Polymorphism and Mesomorphism of Oligomeric Surfactants: Effect of the Degree of Oligomerization

Abstract

A series of cationic oligomeric surfactants (quaternary dodecyldimethylammonium ions with two, three, or four chains connected by an ethylene spacer at the headgroup level, abbreviated as dimer, trimer, and tetramer) were synthesized and characterized. The influence of the degree of oligomerization on their polymorphic and mesomorphic properties was investigated by means of X-ray diffraction, polarizing optical microscopy, thermogravimetry, and differential scanning calorimetry. All compounds display layered arrangements with interdigitated dodecyl chains. The increase in the degree of oligomerization increases the interlayer distance and decreases the ordering in the solid phase; whereas the dimer sample is fully crystalline with well-developed 3D ordering and the trimer and tetramer crystallize as highly ordered crystal smectic phases. The number of thermal phase transitions and sequence of phases are markedly affected by the number of dodecyl chains. Anhydrous samples exhibit polymorphism and thermotropic mesomorphism of the smectic type, with the exception of the tetramer that displays only transitions at higher temperature associated with decomposition and melting. All hydrated compounds form lyotropic mesophases showing reversible phase transitions upon heating and cooling. The sequence of liquid-crystalline phases for the dimer, typical of concentrated ionic surfactant systems, comprises a hexagonal phase at lower temperatures and a smectic phase at higher temperatures. In contrast, the trimer and tetramer reveal textures of the hexagonal phase.

Mesomorphism of a new series of catanionic 4-(1-pentylheptyl)benzenesulfonates

“…Oligomeric surfactant bromides with the same spacer group separating the monomer units were synthesized: 48 bis…”

“…Catanionic compounds 4 and 6 have higher melting temperatures than related bromides, while compound 5 melts at a temperature lower than its bromide analogue. Unlike trimer tribromide, 48 its analogue 4-(1-pentylheptyl)benzenesulfonate does not decompose during melting, while both tetramer tetrabromide 48 and its analogue 4-(1-pentylheptyl)benzenesulfonate decompose simultaneously under melting. Temperature hysteresis was also observed.…”

“…Their quaternary ammonium bromide analogues also exhibit thermotropic mesomorphism and polymorphism of Sm type. 47,48 For sample 3 at 12 C only one sharp peak at low angle is seen, characteristic for SmA with a low MacMillan (positional) order parameter. As can be seen from Table 2 and Fig.…”

Thermotropic phase transitions of catanionic dodecylsulfates with multi-charged and multi-tailed quaternary ammonium centers

“…Heterogeneous surfactant aqueous mixtures containing self-assembled aggregates represent an interesting alternative for the design of new materials due to their higher flexibility when compared with systems based on homogeneous molecules [4]. The effects of surfactants with more polar groups [5][6][7][8][9][10] or tails have been reported.…”

“…The crystal structure of sodium dodecylsulfate [20,21] and its thermal degradation [22] are well known. The cationic surfactants used were also previously well characterized; fully crystalline dodecyltrimethylammonium bromide with layered ordering of the interdigitaded chains and interlayer distance of 24.60 Ǻ [5,23], didodecyldimethylammonium bromide with the high temperature SmB phase [24]. Tridodecylmethylammonium chloride is enantiotropic, with Sm phase interplanar spacing of 23.50 Å [11].…”