Adsorption and interfacial aggregation of a cationic quaternary ammonium surfactant on powdered mesoporous MCM-41 aluminosilicates from dilute aqueous solutions

The adsorption of cationic tetradecyltrimethylammonium bromide (TTAB) from aqueous solution on two series of powdered MCM-41 aluminosilicates (referre d to as SiAlxCn, where x is the mole Si:Al ratio and n the chain length of the surfactant template) has been studied at 298 K under the conditions of free pH of the aqueous phase. Potentiometric acid-base titrations of SiAl32 C14 in NaNO3 and TTAB solutions were performed. The results of the previous study on the adsorption of ionic surfactants and the new proton-balance da ta indicate that the MCM-41 surface acquires a negative charge under the ex perimental conditions employed. The influence of the pore size and the surf ace charge density on the adsorption mechanism was evidenced by comparing a dsorption isotherms of the surface-active ions TTA(+), electrophoretic mobi lities of solid particles, and differential molar enthalpies of displacemen t obtained on various solid samples. For the hydrogen-exchanged SiAl32C14 s ample (H+-SiAl32C14), the adsorption of bromide counterions Br- at the soli d-solution interface and the isotherm of the pH evolution in the equilibrat ed supernatant liquid were additionally measured. Titration microcalorimetr y was also used to determine the molar enthalpy of micellization and the cr itical micelle concentration of TTAB in deionized water at 298 K. Surfactan t adsorption appears to occur simultaneously on the external surface area a nd the pore walls. Monolayered aggregates consisting of "head-on" oriented surfactant units are expected to form on hydrophilic, highly charged surfac e zones of Na+-derived samples at low bulk molalities: It is postulated tha t displacing and destructuring of vicinal water induced by the growth of co mpact aggregates are responsible for the positive enthalpy of displacement in this adsorption range. The endothermic domain broadens with decreasing p ore size and increasing aluminum content in the silica matrix. The displace ment process is exothermic over the whole experimental range on H+-SiAl32C1 4 because water structuring is not significant for this sample. The aggrega tes formed on solid samples at higher surface densities are very heterogene ous in size and shape, depending on the surface charge density and the pore space available to the adsorbate.