Kumar Sugam, Aswal Vinod K, Kohlbrecher Joachim
Solid State Physics Division, Bhabha Atomic Research Centre , Mumbai 400 085, India.
Laboratory for Neutron Scattering, Paul Scherrer Institut , CH-5232 PSI Villigen, Switzerland.
Langmuir. 2016 Feb 16;32(6):1450-9. doi: 10.1021/acs.langmuir.5b03998. Epub 2016 Feb 2.
The phase behavior of nanoparticle (silica)-polymer (polyethylene glycol) system without and with an electrolyte (NaCl) has been studied. It is observed that nanoparticle-polymer system behaves very differently in the presence of electrolyte. In the absence of electrolyte, the nanoparticle-polymer system remains in one-phase even at very high polymer concentrations. On the other hand, a re-entrant phase behavior is found in the presence of electrolyte, where one-phase (individual) system undergoes two-phase (nanoparticle aggregation) and then back to one-phase with increasing polymer concentration. The regime of two-phase system has been tuned by varying the electrolyte concentration. The polymer concentration range over which the two-phase system exists is significantly enhanced with the increase in the electrolyte concentration. These systems have been characterized by small-angle neutron scattering (SANS) experiments of contrast-marching the polymer to the solvent. The data are modeled using a two-Yukawa potential accounting for both attractive and repulsive parts of the interaction between nanoparticles. The phase behavior of nanoparticle-polymer system is explained by interplay of attractive (polymer-induced attractive depletion between nanoparticles) and repulsive (nanoparticle-nanoparticle electrostatic repulsion and polymer-polymer repulsion) interactions present in the system. In the absence of electrolyte, the strong electrostatic repulsion between nanoparticles dominates over the polymer-induced depletion attraction and the nanoparticle system remains in one-phase. With addition of electrolyte, depletion attraction overcomes electrostatic repulsion at some polymer concentration, resulting into nanoparticle aggregation and two-phase system. Further addition of polymer increases the polymer-polymer repulsion which eventually reduces the strength of depletion and hence re-entrant phase behavior. The effects of varying electrolyte concentration on the phase behavior of nanoparticle-polymer system are understood in terms of modifications in nanoparticle-nanoparticle and polymer-polymer interactions. The nanoparticle aggregates in two-phase systems are found to have surface fractal morphology.
研究了不含电解质（NaCl）和含有电解质的纳米颗粒（二氧化硅）-聚合物（聚乙二醇）体系的相行为。观察到纳米颗粒-聚合物体系在电解质存在下表现出非常不同的行为。在没有电解质的情况下，即使在非常高的聚合物浓度下，纳米颗粒-聚合物体系仍保持单相。另一方面，在有电解质的情况下发现了折返相行为，即单相（单个）体系随着聚合物浓度的增加经历两相（纳米颗粒聚集）然后又回到单相。通过改变电解质浓度来调节两相体系的状态。随着电解质浓度的增加，两相体系存在的聚合物浓度范围显著扩大。这些体系通过将聚合物与溶剂进行对比的小角中子散射（SANS）实验进行了表征。使用双 Yukawa 势对数据进行建模以考虑纳米颗粒之间相互作用的吸引和排斥部分。纳米颗粒-聚合物体系的相行为由体系中存在的吸引（聚合物诱导的纳米颗粒之间的吸引耗尽）和排斥（纳米颗粒-纳米颗粒静电排斥和聚合物-聚合物排斥）相互作用的相互作用来解释。在没有电解质的情况下，纳米颗粒之间的强静电排斥作用超过聚合物诱导的耗尽吸引力，纳米颗粒体系保持单相。加入电解质后，在某些聚合物浓度下耗尽吸引力克服了静电排斥力，导致纳米颗粒聚集和两相体系。进一步加入聚合物会增加聚合物-聚合物排斥力，最终降低耗尽强度，从而产生折返相行为。从纳米颗粒-纳米颗粒和聚合物-聚合物相互作用的变化方面理解了不同电解质浓度对纳米颗粒-聚合物体系相行为的影响。发现两相体系中的纳米颗粒聚集体具有表面分形形态。
