Temperature-induced transport of thermosensitive hairy hybrid nanoparticles between aqueous and organic phases

This paper reports on the temperature-induced transport of thermosensitive polymer brush-grafted silica nanoparticles between aqueous and organic phases. Poly(methoxytri(ethylene glycol) methacrylate), a thermosensitive water-soluble polymer with a cloud point of approximately 48 degrees C in H2O, was grown on silica nanoparticles by surface-initiated atom transfer radical polymerization in the presence of a free initiator. These hairy particles were found to quantitatively transfer from aqueous to ethyl acetate phases upon heating at 60 degrees C under the stirring condition. Cooling in an ice/water bath caused the particles to move from ethyl acetate to the aqueous layer. The concentrations of the particles in the original phases during the transport processes were monitored by UV-vis spectrometry. When mutually saturated water and ethyl acetate were used, the transport rates of the particles between the two phases were enhanced. The faster transport was attributed to the solvent phase separation, which produced liquid droplets, dramatically increased the interfacial area, and hence facilitated the transport of the particles. The reversible transfer of the particles between the aqueous and ethyl acetate phases upon heating at 60 degrees C and cooling in an ice/water bath can be repeated consecutively at least 10 times. The hairy particles can also be quantitatively transported from 1-butanol and toluene to H2O by stirring in an ice/water bath. However, only 60% of the particles transferred from water to 1-butanol and no particles to toluene upon heating at 60 degrees C. The reasons are discussed.     

Light-sensitive lamellar phases

Light sensitive lamellar (L(alpha)) phases have been generated in glycerol/water mixtures from blends of an inert ABA tri-block co-polymer (dimethylsiloxane-polyethylenoxide, (EO)(15)-(PDMS)(15)-(EO)(15)), and a photodestructible anionic surfactant C(6)PAS (sodium 4-hexylphenylazosulfonate). These L(alpha) systems have been formulated in a 6/4 glycerol/water (v/v) mixed solvent. Rheology, small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS) and dynamic light scattering (DLS) have been used to characterize the change in phase behavior and structure after the incorporated C(6)PAS is selectively degraded by UV light incident on the L(alpha) phases. The ABA co-polymer alone forms weakly structured lamellar mesophases, which are stabilized by thermal fluctuations and characterized by low shear moduli (L(alpha) phase-A). Addition of C(6)PAS (10 mM) introduces charge stabilization, generating more ordered and stiffer L(alpha) systems (L(alpha) phase-B). After UV irradiation (Hg lamp) of the L(alpha)-B phases, and subsequent degradation of the ionic C(6)PAS, the results of rheological and scattering studies are consistent with an irreversible change back to L(alpha) phase-A type systems. These formulations display a novel transition between electrostatically- and thermally-stabilized lamellae, which may be controlled by incident UV light.     

Colloidal stability of bare and polymer-grafted silica nanoparticles in ionic liquids

The colloidal stability of bare and poly(methyl methacrylate) (PMMA)-grafted silica nanoparticles was studied in 1-alkyl-3-methylimidazolium ([C(n)mim])-based ionic liquids (ILs) with different anionic structures. The theoretical estimation of the colloidal interaction between monodispersed bare silica particles by using the Derjaguin-Landau-Verwey-Overbeek theory indicates that bare silica particles cannot be stabilized and they rapidly form aggregates in all the ILs used in this study. The instability of bare silica particles was experimentally confirmed by dynamic light scattering measurement and in situ transmission electron microscopy observations by utilizing the negligible vapor pressure of ILs. This evidence suggests that electrostatic stabilization is inefficient in ILs because of the high ionic atmosphere and the resulting surface-charge screening. The PMMA-grafted silica particles exhibited long-term colloidal stability in [C(4)mim][PF(6)] and [C(n)mim][NTf(2)], which are compatible with the grafted PMMA. On the other hand, the PMMA-grafted particles could not be stabilized in [C 4mim][BF 4] due to the poor solubility of the grafted PMMA in the IL. Effective steric stabilization is important for obtaining stable colloidal particles in ILs.     

