Effect of addition of dendritic C60 amphiphiles on the structure of cationic surfactant solutions

Self-assembly of fullerene C60-based amphiphiles in solutions

Fullerene C₆₀ is an all-carbon cage molecule with rich physicochemical properties. It is highly symmetric and hydrophobic, which can be used as a building block for the preparation of amphiphiles that self-assemble into diverse supramolecular structures in aqueous solutions. Meanwhile, C₆₀ is also lipophobic, which is different from the alkyl chains in traditional surfactants. By attaching alkyl chains to the C₆₀ sphere, a new type of lipophobic-lipophilic amphiphiles can be constructed which undergo self-assembly in n-alkanes. When inorganic clusters such as polyoxometalate are linked to the C₆₀ sphere, organic-inorganic hybrids will be obtained which can self-assemble in polar organic solvents. Pristine C₆₀ has also been modified by polar groups such as hydroxy and carboxy, which are linked to hydrophobic moieties and form a new class of amphiphiles. In this review, the self-assembly of C₆₀-based amphiphiles in aqueous and nonaqueous solutions will be summarized. The characteristics exhibited by C₆₀-based amphiphiles during the self-assembly will be discussed with close comparison to traditional surfactants, and the influences of the aggregate formation on the physicochemical properties of the C₆₀ sphere will be described. Finally, a brief summary will be given together with a promising perspective in near future.

What dominates the interfacial properties of extended surfactants: Amphipathicity or surfactant shape?

HYPOTHESIS

The properties of conventional surfactants (c-surfactants) are generally accepted to be amphipathicity-dominated, but extended surfactants (e-surfactants) are additionally polypropylene oxide (PPO)-dependent; this additional property makes us wonder how an intramolecular PPO spacer would be "extended" at various interfaces and what is responsible for the excellent all-round properties of e-surfactants.

EXPERIMENTS

A series of novel sodium medium alkyl chain PPO-b-PEO sulfates (2-ethylhexyl polypropylene oxide-block-polyethylene oxide sulfates, C₈P_pE_eS) were designed, synthesized and structurally identified. Tensiometry was applied to estimate the surfactant shape at the air/water surface. Surface tension, interfacial tension, emulsifying power, electrolyte tolerance, adsorption onto oil sands and thermal hydrolysis stability were measured to evaluate the effect of the PPO coil on the interfacial and micellar properties of the e-surfactants.

FINDINGS

On the basis of obtaining greater values for e-surfactants than c-surfactants for both surface area (a_m) per surfactant molecule and the corresponding shape factor (S), we were surprised to find that e-surfactants form a rugby ball shape not only at the air/water surface but also at the oil/water interface; this result is potentially explained by the PPO spacer coiling and collapsing to produce dense packing at the monolayer adsorption, which is rationally borrowed by other interfaces. Many positive or negative correlations were observed between the interfacial/micellar properties of C₈P_pE_eS and a_m values, which seems that the surfactant shape dominants the properties of the e-surfactants. In fact, the properties of C₈P_pE_eS are dominated by the dynamic amphipathicity and assisted by the rugby ball shape of the molecules because of both being driven by the dynamic biphasic affinity of the PPO coil in response to the external environment; these findings provide soft interfacial materials specially adapted for surfactant flooding.