Self Assembly at the Liquid Air Interface

Abstract: The aim of this work is to study the interfacial properties of amphiphilic compounds at the liquid–air interface in an attempt to develop a comprehensive understanding of their orientation as well as the influence of their interaction with the solvent on the interfacial layer properties. Using Vibrational Sum Frequency Spectroscopy (VSFS) as the main tool, the molecular structure of the amphiphilic layer and the amphiphile–solvent relation can be illuminated in great detail – it is arguably the most sensitive surface spectroscopy currently available. Due to its second order nature, the VSFS technique is capable of distinguishing molecules at the interface even in the presence of a vast excess of similar molecules in the bulk.Ionic liquids (Ils) form a class of solvent which are increasingly receiving attention as ``green solvents´´. Some of these, such as ethyl ammonium nitrate (EAN), a protic IL, have the capacity to hydrogen bond extensively which is one of the important features they share with water. Since the interaction with solvent is an important consideration for self assembly and it is known that surfactant self assembly in the EAN bulk is analogous to in water, it was considered of interest to probe self assembly at EAN–air interface. To this end the interfacial structure of the pure EAN interface was probed, as was the conformation and ordering of nonionic surfactants. These studies reveal that EAN is highly ordered at the interface, exposing the ethyl moiety to the gas phase. Additionally, polarization studies have enabled the average orientation of the ethyl group to be determined. Adsorption of nonionic surfactants at the interface appears to significantly displace the EAN from the interface. The headgroup of the surfactant, a linear ethylene oxide group, appears to be highly disordered.The disorder of the linear ethylene oxide groups has led to difficulties in their surface spectroscopic fingerprinting in this and other works. In an attempt to study the interfacial behaviour of ethylene oxide and the temperature dependence of its hydration, closed loop structures of PEO attached to hydrophobic groups were also probed. This essentially locks their conformation. Such molecules are known as crown ethers and display interesting interfacial behaviour and also the ability to bind cations.The presence of even small amounts of adsorbed crown ethers at the water interface is shown to considerably perturb the water structure. The NO, CN, COC and CH vibrational modes of these compounds at the air-water interface as well as OH vibrational modes of the surface water hydrating this compound have been targeted in order to obtain molecular information about arrangement and conformation. The CH2 vibrational modes of crown ethers have been identified and found to be split due to their interaction with ether oxygen. The spectra provide evidence for the existence of a protonated crown complex moiety at the surface leading to the appearance of strongly ordered water species. The orientation of Nitrobenzo crown (NB15C5) was monitored as a function of solution concentration, by targeting the ratio of peak intensities of the CN and NO2 vibrational modes. The water of hydration has also been probed as a function of crown concentration, salt concentration, and temperature. The latter study strongly suggests that the surface can be treated as a charged interface, and that the associated ordered water decreases with increasing ionic strength of the bulkFinally, insoluble monolayers of fatty acids spread on a water surface have also been studied in an effort to further understand the relationship between molecular architecture and film structure. Fatty acid (Arachidic Acid – AA and Eicosenoic Acid – EA) monolayers are compared to investigate the effect on the monolayer structure of introducing unsaturation into the alkyl chain. For AA, at very large area per molecule, floating domains of crystalline nature exist rather than any classical gaseous phase. The measured conformational disorder in EA decreases continuously with monolayer compression and no crystalline domains are observed at low density. Addition of NaCl to the subphase does not affect the monolayer order for either of the compounds; instead, a dramatic increase in the signal of the water hydrating the headgroups is observed. The effect of introducing further unsaturations (up to three) was also studied in order to probe the resulting interfacial structure. Remarkably the double bonds appear to adopt the same orientation, irrespective of how many they are in the chain. By monitoring the vinyl CH stretch it was possible to study the film stability towards oxidative degradation and it was found that all three unsaturated species studied showed rapid degradation. The rate of degradation could be controlled by adjusting the film pressure. However, the monolayers could be stabilised by performing the experiments in an inert nitrogen atmosphere.