Dielectric Relaxation Behavior of Aqueous Solutions of Carbobetaines with Varying Intercharge Distances

• Published in: The journal of physical chemistry. B Vol. 110; no. 19; pp. 9426 - 9433
• Main Authors: Ono, Yousuke, Shikata, Toshiyuki
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 18.05.2006
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp0608170

The dielectric relaxation behavior of aqueous triethylammonioalkanoate (carbobetaine:  Et3 nCB) solutions was examined as a function of frequency from 1.00 × 106 to 2.00 × 1010 Hz (6.28 × 106 to 1.26 × 1011 rad s-1 in angular frequency); number of intercharge methylene groups, n = 1, 3, 4, 5, and 10; and solute concentration, c. Two major relaxation modes, fast and slow, were found in all solutions examined. Et3 nCB systems with n = 5 and 10 possessed another, medium, relaxation mode with relaxation time τDh at high c values (above the contact concentration of solutes) in addition to the fast and slow modes. The fast mode with a relaxation time, τw, of ∼10 ps was attributed to the rotational motion of bulk water molecules. The slow mode with a relaxation time, τ, of 0.08−1 ns, depending on the n value, was attributed to the overall rotational motion of each carbobetaine in aqueous solution. The concentration normalized relaxation strength, Δεc -1, and τ value of the slow relaxation mode increased with increasing n. These findings were quantitatively explained on the basis of changes in the intercharge distance resulting in increased size and dipole moment of the carbobetaines. Above the contact concentration, collisions between solute molecules likely hindered their rotational motions, leading to an increase in τ. The middle relaxation mode found in longer Et3 nCBs (n = 5 and 10) with a relaxation time, τDh, of ∼0.2 ns, more than 20 times as long as that of bulk water molecules, τw, was attributed to the dehydration of water molecules tightly bound to all Et3 nCBs examined (including those with n < 5). This mode was not observed in the solutions of Et3 nCBs with n < 5, since the τ values corresponding to overall rotation were close to or shorter than the τDh values.