Phosphatidylcholine-based nonaqueous photorheological fluids: effect of geometry and solvent

Abstract

We report a detail study on photoinduced rheological changes in nonaqueous photorheological (PR) fluids obtained with 1,2-diacyl-sn-glycero-3-phosphocholine-based reverse wormlike micelles, 1-palmitoyl-2-oleophosphatidylcholine (POPC)/cyclohexane/H2O, POPC/isooctane/H2O, and L-α-dioleophosphatidylcholine (DOPC)/isooctane/H2O systems. Initially, the mixtures form highly viscoelastic fluids of long, reverse wormlike micelles as confirmed by the rheological measurements. When photosensitive molecule, trans-CA, is added to these mixtures, they exhibit photosensitivity, and the viscoelasticity increases that decreases on UV irradiation. The nature of the substituent on the benzene ring of trans-CA influences the rheology. When hydroxycinnamic acid (HCA) (hydrophilic, −OH group attached to the benzene ring of CA) is added to DOPC/isooctane/H2O phase, separation occurs; while with the methoxycinnamic acid (MOCA) and methylcinnamic acid (MCA) (hydrophobic groups, −OCH3, and –CH3 attached to the benzene ring of CA respectively) led to higher viscoelasticity. The study on the effects of position of the substitution on CA reveals the viscosity enhancement is in the order of p- > m- > o-isomers. The different geometries obtained because of substitution and photoinduced trans-cis isomerisation is responsible for the different rheology as confirmed by the dynamic rheology, the UV absorption, and the 1H NMR spectrums. The 1H NMR spectra revealed a change in the solubilization site of CA with irradiation. cis-CA (high solubility in water) is solubilized in the vicinity of water (at the hydrophilic end of DOPC) as compared with trans-CA. This disrupts the 3D networks of reverse wormlike micelles, decreasing the η0, plateau modulus, G0, and relaxation time, τR of solution.