* Correspondence address: ul. Tretja, 8/1, Chernogolovka, Moscow. District, 142432 Russia. E-mail address: sheka@icp.ac.ru (E. Sheka) Physica B 276}278 (2000) 244}246 INS study of intermolecular interaction at the silicone-fumed silica interface E. Sheka*, I. Natkaniec, V. Khavryutchenko, E. Nikitina, H. Barthel, J. Weis Russian Peoples+ Friendship University, General Physics Department, ul. Ordjonikidze, 3, 117923 Moscow, Russia Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Russia Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine Wacker-Chemie GmbH, Werk Burghausen, D-84480 Burghausen, Germany Abstract Surface vibrations of a varied-surface-silylation-degree of silylated fumed silica "ne particles as well as the vibrations of a "ve-member polydimethylsiloxane oligomer Si5 adsorbed at di!erent coverage on hydroxylated and silylated fumed silica have been studied by INS at low temperature and by DRIFT at ambient one. The changes in the AWDS spectra are recorded which might be attributed to sterical restrains caused by the densi"cation of the trimethylsiloxy units packing on the surface of the silylated particles at the silylation degree exceeding 2.6, to immobilisation of the adsorbed molecules on the surface, as well as to the pressure e!ect on the TMS units of substrate caused by adsorbed molecules. Assignment of the observed spectra is done on the basis of the solution of the direct and inverse spectral problems with the force "eld initially determined quantum-chemically.  2000 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Incoherent scattering; Interface; Computer modelling The study has been stemmed out of extended series of experiments previously performed at the IBR-2 reactor at the Frank's Laboratory of Neutron Physics of Joint Institute for Nuclear Research at Dubna for various highly disperse amorphous silicas (see Ref. [1] and refer- ences therein). A convincing demonstration of the ap- plicability of inelastic neutron scattering (INS) for the investigation of vibrational spectra of particle surfaces, covered by hydrogen-atom-containing species, as well as of the surface atoms dynamical behaviour have been presented. Further computational analysis [2] has made allowance for extending the study over delicate processes concerning intermolecular interaction at the adsor- bate/substrate interface. The current paper presents re- sults of such study where fumed silica "ne particles are involved as substrates while silicone polymers pre- sented by a "ve-member polydimethylsiloxane (PDMS) oligomer (Si5 below) is an adsorbate [3,4]. A particular attention has been given to the impact of the modi"ca- tion of the fumed silica particle surface. As known, the surface of as prepared pirogenic fumed silica is termin- ated by hydroxyls. That can be modi"ed by the substitu- tion of the hydroxyl units: trimethylsiloxy (TMS) units are more widely used in practice [5]. Thus obtained silylated fumed silica has been examined in the current study alongside with the originally produced hydro- xylated one. INS experiments were performed at an inverse-ge- ometry spectrometer NERA at a pulsed neutron reactor IBR-2 of Frank's Laboratory on Neutron Physics of the Joint Institute for Nuclear Research at Dubna. Three series of samples were studied. Series 1 was dedicated to a detailed studying of the impact of the silylation on the substrate surface dynamical behaviour (samples were di!ered by TMS graft ratio characterised by carbon content, ). Series 2 and 3 were intended for studying vibrational spectrum of the Si5 adsorbate put on both hydroxylated and silylated fumed silica particles at 0921-4526/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 4 7 3 - 8