This special issue honors Professor Michael H. Abraham, and the many experimental and theoretical contributions that he made to solution chemistry during his prolific scientific career. Michael possessed a very keen understanding of how molecular interactions influenced chemical reaction rates and chemical, spectral, thermodynamic and transport properties. He is most noted for developing the Abraham general solvation parameter that describes solute transfer between two condensed phases, as well as solute transfer from the gas phase. The model was described in a 1993 paper published in Chemical Society Reviews [1], and to date has received more than 1,800 citations according to Web of Science [2]. The basic model has been used successfully to describe a wide range of important chemical, biological and pharmaceutical properties, including both water-to-organic solvent and gas-to-organic solvent partition coefficients [3], gas–liquid chromatographic [4] and high-performance liquid chromatographic retention data [5], enthalpies of solvation [6], limiting diffusion coefficients for ions and nonelectrolyte solutes in water and in organic solvents [7], blood-to-body tissue and air-to-body tissue partition coefficients [8], median lethal molar concentrations for aquatic toxicities [9], human skin permeability of ionic and molecular compounds [10], Draize eye scores and eye irritation thresholds [11], nasal pungency [12], odor detection thresholds [13], minimum alveolar concentration for anesthetic potency [14], and human intestinal absorption [15]. On two separate occasions Michael was named the recipient of the Ebert Prize [16], which is awarded to authors of a published scientific paper in the Journal of Pharmaceutical Sciences describing novel and original research with high probability of significantly impacting the pharmaceutical sciences.

Michael was not only an accomplished scientist, but a very kind and caring individual who was always willing to assist fellow scientists. I, and many other researchers, have been very fortunate to have collaborated with Michael on numerous research projects. I benefitted greatly from our collaborations which began back in 1997 when I received a handwritten letter from Michael asking me to send him measured solubilities for trans-stilbene dissolved in organic solvents of varying polarity and hydrogen-bonding character. At that time one factor limiting the applicability of the Abraham model was the availability of solute descriptor values. Michael used the solubility data to demonstrate the calculation of Abraham model solute descriptors. The computation methodology [17] that Michael proposed has been used many times in recent years to calculate solute descriptors of numerous pesticides, herbicides, high-energy explosives [18], medicinal compounds [19] and the zwitterionic forms of α-amino acids [20]. Calculated solute descriptor values, alone or in combination with experimental spectral data, have been used to examine for possible intramolecular hydrogen-bond formation in 2-substituted phenols [21, 22], hydroxyflavones [23], hydroxyisoflavones [23] and dihydroxyanthraquinones [24]. I did not realize at the time that this simple request would lead to a 23-year research collaboration that would result in the publication of more than 250 joint, peer-reviewed research articles.

Michael maintained an active research program long after his formal retirement. At the time of his death in January of 2021, he was actively working on various computational approaches for estimating Abraham model solute descriptors, on developing Abraham model correlations to describe the solubilizing properties of environmentally friendly, renewable organic solvents and on examining the transition states in the solvolysis of tert-butyl chloride, bromide and iodide [25]. His vast knowledge of science allowed him to tackle problems having chemical, pharmaceutical and environmental significance. The papers published in this special issue are written by several of Michael’s research collaborators, close friends, and others who have used his theoretical models, experimental methods and computational methodologies in their research work. Papers appearing in this issue utilize various aspects of Michael’s research, and are mainly focused on the predictive and descriptive applications of the Abraham general solvation parameter model, on the computation of Abraham model solute descriptors, and on achieving a more thorough understanding of solute–solvent molecular interactions.