Wettability Of Nanoparticles Research Articles

Nanofluid Drop Impact on Heated Surfaces.

We experimentally elucidate the impact dynamics of ethylene glycol (EG) droplets laden with both hydrophilic and hydrophobic SiO2 nanoparticles (NPs) onto a flat heated surface in non-boiling, boiling, and Leidenfrost regimes. We use seven nanofluid concentrations (Cp), ranging from 0.89 to 64.3 wt %, and control the surface temperature (Ts) between 100 and 400 °C, while the nanofluid droplet's impact velocity is constant at 0.22 ± 0.02 m/s. Phase diagrams of impact outcomes are established to illustrate the effect of the additive nanoparticles on the droplets' impact dynamics, revealing that nanoparticles modify droplet impact behaviors differently in each regime. In the non-boiling regime, the droplet spreading profile remains unaffected by nanoparticles up to Cp < 11.9 wt % before reaching the maximum spreading diameter (βmax). For nanofluid drops with higher nanofluid concentration, the increasing viscosity with concentration is likely to be the primary factor that affects the droplets' spreading profile in the non-boiling regime Ts ≲ Tsat ≈ 200 °C, as the saturation temperature. In the boiling regime 200 °C < Ts ≲ 350 °C, a small amount of nanoparticle addition (Cp = 0.89 wt %) promotes atomization regardless of nanoparticle wettability. Finally, manifested in a complete rebound due to an intervening vapor layer, the Leidenfrost temperature (TL) of the nanofluid droplets is affected by both nanofluid concentration and nanoparticles' wettability. The nanofluid droplets' TL increases with higher nanofluid concentration; moreover, this Leidenfrost temperature increment is more significant for EG droplets laden with hydrophobic nanoparticles. Our results quantitatively reveal the significant influence of nanoparticle concentrations and wettability on drop spreading, impact outcome, and Leidenfrost temperature on heat surfaces, potentially benefiting applications in coating, spraying, and cooling.

Vapor Nanobubbles around Heated Nanoparticles: Wetting Dependence of the Local Fluid Thermodynamics and Kinetics of Nucleation.

Plasmonic nanobubbles are composite objects resulting from the interaction between light and metallic nanoparticles immersed in a fluid. Plasmonic nanobubbles have applications in photothermal therapies, drug delivery, microfluidic manipulations, and solar energy conversion. Their early formation is, however, barely characterized due to the short time and length scales relevant to the process. Here, we investigate, using molecular dynamics (MD) simulations, the effect of nanoparticle wettability on both the local fluid thermodynamics and the kinetics of nanobubble generation in water. We first show that the local onset temperature of vapor nucleation decreases with the nanoparticle/water interfacial energy and may be 100 K below the water spinodal temperature in the case of weak nanoparticle/water interactions. Second, we demonstrate that vapor nucleation may be slower in the case of weak water/nanoparticle interactions. This result, which is qualitatively at odds with the predictions of isothermal classical nucleation theory, may be explained by the competition between two antagonist effects: while, classically, hydrophobicity increases the vapor nucleation rate, it also penalizes interfacial thermal transfer, slowing down kinetics. The kinetics of heat transfer from the nanoparticle to water is controlled by the interfacial thermal conductance. This quantity turns out not only to decrease with the nanoparticle hydrophobicity but also drops down prior to phase change, yielding even longer nucleation times. Such conclusions were reached by considering the comparison between MD and continuous heat transfer models. These results put forward the role of nanoparticle wettability in the generation of plasmonic nanobubbles observed experimentally and open the path to the control of boiling using nanopatterned surfaces.

