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　ana-　lysis　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　-as-　sembly　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.