Long-term Dispersion Stability Research Articles

Frozen in Time: Kinetically Stabilized Amorphous Solid Dispersions of Nifedipine Stable after a Quarter Century of Storage.

Kinetically stabilized amorphous solid dispersions are inherently metastable systems. Therefore, such systems are generally considered prone to recrystallization. In some cases, the formation of crystals will impact the bioavailability of the active pharmaceutical ingredient in these formulations. Recrystallization therefore may present a significant risk for patients as it potentially lowers the effective dose of the pharmaceutical formulation. This study indicates that such metastable formulations may indeed remain fully amorphous even after more than two decades of storage under ambient conditions. Different formulations of nifedipine stored for 25 years were compared with freshly prepared samples. A thorough physicochemical characterization including polarized light microscopy, differential scanning calorimetry, X-ray powder diffraction, and transmission Raman spectroscopy was undertaken. This in-depth characterization indicates no signs of recrystallization in the stored samples. The observations presented here prove that long-term stability of amorphous solid dispersions much beyond the typical shelf life for pharmaceutical formulations is indeed possible by kinetic stabilization alone. These findings implicate a reevaluation of the propensity to recrystallize for kinetically stabilized amorphous solid dispersions.

Polyacrylonitrile-grafted reduced graphene oxide hybrid: An all-round and efficient hole-extraction material for organic and inorganic-organic hybrid photovoltaics

In this research, we demonstrate that aliphatic polyacrylonitrile-grafted reduced graphene oxide (PRGO) hybrid can function efficiently as an all-round hole-extraction layer (HEL) in organic and organic-inorganic hybrid perovskite photovoltaic devices (OPVs and PePVs). The hybrid-structure PRGO was developed by facile and scalable in-situ radiation-induced reduction and graft polymerization with polymerizable styryl-functionalized graphene oxide and acrylonitrile. The newly developed PRGO exhibits long-term dispersion stability of six months even up to the high concentration of 10mg/ml. It also shows a full-cover uniform thin-film morphology, good electrical conductivity (0.87S/cm), high work function (4.87eV), and excellent weather stability. Moreover, the incorporation of PRGO (as a HEL) into the OPVs and PePVs results in greater efficiency than chemically-converted graphene nanoflakes references, and device durability superior to poly(ethylenedioxythiophene):poly(styrene sulfonate)-based ones with comparable power conversion efficiencies. Noticeably, our finding demonstrates that the hybrid-structure PRGO provides an all-round and efficient hole-extraction material for use in various conjugated-polymer and perovskite-based PV systems.

Alkylamine-functionalized hexagonal boron nitride nanoplatelets as a novel material for the reduction of friction and wear.

Hexagonal boron nitride nanoplatelets (h-BNNPs), which are structurally analogous to graphene, were prepared via the ultrasound-assisted exfoliation of h-BN powder using N-methyl pyrrolidone as the solvent. The alkylamines with variable alkyl chains and electron-rich nitrogen atoms were grafted onto the boron sites of the h-BNNPs based on Lewis acid-base chemistry. The grafting of the alkylamines onto the h-BNNPs was confirmed using FTIR, XPS, TGA and (13)C SSNMR analyses. The crystalline and structural features of the alkylamine-functionalized h-BNNPs were studied using XRD and HRTEM analyses. The TGA and FTIR results revealed a higher grafting of octadecylamine (ODA) on the h-BNNPs compared to trioctylamine (TOA). The cohesive interaction between the alkyl chains grafted onto the h-BNNPs and the hydrocarbon chains of mineral lube base oil facilitates the dispersion of the alkylamine-functionalized h-BNNPs. The TOA-grafted h-BNNPs (h-BNNPs-TOA) exhibited long-term dispersion stability compared to the ODA-grafted h-BNNPs and this was attributed to a higher degree of van der Waals interactions between the octyl chains of the TOA molecules grafted onto the h-BNNPs and the hydrocarbon chains of the mineral lube base oil. The tribo-performance of the h-BNNPs-TOA as an additive to mineral lube base oil was evaluated in terms of the coefficient of friction and wear using ball-on-disc contact geometry. A minute dosing (0.02 mg mL(-1)) of h-BNNPs-TOA significantly improved the lubrication characteristics of the mineral lube base oil and showed a 35 and 25% reduction of friction and wear, respectively. The presence of boron and nitrogen on the worn scar of an aluminium disc, as deduced from elemental mapping, confirmed the formation of a tribo-chemical thin film of h-BN lamellae on the contact interfaces, which not only reduced the friction but also protected the contact interfaces against undesirable wear events.

