【作者】 延辉；

【导师】 苑世领；

【作者基本信息】 山东大学 ， 理论与计算化学， 2011， 博士

【摘要】 表面活性剂是一类分子中同时含有亲水部分和疏水部分的物质。由于表面活性剂这种双亲(亲油和亲水)特性,使得这类化合物具有与其它有机化合物所不同的独特性能,即表现在溶液表(界)面的吸附以及溶液中有序组合体的形成。界面吸附性质使表面活性剂能够改变界面张力,提高毛细管数以及改变表面的润湿性等,而溶液中分子有序组合体的形成使表面活性剂溶液能够成为具有实际意义的功能体系。近些年来,由于表面活性剂体系的研究发展迅猛,在工业领域的诸多方面都有着广泛的应用。其中,表面活性剂作为驱油剂在三次采油中占有重要的地位。随着油田的不断开发,油藏的温度和矿化度愈来愈高,给强化原油采收率技术的推广带来了严重影响。驱油过程中常用的阴离子表面活性剂,如硫酸盐、羧酸盐、磷酸酯盐,磺酸盐(包括重烷基苯磺酸盐、石油磺酸盐等)在高矿化度下,均易与地表水中的二价离子Ca2+和Mg2+等高价阳离子形成沉淀使其失去界面活性,而非离子表面活性剂则由于浊点现象也往往会在高温条件下沉淀析出。油藏条件的不断变化对驱油用表面活性剂提出了更高的要求：不但要就有较高的降低油／水界面张力的能力和低的吸附损失,而且要耐盐。为了适应当前的油藏状况,有关表面活性剂耐盐性的研究一直是备受重视的课题。尽管目前已经有很多先进的实验方法用于表面活性剂溶液的研究,但主要集中于盐对表面活性剂溶液宏观性质的影响,例如其临界胶束浓度、表面活性剂聚集体、流变性质等,这些研究主要通过体系物理化学性质的变化来推测无机盐与表面活性剂分子之间相互作用的机理,从微观角度来说缺乏确切的说明。因此开展相关的理论研究,从分子水平上研究表面活性剂与无机离子之间的相互作用机理,对于指导适合于不同应用表面活性剂的分子设计,尤其对于筛选高矿化度油藏强化采油技术用的驱油体系具有重要的理论意义和实用价值。本论文围绕几种常用驱油表面活性剂开展一系列的理论研究工作。一方面通过量子力学方法研究了表面活性剂单分子性质,探讨其微观电子结构对其本身物理化学性质的影响。另一方面应用密度泛函方法研究了表面活性剂和二价阳离子的相互作用,通过对比它们之间的结合能以及对表面活性剂分子电荷分布的影响,提出表面活性剂与无机阳离子之间的结合模型。再一方面,针对不同阴离子表面活性剂构建自组装体系,应用分子动力学方法研究聚集体内部的微环境,以及离子对表面活性剂聚集体系结构的影响。重点讨论了二价阳离子影响表面活性剂极性头水合结构,通过计算表面活性剂的极性头基和阳离子之间的均力势,反映不同表面活性剂与离子结合能力的强弱,进而讨论不同表面活性剂耐盐性差异的本质,为驱油表面活性剂的筛选及分子设计提供理论依据。本论文主要创新成果如下：1.用分子动力学模拟方法研究了荧光分子芘分子在十二烷基硫酸钠胶束中的增溶作用,在分子水平上直观地给出了单个芘分子在胶束中的增溶位点。通过改变荧光分子的浓度,模拟得到了实验中的激发态二聚物的结构。同时对两个芘分子之间的取向性以及距离关系进行了探讨,给出了其π-π共轭结构信息。研究发现激发态二聚物的形成对于荧光分子在胶束中的增溶位置有很大的影响。单个芘分子增溶在胶束的内核区域,而激发态二聚物则增溶在胶束的栅栏层区域。这些研究可以提供实验上难以获取的微观信息,进而为更好地了解及利用两亲分子有序组合体提供一定的帮助。2.采用量子力学和分子动力学方法对十二烷基苯磺酸钠的单分子性质,以及从溶液相到气液界面组合体的动力学过程进行了研究。通过探讨SDBS单分子的结构性质以及其极性头水合层的电荷分布信息,加深了人们对表面活性剂电子结构等微观性质决定其聚集体结构和其物理性质的认识。在量子化学计算单分子性质的基础上,选择了合适的力场与计算模型,对十二烷基苯磺酸钠在溶液相及气液界面上的行为进行了分子动力学模拟。通过空间分布函数展现了十二烷基苯磺酸钠极性头的水合层结构。从表面活性剂极性头和离子的之间的均力势分析得知,离子如果与极性头相互作用,必须要克服极性头周围水层的能垒,才能穿过极性头水合层或者破坏极性头周围的氢键结构与极性头发生作用。由于表面活性剂烷烃链的疏水作用,十二烷基苯磺酸钠分子能够自发地从溶液相迁移到气液界面形成聚集体。由表面活性剂在气液界面上分子链的取向性函数发现,十二烷基苯磺酸钠分子在界面上倾向于有序排列。3.研究了驱油用表面活性剂的耐盐性机理,揭示了表面活性剂与离子的相互作用机制,解释了不同表面活性剂之间耐盐性能的差异。(1)研究了两性表面活性剂磺基甜菜碱与阳、阴离子之间的相互作用,揭示了两性表面活性剂和阴阳离子的结合方式,即磺酸根中的两个氧原子与阳离子发生稳定结合；而正电荷中心与阴离子之间采用侧面结合方式形成稳定结构。通过计算甜菜碱和阴阳离子作用后分子上的电荷分布,发现桥联亚甲基和α-亚甲基带有较大的电荷,可以看作是正、负电荷中心的一部分,体现在表面活性剂极性头扩大。同时烷烃链带有了部分弱电荷,使胶束内部带有了部分极性,此种极性介于烷烃相和水相的极性之间。(2)采用分子动力学方法研究了十二烷基硫酸钠和十二烷基磺酸钠在CaCl2、MgCl2溶液中的聚集行为。重点讨论钙、镁离子存在情况下,聚集体水合结构的变化,以及极性头和阳离子之间的相互作用。结果表明表面活性剂与二价离子的结合能力是由两者之间均力势之间的溶剂分离最小点决定的。其中,阻隔极性头和离子相互作用主要是由于离子进入极性头水合层,导致水合层氢键结构变化引起的。当离子进入极性头的水合层后,通过径向分布函数和空间分布函数展现了极性头水合层结构的变化。通过考察离子和极性头之间的均力势,发现Ca2+和Mg2+与十二烷基磺酸钠的之间的势垒要高于十二烷基硫酸钠体系,说明钙、镁离子更容易与十二烷基硫酸钠发生结合,这意味着十二烷基磺酸钠的耐钙镁离子的能力要强于十二烷基硫酸钠。所以磺酸基表面活性剂在高盐油藏中的驱油中的效率要高于硫酸盐表面活性剂。(3)采用分子动力学方法研究了十二烷基磺酸钠和十二烷基羧酸钠在盐溶液界面上的聚集行为。发现二价盐离子能够影响表面活性剂在气液界面的聚集,二价离子和表面活性剂分子之间形成的盐桥结构是导致表面活性剂聚集体结构更加紧密的原因。同样地,根据离子和表面活性剂之间的均力势能够定性地反映不同表面活性剂的耐盐性差异。更多还原

【Abstract】 Surfactants are amphiphilic substances which have their hydrophilic and hydrophobic structural parts. Thus, surfactants exhibit many unusual physical properties compared with other compounds. That is, surfactants can be adsorbed as an orientated monolayer at air/vapor or oil/water interfaces and self-assemble aggregate in the solution. Therefore, surfactants reduce the surface/interfacial tension, change the wetting of the surface by adsorbing at the interface, and can also assemble in the bulk solution into a variety of aggregates. The property of these aggregates is essential in many biological processes and is used in many industrial and domestic applications.Surfactants play an important role in enhanced oil recovery (EOR). Recently, with the continuous development of the oil fields, the