Columnar molecular aggregation in the aqueous solutions of disodium cromoglycate

Stack, chimneylike, and threadlike assemblies have previously been proposed for the structure of disodium cromoglycate (DSCG) aggregates in aqueous solutions. The results of the synchrotron x-ray scattering investigations reported here reveal the formation of simple columnar assemblies with π-π stacking at a separation of 3.4 Å between the DSCG molecules. Lateral separation between the assemblies is concentration and temperature dependent, varying from ∼35 to 42 Å in the orientationally ordered nematic ($N$) phase and from 27 to 32 Å in the columnar or middle ($M$) phase having long range lateral positional order. The assemblies’ length depends on concentration and consists of ∼23 molecules in the $N$ phase, becoming three to ten times larger in the $M$ phase. The scission energy is concentration dependent in the $N$ phase with values ∼7.19 ± 0.14 $k_{B}T$ (15 wt %), 2.73 ± 0.4 $k_{B}T$ (20 wt %), and 3.05 ± 0.2 $k_{B}T$ (25 wt %). Solutions of all concentrations undergo a spinodal decomposition at temperatures above ∼40 °C, resulting in DSCG-rich regions with the $M$ phase and water-rich regions in the $N$ and isotropic phases.

Figure 1

DSCG molecular assembly models by (a) Hartshorne and Woodard [9]; (b) Lydon [10], and (c) Wu [20] et al. The blue and red bars in (c) represent edge-on views of the aromatic rings in the DSCG molecular structure in (a).

Figure 2

(a) Schematic phase diagram of DSCG reproduced from Ref. [18] with permission from Taylor & Francis Ltd. The lines marked as 1, 2, and 3 represent the 15, 20, and 25 wt % concentrations, respectively. The (representative) optical textures correspond to the (b) coexisting $M$+$I$ phases at 43.8 °C, (c) $N$+$I$ phases at 38 °C, and (d) the $N$ phase at 23 °C of the 15 wt % sample; the (e) $M$+$I$ region at 44 °C, (f) $M$+$N$ phases at 38 °C, and (g) the $N$ phase at 25 °C of the 20 wt % sample; (h) the biphasic $M$+$I$ region at 57 ,°C and (i) the $M$+$N$ phases at 40 °C of the 25 wt % sample.

Figure 3

Diffraction patterns for 15 wt % of DSCG at (a) $T$ = 27.7 °C and (b) $T$ = 42.1 °C. The corresponding integrated intensity profiles are shown in the right panels. The length scales corresponding to reflections shown in the inset of (b) are $r_1$ = 32.8 Å, $r_2$ = 18.9 Å, and $r_3$ = 16.4 Å, corresponding to peak ratios 1, $\sqrt{3}$, and 2, respectively.

Figure 4

X-ray intensity vs $q$ profiles for the first peak in the $N$ (squares) and $M$+$I$ (circles) regions of the 15% sample at 27.7 °C and 42.1 °C, respectively. The solid lines are Lorentzian fits to data in the $N$ (green squares) phase and the $M$+$I$ (blue circles) phases. The two fits corresponding to the $I$ (black dashed line) and the $M$ (red dotted line) phases. Their sum fits the $M$+$I$ data, confirming coexistence of the two phases.

Figure 5

Temperature dependence of the $d$ spacing and positional correlation length ${\xi }_{\perp }$ for 15 wt % DSCG.

Figure 6

(a) X-ray diffraction pattern for 20 wt % DSCG in the two-phase regime $N$+$M$ at $T$ = 44.8 °C and (b) the corresponding intensity profiles. The blue solid line is the sum of the Lorentzian fits corresponding to the $N$ (black dashed) and $M$ (red dotted) peaks in the two-phase region.

Figure 7

Temperature dependences of $d$ spacing (open symbols, left ordinate) and ${\xi }_{\perp }$ (filled symbols, right ordinate) for (a) 20 wt % and (b) 25 wt % DSCG aqueous solution in the $N$, $M$, and $I$ phases.

Figure 8

Temperature dependence of (a) assembly length ${\xi }_{\left|\right|}$ and (b) interassembly distance $d$ for all solutions: 15 wt % (blue squares), 20 wt % (red circles), and 25 wt % (green triangles).

Figure 9

Calculation of the scission energy $E$ in the $N$ phase for 20 wt %. Values of the scission energy for all mixtures are shown in the table (inset). The scission energy is determined from the slope of the straight line fit.