Detailed stability investigation of amorphous solid dispersions prepared by single-needle and high speed electrospinning

In this research the long-term stability (one year) of amorphous solid dispersions (ASDs) prepared by high speed electrospinning was investigated at 25°C/60% relative humidity (RH) (closed conditions) and 40°C/75% RH (open conditions). Single needle electrospinning and film casting were applied as reference technologies. Itraconazole (ITR) was used as the model API in 40% concentration and the ASDs consisted of either one of the following polymers as a comparison: polyvinylpyrrolidone-vinyl acetate 6:4 copolymer (no hydrogen bonds between API and polymer) and hydroxypropyl methylcellulose (possible hydrogen bonds between oxo or tertiary nitrogen function of API and hydroxyl moiety of polymer). DSC, XRPD and dissolution characteristics of samples at 0, 3 and 12 months were investigated. In addition, Raman maps of certain electrospun ASDs were assessed to investigate crystallinity. A new chemometric method, based on Multivariate Curve Resolution-Alternating Least Squares algorithm, was developed to calculate the spectrum of amorphous ITR in the matrices and to determine the crystalline/amorphous ratio of aged samples. As it was expected ITR in single needle electrospun SDs was totally amorphous at the beginning, in addition hydroxypropyl methylcellulose could keep ITR in this form at 40°C/75% RH up to one year due to the hydrogen bonds and high glass transition temperature of the SD. In polyvinylpyrrolidone-vinyl acetate matrix ITR remained amorphous at 25°C/60% RH throughout one year. Materials prepared by scaled-up, high throughput version of electrospinning, which is compatible with pharmaceutical industry, also gained the same quality. Therefore these ASDs are industrially applicable and with an appropriate downstream process it would be possible to bring them to the market.

Interparticle potential of 10 nanometer titanium nanoparticles in liquid sodium: theoretical approach

Abstract A suspension of titanium nanoparticles (Ti NPs) in liquid sodium (Na) has been proposed as a method to mitigate the violent sodium–water reaction (SWR). The interparticle potential between Ti NPs in liquid Na may play a significant role in the agglomeration of NPs on the reaction surface and in the bulk liquid Na, since the potential contributes to a reduction in the long-term dispersion stability. For the effective control of the SWR with NPs, a physical understanding of the molecular dynamics of NPs in liquid Na is key. Therefore in this study, the nonretarded Van der Waals model and the solvation potential model are employed to analyze the interparticle potential. The ab initio calculation reveals that a strong repulsive force driven by the solvation potential exceeds the interparticle attraction and predicts the agglomeration energy required for two 10-nm Ti NPs to be 4 × 10−17 J. The collision theory suggests that Ti NPs can be effective suppressors of the SWR due to the high energy barrier that prevents significant agglomeration of Ti NPs in quiescent liquid Na.

Alkyl-chain-grafted hexagonal boron nitride nanoplatelets as oil-dispersible additives for friction and wear reduction.

