Lyotropic and Thermotropic Phase Transitions in Films of Ionene−Alkyl Sulfate Complexes

Abstract

Five [X,Y]-ionenes [(CH2)(X)N+(CH3)2(CH2)(Y)N+(CH3)2]nBr-(2n) were prepared (X = 3, 5; Y = 10, 12, 22). Using new preparation methods, dry, salt-free complexes with three n-alkyl sulfates (octyl, decyl, and dodecyl) were obtained. The ionenes and their complexes were characterized in methanol solution by light scattering, viscometry, and fluorescent probe studies. The solid materials were investigated by means of DSC, TG, and ATR-IR. Transparent films were formed from stoichiometric [3,10], [3,12], and [5,10]-ionene complexes with decyl and dodecyl sulfate. In the films, mesogenic phase transitions could be induced: dry films were optically isotropic; when exposed to elevated humidity, the films slowly became optically anisotropic because of a lyotropic transition to a hexagonal mesogenic phase. The relative humidity, at which the hexagonal phase developed, was distinct for each complex. The anisotropic phases were converted to isotropic in a thermotropic transition under controlled relative humidity at specific clearing temperatures, which were higher for dodecyl complexes than for decyl complexes. This thermotropic isotropic-anisotropic transition could be cycled several times, but partial hydrolysis of the alkyl sulfates reduced the reproducibility of transition points.

Mesogenic Structures, Lyotropic and Thermotropic Phase Transitions in Demethyl-Ionene Alkyl Sulfonate Complexes

“…14,32 In ionenealkyl sulfate complexes, the clearing temperature rises when additional water is incorporated in the complex structure. 21,22 The fan patterns reported for these complexes in liquid crystalline state previously and those observed here in demethyl-ionene sulfonates are closely related. In ionene-alkyl sulfates, the unavoidable hydrolysis of the alkyl sulfate ester bond hampered investigations at higher temperatures.…”

“…220°C only the evaporation of water from the complexes can be observed (see the Supporting Information, Figure S2). Hence, contrary to ionene alkyl sulfates whose alkyl sulfate ester bonds hydrolyze in the presence of water at around 50°C, 22 the Figure 8). Similar stability can be expected for the other sulfonates.…”

“…At low a H 2 O , the clearing points increase with raising the water activity as in previously investigated ionene-surfactant complexes. 22 In both the demethyl-ionene complexes ϑ HI decreases at high values of a H 2 O ; that is, too much water destabilizes the complexes in contrast to the ionene PSCs. The critical temperature appears to increase with the chain length of the alkyl sulfonate.…”

“…In the previously investigated ionene alkyl sulfate complexes the alkyl sulfate chain length necessary to form liquid crystalline phases increased with increasing the number of CH 2 -groups in the ionene. 22 Water uptake. The swelling behavior of [3,10]-demethylionene nonyl sulfonate upon varying a H 2 O (cf.…”

“…21 Measurements at temperatures above 50 °C, however, fail due to thermal decomposition of the complexes. 22 Thus, in these PSCs the effect of water could not be entirely elucidated, whereas it has been shown meanwhile that the amount of water absorbed by ionenes may strongly depend on their (inorganic) counterions. 23 The above-mentioned disadvantages of ionene alkyl sulfates like the lack of thermal stability (especially under high humidity) prevent most practical applications, such as the use in humidity sensors.…”