Formation, structure and mechanical properties of hydrogel-emulsions
Oil-in-water emulsions are ubiquitous in fields such as food, cosmetic and pharmaceutical industries due to their ability to impart texture, or transport or solubilise hydrophobic constituents in a water continuous phase. However, emulsions are thermodynamically unstable and a third constituent or more needs to be added to the mixture to prevent phase separation, at least during the product shelf-life. The use of networks with crosslinked hydrophilic polymer chains (hydrogels) to stabilise emulsions, have been exploited by scientists for long periods of time. Their ability to easily be modified with different functional groups, makes low molecular weight peptides an advantageous choice to form hydrogels. The aim of this thesis is to study the behaviour of dipeptide fibres at interfaces and in the bulk, using a combination of rheological, imaging and in vivo research techniques. Dipeptides are small proteins consisting of only two amino acids linked together by one peptide bond. At high pH, the dipeptide molecules form micelles which transform into a hydrogel of fibres in response to the addition of salt. Under high shear the hydrogel is disrupted and will reform around bubbles or droplets. Following previous studies, we study the behaviour of the dipeptide at low concentration and use salt to trigger gelation. We first present a route to achieving repeatable hydrogel formation. We characterise the properties of the hydrogel for two different salt (MgSO4) concentrations. Using the hydrogel, we proof that at such a low dipeptide concentration, the hydrogel is too weak to prevent ripening of the bubbles, which reduces the long-term stability of the foam. However, under the same conditions, emulsions prepared from some oils are highly stable. Finally, we examine the wetting properties of the oil droplets at a hydrogel surface as a guide to the resulting emulsions. The behaviour of hydrogels were further characterised using extensional rheology. Hydrogels were prepared with different salts (MgSO4, CaCl2 and NaCl) concentrations and stretched at different velocities. We show that the collapse scenario of the hydrogel films is salt type dependent. Hydrogels turn into elastic solid films that collapse at short distances when prepared with calcium chloride. By contrast, hydrogels prepared with sodium chloride develop into extremely viscous films that do not burst; they instead, form a thread that breaks under capillarity. In between, hydrogel films prepared with magnesium sulfate exhibit a viscoelastic response that allows the film to stretch and form a bridge that burst on itself. In addition to this, we observe a non-monotonic trend dependence of the film strength on salt (MgSO4) concentration. Lastly, we observe a disagreement between image and equation based surface areas at intermediate and long distances; while equation based surface areas keep increasing linearly after overtaking the surface area of the rings holding the film, image based surface areas drop to zero when they are near or equal the surface area of the rings. The behaviour of fibre-coated interfaces prepared at different salt (MgSO4) concentrations were studied by conducting strain and frequency sweep tensiometer based dilatational experiments. We have shown that stable liquid-liquid interfaces stabilised by dipeptide fibres, can have a highly nonlinear response to dilatational strain amplitudes. Interfaces prepared at high salt concentrations show an increase in the elastic response, as opposed to films prepared at low salt concentrations, which collapse at small deformations. In between, films prepared at intermediate salt concentrations, show a more viscoelastic response, with lower resistance to deformation. Furthermore, films prepared at high salt concentrations show a strain softening/hardening response when deformed at intermediate and high amplitudes not reported before in large amplitude oscillatory dilatational (LAOD) experiments. Finally, we observe that the method to quantify the degree of nonlinearities in Lissajous curves fails in some cases and hence propose a different approach to quantify nonlinearities that span over the whole range of strains. Last, the effect of salt triggered hydrogel-emulsions with different mechanical properties on the lipid digestion was investigated using a static in vitro model. The rate and extent of lipid digestibility was observed to be higher for emulsions prepared with calcium chloride than with magnesium sulfate. Emulsions prepared with magnesium sulfate show a non-monotonic dependence of digestion on salt concentration, probably caused by the increase in film strength observed in chapter 3 when using extensional rheometry.