Controlled Porosity of MCM-41 Obtained by Partial Blocking of Pores by Silicon Oil

Abstract

Partial blocking of mesopores in the ordered MCM-41 silica by DC550 silicon oil was proposed as a preparation method of the material with controlled porosity and fixed pore size. The porosity of the samples with various content of DC550 was examined with the use of low temperature nitrogen sorption and positron annihilation lifetime spectroscopy. It was shown that the oil blocks the primary pores by forming the plugs near its entrances, but also partially locates in the interparticle spaces. The comparison of the results obtained from both investigation techniques was used to make the first attempt to obtain the calibration of ortho-positronium intensity, depending on pore volume. This is necessary to improve the utility of positron annihilation lifetime spectroscopy as a porosimetric technique. The need to take the migration of positronium to larger free volume into account is discussed.

Molecular probe dynamics and free volume heterogeneities in n-propanol confined in a regular MCM-41 matrix by ESR and PALS

“…In the paper of Gorgol et al 44 a precise study of the porosity of the MCM-41 matrix with surface modifications was performed. The presence of the six components in the annihilation lifetime spectra is realistic in such case, especially for unfilled (empty) or partially filled pores with oligomeric particles.…”

“…This latter effect is the well known fact of the PALS technique connected with the specific interaction between the quantum-mechanical o -Ps particle and the very low viscosity medium giving to rise the so-called bubble-like type of free volume. 44 Further, both these high- T ESR and PALS phenomena well correlate with the higher- T dynamic crossover from the non-Arrhenius to Arrhenius regime of the structural relaxation dynamics of n -PrOH at T A = 179 K. 24 Then, it was concluded that in contrast to the saturation effect in o -Ps annihilation the change in molecular probe dynamics is the real physical phenomenon which reflects directly a subtle change in the structural-dynamic state of the bulk n -PrOH medium. 24 Finally, most of these coinciding relationships between the corresponding characteristic ESR and PALS temperatures can be interpreted in terms of the two-order parameter (TOP) model via the mutually related evolution with temperature of the liquid-like domains and the solid-like ones in the supercooled liquid and normal liquid state of bulk n -PrOH.…”

“…displays the temperature dependence of the o-Ps lifetime, s 3 , and of the o-Ps lifetime dispersion, s 3 , for both the n-PrOH medium in the bulk state24 and in the conned one aer its embedding into the pores of the regular virgin MCM-41 matrix.In the paper of Gorgol et al44 a precise study of the porosity of the MCM-41 matrix with surface modications was performed. The presence of the six components in the annihilation lifetime spectra is realistic in such case, especially for unlled (empty) or partially lled pores with oligomeric particles.…”