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

Empirical solvatochromic polarity parameters (alpha-, beta-, and pi*, AN and DN, as well as E-T(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-diphenyI-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-acetyl- acetonate tetraphenylborate/chloride/bromide (Cu(tmen)(acac)(+) X- (4). The solvatochromic shifts (nu(max)) 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 (pi*), the hydrogen bond donating ability or Lewis acidity (alpha), and the hydrogen bond accepting ability or Lewis basicity (beta), 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: nu(max)(indicator)= nu(max,0) + s pi* + alpha alpha + b beta. Cellulose, DMA/LiCl, and the cellulose/DMA/LiC1 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 alpha, beta, and pi* of cellulose and DMA/LiCI 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. (C) 1998 John Wiley & Sons, Inc.