Concentration, temperature, and $p\text{H}$ dependence of sunset-yellow aggregates in aqueous solutions: An x-ray investigation

The dye sunset yellow (SY) forms columnar aggregates via $\pi \text{−}\pi$ stacking in aqueous solutions. These aggregates develop orientational and translational order at elevated concentrations to exhibit the nematic (N) and columnar (C) mesophases. Positional-order correlation lengths of the aggregates in the directions parallel and perpendicular to the stacking direction were measured as functions of temperature, concentration, and ionic content of solutions with synchrotron x-ray scattering in magnetically aligned samples. Average length of aggregates (i.e., the number of SY molecules in an aggregate) grows monotonically with concentration while their effective transverse separation decreases. The scission energy, $E$, determined from the Arrhenius thermal evolution of the longitudinal correlation length, is found to be $4.3\pm 0.3k_{B}T$ and $3.5\pm 0.2k_{B}T$, in the N and C phases, respectively. Temperature and concentration dependence of $E$ suggests that chromonic aggregation is not an isodesmic process. The aggregate length decreases with decreasing $p\text{H}$ when HCl is added to the solution.

Figure 1

Molecular structure of sunset yellow dye: (a) azo tautomer with scale bar (b) NH hydrazone tautomer.

Figure 2

(Color online) Optical textures of sunset yellow solutions in the (a) nematic ($30\text{wt}\%$ solution), and (b) columnar phase ($40\text{wt}\%$ solution) at room temperature.

Figure 3

(Color online) Phase diagram of aqueous solutions of (a) sunset tellow and (b) sunset yellow and HCl in 1:1 mol ratio.

Figure 4

(Color online) Representative x-ray diffraction pattern in the magnetically aligned (a) N phase of $35\text{wt}\%$ $\text{SY}+\text{water}$ solution (at $30°\text{C}$), and (b) the C phase of $40\text{wt}\%$ $\text{SY}+\text{water}$ solution (at $30°\text{C}$). The inner and outer sharp peaks correspond to $d$ values of 24.11 and $3.32\text{Å}$ in the N, and 19.49 and $3.32\text{Å}$ in the C phase. In both LC phases, four diffuse intermediate reflections at $5.62\text{Å}$ are also clearly visible. (c) Schematic representation of hexagonal structure made of columnar stacks in the C phase. The double headed arrows in (a) and (b) represent the direction of the in situ magnetic field. The blowup of cylindrical cross section shows a possible stacking of dye molecules with rotation of their planes about the column axis, producing a helical structure [5].

Figure 5

Variation of interstack (filled circle, top) and intrastack (open circle, bottom) spacing with the concentration of $\text{SY}+\text{water}$ at room temperature.

Figure 6

(a) Transverse positional-order correlation length shows a sharp increase at $40\text{wt}\%$ SY accompanied by the formation of the columnar phase at higher dye concentration, and (b) variation of longitudinal correlation length $\left({\xi }_{\mid \mid }\right)$ with concentration.

Figure 7

Temperature dependence of $(◻)$ inter- and $(○)$ intrastack spacing in (a) the N phase $\left(35\text{wt}\%\right)$, and (b) the C phase $\left(40\text{wt}\%\right)$.

Figure 8

Variation of longitudinal correlation length, ${\xi }_{\parallel }$, with temperature in the (a) N phase of $35\text{wt}\%$ $\text{SY}+\text{water}$ solution and (b) the C phase of $40\text{wt}\%$ $\text{SY}+\text{water}$ solution.

Figure 9

The scission energy, $E$, shown as a function of SY concentration. Values of $E$ are calculated from the parallel correlation length measured at room temperature, and using the bare correlation length, ${\xi }_0$, of 4.86 and $10.16\text{Å}$ in the N and C phases, respectively.

Figure 10

Variation of ${\xi }_{\parallel }$, with temperature in SY solutions without (filled points, top) and with (unfilled points, bottom) HCl (in 1:1 mol ratio) in (a) the N $\left(35\text{wt}\%\right)$ and (b) the C phases $\left(40\text{wt}\%\right)$.

Figure 11

Variation of interstack ($◻$, on top) and intrastack ($○$, bottom) spacing with temperature in the N phase of $\left(35\text{wt}\%\right)$ solutions in water with 1:1 mol ratio of HCl and SY.

Figure 12

Variation of ${\xi }_{\parallel }$ for solution of SY and HCl dissolved in water in (a) the N phase of $30\text{wt}\%$ (b) the C phase of $40\text{wt}\%$ solutions. Variation in the transverse correlation length in (c) the N phase of $30\text{wt}\%$, and (d) the C phase of $40\text{wt}\%$ solutions.