0428同步年報-2021-全

Soft Matter 035 Binding Site Matters The binding site of inorganic salts on lecithin affects the micellar structure, which in turn alters the rheological properties of the micellar solutions. L ecithin, a mixture of glycerophospholipids, is well known to form reverse spherical or ellipsoidal micelles in a range of slightly polar organic solvents. Previous studies have shown that the lecithin reverse spherical micelles become transformed into reverse wormlike micelles on incorporating additive molecules, such as water, bile salts, inorganic salts and others with highly polar groups, into a lecithin solution. Such a transformation is attributed to either the expansion of the lecithin head-group area on insertion of the additives or on straightening the lecithin tails that decrease the tail area through the strong additive- lecithin attraction force, both of which alter the effective molecular geometry to a critical packing parameter (CPP) that favors the formation of cylindrical micelles. The long, flexible cylindrical (wormlike) micelles thus increase the solution viscosity and even impart viscoelastic properties or further cause gelation. For small additives, such as water and inorganic salts, it is interesting that, instead of monotonic increase, the viscosity attains a maximum and then decreases until a phase separation occurs, as the additive concentration increases ( Fig. 1 ). Although a mechanism of growth has been rationally proposed, the decreased viscosity is intriguing; the reason has been little investigated. The team led by Shih-Huang Tung (National Taiwan University) studied the systems of lecithin mixed with inorganic salts, such as LiCl, CaCl 2 , and LaCl 3 , in cyclohexane and explored the mechanisms for the viscosity change with the salt concentration. Employing small- angle neutron-scattering (SANS) facility on beamline Bilby at the Australian Nuclear Science and Technology Organisation (ANSTO), they demonstrated that the incorporation of various inorganic salts into lecithin sols can induce both axial and radial growth of micelles that causes the viscosity to increase. 1 The team further used the small-angle X-ray scattering (SAXS) facility of TLS 23A1 at the NSRRC to probe the micellar Fig. 1 : Molecular structure of lecithin and viscosity of a CaCl 2 /lecithin mixture in cyclohexane as a function of molar ratio. The images show the flow behaviours of the samples in various regimes. [Reproduced from Ref. 2] Fig. 2 : (a) SAXS data of LaCl 3 /lecithin in cyclohexane at lecithin concentration 20 mM with varied S 0 , (b) pair distance distribution functions p ( r ) versus r obtained on inverse Fourier transforms (IFT) analysis of SAXS data, and (c) viscosity and FT-IR line shifts Δv of choline, phosphate and carbonyl group on lecithin in cyclohexane as functions of S 0 . [Reproduced from Ref. 2] structures, aiming to answer why the viscosity decreases as the salt exceeds a critical concentration. The SAXS data are consistent with the change of viscosity ( Figs. 2(a) and 2(b) ). 2 At a small molar ratio of salt to lecithin ( S 0 ), the