Terminologies and Definitions

Abstract

One of the weak links in surface research has been the lack of clear, well-defined terminologies, definitions, and common language. This at least in part contributes to the current messiness in the surface literature. In this chapter, some of the commonly used terminologies and language are overviewed, shortfalls are discussed, and areas for improvement are proposed. In terms of definition, hydrophilicity and hydrophobicity are the most important concepts in surface science, and they mean having and lacking of affinity with water, respectively. Water contact angle θ at 90° has been defined as the cutoff between hydrophilicity and hydrophobicity. This definition may have been derived from trigonometry but has been challenged numerous times in the past. An improved definition based on the wetting and adhesion interactions of water with 20 different surfaces of varying water affinity is proposed. Surfaces with θ _R > 90° were found to have no affinity with water and are defined as hydrophobic, whereas those with θ _R < 90° are defined as hydrophilic. Surfaces with θ _A ≥ 145° are shown to have no attraction toward water. Accordingly, a surface can be defined as superhydrophobic when its θ _R is >90° and θ _A is ≥145°. The fundamental reason why a surface becomes hydrophobic is discussed. The methodology has been extended to define hexadecane oleophilicity, oleophobicity, and superoleophobicity. It is also shown that the philicity/phobicity cutoff should not be universal at 90°, rather it should be dependent of the liquid surface tension; the lower the surface tension, the larger the θ _R cutoff angle.