In this talk, I will introduce our recent achievements about controlled colloidal systems including the air-water and/or oil-water interface from the viewpoint of surface (interfacial) free energies.  Topics selected for this talk are
1) wetting transition of alkane droplet on the air-water interface, 2) surface freezing transitions at the air-water and oil-water interfaces, 3) surface freezing transition driven oil-in-water (OW) emulsion stabilization, and 4) demulsification of particle-laden (Pickering) emulsions in binary liquid mixtures having critical solution points. Short summary of each topic is given below. Applications closely related to these topics, such as detergency, water (oil) repellant, cosmetics etc., will be introduced also in the talk.

1. Wetting transition of medium chain liquid alkane droplets can be induced by increasing surfactant concentration in the aqueous phase.  Penetration of alkane molecules in the surfactant adsorbed film decreases the air-water interfacial tension by the gain of the two-dimensional mixing entropy.  The resultant wetting film can be tuned from single molecular thick to a macroscopic thickness depending on the surface force between the air-oil and oil-water interfaces at the both sides of the wetting film.

2. Wetting monolayer of surfactant and alkane undergoes another thermal transition to a surface frozen monolayer upon cooling when the surfactant and alkane have a similar chain length.  When the chain length of surfactant and alkane is mismatched and alkane molecules have longer hydrophobic chain compared to surfactant molecules, a unique bilayer frozen film in which the frozen alkane monolayer is rested on the liquid-like mixed film of surfactant and alkane is also observed.

3. Surface freezing transition of surfactant and alkane can be utilized to stabilize OW emulsions when it occurs at the oil-water interface.  At the surface freezing transition, coexisting branched, double-bonded, and chain-mismatched oils are expelled from the interface, hence, the surface frozen film can encapsulate variety of oils in emulsion droplets in a stable manner.

4. Pickering emulsions are stabilized by the fine particles adsorbed at the oil-water interface.  The adsorption energy of particles at the oi-water interface is normally quite large and practically irreversible.  In critical binary liquid mixtures, the liquid-liquid interfacial tension decreases with a critical exponent when the system is approaching to its critical solution point.  In the last topic, this feature is used to demulsify Pickering emulsions.