Pickering emulsions stabilized by chitosan-flaxseed gum-hyaluronic acid nanoparticles for controlled topical release of ferulic acid

Chitosan (CS) based nanoparticles (NPs) were fabricated via an ionic gelation reaction modified by flaxseed gum (FG) or sodium tripolyphosphate (STPP). The average particle size, morphology, interfacial tension, and wettability of NPs were characterized. The particle size of CS-STPP-HA (hyaluronic acid)-FA (ferulic acid) NPs and CS-FG-HA-FA NPs was 400.8 nm and 262.4 nm, respectively under the optimized conditions of CS/STPP = 5:1 (w/w) or CS/FG = 1:1 (v/v) with HA concentration of 0.25 mg/mL and FA dosage of 25 μM. FG acted as a good alternative for STPP to form particles with CS in stabilizing Pickering emulsion with an internal diacylglycerol (DAG) phase of 50–80 % (v/v). The complex nanoparticles had high surface activity and contact angle close to 90 °C, being able to tightly packed at the droplet surface. The emulsions had high thermal, ionic and oxidative stability. With the aid of moisturizing polysaccharides and DAG oil, the emulsions had a good sustained-release ability for FA with deeper penetration and retention into the dermis of the skin. Thus, FG and HA-based NPs serve as green vehicles for the fabrication of novel Pickering emulsions and possess great potential to be applied as a delivery system for lipophilic active agents in functional food and cosmetic products.

Complex bio-nanoparticles assembled by a pH-driven method: environmental stress stability and oil-water interfacial behavior.

Protein-based nanoparticles have gained considerable interest in recent years due to their biodegradability, biocompatibility, and functional properties. However, nanoparticles formed from hydrophobic proteins are prone to instability under environmental stress, which restricts their potential applications. It is therefore of great importance to develop green approaches for the fabrication of hydrophobic protein-based nanoparticles and to improve their physicochemical performance. Gliadin/shellac complex nanoparticles (168.87 ~ 403.67 nm) with various gliadin/shellac mass ratios (10:0 ~ 5:5) were prepared using a pH-driven approach. In comparison with gliadin nanoparticles, complex nanoparticles have shown enhanced stability against neutral pH, ions, and boiling. They remained stable under neutral conditions at NaCl concentrations ranging from 0 to 100 mmol L-1 and even when boiled at 100 °C for 90 min. These nanoparticles were capable of effectively reducing oil-water interfacial tension (5 ~ 11 mNm-1 ) but a higher amount of shellac in the nanoparticles compromised their ability to lower interfacial tension. Moreover, the wettability of the nanoparticles changed as the gliadin/shellac mass ratio changed, leading to a range of three-phase contact angles from 52.41° to 84.85°. Notably, complex nanoparticles with a gliadin/shellac mass ratio of 8:2 (G/S 8:2) showed a contact angle of 84.85°, which is considered suitable for the Pickering stabilization mechanism. Moreover, these nanoparticles exhibited the highest emulsifying activity of 52.42 m2 g-1 and emulsifying stability of 65.33%. The findings of the study revealed that gliadin/shellac complex nanoparticles exhibited excellent resistance to environmental stress and demonstrated superior oil-water interfacial behavior. They have strong potential for further development as food emulsifiers or as nano-delivery systems for nutraceuticals. © 2023 Society of Chemical Industry.

Partially miscible droplet microfluidics to enhance interfacial adsorption of hydrophilic nanoparticles for colloidosome synthesis

Colloidosomes are spherical shells composed of closed-packed nanoparticles, which can be prepared through the self-assembly of nanoparticles at the interphase of two immiscible liquids. However, the method leads to colloidosomes of various sizes and limited synthesis due to charge repulsion and nanoparticle wettability. Herein, a microfluidic droplet device and partially miscible liquids are integrated for colloidosome synthesis. Dissolving liquids within droplets enables hydrophilic nanoparticles to close packing at the liquid interface to form colloidosomes. Moreover, the microfluidic droplet system achieved colloidosomes within a narrow and specific range of sizes. The close-packing of nanoparticles is proved by modifying colloidosomes into probes in signal-enhanced lateral flow assays (LFAs). Due to the plasmon resonance effect, the testing lines are black, demonstrating the close packing of nanoparticles and their signal enhancement property compared to traditional LFAs. Accordingly, this approach is a potential bacterial detection platform with a detection time of 10 min, higher sensitivity, and specificity. Moreover, since colloidosomes are hollow spheres, they are capable of drug cargo with an encapsulation rate of 99.76 %. With this proposed partially miscible fluid-based droplet microfluidic system, colloidosomes can be fabricated with versatile properties, extending their application to advanced material fabrication, drug encapsulation, and biosensing.

