Aggregation Behavior And Charactristics Of Cationic C_nTAB Binary Systems By Electrochemical Methods

In the research of surfactant mixtures solutions, monitoring of the continued change in surfactant concentration or its estimation in a dilute solution under equilibrium condition is a matter of concern to researchers. The isolated surfactant concentration can be determined by some methods such as the equilibrium dialysis, ultrafiltration, isothermal titration calorimetry (ITC) and surfactant ion-selective electrodes (SISEs). Compared to the other determination methods, the electromotive force method based on SISEs is favored by many researchers because of its advantages of low cost, high sensitivity, reproducibility, without disturbing by additions and need little amount of sample with almost no pretreatment. In the past decades, this method was applied successfully to the research of amphiphilic molecules systems in solution. Domestic papers involved in SISEs mainly focused on analytic determination environmental waste water samples, and external papers mainly focused on the research domain of interaction between surfactants and polymers. So far, no papers have been published about SISEs in the research of cationic/anionic surfactant system, the reason may be the preparation of cationic/anionic binary systems where ionic strength was usually inconsistent. Only when the samples maintain the same ionic strength, have the electromotive force (EMF) data comparability.The focus of this thesis have been on the following aspects:Construction of a series of cationic/anionic surfactant systems with almost the same ionic strength, i.e. TTAB/SU. TTAB/10-SU and CTAB/10-SU system, whose phase behaviors were studied in detail by conductivity, polarized light, EMF and rheology techniques, some important evidence for these binary systems were obtained. Detailed study on the interaction between long-chain alkyltrimethylammonium bromide CnTAB (n=16, CTAB;n=14, TTAB;n=12, DTAB) and nonionic triblocked copolymer Pluronic F127 (PEO97-PPO69-PEO97) under various NaBr salinity, different polymer concentration and different temperature were performed using EMF measurement based on SISEs and cyclic voltammetry (CV) measurement. The present dissertation includes five chapters.In chapter I, the research background about ion selective electrodes (ISEs), the study progress and the importance on SISEs applying to study of compounded system composed of cationic/anionic surfactants, polymers or biomacromolecules are summarized, including some problems about SISEs and corresponding settlement encountered during the testing procedure in our own research work.In chapter II, sodium tetraphenylborate (NaBQ4) and sodium dodecyl sulfate (SDS) respectively interact with n-cetyl-N, N, N-trimethylammonium bromide (CTAB) cationic surfactant by electrostatic force, which forms a series of neutral ion-pair complexes (CTA-BQ4, CTA-DS) as electroactive substance. Then, combining with bis-(2-ethylhexyl) phthalate (DOP) as plasticiser and polyvinyl chloride (PVC) with low molecular weight as film materials, SISEs were fabricated. The performance of SISEs was compared through EMF and surface tension measurements, and the results indicate that, when the ion pairs containing BQ4- as counter ion are selected as electroactive species, and the concentrations of DOP, PVC and the electroactive specie are respectively 112.5 mg/mL,75 mg/mL and 0.63 mmol/L, the SISEs perform best. Correspondingly, the SISEs show good stability and reproducibility, and the inaccuracy is less than 1 mV in 2 h continuous testing. In addition, the SISEs which are sensitive to anionic surfactant such as sodium undecanoate (SU) and sodium laurate (SL), were also fabricated by using the netural ion-pairs complexes (CTA-U and CTA-L) formed by CTAB interacting with SU and SL, respectively, as electroactive species, respectively. However, the EMF data of these electrodes can not completely coincide with the results of the surface tension. The response slope of Nernst is somewhat low, which may be relevent with the solubility of the ion-pairs complexes and the properties of the aqueous fatty acid sodium solution. Though largely influenced by low response slope of Nernst, potential drift and memory effect of membrane electrode, after carefully cleaning of the film surface, the SISEs can still be well applied in the monomer composition measurement of the surfactant mixtures.In chapter III, the phase behavior of catanionic surfactant mixtures, composed of alkyltrimethylammonium