Magi Meconi Giulia, Ballard Nicholas, Asua José M, Zangi Ronen
POLYMAT & Departamento de Química Aplicada, Facultad de Ciencias Químicas, University of the Basque Country UPV/EHU, Avenida de Tolosa 72, 20018, Donostia-San Sebastián, Spain.
Phys Chem Chem Phys. 2017 Dec 6;19(47):31692-31705. doi: 10.1039/c7cp05206e.
Although surfactants are known to play a vital role in polymerization reactions carried out in dispersed media, many aspects of their use are poorly understood, perhaps none more so than the vastly different action of ionic and nonionic surfactants in emulsion polymerization. In this work, we combine experimental measurements of emulsion polymerization of styrene with atomistic molecular dynamics simulations to better understand the behavior of surfactants at monomer/polymer-water interfaces. In a batch emulsion polymerization of styrene, the nonionic surfactant Disponil AFX 1080 leads to two nucleation periods, in contrast to the behavior observed for the ionic surfactant SDS. This can be explained by the absorption of the nonionic surfactant into the organic phase at the early stages of the polymerization reaction which is then released as the reaction progresses. Indeed, we find that the partition coefficient of the surfactant between the organic phase and water increases with the amount of monomer in the former, and preferential partitioning is detected to organic phases containing at least 55% styrene. Results from molecular dynamics simulations confirm that spontaneous dissolution of the non-ionic surfactant into a styrene-rich organic phase occurs above a critical concentration of the surfactant adsorbed at the interface. Above this critical concentration, a linear correlation between the amount of surfactant adsorbed at the interface and that absorbed inside the organic phase is observed. To facilitate this absorption into a completely hydrophobic medium, water molecules accompany the intruding surfactants. Similar simulations but with the ionic surfactant instead did not result in any absorption of the surfactant into a neat styrene phase, likely because of its strongly hydrophilic head group. The unusual partitioning behavior of nonionic surfactants explains a number of observable features of emulsion polymerization reactions which use nonionic surfactants and should help with future development of processes for improved control over polymerization.
尽管表面活性剂在分散介质中进行的聚合反应中起着至关重要的作用，但其使用的许多方面仍知之甚少，或许离子型和非离子型表面活性剂在乳液聚合中截然不同的作用更是如此。在这项工作中，我们将苯乙烯乳液聚合的实验测量与原子分子动力学模拟相结合，以更好地理解表面活性剂在单体/聚合物-水界面的行为。在苯乙烯的间歇乳液聚合中，与离子型表面活性剂十二烷基硫酸钠（SDS）的行为不同，非离子型表面活性剂Disponil AFX 1080导致两个成核期。这可以通过聚合反应早期非离子型表面活性剂吸收到有机相中，然后随着反应进行而释放来解释。事实上，我们发现表面活性剂在有机相和水之间的分配系数随着前者中单体的量增加而增加，并且检测到优先分配到含有至少55%苯乙烯的有机相中。分子动力学模拟结果证实，在界面吸附的表面活性剂达到临界浓度以上时，非离子型表面活性剂会自发溶解到富含苯乙烯的有机相中。高于此临界浓度时，可以观察到界面吸附的表面活性剂数量与有机相内吸收的表面活性剂数量之间存在线性关系。为了促进这种吸收到完全疏水的介质中，水分子伴随着侵入的表面活性剂。但是用离子型表面活性剂进行类似模拟时，没有导致表面活性剂吸收到纯苯乙烯相中，这可能是因为其强亲水的头部基团。非离子型表面活性剂这种不寻常的分配行为解释了许多使用非离子型表面活性剂的乳液聚合反应的可观察特征，并且应该有助于未来改进聚合控制工艺的开发。
