Chain Unfolding in an ODS-Bonded Phase Caused by the Sorbed Tetra-n-butylammonium Ion.

The simultaneous sorption of the tetra-n-butylammonium ion (TBA(+)) and butanol on the bonded phase sorbent Partisil-10 ODS-3 from an aqueous mobile phase, at the two different ionic strengths 0.50 and 0.050 mol/L, is studied by the column equilibration technique. When the TBA(+) concentration in the mobile phase is kept constant while the butanol concentration is varied, the plots of moles of TBA(+) sorbed versus moles of butanol sorbed decrease linearly. This indicates that butanol simply competes with TBA(+) for sorption space. In contrast, when the butanol concentration in the mobile phase is kept constant while the TBA(+) concentration is varied, the plots of moles of butanol sorbed versus moles of TBA(+) sorbed decrease nonlinearly. This indicates that, in addition to competing with butanol for space, sorbed TBA(+) also causes an unfolding of the originally collapsed C(18) chains. At 0.050 mol/L ionic strength, chain unfolding causes an increase in the total space that is available for sorption of butanol, a decrease in overlap between sorbed TBA(+) and butanol, and a decrease in the sorbent strength of the ODS phase for butanol. The last effect is likely due to a reduction in contact area between a sorbed butanol molecule and the C(18) chains. At 0.50 mol/L ionic strength, chain unfolding decreases both the TBA(+)/butanol overlap and the sorbent strength but does not increase the total space available for butanol sorption. A physicochemical model is developed which fits well to the experimental data.