140. Inhibited Phenol Ionization in Reverse Micelles. Confinement Effect at the Nanometric Scale O. F. Silva, M. A. Fernández,J. J. Silber, R. H. de Rossi, N. M. Correa

O. F. SILVA, M. A. FERNÁNDEZ,J. J. SILBER, RITA HOYOS DE ROSSI, N. M. CORREA

Chemphyschem

WILEY-V C H VERLAG GMBH

Año: 2011 vol.

1439-4235

We discover that the absorption spectra of 2-acetylphenol (2-HAP), 4-acetylphenol (4-HAP) and p-nitrophenol (p-NPh) in water/AOT/n-heptane reversed micelles (RMs) at the W0(W0 = [H2O]/[Surfactant]) values studied change with time if –OH ions are present in the RMs water pool. There is an evolution of the ionized phenols (phenolate) to the non-ionized phenols absorption bands with time, and this process is faster at low W0 value and with phenols with higher bulk water pKa value. That is, while in bulk water and at the hydroxide anion concentration used, only phenolate species are observed, in AOT RMs at this fixed hydroxide anion concentration, ionized phenols convert to non-ionized phenol species with time. Furthermore, we demonstrate that independently of the –OH concentration used to prepare the AOT RMs, the non-ionized phenols are the more stable species in the reversed micelle media. We explain our results considering that the strong hydrogen bond interaction between phenols and the AOT polar head groups make the non-ionized phenols the only species that can exists at the AOT RMs interface. The situation is quite different when the phenols are dissolved inside the cationic BHDC RMs. Herein, only the phenolates species are present at the –OH concentration used and at all times. The results clearly demonstrate that the classical definition of pH does not apply in a confined environment such as the RMs interior and, challenge the general idea that the pH can be determined inside RMs.