Piezoelectric crystalline hydrogels of two surfactants with different polarity and long-chain alcohols

Piezoelectric materials are capacitive and transform mechanical deformations into electricity. The zwitterionic surfactant cocamidopropyl hydroxysultaine (CAPS) and the anionic surfactant sodium dodecyl sulfate (SDS) have a different dipole moment. Binary mixtures of CAPS and water are capacitive, although they are liquid or incohesive. CAPS interacts with decanol through its amino and sulfate groups, as revealed by Fourier transform infrared spectroscopy. A CAPS-decanol complex with a melting point of ∼31 °C is observed by differential scanning calorimetry. Mixtures of 1:1:14 CAPS:decanol: water (mole based) are birefringent gels, with an elastic modulus G' greater than the loss modulus G", as shown by polarized light microscopy and shear rheology. They have a cubic crystalline structure, as shown by X-ray diffraction in the small angle X ray scattering (SAXS) region. Aqueous mixtures of CAPS and decanol are also capacitive and piezoelectric. CAPS-octanol interactions are weaker, and their aqueous mixtures are more amorphous compared to CAPS-decanol mixtures, as revealed by SAXS. SDS also yields crystalline, birefringent gels upon mixing with water and either octanol or decanol. However, SDS has a smaller dipole moment than CAPS, and these gels are less capacitive.