Hexagonal boron nitride (h-BN), an isoelectric analogous to graphene multilayer, can easily shear at the contact interfaces and exhibits excellent mechanical strength, higher thermal stability, and resistance toward oxidation, which makes it a promising material for potential lubricant applications. However, the poor dispersibility of h-BN in lube base oil has been a major obstacle. Herein, h-BN powder was exfoliated into h-BN nanoplatelets (h-BNNPs), and then long alkyl chains were chemically grafted, targeting the basal plane defect and edge sites of h-BNNPs. The chemical and structural features of octadecyltriethoxysilane-functionalized h-BNNPs (h-BNNPs-ODTES) were studied by FTIR, XPS, XRD, HRTEM, and TGA analyses. The h-BNNPs-ODTES exhibit long-term dispersion stability in synthetic polyol ester lube base oil because of van der Waals interaction between the octadecyl chains of h-BNNPs-ODTES and alkyl functionalities of polyol ester. Micro- and macrotribology results showed that h-BNNPs-ODTES, as an additive to synthetic polyol ester, significantly reduced both the friction and wear of steel disks. Elemental mapping of the worn area explicitly demonstrates the transfer of h-BNNPs-ODTES on the contact interfaces. Furthermore, insight into the lubrication mechanism for reduction in both friction and wear is deduced based on the experimental results.

Multidentate ionic surfactant mediated extraction and dispersion of gold nanoparticles in organic solvents.

Resorcinarenes with three different quaternary ammonium headgroups were synthesized and evaluated for their ability to stabilize gold nanoparticles in organic and aqueous medium. Aqueous dispersions of citrate stabilized gold nanoparticles of dimensions up to 29 nm could be extracted into organic solvents by resorcinarenes functionalized with tetrapyridinium tetrabromide (1), tetratrimethylammonium tetrabromide (2), and tetratributylammonium tetrabromide (3). Such nanoparticles were characterized by TEM, EDS, UV-vis, and IR. Their long-term dispersion stability varied significantly and depended on the nature of the resorcinarene headgroup, and in particular nanoparticles extracted by resorcinarene 1 were stable for several weeks. Nanoparticles passivated by resorcinarenes 1 and 2 were also stable in the presence of thiourea for several hours in both aqueous and organic medium. This is notable as thiourea is known to result in the instantaneous aggregation of citrate stabilized nanoparticles. Remarkably nanoparticles stabilized by resorcinarenes 1 and 2 could be precipitated and redispersed in chloroform without any visible aggregation. The critical parameters controlling the extraction of the nanoparticles into the organic phase have also been evaluated. The resorcinarene surfactant mediated facile phase transfer of gold nanoparticles described here can be readily applied for the stabilization of other citrate stabilized mono- and bimetallic nanoparticles, thus providing opportunities to disperse and stabilize relatively larger nanoparticles in organic solvents using ionic surfactants opening up new applications.

Curcumin-loaded lipid nanocarrier for improving bioavailability, stability and cytotoxicity against malignant glioma cells

Objective: In the present study, Curcumin (CU)-loaded nanocarrier (NC) such as nanoemulsion (NE) was developed with the objective of increasing its cytotoxicity and bioavailability through lymphatic transport by enhancing its solubility and intestinal permeability.Materials and methods: Based on the area obtained in pseudoternary phase diagram, various % combination of Labrafac Lipophile WL 1349, Solutol HS 15, Transcutol HP and distilled water were selected. Formulations which passed physical stability studies were selected for further studies such as globule size, zeta potential, in vitro release, ex vivo permeation, in vitro lipolysis studies, bioavailability studies and cytotoxicity against glioblastoma cells (U-87).Result and discussion: The optimized NC (NE-SB1) had small average globule diameter of 67 ± 6 nm with zeta potential of −37 ± 2.5 mv which indicated long-term dispersion stability. During in vitro lipolysis study, the digestion rate of medium chain triglycerides increased with decreased globule diameter. Statistically significant difference was found in AUC0−inf of NC formulation (p < 0.05) compared to CU suspension. The relative bioavailability of NC was found 11.88 ± 0.47 with respect to CU suspension. During cytotoxicity studies, IC50 of CU solution on U87 cells was found 24.23 µM, while for the NE- SB1 it was 16.41 µM. The optimized formulation was found to be stable during 6 months of accelerated stability.Conclusion: The overall results revealed that the CU-loaded NC is a very effective approach for enhancing the oral absorption of poorly water-soluble drug CU and have great potential for future clinical application.

