The HLB (hydrophilic-lipophilic balance) number has been widely used in quantifying the behavior of polar molecules in emulsions. It has also been used in attempting to explain the behavior of surfactants in flotation systems. The origin of the HLB concept lies in defining the behavior of molecules in terms of the contributions of various groups in their molecular structure. This paper proposes that modern thermodynamics provides a much more robust method of defining such behavior through the concept of excess Gibbs free energy which in turn is obtained from the activity coefficients of such molecules. It not only presents a more fundamental basis for the calculation but also there exists a vast number of species for which the key parameters are known or are currently being determined through global initiatives. (C) 2014 Elsevier Ltd. All rights reserved.