59th Annual Report on Research 2014

Background. The term NLS includes, among others, hyper-Rayleigh scattering (HRS) and second harmonic scattering (SHS). An isotropic solution of molecules with non-zero first order hyper-polarizability upon irradiation of light at frequency ω should not produce a response at frequency 2ω as the SH fields generated by the random distribution of molecules cancel out. Nonetheless, a signal at frequency 2ω, known as HRS, generated by fluctuations in the orientation and density of the molecules in solution, can still be detected. Such signal is incoherent and scales linearly with the number of SH active molecules. However it has been proven that when these molecules are adsorbed on the surface of a nano/micro particle there is a specific relationship between the phase of the SH field they emit that results in a coherent response, called SHS. The coherence of SHS makes the signal quadratically proportional to the number of emitters adsorbed on the particle. In the dipole approximation, when the bulk of the particle and the external medium are centrosymmetric in their structures, SHS arises solely from the interfacial region where the inversion symmetry is broken.

In petroleum technologies water-in-oil (w/o) and oil-in-water (o/w) emulsions stabilized by surfactants play a ubiquitous role. Surfactants are especially important in the stabilization of sub-micron sized droplets in emulsions. A concern in emulsion science is the stability of the emulsion against the tendency of colloidal objects to aggregate. The aggregation of emulsion droplets is influenced by factors similar to those that affect the aggregation of solid particles.

Our o/w emulsion made with 1% (vol) hexadecane has been stabilized using 1mM CTAB (shown in Figure 1(a)) and produced by sonication first at 37 KHz and subsequently at 80 KHz. The emulsion obtained in this way was then diluted hundred times. Dynamic light scattering measurements suggest a quite monodisperse size distribution with PdI<0.2 and average size around 150 nm from which a concentration of 10^10 droplets/ml can be deduced. In our NLS studies of emulsion stability and kinetics we have used a dialkyl aminostyryl pyridinium dye (di-n-ASP, n=10), shown in Figure 1(b): This molecule is both amphiphilic and SH active. Given the positive charge of the chromophore, di-10-ASP appears to minimize possible interactions with CTAB molecules. Di-10-ASP is not water-soluble so it is first dissolved in ethanol and then diluted with water.

Experimental Methods. Our home-built NLS setup is shown in Figure 2. Briefly, the fundamental light from a femtosecond Ti:Sapphire laser (oscillator only, 800 nm wavelength, 80 MHz repetition rate, 100 fs nominal pulse duration, 400 mW average power) is focused down to ~50 μm into the liquid sample, which is held in a cylindrical glass cell. The scattered SH photons can be detected with angular resolution by using collection optics mounted on an arm rotating around the center of the cell. The acquisition wavelength is set by a monochromator. A photomultiplier tube is used for the detection of the SH signal that is amplified, processed by a single-photon counting system, and sent to a PC. NLS experiments have been carried out by detecting the NLS intensity in the forward direction.

Results & Discussion. First we saturated the surface of the oil droplets with di-10-ASP (~35 μM). As the emulsion was observed to be stable on long time scales (at least several hours), the effect of salt and pH on its stability was monitored by detecting the NLS signal.

In Figure 3 the two-photon emission from di-10-ASP in the emulsion is compared to that detected for the same di-10-ASP concentration in water. The narrow peak centered at 400 nm is the SH signal (coherent SHS for the emulsion and incoherent HRS for the aqueous solution). The broad peak at longer wavelengths is two-photon (TP) fluorescence. As expected, changes in the NLS signal are observed upon addition of NaCl or NaOH as shown in Figure 4. Both an increase of the ionic strength and changes in pH are known to affect the stability of emulsions as they affect the surface charge of the droplets and subsequently their colloidal stability. NLS affords the ability to follow the stability of the emulsion over time and enable the study of kinetics. Addition of NaCl produces first a decrease in the signal followed by a slow increase over time, while addition of NaOH immediately induces an increase followed by a slight decrease. While the cause of the initial decrease in the signal upon addition of NaCl is not clear, the increase could be related to an increase of the size of the droplet as expected in coalescence or flocculation. And this same reason can explain also the increase observed upon addition of NaOH. The following slow decay could be instead related to a decrease in the number of droplets as creaming and phase separation take place.

Conclusion & Future Direction. In this second year, we have continued to develop the NLS method as a probe of emulsions and proven that NLS can be successfully used to study in real time their stability. The underlying causes of the observed changes will be further investigated. Because of this research, the students, the PI and Co-PI have been introduced to issues related to the stability of emulsions. The application of the NLS will provide a new probe for characterizing these and more complex systems and contribute to solving some of the currently most challenging problems in colloid science. Two publications on the results of NLS characterization of micelles and emulsions are in preparation.