On the dual behaviour of water in octanol-rich aqueous n-octanol mixtures: an x-ray scattering and computer simulation study
Résumé
Aqueous n-octanol (n = 1, 2, 3, 4) mixtures from the octanol rich side
are studied by x-ray scattering and computer simulation, with focus on
structural changes, particularly in what concerns the hydration of the
hydroxyl-group aggregated chain-like structures, under the inuence of
various branching of the alkyl tails. Previous studies have indicated that
hydroxyl-group chain-cluster formation is hindered in proportion to the
branching number. Here, water mole-fractions up to x = 0.2 are exam-
ined, i.e. up to the miscibility limit. It is found that water molecules
within the hydroxyl-chain domains, participate to the chain formations in
dierent manner for the 1-octanol and the branched octanols. The hy-
dration of the octanol hydroxyl chains is conrmed by the shifting of the
scattering pre-peak to low momentum transfer both from measured and
simulated x-ray scattering intensities, which corresponds to an increased
size of the clusters. Experimental x-ray scattering amplitudes are seen to
increase with increasing water content for 1-octanol, while this trend is
reversed in all branched octanols, with the amplitudes decreasing with the
increase of branching number. Conjecturing that the amplitudes of pre-
peaks are related to the density of corresponding aggregates, these results
are interpreted as water breaking large OH hydroxyl chains in 1-octanol,
hence increasing the density of aggregates, while enhancing hydroxyl ag-
gregates in branched alcohols by inserting itself into the OH chains. Hence,
water acts as a structure maker or breaker in inverse proportion to the
hindering of OH hydroxyl chain structures arising from the topology of
the alkyl tails (branched or not).
Domaines
Chimie théorique et/ou physique
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