Organo-modified cationic silica nanoparticles/anionic polymer as flocculants

The synthesis of quaternized silica nanoparticles and its application to fine clay flocculation were investigated. N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride was used as a cationic reagent to introduce quaternary amine groups onto the surfaces of silica nanoparticles via the formation of covalent bonds between the methoxy groups of the cationic reagents and the silanol groups in the silica surface. The zeta potential, zeta, and charge density of the silica particles modified under various reaction conditions were determined. Dynamic clay flocculation experiments using a photometric dispersion analyzer (PDA) showed that the cationic silica alone contributed little to the flocculation. However, the cationic silica, in conjunction with an anionic polymer of high M(w) and low charge density, led to a significant improvement in the flocculation of fine clay particles. The mechanism of flocculation was explored by a systematic investigation of interaction between cationic silica and anionic polymers as well as of their adsorption behavior on clay surfaces. The influence of factors such as pH and electrolyte concentration on clay flocculation was also investigated.     

Influence of sodium dodecyl sulfate on the physicochemical properties of whey protein-stabilized emulsions

The influence of sodium dodecyl sulfate (SDS) on the flocculation of droplets in 20 wt.% soybean oil-in-water emulsions stabilized by whey protein isolate (WPI) was investigated by light scattering, rheology and creaming measurements. The SDS concentrations used were low enough to prevent depletion flocculation by surfactant micelles and extensive protein displacement. In the absence of SDS, emulsions were prone to droplet flocculation near the isoelectric point of the proteins (4<pH<6), but were stable at a higher and lower pH. Flocculation led to an increase in emulsion viscosity, pronounced shear thinning behavior and accelerated creaming. When the surfactant-to-protein molar ratio was increased from 0 to 10, the emulsion instability range shifted to lower pH values due to binding of the negatively charged SDS molecules to the droplets. Our results indicate that the physicochemical properties of protein-stabilized emulsions can be modified by utilizing surfactant–protein interactions.     

Tuning aggregation of microemulsion droplets and silica nanoparticles using solvent mixtures

The effect of solvent on stability of water-in-oil microemulsions has been studied with AOT (sodium bis(2-ethylhexyl)sulfosuccinate) and different solvent mixtures of n-heptane, toluene and dodecane. Dynamic light scattering DLS was used to monitor the apparent diffusion coefficient D(A) and effective microemulsion droplet diameter on changing composition of the solvent. Interdroplet attractive interactions, as indicated by variations in D(A), can be tuned by formulation of appropriate solvent mixtures using heptane, toluene, and dodecane. In extreme cases, solvent mixtures can be used to induce phase transitions in the microemulsions. Aggregation and stability of model AOT-stabilized silica nanoparticles in different solvents were also investigated to explore further these solvent effects. For both systems the state of aggregation can be correlated with the effective molecular volume of the solvent V(mol)(eff) mixture.     

Synthesis and formation of hierarchical mesoporous silica network in acidic aqueous solutions of sodium silicate and cationic surfactant

Synthesis and formation of hierarchical mesoporous silica network in acidic aqueous solutions of commercial sodium silicate and cationic surfactant were studied by SEM, TEM, XRD and nitrogen adsorption-desorption methods. The formation process occurs through several steps, involving (1) formation of mesoporous silica nanoparticles; (2) aggregation of mesoporous silica nanoparticles into blocks; (3) growth of silica nanorods in closely-packed arrays by merging of silica nanoparticles; (4) separation of the nanorods to form 3D silica network; and (4) shrinking of the mesoporous silica network. The as-prepared 3D silica network exhibits bimodal morphology constituting mesoporous and macroporous phases.     

