In situ kinetics of layer-by-layer assembled nonlinear-optical-active amphiphiles from dynamic surface force...

We report the synthesis and layer-by-layer (LBL) deposition of a class of azo-benzene surfactants with the polycation poly(ethylenimine) (PEI). The different surfactants of the type X-azo-(CH2)10-SO3-, where X = -NO2, -CN, and -COCH3 in the azo-benzene moiety, have decreasing electron-withdrawing strengths. We use dynamic surface force measurements to study the in situ kinetics of adsorption of the amphiphiles onto PEI. Ex situ kinetics data obtained by adsorption-paused UV-visible spectroscopy validate the surface force results. These measurements describe the first application of dynamic force measurements to follow adsorption in LBL systems. UV-visible spectroscopy, second harmonic generation (SHG), and single-wavelength ellipsometry were also used to characterize the films. The observed blue shift upon adsorption of the amphiphiles suggests H-type aggregation within the multilayer. Two of the surfactants studied within the LBL films follow Langmuir adsorption behavior with equilibrium adsorption times under 200 s. The SHG results are consistent with the expected trends in the hyperpolarizabilities of the amphiphiles.