Hydrogelation characteristics of amphiphilic N-octadecyl maleamic acid derivative

Abstract

N-Octadecyl maleamic acid (ODMA), on conversion to sodium salt (ODMG) is shown to form gel-like structures in water. Specific conductivity and viscosity estimations at different concentrations in the temperature range of 25–70 °C indicated that ODMG is capable of gelling water at a concentration as low as 0.75% at 25 °C. The hydrogel tends to undergo reorganization on increasing the temperature. Thus, 1% gel was observed to transform into viscous solution on increasing the temperature, the onset temperature of transformation occurring at 52 °C. This is established from ¹H NMR spectral measurements in D₂O which showed sharp increase in intensity of methyl (δ = 0.86 ppm) and methylene (δ = 1.25 ppm) protons of ODMG gel on raising the temperature from 50–70 °C. The small angle X-ray diffraction studies performed on ODMG gel showed d-spacing in ratio of 1 : 0.5 : 0.3 indicative of lamellar structures. Molecular modelling calculations performed using the Cerius² package further supported the lamellar structures of ODMG. The scanning electron micrograph of ODMG gel showed uniform fibrillar-aggregated structures with a size of about 65 nm. The viscosity estimations on a 1.5% aqueous solution as a function of shear rate in range of 10–100 s⁻¹ indicated non-Newtonian behaviour. ODMG gel showed birefringence when viewed with an optical microscope under cross polarisers. This is suggestive of liquid crystalline behaviour typical of viscoelastic materials. The results from Langmuir film balance (LFB) measurements on ODMA indicated almost similar packing characteristics at the air water interface at pH 6.5 and 10.2. ODMA was found to exhibit A₀ of 30.89 and 33.60 Ų per molecule at pH 6.5 and 10.2 respectively. However, at pH 10.2, ODMG exhibited more expanded packing structures in pressure region of 5–15 mN m⁻¹.