Determination of empirical polarity parameters of the cellulose solvent N,N‐dimethylacetamide/LiCl by means of the solvatochromic technique

Abstract

Empirical solvatochromic polarity parameters (α-, β-, and \documentclass{article}\pagestyle{empty}\begin{document}$ \pi ^* $\end{document}, AN and DN, as well as ET(30)-values) for cellulose, N,N-dimethylacetamide (DMA)/LiCl and cellulose dissolved in DMA/LiCl are presented. The following solvent polarity indicators were applied: 2,6-diphenyl-4-(2,4,6-triphenyl-1- pyridinio)-1-phenolate (1), bis(4-N,N-dimethylamino)-benzophenone (MK, 2), iron(II)-di-cyano-bis(1,10)-phenanthroline, Fe(phen)2(CN)2, (3), and copper(II)-N,N,N′,N′-tetramethyl-ethylendiamine-acetylacetonate tetraphenylborate/chloride/bromide (Cu(tmen)(acac)+ X− (4)). The solvatochromic shifts (νmax) of the indicators 1, 2, 3, and 4 adsorbed to cellulose or dissolved in DMA/LiCl reflect the corresponding properties of the surrounding, the dipolarity/polarizability (\documentclass{article}\pagestyle{empty}\begin{document}$ \pi ^* $\end{document}), the hydrogen bond donating ability or Lewis acidity (α), and the hydrogen bond accepting ability or Lewis basicity (β), respectively. Any indicator employed is well characterized (r > 0.97) by a linear solvation energy relationship (LSER) taking the Kamlet and Taft parameter into account: νmax(indicator) = νmax,0 + s\documentclass{article}\pagestyle{empty}\begin{document}$ \pi ^* $\end{document} + aα + bβ. Cellulose, DMA/LiCl, and the cellulose/DMA/LiCl solution approach a similar polarity with an ET(30) parameter about 52 to 53 kcal mol−1. The hypothetical interaction strength parameter (acid-base interactions, dipolar–dipolar interactions) between cellulose and DMA/LiCl are calculated by means of the individual Kamlet–Taft parameters α, β, and \documentclass{article}\pagestyle{empty}\begin{document}$ \pi ^* $\end{document} of cellulose and DMA/LiCl via a multiparameter equation. The specific chloride/cellulose interaction plays a dominant role in the cellulose solvent DMA/LiCl. Comparison of the polarity parameters of DMA/LiCl with the polarity parameters of other mixtures—such as N,N-dimethyl- formamide/LiCl, DMA/NaCl, or DMA/LiBr—are presented as well. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1945–1955, 1998