Constructing Surfactant Systems with the Characteristics of Gemini and Oligomeric Surfactants Through Noncovalent Interaction

Comparison between pyrrolidinium-based and imidazolium-based dicationic ionic liquids: intermolecular interaction, structural organization, and solute dynamics

With an aim to understand the difference in the behaviour of imidazolium and pyrrolidinium-based dicationic ionic liquids (DILs) in terms of the intermolecular interactions, microscopic-structure and dynamics, two DILs, the imidazolium-based 1,9-bis(3-methylimidazolium-1-yl)nonane bis(trifluoromethanesulfonyl)imide and the pyrrolidinium-based 1,9-bis(1-methylpyrrolidinium-1-yl)nonane bis(trifluoromethanesulfonyl)imide, have been synthesized and subsequently investigated by exploiting combined steady sate and time resolved fluorescence, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopic techniques. Data obtained for DILs have also been compared with their corresponding mono-cationic counterpart (MILs) to evaluate and understand the distinctive characteristics of the DILs in contrast with the corresponding MILs. Steady state emission and EPR data have revealed that the pyrrolidinium-based DIL is slightly less polar than the imidazolium-based DIL. Temperature-dependent fluorescence anisotropy decay of two probes, perylene and MPTS (8-methoxypyrene-1,3,6-trisulfonate), has been measured in DILs as well as in MILs. Solute-solvent coupling constants obtained from the experimentally measured rotational correlation times with the aid of Stokes-Einstein-Debye hydrodynamic theory have indicated appreciable differences in the dynamics of both the solutes on going from MILs to DILs. More interestingly, the outcome of the NMR study has suggested that the alkyl spacer chain in the imidazolium-based DIL exists in the folded form, but the pyrrolidinium-based DIL remains in the straight chain conformation. Inherently, the outcomes of all of these studies have depicted that the microscopic structural organisations in imidazolium and pyrrolidinium-based DILs are different from each other as well as from their respective mono-cationic counterparts.

Partition and Solubilization of Phospholipid Vesicles by Noncovalently Constructed Oligomeric-like Surfactants

This work has investigated the interaction of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles with oligomeric surfactants noncovalently formed by sodium dodecyl sulfate (SDS) and a series of polyamines, 1,3-diaminopropane (PDA), triamine, spermidine, and spermine. The partition coefficients (P) of these surfactants between lipid bilayers and the aqueous phase are measured by isothermal titration microcalorimetry (ITC), showing that the P value increases and the Gibbs free energy of the partition becomes more negative with increasing oligomerization degree of the surfactants. This changing trend is similar to that of synthetic oligomeric surfactants regardless of the charge properties, suggesting that the polyamine and SDS molecules interact with the DOPC bilayer simultaneously. Meanwhile, the DOPC solubilization by these surfactants is evaluated by the effective surfactant-to-lipid molar ratios for the onset (R_e^sat) and end (R_e^sol) of the solubilization process, which are determined from the phase boundaries obtained by ITC, turbidity, and dynamic light scattering measurements. With the increment of oligomerization degree, the R_e^sat and R_e^sol values increase anomalously and are much larger than those of the synthetic surfactants with the same oligomerization degree, suggesting that noncovalently constructed oligomeric surfactants exhibit lower solubilization ability to phospholipid vesicles than the corresponding covalent oligomeric surfactants. Therefore, the noncovalently constructed oligomeric-like surfactants facilitate strong partition but weak solubilization to phospholipid vesicles, which may provide a useful strategy to mildly adjust the permeation and fluidity of phospholipid vesicles with solubilization delay.