물리화학적 처리를 통한 극성 용매 내 나노다이아몬드의 탈응집 및 분산성 향상 연구

In the present work, physicochemical treatments were introduced for de-aggregation and stable dispersion of detonation nanodiamonds (DND) in polar solvents. The DNDs in water exhibited a particle size of 138 nm and high dispersion stability without particular treatment. However, the DNDs in ethanol were severely aggregated to several micrometers in size and showed poor dispersion stability with time. To break down aggregates of DNDs and enhance the dispersion stability of them in ethanol, mechanical force and chemical surfactant were introduced as functions of zirconia ball size, kind of surfactant and amount of surfactant added. From the analyses of average particle size and Turbiscan results, it was suggested that the size of DNDs in ethanol can be reduced by only mechanical force; however, the DNDs were re-aggregated due to high surface activity. The long-term dispersion stability can be achieved by applying mechanical force to break down the aggregates of DNDs and by preventing re-aggregation of them using proper surfactant.

Long-Term Dispersion Stability and Adhesion Promotion of Aqueous Cu Nano-Ink for Flexible Printed Electronics

Aqueous Cu nanoparticles are synthesized using a reducing agent and surface capping molecule which prevents the interparticular agglomeration and surface oxidation. Aqueous conductive nano ink is prepared using the resulting Cu nanoparticles and conductive Cu layers are prepared via a wet coating process. The conductive Cu layers, metalized by annealing at 300 degrees C under vacuum atmosphere, exhibit excellent electrical resistivity, showing values as low as 12 microomega cm. The long-term dispersion stability for three months is monitored through an investigation on the rheological behavior of the conductive nano ink and the resistivity variation of the conductive Cu layer. The adhesion property of the conductive Cu layer is dramatically improved when using a primer-treated polyimide film, whereas the conductive Cu layer completely peels off on a pristine polyimide film. The epoxy-contained primer plays a critical role as an intermediary between the aqueous Cu nano ink and the polyimide film.

Rapid and efficient synthesis of soluble graphene nanosheets using N-methyl-p-aminophenol sulfate as a reducing agent

Mass production of soluble graphene still remains a challenge, although several methodologies have been proposed. Here we report a rapid and efficient method for the synthesis of soluble graphene nanosheets (GNSs) with long-term dispersion stability in both aqueous and common organic solvents. Within only 12 min at 95 °C, exfoliated graphite oxide in ammonia solution (pH 10) was reduced to soluble GNSs using N-methyl-p-aminophenol sulfate (metol) as a reducing agent without external stabilizers. The prepared GNSs were characterized by different techniques and a comparison of metol and hydrazine hydrate as reducing agents was made. The results indicated that, with the advantages of being rapid, efficient, inexpensive and relatively environmentally friendly, the reduction of graphite oxide into soluble GNSs by metol is a promising substitute for hydrazine hydrate in the mass production of soluble GNSs.

Curcumin Loaded Nano Globules for Solubility Enhancement: Preparation, Characterization and Ex Vivo Release Study

