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

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Abstract
Transmission X-ray diffraction (XRD) of C6–16alkylammonium ion-exchanged montmorillonite SAz-1 with Ag-behenate as an internal standard provided accurate estimates ofd(001) values of the alkylammonium ion-exchanged montmorillonite. Inspection ofd(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 fornC(I′) andnC(II) equal to 1.36 and 1.77 nm, respectively, provided ambiguousnC(I′) andnC(II) values. The dependence of full-width at half-maximum values onnCallowed better estimates ofnC(I′) andnC(II) because “integral” and “non-integral” peak characteristics relating to interstratification could be included in the assessment.The effect of inaccurate estimates ofnC(I′) andnC(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 ofCinC(I′) andnC(II) differed by ± 1nCfrom the correct values. This method of calculation ofCiis recommended when reliable values ofCiare needed.