pH-Responsive foams triggered by particles from amino acids with metal ions

Stimuli-responsive Pickering foams have attracted wide attention due to their good stability and controllable foamability. In this work, pH-switchable Pickering foams were constructed using particles from amino acids with metal ions as stabilizers. Some hydrophobic amino acids, phenylalanine, leucine and tryptophan, can induce the production of particles with suitable wettability in the case of combination with some metal ions at their deprotonated forms, and further form Pickering foams. The foams exhibit good stability and can be cycled between foaming and defoaming forms by modulating pH of solutions. When pH was increased, the amino acids were in anionic forms and could easily coordinate with metal ions, forming insoluble particles to stabilize the foams. As pH was decreased, the protonation of amino acids led to the disappearance of particles, resulting in the defoaming state. This work proposes a simple method to prepare Pickering foams with environmentally friendly amino acids as the raw materials, providing a new idea for the construction of stimuli-responsive disperse systems.