It is shown that the analysis of the intra-aggregate pore-size distribution function in silica systems, obtained by thermoporometry, allows a determination of the fractal dimension of the aggregates provided the physical limits are taken into account. A modified fractal relation for the cumulative pore volume as a function of the pore size is introduced, which gives the correct result when applied to simulated aggregates whose fractal dimension is known. This modified relation is then used to determine the fractal dimension D of two experimental aggregated systems, Aerosil 200 silica dispersed in water (sol) or in unde-cane (gel). The difference in the D values can be explained by the nature of the interaction between the silica surface and the liquid. Our results confirm that the fractal character of the aggregates or blobs is well described by the left wing of the pore-size distribution curve (for D < 2), while the right wing is mainly governed by the size distribution of the aggregates themselves.

The pore size distribution curve and the cumulative pore volume of intra-aggregate voids of a pyrogenic silica dispersed in liquids were determined by thermoporometry. Two types of silica surfaces (hydroxylated and non-hydroxylated) and two types of liquids (water and undecane) were considered. It is shown that for the hydroxylated silica dispersed in undecane, the aggregates exhibit large voids giving rise to a high pore volume. Conversely, a more compact aggregate structure is observed when hydroxylated silica is dispersed in water or when hydrophobic silica is dispersed in undecane. These results indicate that for a pyrogenic silica of a given morphology the intra-aggregate void size and void volume are determined by the nature of the solid-solid interaction.

Silica—water systems were frozen and their melting behaviour investigated by differential thermal analysis (DTA). The experimental curves exhibit two endothermic peaks located at a temperature T1 at the melting temperature T0 of bulk ice. Temperature T1 is lower than T0 and this endothermic peak appears irrespective of whether the system is a sol or gel. Peak T1 was attributed to the melting of ice filling the intra-aggregate void volume of silica particles. The amount of non-freezing water was determined by NMR measurements. Analysis of the experimental results led to a determination of the solid volume fraction øs and the specific void volume v1 of the aggregates.

La fusion de undécane gélifié par différentes silices est étudiée par analyse thermique différentielle. Les courbes obtenues permettent de mettre en évidence l'existence de deux pics. Un premier pic est situé à la température normale T 0 de fusion du undécane pur. Le second pic, situé à une température T 1 inférieure à T 0 est attribué à la fusion de undécane contenu dans des volumes intra-agrégats de la silice. L'analyse des résultats expérimentaux permet l'étude de la structure du gel.

The aim of the paper is to describe the influence of the morphological parameters of silicas on gelification processes. The morphology of the silicas is characterized quantitatively by its BET surface area and its initial void volume and qualitatively by electron microscopy. It appears that the parameter which governs the value of the gelification threshold is the aggregate morphology, correlated with the initial void volume. The building of the solid framework leading to the gel state does not depend on the volume fraction of the dry powder but on the volume fraction of the silica aggregates filled with part of the liquid medium. It is suggested that the percolation theory may describe the gelification process of silica in non-polar liquids.

The low-temperature (LT) TSD spectra of water and methyl alcohol adsorbed on Aerosil have been experimentally resolved. The variation of the area under the peaks with relative pressure (partial dielectric isotherms) have been studied and correlated with the adsorption isotherms. The relationship between the peak areas and the adsorbed amount suggest peaks I, II, and III be attributed to the dipolar polarization of the water or methyl alcohol molecules H bonded to the surface OH groups. The lengthening of the relaxation times as compared with that of the bulk liquid is explained in terms of entropy change on adsorption. In contrast, peaks IV and IV* are probably due to the dielectric polarization into small-size molecular aggregates. The fine structure of the TSD spectrum of adsorbed water on Aerosil is also compared with hertzian spectra of water adsorbed on silica.

Adsorption isotherms of water and methyl alcohol on Aerosil 380 outgassed at 110 and 850°C are determined and analyzed by the method developed by Ross and Olivier (“On Physical Adsorption.” Wiley, New York, 1964). The thermally stimulated depolarization (TSD) spectra, measured as a function of the adsorbed quantities of water or methyl alcohol on the different samples, exhibit two maxima of current, the first located between −160 and −140°C (low temperature peaks) and the second between −120 and −100°C (high temperature peaks). The latter appears only above the monolayer mβ. The dielectric isotherms, defined by the variation of the low temperature peaks area, P0, versus the relative pressure p/p0 are analyzed by the Ross and Olivier method and may be fitted by the superposition of two model isotherms characterized by a limit value P0β. The polarization P0 varies linearly with the adsorbed amounts but the plots exhibit slope discontinuities which are correlated with the P0β and mβ values. The results obtained by the TSD method are discussed and compared with published results obtained by other dielectric methods.