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Title: Phase behaviour, film formation and industrial applications of model salt-humectant-water mixtures
Author: Ferretti, Giulia
ISNI:       0000 0004 7657 1379
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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When mixtures composed of non-volatile solutes and a solvent dry into thin films, a range of processes occur during the film formation, affecting the resulting film's functionality. In the case of aqueous systems, water loss or uptake is particularly affected by temperature and relative humidity, altering the film's physical properties (e.g. rheo-mechanical, permeability, surface topography) as it dries or swells in response to the environment. Understanding these processes is thus essential to a range of applications in the packaging and consumer goods industries. A model ternary system comprising an inorganic salt (aluminium chlorohydrate) and a humectant (glycerol), in water, was studied due to its hygroscopic behaviour and interesting water management properties. This mixture can be thought of as a 'minimal' antiperspirant in the context of personal care. The film formation parameter space was systematically mapped out to gain a deep understanding into the relationship between composition, relative humidity, phase behaviour, drying/swelling kinetics and rheo-mechanical properties. Spectroscopy, gravimetric, dynamic vapour sorption, x-ray diffraction, calorimetry, viscometry and adhesion measurements were employed throughout the film drying pathway. The 'strain-induced elastic buckling instability for mechanical measurements' method was adapted for time-resolved measurements of thin drying films' mechanical properties. A range of concentration-dependent morphologies were found and sinusoidal wrinkling was exploited to calculate the evolving film modulus, with results validated against nanoindentation measurements. Equilibrium film compositions, drying/swelling kinetics and film rheo-mechanical properties were found to be non-monotonic with glycerol content, which was rationalised in terms of the competitive binding of water and glycerol to aluminium chlorohydrate and the overall miscibility and glass formation within the ternary system. Importantly, water sorption/desorption was shown to be fully reversible. In order to improve our predictive ability, a 'minimal mathematical model' was developed, capturing the key drying stages and non-monotonic behaviour of the system, delivering characteristic drying timescales and skin formation indications. Overall, this work provides a holistic thermodynamic and non-equilibrium understanding of the behaviour and response of this important class of salt-rich, glass forming films.
Supervisor: Cabral, João Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral