ELECTRON-PARAMAGNETIC-RESONANCE STUDY OF THE CONCENTRATION OF SPIN-LABEL MOLECULES IN HYDRATED GLASS PORES

Dehydration of porous glass hydrated with a 5 mM solution of the spin- label molecules 2,2,6,6-tetramethyl-4-acetamido-piperidine-1-oxyl (TMA PO) was studied with electron paramagnetic resonance at 9 GHz. The spi n-label molecules' concentration remains constant, i.e., it follows th e Langmuir isotherm when glass pores of 292 +/- 20 nm diameter are deh ydrated. When the pore diameter is only 127 +/- 11 nm, or less, the La ngmuir isotherm appears not to apply despite the fact that in smaller pores the surface-to-volume ratio is larger. The reason for this appar ent failure could be the establishment of a water cluster phase which does not dissolve TMAPO, and/or it could be a kinetic effect of the sp in-label molecules. In this report the possibility that the failure of the Langmuir isotherm is the consequence of ordered water clusters is examined. To account for the lower concentration of TMAPO in smallest pores, as much as 70% of total water volume has to consist of such wa ter clusters, in agreement with an earlier proposal. (C) 1997 American Institute of Physics. [S0021-9606(97)52647-X].