Measurement and Development of Solubility Correlations for Tritolylamine in Twelve Organic Solvents Touraj Manifar and Sohrab Rohani* Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario N6A 5B9, Canada Marko Saban Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario L5K 2L1, Canada The solubility of tritolylamine (TTA) in 12 solvents (hexane, heptane, n-octane, decane, hexadecane, toluene, benzene, 2-propanol, propanol, 2-butanone, methanol, and ethanol) was measured gravimetrically and compared with the predictions obtained from the ideal solution and UNIQUAC equations. To use the UNIQUAC equation, the specific heat and heat of fusion of TTA were measured by differential scanning calorimetry (DSC). DSC results also indicated that TTA does not have any enantiotropically related polymorphs. It is shown that the UNIQUAC predictions are close to the measured solubility values. In addition, measurement of the solubility of TTA in hexane was performed by an on-line density meter. The results compared closely with those obtained with the gravimetric method. The solvents were chosen so that their polarity indexes cover a wide range. Introduction Arylamine molecules in general and tritolylamine (TTA; for its molecular structure see Figure 1) in particular have a wide range of industrial applications such as in the xerographic, polymer, 1 and pharmaceuti- cal 2,3 industries. The arylamine molecules form a stable aminium radical cation. 4 In the xerographic industry, arylamine molecules are used as hole-transport materials because of their high hole drift mobilities. 5-7 Typically, in organic light- emitting devices (OLEDs) and in organic photoreceptors, the arylamine molecules are used in the form of a thin solid film. Constantly increasing demand for the im- provement of these devices (durability, thermodynamic stability, and higher efficiency) necessitates stricter control over the properties and characteristics of the hole-transport materials that make up such devices. Although the use of arylamine molecules in opto- electronic devices (OLEDs 8-10 and organic photo- receptors 11-13 ) has been well documented, no data on the solubilities of their constituent arylamine molecules in different solvents have been published. The solubility information is of special importance in the study of nucleation and growth kinetics of these molecules. For example, in the xerographic industry, a minor impurity in these molecules could have a serious effect on the operation of the optoelectronic devices. In OLEDs, the tendency of the hole-transport materials to crystallize on aging is thought to be one of the main causes of degradation. 14-16 To prevent crystallization of these molecules, precise knowledge of the kinetics and solubil- ity of the hole-transport materials is essential. In this study, we investigated the solubility of TTA in different solvents. In a series of subsequent papers, we will report the solubilities of other arylamine mol- ecules that we have synthesized in our laboratory, as well as on the crystallization kinetics of these molecules. For the measurement of solubility, a gravimetric method was used. In addition, an on-line density meter was used to monitor the solubility of TTA in hexane. The two adjustable parameters of the UNIQUAC model were estimated for the prediction of the solubilities of TTA. Theory It is desirable to estimate the solubility of a solid in a solution completely from the properties of the pure components to minimize the amount of experiments required. If we assume that there is no appreciable solubility of the liquid solvent in the solid phase, then the equation showing the equilibrium between two phases becomes where f represents the fugacity and the subscript 2 refers to the solute. By assuming that the solution is nonideal and considering the intermolecular and in- tramolecular forces, the solubility of a solute in solution can be written as * To whom correspondence should be addressed: E-mail: rohani@eng.uwo.ca. Figure 1. Molecular structure of tritolylamine molecule. f 2 (pure solid) ) f 2 (solute in liquid solution) (1) x 2 ) f 2 (pure solid) γ 2 f 2 0 (2) 970 Ind. Eng. Chem. Res. 2005, 44, 970-976 10.1021/ie049352v CCC: $30.25 © 2005 American Chemical Society Published on Web 01/25/2005