Dynamic surface tension,critical micelle concentration,and activity coefficients of aqueous solutions of nonyl phenol ethoxylates

Dynamic surface tensions, σ(t) for aqueous solutions of nonyl phenol ethoxylates (NPEOs) at the temperature 298.15 K were measured using a Lauda drop volume tensiometer. The non-ionic surfactants analyzed in this work were Tergitol NP-9, NP-35 and NP-40. By using the classical Ward and Torday equation, the diffusion coefficient for each bulk surfactant concentration was calculated. The equilibrium surface tension values were determined by extrapolating the dynamic surface tension to t → ∞ on the σ(t) vs. t^−1/2 curves. These values were used to determine the critical micelle concentrations (CMC) of the surfactant aqueous solutions as well as to calculate the infinite dilution activity coefficient of the surfactant, following a model that combines the Volmer surface equation of state and the Gibbs adsorption equation.     

Octadecyltrichlorosilane adsorption kinetics on Si(100)/SiO2 surface: contact angle,AFM, FTIR and XPS analysis

Octadecyltrichlorosilane (OTS) adsorption kinetics on Si(100)/SiO₂ surface has been studied as a function of concentration by sequential and nonsequential dipping techniques. The contact angle technique is used to evaluate growth kinetics and thermal stability and to determine critical surface tension of the OTS layer. Atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and X‐ray photoelectron spectroscopy (XPS) are used to confirm OTS adsorption. Langmuir isotherms are employed to analyze the kinetics data to obtain adsorption and desorption rate constants (k_a & k_d) as well as Gibbs free energy, (ΔG_ads). These parameters, k_a, k_d and ΔG_ads(y) are found to depend exponentially (y = y₀ + A.exp(?x/t)) on the OTS concentration (x). The OTS layers are found to be thermally stable up to a temperature of 230 °C and the critical surface tension obtained from the Zisman plot is found to be ～19.8 dynes/cm. OTS monolayer coverage obtained by AFM measurement agrees quite closely with that obtained from contact angle measurements. FTIR and XPS results confirm OTS adsorption. Copyright © 2006 John Wiley & Sons, Ltd.