Xuefeng Li

A new family of gemini surfactants with pyrrolidinium head groups, 1,1′-(alkane-1, s-diyl)bis(1-dodecyl pyrrolidinium)bromide (C₁₂–C_s–C₁₂PB, s = 3, 4, 6, 8, 10, 12, 14, and 16), were synthesized. Their adsorption and micellization in aqueous solutions were investigated by various techniques such as equilibrium surface tension, fluorescence, and conductivity in detail. It was found that the effect of the spacer length on surfactant aggregation in aqueous solutions is similar to that of the well-known gemini surfactants with quaternary ammonium head groups (m-s-m), whereas the occupied limiting area per molecule (A_min) at the surface increases linearly with s rather than going through a maximum value at a medium value of s like that of m-s-m. The thermodynamic parameters suggest that the micellization of C₁₂–C_s–C₁₂PB is an entropy driven process regardless of the spacer length s. Moreover, a second breakpoint appeared in both γ–log c and I₁/I₃–c curves of C₁₂–C₁₄–C₁₂PB and C₁₂–C₁₆–C₁₂PB as the surfactant concentration increases, which can be attributed to the formation of larger aggregates. The morphology of the aggregates is confirmed by employing dynamic light scattering (DLS) and cryo-transmission electron microscopy (cryo-TEM) techniques.

Sodium 4-[(E)-phenyldiazenyl] benzoate (AZONa), a photo-responsive dye, induced micellar growth in alkyl-trimethylammonium bromide (dodecyl-, DTAB; tetradecyl-, TTAB; cetyl-, CTAB and octadecyl-, OTAB) solutions was investigated using light scattering and viscosity. The viscosity decrease upon increasing [surfactant] to above about 20 mM was found to be common from TTAB to OTAB solutions at fixed mole ratio of [AZONa]/[surfactant]. Cryo-TEM demonstrated that the decrease of viscosity was attributed to the entangled linear to multi-connected saturated network wormlike micelles transition. UV-vis spectra showed that AZONa interacted strongly with OTAB as revealed from both the shifting of absorbance peak and the variation of absorbance intensity, which suggest the binding of AZONa to OTAB micelles. ¹H NMR confirmed the solubilization of AZONa to the palisade layer and growth of micelles upon increasing [AZONa] as revealed from the shifting and broadening of peaks.

A series of pH sensitive single-tailed surfactants, N-alkyl-1,2-ethylenediamine (CmN2N, where m = 8, 10, 12, 14, 16), were synthesized and characterized. The adsorption and aggregation properties of them were determined by equilibrium surface tension, static and dynamic light scattering (SLS and DLS), rheology, and cryogenic transmission electron microscopy (cryo-TEM) techniques in detail. It was found that both the surface activity and self-assembly behavior of CmN2N were strongly dependent on the solution pH, owing to the protonation state of amino groups in CmN2N. When at least one of the amino groups of CmN2N was protonated the logarithm of the critical micelle concentration (cmc) followed the well-known Klevens equation, i.e. cmc decreased linearly with the increase of hydrocarbon chain length. Simultaneously, the surface adsorption area per molecule (A_min) was decreased for each CmN2N when the solution pH varied from acidic to basic conditions. The pH-dependent micelle to vesicle transition was universally observed in the bulk phase of CmN2N. More interestingly, the rodlike micelles formed in C10N2N, C12N2N and C14N2N could transform into vesicles reversibly upon heating or cooling, which indicated the thermal sensitivityof organized assemblies, whereas not in C8N2N and C16N2N.