Effective stabilization of pickering emulsion using a smart material with V-shaped surface structure via controlled polymerization: Its multi-responsive behavior, recyclable process and applications

Poly (2-(dimethylaminoethyl methacrylate)-b-(glycidyl methacrylate)-b-(acrylic acid)) blocks prepared by RAFT polymerization were grafted to the surface of Fe3O4 to obtain four-responsive nanocomposite particles (P(DMAEMA-b-GMA-b-AA)/Fe3O4) with V-shaped surface structure. Subsequently, the structure and properties of the P(DMAEMA-b-GMA-b-AA)/Fe3O4 nanoparticles were characterized by FT-IR, TEM and TGA. Moreover, various factors on P(DMAEMA-b-GMA-b-AA)/Fe3O4 Pickering emulsion stability were investigated including pH, nanoparticle concentration, water–oil ratio and oil phase polarity. Notably, its superiority was demonstrated by the formation of the Pickering emulsion that underwent CO2, temperature, magnetic and pH-induced reversible emulsification/demulsification for at least 10 cycles without obvious performance decay or even morphology changes. It was demonstrated that the emulsification/demulsification mechanisms under the action of temperature, CO2, and temperature might be related to the surface wettability of nanoparticles by measuring zeta potential, interfacial tension and contact angle. Furthermore, the P(DMAEMA-b-GMA-b-AA)/Fe3O4 nanoparticles exhibited environmentally friendly behavior and could be easily recycled and further utilized through magnetic force. They were also found to stabilize diverse real oil phases. Therefore, the developed nanoparticles have significant potential for applications in oily wastewater treatment, oil transfer, and design of crude oil delivery systems.

Water wettability alternation of CO2-water-shale system due to nanoparticles: Implications for CO2 geo-storage

The wettability of rocks is closely related to the CO2 geo-storage (CGS) security after CO2 injection into the reservoir. Nanofluids can be used to efficaciously alter the wettability of the CO2-water-rock system toward an advantageous state (water-wet) for CGS. In this study, we investigated the potential of SiO2 and Al2O3 nanofluids at different aging time and concentrations to retain the wettability reversal of CO2-exposed shale from the Sichuan Basin after supercritical carbon dioxide (ScCO2) exposure. Fourier Transform Infrared Spectrometer and field emission scanning electron microscopy tests were performed to evaluate the variations of functional groups and surface topography of shale. Results indicated that both nanoparticles efficiently retain the wettability reversal of shale after a period of ScCO2 treatment, which is conducive to structural trapping in the CGS project. The efficiency was improved by increasing the nanofluid aging time and concentrations, while beyond a certain value, the contact angle value changed trivially. Quasi-homogeneously distributed on the shale surface in ScCO2 may contribute to the wettability alteration of shale in ScCO2, whereas there are still removals of nanoparticles in some sites. The variations of functional groups suggest that the existence of nanoparticles may mildly expedite the CO2 mineralization, signifying the change of wetting behavior of shale in ScCO2 depends on the wettability of nanoparticles, and the nanofluids treatment may exert a positive influence on mineral trapping in the CGS project. The findings in this research provide an important basis for raising the CO2 geological storage security and potential via nanofluids.

Molecular Dynamics Study of Energy Transport Mechanism in Nanofluids: Spatial and Component Decompositions of Effective Thermal Conductivity

Nanofluids, which are nanoparticle-suspended fluids, have gathered extensive attention for decades. Yet, the energy transport mechanism of nanofluids has not been comprehensively understood. The present study employs spatial and component decompositions of the effective thermal conductivity (ETC), and numerically investigates the energy transport mechanism of nanofluids, with a focus on influence of liquid layering (i.e., the adsorption layer of the fluid molecules) around nanoparticles. The local ETC of the nearest adsorption layer increases as the nanoparticle wettability improves, but this value has a certain upper limit. Compared with the case of the nearest adsorption layer, the local ETC of the liquid, except for the adsorption layers, gently increases as the nanoparticle wettability improves. However, a main contribution to the ETC component contributed by the liquid is the change in the local ETC of the liquid, except for the adsorption layers, since its volume fraction is relatively large. In conclusion, as a whole, the nanofluid’s ETC is mainly determined by the change in the ETC component contributed by the liquid.

