Thermally and pH-responsive gelation of nanoemulsions stabilized by weak acid surfactants

Temperature-Insensitive Nonpolar Suspensions of Polyoxyethylene Alkyl Ether-Grafted Silica Nanoparticles

Nonpolar suspensions of organically modified particles exhibit a strong temperature sensitivity owing to the high-temperature-induced desorption/decomposition and the low-temperature-induced disorder/order conformational transition of the modifiers. This strong temperature sensitivity limits their applications, such as lubricants and oil-based drilling fluids, which require the suspensions to operate over a wide temperature range (e.g., 0-200 °C). We hypothesize that the introduction of a flexible ethylene oxide (EO) chain into the modifiers can disrupt the low-temperature-induced ordered conformation to improve the stability of the nonpolar suspensions. In this article, nonpolar suspensions with temperature insensitivity in the range of 5-160 °C were obtained via the covalent modification of silica NPs and the introduction of EO chains into the modifier molecules. Here, octadecyl-grafted silica NPs (C18-SiO2) and polyoxyethylene alkyl ether-grafted silica NPs (AEOn-SiO2) were synthesized and subsequently dispersed in mineral oil. The rheological properties of each suspension at different temperatures were evaluated, and the thermal stability of AEOn-SiO2 in mineral oil was investigated along with the conformational changes of the grafted chains. In the temperature range of 5-160 °C, the apparent viscosity and gel strength of the C18-SiO2 suspension changed dramatically, whereas the AEOn-SiO2 suspensions exhibited constant rheological properties over this temperature range. This temperature insensitivity of AEOn-SiO2 suspensions is attributed to the excellent thermal stability of AEOn-SiO2 in mineral oil and the disordered conformation of the EO chains upon cooling. This study provides a novel approach to preparing temperature-insensitive nonpolar suspensions, which have potential applications in the petroleum and lubricant industries.

Nanoemulsions Stable against Ostwald Ripening

Ostwald ripening, the dominant mechanism of droplet size growth for an O/W nanoemulsion at high surfactant concentrations, depends on micelles in the water phase and high aqueous solubility of oil, especially for spontaneously formed nanoemulsions. In our study, O/W nanoemulsions were formed spontaneously by mixing a water phase with an oil phase containing fatty alcohol polyoxypropylene polyoxyethylene ether (APE). By monitoring periodically the droplet size of the nanoemulsions via dynamic light scattering, we demonstrated that the formed O/W nanoemulsions are stable against Ostwald ripening, i.e., droplet growth. In contrast, the nanoemulsion droplets grew with the addition of micelles, demonstrating the pivotal role of the presence of micelles in the water phase in the occurrence of Ostwald ripening. The influence of the initial phase of APE, the oil or water phase in which APE is present, on the micelle formation is discussed by the partition coefficient and interfacial adsorption of APE between the oil and water phase using a surface and interfacial tensiometer. In addition, the spontaneously formed O/W nanoemulsion, which is stable against Ostwald ripening, can be used as a nanocarrier for the delivery of water-insoluble pesticides. These results provide a novel approach for the preparation of stable nanoemulsions and contribute to elucidating the mechanism of instability of nanoemulsions.

Molecular Simulations on the Coalescence of Water-in-Oil Emulsion Droplets with Non-ionic Surfactant and Model Asphaltene

Water droplets in crude oil can be stabilized by the adsorption of interfacially active components, such as asphaltenes. Demulsifiers like non-ionic surfactants are commonly used to destabilize the water-in-oil emulsions. In this work, molecular dynamics simulations and free energy calculations were performed to study the coalescence of water droplets coated with both model asphaltene and non-ionic surfactants [PEO-PPO-PEO copolymer (SurP) or Brij surfactant (SurB)]. For the first time, we quantitatively studied the interaction force between water droplets in the presence of both asphaltenes and demulsifiers and addressed the effect of solvent property on the coalescence process. At the droplet surface, demulsifiers adsorbed closer to the water phase and formed more hydrogen bonds with water molecules compared to asphaltenes, indicating the capability of demulsifiers to break the asphaltene film. Comparing the two non-ionic surfactants, VO-79/SurP complexes formed a single-layer film on the droplet surface, while a two-layer structure was formed by VO-79/SurB complexes. This led to a higher repulsive force during droplet coalescence when SurB was present, regardless of the type of solvent. Comparing the two different solvents (toluene vs heptane), for the same adsorbates, the interfacial film was more compact in heptane and there were fewer dispersed VO-79. For VO-79/SurB adsorbates, the bridging of VO-79 led to a smaller repulsion during droplet coalescence when the solvent was heptane, while the difference is insignificant for VO-79/SurP adsorbates. This work suggests that the energy barrier and interaction force for droplet coalescence is highly dependent on the structure of interfacial films, thus providing atomic-level insights into the demulsification mechanisms of water-in-oil emulsions in the presence of surface-active asphaltenes.

