Cleavage Energy of Alkylammonium-Modified Montmorillonite and Relation to Exfoliation in Nanocomposites: Influence of Cation Density, Head Group Structure, and Chain Length
Cleavage Energy of Alkylammonium-Modified Montmorillonite and Relation to Exfoliation in Nanocomposites: Influence of Cation Density, Head Group Structure, and Chain Length
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DOI：
10.1021/cm902784r
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发表时间：
2010-02-23
影响因子：
8.6
通讯作者：
Heinz, Hendrik
中科院分区：
文献类型：
--
作者：
Fu, Yao-Tsung;Heinz, Hendrik
虽然在纳米尺度上测量解理能具有实验挑战性，但分子模拟表明，根据阳离子交换容量（CECs为91 meq/100 g和143 meq/100 g）、头基化学（伯胺和季铵盐）、阳离子交换容量的变化，解理能的可调范围在25至210 mJ/m(2)之间。44个烷基铵改性蒙脱石系列的C-2 ~ C-22类链长。设计的低解理能有机粘土在解理后重建表面的表面张力较高时，能够支持剥离。高解理能和低解理能之间的差异与库仑相互作用有关，这取决于在平衡分离时，层间带正电荷的表面活性剂头基团相对于两个粘土层的位置，并进一步与层间密度有关，这取决于表面活性剂在无溶剂矿物中的体积堆积。具有NH3头基的表面活性剂具有较低的裂解能，在25 ~ 55 mJ/m之间(2)。具有NMe3头基的表面活性剂在短链上的裂解能可达210 mJ/m(2)，在较长的烷基链上的裂解能可达30 - 65 mJ/m(2)。作为链长函数的解理能表明，由松散排列、扁平的烷基链层组成的层间结构的局部最小值和由密集排列、扁平的烷基链层组成的层间结构的局部最大值，对于较长的链（> -20），趋近于45 +/- 5 mJ/m(2)。我们使用精确的、广泛验证的力场，在全原子细节上进行分子动力学模拟[Heinz等人]。化学。物质学报，2005,17,5658]和先进的采样技术来获得平衡表面活性剂构象。
Cohesion between layers of organically modified aluminosilicates creates a barrier to exfoliation in polymer matrices and can be thermodynamically described by the cleavage energy, While the measurement of cleavage energies at the nanometer scale is experimentally challenging, molecular simulation indicates a tunable range between 25 and 210 mJ/m(2) upon variation of the cation exchange capacity (CECs of 91 meq/100 g and 143 meq/100 g), headgroup chemistry (primary and quaternary ammonium), kind chain length (C-2 to C-22) in a series of 44 alkylammonium-modified montmorillonites. Designed organoclays of low cleavage energy can support exfoliation when the surface tension, which relates to reconstructed surfaces after cleavage, is comparatively higher. The difference between high and low cleavage energies is associated with Coulomb interactions depending on the position of the positively charged surfactant head groups in the interlayer with respect to the two clay layers at equilibrium separation and further with the interlayer density depending on volume packing of the surfactants in the solvent-free minerals. Surfactants with NH3 head groups exhibit low cleavage energies between 25 and 55 mJ/m(2). Surfactants with NMe3 head groups exhibit cleavage energies up to 210 mJ/m(2) for short chain length and approach a range between 30 and 65 mJ/m(2) for longer alkyl chains. The cleavage energy as a function of chain length indicates local minima for interlayer structures comprised of loosely packed, flat-on layers of alkyl chains and local maxima for interlayer structures comprised of densely packed, flat-on layers of alkyl chains, convergent to 45 +/- 5 mJ/m(2) for longer chains (> C-20). We employed molecular dynamics simulation in full atomistic detail using an accurate, extensively validated force field [Heinz et al. Chem. Mater. 2005, 17, 5658] and advanced sampling techniques to obtain equilibrium surfactant conformations.