Wettability of Nanostructured Transition-Metal Oxide (Al2O3, CeO2, and AlCeO3) Powder Surfaces.

Wettability has been the focal point of many studies in metal oxide materials due to their applications in water–gas shift reactions, organic reactions, thermochemical water splitting, and photocatalysis. This paper presents the results of systematic experimental studies on the wettability of surfaces of nanostructured transition-metal oxides (TMOs) (Al2O3, CeO2, and AlCeO3). The wettability of nanoparticles was investigated by measuring contact angles of different concentrations of water-based nanofluids (0.05–0.1 wt%) on the glass slide. The morphology, the heterostructure, and the nature of incorporated nanoparticles were confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Characteristic diffraction patterns of the nanomaterials were evaluated using energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) techniques. The contact angles of water–Al2O3, water–CeO2, and water–AlCeO3 were measured as 77.5 ± 5°, 89.8 ± 4°, and 69.2 ± 1°, respectively. This study suggests that AlCeO3 is strongly water-wet (hydrophilic), while CeO2 is weakly water-wet (hydrophobic). It further demonstrated that the sizes and compositions of the nanoparticles are key parameters that influence their wetting behaviors.

Co-encapsulation of chlorogenic acid and cinnamaldehyde essential oil in Pickering emulsion stablized by chitosan nanoparticles.

Most of the current research only explored the loading of an active substance in active packaging. In this study, cinnamaldehyde essential oil (CEO) and chlorogenic acid (CA) were co-encapsulated in chitosan (CS) nanoparticles based Pickering emulsion. The morphology and wettability of CS-CA particles were determined. In addition, physicochemical characterizations and stability of the Pickering emulsion were also investigated. Results showed that the wettability of nanoparticles was improved with increasing the ratios of CS to CA, which is helpful to stabilize the emulsion. CEO Pickering emulsion was stabilized by CS-CA nanoparticles and CEO emulsion showed the best stability by using CS-CA nanoparticles with the ratios of CS to CA 1:0.75 with the minimum creaming index value of 26.5±4.6% after 5days of storage. These overall results presented in this work demonstrate, for the first time, the potential of Pickering emulsion for the co-encapsulation of water-soluble and water-insoluble ingredients.

Assessment of nanoparticle immersion depth at liquid interfaces from chemically equivalent macroscopic surfaces

HypothesisWe test whether the wettability of nanoparticles (NPs) straddling at an air/water surface or oil/water interface can be extrapolated from sessile drop-derived macroscopic contact angles (mCAs) on planar substrates, assuming that both the nanoparticles and the macroscopic substrates are chemically equivalent and feature the same electrokinetic potential. ExperimentsPure silica (SiO2) and amino-terminated silica (APTES-SiO2) NPs are compared to macroscopic surfaces with extremely low roughness (root mean square [RMS] roughness ≤ 2 nm) or a roughness determined by a close-packed layer of NPs (RMS roughness ∼ 35 nm). Equivalence of the surface chemistry is assessed by comparing the electrokinetic potentials of the NPs via electrophoretic light scattering and of the macroscopic substrates via streaming current analysis. The wettability of the macroscopic substrates is obtained from advancing (ACAs) and receding contact angles (RCAs) and in situ synchrotron X-ray reflectivity (XRR) provided by the NP wettability at the liquid interfaces. FindingsGenerally, the RCA on smooth surfaces provides a good estimate of NP wetting properties. However, mCAs alone cannot predict adsorption barriers that prevent NP segregation to the interface, as is the case with the pure SiO2 nanoparticles. This strategy greatly facilitates assessing the wetting properties of NPs for applications such as emulsion formulation, flotation, or water remediation.

