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Sum Frequency Spectroscopy


Cambridge Sum Frequency Spectroscopy



The research of the Sum Frequency Vibrational Spectroscopy (SFS) section of the Paul B. Davies group focuses on interfacial adsorption chemistry: more specifically the determination of the orientation and conformation of surface active species such as surfactants and polymers adsorbed at the solid/liquid, solid/air and liquid/air interfaces. Such information is critical to the understanding of many industrial and biological processes where surface properties and hence interfacial behavior are modified through the addition of surface active molecules to promote desired interactions. Interfacial regions between bulk media, though often comprising only a fraction of the material present, are frequently the site of reactions and phenomena that dominate the macroscopic properties of the entire system. Spectroscopic investigations of such interfaces are typically hampered by the lack of surface specificity of most available techniques. Few techniques are capable of providing information solely on surface species without contributions from species resident in the bulk. One technique which may be applied to a wide range of interfacial systems, yields information on both the polar orientation and degree of ordering of adsorbed molecules, and is truly surface specific is Sum Frequency Vibrational Spectroscopy. Sum Frequency Spectroscopy is a second order non-linear optical technique in which a visible laser beam of fixed frequency and a tunable infrared laser beam are pulsed simultaneously onto an interface and light is emitted at the sum of the two frequencies. The intensity of the emitted light is proportional to the second-order non-linear susceptibility of the interface which has two components: a non-resonant term from the substrate, which is almost invariant with infrared frequency, and a resonant term from the adsorbate. Consequently, when the infrared frequency coincides with the resonant frequency of the adsorbed species, the intensity of the emitted light is altered. Detecting the emitted light as a function of infrared frequency produces a vibrational spectrum. The second order non-linear susceptibility is zero in centrosymmetric media. This has two important consequences. First, a vibrational spectrum is produced only from molecules at an interface, where the centrosymmetry of the bulk phase is broken. Second, the interfacial molecules must have a net polar orientation - no sum frequency emission results from molecules arranged in an equal number of opposite orientation or from a completely disordered surface structure. Solutions of surfactants and polymers are isotropic so only a net polar orientation of an adsorbate at an interface will result in a sum frequency spectrum. Polar orientation is determined from the relative phase of the resonant and non-resonant signal and the degree of orientational order is reflected in the relative strength of the resonances. The current sum frequency spectroscopy projects within the group fall into four broad areas: Adsorption from polymer and surfactant solutions at the hydrophobic solid/liquid interface, the development and application of a sum frequency active hydrophilic surface for adsorption studies, sum frequency studies of centrosymmetric bulk dispersions, and sum frequency generation at the nanoparticle/surfactant interface.

Adsorption at the Hydrophobic Solid/Liquid Interface

Adsorption at the Hydrophobic Solid/Liquid Interface

The adsorption of polyelectrolytes (charged polymers) at the solid/solution interface is applied in many industrially significant fields: yet despite much experimental and theoretical research is a poorly understood phenomenon. Similarly, surfactant adsorption from solution is widely used to alter surface properties and has been extensively investigated. However, a potentially important field which has received little attention to date is co-adsorption from mixed solutions of polyelectrolytes and surfactants. Such studies are extremely complex with potential interactions between the polyelectrolyte and the surfactant, the surfactant and the surface and between the polyelectrolyte and the surface. Each of these interactions may in turn be a function of the solution pH, the chemical nature of the surface, the ionic strength and the order of addition of the species to the system. Due to the inherent complexity of such systems a paucity of experimental data exits. Our group has published two papers in the field focussing on adsorption from non ionic surfactant solutions, and from mixed cationic surfactant/anionic polyelectrolyte solutions at the hydrophobic interface. These studies elucidated the effects of concentration and temperature on the composition and orientation of the adsorbed species. Currently we are studying adsorption from environmentally friendly sugar surfactant solutions and from anionic surfactant/cationic polyelectrolyte mixed systems. Specifically, the effect of the order of addition, the ionic strength and the concentration of each species is under investigation. Complimentary techniques such as surface tension determination are employed through industrial collaborations. Future work will concentrate on synergistic polyelectrolyte/surfactant interactions and the adsorption of non ionic biodegradable surfactants.