Interfacial Activity of Amine‐Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS): A Simple Strategy To Structure Liquids

Amine‐functionalized polyhedral oligomeric silsesquioxane (POSS), the smallest, monodisperse cage‐shaped silica cubic nanoparticle, is exceptionally interfacially active and can form assemblies that jam the toluene/water interface, locking in non‐equilibrium shapes of one liquid phase in another. The packing density of the amine‐functionalized POSS assembly at the water/toluene interface can be tuned by varying the concentration, the pH value, and the degree of POSS functionalization. Functionalized POSS gives a higher interface coverage, and hence a lower interfacial tension, than nanoparticle surfactants formed by interactions between functionalized nanoparticles and polymeric ligands. Hydrogen‐bonded POSS surfactants are more stable at the interface, offering some unique advantages for generating Pickering emulsions over typical micron‐sized colloidal particles and ligand‐stabilized nanoparticle surfactants.     

Chitosan based nonstoichiometric polyelectrolyte complexes as specialized flocculants

The aim of this work was to evaluate the flocculation of kaolin by using positively charged nonstoichiometric interpolyelectrolyte complex (N-PEC) nanoparticles formed by the interaction of chitosan with poly(sodium 2-acrylamido-2-methylpropanesulfonate) (NaPAMPS) and two random copolymers of AMPS with t-butyl acrylamide, comparative with chitosan. The flocculation efficiency was evaluated by optical density at 500 nm. The N-PEC nanoparticles were more effective than chitosan in the kaolin separation, especially at low molar ratios between charges, when the flocculation window was more then double at an optimum dose lower than that of chitosan. The main advantage of N-PECs is the increase of critical concentration for kaolin re-stabilization, the N-PEC particles adsorbed on the kaolin surface protecting them more efficient against re-dispersion. The influence of low molar mass electrolytes on the flocculation with N-PECs was also investigated.     

Separation of gold nanoparticles with a monolithic silica capillary column in liquid chromatography

A separation system for gold nanoparticles was developed using monolithic silica capillary columns with 50 μm i.d., which were prepared via in-situ sol-gel processes. Gold nanoparticles with five different average sizes were synthesized via reduction of tetrachloroauric acid (HAuCl(4)) under different synthesis conditions, and were evaluated by UV-visible spectrophotometry, dynamic light scattering as well as transmission electron microscopy before they were separated using the developed system. The results showed that all of the gold nanoparticles had a certain size distribution, and the mean sizes obtained were 13, 17, 33, 43 and 61 nm, with σ = 2.5, 2.7, 5.2, 5.1 and 5.6 nm, respectively. Transmission electron microscopy showed that the samples with mean sizes of 13 and 17 nm were almost spherical, while larger samples were slightly non-uniform. The agglomeration of gold nanoparticles as the sample could be prevented by using a sodium dodecyl sulfate aqueous solution as the mobile phase, and gold nanoparticles were retained by adsorption on the silica surface. Separation with 8 mM sodium dodecyl sulfate as the eluent and a 1000-mm column was successful, and the separation of gold nanoparticles with 61 and 17 nm or 61 and 13 nm was demonstrated. The separation results obtained using a nonporous silica packed column as well as monolithic silica columns with or without mesopore growth were compared. It was found that separation using the mesopore-less monolithic column achieved better resolution. Through the use of a 2000-mm separation column, the mixtures of 61, 43, 17 nm and 61, 33, 13 nm could be separated.     

Role of silica coated magnetic nanoparticle on cell flocculation,lipid extraction and linoleic acid production from Chlorella pyrenoidosa

Abstract

In the present work, it has been observed that magnetic (Fe₃O₄) – silica core- shell nanoparticles helps in flocculation of Chlorella pyrenoidosa cells with simultaneous production of linoleic acid. The mean particle size in Dynamic light scattering (DLS) of the silica coated magnetic nanoparticle was estimated 444.7?nm. The characterization of nanoparticles was also performed by X-ray diffraction technique (XRD). Apart from flocculation, it has been observed that in presence of magnetic silica core- shell nanoparticles the amount of lipid obtained was four times than that of control. On the contrary, in presence of these nanoparticles, linoleic acid (18:2) has been produced in Chlorella pyrenoidosa cells almost by 80% whereas, it has been noticed only 8.73% in control. This is the first report where the linoleic acid has been obtained as major component of microalgal fatty acid methyl esters (FAME) having important application in nutraceuticals and pharmaceutical sectors.     