Effects of the surface wettability of nanoparticles on the impact dynamics of droplets

Herein, we report an experimental investigation on droplet impact dynamics with a particular interest in the effects of hydrophilicity and hydrophobicity of particles on the impact dynamics. Remarkable contact line pinning and receding breakup were observed when hydrophobic particles were present in the droplets, while hydrophilic particles showed little effect on the dynamics. Furthermore, the maximum retraction height and vertical oscillatory relaxation time for the droplet were restrained by hydrophobic particles. Scanning electron microscopy and rheological analyses revealed that these phenomena may be related to the adsorption and deposition of hydrophobic particles near the three-phase contact line, which leads to an increase in local viscosity and friction near the edge of the drop. In addition, the contact line pinning is positively associated with the Weber number and particle concentration. Our work provides a novel approach for droplet impact dynamics control, particularly for suppressing the oscillation and contraction of droplets.

Surface Wettability of Nanoparticle Modulated Sonothrombolysis.

Sonodynamic therapy (SDT) is a non-invasive and highly penetrating treatment strategy under ultrasound irradiation. However, uncertainty in the mechanism of SDT has seriously hindered its future clinical application. Here, the mechanism of SDT enhanced by the wettability of nanoparticles is investigated. Nanoparticles can adsorb and stabilize nanobubbles in liquid, thus enhancing SDT efficiency. The stability of the nanobubbles is positively correlated with the desorption energy of the nanoparticles, which is determined by the wettability of the nanoparticles. This conclusion is verified for mesoporous silica and polystyrene nanoparticles and it is found that nanoparticles with a water contact angle of about 90° possess the largest desorption energy. To further apply this conclusion, thrombus models are constructed on rats and the experimental results demonstrate that nanoparticles with the largest desorption energy have the highest thrombolytic efficiency. It is believed that these findings will help to better understand the SDT mechanism and guide new strategies for rational design of nanoparticles adopted in SDT.

Atomistic Insight into the Effects of Depositional Nanoparticle on Nanoscale Liquid Film Evaporation.

Nanoscale liquid film evaporation plays an essential role in many engineering applications. This study carries out molecular dynamics simulations on the effects of the depositional nanoparticle's wettability and volume in base fluid on the evaporation process to understand how the depositional nanoparticle affects the evaporation heat transfer. Increasing the nanoparticle's wettability can enhance the evaporation heat transfer process, and the enhancement effect of the hydrophobic surface is more remarkable than that of the hydrophilic surface. This because the increasing wettability causes more significant solid-liquid interaction. However, the potential energy of argon atoms at the liquid-vapor interface is almost unaffected by wettability. Moreover, when the depositional nanoparticle locates below the free liquid film, increasing the nanoparticle volume has a better heat transfer performance. As the volume increases, the heat transfer through the nanoparticle becomes more obvious, which effectively enhances the heat transfer at the solid-liquid interface and the liquid-vapor interface. The latent heat of phase change at the liquid-vapor interface is almost unchanged so that the evaporation can be enhanced. This research provides an understanding of the effects of depositional nanoparticles on nanoscale evaporation, which can impact several engineering applications, including devices' cooling and fluid transport.

Toward mechanistic understanding of asphaltene adsorption onto quartz surface: The roles of size, concentration, and hydrophobicity of quartz, asphaltene composition, flow condition, and aqueous phase