Development of a Hydrophilic Surface for Sum Frequency Studies

From the extensive nature of the research described above, it is clear that sum frequency spectroscopy is ideally suited to the study of hydrophobic solid/liquid interfaces. This is due to well established procedures for forming hydrophobic sum frequency active substrates. A typical substrate consists of a perdeuterated alkane thiol self assembled monolayer on a gold surface. The alkane thiol provides a well defined and robust hydrophobic surface, while the gold creates a strong non resonant signal, advantageous to sum frequency spectroscopy. However, analogous hydrophilic surfaces have not been developed for sum frequency spectroscopy, despite their widespread application in fields as diverse as the detergency industry, mineral processing and cosmetics. In order to study surfactant adsorption on a hydrophilic surface it has therefore been necessary to develop an entirely new method of substrate preparation, known as Displaced Metal Surface (DIMS). Essentially, the mineral oxide mica is cleaved to micron thicknesses and backed with evaporated gold. The fresh mica surface provides a highly hydrophilic adsorption substrate. Additionally mica is transparent in both the visible and infrared spectral regions, thereby allowing the generation of both a resonant signal from adsorbates on the mica surface and a non resonant signal from the gold surface. Since the mica is of the order of the infrared wavelength in thickness, constructive and destructive interference between the resonant and the non resonant sum frequency signals occurs as the thickness of the mica is varied between samples. This results in a change of phase and modulation of the strength of the overall signal. For the technique to be readily applicable a calibration curve of sum frequency phase and strength versus mica thickness is required. This has been achieved by recording sum frequency spectra of well packed, highly ordered alkyl silane monolayers on mica samples of varying, well defined thicknesses. Silanisation of mica is a notoriously difficult reaction and in the past has proved highly irreproducible. Over the last twelve months we have developed a rigorous experimental protocol which consistently produces the high quality monolayers required for the calibration experiments. This achievement is of great significance and a major paper in the field is currently in press with Langmuir. Consequently a large series of different mica thicknesses have now been characterised and the calibration curve is nearing completion. Ongoing mathematical modeling of sum frequency generation in this system is almost complete, providing a sound theoretical basis for the technique. DIMS has for the first time made adsorption studies at the hydrophilic surface accessible to nanosecond sum frequency spectroscopy. Currently, the adsorption of alkyltrimethylammonium surfactants at the mica surface is being studied as a function of concentration. It is intended that these experiments will clarify the nature of the surface species formed, particularly in the high concentration regime where structures such as hemi-micelles, rod like micelles etc. have been postulated to exist.

Sum Frequency Generation at the Nanoparticle/Surfactant Interface

The electronic and optical properties of metallic and semi-conductor nanoparticles has become one of the most intensely studied fields in surface science. Noble metal nanoparticles have been fabricated with widely differing morphologies such as wires and microcrystals. Following formation, the nanoparticles are typically passivated with surfactant molecules prior to the formation of an array on a suitable solid or liquid phase substrate. We have recently made the novel and potentially highly significant discovery of sum frequency generation at the nanoparticle/surfactant interface. To date we have concentrated our efforts on characterising the sum frequency generation phenomena. Specifically, we are in the process of determining the effect of nanoparticle size, surface density and surfactant coverage on the strength and resonant enhancement of the sum frequency signal. Anionically stabilised gold nanoparticle dispersions have been prepared in discrete sizes in the range of 7-80 nanometres. A nanoparticle/surfactant monolayer is formed at the air/water interface of a Langmuir trough through the spreading of a layer of dichain cationic surfactant in chloroform. The monolayer is then deposited on a silicon substrate and the sum frequency spectra of the alkyl chain recorded, enhanced by the non resonant surface plasmon signal generated from the nanoparticle. Measurements are not restricted to a solid substrate and an apparatus has recently been constructed that allows spectra to be recorded at the air/water interface and potentially at the liquid/liquid interface. Whilst sum frequency generation at the noble metal nanoparticle/surfactant interface is of fundamental interest, a system with far wider applications is that of semi-conductor/surfactant interfaces. Such systems find widespread industrial application in fields such as photo-voltaic cells, optical switching and nano-electronics. Consequently the next stage of this project is to study semi-conductor nanoparticle systems in an attempt to provide information on ordering of stabilising surfactants and their effect on morphological properties of the resulting assemblies.

Sum Frequency Studies of Centrosymmetric Bulk Dispersions

As stated above, the surface specificity of second order non-linear optical techniques such as sum frequency generation derive from the non centrosymmetric nature inherent in an interface. It has generally been believed that extension to centrosymmetric systems such as colloidal dispersions was forbidden by the electric dipole approximation. A recent second harmonic generation study however has shown that second order non-linear phenomena are possible in centrosymmetric systems if the coherence length of the process is correlated with the length scale of the species studied. To investigate the possibility of sum frequency generation from bulk centrosymmetric media, silica beads of the order of 5 micron in diameter have been silanised with an alkyl silane monolayer and dispersed in a non aqueous solvent. Measurements are performed in the transmission mode through a variable path length cell, with parameters such as particle concentration, beam power, focussing length and beam angles being varied. Additionally, theoretical modeling of the system has been undertaken in an attempt to predict the experimental conditions most favorable for sum frequency generation. If the project is successful, self assembled centrosymmetric systems such as micelles and vesicles will be studied. The technique is ideally suited to the study of both kinetic and equilibrium phenomena, and offers the possibility of directly monitoring processes such as micelle formation and, through surfactant deuteration, exchange processes. Additionally it will be possible for the first time to correlate orientational and ordering information of surfactant monolayers at planar and spherical interfaces.

Future Directions

As may be seen from the above, our ongoing and proposed experiments are quite diverse, covering the frontiers of current research in the field of polymer and surfactant adsorption at interfaces. In order to support these projects we have recently completed the construction of a dedicated preparation laboratory. This laboratory provides the necessary equipment and environment for the preparation of samples, cleaning of apparatus and the performing of air sensitive reactions. A vast increase in the productivity of the group has resulted due to greatly improved reproducibility and the availability of critical new equipment. We now have excellent facilities for both sample preparation and sum frequency spectral measurement. In addition, we have access to complimentary techniques such as transmission and scanning electron microscopy, and atomic force microscopy, although we have few in-house techniques available. We are therefore intending to expand this section of our group by acquiring equipment such as a Langmuir-Blodgett trough, a surface tensiometer and our own atomic force microscope. Due to recent expansion and the forging of new industrial links, we also intend to construct a second sum frequency spectrometer in the near future. This will facilitate both the founding of new projects and the extension into exciting new areas of existing ones.

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