions of zinc with modified xerogels were studied by solid-phase spectrophotometry at different concentrations of PAR and CP, pH of the solution, and equilibrium time. A procedure was developed for determining 0.1–4.5 mg/L zinc in solutions by solid-phase spectrophotometry. Results of determining zinc in acid soil extracts and milk ash are given.     

Changes in Physicochemical Properties and Volatile Compounds of Roselle (Hibiscus sabdariffa L.) Calyx during Different Drying Methods

Fresh roselle are high in moisture and deteriorate easily, which makes drying important for extending shelf-life and increasing availability. This study investigated the influence of different drying methods (oven-drying, freeze-drying, vacuum-drying, and sun-drying) on the quality of roselle calyx expressed as physicochemical properties (moisture content, water activity, soluble solids, color), volatile compounds, and microstructure. Oven-drying and freeze-drying reduced moisture content most while vacuum-drying and sun-drying were not as efficient. All drying methods except sun-drying resulted in water activities low enough to ensure safety and quality. Vacuum-drying had no impact on color of the dry calyx and only small impact on color of water extract of calyx. Drying reduced terpenes, aldehydes, and esters but increased furans. This is expected to reduce fruity, floral, spicy, and green odors and increase caramel-like aroma. Sun-drying produced more ketones, alcohols, and esters. Scanning electron microscopy revealed that freeze-drying preserved the cell structure better, and freeze-dried samples resembled fresh samples most compared to other drying techniques. The study concludes that freeze-drying should be considered as a suitable drying method, especially with respect to preservation of structure.     

Influence of the Drying Technique on the Structure of Silica Gels

In the present research the influence of drying technique on the structural properties of silica gels prepared by different routes was studied. The silica aquagels were obtained by hydrolysis of tetraethylorthosilicate Si(OEt)₄ (TEOS) under base- or acid-catalyzed conditions or by the gelation of SiO₂ sol prepared by the fast hydrolysis of Na₂SiO₃ aqueous solutions in the presence of HCl. The drying techniques employed were freeze-drying as compared to conventional thermal drying at 80°C. Properties of resulting xerogels and cryogels were studied by N₂ adsorption at 77K, DTA and SEM. It was shown that the structural changes during both freeze-drying and conventional drying are strongly influenced by the structure of initial aquagel.     

29Si MAS-NMR Study of Silica Gels and Xerogels: Influence of the Catalyst

Four silica gels were prepared by hydrolysis of tetraethoxysilane (TEOS) in ethanol, using different catalysts: HCl, NaOH, NH₃, and NBu₄F. Nitrogen adsorption-desorption isotherms indicated that the HCl-catalyzed xerogel was purely microporous, whereas the other samples exhibited a very broad distribution of mesopores and a variable amount of micropores. ²⁹Si MAS NMR spectroscopy of the wet gels (before drying) pointed to a low degree of condensation for the HCl-catalyzed gel, and to the presence of unhydrolyzed TEOS monomer in the NaOH-catalyzed gel. Comparison with the ²⁹Si MAS NMR spectra of the xerogels indicated a significant increase of the degree of condensation during the drying procedure (3 hrs at 120°C under vacuum) for the HCl-catalyzed gel.     

Small angle X-ray scattering study of surface modified tin oxide nanoparticles prepared by sol-gel route

The surface properties of SnO₂ nanoparticles were modified by grafting ionic (Tiron®_, (OH)₂C₆H₂(SO₃Na)₂·H₂O) or non-ionic (Catechol®_, C₆H₄-1,2-(OH)₂) capping molecules during aqueous sol-gel processing to improve the redispersibility of powdered xerogel. The effect of the amount of grafted organic molecules on the redispersibility of powders in aqueous solution at several basic pH values was studied. The nanostructural features of the colloidal suspensions were analyzed by small angle X-ray scattering (SAXS) measurements. Irrespective of the nature and amount of grafted molecules, complete redispersion was obtained in aqueous solution at pH = 13. The redispersion at pH = 11 results in a mixture of dispersed primary particles and aggregates. The proportion of well dispersed nanoparticles and aggregates (and their average size) can be tuned by the quantity of grafted ionic molecules.     

