Using light to control the floatability of solid particles in aqueous solution of a Gemini surfactant

Synthesis and Surface Properties of Photoresponsive Gemini Surfactants: Implication for Remediating PAHs-Contaminated Groundwater

The efficient utility of surfactants remains a daunting task for groundwater remediation. In this study, we have synthesized a conventional photoresponsive surfactant 4-[4-[(4-butylphenyl)azo]phenoxy]butyldimethylethylammonium bromide (AzoPB) and a gemini photoresponsive surfactant N¹,N²-bis[4-[4-[(4-butylphenyl)azo]phenoxy]butyl]-N¹,N²-tetramethylethane-1,2-diammonium bromide (AzoPBT) for solubilizing PAHs in groundwater. The two surfactants' photosensitivity, surface properties, and solubilization/release ability for phenanthrene (Phe) and acenaphthylene (Ace) were studied in detail. Under UV-light irradiation for 15-20 s, the two surfactants can be converted from trans to cis, while cis-to-trans isomerization can be achieved under visible-light irradiation for 1 min. Compared to AzoPB, AzoPBT exhibited strong surface properties such as lower critical micelle concentration (0.52 mM), surface tension (γ, 28.94 mN·m^-1), minimum area (A_min, 1.72 × 10^-8 nm²), and higher maximum adsorption (Γ_max, 96.55 mol·m^-2). The solubility of Phe and Ace in the AzoPBT aqueous solution (12.84 and 14.27 mg/L) was much higher than that in the AzoPB aqueous solution (7.51 and 8.77 mg/L) and gradually increased as the surfactant concentration increased in both aqueous solutions. Compared to AzoPB, gemini surfactant AzoPBT exhibited stronger solubilization ability. After four cycles of cis-trans isomerization conversion, AzoPBT could still reduce the hydrophobicity of Phe in natural groundwater, although the solubility of Phe decreased slightly. Additionally, the release capacity of AzoPBT was significantly higher than that of AzoPB during the cyclic solubilization-release process. The results indicated that gemini photoresponsive surfactants should be preferable to conventional photoresponsive surfactants for groundwater remediation due to their higher solubilization and release efficiency for Phe in the cyclic solubilization and release process, which can improve repair efficiency, minimize secondary pollution, and reduce remediation costs.

Self-Assembly of Photoresponsive Molecular Amphiphiles in Aqueous Media

Amphiphilic molecules, comprising hydrophobic and hydrophilic moieties and the intrinsic propensity to self-assemble in aqueous environment, sustain a fascinating spectrum of structures and functions ranging from biological membranes to ordinary soap. Facing the challenge to design responsive, adaptive, and out-of-equilibrium systems in water, the incorporation of photoresponsive motifs in amphiphilic molecular structures offers ample opportunity to design supramolecular systems that enables functional responses in water in a non-invasive way using light. Here, we discuss the design of photoresponsive molecular amphiphiles, their self-assembled structures in aqueous media and at air-water interfaces, and various approaches to arrive at adaptive and dynamic functions in isotropic and anisotropic systems, including motion at the air-water interface, foam formation, reversible nanoscale assembly, and artificial muscle function. Controlling the delicate interplay of structural design, self-assembling conditions and external stimuli, these responsive amphiphiles open several avenues towards application such as soft adaptive materials, controlled delivery or soft actuators, bridging a gap between artificial and natural dynamic systems.

Comparative Study of Conventional/Ethoxylated/Extended n-Alkylsulfate Surfactants

A series of novel anionic e-surfactants n-C _cP _pS was molecular designed and synthesized from long-chain fatty alcohols by polypropoxylation and sulfation followed by neutralization. Excellent all-round performance of extended surfactants (e-surfactants) interests us how a simple polypropylene oxide (PPO) spacer has great effects on properties. By a comparative study of conventional/ethoxylated/extended n-alkylsulfate surfactants, we were surprised to find that e-surfactants are in an obvious rugby shape at the air/water surface according to molecular surface area ( a_m), and it comes down to the intramolecular PPO spacer coiling and surface-induced collapse. On the basis of the interfacial properties of the e-surfactants, it is found that the PPO spacer can provide both hydrophilic and lipophilic contributions to an e-surfactant molecule. The synergism between PPO spacers and alkyl chains indicates that a certain PPO spacer can adjust the contributions in view of different alkyl chain lengths. Therefore, it is both the rugby-shaped molecular geometry of e-surfactants and the dynamic amphipathicity of a PPO spacer that makes e-surfactants behave with excellent interfacial and solution properties for household cleaning. Therefore, this work gives us a hint that the molecular geometry of surfactants plays a vital role in interfacial and solution properties similar to molecular amphipathicity.