Particle-water Interface Research Articles

Optical tweezer platform for the characterization of pH-triggered colloidal transformations in the oleic acid/water system

Hypothesis: Soft colloidal particles that respond to their environment have innovative potential for many fields ranging from food and health to biotechnology and oil recovery. The in situ characterisation of colloidal transformations that triggers the functional response remain a challenge.Experiments: This study demonstrates the combination of an optical micromanipulation platform, polarized optical video microscopy and microfluidics in a comprehensive approach for the analysis of pH-driven structural transformations in emulsions. The new platform, together with synchrotron small angle X-ray scattering, was then applied to research the food-relevant, pH-responsive, oleic acid in water system.Findings: The experiments demonstrate structural transformations in individual oleic acid particles from micron-sized onion-type multilamellar oleic acid vesicles at pH 8.6, to nanostructured emulsions at pH < 8.0, and eventually oil droplets at pH < 6.5. The smooth particle-water interface of the onion-type vesicles at pH 8.6 was transformed into a rough particle surface at pH below 7.5. The pH-triggered changes of the interfacial tension at the droplet-water interface together with mass transport owing to structural transformations induced a self-propelled motion of the particle. The results of this study contribute to the fundamental understanding of the structure–property relationship in pH-responsive emulsions for nutrient and drug delivery applications.

Modeling of rainfall induced phosphorus transport from soil to runoff with consideration of phosphorus-sediment interactions

Phosphorus (P) transport from soil to surface runoff is of great concern for the accurate prediction of P transport within the watershed, while the interaction between P and sediments has rarely been considered in conventional soil solute transport models. This study develops a novel P transport model that couples the processes of selective soil erosion and dynamic partitioning of P at the particle-water interface. The model assumes that the gradation of soil/sediment particles strongly affects the maximum amount of adsorption and therefore, affects P partitioning, while changes in the water environment also influence the partition coefficient. The novel model was then validated by four laboratory experimental runs. The R2 and RMSE values show that the simulated results are in good agreement with the measured data, indicating that the model is capable of effectively simulating rainfall induced P transport from soil to runoff. The modelled results indicate that an increase in the coarseness of sediment particles resulted in a sharp decrease in particle P adsorption (Csd) during erosion. Therefore, if the change in particle size was not accounted for in the model (Ked=Kp), the concentrations of soluble phosphorus (SP) and particulate phosphorus (PP) would be significantly underestimated. This highlights the importance of including selective erosion and P-sediment interactions in models of P transport from soils to surface runoff.

Implications of phosphorus partitioning at the suspended particle-water interface for lake eutrophication in China’s largest freshwater lake, Poyang Lake

Nutrient partition, especially for phosphorus (P), has been prominently changed that was caused by variation of river-lake relationship during the post-Three Gorges Reservoir and catchment alternations. Changes in proportion of total particulate phosphorus (TPP) and total dissolved phosphorus (TDP) might accelerate lake eutrophication, but limited attention has been paid to P partition over suspended particle (SP) levels. Data analysis showed that SP concentration presented a positive effect on TPP in wet season and soluble reactive phosphorus (SRP) in dry season, indicating seasonal physical and chemical variations. Based on this phenomenon, we proposed a hypothesis that the SP levels would affect TDP and TPP proportions by partition in aqueous-solid. It was found that using the parabola models to fit the sorption relationships of SRP and TDP (R2 > 0.6, p < 0.01), the maximum sorption capacity (Qmax) was 64.54 mg/kg and 60.52 mg/kg at 400 mg/L of SP level, respectively. In addition, the partition coefficients (KP) of TDP and SRP were logarithmically increased with SP levels, indicating that higher SP levels (>400 mg/L) would hinder the sorption process. Furthermore, enhancing turbulence lead to less sorption of SRP and TDP at high SP levels (>800 mg/L). The sorption of SRP and TDP related to the presence of Fe/Al oxy-hydroxides were enriched in the Fe/Al–P fraction (47% of TP). The findings of this study indicated that the low SP levels would increase P bioavailability for alga and is not conducive for lake eutrophication management.

Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation

Lateral optical forces induced by linearly polarized laser beams have been predicted to deflect dipolar particles with opposite chiralities toward opposite transversal directions. These “chirality-dependent” forces can offer new possibilities for passive all-optical enantioselective sorting of chiral particles, which is essential to the nanoscience and drug industries. However, previous chiral sorting experiments focused on large particles with diameters in the geometrical-optics regime. Here, we demonstrate, for the first time, the robust sorting of Mie (size ~ wavelength) chiral particles with different handedness at an air–water interface using optical lateral forces induced by a single linearly polarized laser beam. The nontrivial physical interactions underlying these chirality-dependent forces distinctly differ from those predicted for dipolar or geometrical-optics particles. The lateral forces emerge from a complex interplay between the light polarization, lateral momentum enhancement, and out-of-plane light refraction at the particle-water interface. The sign of the lateral force could be reversed by changing the particle size, incident angle, and polarization of the obliquely incident light.

Determination of Internal Density Profiles of Smart Acrylamide-Based Microgels by Small-Angle Neutron Scattering: A Multishell Reverse Monte Carlo Approach.

The internal structure of nanometric microgels in water has been studied as a function of temperature, cross-linker content, and level of deuteration. Small-angle neutron scattering from poly( N-isopropylmethacrylamide) (volume phase transition ≈ 44 °C) microgel particles of radius well below 100 nm in D2O has been measured. The intensities have been analyzed with a combination of polymer chain scattering and form-free radial monomer volume fraction profiles defined over spherical shells, taking polydispersity in size of the particles determined by atomic force microscopy into account. A reverse Monte Carlo optimization using a limited number of parameters was developed to obtain smoothly decaying profiles in agreement with the experimentally scattered intensities. The results are compared to the swelling curve of microgel particles in the temperature range from 15 to 55 °C obtained from photon correlation spectroscopy (PCS). In addition to hydrodynamic radii measured by PCS, our analysis provides direct information about the internal water content and gradients, the strongly varying steepness of the density profile at the particle-water interface, the total spatial extension of the particles, and the visibility of chains. The model has also been applied to a variation of the cross-linker content, N, N'-methylenebisacrylamide, from 5 to 15 mol %, providing insight on the impact of chain architecture and cross-linking on water uptake and on the definition of the polymer-water interface. The model can easily be generalized to arbitrary monomer contents and types, in particular mixtures of hydrogenated and deuterated species, paving the way to detailed studies of monomer distributions inside more complex microgels, in particular core-shell particles.

Coal fly ash: linking immersion freezing behavior and physicochemical particle properties

Abstract. To date, only a few studies have investigated the potential of coal fly ash particles to trigger heterogeneous ice nucleation in cloud droplets. The presented measurements aim at expanding the sparse dataset and improving process understanding of how physicochemical particle properties can influence the freezing behavior of coal fly ash particles immersed in water. Firstly, immersion freezing measurements were performed with two single particle techniques, i.e., the Leipzig Aerosol Cloud Interaction Simulator (LACIS) and the SPectrometer for Ice Nuclei (SPIN). The effect of suspension time on the efficiency of the coal fly ash particles when immersed in a cloud droplet is analyzed based on the different residence times of the two instruments and employing both dry and wet particle generation. Secondly, two cold-stage setups, one using microliter sized droplets (Leipzig Ice Nucleation Array) and one using nanoliter sized droplets (WeIzmann Supercooled Droplets Observation on Microarray setup) were applied. We found that coal fly ash particles are comparable to mineral dust in their immersion freezing behavior when being dry generated. However, a significant decrease in immersion freezing efficiency was observed during experiments with wet-generated particles in LACIS and SPIN. The efficiency of wet-generated particles is in agreement with the cold-stage measurements. In order to understand the reason behind the deactivation, a series of chemical composition, morphology, and crystallography analyses (single particle mass spectrometry, scanning electron microscopy coupled with energy dispersive X-ray microanalysis, X-ray diffraction analysis) were performed with dry- and wet-generated particles. From these investigations, we conclude that anhydrous CaSO4 and CaO – which, if investigated in pure form, show the same qualitative immersion freezing behavior as observed for dry-generated coal fly ash particles – contribute to triggering heterogeneous ice nucleation at the particle–water interface. The observed deactivation in contact with water is related to changes in the particle surface properties which are potentially caused by hydration of CaSO4 and CaO. The contribution of coal fly ash to the ambient population of ice-nucleating particles therefore depends on whether and for how long particles are immersed in cloud droplets.