Curcumin in spite of being an effective chemotherapeutic agent against different type of cancer, suffer from the problem of low systemic bioavailability due to low aqueous solubility, extensive intestinal metabolism and first-pass metabolism when administered via the oral route. The aim of present investigation was to evaluate the potential of nano globules based nanoemulsion formulation for the solubility enhancement of curcumin. The nano globules based formulation was developed using Labrafac Lipophile WL 1349, Unitop FFT 40, PEG 400 and distilled water as an oil, surfactant, co-surfactant and aqueous phase respectively using aqueous titration method. Furthermore, different formulations were subjected to physical stability and consequently evaluated for ex vivo permeation using small intestine. The optimized formulation had small average globule diameter of 58 nm with zeta potential of -32 mv which indicated long-term dispersion stability. The globules were spherical in shape as observed by Transmission electron microscopy. During ex vivo study, the release of curcumin from nanoemulsion was 96.21% and 98.1% in 6 h and 12 h respectively whereas CU suspension was release up to 28.2% at the end of 12 h. This indicated the enhancement of solubility of curcumin in aqueous solution which is the rate limiting step in the absorption of curcumin in the intestine.

Interfacial rheology: An overview of measuring techniques and its role in dispersions and electrospinning

Interfacial rheological properties have yet to be thoroughly explored. Only recently, methods have been introduced that provide sufficient sensitivity to reliably determine viscoelastic interfacial properties. In general, interfacial rheology describes the relationship between the deformation of an interface and the stresses exerted on it. Due to the variety in deformations of the interfacial layer (shear and expansions or compressions), the field of interfacial rheology is divided into the subcategories of shear and dilatational rheology. While shear rheology is primarily linked to the long-term stability of dispersions, dilatational rheology provides information regarding short-term stability. Interfacial rheological characteristics become relevant in systems with large interfacial areas, such as emulsions and foams, and in processes that lead to a large increase in the interfacial area, such as electrospinning of nanofibers.

An Effective Polymer Cross-Linking Strategy To Obtain Stable Dispersions of Upconverting NaYF4 Nanoparticles in Buffers and Biological Growth Media for Biolabeling Applications

Ligands on the nanoparticle surface provide steric stabilization, resulting in good dispersion stability. However, because of their highly dynamic nature, they can be replaced irreversibly in buffers and biological medium, leading to poor colloidal stability. To overcome this, we report a simple and effective cross-linking methodology to transfer oleate-stabilized upconverting NaYF(4) core/shell nanoparticles (UCNPs) from hydrophobic to aqueous phase, with long-term dispersion stability in buffers and biological medium. Amphiphilic poly(maleic anhydride-alt-1-octadecene) (PMAO) modified with and without poly(ethylene glycol) (PEG) was used to intercalate with the surface oleates, enabling the transfer of the UCNPs to water. The PMAO units on the phase transferred UCNPs were then successfully cross-linked using bis(hexamethylene)triamine (BHMT). The primary advantage of cross-linking of PMAO by BHMT is that it improves the stability of the UCNPs in water, physiological saline buffers, and biological growth media and in a wide range of pH values when compared to un-cross-linked PMAO. The cross-linked PMAO-BHMT coated UCNPs were found to be stable in water for more than 2 months and in physiological saline buffers for weeks, substantiating the effectiveness of cross-linking in providing high dispersion stability. The PMAO-BHMT cross-linked UCNPs were extensively characterized using various techniques providing supporting evidence for the cross-linking process. These UCNPs were found to be stable in serum supplemented growth medium (37 °C) for more than 2 days. Utilizing this, we demonstrate the uptake of cross-linked UCNPs by LNCaP cells (human prostate cancer cell line), showing their utility as biolabels.

Solvent-free nanoparticle fluids with highly collective functionalities for layer-by-layer assembly

Here we demonstrate the successful preparation of solvent-free inorganic nanoparticle (NP) fluids with high functionalities that allow mass production. The inorganic NPs (Ag, Au, and quantum dots [QDs]) used in this study were directly synthesized using hydrophobic stabilizers such as palmitic acid, oleic acid, or tetraoctylammonium bromide in a nonpolar solvent (chloroform or toluene), and these NPs were directly phase-transferred to solvent-free low molecular weight liquid media (i.e., thiol-containing imidazolium-type ionic liquid). The NP fluids formed at room temperature showed excellent functionalities (i.e., long-term dispersion stability, high electrical conductivity, strong optical energy transfer, and high photoluminescent intensity) without requiring any additional process such as anion exchange or thermal processing. Furthermore, layer-by-layer deposition of QDs stabilized by the ionic liquid induced highly fluorescent properties compared to those of layer-by-layer QD multilayers prepared using conventional ligands such as mercaptoacetic acid or cysteamine, which mainly originated from the high packing density and the relatively high quantum efficiency of ionic liquid-stabilized QDs.