Mechanical deconstruction of lignocellulose cell walls and their enzymatic saccharification

Laboratory mechanical softwood pulps (MSP) and commercial bleached softwood kraft pulps (BSKP) were mechanically fibrillated by stone grinding with a SuperMassColloider^®. The extent of fibrillation was evaluated by SEM imaging, water retention value (WRV) and cellulase adsorption. Both lignin content and mechanical treatment significantly affected deconstruction and enzymatic saccharification of fibrillated MSP and BSKP. Fibrillation of MSP and BSKP cell walls occurs rapidly and then levels off; further fibrillation has only limited effect on cell wall breakdown as measured by water retention value and cellulase adsorption. Complete (100 %) saccharification can be achieved at cellulase loading of 5 FPU/g glucan for BSKP after only 15 min fibrillation with energy input of 0.69 MJ/kg. However, the presence of lignin in MSP affects the extent of fibrillation producing fibrils mainly above 1 μm. Lignin binds nonproductively to cellulases and blocks cellulose thereby reducing its accessibility. As a result, the cellulose saccharification efficiency of MSP fibrils (6 h of fibrillation, energy input of 13.33 MJ/kg) was only 55 % at same cellulase loading of 5 FPU/g glucan.     

High‐performance liquid chromatographic enantioseparation of amino alcohol analogues possessing 1,2,3,4‐tetrahydroisoquinoline skeleton on polysaccharide‐based chiral stationary phases

The stereoisomers of 1,2,3,4‐tetrahydroisoquinoline amino alcohol analogues and derivatives thereof were separated in normal‐phase mode on chiral stationary phases based on preprepared silica coated with cellulose tris‐(3,5‐dimethylphenyl carbamate), cellulose tris‐(3‐chloro‐4‐methylphenyl carbamate), cellulose tris‐(4‐methylbenzoate) or cellulose tris‐(4‐chloro‐3‐methylphenyl carbamate). On all the investigated chiral columns, the retention and the enantioseparation were influenced by the nature and the concentrations of the mobile phase components and additives, and also the temperature. Experiments were performed in the temperature range 10–50°C. Thermodynamic parameters were calculated from plots of lnα vs 1/T. On these polysaccharide‐based chiral columns, both enthalpy‐driven separations and entropy‐controlled enantioseparations were observed. The latter was advantageous with regard to the shorter retention and greater selectivity at high temperature. The sequence of elution of the stereoisomers was determined in all cases. Copyright © 2014 John Wiley & Sons, Ltd.     

Preparation of hydroxypropylmethyl cellulose-based porous matrix for gastroretentive delivery of gabapentin using the freeze-drying method

The aims of the present study were to prepare hydroxypropylmethyl cellulose (HPMC)-based porous matrix tablets for gastroretentive drug delivery and to characterize their physicochemical properties. Gabapentin (GBP) was used as a model drug. Paste containing GBP, HPMC and water was molded and freeze-dried to prepare freeze-dried gastroretentive matrix tablet (FD-GRT). In vitro drug release and erosion studies were also performed. Although FD-GRT exhibited porous structure, they had good tablet strength and friability. Density of FD-GRT ranged from 0.402 to 0.509 g/cm³ and thus they could float on the medium surface without any lag time. FD-GRT was remained floated until the entire matrix erosion or end of drug release during in vitro release test. Release behavior of GBP could be modulated by the amount and the viscosity grade of HPMC. However, large amount and high viscosity of HPMC caused trouble in molding prior to freeze-drying. Addition of ethylcellulose could retard the release rate of GBP, with relatively low increase in viscosity of paste. Since pores generated by freeze drying imparted buoyancy for gastric retention to FD-GRT, additional materials for buoyancy was not necessary and FD-GRT had no lag time for buoyancy due to low density. Therefore it could be a promising tool for gastroretentive drug delivery.     

Influence of drying method on the material properties of nanocellulose I: thermostability and crystallinity

The effect of drying method on selected material properties of nanocellulose was investigated. Samples of nanofibrillated cellulose (NFC) and cellulose nanocrystals (CNC) were each subjected to four separate drying methods: air-drying, freeze-drying, spray-drying, and supercritical-drying. The thermal stability and crystallinity of the dried nanocellulose were evaluated using thermogravimetric analysis (TGA) and X-ray diffraction. Supercritical-drying produced NFCs with the least thermal stability and the lowest crystallinity index. Air-drying or spray-drying produced NFCs which were more thermally stable compared with freeze-dried NFCs. The CNCs dried by the three methods (air-drying, freeze-drying, and spray-drying) have similar onset temperature of thermal degradation. The different drying methods resulted in various char weight percentages at 600 °C for the dried NFCs or CNCs from TGA measurements. The dried NFCs are pure cellulose I while the dried CNCs consist of cellulose I and II. The calculated crystallinity indices differ with each drying method. The cellulose II content in CNCs changes as a function of drying method. For the application of nanocellulose in non polar thermoplastics, spray-dried products are recommended according to their higher thermal stability and higher crystallinity index.     

