Edible surfactants at fluid-fluid interfaces

Liu, Yu

Chemistry
June 2021

Thesis or dissertation


Rights
© 2021 Yu Liu. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
Abstract

This thesis probes into the adsorption properties of several edible surfactants/fats at a range of fluid-fluid interfaces, e.g. air-oil, air-water and water-oil. Based on this, various colloidal materials are fabricated including aqueous and oil foams, simple and multiple emulsions, foamed and aerated emulsions. Among these materials, oil foams are the focus. Different techniques are applied to characterise the resulting materials and reveal the underlying stabilisation mechanisms, such as differential scanning calorimetry, rheology, X- ray diffraction, infrared spectrometry, surface/interfacial tension, contact angle, dynamic light scattering, optical and cryo-scanning electron microscopy.
Ultra-stable edible oil foams can be prepared from neat vegetable oils containing mainly long-chain, unsaturated fatty acids in their triglyceride molecules in the absence of an added foaming agent. Upon cooling/warming these oils, crystals of high melting fractions form in a low melting liquid oil yielding an oil gel. Such oil gels can be whipped to fabricate oil foams stabilized by pre-formed fat crystals. Foaming behavior depends on the oil composition and the degree of supercooling. Optimum foaming yields an over-run of ~ 40% for peanut oil and ~ 110% for olive oil. Oil foams devoid of drainage, coarsening or coalescence are achievable. We demonstrate that high melting triglyceride crystals possess a higher fraction of saturated fatty acids than the original oil. In addition, ultra-stable oil foams can be destabilized by heating due to the dissolution of the crystals in the continuous phase and at the air-oil surface.
Very stable oil foams can also be fabricated from mixtures of hydrophobic sugar ester surfactants and vegetable oils. Nevertheless, a novel foaming strategy is adopted during which aeration is first performed in the one-phase region followed by rapid cooling and then storing at low temperature. For sucrose ester surfactants, we first study the effect of aeration temperature and surfactant concentration on foamability and foam stability cooled from an one- phase oil solution. Unlike previous reports, both foamability and foam stability decrease upon decreasing the aeration temperature into the two-phase region containing surfactant crystals. At high temperature in the one-phase region, substantial foaming is achieved within minutes of whipping, but foams ultimately collapse within a week. We show that surfactant molecules are surface-active at high temperature and that hydrogen bonds form between surfactant and oil molecules. Cooling these foams substantially increases foam stability due to both interfacial and bulk surfactant crystallisation in situ. The generic nature of our findings is demonstrated for a range of vegetable oil foams with a maximum over-run of 330% and the absence of drainage, coalescence and disproportionation being obtained. In analogy with sucrose esters, long-term oil foams with a maximum over-run of ⁓ 275% are yielded based on mixtures of sorbitan esters and vegetable oil.
Reasonably stable aqueous foams can be prepared from mixtures of a series of sucrose ester surfactants and water, during which the effect of surfactant HLB and pH is investigated. The foaming functionality of sucrose esters is closely linked to the physicochemical properties of bulk liquid before aeration, including aggregate morphology and size, zeta potential, viscosity, and surface tension. At the natural condition, in comparison to the micelle-forming aqueous solutions of sodium dodecyl sulfate (SDS), the foaming capacity of sucrose ester dispersions containing vesicles is lower due to slower adsorption kinetics of surfactant molecules towards the air-water surface. By contrast, the resulting foams are much more stable than SDS foams. This is attributed to the steric hindrance of giant vesicles and that electrostatic repulsion existing between adjacent vesicles within the liquid films and Plateau borders of the foam. Amidst the investigated sucrose esters, lower HLB tends to yield improved foam stability. The effect of lowering pH on the dispersion and foaming characteristics of two relatively hydrophobic sucrose esters in water is then discussed. Upon lowering pH, the morphology of surfactant aggregates in bulk liquid and the interfacial properties are altered. Meanwhile, their foamability is improved markedly due to more rapid adsorption dynamics of surfactant monomers towards the air-water surface.
As seen above, sucrose ester is versatile adsorbing readily at various fluid-fluid interfaces. Is it feasible to stabilise multiple interfaces simultaneously in the same system using sucrose ester alone? Herein we report for the first time on the preparation of a multitude of colloidal materials using one and the same sucrose ester sample through facile protocols, i.e. air-in-oil (a/o) and air-in-water (a/w) foams, oil-in-water (o/w), water-in-oil (w/o), and oil-in- water-in-oil (o/w/o) emulsions, air and water-in-oil (a & w/o) foamulsions and air-in-oil-in- water (a/o/w) emulsions.
Overall, the research in this thesis gives insight into the adsorption modes of several types of edible surfactants/fats at a range of fluid-fluid interfaces. Besides, various colloidal materials are fabricated via facile protocols. Despite this, the stabilisation mechanism of the air-oil surface with surfactants/fats of varying architecture is not fully understood yet. For the sake of application, further investigations on novel colloidal materials using oil foams as templates are needed as well.

Publisher
Department of Chemistry, The University of Hull
Supervisor
Binks, Bernard P.
Sponsor (Organisation)
China Scholarship Council
Grant number
CSC, No.201706450036
Qualification level
Doctoral
Qualification name
PhD
Language
English
Extent
27 MB
Identifier
hull:18386
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