Study On The Aggregation Behavior And Properties Of Perfluoroalkyl Carboxylates And Their Mixtures In Solutions

Surfactants, active in surface and interface, can greatly decrease the surface and interfacial tension both in efficiency and capability. Above the critical micelle concentration (cmc), surfactants can self-assemble into various regular molecule aggregates, which present a range of novel properties and important applications. The wide-ranging applications of surfactant aggregates as drug delivery, selective biomembranes and microreactors depend on the structure and properties of aggregates. The synergistic effect of mixed surfactant systems makes them with fascinating properties, such as low critic aggregates concentration, low surface tension, colorful morphologies of colloid particles. Fluorocarbon surfactant is one kind of the specific surfactants. Because of their special molecular structure, fluorocarbon surfactant are both hydrophobic and oleophobic, and show exceptional activity, thermal and biological inertness, and higher Krafft point. These properties have triggered wide applications of fluorocarbon surfactants in some exceptive situations, which can not be replaced by the analogous hydrocarbon counterparts. In practical applications, fluorocarbon surfactants are very often used in mixed systems with hydrocarbon surfactants. In the past decades, special attention to the hydrocarbon and fluorocarbon surfactant mixtures in solution has revealed the non-ideal behavior in aggregation experimentally as well as theoretically.In this thesis we intensively investigate the aggregation behavior and solution properties of perfluoroalkyl carboxylates and their mixtures in solutions. By changing the surfactant concentrations, mixed molar fractions, counterions and temperature, various regular molecule aggregates can formed in perfluoroalkyl carboxylates and their mixtures in solutions, such as: lamellar lyotropic liquid crystal phases can form in aqueous solutions present in lithium perfluorinated fatty acid salts; the formation of pH-sensitive vesicles is discovered in the mixtures of perfluorolauric acid (PFLA) and its salts (PFL-Na and PFL-Li); A gel state from densely packed multilamellar vesicles in the crystalline state can form in PFLA/C14DMAO/H2O mixed system; micelle formation for PFNT is confirmed in bmimBF4solutions. Various techniques, such as freeze fracture transmission electron microscopy, atomic force microscopy, polarizing-microsopy, small angle X-ray scattering, differential scanning calorimetry, rheological measurements etc. are employed to investigate the formation of aggregates, microstructures and structural transition in detail, and give the theoretical explanation. In addition, the rheological properties of the solutions are studied in detail, too. The specific contents of this doctoral thesis are shown in the follow:Chapter I is a brief introduction of the research background of this work, in which the basic knowledge of surfactant and as well as the formation regulation of surfactant aggregation in aqueous solutions are reviewed. Then the aggregation behavior, self-assembly aggregation, and interaction of hydrocarbon and fluorocarbon surfactant mixtures in solution are reviewed from a worldwide angle of view. The objective and the scientific significance of this doctoral dissertation are also pointed out at the end of this part.In chapter II, we investigated the lyotropic liquid crystal (LLC) phases of lithium perfluorinated fatty acid salts (PFL/M-Li) in aqueous solutions. From our experimental results, we can find that the formation of lamellar LLC phases of perfluorofatty acid salts has some connection with their chain length and counterions, which is confirmed by NVT molecular dynamics simulation. Because the volume of Li+is very small, leading to much higher charging density. This is the predominant reason that the lithium perfluorofatty acid salts have lower Krafft points and can form LLC phases in aquous solutions with appropriate length of fluorocarbon chains. Additionally, temperature and concentration of surfactant play an important role in the formation of LLC phases. With the increasing of chain lengths, the LLC-phase will become more temperature-resistable. From the results of MD simulation, one can observe that the molecule bilayers of PFL－are interdigitated together, which is because the fluorocarbon chains of PFL－with a high rigidity are in the fluid state at room temperature.In chaper III, we have detailedly investigated the phase behaviors and rheological properties of PFLA/PFL-Na/H2O and PFLA/PFL-Li/H2O systems. Similar to hydrocarbon fatty acid vesicles, the formation of perfluorofatty acid vesicles is restricted a rather narrow pH range (ca.2.85-3.20in PFLA/PFL-Na/H2O system and ca.2.90-3.20in PFLA/PFL-Li/H2O system), so they are "pH-sensitive". Because of the differences between fluorocarbon and hydrocarbon chains, perfluorocarboxylic acid vesicles are more stable. The system of perfluorofatty acid vesicles exhibites much more interesting rheological behavior, compared to hydrocarbon fatty acid vesicles. For instance, the perfluorofatty acid vesicle solutions shows much higher yield stress and viscoelasticity, which depend on two factors:i) the fluorinated alkyl chains of PFL－which are in the crystalline state at room temperature because of their rigid chains compared to analogous hydrocarbon chains; ii) the packing of the vesicles which is very densely. The mechanism of perfluorocarboxylic-acid vesicle formation has a better chance to follow Haines'theory:The protonated PFL-ions firstly form stable dimers through strong hydrogen bonds, and then self assemble to vesicles through attractive hydrophobic interaction and repulsive electrostatic force.In chapter IV We have investigated the new salt-free cat-anionic C14DMAO and PFLA surfactant system. The phase behavior of C14DMAO/PFLA/water shows a classic birefringent La-phase and a unusual birefringent gel-phase, which is made from close-packed vesicles. However, the vesicles in each phase are not the "same" vesicles, due to the special properties of the fluorocarbon surfactants, such as larger alkyl chain volume, higher Kraft points, higher surface activity and stronger hydrophobic interactions compared to their hydrogenated counterparts. On the C14DMAO-rich side of this system, the formation of salt-free, cat-anionic, C14DMAOH＋/－OOCC11F23vesicles is similar to classic salt-free cat-anionic surfactant systems. In contrast, on the PFLA-rich side, some C14DMAO molecules participate in vesicle formation, but most are present as micelles, stabilizing them to form a three-dimensional gel-phase network. Thus, the hydrocarbon surfactants have an important effect on the form and stability of the perfluorinated vesicles and gelation. In addition, the observed phases and stability are the result of cooperation between electrostatic interaction and hydrophobic forces, and the gel state also due to the crystalline form of fluorocarbon chains. These observations should be helpful for surfactant science research into simulated biomaterials and on the gels formed from closely packed vesicles.In the last chapter, we investigated the micellization of perfluorononanoic carboxylate ammonium salts with different counterions (PFNM, PFND, and PFNT) in water and bmimBF4solutions. Changes in the counterions of the fluorocarbon surfactants have an effect on their surface activities in aqueous solution. With the increase of counterion volume, the cmc value of relevant fluorinated surfactant decreases. This is because the counterions with larger size, such as (NMe4+) can be little hydrated, which can screen the electrostatic repulsion of the headgroups of the fluorocarbon surfactant and thus facilitate micelle formation. However, the fluorocarbon surfactants can dissolve and form micelles in bmimBF4only when they provide with largest counterion (PFNT). This is because the four methyls of counterion (NMe4+) disperse the charge of the cations, which could weaken the electrostatic interaction between the ion pair of the surfactant, leading to a higher degree of counterion binding. So, wo believe that the key of the micelle formation for fluorocarbon surfactants in ILs is weakening the electrostatic interaction between the ion pair of the surfactant, and increasing the volume of their counterions is an efficient way. The driving force of the micellization for fluorocarbon surfactants in bmimBF4is the solvophobic effect, due to the hydrophobic and oleophobic properties of fluorocarbon chains.