Separation and Solubility of Cis and Trans Isomers in Nanostructured Double-Decker Silsequioxanes Beth W. Schoen, Carl T. Lira, and Andre Lee* Department of Chemical Engineering and Materials Science, Michigan State University, Room 2527 Engineering Building, 428 South Shawn Lane, East Lansing, Michigan 48824, United States ABSTRACT: A fractional crystallization method was used to separate the cis and trans isomers of three double-decker silsesquioxanes (DDSQs) with an aminophenyl moiety in a THF + hexanes solvent mixture. The experimental solubilities were fitted to the Schrö der-van Laar equation with activity coefficients determined using the NRTL model by adjusting the binary interaction parameters. The ability to separate these cis and trans isomers was affected by the regioisomer (m- or p-aminophenyl) and the R moiety (cyclohexyl or methyl) coupled via silicon. For a given DDSQ compound, the variances between the solubilities of the cis and trans isomers depend on differences in thermal properties (Schrö der-van Laar). Cis isomers were 33 times more soluble than trans isomers for p-aminophenyl (R = methyl) and 22 times more soluble for the analogous m-aminophenyl in a solution of THF and hexanes. For a more sterically hindered m-aminophenyl (R = cyclohexyl), the cis isomers were only 3.5 times more soluble, and the overall solubility was also the lowest. The magnitude of the binary interaction between DDSQ and nonsolvent (hexanes) was used to explain how quickly the solubility decreased as hexanes were added. The solubilities of the two m-aminophenyl structures decreased at similar rates, while the solubility of the p-aminophenyl structure decreased at a much lower rate since the magnitude of the binary interaction between p-aminophenyl and hexanes is smaller. 1. INTRODUCTION A recently developed class of nanostructured silsesquioxanes provides a unique opportunity to investigate and characterize the influence of cis and trans configurations on the physical and chemical properties of an inorganic-organic hybrid material (Figure 1). The cis and trans descriptors characterize the orientation of the X and R moieties with respect to the Si-O core of the silsesquioxane. This class of silsesquioxanes are formally known as double-decker silsesquioxanes (DDSQs) because they are composed of two “decks” of silsesquioxanes stacked on top of one another, forming a cagelike structure. 1 Prior to the advent of DDSQs, the majority of cagelike silsesquioxanes did not incorporate cis and trans isomers. 2-5 Of the few cagelike silsesquioxanes that did incorporate geometric isomers, none have been synthesized in large quantities. 6,7 Cagelike silsesquioxanes have demonstrated superior properties over their organic counterparts in areas such as thermal and mechanical properties, 8-11 solubility, 12-14 flame retard- ance, 15-23 oxidative resistance, 24-27 and dielectric proper- ties. 28-30 DDSQs with various reactive chemical moieties have been prepared, and their cis and trans isomers have been partially isolated. 31,32 Recently our group has identified cis and trans isomers using 1 H NMR spectroscopy, and thus, the ratio of cis and trans isomers of compounds A, B, and C can now be accurately quantified. 33 This allows for verification of purity and the development of a model representing the quantitative measurements and parameters needed for fractional crystal- lization of these isomers. Furthermore, it provides an opportunity to understand how these configurations influence the structure-property relationships of these DDSQs. Fractional crystallization provides a platform for larger quantities of material to be separated into fewer fractions compared with other methods such as chromatography. 34-36 Furthermore, fractional crystallization provides a much lower energy demand as opposed to an energy-intensive thermal separation method such as distillation. Hence, it is accepted as Received: November 25, 2013 Accepted: March 25, 2014 Published: April 4, 2014 Article pubs.acs.org/jced © 2014 American Chemical Society 1483 dx.doi.org/10.1021/je4010245 | J. Chem. Eng. Data 2014, 59, 1483-1493