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 (C_nH_2n+1NH₃Br; n=1-8, 10) and n-alkyltrimethylammonium bromide (C_nH_2n+1N(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 (φ_V⁰ and φ_K⁰) were evaluated. It was confirmed that φ_K⁰ linearly decreased with increasing alkyl chain length while φ_V⁰ linearly increased. Differences of φ_K⁰ between C_nH_2n+1NH₃Br and C_nH_2n+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 C_nH_2n+1NH₃Br was distinctly larger than that of C_nH_2n+1N(CH₃)₃Br. The reason for the difference in compression of these micelles was discussed concerning the counter ion binding and the aggregation number.