Effect of double quaternary ammonium groups on micelle formation of partially fluorinated surfactant

Water-in-oil microemulsions (w/o μEs) stabilized by the cationic surfactant cetyltrimethylammonium chloride (CTACl) have been used as reaction media to generate Au nanoparticles (Au-NPs). In addition the pure μEs have been used as media to disperse Au and Pd-NPs, which have been pre-synthesised in aqueous phases and stabilized by sodium 2-mercaptoethanesulfonate (MES) ligands, and also commercially available SiO(2)-NPs. A general method for recovery and separation of the nanoparticles from these mixed NP-μE systems has been demonstrated by tuning phase behavior of the background microemulsions. Addition of appropriate aliquots of water drives a clean liquid-liquid phase transition, resulting in two macroscopic layers, the NPs preferentially partition into an upper oil-rich phase and are separated from excess surfactant which resides in a lower aqueous portion. UV-vis and (1)H NMR spectroscopy have been used to follow these separation processes and quantify the recovery and recycle efficiencies for the different NPs. For example, ～90% of the microemulsion-prepared Au-NPs can be recovered; with even greater separation efficiencies attainable for pre-synthesised MES-stabilized Au-MES-NPs (～98%) and Pd-MES-NPs (92%). For the silica NP-μE dispersions gravimetry indicates ～84% recovery of the NPs. TEM images of all systems showed that NP shapes and size distributions were generally preserved after these phase transfer processes. This low-energy and cost-effective purification route appears to be a quite general approach for processing different inorganic NPs, having advantages of being isothermal, using only commercially available inexpensive components and requiring no additional organic solvents.     

Adsorption of Polyelectrolyte-Nanoparticle Systems on Silica: Influence on Interaction Forces

In this work, we have studied the interfacial properties of cationic polyelectrolyte (PE) and silica nanoparticle (NP) systems at macroscopic silica surfaces by means of ellipsometry. The influence of adsorbed layers on the interactions between silica surfaces was also investigated using the bimorph surface force apparatus. Added nanoparticles were observed to strongly swell the interfacial polyelectrolyte layers, an effect partly related to neutralization of charged polyelectrolyte groups. The effect was more pronounced for low charged than for highly charged polyelectrolytes. Overall, the presence of nanoparticles seemed to increase the repulsive interaction measured between silica surfaces. The force measured on approach was long range and quite strongly repulsive. On separation, an attractive bridging interaction was measured for polyelectrolyte-covered surfaces. For the low charged polyelectrolyte used in the study, the force turned repulsive on addition of nanoparticles. For the highly charged polyelectrolyte used, a change from a very strong attraction (involving a jump of the surfaces out of contact) to a very long-range elastic attractive force was observed on adding nanoparticles. The long-range elastic force indicates that polymer chains and nanoparticles form a transient network in the gap between the surfaces. The observed difference in the outward force curves may explain why the addition of nanoparticles appears to improve, e.g., shear-resistance and reflocculation characteristics of polymeric flocculants. Copyright 2000 Academic Press.     

Controlled production of ZnO nanoparticles from zinc glycerolate in a sol-gel silica matrix

The controlled production of ZnO nanoparticles within an amorphous silica matrix is achieved using a new methodology consisting of four stages. First, precursor zinc glycerolate nanoparticles are produced within reversed micelles of glycerol in heptane stabilized by the surfactant Aerosol-OT (bis-ethylhexyl sodium sulfosuccinate, AOT). The surface of these nanoparticles is then modified by exchanging AOT with bis-trimethoxysilyl-ethane (BTME). The surface-modified nanoparticles are copolymerized with tetramethoxysilane (TMOS) to provide a composite silica material, in which the nanoparticles are apparently dissolved, producing a uniform distribution of zinc in the silica matrix. Finally, the conversion of zinc to ZnO is achieved by heating the material at 700 degrees C, leading to a uniform dispersion of very small (<10 nm) ZnO particles within the amorphous matrix. The fluorescence spectrum of the ZnO particles within the matrix is blue-shifted, as expected from the strong quantum confinement achieved. The properties of the system at all stages in this synthetic process are monitored using TEM, XRD, fluorescence and FT-IR spectroscopy. Glycerol forms complexes with many metal ions, so the present procedure may be generalized to provide uniform distributions of metal ions and subsequently metal oxide nanoparticles in amorphous silica.     

