Application of an Internal Standard Technique by Transmission X-ray Diffraction to Assess Layer Charge of a Montmorillonite by Using the Alkylammonium Method

M. Janek and L'. Smrčok
Institute of Organic Chemistry, Slovak Academy of Science, SK-842 36 Bratislava, Slovak Republic

Abstract: Transmission X-ray diffraction (XRD) of C6–16 alkylammonium ion-exchanged montmorillonite SAz-1 with Ag-behenate as an internal standard provided accurate estimates of d(001) values of the alkylammonium ion-exchanged montmorillonite. Inspection of d(001) values were made to assess the possible formation of gauche conformers (alternate arrangements of the molecules) in the interlayer and to determine the critical carbon chain length, nC. Using conventional constraints for nC(I′) and nC(II) equal to 1.36 and 1.77 nm, respectively, provided ambiguous nC(I′) and nC(II) values. The dependence of full-width at half-maximum values on nC allowed better estimates of nC(I′) and nC(II) because “integral” and “non-integral” peak characteristics relating to interstratification could be included in the assessment.

The effect of inaccurate estimates of nC(I′) and nC(II) values on calculated interlayer cation exchange capacity (Ci) using two conventional concepts of calculation were compared. It was found that a procedure based on the summation of fractions of the layer charge gave relative errors of Ci <2%, even where the number of carbon atoms corresponding to both nC(I′) and nC(II) differed by ± 1 nC from the correct values. This method of calculation of Ci is recommended when reliable values of Ci are needed.

Key Words: Alkylammonium Method • Interlayer Cation Exchange Capacity • Layer-Charge Determination • Montmorillonite • Silver Behenate • XRD

Clays and Clay Minerals; April 1999 v. 47; no. 2; p. 113-118; DOI: 10.1346/CCMN.1999.0470201
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)