Adiabatic Compressibility of Aqueous Solutions of Amphiphiles with an Ammonium Group as the Hydrophilic Domain

Abstract

The apparent molar volume and the apparent molar adiabatic compression of aqueous solutions of n-alkylammonium bromide (CnH2n+1 NH3Br; n=l-8, 10) and n-alkyltrimethylammonium bromide (CnH2n+1 N(CH3)3Br; n = 1, 8, 10) were measured using a vibration-tube density meter and a sing-around ultrasound velocity meter at 5°C, and the partial molar volume and the partial molar adiabatic compression at infinite dilution (ϕv0 and ϕv0) were evaluated. It was confirmed that ϕv0 linearly decreased with increasing alkyl chain length while ϕv0 linearly increased. Differences of ϕK0 between CnH2n+1NH3Br and CnH2n+1N(CH3)3Br with the same alkyl chain were negligibly small, which was in good agreement with the fact that the successive N-methylation of ammonium cation had only a little effect on compression. These results could be interpreted in terms of the compensation between the increase of hydrophobic hydration and the decrease of electrostriction of water around the ammonium group, leading the net compression being almost constant. When the amphiphiles with an octyl or a decyl group form micelles above their critical micelle concentration, on the contrary, the adiabatic compression of CnH2n+1NH3Br was distinctly larger than that of CnH2n+1N(CH3)3Br. The reason for the difference in compression of these micelles was discussed concerning the counter ion binding and the aggregation number.© 2002, Japan Oil Chemists’ Society. All rights reserved.