Breaking of the Bancroft rule for multiple emulsions stabilized by a single stimulable polymer

Abstract

We investigated emulsions of water and toluene stabilized by (co)polymers consisting of styrene (S) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) monomer units with different compositions and structures such as a PDMAEMA homopolymer, a P(S-co-DMAEMA) random copolymer and various PS-b-PDMAEMA and PS-b-(S-co-DMAEMA) block copolymers. The model system is used to study the fundamental conditions under which the different kinds of polymer-stabilized emulsions (direct oil in water, inverse water in oil and multiple emulsions) are stabilized or destabilized by pH change (at constant temperature). Polymer properties like chain conformation at the toluene-water interface as probed by SANS and neutron reflectivity at the liquid-liquid interface, the oil-water partitioning of the polymer chains (Bancroft's rule of thumb) as determined by UV spectroscopy and interfacial tensions measured by the rising and spinning drop techniques are determined. Overall, results evidence that the curvature sign, as defined by positive and negative values as the chain segments occupy preferentially the water and toluene sides of the interface respectively, reliably predicts the emulsion kind. In contrast, the Bancroft rule failed at foreseeing the emulsion type. In the region of near zero curvature the crossover from direct to inverse emulsions occurs through the formation of either unstable coexisting direct and inverse emulsions (i) or multiple emulsions (ii). The high compact adsorption of the chains at the interface as shown by low interfacial tension values does not allow to discriminate between both cases. However, the toluene-water partitioning of the polymeric emulsifier is still a key factor driving the formation of (i) or (ii) emulsions. Interestingly, the stabilization of the multiple emulsions can be tuned to a large extent as the toluene-water polymer partitioning can be adjusted using quite a large number of physico-chemical parameters linked to polymer architecture like diblock length ratio or polymer total molar mass, for example. Moreover, we show that monitoring the oil-water partitioning aspect of the emulsion system can also be used to lower the interfacial tension at low pH to values slightly higher than 0.01 mN m(-1), irrespective of the curvature sign.

Biocompatible Stimuli-Responsive W/O/W Multiple Emulsions Prepared by One-Step Mixing with a Single Diblock Copolymer Emulsifier

“…The fact that no multiple emulsions are formed with this system is consistent with the architecture of the polymer used in this study since previous results on other systems 25 have shown that no multiple emulsions are formed when the hydrophilic block of the copolymer is purely constituted of DMAEMA. Here, it is also found out that no pH-driven phase inversion takes place for this system at I = 0 mol.L -1 .…”

“…Emulsions stabilized with these copolymers were stable for several months but were also stimuli-responsive with a capacity of encapsulation tuned by pH or temperature 17,25 . The latter study showed that the architecture of the polymer is important for the stabilization of multiple emulsions.…”

“…The latter study showed that the architecture of the polymer is important for the stabilization of multiple emulsions. A study of polymer conformations based on Cryo-TEM imaging and SANS measurements also evidenced the link existing between interfacial curvature and emulsion type 25 .…”

“…A few years ago, our team proved that appropriate polymer architectures can easily generate one-step W/O/W emulsions via transitional phase inversion 17,25 . Water-toluene emulsions were stabilized using PS-b-P(S-st-DMAEMA) copolymers constituted of a hydrophobic block of polystyrene (PS) and a hydrophilic block of randomly distributed units of styrene (30 mol %) copolymerized with 2-(dimethylamino)ethyl methacrylate (DMAEMA) (70 mol %).…”

Neutron reflectivity for soft matter

“…Finally, true liquid/liquid measurements are now emerging owing due to gain performances of the present reflectometers compared to those operating thirty years ago. For instance, in reference [15], water-toluene interface was probed in a cell specifically designed for the purpose. The beam crossed a sapphire before travelling through several hundreds of microns of water before hitting the toluene interface.…”