Fluorimetric determination of the aggregation number of cetyltrimethylammonium chloride (CTAC)

Static (SLS) and dynamic (DLS) light-scattering techniques were applied to the study of the aggregation of dodecyl- (C₁₂DBACl), tetradecyl- (C₁₄DBACl), and hexadecyldimethylbenzylammonium (C₁₆DBACl) chlorides in water and in 0.01 and 0.05 m NaCl aqueous solutions at 25 °C. Results of SLS measurements yielded critical micelle concentration (cmc) values for aqueous and NaCl solutions. The aggregation numbers of the micelles for the homologous surfactants are low but increase with chain length and ionic strength of the solution. Various patterns of changes of the diffusion coefficient, D, as a function of chain length, molality, and with ionic strength were found for the studied surfactants. Transformations in the structure of micelles of C₁₄DBACl in 0.01 m NaCl occur at a concentration of surfactant of about 0.01 m. Such transformations, presumably due to rodlike structure, are the more extensive the higher the concentration of NaCl. The concentration of C₁₆DBACl in 0.05 m NaCl covers the range where already repulsive interactions between micelles occur, as judged by the strongly negative slope of the D versus molality plot. To provide additional information on the suggested transformations, complementary viscosity measurements for C₁₄DBACl in 0.01 m of NaCl have been performed.

Time-resolved and steady-state fluorescence have been employed to study the fluorescence quenching of pyrene, solubilized in novel micelles of the zwitterionic surfactant,N-(3-dodecyloxy-2-hydroxy propyl)-N,N-dimethylglycine (DHDG), by iodide ions in aqueous solution. The fluorescence quenching rate constants (k_q), micellar exit rate constants (k_–), and micellar aggregation numbers (N) have been determined in each case. Generally the fluorescence quenching rates in micellar solutions with pH values above the isoelectric point (pH 5.1) (zwitterionic) of DHDG are lower than when pH is below 5.1 (cationic). Addition of NaCl results in a moderate variation of the micellar aggregation numbers and a subsequent change ink_q. When cationic and zwitterionic DHDG micelles have the same micelle aggregation number,k_qin the cationic DHDG micelles is higher, suggesting that both the aggregation number and the micellar surface potential related to the charge of head groups influencek_q. The dynamics of iodide ion exit from DHDG micelles has also been studied. The exit rate constants of iodide ion from micelles,k_–, is independent of micelle concentration (in the range studied), but is dependent on micellar aggregation number and is related to micellar interfacial potential. The steady-state and time-resolved fluorescence measurements agree very well, suggesting the importance of dynamic fluorescence quenching processes.

The ability of aqueous micelles and similar association colloids to control reaction rates is treated in terms of pseudophase models in which micelles and water are regarded as distinct reaction media, with consideration of reactant concentrations and rate constants in the two regions. An alternative model, based on transition state theory, considers the transfer free energy of the activated complex between water and micelles. Both models fit data for reactions in solutions of cetyltrimethylammonium hydroxide, for example. Their values in relating rate effects to reaction mechanism and structures of reactants and surfactants are discussed.

Fluorescence quenching experiments were carried out in cetyltrimethylammonium chloride (CTAC) micelles and in dioctadecyldimethylammonium chloride (DODAC) giant unilamellar vesicles employing a wide variety of quenchers and a series of pyrene derivatives whose fluorophores can be considered to be located at different positions within the microaggregates. The quenchers employed comprise ionic (I⁻) and zwitterionic compounds (tryptophan at pH 6–7) and neutral compounds (oxygen, acrylamide, hydrogen peroxide, dimethylaniline, di-tertbutyl-peroxide, tert-butylhydroperoxide, desferrioxamine, and carbon tetrachloride) of widely different hydrophobicities, and the quenching rates were measured at temperatures below (293 K) and above (323 K) the vesicles' phase transition temperature. Quenching rate constants in DODAC vesicles, both above and below the phase transition temperature of the vesicles, are very slightly dependent on the employed probe, particularly when ionic or highly polar quenchers are considered. The measured quenching rates are rather similar in CTAC micelles and DODAC vesicles. These results imply that either the quenchers readily reach all parts of the bilayer of the vesicles and/or that there is very little relationship between the structure of the probe, its average location, and/or its diffusion time towards the interface. The most noticeable differences between micelles and vesicles, as well as the largest effect of temperature in DODAC vesicles, are observed for uncharged, nonpolar quenchers. These differences, as well as the increase in deactivation rate by oxygen observed in the presence of hydrophobic additives, can be related to significant changes in the extent of incorporation of neutral solutes when the temperature increases or when the bilayer is perturbed by solute incorporation.

Voltammetry at the rotating disk electrode of a water-and-oil-soluble electroactive probe (N,N-diethyl-3-methyl-p-phenylene diamine, PPD) was used to detect the onset of micellization in aqueous sodium dodecyl sulfate (SDS), di-/triisopropyl naphthalene sulfonate (DTINS), and cetyltrimethylammonium bromide (CTAB) systems in 0.1 M NaCl at 25°C and pH 10. Critical micelle concentrations in agreement with values determined by other methods were obtained. Micellar/aqueous phase distribution coefficients were estimated by modeling the variation of limiting current with amphiphile concentration. Distribution coefficients of 324 and 73 were obtained for the SDS and DTINS systems, respectively. In the CTAB system, sequential distribution coefficients of 272 and 88, respectively, were obtained below and above a “second CMC” (0.001 M), which corresponds to the onset of a transition in micellar morphology.

A theoretical model developed for the intramicellar fluorescence quenching rate constant in cylindrical micelles is used to simulate the fluorescence decay of a probe solubilized in the presence of a solubilized quencher. Different analytical approximations to the simulated fluorescence decay are evaluated for a range of aggregation numbers, diffusion coefficients and quencher concentrations. The fluorescence decay can always be fitted to either a Poisson distribution of quenchers or to the decay law corresponding to a reaction controlled process in an infinite micelle. Using single curve analysis it is often impossible to make a discrimination between those decay laws. The simultaneous analysis of several decays obtained at different quencher concentrations however allows us to make this discrimination when the fluorescence decay time of the probe is long enough (200 ns) and the mutual intramicellar diffusion coefficient is large enough (5.0 × 10⁻⁶ cm² s⁻¹). Under those conditions simultaneous analysis of fluorescence decays obtained for different quencher concentrations also allows us to make a discrimination between diffusion transients and mobile quenchers.