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, and 4) mixtures from the octanol rich side are studied by X-ray scattering and computer simulation, with a 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 in the chain formations in a different manner for 1-octanol and the branched octanols. The hydration of the octanol hydroxyl chains is confirmed by the shifting of the scattering pre-peak position kPP to smaller values, both from measured and simulated X-ray scattering intensities, which corresponds to an increased size of the clusters. Experimental pre-peak 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 the branching number. Conjecturing that the amplitudes of pre-peaks are related to the density of the 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 aggregates in branched alcohols by inserting itself into the OH chains. The analysis of the cluster distributions from computer simulations provide more details on the role of water. For cluster sizes smaller than d(c) = 2 pi/k(PP), water is found to always play the role of a structure enforcer for all n-octanols, while for clusters of size dc water is always a destructor. For cluster sizes larger than d(c), the role of water differs from 1-octanol and the branched ones: it 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).