The effect of surfactant size on the extent of montmorillonites decomposition in HCl was investigated. Na-SAz montmorillonite and tetraalkylammonium salts of alkyl-chains length increasing from methyl-Me4N to pentyl- Pe(4)N were used for organo-montmorillonites preparation. Decreasing intensity of d(001) diffraction proved destruction of montmorillonite structure connected with gradual surfactant release. A shift of the SiO stretching band to 1097 cm(-1) confirmed formation of amorphous silica phase. A new band near 7315 cm(-1) corresponding to SiOH overtone revealed creation of protonated silica. This band was observed in the spectra of all acid-treated samples, also in those with minor decomposition of the structure. The size of the cations significantly affected decomposition of montmorillonites in HCl. The less stable were Na-SAz and Me4N-SAz in which the content of octahedral atoms dropped to similar to 5% of their original values upon 8 h treatments. Et4N-SAz and Pr4N-SAz were slightly more resistant mainly at short times. Bu4N-SAz and Pe(4)N-SAz showed the least structural modifications, only 50% and 35% of octahedral atoms, respectively, were released into solution within 8 h treatments. This observation proves that bulky alkylammonium cations covering the inner and outer surfaces of montmorillonite prevent effectively the access of protons to the layers protecting the mineral from degradation in the acid. (c) 2012 Elsevier B.V. All rights reserved.