In the process of being involved in research on the development of functional surfactants for many years, the author has studied from the viewpoint of developing molecular design techniques that can reliably show the desired physical properties, breaking away from the molecular structure-surface activity relationship concept. In this paper, I provide the molecular structure design approach I have taken in the two studies in our series of research. (1) Development of nonionic surfactants that can drastically boost the foamability of mixed surfactant systems, (2) Development of general anionic surfactants that can respond to the sustainable age.

The wettability of colloidal particles at fluid interfaces has been thought to be one of the most important physicochemical factors in controlling the properties of Pickering emulsions. To obtain Pickering emulsions with desired properties using a unary particle requires chemical or physicochemical optimization of the particle surface, which can often be limiting or problematic for applied use. On the other hand, by mixing two types of particles, it is possible to achieve synergistic effects. For example, by adjusting the relative concentration of hydrophobic and hydrophilic particles, a transitional phase transition of emulsion can be induced. Some combinations of particles may, however, cause destabilization against coalescence or demulsification. This review proposes a classification of the stabilization of Pickering emulsions with mixtures of two types of particles, focusing on the wettability of the particles, and describes the stabilization or destabilization mechanisms reported so far.

Emulsion is a mixture of oil and water, which are immiscible with each other. So, in general, emulsifiers such as surfactants are required for the preparation of emulsions. On the other hand, it is concerned to be difficult to manufacture the emulsion products using surfactants due to the limitation of produced palm oil which is the raw material for surfactants, and the increasing global demand for surfactants. Therefore, the emulsification technique in the absence of any emulsifiers such as surfactants, which is named as surfactant-free emulsification, are strongly demanded. This paper reports the current situation and future possibilities of surfactant-free emulsions.