On the dual behaviour of water in octanol-rich aqueous n-octanol mixtures: an x-ray scattering and computer simulation study

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 influence 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 examined, i.e. up to the miscibility limit. It is found that water molecules within the hydroxyl-chain domains, participate to the chain formations in different manner for the 1-octanol and the branched octanols. The hydration of the octanol hydroxyl chains is confirmed 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 n. Conjecturing that amplitude of pre-peaks are related to the density of corresponding aggregates, these results interpreted as water breaking large OH hydroxyl chains in 1-octanol, hence increasing the density of aggregates, while enhancing hydroxyl aggregates 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)