Rabideau Brooks D, Ismail Ahmed E
RWTH Aachen University, Aachener Verfahrenstechnik, Schinkelstrasse 2, Aachen, Germany.
Phys Chem Chem Phys. 2015 Feb 28;17(8):5767-75. doi: 10.1039/c4cp04060k.
We study the dynamics of the formation of multiple hydrogen bonds between ionic liquid anions and cellulose using molecular dynamics simulations. We examine fifteen different ionic liquids composed of 1-alkyl-3-methylimidazolium cations ([Cnmim], n = 1, 2, 3, 4, 5) paired with either chloride, acetate or dimethylphosphate. We map the transitions of anions hydrogen bonded to cellulose into different bonding states. We find that increased tail length in the ionic liquids has only a very minor effect on these transitions, tending to slow the dynamics of the transitions and increasing the hydrogen bond lifetimes. Each anion can form up to four hydrogen bonds with cellulose. We find that this hydrogen bond "redundancy" leads to multiply bonded anions having lifetimes three to four times that of singly bound anions. Such redundant hydrogen bonds account for roughly half of all anion-cellulose hydrogen bonds. Additional simulations for [C2mim]Cl, [C2mim]Ac and [C2mim]DMP were performed at different water concentrations between 70 mol% and 90 mol%. It was found that water crowds the hydrogen bond-accepting sites of the anions, preventing interactions with cellulose. The more water that is present in the system, the more crowded these sites become. Thus, if a hydrogen bond between an anion and cellulose breaks, the likelihood that it will be replaced by a nearby water molecule increases as well. We show that the formation of these "redundant" hydrogen bonding states is greatly affected by the presence of water, leading to steep drops in hydrogen bonding between the anions and cellulose.
我们使用分子动力学模拟研究了离子液体阴离子与纤维素之间多个氢键形成的动力学过程。我们考察了由1-烷基-3-甲基咪唑鎓阳离子（[Cnmim]，n = 1、2、3、4、5）与氯离子、醋酸根离子或磷酸二甲酯配对组成的15种不同离子液体。我们将与纤维素形成氢键的阴离子的转变映射到不同的键合状态。我们发现离子液体中增加的尾链长度对这些转变只有非常小的影响，倾向于减缓转变的动力学并增加氢键寿命。每个阴离子最多可与纤维素形成四个氢键。我们发现这种氢键“冗余”导致多重键合的阴离子的寿命是单键合阴离子的三到四倍。这种冗余氢键约占所有阴离子 - 纤维素氢键的一半。对[C2mim]Cl、[C2mim]Ac和[C2mim]DMP在70摩尔％至90摩尔％的不同水浓度下进行了额外的模拟。发现水占据了阴离子的氢键接受位点，阻止了与纤维素的相互作用。系统中存在的水越多，这些位点就越拥挤。因此，如果阴离子与纤维素之间的氢键断裂，被附近水分子取代的可能性也会增加。我们表明这些“冗余”氢键状态的形成受到水的存在的极大影响，导致阴离子与纤维素之间的氢键急剧下降。