temperature and the salinity rise which reduces the efficiency of the EOR. The most widely used ionic surfactants such as sulfonate, sulfate, carboxylate and organic phosphate may bind with Ca2+, Mg2+ and the salt-out occurred with ions in the solution. Thus, the surfactants loose their surface activity. The non-ionic surfactants are also precipitated under high temperature conditions. Thus, it is meaningful to investigate the salt tolerance and temperature resistance of EOR surfactants.There are already many investigations on the solution of surfactants using many advanced experimental methods. However, these investigations are mostly focused on the macroscopic properties, such as critical micelle concentration, aggregation number, rheological properties et al. These studies usually inter the mechanism of the interaction between salt and surfactants by observing the changes of the physic-chemical properties of the surfactants. Thus, the precise description of the mechanism from the microscopic view is scare. Therefore, it is of great meaningful to investigate the salt-tolerant mechanism between surfactants and salts at molecular level.In this dissertation, a series of theoretical studies have been carried out for several EOR surfactants. On the one hand, by performing quantum mechanism calculations, we investigated the properties of the single molecule which decided the physic-chemical characteristics of the surfactants. On the other hand, we investigated the interaction between the surfactant and the divalent ions by performing density functional theory (DFT) calculations. By comparing the difference between the bind energies between surfactants and ions, we proposed the binding model between cations and surfactants. Then, we performed molecular dynamics on the surfactants aggregates to investigate the interior micro-environment of the aggregates. We focused on the influence of cations on the hydration shell of the surfactants. By calculating the potential of mean force between surfactants and ions, the binding energy between them are shown. It can reflect the difference of the salt tolerant among the EOR surfactants. The important and valuable results in this dissertation can be summarized as follows:1. We performed molecular dynamics to study the solubilization of pyrene in the SDS micelle. The distribution of the single pyrene can be observed at molecular level. By changing the concentration of pyrene, the structure of the excimer was obtained by the simulation. Meanwhile, the orientation and the distance between the pyrene in the excimer were investigated and theπ-πconjugation structure was confirmed. Our simulation showed that free pyrene can be solubilized into the micelle spontaneously and prefers to be located in the hydrophobic core region, while two pyrene molecules are found to be distributed mainly in the palisade layer. These results are helpful and meaningful to utilize the surfactant aggregates better.2. Quantum mechanics (QM) method was used to calculate molecular properties of sodium dodecylbenzenesulfonate (SDBS) in vcuum and in solution. Moreover, molecular dynamics (MD) simulations have been performed to determine the dynamic behavior of SDBS moving from the bulk solution to the air/water interface.QM calculations suggest that two headgroup oxygen atoms on each surfactant molecule interact with a Na+ ion, despite the availability of three oxygen atoms in the