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The inhibitory effects of CdTe/ZnS quantum dots (QDs) modified with 3-mercaptopropionic acid (negatively charged) or cysteamine (positively charged) on the metabolic activity of Escherichia coli were investigated using biological microcalorimetry. Results show that the inhibitory ratio of positive QDs is higher than that of negative QDs. Transmission electron microscopy images indicate that QDs are prone to be adsorbed on the surface of E. coli. This condition disturbs the membrane structure and function of E. coli. Fluorescence anisotropy results demonstrate that positive QDs show a significant increase in the membrane fluidity of E. coli and dipalmitoylphosphatidylcholine (DPPC) model membrane. Furthermore, fluorescence anisotropy values of DPPC membrane in the gel phase decreased upon the addition of positive QDs. By contrast, anisotropy values in the liquid-crystalline phase are almost constant. The change in membrane fluidity is associated with the increased permeability of the membrane. Finally, the kinetics of dye leakage from liposomes demonstrate that the surface charge of QDs is crucial to the interaction between QDs and membrane.

The equilibrium and dynamic surface properties of cationic/anionic surfactant mixtures of carboxylate gemini surfactant (CGS12) and quaternary ammonium salts with different alkyl chain lengths were investigated, and the synergistic properties and solubilization capacity toward phenanthrene of these mixtures were evaluated. Results show that all cationic/anionic surfactant mixtures exhibit negative interaction parameters, indicating the strong synergistic effects in the reduction of surface tension and the formation of micelles. When the mixing ratio is 1:1, the surface tension reaches the minimum value. Moreover, as the alkyl chain length increases, the interaction parameters become more negative. In addition, the thermodynamic parameters suggest that the formation of mixed micelles is exothermic with a positive change in entropy. The tendency of dynamic surface tension curves of these mixtures is similar to that of pure CGS12. At the same concentration, these mixtures exhibit a higher adsorption rate than pure surfactants, and a lower adsorption potential barrier than cationic surfactants. However, for the CGS12/stearyltrimethylammonium bromide mixture, the drop rate constant of surface tension is lower than that of either component. Compared with the pure surfactant solutions, the cationic/anionic surfactant mixtures exhibit higher solubilization capacity toward phenanthrene. As the length of the alkyl chain increases, the solubilization capacity of the mixed cationic/anionic surfactant increases.

The dynamic interfacial properties and dilational rheology of gemini sulfonate surfactant (SGS) and its mixtures with quaternary ammonium bromides (DTAB, CTAB) at the air-water interface were investigated using drop shape analysis. Results suggest that the adsorption process of these surfactants is diffusion-controlled at dilute concentrations, whereas the adsorption mechanism gradually shifts to a mixed kinetic-diffusion control with increasing surfactant concentration. The mixed surfactant system possesses the best surface activity when the molar ratios of SGS/DTAB and SGS/CTAB mixtures are 9:10. The formation of catanionic complexes shields the electrostatic repulsion between surfactant molecules and lowers the electrostatic adsorption barrier. Therefore, SGS/ DTAB and SGS/CTAB mixtures exhibit higher adsorption rates than either component alone. The effects of oscillating frequency and surfactant concentration on the surface dilational properties of SGS, DTAB, CTAB, SGS/DTAB, and SGS/CTAB mixtures were also determined. As the oscillating frequency increases, the dilational elasticity of these surfactants gradually increases. The dilational elasticity peaks at a certain concentration, which is less than the critical micelle concentration (CMC). Results show that the dilational elasticity of SGS/DTAB and SGS/CTAB mixtures is higher than that of either component, resulting from the formation of a denser monomolecular adsorption layer at the air-water interface. Our study provides a basis for understanding the interaction mechanism of catanionic surfactant mixtures containing Gemini surfactant at the airwater interface.Keywords Dynamic surface tension Á Dilational rheology Á Sulfonate Gemini surfactant Á Air-water interface Á Catanionic surfactant mixture Electronic supplementary material The online version of this article (

The critical micelle concentration (CMC) of 1,2‐bis[N‐methyl‐N‐(3‐sulfopropyl)‐alkylammonium]‐ethane betaine (GCS12) was measured using a tensiometric method in the presence of inorganic salts. Inorganic salt has a little impact on the surface tension and CMC of zwitterionic gemini surfactant. The CMC value of GCS12 is 0.07 mmol/L in distilled water, while all CMC values are around 0.04–0.05 mmol/L in the presence of 0.5 % NaCl, 2 % NaCl, and 2 % NaCl + 0.05 % CaCl2. The interactions between GCS12 and non‐ionic surfactant lauric acid diethanolamide (CDA) were investigated by measuring the CMC of their mixtures at different molar ratios. CDA and GCS12 form mixed micelles and exhibit synergism when the mole fraction of CDA is higher than 0.25. Both the steric effect of the head group and GCS12 charge affect the formation and stability of the mixed micelles. Small amounts of GCS12 with a lower CMC penetrate into the micelle of nonionic surfactant with a higher CMC and reduce its degree of hydration inducing an attractive interaction between the two surfactants.

The interaction mechanism of multiple quaternary ammonium salts (MQAS) with bovine serum albumin (BSA) was examined by the fluorescence quenching method and circular dichroism (CD) spectra. Moreover, the effects of MQAS on the dynamic properties of BSA adsorption layers at different pH values were investigated using dilational interfacial rheology. Results show that the quenching constants increase with an increase in pH values and decrease with an increase in the experiment temperature at pH 5.3. The quenching mechanism is static quenching, and the electrostatic force dominates the interaction between MQAS and BSA at pH 5.3. Due to three positive head groups, MQAS can significantly affect the dynamic interfacial activity of BSA molecules at a relatively low concentration. At pH 4.3, the electrostatic repulsion is unfavorable for the formation of MQAS/BSA complexes. Consequently, MQAS molecules will replace BSA molecules from the interface by competitive adsorption. At the pH value above the isoelectric point of BSA, the electrostatic attraction is better for the formation of MQAS/BSA complexes, which exhibit a rapid adsorption rate and an enhanced interfacial activity. Moreover, the kinetic dependencies of interfacial dilational elasticity for the MQAS/BSA mixtures become nonmonotonous. The appearance of the maximum interfacial elasticity values can be attributed to the formation of tails and loops, which suggests that the addition of MQAS destroys the secondary and tertiary structure of protein in the bulk phase. In addition, the effects of MQAS on the secondary structure of protein were demonstrated by CD spectra.

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