Dispersion of alkylated graphene in organic solvents and its potential for lubrication applications

We report on the preparation of alkylated graphenes on a large scale followed by their dispersion in organic solvents. The alkylated graphenes with variable alkyl chain lengths (Cn = 8, 12, 18) are prepared by coupling of alkylamine with carboxylic groups of graphene oxide (GrO). The FTIR, UV-Visible, and TGA results reveal that, during alkylation, the oxygen functionalities of GrO are reduced significantly and the average size of the sp2 carbon domain increased, which is further supported by Raman characteristics. It is observed that the dispersibility of alkylated graphene in hydrocarbon solvents increases on increasing the chain length of (a) hydrocarbon solvents used for the dispersion and (b) alkyl groups attached to the graphene. The van der Waals interaction between methylene units associated with alkylated graphenes and hydrocarbon solvents plays a crucial role in determining their dispersion characteristics, and such an interaction increases with increasing methylene units. Octadecylamine functionalized graphene (ODA-Gr) dispersion in hexadecane is found to have long-term dispersion stability due to its high degree of cohesive interaction. The lubrication characteristics of hexadecane containing ODA-Gr were probed by evaluating its friction and wear properties. The results reveal that hexadecane doped with an optimized dose of 0.06 mg mL−1 ODA-Gr reduced friction and wear by 26% and 9%, respectively, compared to hexadecane. The lubricity enhancement could be attributed to uninterrupted supplies of graphene nanosheets under the rubbing surfaces, where these nanosheets prevent direct contact between the rubbing surfaces, providing low resistance to shear.

Theoretical and experimental investigation of the motion of multiphase fluids containing paramagnetic nanoparticles in porous media

Paramagnetic nanoparticles are potentially useful for monitoring of immiscible fluids distribution in subsurface, as they can be induced to move by an imposed magnetic field. The nanoparticles can be designed to be preferentially adsorbed at oil–water interface, as well as to have the long-term dispersion stability with minimal retention in the porous medium to be monitored. When exposed to magnetic field, they generate sufficient interfacial movements for external detection.When paramagnetic nanoparticles are either adsorbed at oil–water or air–water interface or dispersed in one of two fluid phases co-existing in pores, and exposed to external magnetic field, the resultant particle movements displace the interface. Interfacial tension acts as a restoring force, leading to interfacial fluctuation and a pressure (sound) wave. Our previous work (Prodanovic et al., 2010) provided a theoretical explanation for the motion of the interface between a suspension of paramagnetic nanorods and a non-magnetized fluid in a cylindrical dish, as measured by phase-sensitive optical coherence tomography (PS-OCT). Here we report on additional experiments carried out with a range of in-house synthesized and surface-modified iron-oxide nanoparticles. We also improved numerical method to be volume conserving for more quantitative matching.The measurements of interfacial motion by PS-OCT reported confirm theoretical predictions of the frequency doubling and the importance of material properties, such as magnetic susceptibility, for the interface displacement.The results are encouraging: this laboratory and modeling study is thus an important step to develop a magnetic field-based method for an accurate, non-invasive determination of multiphase fluids distribution in reservoir rock. With the combined experimental and modeling work, strategies for improved nanoparticle design will be developed so that the interfacial, thereby acoustic, response can be magnified.