CO2-Responsive Pickering Emulsions Stabilized by a Bio-based Rigid Surfactant with Nanosilica.

A novel CO2-responsive surfactant, maleopimaric acid glycidyl methacrylate ester 3-(dimethylamino)propylamine imide (MPAGN), based on sustainable resource of rosin was synthesized and used to prepare a kind of CO2-responsive Pickering emulsions with nanosilica. MPAGN can be reversibly responsive to CO2 and N2 between active cationic (MPAGNH+) and inactive nonionic (MPAGN), leading to adsorb on or desorb from the surface of nanosilica, then stabilize or break emulsion. CO2-responsive behavior of MPAGN was verified by cycle change of pH and conductivity with bubbling CO2 and N2 alternately. The type of adsorption of MPAGNH+ at the particle-water interface was explained according to the adsorption isotherms. The mechanisms of stabilization, destabilization, and restabilization of Pickering emulsion were analyzed according to zeta potentials and droplet size. This Pickering emulsion can be reversible between stable and unstable by bubbling CO2 and N2 alternately. Moreover, this emulsifier can be recycled when new oil was added after removing the initial oil. Therefore, it not only has economic benefits but also has an environmentally friendly property.

Micro-electrode system designed to determine H+ concentration distribution at particle-water interface

The current understanding of the particle-water interface relies primarily on the use of electric double layer theory (EDL theory), in which the concentration distribution of ions with distance can be evaluated. However, although EDL theory provides a theoretical and mathematical basis, obtaining direct experimental results is also necessary. In this study, an instrument system set with a designed H+ selective micro-electrode was developed for in situ determination at the particle-water interface, and a preliminary investigation of the H+ concentration distribution at the particle-water interface was enabled using data collection at 1 nm/step. The experimental results agree with EDL theory and provide direct evidence to support its general assumptions. The compressible effect of the electric double layer with an increase in ionic strength is verified, and the defined length of the diffusion layer in this investigation is shown to have an approximately inverse proportion to the square root of ionic strength. The rise in H+ concentration near the particle surface is also determined during interactions between trace Fe3+ and particles. Results indicate that the method reported here can be applied to conduct dynamic in situ observations; based on its excellent performance, this innovative instrument system has great potential for interface mechanism research in the water environment.

Electrokinetic motion of a spherical micro particle at an oil-water interface in microchannel.

The electrokinetic motion of a negatively charged spherical particle at an oil-water interface in a microchannel is numerically investigated and analyzed in this paper. A three-dimensional (3D) transient numerical model is developed to simulate the particle electrokinetic motion. The channel wall, the surface of the particle and the oil-water interface are all considered negatively charged. The effects of the direct current (DC) electric field, the zeta potentials of the particle-water interface and the oil-water interface, and the dynamic viscosity ratio of oil to water on the velocity of the particle are studied in this paper. In addition, the influences of the particle size are also discussed. The simulation results show that the micro-particle with a small value of negative zeta potential moves in the same direction of the external electric field. However, if the zeta potential value of the particle-water interface is large enough, the moving direction of the particle is opposite to that of the electric field. The velocity of the particle at the interface increases with the increase in the electric field strength and the particle size, but decreases with the increase in the dynamic viscosity ratio of oil to water, and the absolute value of the negative zeta potentials of both the particle-water interface and the oil-water interface. This work is the first numerical study of the electrokinetic motion of a charged particle at an oil-water interface in a microchannel.