Essential Oil Stabilisation by Response Surface Methodology (RSM): Nanoemulsion Formulation, Physicochemical, Microbiological, and Sensory Investigations

This manuscript aimed to optimise the encapsulation of Thymus capitatus essential oil into nanoemulsion. Response Surface Methodology results were best fitted into polynomial models with regression coefficient values of more than 0.95. The optimal nanoemulsion showed nanometer-sized droplets (380 nm), a polydispersity index less than 0.5, and a suitable Zeta potential (−10.3 mV). Stability results showed that nanoemulsions stored at 4 °C were stable with the lowest d3,2, PolyDispersity Index (PDI), and pH (day 11). Significant ameliorations in the capacity to neutralise DPPH radical after the encapsulation of the antimicrobial efficacy of thyme essential oil were recorded. S. typhimurium growth inhibition generated by nanoencapsulated thyme essential oil was 17 times higher than by bulk essential oil. The sensory analysis highlighted that the encapsulation of thyme essential oil improved enriched milk’s sensory appreciation. Indeed, 20% of the total population attributed a score of 4 and 5 on the scale used for milk enriched with nanoemulsion. In comparison, only 11% attributed the same score to milk enriched with bulk essential oil. The novel nanometric delivery system presents significant interest for agroalimentary industries.

Solid lipid nanoparticles and nanoemulsions with solid shell: Physical and thermal stability

HYPOTHESIS

Nanoemulsions (NE) and solid lipid nanoparticles (SLN) used for drug delivery should have a solid shell to be stable during long shelf life and become liquid at human body temperature. The core components of lipid nanoparticles can be partially incorporated into the shell and affect the physical and thermal stability.

EXPERIMENTS

We prepared NE and SLN by the phase inversion temperature (PIT) method. Solidification of the surfactants Tween60 and Span 60 on the surface of NE droplets with paraffin oil resulted in the formation of the solid shell. SLN contained stearic acid in the core and the same surfactants in the solid shell. The size, structure and stability of the NE and SLN were studied by DLS and cryo-TEM. Their crystallization and melting were analyzed using DSC.

FINDINGS

The lipid nanoparticles were resistant to aggregation and sedimentation and hold up to at least two cycles of heating to 50-60 °C and subsequent cooling to 5 °C, even though the upper temperatures were higher than the melting point of the surfactant shell. The expected liquid core/solid shell morphology of NE was confirmed. SLN were composed of a semi-liquid core of supercooled stearic acid melt and coated with a solid surfactant shell, so they can be treated as NE. Stearic acid molecules penetrated the shell, leading to an increase in its melting point.

Dynamic Duo: Vibrational Sum Frequency Scattering Investigation of pH-Switchable Carboxylic Acid/Carboxylate Surfactants on Nanodroplet Surfaces

Surfactants containing pH-switchable, carboxylic acid moieties are utilized in a variety of environmental, industrial, and biological applications that require controlled stability of hydrophobic droplets in water. For nanoemulsions, kinetically stable oil droplets in water, surface adsorption of the anionic form of the carboxylic acid surfactant stabilizes the droplet, whereas a dominant surface presence of the neutral form leads to destabilization. Through the use of dynamic light scattering, ζ-potential, and vibrational sum frequency scattering spectroscopy (VSFSS), we investigate this mechanism and the relative surface population of the neutral and charged species as pH is adjusted. We find that the relative population of the two surfactant species at the droplet surface is distinctly different than their bulk equilibrium concentrations. The ζ-potential measurements show that the surface concentration of the charged surfactant stays nearly constant throughout the stabilizing pH range. In contrast, VSFSS shows that the neutral carboxylic acid form increasingly adsorbs to the surface with increased acidity. The spectral features of the headgroup vibrational modes confirm this behavior and go further to reveal additional molecular details of their adsorption. A significant hydrogen-bonding interaction occurs between the headgroups that, along with hydrophobic chain-chain interactions, assists in drawing more carboxylic acid surfactant to the interface. The charged surfactant provides the stabilizing force for these droplets, while the neutral surfactant introduces complexity to the interfacial structure as the pH is lowered. The results are significantly different than what has been found for the planar oil/water studies where stabilization of the interface is not a factor.

