Multilamellar Structures Induced by Hydrophilic and Hydrophobic Ions Added to a Binary Mixture of ${\mathbf{D}}_2\mathbf{O}$ and 3-Methylpyridine

A phase transition is observed between a one-phase disordered phase and an ordered phase with multilamellar (onion) structures in an off-critical mixture of ${\mathrm{D}}_2\mathrm{O}$ and 3-methylpyridine (3MP) containing 85 mM of a salt in the absence of a surfactant. The salt consists of hydrophilic cations and hydrophobic anions that interact asymmetrically with the solvent composition fluctuations inducing mesophases. The structure factor of the composition distribution obtained from small-angle neutron scattering has a peak at an intermediate wave number in the disordered phase and multiple peaks in the ordered phase. Lamellar layers forming onions are composed of solvation-induced charged membranes swollen by ${\mathrm{D}}_2\mathrm{O}$. The onion phase is realized only for small volume fractions of 3MP (in ${\mathrm{D}}_2\mathrm{O}$-rich solvent).

Figure 1

Optical microscopic images obtained from the sample of ${\varphi }_{3\mathrm{MP}}=0.09$: (a) $T=323\mathrm{K}$ (disordered), (b) $T=313\mathrm{K}$ (onion), and (c) $T=293\mathrm{K}$ (onion). Scale bars in (b) and (c) are $100\mu \mathrm{m}$. (${\mathrm{b}}^{\prime }$) A magnified view of (b) (left) and a birefringent image of the same region (right) with the scale bar being $20\mu \mathrm{m}$.

Figure 2

(a) Temperature dependence of SANS intensity from a sample of ${\varphi }_{3\mathrm{MP}}=0.09$. The first-order transition occurs slightly below $T=318\mathrm{K}$. The data at lower temperatures are shifted by multiplying 10 for better visualization. (b) Scattering density distribution at $T=283\mathrm{K}$ estimated from the formula in Ref. 20 (left) and schematic picture of the distribution of the ingredients (right). The vertical axis indicates the distance normal to the layers.

Figure 3

Repeat distance $d$ vs temperature $T$ for ${\varphi }_{3\mathrm{MP}}=0.14$ (triangle), 0.11 (circle), and 0.09 (square). Open symbols indicate data of the disordered phase while filled symbols indicate data of the onion phase. No onion appears for ${\varphi }_{3\mathrm{MP}}=0.14$.

Figure 4

Connected USANS profile obtained at PNO and SANS-J over a wide range of $Q$ at $T=298\mathrm{K}$ and ${\varphi }_{3\mathrm{MP}}=0.09$. The Porod tail $\propto Q^{-4}$ from onion surfaces can be observed in the range $1\times {10}^{-4}<Q<5\times {10}^{-3}{\AA }^{-1}$.