Multi-lobed composite polymer nanoparticles prepared by conventional emulsion polymerization

The vast majority of latex particles, including those made from multiple polymers, are spherical in shape. This is a result of the minimization of the combined interfacial free energies at the polymer particle–water interface and the polymer–polymer interfaces within the particle. Yet there are special situations in which latex particles containing two, incompatible polymer phases can form asymmetric, multi-lobed particles during conventional, two stage emulsion polymerizations. Here we report on a variety of situations used to produce these interesting polymer particles and discuss the mechanism by which they are formed. The key condition is that the reaction process be carried out in a manner in which the second stage oligoradicals in the water are not allowed to significantly penetrate the first stage latex particle during the reaction. Then, the important variables are the relative differences between the interfacial tensions at the water-polymer interface for the two polymers in question, and the second stage polymer chain diffusivity on the surface of the first stage particles. By extension, the same fundamental parameters of interfacial tensions and polymer chain surface diffusion apply to the morphology possibilities when inorganic first stage particles are used to create inorganic-organic composite latices.

Nanobubbles at Hydrophilic Particle-Water Interfaces.

The puzzling persistence of nanobubbles breaks Laplace's law for bubbles, which is of great interest for promising applications in surface processing, H2 and CO2 storage, water treatment, and drug delivery. So far, nanobubbles have mostly been reported on hydrophobic planar substrates with atomic flatness. It remains a challenge to quantify nanobubbles on rough and irregular surfaces because of the lack of a characterization technique that can detect both the nanobubble morphology and chemical composition inside individual nanobubble-like objects. Here, by using synchrotron-based scanning transmission soft X-ray microscopy (STXM) with nanometer resolution, we discern nanoscopic gas bubbles of >25 nm with direct in situ proof of O2 inside the nanobubbles at a hydrophilic particle-water interface under ambient conditions. We find a stable cloud of O2 nanobubbles at the diatomite particle-water interface hours after oxygen aeration and temperature variation. The in situ technique may be useful for many surface nanobubble-related studies such as material preparation and property manipulation, phase equilibrium, nucleation kinetics, and relationships with chemical composition within the confined nanoscale space. The oxygen nanobubble clouds may be important in modifying particle-water interfaces and offering breakthrough technologies for oxygen delivery in sediment and/or deep water environments.

Interaction of the amyloid β peptide with sodium dodecyl sulfate as a membrane-mimicking detergent.

The amyloid β (A β) peptide is important in the context of Alzheimer’s disease, since it is one of the major components of the fibrils that constitute amyloid plaques. Agents that can influence fibril formation are important, and of those, membrane mimics are particularly relevant, because the hydrophobic part of A β suggests a possible membrane activity of the peptide. We employed spin-label EPR to investigate the aggregation process of A β1–40 in the presence of the sodium dodecyl sulfate (SDS) detergent as a membrane-mimicking agent. In this work, the effect of SDS on A β is studied using two positions of spin label, the N-terminus and position 26. By comparing the two label positions, the effect of local mobility of the spin label is eliminated, revealing A β aggregation in the SDS concentration regime below the critical micelle concentration (CMC). We demonstrate that, at low SDS concentrations, the N-terminus of A β participates in the solubilization, most likely by being located at the particle–water interface. At higher SDS concentrations, an SDS-solubilized state that is a precursor to the one A β/micelle state above the CMC of SDS prevails. We propose that A β is membrane active and that aggregates include SDS. This study reveals the unique potential of EPR in studying A β aggregation in the presence of detergent.

Formation of micro/nanostructured AlOOH hollow spheres from aluminum nanoparticles

Micro/nanostructured AlOOH hollow spheres have been obtained via one-step synthesis. Spherical aluminum nanoparticles coated with amorphous oxide film are used as a precursor. Hollow spheres of AlOOH are formed during the oxidation of aluminum nanoparticles with water. In order to study the evolution of hollow spheres, electron microscopy studies of the reaction intermediates at various stages of the process are carried out. Micro/nanostructured AlOOH hollow spheres have a diameter of 500–800 nm, and their shells consist of boehmite nanosheets with a planar size of 200–300 nm and a thickness of 2–10 nm. It is shown that the formation of AlOOH hollow spheres occurs through dissolution of aluminum core, the diffusion of Al3+ ions through the surface oxide film, and the formation of islands of amorphous Al(OH)3 at the particle–water interface. Further, the formation and growth of AlOOH nanosheets and the formation of a porous shell from nanosheets take place. As a result, the oxide film serves as a substrate for growth of boehmite nanosheets and the formation of micro/nanostructured AlOOH hollow sphere. Morphology and physicochemical properties of the hollow spheres are characterized by transmission and scanning electron microscopy, X-ray diffraction, energy-dispersive analysis, and low-temperature nitrogen adsorption.