Insights into the Assembly of the Pseudogemini Surfactant at the Oil/Water Interface: A Molecular Simulation Study

The interfacial assembly process and configuration of the pseudogemini surfactant fabricated by sodium dodecyl benzene sulfonate (SDBS) and 4,4'-oxydianilinium chloride (ODC) were studied using quantum mechanical calculations and molecular dynamics (MD) simulations. The MD simulation results revealed that SDBS and ODC showed the vertical and horizontal arrangements at the oil/water interface, respectively, and the interfacial assembled configuration presented an unexpected "H" shape rather than the traditional "U" shape. The radial distribution functions between the head groups and water molecules were employed to explore the effects of the surrounding water molecules on the SDBS/ODC interaction. Furthermore, the results of the nonbonded interaction calculations and the reduced density gradient method directly confirmed that the cation-π interaction should be responsible for the SDBS/ODC assembly mechanism and the final configuration at the oil/water interface.

Oil-in-Water Emulsions Stabilized by Acylglutamic Acid-Alkylamine Complexes as Noncovalent-Type Double-Chain Amphiphiles

We have studied the preparation and stabilization mechanism of oil-in-water-type emulsions in the presence of amphiphilic 1:1 stoichiometric complexes of acylglutamic acids (CnGlu) with tertiary alkylamines (CnDMA). Relatively stable emulsions were obtained when C16Glu-C16DMA (or C18Glu-C18DMA), hexadecane, and water were homogenized at 80 °C and then stored at room temperature. The gel-liquid crystal phase transition temperatures (T_c) of C16Glu-C16DMA and C18Glu-C18DMA dispersed in water were determined to be ca. 39 and 53 °C, respectively. This indicates that the complexes form an adsorbed layer at the oil/water interface during the homogenization process above the T_c and then change into a gel during storage at room temperature. The gel phase formed at the oil/water interface prevents the oil droplets from coalescing. In contrast, shorter chain analogues (C10Glu-C10DMA and C12Glu-C12DMA) did not yield stable emulsions because their adsorption layers were not able to prevent coalescence of the oil droplets (i.e., the T_c of these analogues was below the room temperature). We have also demonstrated that the dispersion stability of these emulsion systems can be controlled by changing the aqueous pH.

Effects of length and hydrophilicity/hydrophobicity of diamines on self-assembly of diamine/SDS gemini-like surfactants

This work studied gemini-like surfactants formed from anionic surfactant sodium dodecyl sulfate (SDS) and cationic charged bola-type diamines with hydrophilic or hydrophobic spacers of different lengths using surface tension, small angle neutron scattering, isothermal titration microcalorimetry and cryogenic transmission electron microscopy. The critical micelle concentrations (CMC) and the surface tension at CMC (γ_CMC) for all the diamine/SDS mixtures are markedly lower than that of SDS. The shorter diamines reduce γ_CMC to a greater extent regardless of the hydrophilicity/hydrophobicity of the diamines. Meanwhile, either the hydrophobic diamine with a longer spacer or the hydrophilic diamine with a shorter spacer is more beneficial to decrease CMC and leads to the transition from spherical micelles into rodlike or wormlike micelles. This is principally because of the formation of gemini-like surfactants by the electrostatic binding between SDS and the diamines, where the electrostatic repulsion between the adjacent headgroups of SDS becomes much weaker due to the electrostatic binding of oppositely charged diamine with SDS, and the longer hydrophobic spacer may also bend into the hydrophobic domain of micelles to promote micellar growth. However, the hydrophilic spacers are more compatible with the headgroup region, leading to micelles with a larger curvature. This work contributes to the understanding of the relationship between the properties of constructed gemini-like surfactants and the natures of connecting molecules, and provides guidance to efficiently improve the performance of surfactants.

Aggregation of a Cationic Gemini Surfactant with a Chelating Molecule and Effects from Calcium Ions

