Contrasting Mechanisms of Spontaneous Adsorption at Liquid-Liquid Interfaces of Nanoparticles Constituted of and Grafted with pH-Responsive Polymers.

Elucidating the mechanisms responsible for spontaneous adsorption of nanoparticles (NPs) at interfaces is important for their application as emulsifiers, bubble stabilizers, or foaming agents. In order to investigate the key factors that control the spontaneous adsorption of NPs at liquid-liquid interfaces, we synthesized seven different types of NPs from pH-responsive polymers poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) and poly(2-dimethylamino)ethyl methacrylate) (PDMAEMA) via surfactant-free emulsion polymerization or via "grafting from" polystyrene (PS) NPs. The dynamic interfacial tension (IFT) measurements at the toluene-water (Tol-H2 O) interface reveal that when PDEAEMA and PDMAEMA are grafted from the surface of PS NPs the solubility of the grafted pH-responsive polymers in toluene is the key factor determining the NPs' interfacial adsorption. Under acidic conditions (pH < 6.0), PDEAEMA and PDMAEMA are protonated and show no solubility in toluene, and as a result, the grafted NPs do not adsorb at the Tol-H2 O interface. Oppositely, under basic conditions (pH > 7.0), PDMAEMA dissolves in toluene and therefore the PDMAEMA-grafted NPs can adsorb at the Tol-H2 O interface. Interestingly, when NPs are constituted of PDEAEMA, they can adsorb spontaneously at the Tol-H2 O interface under acidic conditions (pH < 6.0) but not under basic conditions (pH > 7.0). In this case, the key factor determining the NPs' spontaneous adsorption at the Tol-H2 O interface is the degree of softness of the NPs rather than the solubility of PDEAEMA in toluene. Furthermore, we found that the adsorption of NPs constituted of PDEAEMA- (pH 2.0-6.0) and PDMAEMA-grafted PS NPs (pH 7.0-10.0) at the Tol-H2 O interface is a combination of diffusion-controlled and energy-barrier-controlled. The opposite trends observed for the interfacial attachment Δ E and activation energies Ea for the "constituted of" and "grafted from" NPs with pH suggest an opposite mechanisms of adsorption at the Tol-H2 O interface. Finally, the synthesized NPs prove to be effective emulsifiers, where the phase of the Pickering emulsions can be changed dynamically by pH adjustment.