isomorphic substitution in kaolinite?

as the layers in kaolinites are held much tighter (through hydrogen bonding) than the layers of montmorillonites which are held together by coulombic forces, are there no isomorphic substitutions in kaolinites as water molecules cannot enter the interlayer space? or are there still isomorphic substitutions but just more limited compared to montmorillonites

You are correct about the structure of these two secondary phyllosilicate clay minerals. Kaolinite being a 1:1 phyllosilicate, where the layers are tightly held together by H bonding. Montmorillonite, a 2:1 phyllosilicate clay mineral, with shrink-swell properties.

Isomorphic substitution is a separate concept; the process by which one element of similar size fills the position of another within the crystal structure of a mineral. Commonly Fe²⁺ or Mg²⁺ replacing Al³⁺ in the octahedral layer of Al³⁺ replacing Si⁴⁺ in the tetrahedral layer. This substitution results in a net negative charge. Montmorillonite has a high degree of isomorphic substitution, while Kaolinite has very low isomorphic substitution.