The Odd-Even Effect on Bilayer Structures of C_nAzoC_mN⁺Br⁻

Abstract

The phase transition behavior of totally synthetic bilayer-forming amphiphiles, C_nAzoC_mN⁺Br⁻,with various chain lengths of the spacer (m) and tail (n) parts, has been investigated by the differential scanning calorimetry, X-ray diffraction, and absorption spectroscopic methods. The compounds which have the relationship of m − n = 2 showed similar aggregation states, the so-called H-aggregation state. However, there are some structural differences between the odd and the even number compounds in the hydrophilic region. These structural differences made big differences in the transition temperatures (Tc), enthalpy (ΔH) and entropy (ΔS) at Tc between the odd and even number compounds. On the other hand, the C_nAzoC₅N⁺Br⁻ (n = 6 ∼ 12) showed very similar molecular aggregation states, the so-called J-aggregation state. Their transition temperatures showed no such tendency. They had similar values of about 130°C. However, very small odd-even effects of the ΔH and ΔS at Tc were observed. These were attributed to the slight differences of the molecular areas between the odd and even number compounds of C_nAzoC₅N⁺Br⁻.

Key Words:

• the odd-even effect
• phase transition
• H-aggregate J-aggregate
• bilayer structure