Adsorption of sodium cyclododecylmethanoate and cycloundecylmethanoate at the air/water interface

Abstract

Surface tension measurements on aqueous solutions of sodium cyclododecylmethanoate and sodium cycloundecylmethanoate in sodium carbonate/bicarbonate buffers and at constant sodium ion concentration at 25 °C were used to obtain the standard free energies of adsorption, the saturation areas per surfactant ion, and the critical "micelle" concentrations of these compounds. Similar measurements were done for solutions of sodium dodecanoate for comparisons.The standard free energies of adsorption of CH2 group that is located in a macrocyclic cycloalkyl ring appears to follow the "rule of two" (−RT ln 2) that is also valid for straight chain ionic surfactants. The saturation areas of both cyclododecylmethanoate and cycloundecylmethanoate are about 52 Å2/ion as compared to about 32 Å2/ion for normal dodecanoate.For large ring cycloalkylmethanoates the critical "micelle" concentrations appear to decrease by a factor of about 4/5 for each CH2 group added to the ring as compared to the factor of about 1/2 for each CH2 group added to the chain of normal alkanoates. The data suggest that large cycloalkyl rings have "collapsed ring", or "double chain", conformation in aqueous solutions and in the adsorbed state.