Acetoin modulates conformational change of surfactin: Interfacial assembly and crude oil-washing performance

Due to its special structure, the cyclic lipopeptide surfactin showed remarkable responsiveness to stimuli such as pH, temperature and metal ions. However, few studies investigated the effect of fermented by-products on the conformational change and interfacial assembly of surfactin. Here, the effect of acetoin, a primary metabolite of Bacillus subtilis, on the conformational change and interfacial assembly of surfactin was studied in detail. Surface tension measurements showed that the critical micelle concentration (CMC) of surfactin increased from 1.14 × 10^-5 to 4.32 × 10^-5 M in the presence of acetoin. Moreover, acetoin has increased the interfacial tension of surfactin aqueous solution-crude oil from 1.08 mN/m to 3.01 mN/m. Circular dichroism (CD) spectra and dynamic light-scattering (DLS) further demonstrated that acetoin had induced the conformational transition of surfactin from β-sheet to β-turn structure, and caused surfactin forming some larger micelle aggregations. Afterwards, it was further found that acetoin decreased the oil sand cleaning efficiency of surfactin from 59.7% to 6.6%, and deteriorated the O/W emulsion stability and altered the silicate wettability toward less water wet state. Based on the experimental results, a possible mechanism of the interaction between surfactin and acetoin was proposed.

Nutraceutical delivery through nano-emulsions: General aspects, recent applications and patented inventions

Nanostructured emulsions have a significant potential for encasing, transport and delivery of hydrophilic and lipophilic nutraceuticals and other bioactive compounds by providing enhanced stability and functionality in food and pharmaceutical applications. As highlighted in recent researches, essential fatty acids (EFA) and oils (EO), antioxidants, vitamins, minerals, pro and prebiotics, and co-enzymes, are common bioactives encapsulated in nanoscale delivery systems in order to protect them from degradation during processing and storage, and to improve bioavailability after their consumption. Nanoemulsions (NEs) as delivery systems for nutraceuticals comprise either oil-in-water (O/W) or water-in-oil (W/O) biphasic dispersion with nano-sized droplets, which are stabilized through an active surfactant. Both high- and low- energy methods are used to produce well-structured and stable NEs with advanced structural and rheological features. The in vitro and in vivo studies are focused to assess the nutraceutical releasing profile, gastrointestinal transportation and cytotoxicity of nutraceutical loaded NE. Within the last three decades, a number of NE systems have been developed for certain purposes and submitted for patent approval. Currently, there are many issued patents published as well as and applications under process. This review focus on the current status of food-grade NEs in terms of formation, characterization, relevant applications of nutraceutical delivery, and the recent developments including patented systems.

New insights into unusual droplets: from mediating the wettability to manipulating the locomotion modes

The ability to manipulate droplets can be utilized to develop various smart sensors or actuators, endowing them with fascinating applications for drug delivery, detection of target analytes, environmental monitoring, intelligent control, and so on. However, the stimuli-responsive superhydrophobic/superhydrophilic materials for normal water droplets cannot satisfy the requirements from some certain circumstances, i.e., liquid lenses and biosensors (detection of various additives in water/blood droplets). Stimuli-responsive wetting/dewetting behaviors of exceptional droplets are open issues and are attracting much attention from across the world. In this perspective article, the unconventional droplets are divided into three categories: ionic or surfactant additives in water droplets, oil droplets, and bubble droplets. We first introduce several classical wettability models of droplets and some methods to achieve wettability transition. The unusual droplet motion is also introduced in detail. There are four main types of locomotion modes, which are vertical rebound motion, lateral motion, self-propulsion motion, and anisotropic wettability controlled sliding behavior. The driving mechanism for the droplet motion is briefly introduced as well. Some approaches to achieve this manipulation goal, such as light irradiation, electronic, magnetic, acid-base, thermal, and mechanical ways will be taken into consideration. Finally, the current researches on unconventional droplets extending to polymer droplets and liquid metal droplets on the surface of special wettability materials are summarized and the prospect of unconventional droplet research directions in the field of on-demand transport application will be proposed.

Tuning Material Properties of Nanoemulsion Gels by Sequentially Screening Electrostatic Repulsions and Then Thermally Inducing Droplet Bridging

Nanoemulsions are widely used in applications such as food products, cosmetics, pharmaceuticals, and enhanced oil recovery for which the ability to engineer material properties is desirable. Moreover, nanoemulsions are emergent model colloidal systems because of the ease in synthesizing monodisperse samples, flexibility in formulations, and tunable material properties. In this work, we study a nanoemulsion system previously developed by our group in which gelation occurs through thermally induced polymer bridging of droplets. We show here that the same system can undergo a sol-gel transition at room temperature through the addition of salt, which screens the electrostatic interaction and allows the system to assemble via depletion attraction. We systematically study how the addition of salt followed by a temperature jump can influence the resulting microstructures and rheological properties of the nanoemulsion system. We show that the salt-induced gel at room temperature can dramatically restructure when the temperature is suddenly increased and achieves a different gelled state. Our results offer a route to control the material properties of an attractive colloidal system by carefully tuning the interparticle potentials and sequentially triggering the colloidal self-assembly. The control and understanding of the material properties can be used for designing hierarchically structured hydrogels and complex colloid-based materials for advanced applications.