Application of an internal standard technique by transmission X-ray diffraction to assess layer charge of a montmorillonite by using the alkylammonium method

Transmission X-ray diffraction (XRD) of C6-16 alkylammonium ion-exchanged m ontmorillonite SAz-1 with Ag-behenate as an internal standard provided accu rate estimates of d(001) values of the alkylammonium ion-exchanged montmori llonite. Inspection of d(001) values were made to assess the possible forma tion of gauche conformers (alternate arrangements of the molecules) in the interlayer and to determine the critical carbon chain length, n(C). Using c onventional constraints for n(C)(I') and n(C)(II) equal to 1.36 and 1.77 nm , respectively, provided ambiguous n(C)(I') and n(C)(II) values. The depend ence of full-width at half-maximum values on n(C) allowed better estimates of n(C)(II) and n(C)(II) because "integral" and "non-integral" peak charact eristics relating to interstratification could be included in the assessmen t. The effect of inaccurate estimates of n(C)(I') and n(C)(II) values on calcu lated interlayer cation exchange capacity (C-i) using two conventional conc epts of calculation were compared. It was found that a procedure based on t he summation of fractions of the layer charge gave relative errors of C-i < 2%, even where the number of carbon atoms corresponding to both n(C)(I') an d n(C)(II) differed by +/- 1 n(C) from the correct values. This method of c alculation of C-i is recommended when reliable values of C-i are needed.