Structural-chemical evolution within exfoliated clays.

The exfoliated (delaminated) structures of lamellar clays offer potential as precursors for the formation of various nanostructured materials. In this article, Lucentite and Laponite phyllosilicate clays, which both have empirical formulas of Na(0.33)[Mg(2.67)Li(0.33)Si4O10(OH)2] but differ in nanodimensions, have been exfoliated. Experiments were carried out for mixtures containing approximately 1 wt % phyllosilicate in a 5% aqueous solution of poly(acrylic acid) at different temperatures. X-ray diffraction and photoemission spectroscopy measurements for the solid products recovered after stirring the mixtures at 20 degrees C showed that the fully extended chains of poly(acrylic acid) were intercalated within the interlayer spaces between the silicate plates of the clays. At 85 degrees C, however, the clays were exfoliated and/or partially exfoliated. Photoemission spectroscopy also indicated that the exfoliated structures primarily consisted of silica nanoplates. 29Si nuclear magnetic resonance and oxygen K-edge near-edge X-ray absorption fine structure indicated that the surfaces of the plates were terminated by high concentrations of the silanol (-SiOH) groups, which created structural branches during intercalation. A model was developed in which intercalation and the removal of ions from the clays after the poly(acrylic acid) interactions reduced the electrostatic van der Waals forces between the plates. It was also shown that the formation of branches created a steric effect that inhibited the stacking of the plates. Together these resulted in exfoliation.

Multiple group association models with latent variables: an analysis of secular trends in abortion attitudes, 1972-1988.

This paper examines methods for integrating latent class models with association and conditional association models, including those in multiple groups. These methods are shown to be generalizable to a wide range of association models in which one or more variables is latent and shown to be especially well suited to modeling temporal changes (tends) in latent structures. Like the usual association model, association models with latent variables utilize the information inherent in the ordering of the categories. Consequently, these models require the estimation of fewer parameters and are preferred to loglinear models that may require the estimation of substantial numbers of parameters even for models with a moderate number of observed variables. Following a discussion of the formal model, an example is considered. This example focuses on the American public's approval (disapproval) of legalized abortion from 1972 to 1988.

Patterns of drug use among white institutionalized delinquents in Georgia: evidence from a latent class analysis.

Previous research by Kandel [1] and others indicates that adolescent drug use follows a progression from legal drugs, through marijuana, to hard drugs. In this study of drug use patterns, institutionalized delinquents were found to follow a similar progression of drug use. Unlike previous studies which used "rule of thumb" methods of model assessment, this study uses probabilistic assessment of the models. Importantly, the present study supports a modified gate-way sequence, where cocaine use appears as an intermediate step between marijuana use and use of other hard drugs. It is suggested that widespread availability of cocaine in the late 1980's may have resulted in a new "step" in the drug use sequence.