headgroup. MD simulations showed that the Na+ ion must overcome the energy barrier between two solvent layers around the headgroup to form stable ion pair in solution, which is consistent with experimental results. In the simulation, in moving from the bulk to the interface, SDBS can aggregate in a short time, and the adsorption adopts a preferred orientation. The results indicate that formation of favorable hydrophobic interactions of the surfactant alkyl chains is the origin of interfacial adsorption of SDBS.3. The mechanics of the salt tolerance of EOR surfactants was investigated by QM and MM methods. These results reveal the interaction mechanism between surfactants and ions. (1) The structure of zwitterionic surfactant sulfobetaine, i.e. N-Dodecyl-N, N-dimethyl-3-ammonio-l-propanesulfonate, was optimized using density functional theory (DFT) and the interactions between the surfactant and Ca2+ or Cl- ions were studied at the molecular level. The results showed that:ⅰ) a 2:1 type pair between zwitterionic negative center (-SO3-) and Ca2+ was formed,ⅱ) the positive center (-N+(CH3)2-) bound with one Cl- through two methyl groups and one methylene which connect to N atom. Since there are some weak charges on the methylene nearest to the polar groups, the negative and positive centers in the polar group of surfactant should be re-divided. The calculation also showed that the tail chain has a weak charge resulting in the core of the micelle having polarity. This core polarity of the micelle is somewhere between the oil phase polarity and the water phase polarity, which favors surfactant aggregation in solution.(2) Molecular dynamics studies were performed to study surfactant sodium dodecyl sulfate (SDS) and sodium dodecyl sulfonate (SDSn) in the solution with Ca2+ and Mg2+. We focused on the on the influence of cations on the hydration shell of the surfactants. Our results showed that the combination between the headgroup of surfactant and Ca2+ or Mg2+ is prevented not by the hydrate shells, but by a deep stabilizing minimum formed in the potential of mean force between the interacting ion pair. They can disturb the original H-bonding structure of water around the headgroup, leading to the decrease of the H-bonding number.The potential of mean force showed that the energy barriers of ion pair between the headgroup and Ca2+ and Mg2+ in SDSn system are more than those in SDS system, and the water coordinate numbers for Ca2+ or Mg2+ in SDS solution are the lowest. It indicates that SDS surfactant easily combines the ions compared with the SDSn surfactant, and it has strong effect on the original hydration structure. These results can be explained the reason that sulfonate surfactant (such as SDSn) has better efficient in salt solution with Ca2+ and Mg2+ in enhanced oil recovery (EOR) experiment.(3) The effect of Ca2+ ions on the hydration shell of sodium dodecyl carboxylate (SDC) and sodium dodecyl sulfonate (SDSn) monolayer at vapor/liquid interfaces was studied using molecular dynamics simulations. The simulations indicate that the adsorption structure of both surfactants not only depends on the surfactant surface coverage, but also on the Ca2+ ions circumstances. The PMFs show that the energy barrier of ion-pairs between the SDSn headgroup and Ca2+ is higher than that in SDC systems, which means sulfonate surfactants are more efficient in saline circumstance in EOR experiments.更多还原