Tracking Molecular Aggregates at a Liquid Interface by Nonlinear Correlation Spectroscopy

An analytical model is developed to analyze the fluctuations in the surface second harmonic generation (SHG) intensity from a Langmuir film. From the SHG autocorrelation analysis, also called nonlinear correlation spectroscopy (NLCS), the characteristic times of the phenomena occurring at the interface are determined. This method is then applied to monolayers of the amphiphilic chromophore dye 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide (DiA) at the air–water interface. This compound is known to spontaneously form aggregates at liquid interfaces. The aggregates characteristic size is estimated using the newly developed NLCS method and compared to Brewster angle microscopy (BAM) data for two experimental initial conditions leading to two different sizes, namely, a chloroform and a methanol DiA solution. This study combining the advantages of the surface specificity of SHG with the temporal analysis of intensity correlations demonstrates the potential of NLCS to investigate the dynamics of molecular aggregates at a liquid interface.