Interactions between sodium dodecyl sulphate (SIDS) and four new surfactants, 3-alkoxyl-2-hydroxypropyl trimethyl ammonium chlorides (CnNCI) in aqueous systems were investigated using titration microcalorimetry. Pseudo-critical micellar concentration (pcmc), and thermodynamic parameters of the formation of mixed micelle for two binary surfactant systems at a certain concentration of CnNCI were estimated from the calorimetric data and the titration curve with methods from the literature. Changes in enthalpy and entropy of formation of the mixed micelle were discussed in the light of the structure of the surfactants and the iceberg model. The deduced conclusion is that interactions between hydrophobic chains of SDS and CnNCI are more important than the electrostatic attraction of the negatively charged head group of SDS with the positively charged head of CnNCI in the synergistic interaction between the anionic and cationic surfactants while the size of iceberg around the surfactant monomer is not sensitive to length of the hydrophobic chain. (C) 2003 Elsevier B.V. All rights reserved.