Solution-Phase Adsorption of 1-Pyrenebutyric Acid Using Single-Wall Carbon Nanotubes

The time-dependent adsorption of aqueous 1-pyrenebutyric acid (PBA) onto single-wall carbon nanotubes (SWCNT) is studied theoretically and experimentally. Optical absorption spectroscopy was used to measure determine the amount of PBA adsorbed to the nanotubes as a function of time.  Short time (≤ 3 hours) adsorption data obtained at various temperatures exhibit a linear dependence of adsorbed PBA with time, where the slopes of these curves also vary linearly with initial bulk concentration. Equilibrium analysis after 78 days is fit well by a Langmuir isotherm, and suggests favorable adsorption of PBA onto SWCNTs at low solution concentrations with a maximum adsorption of 0.27 mg PBA/mg SWCNT.  A mass transport model was developed to describe the batch adsorption process, and agrees favorably with experimental results. The model suggests that the mass transfer process from the bulk to the SWCNTs is adsorption-limited, which enables an interpretation that the adsorption process is first order in kinetics.  The Arrhenius activation energy can be directly related to the mass transfer coefficient and is found to be 20.3 kJ/mol further suggesting that the PBA binds to the SWCNT by physisorption.  The model and experiments together provide a semi-empirical approach to streamline the study of adsorption behavior of organic molecules onto carbon nanotubes, which may be applicable to other systems of interest.