Encapsulation of fluids by solid particles

Darragh, Sophie Anne

June 2015

Thesis or dissertation

© 2015 Sophie Anne Darragh. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

This thesis aimed to investigate and understand the properties and behaviour of particles at fluid interfaces. The primary aim of this research was to develop a novel hand sanitiser based on the Fenton reaction, that is, the reduction of hydrogen peroxide into hydroxyl radicals by a metal ion catalyst (traditionally iron). The hydroxyl radicals produced can act as highly efficacious anti-bacterial agents, and thus have important application as a hand sanitiser. This thesis focused on the use of copper sulfate as a skin- friendly and water-soluble ion, suitable as a catalyst for the Fenton reaction due to its multiple valency states. For the reaction in question to be successfully utilised as a hand sanitiser, the two components must be kept apart until the point of use on the skin. Thus to achieve this, dry water technology has been explored, whereby the two aqueous components have been encapsulated separately so that by shearing on the skin surface at the point of use the contents can be released. Much of the research to date on dry water has only been carried out with pure water and no other components in the aqueous phase. In order to generate an understanding of the behaviour of particles at interfaces of varying surface tension, this thesis contains a systematic investigation of many types of silica particles at liquid-air interfaces.

The first study comprised an investigation into the foaming behaviour of two commercial nonionic surfactants. These surfactants are in use in a current formulation (InstantFOAM™) marketed by the industrial sponsor (Deb Group Ltd.). The foaming hand sanitiser contains a high proportion of ethanol, and thus there are difficulties in stabilising the foam produced. A systematic study of the foaming behaviour of the nonionic surfactants (Lamesoft PO65 and Silsurf DI-2510) was carried out in pure water and aqueous ethanol. A maximum in foamability of the surfactants was seen at 30 wt. % and 70 wt. % ethanol for Lamesoft PO65 and Silsurf DI-2510, respectively.

The ability of particles as foam stabilisers is well known. Irreversible attachment of particles to the air-water interface results in aqueous foams stable for many years. Therefore the potential for particles as stabilising agents of aqueous ethanol foams was explored. Extremely stable foams of hydrophobic (covered in 20% SiOH groups, so that 80% covered in hydrophobic dichlorodimethylsilane groups) fumed silica were generated in 20 wt. % ethanol, in pure water aeration of these particles generated particle-laden films that travelled up the walls of the vessels (termed ‘climbing films’). Aeration of particles of intermediate hydrophobicity (47% SiOH) in pure water resulted in foams stable to collapse for over 1 year. Introduction of 20 wt. % ethanol to the aqueous phase resulted in a complete destabilisation of the foams, which all collapsed within 90 seconds.

The synergistic effect of particle and surfactant mixtures on foam stability is well known, a significant quantity of the literature covers the adsorption of ionic surfactants to charged solid particles. Much less is understood about the synergy between charged surfaces and nonionic surfactants. The foaming behaviour of mixtures of the nonionic surfactants with fumed silica particles was investigated. The effect of surfactant concentration, particle concentration and particle hydrophobicity was studied. It was found that the addition of particles to all foaming surfactant solutions, enhanced the foamability and stability (with the exception of 1 wt. % 100% SiOH and 1 wt. % Silsurf DI-2510 which formed a viscous gel). The effect of varying the particle hydrophobicity demonstrated that the two surfactants behaved completely differently. Addition of hydrophobic particles to Lamesoft PO65 solutions caused a reduction in the foamability, but an increase in the foamability of Silsurf DI-2510 solutions.

The second study in the thesis explored the fluorination of a series of monodisperse silica particles. A set of particles of varying hydrophobicity was generated by treating with different concentrations of a perfluorosilane. The particle contact angles at an air-water interface were measured directly with the film calliper method and found to range from 50.7° to >90°. As the two most hydrophobic particles broke the thin film, they were therefore assumed to have contact angles above 90°.

The particle contact angles were measured after introduction of 15 wt. % to the aqueous phase. Ethanol addition was seen to effect a reduction in the contact angle of all particles, demonstrating the effect of lowering the surface tension of the aqueous phase on the particle properties at the interface. Monolayers of the particles were then spread at air-water and aqueous ethanol-air interfaces and observed with optical microscopy. Aggregation was seen for all particles except the two most hydrophobic sets of particles, where ordered, repulsive arrays were observed. Introduction of 15 wt. % ethanol to the aqueous phase resulted in heavy aggregation, except for the two most hydrophobic particles which still demonstrated localised areas of the spread monolayers that formed repulsive arrays.

The foaming properties of these particles in pure water and 15 wt. % ethanol was then explored by aerating with a high shear method. In pure water, aqueous foams were formed for all particles but an inversion of the curvature of the interface was observed for particles treated with 3.4 × 10-4 M silane and above. The dry powders were stable for many months but the foams demonstrated poor stability of around 10 days. Introduction of 15 wt. % ethanol to the aqueous phase improved the foamability and stability significantly. An intermediate material between a foam and dry powder was observed for the most hydrophobic particles. The particles were wetted much better by the ethanol solutions which may have enhanced their transport to the interface, enhancing the foam properties.

Finally, the Fenton reaction was explored kinetically by studying the degradation of an azo-dye (with UV-Visible spectrophotometry) by the Fenton reagents. The initial rate of the reactions was determined by measurement of the reduction in concentration of the probe dye as a function of time. The intial rates passed through a maximum at 60 mM and 6 mM for hydrogen peroxide and copper sulfate, respectively. The effect of the pH was also investigated and seen to have a significant effect on the initial reaction rate, with the rate lowering with decrease in the pH. In vitro studies of the formulations demonstrated a 100% bacterial kill when exposed to E.Coli cultures for 60 seconds. A similar pH dependent effect was observed and the anti-bacterial efficacy was reduced at lower pH values. The high efficacy did not translate to in vivo studies on the skin surface, where there was no evidence of synergy when the Fenton components were applied to the skin surface. There was also a loss of efficacy when the formulations were applied as a powder, however no pH dependence was observed in in vivo studies.

Department of Chemistry, The University of Hull
Binks, Bernard P.; Horozov, Tommy S.
Sponsor (Organisation)
Engineering and Physical Sciences Research Council; Deb Group Ltd.
Qualification level
Qualification name
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