Electron Paramagnetic Resonance and Differential Scanning Calorimetry Studies of Thorium and Tin Phosphate Xerogels

Different transparent phosphate xerogels were synthesized using concentrated solutions of metal chlorides and phosphoric acid with a proper mole ratio of both components. By this method we prepared bulk samples of thorium and tin(IV) phosphate xerogels by drying at room temperature or at 350 K. Some properties of these amorphous materials were studied by means of differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR) techniques. Depending on mole ratio metal/phosphate, these xerogels show, near 180 K, inflection points which we interpret asT _g . Samples dried at 425 K lose their transparency and have noT _g . Thus, it seems that the “glassy” state is due to water molecules remaining in the material. The same properties were confirmed by EPR studies of the xerogels doped with Cr³⁺ and Fe³⁺ ions as probes. These results show the existence of two different phases in the xerogels: a liquid-like one, in the range from 190 K to 350 K and a solid-like one, in the range from 4 K to 190 K.     

The fluorolytic sol–gel reaction of aluminium alkoxides: a multinuclear MAS NMR study of structural influences of the synthesis parameters

The fluorolytic sol–gel reaction between aluminium alkoxides and non-aqueous HF leads, after drying in vacuum, to xerogels. These can be seen as intermediates for the preparation of aluminium fluorides with large surface areas having a great potential for application in diverse fields of heterogeneous catalysis. This work reports about a comprehensive systematic study of the involved local structures of the aluminium alkoxide fluorides and their changes provoked by (a) the variation of sol gel synthesis parameters, (b) a subsequent drying and (c) the exposure of the materials to different atmospheres. The main analytical tools are solid state MAS NMR investigations of the naturally abundant nuclei in these kinds of materials, namely ²⁷Al, ¹⁹F for probing the Al- and F-environments and ¹H and ¹³C for probing the organic moieties. The results of previous studies become more comprehensive and were underlined by the findings reported here.     

Application of cellulose acetate butyrate-based membrane for osmotic drug delivery

The release rate of drugs from an OROS^® is controlled by semipermeable membranes composed typically of cellulose acetate (CA) with various flux enhancers. Cellulose acetate butyrate (CAB) was identified as a viable alternative. The CAB membrane matched the CA membrane in robustness but had superior drying properties, offering particular advantages for thermolabile formulations. Studies were conducted to characterize CAB membrane properties with respect to performance of OROS^® systems. Four different membrane formulations with varying plasticizer type and concentration were investigated. The CAB based membranes exhibited superior drying characteristics and similar functionality to the CA:polyethylene glycol (PEG) membranes used as a control. A linear relationship was observed between the level of flux enhancer and release rate. The stability of the membrane was evaluated based on release profiles after system storage at various conditions. The CAB membranes appeared to have stability comparable to the standard CA membrane. A linear relationship between membrane weight and release rate as well as the time required to release 90% of a drug from the system [T₉₀] for a model formulation was observed. In conclusion, the newly identified alternative membrane composition allows for the use of thinner membranes, thereby reducing cost of goods, coating time and, most importantly, membrane drying time.     

Vanadium oxide gels as electrode materials for lithium batteries

Electrochemical lithium insertion has been studied in a large number of vanadium oxides with three dimensional framework structure. Several of these oxides have shown high capacities for lithium insertion and good reversibility.Pure solutions of decavanadic acid have shown to undergo spontaneous polycondensation reaction forming sols or gels of highly polymerized vanadium oxides, M _w 10⁶. After dehydration a series of xerogels with varying amounts of water, V₂O₅ · nH₂O, can be obtained. The structure of these xerogels consists of ribbons of corner and edge sharing VO₆ octahedra stabilized by interlayer water molecules. Under ambient conditions the water content corresponds to n=1.8, but this value can be reversibly changed under mild drying conditions.This report deals with the electrochemical insertion of lithium in dried vanadium oxide xerogels, with special regard to the use of these materials as electrodes in rechargeable lithium batteries.     

Comparison of polyethylene glycol adsorption to nanocellulose versus fumed silica in water

The recent intensification of industrially produced cellulose nanocrystals (CNCs) and cellulose nanofibrils has positioned nanocelluloses as promising materials for many water-based products and applications. However, for nanocelluloses to move beyond solely an academic interest, a thorough understanding of their interaction with water-soluble polymers is needed. In this work, we address a conflicting trend in literature that suggests polyethylene glycol (PEG) adsorbs to CNC surfaces by comparing the adsorption behaviour of PEG with CNCs versus fumed silica. While PEG is known to have strong hydrogen bonding tendencies and holds water tightly, it is sometimes (we believe erroneously) presumed that PEG binds to cellulose through hydrogen bonding in aqueous media. To test this assumption, the adsorption of PEG to CNCs and fumed silica (both in the form of particle films and in aqueous dispersions) was examined using quartz crystal microbalance with dissipation, isothermal titration calorimetry, rheology and dynamic light scattering. For all PEG molecular weights (300–10,000 g/mol) and concentrations (100–10,000 ppm) tested, strong rapid adsorption was found with fumed silica, whereas no adsorption to CNCs was observed. We conclude that unlike silanols, the hydroxyl groups on the surface of CNCs do not readily hydrogen bond with the ether oxygen in the PEG backbone. As such, this work along with previous papermaking literature supports the opinion that PEG does not adsorb to cellulose surfaces.     