bromide (TTAB, CTAB) and fatty acid sodium salts (10-SU, SU), was researched by EMF and electrical conductivity measurements, observation through polarizing microscope, and rheological study. Then the SISEs sensitive to TTAB, CTAB and SU were used to determine the surfactant monomer concentration, in order to confirm a correlation between surfactant mole fraction in whole solution and its isolated monomer concentration. The results indicate that, with the increase of cationic surfactants at fixed concentration of mixtures, a series of phase changes were observed in sequence:a clear dilute micellar solution (L1-phase), a micellar/vesicular two-phase region (L1/La-phase), a single vesicle phase (Lav-phase), and a viscous micellar solution (L1-phase). Accompanied with the ordinal phase transitions, the concentration of the surfactant monomer gradually increases, especially for the transitions from L1-phase to Lav-phase to viscous L1-phase. For TTAB/10-SU and TTAB/SU mixed systems, the rheology behavior was studied when the molar fraction of TTAB (Xn(TTAB)) is 0.3,0.4,0.5, and 0.7, respectively, which reveals that the L1-phases all show viscosity at different Xn(TTAB)-For CTAB/10-SU mixture, the solution also obviously shows viscous characteristic at Xn(CTAB)=0.3,0.4,0.5, and 0.7. Furthermore, the viscosity increases with the increase of the total concentration, and the mixed solution exhibits elastic behavior when Xn(CTAB) is near 0.7.In chapter IV part, potentiometric method based on SISEs and CV method have been used to study the interactions between a homologous series of n-alkyl trimethyl ammonium bromides (CnTAB) and triblock copolymer pluronic F127. The CV method was firstly introduced to determine the critical micelle temperature (CMT) of nonionic polymer solution. The hydrohpobic behavior of CnTAB/F127 mixture solutions were discussed by comparing the variation of the diffusion coefficient of electroactive probe 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) in mixture solutions. Using SISEs, the EMF data of CnTAB/F127 systems were obtained under different salinity of NaBr. From these data, it is shown that the synergistic interactions between CnTAB and F127 were enhanced with the increase of surfactant chain-length, the concentration of CnTAB when the synergistic interact begin (noted as T1) were almost independent of the F127 concentration, the surfactant concentration where the polymer is saturated with surfactant (noted as T2) increases as the F127 concentration increases. But the increase of salinity and temperature has a negative influence on the synergistic interaction between CnTAB and F127. From the EMF data, uncooperative binding process of CTAB/F127 and TTAB/F127 systems was detected before the synergistic interactions in solutions with low salinity (lmmol/L NaBr) or with low F127 concentration (0.5 g/L).In chapter V, the electrochemical characteristics of vitamin K3 (denoted VK3) incorporated in acid-treated multiwalled carbon nanotubes (MWNTs) film, chitosan (CHIT) film and MWNTs-CHIT hybrid films in phosphate buffer solutions were studied respectively and the electrocatalytic oxidation of dopamine (DA) was investigated by cyclic voltammetry method. The results shown that the adsorbility of VK3 on the surface of a GC electrode was greatly enhanced by CHIT and MWNTs, and the redox peak currents of VK3 increased dramatically with the addition of MWNTs in cyclic voltammogram. Because of the existence of CHIT, the alkali resistance of VK3 get a great improvement, even if in pH=12 strong alkali solution, there are also redox response of VK3 in cyclic voltammogram. By changing pH of solutions and scan rate of cyclic voltmmetry measurements, the proton number and electron number involved in redox process were calculated, and the redox mechanism of VK3 incorporated in hybrid films was given. The apparent charge-transfer rate constant ks and transfer coefficient a for electron transfer between the electrode surface and immobilized VK3 in MWNTs-CHIT hybrid films and in MWNTs films were calculated as 21.4 s-1,0.49 and 3.2 s-1,0.47, respectively. The electrocatalytic oxidation of DA evaluated by cyclic voltammetric measurements indicates that VK3-MWNTs-CHIT hybrid film modified electrodes have good electrocatalytic property to DA. The differential pulse voltammograms were used to detect the DA concentration. A linear relationship between steady-state current and DA concentration was obtained over the range of 2-100?mol/L with a detection limit of 0.18?mol/L.