Aggregation Behavior Of The Mixed System Of Silicone Surfactant And Its Effect On Dispersing Carbon Nanotubes

Surfactant is an amphiphilic substance, which has hydrophilic and hydrophobic moieties. It can reduce surface tension at very low concentrations and form aggregates in solutions. Silicone surfactant is an efficient one with favorable biological compatibility and no environmental pollution. While mixed surfactant system can present remarkable advantage. This kind of system may provide a way of tailoring properties through simple composition variations, thus new structures may be obtained by changing the system composition rather than through synthesis of new materials. It has widely applications, such as detergent, cosmetic, medicine, food, coating and oil recovery. Surfactant is also used to disperse carbon nanotubes (CNTs) and dispersions with individual CNTs are obtained. Since the discovery of CNTs in1991, they are widely concerned due to unique properties and potential applications. To solve the agglomeration of CNTs, much research has been carried out. Recent years, techniques of computer simulation develop quickly. They have already been used in mixed surfactant and CNTs dispersion systems. Research is hoped to probe into microscopic structures not just macroscopic properties, in order to explain the mechanism. Herein, experimental investigation and computer simulation were combined to study aggregation behavior of mixed surfactant system in solution and at the air/water interface, as well as dispersing ability of different surfactant for CNTs in water. Obtained results may provide the theoretical foundation for application.Silicone surfactant favors to spread at interface and siloxane has strong interaction with CNTs, thus silicone surfactant may be a good dispersant of CNTs. In Chapter2, four silicone surfactants (SIE19, S2E38, S2E16and S1E16P9) were used to disperse CNTs in water. The effect of surfactant structure and concentration on dispersibility was considered. Sample with1000mg·L-1S1E16P9is the best dispersing agent and more amounts of individual CNTs is discovered. As surfactant concentration is higher, the amounts of dispersed CNTs increase firstly and then decrease in S1E19and S1E16P8systems, while the amounts increase to a certain value and then keep constant in S2E38and S2E16systems. Hydrophilic moiety polyoxyethylene (PEO) and hydrophobic moieties siloxane and polypropylene (PPO) are crucial factors on the dispersibility of CNTs. S2E38with more EO groups has stronger ability to disperse CNTs than S2E16. Dispersion with S1E16which contains less siloxane and EO groups is relatively unstable and disperses less CNTs. Similar results were obtained via molecular dynamics (MD) simulation. Compared with S1E19, S2E38and S2E16have stronger interactions with CNTs. Interaction energy of CNTs with S1E16P8which have PPO moiety but less siloxane groups is close to that with S2E16. Furthermore, the amounts of dispersed CNTs are larger in S1E16P8systems. From MD simulation, these four surfactants adsorb on CNTs mainly by van der Waals forces.In Chapter3, PEO modified trimethylsiloxane Ag-64was employed to disperse CNTs in water. Both of single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) were dispersed well. Surfactant concentration has an effect on dispersibility of CNTs. The dispersibility increases firstly and then reduces with surfactant concentration. Critical micelle concentration (cmc) of Ag-64rises ten times in the presence of CNTs, which means a lot of Ag-64adsorb on the surface of CNTs. From MD simulation, Ag-64adsorbs and wraps on CNTs by van der Waals interactions. The hydrophilicity of CNTs increases and they can disperse in water. Based on previous work of our research team, polymer like F127dispersed more amounts of CNTs in water but most of them are small bundles. On the other hand, it is discovered that F127has strong interaction with Ag-64via experimental research and dissipative particle dynamics (DPD) simulation. Therefore, the mixture of F127and Ag-64could be a good dispersant of CNTs. In the presence of F127, dispersibility of CNTs increases remarkable. At some certain concentrations of Ag-64and F127, a synergistic effect on dispersing CNTs appears. The quantity of dispersed CNTs exceeds the sum of each quantity of the dispersions with individual Ag-64and F127at the same concentration. The dispersion with2000mg·L-1Ag-64and250mg·L-1F127is the best dispersing agent in our research scope, in which there are large amounts of individual CNTs in solutions. Similar to pure Ag-64system, van der Waals force is the main fore for dispersants adsorbing on CNTs in F127/Ag-64mixed systems. From MD simulation, Ag-64has stronger interaction with CNTs than F127and it wraps on CNTs more quickly. Therefore, CNTs are easily to be exfoliated into individuals. At low concentration of F127, F127can interact with the Ag-64adsorbed on CNTs and form strong steric effect. Then more individual CNTs are stabilized in water.β-cyclodextrin (β-CD) is composed of7glucosidic units, owning a hydrophobic inner cavity and a hydrophilic outside surface. According to literatures, CD can adsorb on CNTs via van der Waals force and disperse them in water. CD/CNT comples can construct electrodes, which are used for electrochemistry and biological sensors. Thus, it is essential to carry out related theoretical research. In Chapter4, the dispersing ability of hydroxypropyl and hydroxybutyl modified β-CDs for SWNTs were studied by MD simulation. Head and tail of β-CDs can adsorb on SWNTs via van der Waals interactions. Modified β-CDs are prone to interact with SWNTs compared with β-CD. There is stronger interaction with SWNTs as the head of β-CD with longer C-2substituted group adsorbs on SWNTs. SWNT diameter is another effect on the interaction with β-CDs. The interaction energy increases with the diameter. Furthermore, systems in the presence and absence of water were both investigated. In the presence of water, the interacton energy between β-CDs and SWNTs is lower and it takes longer time for β-CDs to adsorb on SWNTs. However, the influence of substituted position and group on the interactions is similar to that in the absence of water.Silicone and fluorinated surfactants are widely used due to unique properties. Because of high prices, they usually mix with hydrocarbon surfactants in applications. Research on aggregation behavior of their mixtures has been developed for years. Compared with experimental methods, computer simulations have the potential to complement both experimental and other theoretical methods. In Chapter5, the effect of silicone, hydrocarbon and fluorocarbon surfactants (DSEP, HCEP and FCEP) on the interfacial properties of surfactants were investigated by MD simulation. Surfactants with the same hydrocarbon chains but different headgroups, which are sodium dodecylsulfate (SDS), dodecyltrimethylammonium bromide (DTAB) and octaethylene glycol monododecyl ether (C12E8), were employed to study the influence of headgroups on the interactions between surfactants. The optimal mixed system is hopefully found through this investigation. In simulation results, the structure of surfactant monolayer is consistent with the well-known surfactant organization at the air/water interface. Headgroups insert in water layer with counterions associating around them, and hydrocarbon tails are mostly excluded from water layer. The whole surfactant chain adsorb at air/water interface with a certain tilt angle. After adding HCEP or FCEP, the monolayers are more organized. HCEP and C12E8form mixed adsorption layer which is highly organized. The addition of DSEP makes C12E8monolayer disordered, but relatively organized monolayer is obtained in DTAB/DSEP system. Highly ordered monolayer and a small tilt angle could be related to favorable interfacial properties. Therefore, the system contained FCEP has the highest efficiency to reduce surface tension of water in SDS systems, and similar result is obtained in the presence of DSEP or HCEP in DTAB or C12E8systems, respectively. Furthermore, some phenomena in experiments were also observed in our simulations, such as the mutual-phobicity between fluorocarbon and hydrocarbon surfactants, flexible siloxane chain which is easily to spread at an interface and extremely stiff fluorocarbon chain.