Characterization and conversion determination of stable PEDOT latex nanoparticles synthesized by emulsion polymerization

In this research, poly(3,4-ethylenedioxythiophene) (PEDOT) latex nanoparticles with good colloidal stability were prepared by emulsion polymerization and the conversions of EDOT were determined. Two kinds of oxidants, Iron(III) p-toluenesulfonate Fe(OTs) 3 and hydrogen peroxide H 2O 2, were introduced to decrease the use of iron salt and therefore reduce the particle coagulation. The ferrous ions (Fe 2+) produced during the polymerization would be re-oxidized back to the reactive ferric ions (Fe 3+) with the help of H 2O 2. This cyclic oxidation–reduction process resulted in the sustained regeneration of Fe 3+ ions and led to a higher conversion. A dark blue PEDOT latex with long-term dispersion stability was obtained when Fe(OTs) 3 and H 2O 2 were added in sequence. The results obtained from dynamic light scattering and TEM measurements showed that the sizes of nanoparticles were around 100 nm. To determine the conversion of EDOT, two methods (gravimetric analysis and UV–visible method) were used and compared. For the first time, the UV–visible method was established to quantitatively determine the conversion of EDOT monomer. From the measurement, the conversion of EDOT in this system was determined as 74–75%. The PEDOT film prepared by drying the latex solution had conductivity up to 6.3 S/cm.

Dispersion of carbon nanotubes by carbazole-tailed amphiphilic imidazolium ionic liquids in aqueous solutions

Surfactants with a polycyclic aromatic moiety and a long hydrocarbon chain, carbazole-tailed amphiphilic imidazolium ionic liquids 1-[ n-( N-carbazole)alkyl]-3-methylimidazolium bromide [CzC n MIm]Br ( n = 10 and 12), were designed to disperse carbon nanotubes (CNTs) in aqueous solutions. UV–vis–NIR spectra were performed to determine the dispersion of CNTs and the optimal concentration ( C opt) of [CzC n MIm]Br. Compared with [C n MIm]Br, [CzC n MIm]Br was more effective with the smaller C opt and more individual CNTs, reflecting the effect of the carbazole moiety. The adsorption of [CzC n MIm]Br molecules on CNTs was investigated by zeta-potential, surface tension, fluorescence, and 1H NMR. Having zeta-potentials higher than 15 mV contributed to the long-term stability of aqueous CNT dispersions. The significant fluorescence quenching and the upfield shift of carbazole protons support the π– π stacking interaction between carbazole moieties and the π-networks of CNTs. Meanwhile, the upfield shift of imidazolium protons indicates the cation– π interaction between the imidazolium ions and the π-networks of CNTs.

Surface-modified cerium oxide nanoparticles synthesized continuously in supercritical methanol: Study of dispersion stability in ethylene glycol medium

Dispersion stability of surface-modified cerium oxide (CeO 2) nanoparticles in ethylene glycol is examined and the experimental stability results are compared with an extended DLVO model consisting of electrostatic, van der Waals, and hydrophobic/hydrophilic interactions. Unmodified, decanoic acid-modified and oleic acid-modified CeO 2 nanoparticles are synthesized continuously in supercritical methanol (scMeOH). The surface charge of the surface-modified CeO 2 particles changes from positive to negative with an increment in the medium pH while the surface charge of the unmodified CeO 2 particle does not change with varying pH. Long-term dispersion stability test (up to 100 days) shows that the oleic acid-modified nanoparticle with a concentration of 0.3 M retains most stable dispersion in ethylene glycol. The unmodified and decanoic acid-modified nanoparticles with a concentration of 0.03 M precipitate within 7–15 days. In contrast, initial short-term stability evolution reveals different stability behavior compared to the long-term stability. The unmodified and the decanoic acid-modified nanoparticles with a concentration of 0.03 M were less attractive than the oleic acid-modified nanoparticle with 0.3 M. The experimental short-term stability data is in good agreement with the computational results of energy profiles for the CeO 2 nanoparticle suspension.