Origin of the Director Tilt in the Lyotropic Smectic C* Analog Phase: Hydration Interactions and Solvent Variations

Abstract

The origin and long-range correlation of the director tilt in the recently discovered Lα'* phase, which is the lyotropic analog of the thermotropic smectic C* (SmC*) liquid crystalline phase, are investigated. Polarized micro-Raman spectroscopy reveals that the director tilt in the Lα'* phase originates from a tilting of the aromatic 2-phenylpyrimidine cores of the surfactant molecules. Optical measurements of the tilt angle show that its magnitude decreases with increasing solvent concentration, suggesting that the long-range inter-lamellar correlation of the tilt directions is reduced at increasing thickness of the solvent layers. The phase diagrams with four different solvents (water, formamide, N-methylformamide, N,N-dimethylformamide) are investigated, showing that the Lα'* phase is only formed with those solvents that exhibit a dense network of hydrogen bonds. This observation suggests that these hydrogen bond networks play an essential role in the long-range correlation of the director tilt between adjacent surfactant layers. To verify this assumption, mixtures with deuterated solvents are investigated, showing that the tilt angle in the Lα'* phase is indeed reduced by this modification of the solvent's hydrogen bond network.

The Lyotropic Analog of the Polar SmC* Phase

“…Figure 11c,d). In Figure 19 the optical tilt angle θ opt of the system 4/formamide is plotted against the relative temperature for different mass fraction of formamide [45]. For all samples investigated, the tilt angle seems to saturate at lower temperatures.…”

“…To learn more about the origin of the director tilt and its long-range correlation, the orientation of the C-N pyrimidine vibration which is directed along the long molecular axis of 4 was selectively probed by polarized micro-Raman spectroscopy [45]. For this a mixture of amphiphile 4, 15 wt% of formamide was used.…”

“…Since all four solvents can accept up to two hydrogen bonds, water and formamide are known to form extended hydrogen bond networks in the liquid state, while NMF only forms chains of hydrogen bonds and DMF no hydrogen bonds with itself. Thus, the authors conclude, that anisotropic hydration interactions which are mediated by a dense three dimensional hydrogen bond network might be responsible for the correlation of the tilt direction in adjacent bilayers [45]. In general, hydration forces have already been known for phospholipids forming lyotropic lamellar phases [48][49][50][51][52][53][54], but have never been observed in combination with lamellar, fluid and tilted phases.…”

“…To underline the importance of the hydrogen bond network further, the tilt angle in mixtures with ordinary formamide and deuterated formamide were compared [45], since deuteration selectively modifies the strength and dynamics of the hydrogen bond network. The result is shown in Figure 23.…”

“…The values were plotted relative to the temperature T tr of the phase transition from the high temperature phase to the lyotropic SmC* phase. (Reprinted by permission from John Wiley and Sons:[45]. )…”