The nature of asphaltene and mineralogy of reservoir rock affect the adsorption of asphaltene onto the rock surface. The basic goal of the present study is to assess the adsorption of different asphaltenes on quartz as the main mineral of sandstone formations. For this purpose, asphaltenes were separated from four crude oil samples from different Iranian oil reservoirs and their specifications were analyzed applying various methods including Fourier-transform infrared spectroscopy, elemental analysis, dynamic light scattering, and field emission scanning electron microscopy. Also, X-ray fluorescence and Brunauer–Emmett–Teller methods were carried out to analyze the quartz mineral. Adsorption experiments were performed both statically and dynamically and the effects of the water phase and hydrophobicity of quartz on the adsorption of asphaltene on the mineral were evaluated. The calculated H/C ratios of asphaltenes are in the range of 1.2–1.3, and their average particle size is between 46 and 380 nm. The results of static tests showed that asphaltenes uptake by quartz adsorbents is from 1.16 to 6.78 mg/m2. The results of this work indicated that the adsorption amount of asphaltenes on the quartz surface is directly related to the nitrogen content and polarity of asphaltene, but it is not related to the average particle diameter of asphaltenes and their aromatic nature. By double increasing the flow rate in the dynamic adsorption process, the asphaltene uptake by quartz is reduced by about 35%, which could be an indication of the physical nature of asphaltene adsorption on quartz. Finally, compared to the asphaltene uptake by quartz in the two-phase system, the uptake in the three-phase system on average shows a decrease of more than 30%, which indicates the effect of the competitive presence of water with asphaltene for the surface-active sites. Also, the uptake of asphaltene onto the quartz samples increased with increasing the initial asphaltene concentration and decreasing quartz particle sizes. The effect of wettability on asphaltene adsorption in two- and three-phase systems was investigated using hydrophilic and hydrophobic quartz nanoparticles. In both systems, the adsorption of asphaltene on hydrophobic nanoparticles was higher and the adsorption reduction in the three-phase system was calculated to be between 5 and 10%, which was very small compared to hydrophilic nanoparticles with an adsorption reduction of 30–60%. Wettability of nanoparticles compared to their specific surface area showed more importance in asphaltene adsorption.

Molecular dynamics simulation of explosive boiling of the H2O-Cu nanofluid

In this paper, molecular dynamics simulations are used to study the effect of the Cu nanoparticle and wall surface wettability on the explosive boiling of H2O-Cu nanofluids. The influence of the nanoparticle wettability, the wall surface wettability and the size of nanoparticles on explosive boiling is analyzed. The results show that when the wall superheat is the same, the hydrophilic nanofluid will explode fastest on the hydrophilic surface. The nanoparticle enhances the heat transfer during the boiling process, and the enhancement is more significant on the hydrophobic surface. The increase in the wettability of the nanoparticle reduces the thermal resistance of the solid-liquid interface, thereby enhancing heat transfer. The potential energy of the solid-liquid interface increases with the decrease of the particle size, which also enhances the solid-liquid heat transfer capability. In addition, the temperature gradient, heat exchange and evaporation rate of the hydrophilic surface per unit time are greater than those of the hydrophobic surface.

Hydrophobic Modification on the Surface of SiO2 Nanoparticle: Wettability Control.

Good control of the morphology, particle size, and wettability of silica nanoparticles is of increasing importance to their use in a variety of fields. Here, we propose a strategy to tune the surface wettability of nanosilica by changing the dosage of a chemical modifier. A series of measurements, including scanning electron microscopy (SEM), laser scatting technique, Fourier transform infrared (FTIR) spectroscopy, thermogravimetry, and surface hydroxyl number and water contact angle measurement, were conducted to verify the surface chemistry and wettability of these nanoparticles. Through controlled chemical modification, the contact angle of the treated nanoparticles increases from 34.7 to 155° with increasing amount of dichlorodimethylsilane (DCDMS) within a molar ratio (MR) between DCDMS and nanoparticles of 5.17. The number of hydroxyl groups covered on the particle surface decreases gradually from 1.79 to 0.47, and the surface grafting rate could reach 73.7%. As the addition of dichlorodimethylsilane equals MR 5.17, the contact angle reaches the maximum value of 155°, which displays excellent superhydrophobicity. After surpassing the point of MR 5.17, the contact angle does not increase but starts to decrease, ultimately remaining stable at 135°. It can be concluded that the surface wettability of nano-SiO2 particles can be precisely modulated by varying the amounts of the modifier. Furthermore, the modulating mechanism of the process occurring on the surface of SiO2 particles has been investigated at the molecular level.

