Tuning Interfacial Properties and Processes by Controlling the Rheology and Structure of Poly(N-isopropylacrylamide) Particles at Air/Water Interfaces

Abstract

By combining controlled experiments on single interfaces with measurements on solitary bubbles and liquid foams, we show that poly( N-isopropylacrylamide) (PNIPAM) microgels assembled at air/water interfaces exhibit a solid to liquid transition changing the temperature, and that this is associated with the change in the interfacial microstructure of the PNIPAM particles around their volume phase transition temperature. We show that the solid behaves as a soft 2D colloidal glass, and that the existence of this solid/liquid transition offers an ideal platform to tune the permeability of air bubbles covered by PNIPAM and to control macroscopic foam properties such as drainage, stability, and foamability. PNIPAM particles on fluid interfaces allow new tunable materials, for example foam structures with variable mechanical properties upon small temperature changes.

Stimuli-Responsive Behavior of PNiPAm Microgels under Interfacial Confinement

“…One model attributes the destabilization of the emulsion to the collapse of the interfacially adsorbed microgels into smaller spheres which cover less area and thus are less efficient stabilizers. [51][52][53] Additionally, it was proposed that microgels may desorb from the oil/water interface to the oil phase due to their increased hydrophobicity, 53,54 which, however, does not seem to occur in all reported experimental scenarios. 55,56 It was also suggested that microgels form aggregates and multilayers at the oil/water interface, 44,45,56 which has been experimentally visualized for the inverse case of water in oil emulsions.…”

“…PNiPAm microgels are known to stabilize oil/water emulsions, which are stable below the VPTT and can be destabilized by heating above the VPTT. 42,43,[52][53][54][55][56][44][45][46][47][48][49][50][51] Various explanations regarding this stimuli-responsive behavior were proposed. One model attributes the destabilization of the emulsion to the collapse of the interfacially adsorbed microgels into smaller spheres which cover less area and thus are less efficient stabilizers.…”

“…26,[41][42][43] Exploiting stimuli-responsive properties of the microgels enables to break such emulsions on demand by changing the temperature or pH. [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] The rupture of such emulsions can be correlated with the volume phase transition of the stabilizing microgels. 42,43,[49][50][51][52] Rheological studies suggest that the microgels collapse to a smaller diameter at a liquid interface, leading to a reduced interfacial coverage and thus a reduced emulsion stability.…”

“…[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] The rupture of such emulsions can be correlated with the volume phase transition of the stabilizing microgels. 42,43,[49][50][51][52] Rheological studies suggest that the microgels collapse to a smaller diameter at a liquid interface, leading to a reduced interfacial coverage and thus a reduced emulsion stability. 51,52 While this model correlates experimentally observed rheological properties with the macroscopic emulsion stability, an experimental visualization to provide a microscopic picture of the microgel collapse at liquid interfaces is still missing.…”

Preparation of pH Responsive Polystyrene and Polyvinyl Pyridine Nanospheres Stabilized by Mickering Microgel Emulsions

“…The previous works to obtain smart microgels using surfactant free method are based on N-isopropyl acrylamide (NIPAm) polymers and its copolymers with acrylic (AA), methacrylic (MA), or 2-acrylamido-2-methyl propane sulfonic acids (AMPS) [28][29][30]. In this category of microgels, the low temperature and high pH conditions increased the swelling of particles and formed more stable Mickering emulsions.…”