The aggregation behavior of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) with chelating molecule ethylenediaminetetraacetic acid (EDTA) and the effects of calcium bromide (CaBr₂) on the structure and morphology of the aggregates in the mixture have been investigated by surface tension, isothermal titration microcalorimetry, electrical conductivity, ζ potential, dynamic light scattering, cryogenic transmission electron microscopy, freeze-fracture transmission electron microscopy, and ¹H NMR techniques. It was found that the electrostatic attraction between the carboxyl groups of EDTA and the headgroups of 12-6-12 leads to the formation of oligomeric-like surfactant EDTA(12-6-12)₂ at an EDTA/12-6-12 molar ratio of 0.50. The critical aggregation concentration of the EDTA(12-6-12)₂ complexes is much lower than that of 12-6-12, and the complexes form loose, large network-like premicellar aggregates and then transfer into small micelles with an increase in concentration. Moreover, the addition of CaBr₂ induces the transition from the loose aggregates and micelles to vesicles owing to the coordination interaction between the calcium ion and EDTA and the electrostatic interaction between EDTA and 12-6-12. The work reveals that as a bridging molecule between the calcium ion and the gemini surfactant, the chelating molecule greatly promotes the assembly of the gemini surfactant and strengthens the molecular packing in the presence of calcium ions.

Aggregation of Oligomeric Surfactant Constructed by Four-Armed Carboxylic Acid Sodium and Cationic Surfactant

A star-shaped oligomeric-like surfactant with variable oligomeric degrees has been formed with a four-arm carboxylate salt (4EOCOONa) and cationic single chain surfactant dodecyl trimethylammonium bromide (DTAB). The aggregation of the 4EOCOONa/(DTAB)_n complexes has been investigated by surface tension, electrical conductivity, isothermal titration microcalorimetry, ζ potential, dynamic light scattering, ¹H NMR spectroscopy, and steady-state fluorescence measurements. The calorimetric result shows that 4EOCOONa interacts strongly with DTAB and each 4EOCOONa molecule binds with six DTAB molecules, wherein four DTAB molecules electrostatically bind to one 4EOCOONa molecule and additional two DTAB molecules further bind to the 4EOCOONa/(DTAB)_n complex by hydrophobic interaction. The critical micelle concentration (CMC) of the 4EOCOONa/(DTAB)_n complexes is remarkably lower than the CMC of DTAB, similar to synthesized star-shaped oligomeric surfactants. The micelle properties of the DTAB/4EOCOONa mixtures depend on the component changes of the 4EOCOONa/(DTAB)_n complexes. By increasing the DTAB/4EOCOONa molar ratio and/or concentration, the DTAB/4EOCOONa mixtures gradually form the complexes of 4EOCOO(DTA)₁^3-, 4EOCOO(DTA)₂^2-, 4EOCOO(DTA)₃^-, 4EOCOO(DTA)₄, and 4EOCOO(DTA)₆²⁺, and the corresponding aggregates are small anionic micelles with loose molecular packing, and nearly nonionic or positively charged small micelles with more compact packing. Moreover, the positive charge of the small micelles increases with the increase of the concentration and the DTAB/4EOCOONa molar ratio. Therefore, constructing oligomeric-like surfactants by adding appropriate organic salts into conventional surfactants is a convenient method to achieve desired properties of surfactant aggregates.

Viscoelastic properties of supramolecular gemini-like surfactant solutions in the absence of inorganic salts

A bola-type cation and a newly synthesized anionic surfactant together generate a supramolecular gemini-like structureviaelectrostatic interactions, which facilitates the formation of wormlike micelles.

Facile construction of gemini-like surfactants at the interface and their effects on the interfacial tension of a water/model oil system

Gemini-like surfactants were fabricated by combining sodium dodecyl benzene sulfonate and butane-1,4-bis(methylimidazolium bromide) at the interface and evaluated for their ability to reduce the interfacial tension between water and a model oil.

Temperature-induced reversible micelle–vesicle transition in aqueous solution of a pseudogemini surfactant without any additive

Temperature-induced reversible micelle–vesicle transition is achieved in aqueous solution of a single pseudogemini surfactant without any additive.

Sheet-like and truncated-dodecahedron-like AgI structures via a surfactant-assisted protocol and their morphology-dependent photocatalytic performance

Sheet-like and truncated-dodecahedron-like AgI structures are synthesized via a surfactant-assisted method, the latter display boosted photocatalytic performances and excellent recyclability.