Effects of depositional nanoparticle wettability on explosive boiling heat transfer: A molecular dynamics study

The depositional nanoparticles have a significant effect on the solid/liquid interaction, which affects the heat transfer rate across the interface. In this paper, molecular dynamics simulations are employed to investigate the explosive boiling behaviors of nanofluids with the depositional nanoparticles, and the varied nanoparticle wettability is analyzed to reveal the mechanism through which depositional nanoparticles affect explosive boiling characteristics. Results show that heat transfer of explosive boiling is enhanced by the depositional nanoparticles, and the improvement in heat transfer increases with the increasing nanoparticle wettability. One of the enhancement mechanism of heat transfer is the increased solid/liquid interaction induced by the depositional nanoparticle, which provides more favorable conditions for heat transfer. Another enhancement mechanism is that the thermal resistance at the solid/liquid interface decreases, resulting in more energy transfer across the solid/liquid interface. Furthermore, the enhancement of heat transfer on the hydrophobic surface is more remarkable than that on the hydrophilic surface.

Influence of non-ionic surfactant addition on the stability and rheology of particle-stabilized emulsions

HypothesisNanoparticle wettability is expected to influence the surfactant adsorption onto an oil-water interface, which in turn would affect the structure and rheology of Pickering emulsions. ExperimentsWe investigated the influence of non-ionic surfactant (Span 80) addition to Pickering emulsions stabilized by either partially hydrophobic (Aerosil R711) or hydrophobic (Aerosil R812S) fumed silica particles. FindingsThe addition of a non-ionic surfactant to emulsions stabilized by hydrophobic fumed silica particles decreased the droplet size of emulsions unlike emulsions stabilized by partially hydrophobic fumed silica particles. Even under low shear conditions, the addition of a non-ionic surfactant displaced the hydrophobic fumed silica particles from the oil-water interface unlike partially hydrophobic fumed silica particles. Our results suggested that the displacement of hydrophobic fumed silica particles by the addition of a non-ionic surfactant was not due to the change in nanoparticle wettability by surfactant adsorption onto the surface of fumed silica particles. Viscosity of emulsions stabilized by partially hydrophobic fumed silica particles decreased upon the addition of a non-ionic surfactant. In contrast, the viscosity of emulsions stabilized by hydrophobic fumed silica particles increased upon the addition of a non-ionic surfactant. Depending on the nanoparticle wettability, the addition of a non-ionic surfactant influenced the elastic and viscous properties of Pickering emulsions.

The effect of silanization assisted nanoparticle hydrophobicity on emulsion stability through droplet size distribution analysis

There has been great recent interest in the potential use of Nanoparticles as modifiers or alternatives to surfactants for stabilizing emulsions, among which silica NPs have been commonly used as the most cost effective option. Since silica NPs are hydrophilic with a tendency toward the aqueous phase, their surface should be activated either by the aid of a surfactant or by chemical treatment. The aim of the present work is to assess the effect of two chemically activated silica NPs on stability of dodecane-water emulsions through optical microscopy. Therefore, hydrophilic silica NPs are firstly synthesized with sodium silicate solution and are subsequently silanized with hexamethyldisilazane (HMDS) into different extents to synthesize two different NPs with different wettability states, namely as HSNP1 and HSNP2. The degree of Nanoparticle wettability is quantified by the variation of the silanol group density of bare silica NPs, which has reduced by 49% for HSNP1, resulting in partially hydrophobic silica NPs and by 60% for HSNP2, resulting in hydrophobic silica NPs. The emulsion stability prepared by HSNP1 and HSNP2 has been assessed via both bottle tests and droplet size distribution analysis, which is tracked by fitting proper distribution functions on the experimental data. The log-normal and the bimodal (a mixture of two log-normal distribution) distribution functions are respectively fitted on the droplet size distribution of emulsions stabilized by HSNP1 and HSNP2. This revealed minor changes in the droplet size distribution of emulsions stabilized by partially hydrophobic silica NPs (HSNP1) over a month, while a significant alteration in droplet size distribution of emulsions stabilized by hydrophobic silica NPs (HSNP2) has been observed after a week. Higher emulsion stability stabilized by HSNP1 is also confirmed by higher zeta potential value and higher surface coverage of partially hydrophobic silica NPs (HSNP1) compared to that of hydrophobic silica NPs (HSNP2). The partially hydrophobic silica NPs, wetted to the same extent by both liquid phases has been found to be suitable for stabilizing emulsions for Enhanced Oil Recovery (EOR) applications.