It is well known that amphiphilic materials can affect dispersion stability of colloidal systems. They work as a sensitizer (coagulant) to destabilize dispersion state of colloids in some situa tion, and as a protecting agent to stabilize colloids in other situations. There are a large variety of amphiphilic materials such as small-molecular surfactant and amphiphilic biopolymers, proteins or polysaccharides, and one have to give careful consideration to the effects of each amphiphile case by case. In this review article, the author focused attention to 1) the influence of small-molecular ionic surfactants to the colloidal stability of suspensions (especially, negatively charged polystyrene latex) and 2) the formation of O/W emulsions using three kinds of proteins having emulsifying ability (bovine serum albumin, β-lactoglobulin, and β-casein) and their stability. Experimental data concerning adsorption behaviors of these surfactant or protein molecules are presented, and the relation between the adsorbed amount of these molecules and dispersion stability of colloidal particles was briefly explained.

Surface modification and treatment of nanoparticles by using various surfactants and silane coupling agents with different molecular structure and weight on particle surfaces in aqueous solution or organic solvents were applied to control surface interaction between particles and aggregation / dispersion behaviors. If surface modified molecules were almost completely covered on the surface of particles, surface modified nanoparticles were able to be re-dispersed into various organic solvent after surface treatment on particles in suspension and dried nanoparticles. Various kinds of functional materials, such as uniform nanoparticles dispersed polymer composites, were prepared from nanoparticles dispersed suspension without aggregation. Furthermore, molecular and nanometer scaled surface structure and interaction after modification was characterized by using a colloid probe atomic force microscope, AFM, method. Based on the measurement results, the action mechanism of some kinds of surfactants and other modified molecules on aggregation and dispersion behavior was discussed.

Emulsions stabilized by various emulsifiers, such as polymers, solid particles alone, solid particles with pre-adsorbed polymers, and solid particles with controlled aggregation structures are reviewed, focusing on the effects of emulsifiers on their emulsion stability and rheological properties. The overlapping concentration of polymer can be regarded as a criterion for the preparation of emulsions using polymeric emulsifiers. An increase in the emulsifier concentration leads to a decrease in the droplet size and an increase not only in the emulsion stability but also in the dynamic elastic modulus, irrespective of the emulsifier.

According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloid stability, the balance of the van der Waals attraction and the electrostatic repulsion acting between colloidal particles determines the stability of their suspensions. The DLVO theory can be applied to hard particles with no surface structures. Biocolloidal particles such as cells, on the other hand, are not simple hard particles but can be modeled as soft particles, i.e., hard particles covered with a surface layer of polyelectrolytes. In this review article, we start with the traditional DLVO theory and then discuss a new theory of interactions between soft particles, which can be applied to the interaction of biocolloidal particles.

Water at high temperature and high pressure near the gas/liquid critical point (T_c = 374°C, P_c = 22.1 MPa) exhibits properties that are remarkably different from those of ambient water. For example, the dielectric constant decreases from 78 at 25°C and 0.1 MPa to 2 at 400°C and 25 MPa, the value of which is comparable to those of hydrocarbons. Accordingly, water and hydrocarbons become freely mixable, while electrolytes precipitate out. Under such extreme conditions, dispersion stability of colloidal particles should be very different because DLVO forces depend not only on the surface properties of the particles but also on the properties of the dispersion medium. In this article, properties of colloidal dispersions in water at high temperatures and high pressures are reviewed. Anomalous long–range repulsion that appears between silica surfaces in ethanol near the gas/liquid critical point (T_c = 241°C, P_c = 6.1 MPa) will also be reviewed.