The influence of the cationic center, anion, and chain length of tetra-n-alkylammonium and -phosphonium salt gelators on the properties of their thermally reversible organogels

The phase properties of 14 tetra-n-alkylammonium and -phosphonium salts with chloride, bromide, iodide, and perchlorate as anions and alkyl chain lengths from heptyl to octadecyl have been examined as neat solids and as gelators in thermally reversible organogels. These gelator structures are among the simplest investigated to date. The salts with the longest alkyl chains and a nitrogen cationic center produce the most stable gels based upon temperatures at which gels form on cooling from sols, periods of stability at room temperature, and minimum concentrations of gelator necessary to effect gelation of a liquid at room temperature. Specific gel properties are dependent upon the rate at which their (precursor) sol phases are cooled. Generally, gels with ammonium salts persist for longer periods, require less gelator, and exhibit higher gelation temperatures than those with the corresponding phosphonium salts. Typically, <2 wt % of a gelator is necessary to effect gelation. Several of the gels have persisted without visible change for years when kept at room temperature in sealed containers. The superiority of the tetra-n-octadecylammonium salts is attributed to greater dispersive (van der Waals) interactions among alkyl chains and stronger ionic interactions between charged centers. However, since the phosphonium salts are more resistant to thermal decomposition, their liquid mixtures can be cycled between the gel and sol states more times than those with ammonium salts.