Technical Notes A High-Performance and Simplified Quasi-Elastic Laser Scattering Method Using Homodyne Detection in Beam Divergence Isao Tsuyumoto* and Hiroshi Uchikawa Department of Environmental Systems Engineering, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan We devise the new principle of the quasi-elastic laser scattering (QELS) method using a homodyne detection technique in a beam divergence and successfully facilitate the equipment. The QELS method is a unique technique for the noncontact and time-resolved study of surface tension at liquid surfaces and liquid/ liquid interfaces. The conventional QELS method requires a precise optical alignment using a local oscillator such as a diffraction grating, and the determination of the surface tension accompanies much difficulty because of the low S/ N ratio of the power spectra. Our new principle allows high- performance QELS measurements by only a simple align- ment of a downsized experimental setup. The power spectra are obtained with 5 0 -1 0 0 times higher S/ N ratios than the conventional ones. The power spectra are ana- lyzed by a new theory, and the calculated surface tensions agree with the literature values. The accuracy of the surface tension measurements using the QELS method is substantially improved. Surface tension measurements provide much information on surfactant dynamics at liquid surfaces and liquid/ liquid interfaces. Common industrial processes such as washing, printing, and coating are closely related to the adsorption of surfactants, and surface tension measurements are widely used for quality control, research, and development in various industries. The measure- ments are also helpful for fundamental studies in view of the interfacial thermodynamics, properties, and structures. A number of methods for surface tension measurements such as the Wilhelmy method, the Du Nou ¨ y method, the spinning drop method have been reported so far, 1 but these methods bring about mechanical perturbation and are not suitable for time-resolved measurements. Katyl and Ingard reported a quasi-elastic laser scattering from liquid surfaces by capillary waves (ripplons), which are spontaneously generated at liquid surfaces. 2 Several research- ers reported that the QELS method is an appropriate method for surface tension measurements, indicating that the surface tensions calculated from the capillary wave frequencies agreed with the other methods. 3-5 Tsuyumoto and co-workers have fabricated the conventional QELS measurement system based on a laser heterodyne apparatus 6 using a diffraction grating as a local oscillator and have reported on the dynamics of mass transfer of surfactants at the water/ nitrobenzene interface. 7-11 However, many experimental difficulties arose in the measurements, e.g., the detection of the signal from the capillary waves was very difficult because of the extremely small amplitude of the waves ( ∼10 nm), and the determination of the surface tensions included accidental errors due to the low S/ N ratio of the observed power spectra. In this study, to improve the QELS method, we devise a homodyne detection technique using a beam divergence instead of heterodyne detection using the diffraction grating. We present here the measurement results of standard samples and discuss the advantages of the new homodyne QELS method. PRINCIPLE The principles of the conventional and the new homodyne QELS methods are shown in Figure 1a and b, respectively. A thermally generated spontaneous density fluctuation occurs at the liquid surface. The surface tension acts as a restoring force on the fluctuation, and it excites a surface tension wave, which is called a capillary wave or ripplon. The capillary waves occurring on liquid surfaces have various wavelengths, and each frequency is dependent on each wavelength. This makes it essential to select a capillary wave with a certain wavelength to determine the dispersion relation of the waves. The QELS methods measure the (1) Mingins, J.; Taylor, J. A. G.; Pethica, B. A.; Jackson, C. 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Chem. 2001, 73, 2366-2368 2366 Analytical Chemistry, Vol. 73, No. 10, May 15, 2001 10.1021/ac001338e CCC: $20.00 © 2001 American Chemical Society Published on Web 04/05/2001