【作者】 王华；

【导师】 苑世领；

【作者基本信息】 山东大学 ， 理论与计算化学， 2014， 博士

【摘要】 诺贝尔获奖者Pierre-Gilles de Gennens曾经指出：“离开表面活性剂,我们对90%的工业问题都将无能为力(without surfactants, we would be compleltely unable to confront90%industrial issues)".实践表明,单一表面活性剂的效果常常不及混合物。将表面活性剂与聚合物或者不同类型的表活性剂按适当条件复配,可以产生协同效应,从而提高活性和稳定性,获得单一表面活性剂所没有的独特性质,如丰富的相行为、多变的聚集体形貌、更低的表面张力、增强粘性等等。基于以上特殊性能,含有表面活性剂的复配体系在人们的日常生活和工业中发挥着重要的作用,从而引起人们的广泛关注。随着研究的不断深入,采用分子动力学模拟方法在分子水平上阐述含有表面活性剂复配体系的自组装规律及它们之间的相互作用显得尤为重要。而且,与传统的实验研究方法相比,分子动力学模拟可以大大降低实验的盲目性和重复实验的耗材与耗时性,同时还可以直观地观察到复配体系中聚集体的三维空间结构,从分子水平上探讨聚集体结构演变过程,加深对复配体系之间相互作用理解,对于实验设计和开发新型的复配体系具有重要的指导意义。本论文主要围绕最新实验报道的表面活性剂与聚合物及阴阳离子混合表面活性剂两类复配体系展开一系列的研究。一方面运用分子动力学模拟的方法研究表面活性剂与聚合物自组装机理,探讨表面活性剂与聚合物在体相与界面中不同的吸附行为,提出合理的吸附机理,为实验设计和改良表面活性剂与聚合物这类复配体系提供理论依据。另一方面借助分子动力学模拟研究阴阳离子表面活性剂复配体系的自组装机理,对自组装过程进行了深入探讨,从微观层次揭示了复配比对聚集体结构与性质的影响,为实验根据阴阳离子表面活性剂的复配比设计分子有序组合体起到了积极的理论指导。本论文主要研究内容和创新成果归纳如下：(1)用分子动力学模拟方法研究了十二烷基硫酸钠(SDS)与聚丙烯酰胺(PAM)之间的相互作用,揭示了表面活性剂与聚合物的缔合方式,即表面活性剂通过疏水作用且以胶束的形式吸附在聚合物链上,而聚合物坐落在表面活性剂胶束的亲水区域与疏水区域的界面处。通过观察与分析动力学轨迹与聚合物链构象变化,可以将表面活性剂与聚合物的自组装过程分为三步：(ⅰ)PAM链迅速卷曲,PAM的回转半径降低至最低值,此时少量SDS分子吸附在PAM链上；(ii)越来越多的SDS的分子吸附在PAM链上使得PAM链逐渐伸展,PAM的回转半径开始逐渐增大直至最大值；(iii) PAM链完全伸展开且其回转半径在最大值附近波动,体系中形成实验中所说的“珍珠项链”稳定结构。根据SDS的吸附对PAM链结构影响进一步研究了SDS对限制性聚合物(聚合物刷)的影响。通过计算聚合物刷高度的变化,重点讨论了SDS浓度与接枝密度对聚合物刷(PEO brush)结构与性质的影响,接枝密度比较低时,PEO brush的高度主要由吸附的SDS胶束影响；在接枝密度比较高时,由于PEO链之间的空间排斥作用对SDS胶束吸附的抑制,此时PEO brush的高度主要由接枝密度控制,这与实验的结论非常一致。所以,这些研究提供了实验上难以获得的微观信息,进而可以更好地了解及设计表面活性剂与聚合物这类复配体系。(2)应用分子动力学方法研究带有相反电荷的离子表面活性剂(C12TAB)与聚电解质(NaPAA)在气/液界面的吸附结构与性质,该模拟工作基于Zhang等人对C12TAB与NaPAA在气/液界面吸附现象的相关实验报道。模拟结果证实了在低表面活性剂浓度时,表/聚复合物在气/液界面以单层吸附,在高表面活性剂浓度时,采取多层吸附的三维空间结构。另外,模拟结果表明NaPAA本身没有表面活性,只有与表面活性剂复配形成复合物之后才具有表面活性；C12TAB通过静电作用吸附在NaPAA链上,而C12TAB疏水作用带动着C12TAB与NaPAA的复合物吸附在气/液界面上。在表面活性剂浓度比较高时,C12TAB尾链之间的疏水作用诱导着复合物采取尾对尾(tail-to-tail)层层排列方式吸附到气/液界面上；另外NaPAA和C12TAB通过静电作用形成复合物的过程是离子交换过程。我们的模拟结果加深了人们对表面活性剂与聚合物这类复配体系的表面活性的理解。(3)采用Martini粗粒模拟方法研究SDS和DTAB混合体系的自组装过程。模拟结果表明表面活性剂自组装过程可以分为三个步骤：(ⅰ)表面活性剂活性剂单体分子迅速聚集形成亚稳态的低聚体,在这过程中,聚集体的聚集数直线增加；(ⅱ)在随后的几百纳秒内,低聚体之间发生碰撞融合形成更大的聚集体,在这过程中聚集体的聚集数呈现阶梯式的增长；(ⅲ)聚集体的聚集数和形貌都不再发生变化,意味着体系达到平衡。通过对复配体系中聚集体大小与形貌的分析证明了在SDS:DTAB=2:1时,聚集体的聚集数达到最大值,而且聚集体在复配体系中以结构规整的扁平状胶束存在,这与实验结论非常一致。所以,我们模拟工作不但从微观角度印证了实验结果,而且得到了复配体系的微观结构与性质等实验手段不能获得的信息,更加完善了该领域的工作,这对根据阴阳离子表面活性剂复配设计不同形貌与大小的分子有序体具有重要的指导意义。更多还原

