On the dual behaviour of water in octanol-richaqueous n-octanol mixtures: an x-ray scatteringand computer simulation study

Aqueous n-octanol (n = 1, 2, 3, 4) mixtures from the octanol rich sideare studied by x-ray scattering and computer simulation, with focus onstructural changes, particularly in what concerns the hydration of thehydroxyl-group aggregated chain-like structures, under the inuence ofvarious branching of the alkyl tails. Previous studies have indicated thathydroxyl-group chain-cluster formation is hindered in proportion to thebranching 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 moleculeswithin the hydroxyl-chain domains, participate to the chain formations indierent manner for the 1-octanol and the branched octanols. The hy-dration of the octanol hydroxyl chains is conrmed by the shifting of thescattering pre-peak to low momentum transfer both from measured andsimulated x-ray scattering intensities, which corresponds to an increasedsize of the clusters. Experimental x-ray scattering amplitudes are seen toincrease with increasing water content for 1-octanol, while this trend isreversed in all branched octanols, with the amplitudes decreasing with theincrease of branching number. Conjecturing that the amplitudes of pre-peaks are related to the density of corresponding aggregates, these resultsare 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 thehindering of OH hydroxyl chain structures arising from the topology ofthe alkyl tails (branched or not).