Wolfgang Wieker

Gaschromatographische und 29Si‐NMR‐spektroskopische Untersuchungen an Kieselsäuretrimethylsilylestern

Kieselsauretrimethylsilylester unterschiedlicher Konstitution werden mit Hilfe der Gaschromatographie hinsichtlich ihres Retentionsindex und der Retentionstemperatur charakterisiert. Es wird gezeigt, das die Retentionsdaten der Ester im wesentlichen von der Anzahl der Trimethylsilylgruppen im Molekul abhangen und nur wenig von der Konstitution des Kieselsauregerustes beeinflust werden. Die 29Si-NMR-Spektren von neun Kieselsaure-TMS-Estern unterschiedlicher Konstitution wurden aufgenommen und die Zuordnung der Signale zu den einzelnen Baugruppen der Ester diskutiert. Auf der Grundlage dieser Ergebnisse wurden Konstitutionsuntersuchungen an hexameren Kieselsaure-TMS-Estern durchgefuhrt, die zur Identifizierung von zwei Isomeren des Bicyclohexakieselsaureesters [(CH3)3Si]10[Si6O17] fuhrten.

Silicate groupings in glassy and crystalline 2PbO·SiO2

The type and the amount of silicate groupings existing in glassy and crystalline 2PbO·SiO2 have been determined by direct chemical methods: paper chromatography, trimethylsilylation combined with gas-liquid partition chromatography and by the molybdate method. The results obtained by these three different methods are in good agreement and demonstrate, that glassy 2PbO·SiO2 and each of the three main crystalline polymorphs are characterized by its own specific silicate anion distribution: the distribution in vitreous 2PbO·SiO2 is of a polyanionic nature; in TPb2SiO4 dimetic groups [Si2O7]6− prevail; M1Pb2SiO4 contains predominantly [Si4O12]8− rings and HPb2SiO4 is a typical polysilicate with chain anions [SiO32−]n. The results fit a structural model according to which glass is a random array of discrete polyatomic groupings; the gradual transition from the glassy state to the stablest crystalline structure is connected with degradation and polymerization of silicate anions.

On the constitution of silicate groupings in binary lead silicate glasses

Abstract Direct chemical methods have been used to determine the type and the percentage of silicate groupings present in binary lead silicate glasses within the composition range 4PbO · SiO2PbO · SiO2. The results obtained indicate that the constitution and the relative amounts of various silicate groupings change with the changing total amount of SiO2 in the glass. Glasses containing small amounts of SiO2 are characterized by the presence of few low-molecular silicate units; increasing the percentage of SiO2 promotes the formation of higher polymerized silicate groupings. The results are discussed with regard to stoichiometry, and are consistent with Haggs concept of glassiness.

Determination of silicate anion constitution in glassy and crystalline lead silicates using an improved TMS technique

Abstract Using the DMF trimethylsilylation technique developed by Tamas, Sarkar and Roy, the distribution of silicate anions in lead silicate glasses of the composition range PbO·SiO 2 4PbO· SiO 2 has been reinvestigated. The obtained results agree with earlier findings except for the tetrameric chain anion [SiO 4 O 13 ] 10− , which had not been found before. Crystallization of glassy PbO·SiO 2 proceeds via the formation of low-molecular silicate anions [Si 2 O 7 ] 6− , [Si 4 O 13 ] (II) 10− and [Si 7 O 22 ] 16− , which characterize the low-temperature crystalline polymorph TPbO·SiO 2 . Further heat treatment turns TPbO·SiO 2 into alamosite, characterized by polysilicate chain anions [SiO 3 2− ] ∞ .

The influence of the cooling rate of A 2PbO · SiO2 melt on the constitution of silicate anions

The kinetics of crystallization in a 2Pbo · SiO2 melt have been investigated. A TTT-diagram was constructed, which describes the kinetic parameters for the formation of crystalline phases in the system. By means of silicate anion analysis the relationship between the cooling rates of the melt and the structure of silicate units in solid 2PbO · SiO2 has been studied. Substantial differences in the thermal treatment of the melt lead to alterations of the silicate anion constitution, which cause changes in the crystallization behavior of glassy 2PbO · SiO2.