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.
POLYMAT & Departamento de Química Orgánica I, University of the Basque Country UPV/EHU, Avenida de Tolosa 72, 20018, Donostia-San Sebastián, Spain and IKERBASQUE, Basque Foundation for Science, María Díaz de Haro 3, 48013 Bilbao, Spain.
Soft Matter. 2016 Dec 6;12(48):9692-9704. doi: 10.1039/c6sm01878e.
We report combined experimental and computational studies aiming to elucidate the adsorption properties of ionic and nonionic surfactants on hydrophobic polymer surface such as poly(styrene). To represent these two types of surfactants, we choose sodium dodecyl sulfate and poly(ethylene glycol)-poly(ethylene) block copolymers, both commonly utilized in emulsion polymerization. By applying quartz crystal microbalance with dissipation monitoring we find that the non-ionic surfactants are desorbed from the poly(styrene) surface slower, and at low surfactant concentrations they adsorb with stronger energy, than the ionic surfactant. If fact, from molecular dynamics simulations we obtain that the effective attractive force of these nonionic surfactants to the surface increases with the decrease of their concentration, whereas, the ionic surfactant exhibits mildly the opposite trend. We argue that the difference in this contrasting behavior stems from the physico-chemical properties of the head group. Ionic surfactants characterized by small and strongly hydrophilic head groups form an ordered self-assembled structure at the interface whereas, non-ionic surfactants with long and weakly hydrophilic head groups, which are also characterized by low persistence lengths, generate a disordered layer. Consequently, upon an increase in concentration, the layer formed by the nonionic surfactants prevents the aprotic poly(ethylene glycol) head groups to satisfy all their hydrogen bonds capabilities. As a response, water molecules intrude this surfactant layer and partially compensate for the missing interactions, however, at the expense of their ability to form hydrogen bonds as in bulk. This loss of hydrogen bonds, either of the head groups or of the intruding water molecules, is the reason the nonionic surfactants weaken their effective attraction to the interface with the increase in concentration.
我们报告了组合实验和计算研究，旨在阐明离子型和非离子型表面活性剂在疏水性聚合物表面（如聚苯乙烯）上的吸附特性。为了代表这两种类型的表面活性剂，我们选择了十二烷基硫酸钠和聚（乙二醇）-聚（乙烯）嵌段共聚物，这两种表面活性剂都常用于乳液聚合。通过应用石英晶体微天平与耗散监测，我们发现非离子型表面活性剂从聚苯乙烯表面解吸较慢，并且在低表面活性剂浓度下，它们的吸附能量更强，比离子型表面活性剂更强。实际上，从分子动力学模拟中，我们得出这些非离子型表面活性剂对表面的有效吸引力随着其浓度的降低而增加，而离子型表面活性剂则表现出轻微的相反趋势。我们认为这种相反行为的差异源于头基的物理化学性质。离子型表面活性剂的头基较小且亲水性强，在界面处形成有序的自组装结构，而非离子型表面活性剂的头基较长且亲水性弱，同时具有较低的持久长度，形成无序层。因此，随着浓度的增加，非离子型表面活性剂形成的层阻止了无质子的聚乙二醇头基满足其所有氢键能力。作为响应，水分子侵入这个表面活性剂层，并部分补偿了缺失的相互作用，但以牺牲它们在本体中形成氢键的能力为代价。这种氢键的损失，无论是头基还是侵入的水分子，都是非离子型表面活性剂随着浓度的增加而减弱其对界面的有效吸引力的原因。
