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Titolo:
Thermal stability of octadecylsilane monolayers on silica: Curvature and free volume effects
Autore:
Wang, RW; Wunder, SL;
Indirizzi:
Temple Univ, Dept Chem 016 00, Philadelphia, PA 19122 USA Temple Univ Philadelphia PA USA 19122 016 00, Philadelphia, PA 19122 USA
Titolo Testata:
JOURNAL OF PHYSICAL CHEMISTRY B
fascicolo: 1, volume: 105, anno: 2001,
pagine: 173 - 181
SICI:
1520-6106(20010111)105:1<173:TSOOMO>2.0.ZU;2-6
Fonte:
ISI
Lingua:
ENG
Soggetto:
SELF-ASSEMBLED MONOLAYERS; HYBRID BILAYER-MEMBRANES; NORMAL-ALKYL CHAINS; H STRETCHING MODES; CAB-O-SIL; ALKYLSILOXANE MONOLAYERS; ALKANETHIOL MONOLAYERS; MOLECULAR-STRUCTURE; MIXED MONOLAYERS; SPECTROSCOPY;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
47
Recensione:
Indirizzi per estratti:
Indirizzo: Wunder, SL Temple Univ, Dept Chem 016 00, Philadelphia, PA 19122 USA Temple Univ Philadelphia PA USA 19122 iladelphia, PA 19122 USA
Citazione:
R.W. Wang e S.L. Wunder, "Thermal stability of octadecylsilane monolayers on silica: Curvature and free volume effects", J PHYS CH B, 105(1), 2001, pp. 173-181

Abstract

The thermal stability and dynamic behavior at elevated temperature (120 degreesC) of octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on fumed silicas [Aerosil 380(A380), 7 nm primary particle size, and Aerosil OX50 (OX50), 40 nm primary particle size] and nonporous silica beads (106-nm) were investigated using vibrational spectroscopy and calorimetry. In the case of well-ordered SAMs. formed by OTS adsorption on "superhydrated" silica, thermal disordering of the alkyl chains increased with increased surface curvature and thus increased free volume. A melt endotherm was observed only for the high curvature A380 fumed silica. In-situ Raman data at 120 degreesC supported these results and showed increased conformational disorder and mobility for A380 compared with OX50. The conformational structure of OTS SAMs was found to be largely reversible up to 200 degreesC, except for "superhydrated" A380. This suggests that, for close-packed SAMs, silanol condensation only occurs when the resulting Si-O-Si bond does not interferewith the hydrophobic interactions of the alkyl chains; the Si-O-Si bond distance is smaller than the van der Waals distance between alkyl chains. In the case of "superhydrated" A380, the increased free volume available due to the high surface curvature, as well as the void space left in the interfacial region after evaporation of the adsorbed water, resulted in some unfavorable cross-linking at high temperature and irreversible disordering of the chains. When triethylamine (TEA) was added to already-close-packed SAMs, in the presence of excess OTS, no further OTS adsorption occurred, and the conformational order remained high. For lower coverage fumed silica, the addition of TEA resulted in increased OTS coverage at the expense of chain order.

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Documento generato il 29/05/20 alle ore 16:25:06