Kinetic study of styrene and methyl methacrylate emulsion polymerization induced by cumene hydroperoxide/tetraethylenepentamine

The redox initiator, cumene hydroperoxide/tetraethylenepentamine (CHPO/TEPA), was used to initiate the emulsion polymerizations of styrene and methyl methacrylate (MMA). The hydrophobic CHPO acting as oxidizer would interact with the hydrophilic TEPA employed as the reducer at the particle-water interface where the vinyl monomer is present. The kinetics of the styrene and MMA polymerizations were studied under a temperature range of 30–70 °C. The polymerization rate, the concentration of monomer in the particles, the average number of latex particles and the number of radicals per particle were investigated to evaluate the propagation rate constant during interval II of the polymerization. With increasing temperature, the number of latex particles increases while the monomer concentration in the particles slightly decreases. The observed number of radicals per particle, calculated from the number average volume of the particles and the relation between the entry rate coefficients of free radicals into the particles and the exit rate coefficients of free radicals from the particle, was found to be approximately 0.47 and 0.5 in case of styrene and MMA polymerization, respectively. The calculation shows that the propagation rate constant, kp = 6.84 × 107e−37.65/RT and kp = 3.4 × 107e−31.05/RT L/mol∙s for styrene and MMA polymerizations over the temperature range studied, respectively.

Surface modification of CaCO3 nanoparticles by alkylbenzene sulfonic acid surfactant

Surface modification of CaCO3 nanoparticles by two kinds of anionic surfactants, such as linear alkylbenzene sulfonic acid (LABSA) and branched alkylbenzene sulfonic acid (BABSA), was investigated by a quartz crystal microbalance with dissipation monitoring (QCM-D) measurement and contact angle analysis. Basically, both surfactant systems showed the same trend in the measurements of QCM-D and contact angle. Contact angle measurement has shown that CaCO3 surface becomes more hydrophobic as surfactant concentration increases. However, the use of excess surfactant has led to the reverse change in surface property of CaCO3 particles from hydrophobic to hydrophilic. QCM-D measurement using a quartz crystal coated with CaCO3 revealed that monolayer adsorption of surfactant molecules at the particle–water interface occurs in a low surfactant concentration region and the thickness of forming molecular layers was found to be in the order of the length of a single surfactant molecule. On the other hand, both mass of adsorbed surfactant molecules and the thickness of molecular layers indicated bilayer adsorption of surfactant molecules in a high surfactant concentration region, which evidently resulted in an increase in hydrophilicity of CaCO3 surface. Visual floating test has shown a good agreement with measurements of contact angle and QCM-D.

汽水湖における有機泥の沈降速度の推定と界面特性に関する考察

Distribution of settling velocity and characteristics of particle-water interface were discussed by using organic mud in Lake Abashiri located in a cold region, east Hokkaido in Japan. The distributions of settling velocity were estimated from temporal variations measured by laser diffraction analyzer. Since the settling velocity of cray mineral sample having low cohesiveness with dispersing agent agreed well with theoritical value, the analytical method were well verified. Comparing with the model consdering organic substances in floc, floc structure varies with floc size in lake water less than in distilled water. Zeta potential representing interface characteristics of organic mud is closely related to dissolved divalent-ion derived from sea water and humic-like organic substance.

Multiple Phase Inversion of Emulsions Stabilized by in Situ Surface Activation of CaCO3 Nanoparticles via Adsorption of Fatty Acids

