Molecular arrangement mechanisms within phosphate films on Ti6Al4V regulated by intermolecular forces based on sum frequency generation vibrational spectroscopy

Abstract

Self-assembled films of phosphonic acid on Ti6Al4V surfaces can improve the interface properties, mainly in terms of biocompatibility. In this study, different phosphoric acid films were formed on Ti6Al4V surfaces and the intermolecular forces in the films were explored by sum frequency generation (SFG) vibrational spectroscopy due to its intrinsic sub-monolayer interfacial selectivity. With the surface and interface resonance signals analysis and theoretical calculation, it is found that when the molecular polarity is the same, there is an extreme value of carbon chain length, near which van der Waals (vdW) forces can more effectively drive the self-assembly arrangement. However, for the phosphonic acids with polarity groups, the stronger polarity of the terminal functional groups generated the electrostatic interactions between the phosphonic acid molecules, further enhancing the distance between molecules and weakening the vdW interactions. Moreover, the new electrostatic force participated in the final molecular interactions. The synergistic effect of vdW and electrostatic forces constituted the final molecular interaction force, driving the molecular arrangement. A larger molecular interaction force results in lager inclination angles of the carbon chains, which is beneficial to resisting molecular collapse within the films. This research can be an effective guidance for improving SAMs properties.