Kenneth Wong

Selective Retardation of Perfume Oil Evaporation from Oil-in-Water Emulsions Stabilized by Either Surfactant or Nanoparticles

We have used dynamic headspace analysis to investigate the evaporation rates of perfume oils from stirred oil-in-water emulsions into a flowing gas stream. We compare the behavior of an oil of low water solubility (limonene) and one of high water solubility (benzyl acetate). It is shown how the evaporation of an oil of low water solubility is selectively retarded and how the retardation effect depends on the oil volume fraction in the emulsion. We compare how the evaporation retardation depends on the nature of the adsorbed film stabilizing the emulsion. Surfactant films are less effective than adsorbed films of nanoparticles, and the retardation can be further enhanced by compression of the adsorbed nanoparticle films by preshrinking the emulsion drops.

Drop sizes and particle coverage in emulsions stabilised solely by silica nanoparticles of irregular shape

We have investigated emulsions stabilised solely by partially-hydrophobised fumed silica particles which consist of a mixture of primary particles and irregularly-shaped fused aggregates and larger agglomerates. The particle wettability is controlled by varying the extent of hydrophobisation of their surfaces. This, in turn, controls the contact angle between the oil-water interface and the particle surface (θ(ow)) which affects the particle adsorption energy and the type of emulsion formed (oil-in-water, o/w or water-in-oil, w/o). Progressive particle hydrophobisation causes transitional phase inversion of the emulsions from o/w to w/o which occurs when θ(ow) = 90° and the energy of particle adsorption to the oil-water interface is maximally favourable. The key problem addressed here is to understand why the emulsion drop size passes through a minimum at the point of emulsion phase inversion. In principle, this effect could be the result of particle desorption, changes in the extent of close-packing of the adsorbed particle film (at constant particle orientation), particle re-orientation or a combination of these processes. Using measurements of emulsion drop size and the extent of particle desorption, we have derived adsorbed particle surface concentrations as a function of the energy of desorption of the particles from the oil-water interface for a range of particle concentrations and different oil-water systems. The main conclusion is that the minimum in emulsion drop size through phase inversion is mainly caused by re-orientation of the particles from a high surface area orientation when the energy of desorption is high to a low surface area orientation when the energy of desorption is low. Some particle desorption occurs but this is a secondary effect.

Controlled Release of Volatile Fragrance Molecules from PEO-b-PPO-b-PEO Block Copolymer Micelles in Ethanol−Water Mixtures

Active materials that can solubilize in different compartments of a sample show release properties which might be of interest in some applications where a delayed release of solutes for instance is required. We studied perfume solutes in compartments of Pluronic block copolymers of different compositions and molecular weights over a range of ethanol-water mixtures. Phase diagrams were constructed to identify and map micellar phases, then dynamic light scattering was used to characterize the solute-swollen micelles; NMR provided with the partition of solutes between solvent and micelles, and equilibrium constants K(c) were estimated using headspace analysis. Finally solute-evaporation rates were measured by thermogravimetry. We focused on two typical behaviors: when solubilization in a micellar compartment occurs, delayed release increased with K(c). When solubilization was limited or absent, either because no micelles form or, in the presence of micelles, because solubilization was minor or absent, delayed release was correspondingly absent.

Turning Coacervates into Biohybrid Glass: Core/Shell Capsules Formed by Silica Precipitation in Protein/Polysaccharide Scaffolds

Delivery systems with low-permeability barriers and controllable release are crucial for the encapsulation of cells, pharmaceuticals, vitamins, inks, or fragrance and flavor molecules. Core/shell capsules provide a stable microenvironment and protect sensitive chemicals from degradation, undesired reactions, or evaporation. Traditionally, volatile oils have been encapsulated using synthetic polymers. While there is a strong interest in using capsule wall materials of biological origin, their barrier properties for small-molecularweight, highly volatile active ingredients remain inferior to those of polyurea or aminoplast capsules with walls that are produced synthetically. 3] Herein, we describe core/shell capsules with dense walls composed entirely of a biopolymer scaffold interpenetrated by a network of amorphous silica. We first formed a weakly acidic hydrogel shell around an oil drop. This shell then served as a scaffold to induce protein-directed mineralization of silicon dioxide from a liquid-silica precursor. The precipitation process occurring in the hydrogel scaffold consumes water and forms SiO2, yielding dense shells with very low permeability for volatile organic compounds and adjustable mechanical characteristics. Macroscopic biopolymer layers at the oil/water interface can be formed by a process called complex coacervation. For this to occur, micrometer-sized droplets of a polymer-rich aqueous liquid (coacervates) are first formed by associative phase separation between a protein and a weakly anionic polyampholyte (Scheme 1). Unlike polyelectrolyte multilayers and precipitated complexes, coacervate phases remain in the liquid state and are therefore moldable, providing a wide range of design possibilities for composite materials. If a aqueous dispersion of liquid coacervate droplets is mixed with an oil-in-water emulsion, the interfacial energy balance between the three phases (oil/ solvent/coacervate) causes the polymer-rich phase to deposit at the oil/solution interface. This coating process results in a composite emulsion of oil droplets contained within larger coacervate drops. The protein component of the outer coacervate droplet is then gelled, resulting in a physically cross-linked hydrogel wall. However, these traditional biopolymer shells often remain permeable and provide poor protection for the sensitive core materials even after further covalent cross-linking. Moreover, capsules made from classic hydrogels are soft and mechanically compliant, complicating controlled release, for example, upon chewing or rubbing on the skin. Recent efforts to improve the mechanical and stability profiles of encapsulation systems has focused on the design and synthesis of composite structures and alternative wall Scheme 1. The coacervate/silica scaffold-precipitation process. Step 1) Formation of scaffold capsules: complex coacervate droplets form by associative phase separation of a protein and a polyanion (the coacervation corner is indicated in the protein/polyanion/water phase diagram); the coacervate droplets then deposit and coalesce around the core material (e.g. volatile oil). Physical and covalent cross-linking stabilizes the coacervate shell. Step 2) Silica formation in the coacervate shell: The capsules a) are immersed in a liquid silica precursor (TEOS). The coacervate hydrogel b) serves as a mechanical scaffold shell and provides an acidic microenvironment wherein the silica precursor locally hydrolyzes and condenses to form precipitated silica. Additionally, water consumption compresses and densifies the scaffold, resulting in composite capsules (c) with dense silica/ biopolymer shells (d).