【Abstract】 Nobel laureate Pierre-Gilles de Gennens once pointed out:"without surfactants, we whenever completely unable to confront90%industrial issues". Therefore, intensive efforts have been sent to investigate the property of surfactant. And it shows that the effect of single surfactant is often not as good as the mixtures. Under appropriate conditions, there have strong interactions between surfactant and polymer or different kinds of surfactants to form complexes. The new complexes in mixtures usually exhibit many fascinating and intriguing features which are different from any one component, such as rich phase, versatile aggregation morphology, lower surface tension, stronger viscosity and so on. Based on these special performances, the mixtures contained surfactant play an important role both in people’s daily life and industry.With the development of research, it is important to study the self-assembly rule of different mixtures containing surfatant as well as the interactions between them at molecular level by molecular dynamics simulation. Compared with the traditional experimental methods, molecular dynamics simulations could greatly reduce the blindness and revision test time-consuming of experiments. In additional, molecular simulation can not only provide a clear three-dimensional structure picture of the aggregates formed in studied systems, but also allow us to study the self-assembly process at molecular insight, and deepen in understanding the interaction mechanism between the mixtures, therefore it could be helpful and meaningful to design and explore new complex.In this dissertation, a series of molecular dynamics simulations have been carried out for several mixtures, including surfactant and polymer, catanionic surfactants, which were newly reported in experiments recently. On the one hand, molecular dynamics simulation were performed to investigated the self-assembly of surfactant and polymer complexes. We have studied the different adsorption behavior of surfactant on polymer at solution and interface. In addition, we also put forward the appropriate adsorption mechanism, which supplied some valuable theoretical information for designing and improving complexes of surfactant and polymer. On the other hand, we have studied the interaction between oppositely charged surfactants by coarse-grained simulation with the framework of Martini force field. We mainly forced on the self-assembly process of catanionic surfactants and the effect of mixing ratio on the structure and properties of aggregates.The important and valuable results in this dissertation can be summarized as follows:(1) We have performed coarse-grained molecular dynamics simulation to investigate the interaction between polyacrylamide (PAM) and sodium dodecylsulfate (SDS) in aqueous solution. The simulation results revealed that SDS micelle would be adsorbed on the polymer chain by the hydrophobic interaction between the surfactant hydrophobic tails and polymer backbone, and PAM was located at the interface of the hydrophobic and hydrophilic regions of the SDS micelle. The formation process of SDS-PAM complexes was divided into three stages by monitor the trajectory and the conformation of PAM,(i) PAM quickly curled with the adsorption of some SDS molecules until the radius of gyration (Rg) of polymer reaches a minimum;(ii) due to more and more SDS adsorbed, PAM stretched slowly with the increase of Rg of PAM;(iii) the commonly accepted "necklace" structure was formed when Rg just fluctuated around a value.According to the effect of surfactant on the polymer conformation, the interaction between SDS and polymer in very constrained environment (PEO, one end of a dense array of PEO chains attached to a solid surface) was studied by molecular dynamics simulations. The effects of adding SDS amount and grafting density on the adsorption behavior of SDS on the PEO brush were discussed. The simulation showed that the height of PEO brush was mainly affected by the adsorbed SDS at low grafting density; however, at high grafting density it was primarily controlled by the grafting density on solid surface. With the increase of grafting density, the adsorbed EO monomers were decrease due to the excluded volume interaction among polymer chains and the electrostatic repulsion among the negatively charged surfactant micelles. Our work about the adsorption of surfactant on polymer chains showed a clear microscopic picture of complex of surfactant and polymer, which are difficult to obtain from experiments, and also make a better understand and design the surfactant and polymer complex.(2) Molecular dynamic simulations were performed to study the adsorption of oppositely charged sodium poly(acrylic acid)(NaPAA) and dodecyltrimethylammonium bromide (DTAB) at the air/water interface. Our results have proven that the transition from monolayer adsorption of the complex of NaPAA and DTAB to a multilayer structure was observed with the increasing of DTAB concentration. And in the multilayer adsorption, two layers of DTAB which adopted tail-to-tail arrangement were used to link the two polyelectrolyte chains. It indicated that electrostatic interaction was the driving force which bound DTAB to NaPAA, while the hydrophobic interaction between surfactant tails was the main force in inducing a layer structure of the complex at high DTAB concentration. The dynamic properties of counterions implied that the formation of polyelectrolyte-surfactant complex was an ion-exchange process, which was in good accordance with experimental results. Our simulation work provided a microscopic perspective on the adsorption of oppositely charged surfactant and polyelectrolyte at interface that would be helpful and meaningful in studying the surface properties of surfactant and polymer complexes.(3) The self-assembly process and structure properties of aggregates formed by oppositely charged surfactant, SDS and DTAB, have been studied by coarse-grained simulation within the framework of the Martini force field. The simulation results indicated that the self-assembly processes can be divided into three stages:monomers firstly fast gather to form disorded oligomers with the continuous growth of the cluster size; and then collisions and merge between small aggregates to form the larger clusters, which caused the clusters growing like a ladder; the last stage, the size and structure of aggregates reached stable. Analysis of the size distribution and morphology of the clusters showed that the aggregates reached to the biggest when the mixing ratio of SDS and DTAB was2:1, and discal micelles with the hydrophilic headgroups of catanionic surfactants interdigitation and parallel in the surface was only formed in mixtures, which were are in excellent agreement with experiment. Therefore, our work not only obtained a clear microscopic picture to extract information about dynamic and structural properties, which are difficult to obtain from experiments, but also make a good guide for designing different self-assemble aggregates of oppositely charged surfactant.更多还原