Because surfactants combine both polar and non-polar groups into a single molecule, they have the ability to adsorb (or locate) at interfaces, thereby altering significantly the physical properties of those surfaces. The term “interface” is commonly employed here to describe the boundary in liquid/liquid, solid/liquid and gas/liquid systems. As a result of their dual nature, amphiphiles can “sit” at interfaces so that the polar/non-polar moieties can remain solvated in their preferential environment. This can be visualised by considering the specific example of surfactant molecules at the air-water surface, shown in the figure right. The polar hydrophilic heads remain solvated in polar water whilst the non-polar tails can partition into the air, which is by nature non-polar.‌‌‌ ‌

Accumulation of amphiphiles at the interface (liquid/liquid or gas/liquid) is a spontaneous process and results in a decrease of the interfacial (surface) tension. In the example above, high energy water molecules are replaced by lower energy surfactant tails. Hence, surfactants are distinguished by an ability to form oriented monolayers at the interface and, most importantly, self-assembled structures (micelles, vesicles) in bulk phases (spherical micelle shown right). Owing to such a versatile phase behaviour and diversity in colloidal structures, surfactants find applications in many industrial processes, essentially where high surface areas, modification of the interfacial activity or stability of colloidal systems are required, such as:  

• Detergents
• Cosmetics
• Paints and inks
• Herbicides and insecticides
• Firefighting foams
• Medicine
• Oil recovery

Because of the highly diverse and practical applications of surfactants, they constitute an extremely important group of chemicals in today’s society. Here, in the Eastoe group we focus on surfactant research and regularly visit neutron scattering facilties to study colloidal systems at the atomic level, gaining information critical to studying relationships between physical properties and molecular structure.