Solid particles at fluid interfaces : emulsions, liquid marbles, dry oil powders and oil foams

Tyowua, Andrew Terhemen

Abstract

The behaviour of different particle types at oil-water, oil-oil and liquid (and/or oil)-air interfaces in terms of the type of materials they stabilise has been studied. For the oil-water interfaces, oil-in-water o/w Pickering emulsions (composed of tricaprylin or Miglyol 812N and water) were stabilised using rod- and cube-shaped CaCO₃ particles. For the oil-oil interfaces, oil-in-oil o/o and oil-in-oil-in-oil o/o/o Pickering emulsions, comprising solely of sunflower/olive/rapeseed oil and PDMS 20−100 cS, were prepared using fluorinated and hydrocarbon-coated fumed silica, fluorinated ZnO, fluorinated clay (sericite and bentonite), polytetrafluoroethylene PTFE, Bentone 34, rod-shaped CaCO₃ and Calofort SV particles as the stabilising agents. Finally, for the oil-air interfaces, oil liquid marbles, dry oil powders and oil foams were prepared using the fluorinated particles mentioned above as stabilisers. The following summary can be made about these particle-stabilised materials:

The rod- and cube-shaped CaCO₃ particle-stabilised o/w emulsions were white, basic (pH = 8 to 9) and were composed of μm-size polydisperse and flocculated tricaprylin and Miglyol 812N droplets. The stability of the emulsions, containing equal volume fraction of oil and water, to creaming and coalescence increased with increasing concentration of the rod-shaped CaCO₃ particles. Those stabilised by 3−5 wt. % of the particles remained completely stable to creaming for over 3 years, but released a relatively small fraction of the oils. No such stable emulsions were obtained regardless of the particle concentrations when the volume fraction of oil ϕo in the emulsions was reduced to 0.2. When ϕo in the emulsions was varied from 0.95 to 0.05 at a constant particle concentration (4 wt. %), ‘oily liquids’ form when ϕo = 0.95−0.8, relatively stable o/w emulsions form when ϕo = 0.7−0.5 and unstable o/w emulsions form when ϕo < 0.5. Using the relatively stable o/w emulsions (ϕo = 0.5, 4 wt. % of the CaCO3 particles), it was observed that addition of electrolyte and pH adjustment induce coalescence and creaming or sedimentation in the emulsions. The stability of these emulsions was also affected by the method of emulsion preparation and particle shape.

The o/o and o/o/o emulsions were white, polydisperse and unflocculated with the average droplet size ranging from a few mm to μm. The type of emulsions obtained depends on the particle type and % SiOH for the fumed silica particles. Some of the emulsions were kinetically stable for over a month. The kinetic stability of the emulsions increased with increasing particle concentration. At relatively high particle concentrations (≥ 1 wt. %), some of the o/o emulsions (equal volumes of oil) remained stable to creaming/sedimentation and coalescence for over a month. No stable o/o emulsions were obtained when the volume fraction of oil in the emulsions (composed of sunflower oil and PDMS 50 cS) was varied from 0.05 to 0.9 at constant particle concentration (1 wt. %). However, a catastrophic phase inversion was observed as expected. Transitional phase inversion was observed in some cases. The inversion occurred if the viscosity of the PDMS oils in a vegetable oil is varied in the presence of 75% SiOH fluorinated fumed silica particles or the % surface SiOH on the hydrocarbon-coated fumed silica particles in a vegetable oil-PDMS oil combination is varied from 100−14% SiOH. For the hydrocarbon-coated fumed silica particles, multiple emulsions were observed near the point (23 and 20% SiOH) of inversion in some cases. The multiple emulsions occurred before or after inversion. The vegetable oil-PDMS oil-glass three-phase contact angle θ₀₀ values were measured and were seen to correlate with the type of emulsions stabilised by the 100% SiOH and 14% SiOH fumed silica particles.

Many of the liquid marbles stabilised by the fluorinated fumed silica particles remained stable for more than 90 min whilst some of the oil foams remained kinetically stable for up to a year. The formation of these materials was seen to depend on the surface tension γla of the oils, the degree of particle fluorination or apparent surface energy γsa of the particles and energy input. An oil dispersion of particles forms in liquids of relatively low γla with particles of low fluorine content where the apparent advancing particle-liquid-air contact angle θla < 20°. Particle-stabilised oil foams were obtained in oils of relatively high tension (> 32 mN m⁻¹) and particles of moderate fluorine content. The oils of lower tension (e.g. 27 mN m⁻¹) and particles of high fluorine content also formed foams. For foams, the apparent advancing θla was between 82−145°. In many of the oil-particle systems forming foams by vigorous agitation, oil liquid marbles were also stabilised with particles of highest fluorine content encapsulating oils of lower γla.

Unlike the fluorinated fumed silica and fluorinated bentonite clay particles, the fluorinated sericite and fluorinated ZnO particles were able to stabilise dry oil powders in addition to oil liquid marbles and oil foams. Upon vigorous agitation, oil dispersions were obtained in oils of relatively low tension (< 26 mN m⁻¹) and particles of relatively high apparent γsa. For liquids of higher tension and particles of moderately high apparent γsa where the apparent advancing θla is between 73 −130°, foams were obtained. For many of the oils having tension > 27 mN m⁻¹ and particles for which apparent advancing θla > 65°, oil liquid marbles and dry oil powders were obtained. The dry oil powders did not leak oil for over 2 years. The dry oil powders inverted to ultra-stable oil foams above a critical oil : particle ratio COPR.

Citation

Tyowua, A. T. (2014). Solid particles at fluid interfaces : emulsions, liquid marbles, dry oil powders and oil foams. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4221420

• Thesis Type: Thesis
• Deposit Date: Apr 10, 2019
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4221420
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Nov 1, 2014