Effect of the solid/liquid interface structure on X-ray diffraction in nano-biocomposites

It is shown that periodic modulation of electron density in a liquid layer surrounding a nanocrystal may influence considerably the width of a conventional diffraction profile taken along the normal direction to the liquid/solid interface. The kinematic approximation is used to develop an analytical expression for the diffraction profile, assuming that the degree of order in the modulated liquid-like layers diminishes exponentially with the distance from the interface, which is characterized by the correlation length, L. Owing to the above-mentioned modulation, the sizes of nanocrystals extracted from the width of diffraction profiles will appear larger than they really are. Molecular ordering is destroyed by mild annealing or pressure application, resulting in substantial broadening of X-ray diffraction lines. This effect may be most significant in nano-biocomposites, such as bone and tooth dentin, comprising substantial amounts of water (tens of percent). It is calculated that for L ≃ 1 nm and a net crystallite thickness of T < 50 nm, the relative change in profile width can reach a few percent, which is easily measurable. The obtained simulation results are compared with existing experimental data.