The conformational behavior of poly(propylene imine) (PPI) dendrimers at three different solution pH is studied in an ionic liquid (IL) solvent, 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), through molecular dynamics (MD) simulations. The size, shape, density distribution, structure factor, and the scattering intensity are evaluated to probe the conformational transition of dendrimers as a function of pH. The results of the radial atomic and terminal amine group density distribution at low pH indicate a shift in the density towards the periphery of the dendrimers due to the electrostatic repulsion between the charged tertiary amine groups within the dendrimers. The [BMIM] cations are not encapsulated within dendrimers and predominantly reside near the periphery. The extensive back-folding of the outer branches due to the electrostatic repulsion between the solvent cations and the peripheral charged amine groups at neutral and low pH results in a dense compact structure in [BMIM]Cl as compared to that in water, as evident from the results of the structure factor and scattering intensity. The structural analysis in terms of the fractal dimension reveals that the lower generation dendrimers exhibit conformational transition as a function of pH, while the higher generations exhibit a highly compact structure at all solution pH. However, PPI dendrimers at low pH exhibit more free volume as compared to that at high pH, which may be utilized to accommodate specific guest molecules.

中文翻译：

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在不同pH值的离子液体中，聚丙烯亚胺树枝状聚合物的构象。

通过分子动力学（MD）在离子液体（IL）溶剂1-丁基-3-甲基咪唑鎓氯化物（[BMIM] Cl）中研究了聚（丙烯亚胺）（PPI）树状聚合物在三种不同溶液pH下的构象行为模拟。评估大小，形状，密度分布，结构因子和散射强度，以探测树枝状大分子的构象转变与pH的关系。低pH下径向原子和末端胺基团密度分布的结果表明，由于树枝状聚合物中带电叔胺基团之间的静电排斥，密度向树枝状聚合物的外围移动。[BMIM]阳离子未封装在树枝状聚合物中，并且主要位于外围附近。从中可以明显看出，由于中性和低pH值下溶剂阳离子与周围带电荷的胺基之间的静电排斥，导致外部支链的广泛折叠，从而导致[BMIM] Cl中的致密致密结构。结构因子和散射强度的结果。关于分形维数的结构分析表明，低代树状聚合物表现出随pH变化的构象转变，而高代树状聚合物在所有溶液pH下均表现出高度致密的结构。然而，与高pH下相比，低pH下的PPI树状聚合物表现出更多的自由体积，可用于容纳特定的客体分子。