Synthesis of polymer–inorganic patchy microcapsules with tunable patches

We introduce a facile and versatile approach for the formation of ball-like polymer–inorganic patchy microcapsules with a tunable shell by combining sol–gel chemistry of silica precursor and phase separation between the polymer and the precursor. Firstly, chloroform-in-water emulsion droplets containing poly(methyl methacrylate) (PMMA), silica precursor [tetraethyl orthosilicate (TEOS)] and co-surfactant sodium dioctyl sulfosuccinate (Aerosol OT or AOT) were prepared by shaking the mixture by hand. Due to the added AOT, water molecules diffuse into the chloroform droplets, and the tiny water droplets would coalesce gradually, triggering the formation of double emulsion droplets. Upon further solvent evaporation, the concentration of the polymer and the silica precursor in the oil shell of the double emulsions increases, leading to the phase separation between the polymer and the precursors (and partially formed silica through the hydrolysis and condensation of TEOS). Because of the confined geometry of the oil shell in the double emulsions, polymeric disc-like structures, stabilized by AOT, were dispersed in the silica precursors. Meanwhile, the silica precursor hydrolyzed and condensed when brought in contact with the aqueous solution, ultimately leading to the formation of a mineralized shell around the polymer domains and the hybrid patchy microcapsules. Effect of synthesis conditions, such as the amount of TEOS, AOT, and PMMA used, the pH value, and solvent evaporation rate on interfacial behavior of the solvent/water; and the morphology of the patchy microcapsules were investigated. Patchy microcapsules with tunable patch size and shape can be generated through tailoring the experimental parameters. Our study indicates that the hybrid patchy microcapsules can be formed by taking advantage of the sol–gel chemistry and the phase separation process, and the underlying generality of the synthesis procedure allows for a variety of applications, including drug storage, coatings, delivery, catalysis, and smart building blocks in self-assembling systems.     

Effect of silica particles on stability of highly concentrated water-in-oil emulsions with non-ionic surfactant

Water-in-oil, high internal phase emulsion made of super-cooled aqueous solution containing a mixture of inorganic salts and stabilized with non-ionic surfactant (sorbitan monooleate) alone was investigated. It was not possible to produce a highly concentrated emulsion (with aqueous phase fraction = 94 wt %), stabilized with surface-treated silica, solely: we were able to form an emulsion with a maximal aqueous phase mass fraction of 85 wt % (emulsion inverts/breaks above this concentration). The inversion point is dependent on the silica particle concentration, presence of salt in the aqueous phase, and does not depend on the pH of the dispersed phase. All emulsions stabilized by the nanoparticles solely were unstable to shear. So, the rheological properties and stability of the emulsions containing super-cooled dispersed phase, with regards to crystallization, were determined for an emulsion stabilized by non-ionic surfactant only. The results were compared to the properties obtained for emulsions stabilized by surface treated (relatively hydrophobic) silica nanoparticles as a co-surfactant to sorbitan monooleate. The influence of the particle concentration, type of silica surface treatment, particle/surfactant ratio on emulsification and emulsion rheological properties was studied. The presence of the particles as a co-stabilizer increases the stability of all emulsions. Also, it was found that the particle/surfactant ratio is important since the most stable emulsions are those where particles dominate over the surfactant, when the surfactant’s role is to create bridging flocculation of the particles. The combination of the two types of hydrophobic silica particles as co-surfactants is: one that resides at the water/oil interface and provides a steric boundary and another that remains in the oil phase creating a 3D-network throughout the oil phase, which is even more beneficiary in terms of the emulsion stability.     