Characterisation of hybrid xerogels synthesised in acid media using methyltriethoxysilane (MTEOS) and tetraethoxysilane (TEOS) as precursors

The mild synthetic conditions provided by the sol-gel process and the versatility of the colloidal state allow for the mixing of inorganic and organic components at the nanometre scale in virtually any ratio for the preparation of hybrid materials. Our interest in hybrid xerogels focuses on combining their porosity with other properties to prepare optic-fibre sensors. The specific aim of this paper is to synthesise hybrid xerogels in acid media using methyltriethoxysilane (MTEOS) and tetraethoxysilane (TEOS) as silica precursors and to investigate the effect of the MTEOS molar ratio on the structure and porous texture of xerogels. Gelation time exponentially increased as the MTEOS molar ratio increased. Increasing the MTEOS molar ratio yielded xerogels with lower density and lower particle size. The incorporation of MTEOS resulted in new FTIR bands at 1276 and 791 cm⁻¹, which was attributed to vibrational modes of methyl group. The band around 1092 cm⁻¹ associated with siloxane bonds shifted to lower wavenumbers and split into two bands. The ²⁹Si spectra only showed the Q ⁿ (n=2, 3, 4) signal in xerogels with 0% MTEOS and the T ⁿ (n=2, 3) signal in xerogels with 100% MTEOS; hybrid xerogels showed both Q and T signals. From XRD peaks at 2θ around 9°, we inferred that xerogels (>70% MTEOS) consisted of nanocrystalline CH₃–SiO_3/2 species. Increasing the MTEOS molar ratio produced xerogels with lower pore volumes and lower average pore size. The integration of methyl groups on the surface decreased the surface polarity and, in turn, the characteristic energy.     

Determination of alkylphenols and alkylphenol ethoxylates in sewage sludge: effect of sample pre-treatment

A complete characterization of sewage sludge collected from five biological waste water treatment plants was done to determine physico-chemical parameters, heavy metals and alkylphenols, making special emphasis on sampling, homogenization, and sample pre-treatment. Ultrasonic extraction followed by gas chromatrography coupled with mass spectrometry was used to evaluate the effect of sample pre-treatment (untreated sample, freeze-drying, drying at 40 °C or drying at 100 °C) on the concentration of octylphenol (OP), nonylphenol (NP) and nonylphenol ethoxylates (NP₁EO, NP₂EO). Untreated samples and samples dried at 100 oC gave concentration levels up to 62% and 89% lower, respectively, than freeze-dried samples. In 50% of cases, freeze-dried samples led to significantly higher concentrations than those obtained by drying at 40 °C. Thus, freeze-drying is the recommended sample pre-treatment to prevent possible losses of OP, NP, and NP₁EO. Using this methodology, concentrations detected were from 3.2 to 199 mg kg⁻¹ being NP followed by NP₁EO found in highest concentration. The total concentration of NP and NP₁EO exceeded the limit of 50 mg kg⁻¹ proposed by the draft European directive on sewage sludge in three out of five samples studied. Contrarily, heavy metals were below the legislated values.     

Phase assembly-induced transition of three dimensional nanofibril- to sheet-networks in porous cellulose with tunable properties

Ultralight and highly porous cellulose was fabricated via cellulose/sodium hydroxide/urea aqueous solution followed by gelation, coagulation and freeze-drying in the current work. The water content and freeze rate of cellulose coagulated sample are two crucial factors controlling the morphology, density and porosity of porous cellulose, which led to an interesting morphological transition from three dimensional nanofibrillar network to sheet network in porous cellulose. It was proposed that the aggregation and assembly of cellulose-rich phase and crystallization of water-rich phase were closely related to this transition. Based on this concept, a series of cellulose materials with densities varied from 0.129 to 0.330 g cm^?3 and corresponding porosities ranged from 91.4 to 78.0 %, were obtained. The porous celluloses showed a good ductility (strain to fracture is more than 30 %) and high modulus, which also could be tuned by porous morphology. The new understanding on the morphological transition in porous cellulose could be beneficial for the development of “green” porous materials.