The in situ surface activation of raw CaCO(3) nanoparticles by interaction with a series of sodium carboxylates of chain length between 6 and 12 as well as sodium 2-ethylhexylsulfosuccinate (AOT) was studied, and the impact of this on the stabilization and phase inversion of toluene-water emulsions was assessed. By using complementary experiments including measurement of particle zeta potentials, adsorption isotherms of amphiphile, and relevant contact angles, the mechanism of this activation was revealed. The results show that hydrophilic CaCO(3) nanoparticles can be surface activated by interaction with sodium carboxylates and AOT even if they are not surface-active themselves. Both the electrostatic interaction between the positive charges on particle surfaces and the negative charges of anionic amphiphile headgroups and the chain-chain interactions of the amphiphile result in monolayer adsorption of the amphiphile at the particle-water interface. This transforms the particles from hydrophilic to partially hydrophobic such that they become surface-active and stabilize oil-in-water O/W(1) emulsions and induce O/W(1) → water-in-oil W/O phase inversion, depending on the chain length of the carboxylate molecules. At high amphiphile concentration, bilayer or hemimicelle adsorption may occur at the particle-water surface, rendering particles hydrophilic again and causing their desorption from the oil-water interface. A second phase inversion, W/O → O/W(2), may occur depending on the surface activity of the amphiphile. CaCO(3) nanoparticles can therefore be made good stabilizers of both O/W and W/O emulsions once surface activated by mixing with traces of suitable anionic amphiphile.

Aqueous Foams Stabilized by in Situ Surface Activation of CaCO3 Nanoparticles via Adsorption of Anionic Surfactant

The in situ surface activation of unmodified CaCO(3) nanoparticles by interaction with surfactant in aqueous media has been studied, and the impact of this on the foamability and foam stability of aqueous dispersions was assessed. Using complementary experiments including measurement of particle zeta potentials, adsorption isotherms of surfactant, air-water surface tensions, and relevant contact angles, the mechanism of this activation was revealed. The results show that the non-surface-active CaCO(3) nanoparticles cannot be surface activated by interaction with cationic or nonionic surfactants but can be surface activated by interaction with anionic surfactants such as SDS and AOT, leading to a synergistic effect in both foamability and foam stability. The electrostatic interaction between the positive charges on particle surfaces and the negative charges of anionic surfactant headgroups results in monolayer adsorption of the surfactant at the particle-water interface and transforms the particles from hydrophilic to partially hydrophobic such that particles become surface active and stabilize bubbles. SDS is a more efficient surfactant for this surface activation than AOT. Possible reasons for this difference are suggested.

Size-dependent hydrophobic to hydrophilic transition for nanoparticles: A molecular dynamics study

The physical properties of nanoscale materials often vary with their size, unlike the corresponding bulk material properties, which can only be changed by modifying the material composition. In particular, it is believed that hydration phenomena are length scale dependent. The manifestation of hydrophobicity over multiple length scales plays a crucial role in self-assembly processes such as protein folding and colloidal stability. In the case of particles composed of a bulk hydrophobic material, it is well known that the free energy of hydration monotonically increases with particle size. However, the size-dependent free energy of hydration for particles composed of a bulk hydrophilic material has not been studied. Here we show that the free energy of hydration is not a monotonic function of particle size, but rather, changes sign from positive to negative as the particle size increases. In other words, the particle is hydrophobic at small size and hydrophilic at large size. This behavior arises from a purely geometrical effect caused by the curvature of the particle-water interface. We explore the consequences of this phenomenon on colloidal stability and find that it dictates the shape of colloidal aggregates.

Increase in the Adsorption Density of Anionic Molecules on Ceria for Defect-Free STI CMP

We have investigated the adsorption density of anionic molecules on a ceria surface and its effect on suppressing defects on the films during shallow trench isolation (STI) chemical mechanical planarization (CMP). Intermolecular vacancies in poly(methyl methacrylate) (PMMA) caused by ionic repulsive forces between its carboxyl groups provide adsorption sites on the ceria surface for 3-hydroxypentanedioic acid-3-carboxylic acid (hydrogen citrate), which has a molecular length of 8.39 Å. This allows for an increase in adsorption density. The coadsorption of PMMA and hydrogen citrate on the ceria surface leads to an increase in negative charge at the ceria particle–water interface and provides repulsive forces between the ceria particle and the surface. Ceria slurry prepared with the coadsorption of PMMA and hydrogen citrate shows a suppression of remaining particles (90.9%) and microscratches (100%) on the film, resulting from a decrease in the adhesion force of ceria particles on the film. This effect was analyzed by force measurement using atomic force microscopy.