Self-depletion flocculation of tetralin oil-in-water emulsions

Oil-in-water emulsions of slightly soluble oils such as tetralin prepared by high-pressure homogenization and stabilized by sodium dodecyl sulfate undergo depletion flocculation induced by an initially polydisperse droplet size distribution. The smaller droplets flocculate the larger ones; the flocculation can be reversed by gentle sonication. After aging, the flocs disappear because the smaller droplets dissolve through Ostwald ripening. These effects were observed by electroacoustic measurements, supplemented by light scattering.     

Asymmetric orientation of toluene molecules at oil-silica interfaces

The interfacial structure of heptane and toluene at oil-silica interfaces has previously been studied by sum frequency generation [Z. Yang et al., J. Phys. Chem. C. 113, 20355 (2009)]. It was found that the toluene molecule is almost perpendicular to the silica surface with a tilt angle of about 25°. Here, we have investigated the structural properties of toluene and heptane at oil-silica interfaces using molecular dynamics simulations for two different surfaces: the oxygen-bridging (hydrophobic) and hydroxyl-terminated (hydrophilic) surfaces of quartz (silica). Based on the density profile, it was found that both heptane and toluene oscillate on silica surfaces, with heptane showing more oscillation peaks. Furthermore, the toluene molecules of the first layer were found to have an asymmetric distribution of orientations, with more CH(3) groups pointed away from the silica surface than towards the silica surface. These findings are generally consistent with previous experiments, and reveal enhanced molecular structures of liquids at oil-silica interfaces.     

Manipulation of the magnetic properties of magnetite-silica nanocomposite materials by controlled Stober synthesis

The paper describes the synthesis and characterization of the magnetic properties of magnetite/silica nanocomposites using a modified Stober method. Magnetite nanoparticles averaging 8-10 nm in diameter and stabilized with oleic acid in toluene were used as the magnetic component of the nanocomposites. SQUID magnetic measurements and ferromagnetic resonance spectroscopy measurements were performed at each stage of the synthesis to understand the properties of the formed composites. Changes of blocking temperature in ZFC/FC SQUID curves correlated with corresponding changes of the resonance field in the ferromagnetic spectra of the sample at each stage of formation. The paper concludes that it is possible to manipulate the magnetic properties of silica/magnetite composite materials by controlling their surface properties and silica coating thickness.     

Magnetoswitchable controlled hydrophilicity/hydrophobicity of electrode surfaces using alkyl-chain-functionalized magnetic particles: application for switchable electrochemistry

Magnetic nanoparticles consisting of undecanoate-capped magnetite (average diameter approximately 4.5 nm; saturated magnetization, M(s), 38.5 emu g(-1)) are used to control and switch the hydrophobic or hydrophilic properties of the electrode surface. A two-phase system consisting of an aqueous buffer solution and a toluene phase that includes the suspended capped magnetic nanoparticles is used to control the interfacial properties of the electrode surface. The magnetic attraction of the functionalized particles to the electrode by means of an external magnet yields a hydrophobic interface that acts as an insulating layer, prohibiting interfacial electron transfer. The retraction of the magnetic particles from the electrode to the upper toluene phase by means of the external magnet generates a hydrophilic electrode that reveals effective interfacial electron transfer. The electron-transfer resistance and double-layer capacitance of the electrode surface upon the attraction and retraction of the functionalized magnetic particles to and from the electrode, respectively, by means of the external magnet were probed by Faradaic impedance spectroscopy (R(et) = 170 Omega and C(dl) = 40 microF sm(-2) in the hydrophilic state of the electrode and R(et) = 22 k Omega and C(dl) = 0.5 microF sm(-2) in the hydrophobic state of the interface). The magnetoswitchable control of the interface enables magnetic switching of the bioelectrocatalytic oxidation of glucose in the presence of glucose oxidase and ferrocene dicarboxylic acid to "ON" and "OFF" states.