BACKGROUND: Thermodynamics and kinetics data are both important to explain the extraction property. In order to develop a novel separation technology superior to current extraction systems, many promising extractants have been developed including calixarene carboxylic acids. The extraction thermodynamics behavior of calix[4]arene carboxylic acids has been reported extensively. In this study, the mass transfer kinetics of neodymium(III) and the interfacial behavior of calix[4]arene carboxylic acid were investigated. RESULTS: The rate constant (K-ao) becomes constant when the stirring speed was controlled between 250 rpm and 400 rpm. The activation energy (E-a) was calculated to be 21.41 kJ mol(-1) or 88.17 kJ mol(-1) (dependent on temperature) from the slope of log K-ao against 1000/T. The linear relationship between the specific area and the extraction rate is the characteristic of an interfacial reaction control. The minimum bulk concentration of the extractant necessary to saturate the interface (C-min) is lower than 4.19 x 10(-4) mol L-1. CONCLUSION: The effect of stirring speed, temperature, and species concentration on the extraction rate demonstrates that the extraction regime depends on the extraction conditions. The chemical reaction control governs the extraction regime at temperatures below 303 K and a mixed control regime occurs when the temperature is between 303 K and 318 K. The probable locale for the chemical reaction is at the liquid-liquid interface and the rate equation is deduced to be: -d[Nd3+]((a))/dt = k(f) [Nd3+]((a))[H(4)A]((o))0.727 [H+]((a))-0-978. The rate-controlling step was suggested by the analysis of the experimental results. (C) 2008 Society of Chemical Industry.