Hang Xing

Demicellization of a Mixture of Cationic−Anionic Hydrogenated/Fluorinated Surfactants through Selective Inclusion by α- and β-Cyclodextrin

The interactions between alpha- and beta-cyclodextrin (alpha-/beta-CD) and an equimolar mixture of octyltriethylammonium bromide (OTEAB) and sodium perfluorooctanoate (SPFO) were studied by 1H and 19F NMR, surface tension, conductivity, and dynamic light scattering. It was shown that beta-CD could destroy the mixed micelles of OTEAB-SPFO by selective inclusion of SPFO. As beta-CD was added, the system was observed to undergo a process like this: beta-CD preferentially included SPFO to form 1:1 beta-CD/SPFO complexes. As the inclusion of SPFO was almost saturated, the mixed micelles broke and all OTEAB was released and exposed to aqueous surroundings. Then 1:1 beta-CD/OTEAB and 2:1 beta-CD/SPFO complexes significantly formed simultaneously. Contrary to beta-CD, alpha-CD exhibited selective inclusion to OTEAB and only weak association with SPFO. alpha-CD could also destroy the mixed micelles of OTEAB-SPFO; however, the demicellization ability of alpha-CD is much smaller than that of beta-CD. These conclusions were also well supported by the calculations of binding constants and DeltaG degrees . Different from the complexes of CD/conventional surfactants, the complexes of beta-CD/SPFO or alpha-CD/OTEAB formed by selective inclusion of CD in the mixed cationic-anionic surfactants may have contributed to the surface activity of the aqueous mixtures. The complexes of alpha-CD/OTEAB showed much more significant contribution to the surface activity than that of the complexes of beta-CD/SPFO.

Unusual location of the pyrene probe solubilized in the micellar solutions of tetraalkylammonium perfluorooctanoates

The aqueous solutions of tetraalkylammonium perfluorooctanoates (C7F15COON(CnH2n+1)4, n = 1, 2, 3, 4, abbr. TMAPFO, TEAPFO, TPAPFO, TBAPFO) were studied by steady-state and time-resolved fluorescence with pyrene as a probe, and the ammonium perfluorooctanoate (APFO) was also studied as a comparison. Unusual response of pyrene fluorescence was observed. The intensity ratio of the first and third vibronic peaks of pyrene (I1/I3) greatly varied with the counterions for these fluorinated surfactants after their critical micelle concentration (cmc): for TEAPFO, the I1/I3 abnormally increased to a value higher than that of water, while for TBAPFO the I1/I3 value exhibited a small extent of decease after cmc. However, no significant change of I1/I3 was observed during the micellization of TMAPFO and TPAPFO. The lifetime of pyrene fluorescence (τ0) among the systems of tetraalkylammonium perfluorooctanoates (TAAPFOs) was observed to be increased with the hydrophobicity of counterions. Moreover, the time-resolved fluorescence decay curves of pyrene for all micellar solutions exhibited single exponential decay, suggesting a single environment for pyrene, i.e. each surfactant had a single location of pyrene solubilized in its micelle. The performances of the pyrene probe in concentrated solutions of neat ammonium/tetraalkylammonium chlorides (NH4Cl/TAACls) were also investigated and an unusually high I1/I3 could be found for tetraethylammonium chloride. It reached a conclusion that pyrene located in the counterion layer of the TAAPFO micelles, which could be ascribed to the hydrophobic interaction and cation–π interaction between pyrene and tetraalkylammonium ions (TAA+) and the immiscibility between pyrene and the fluorocarbon core.

Anomalous effects of concentrated salts on the equimolarly mixed cationic–anionic surfactant mixtures

Effects of alkali metal halides (NaX, X = F−, Cl−, Br−, I−; KX, X = Cl−, Br−, I−; and CsCl) on the morphology of the aggregates of decyltriethylammonium bromide and sodium decylsulfonate (C10NE–C10SO3) mixtures with equimolar ratios were studied by static/dynamic light scattering and viscosity measurements. The average hydrodynamic radii of the aggregates, the apparent aggregation number, and the relative viscosity as functions of both the concentrations of surfactants and added salts were investigated, respectively. It showed that the addition of a small amount of NaX (e.g., 0.1 M) had only small effects on the C10NE–C10SO3 vesicles. However, striking anion specificity, which differed anomalously with the anions of salts varying from F− to I−, was observed when large amounts of NaX (e.g., 1 M) were added. The size of the C10NE–C10SO3 aggregates was normally increased and then a liquid–liquid phase separation occurred with the addition of NaF; meanwhile, the addition of NaCl had little effect on the C10NE–C10SO3 aggregates under experimental conditions; however, the size of the C10NE–C10SO3 aggregates was abnormally decreased when high concentrations of NaBr or NaI (up to 2 M) were added, implying a vesicle-to-micelle transition which might be due to a decrease in the intramicellar attraction between oppositely charged headgroups by electrical shielding, and the mixtures still remained homogeneous. A cation specificity of added salts was also observed with the increase of the ionic radii from Na+ to Cs+. However, it was noted that the influence of increasing the ionic radii of cations on the aggregate size was not as pronounced as that of anions in the presence of high concentrations of salts. The aggregates of C10NE–C10SO3 became a little smaller when concentrated alkali metal halides with bigger cations were added into this homogeneous equimolar cationic–anionic surfactant mixture. Both the observed cation and anion specificities were consistent with the Hofmeister series.

Aggregation behavior of N-alkyl perfluorooctanesulfonamides in dimethyl sulfoxide solution.

N-alkyl perfluorooctanesulfonamides (C8F17SO2NHCnH2n+1, FC8-HCn, n = 2, 4, 6, 8) were shown to form aggregates in dimethyl sulfoxide (DMSO). Surface tension results revealed that the dissolution of FC8-HCn reduced the surface tension of DMSO in a manner analogous to common surfactants in aqueous solutions. Maximum surface excess amount (Gamma(max)) and minimum surface area per molecule (Amin) at the air-liquid interface were estimated. Gamma(max) decreases and Amin increases with an increase of the hydrocarbon chain length of FC8-HCn. Steady-state fluorescence and NMR measurements demonstrated that both fluorocarbon and hydrocarbon chains of FC8-HCn molecules were incorporated inside the aggregates. UV-vis spectroscopy confirmed the formation of aggregates and determined the critical micelle concentration (cmc) of FC8-HC6 and FC8-HC8 solutions. The thermodynamic parameters DeltaG(0)(agg), DeltaH(0)(agg), and DeltaS(0)(agg) for the aggregate formation of FC8-HCn in DMSO derived from the temperature dependence of the cmc revealed that the aggregate formation is an enthalpy-driven process, which was further confirmed by isothermal titration calorimetry (ITC) measurements. Moreover, the absolute values of DeltaG(0)(agg) and DeltaH(0)(agg) increase with an increase of the hydrocarbon chain length of FC8-HCn at 298 K.