The impact of size and shape in the performance of hydrotropes: a case-study of alkanediols

Abstract

Inspired by the recently proposed cooperative mechanism of hydrotropy, where water moleculesmediate the aggregation of hydrotrope around the solute, this work studies the impact of apolar volumeand polar group position on the performance of hydrotropes. To do so, the ability of two differentfamilies of alkanediols (1,2-alkanediols and 1,n-alkanediols) to increase the aqueous solubility of syringicacid is initially investigated. Interestingly, it is observed that in the dilute region (low hydrotropeconcentration), the relative position of the hydroxyl groups of the alkanediols does not impact theirperformance. Instead, their ability to increase the solubility of syringic acid correlates remarkably wellwith the size of their alkyl chains. However, this is not the case for larger hydrotrope concentrations,where 1,2-alkanediols are found to perform, in general, better than 1,n-alkanediols. These seeminglycontradictory findings are reconciled using theoretical and experimental techniques, namely thecooperative model of hydrotropy and chemical environment probes (Kamlet–Taft and pyrene polarityscales). It is found that the number of hydrotropes aggregated around a solute molecule does notincrease linearly with the apolar volume of the former, reaching a maximum instead. This maximum isdiscussed in terms of competing solute–hydrotrope and hydrotrope–hydrotrope interactions. Theresults suggest that hydrotrope self-aggregation is more prevalent in 1,n-alkanediols, which negativelyimpacts their performance as hydrotropes. The results reported in this work support the cooperativemodel of hydrotropy and, from an application perspective, show that hydrotropes should be designedtaking into consideration not only their apolar volume but also their ability to stabilize their self-aggregation in water, which negatively impacts their performance as solubility enhancers.