Surfactant monolayers at the air-water interface

Surfactant monolayers at the air-water interfaceYei-Shin Tung and Leslie J. FinaRutgers University, Department of Materials Science and EngineeringP.O. Box 909, Piscataway, New Jersey 08855ABSTRACTSurfactant monolayers are formed by the adsorption of dodecane sulfonate sodium salt from bulk aqueoussolutions to the air-water interface. The reflection-absorption of polarized infrared radiation from the surface is probedwith respect to the bulk concentration of the water soluble surfactant and the concentration of sodium chloride in thesubphase. The anisotropic orientation of the amphiphiles is calculated by using optical modeling equations.Inferences about the conformation of the alkyl chain are made from frequency information.1. INTRODUCTIONThe relationship between the refIection-absorptior of infrared radiation from an oriented monolayer on anisotropic substrate and the structure of the monolayer has been explored recently.1 In this previous work it wasshown that the angle of orientation between the average chain axis and the surface normal can be found if theisotropic optical constants are known and some information about anisotropic constants is available. The emphasis ofthe current work is on the application of the optical model to structural determinations of water soluble surfactants atthe air-water interface. These type of surfactants dissolve in water, but due to their amphiphilic character accumulateat the air-water interface. Surface tension measurements lower than that of pure water have been measured2 and thisis a demonstration of the presence of surtactant at the interface. Combination of the structural information found frominfrared spectroscopy and thermodynamic parameters from surface tension measurements offers a betterunderstanding of surfactant ordering at interfaces.2. EXPERIMENTALSolutions of purified dodecane sulfonate sodium saft (C12S) were prepared in doubly distilled water andpoured into a teflon trough for scanning. In this study all p-polarized data were collected at an incident angle of 30°and s-polarized, at I 0°. Four thousand coadded scans were collected. A commercially manufactured reflection-absorption cell was used to probe the air-water interface (Specac 19653).3. RESULTS AND DISCUSSIONIn Figure 1 are shown the p-polarized reflection-absortion spectra of the methylene stretching region of C12Susing pure water as a substrate. The bulk concentration of the surfactant increases in the plot from top to bottom.The change in intensity is obvious. The peak intensities of both p- and s-polarized light have been measured for theasymmetric and symmetric CH2 stretching peaks with concentration and these are shown in Figure 2. The peakintensities are constant at high concentrations and decrease at low concentrations.Using isotropic optical constants which have been calculated from transmission spectra using a Kramers-Kronig analysis and assumptions about the anisotropy in the optical constants1, the intensities in Figure 2 are used tocalculate the chain orientation. The results are shown in Figure 3. The hydrated crystal structure of C12S waspublished a number of decades ago3 and yields a chain orientation angle of 43°. The transition which occurs at a -logCof ca. 2.4 indicates the onset of changes in orientation and corresponds to a deviation from linearity of surfacetension versus concentration plots2.