Foster Lynn M, Worthen Andrew J, Foster Edward L, Dong Jiannan, Roach Clarissa M, Metaxas Athena E, Hardy Clifford D, Larsen Eric S, Bollinger Jonathan A, Truskett Thomas M, Bielawski Christopher W, Johnston Keith P
McKetta Department of Chemical Engineering and ‡Department of Chemistry, The University of Texas at Austin , Austin, Texas 78712-0231, United States.
Langmuir. 2014 Sep 2;30(34):10188-96. doi: 10.1021/la501445f. Epub 2014 Aug 20.
The mechanism by which polymers, when grafted to inorganic nanoparticles, lower the interfacial tension at the oil-water interface is not well understood, despite the great interest in particle stabilized emulsions and foams. A simple and highly versatile free radical "grafting through" technique was used to bond high organic fractions (by weight) of poly(oligo(ethylene oxide) monomethyl ether methacrylate) onto iron oxide clusters, without the need for catalysts. In the resulting ∼1 μm hybrid particles, the inorganic cores and grafting architecture contribute to the high local concentration of grafted polymer chains to the dodecane/water interface to produce low interfacial tensions of only 0.003 w/v % (polymer and particle core). This "critical particle concentration" (CPC) for these hybrid inorganic/polymer amphiphilic particles to lower the interfacial tension by 36 mN/m was over 30-fold lower than the critical micelle concentration of the free polymer (without inorganic cores) to produce nearly the same interfacial tension. The low CPC is favored by the high adsorption energy (∼10(6) kBT) for the large ∼1 μm hybrid particles, the high local polymer concentration on the particles surfaces, and the ability of the deformable hybrid nanocluster cores as well as the polymer chains to conform to the interface. The nanocluster cores also increased the entanglement of the polymer chains in bulk DI water or synthetic seawater, producing a viscosity up to 35,000 cP at 0.01 s(-1), in contrast with only 600 cP for the free polymer. As a consequence of these interfacial and rheological properties, the hybrid particles stabilized oil-in-water emulsions at concentrations as low as 0.01 w/v %, with average drop sizes down to 30 μm. In contrast, the bulk viscosity was low for the free polymer, and it did not stabilize the emulsions. The ability to influence the interfacial activity and rheology of polymers upon grafting them to inorganic particles, including clusters, may be expected to be broadly applicable to stabilization of emulsions and foams.
尽管人们对颗粒稳定的乳液和泡沫有着浓厚兴趣，但聚合物接枝到无机纳米颗粒上时降低油水界面表面张力的机制仍未得到很好的理解。本文采用了一种简单且通用的自由基“接枝穿入”技术，在无需催化剂的情况下，将高有机含量（按重量计）的聚（寡聚（环氧乙烷）单甲醚甲基丙烯酸酯）接枝到氧化铁簇上。在所得的约1μm杂化颗粒中，无机核和接枝结构使得接枝聚合物链在十二烷/水界面处具有较高的局部浓度，从而产生仅为0.003 w/v%（聚合物和颗粒核）的低表面张力。这些杂化无机/聚合物两亲性颗粒将表面张力降低36 mN/m的“临界颗粒浓度”（CPC），比游离聚合物（无无机核）产生几乎相同表面张力时的临界胶束浓度低30倍以上。低CPC有利于约1μm大尺寸杂化颗粒具有较高的吸附能（约10(6) kBT）、颗粒表面较高的局部聚合物浓度，以及可变形的杂化纳米簇核和聚合物链适应界面的能力。纳米簇核还增加了聚合物链在去离子水或合成海水中的缠结，在0.01 s(-1)时产生高达35,000 cP的粘度，而游离聚合物仅为600 cP。由于这些界面和流变特性，杂化颗粒在低至0.01 w/v%的浓度下就能稳定水包油乳液，平均液滴尺寸低至30μm。相比之下，游离聚合物的本体粘度较低，无法稳定乳液。将聚合物接枝到无机颗粒（包括簇）上时影响聚合物界面活性和流变学的能力，有望广泛应用于乳液和泡沫的稳定。
