Interfacial Tension

Definition

The interfacial tension γ between two immiscible liquids in the absence of any adsorption is the change of the free energy G^σ with interfacial area A at constant temperature and constant composition: ${\gamma} = \mathop{{\left( {\frac{{\partial \mathop{\rm G}\nolimits^{\sigma } }}{{\partial {\rm A}}}} \right)}}\nolimits_{{{\rm T},\mathop{\rm n}\nolimits_{\rm i }}}$. The value of γ depends on the nature of the two immiscible liquids. For an alkane-water system, γ is ∼50 mNm⁻¹; for an aromatic oil-water interface, γ is ∼ 30 mNm⁻¹; and for a polar oil (such as hexanol)-water interface, γ is ∼15 mNm⁻¹. Any adsorption at the interface, in particular surfactant molecules, will lower γ to an extent depending on the nature of the oil and surfactant, reaching values in the region of ∼1–10 mNm⁻¹. The reduction in γ with surfactant addition is important for formation of emulsions, whereby the total interfacial energy is reduced, thus facilitating the process of emulsification...