Ammonium amphiphiles carrying mesogenic units. Synthesis - properties - applications.

When the structural characteristics of amphiphiles and thermotropic liquid crystals are combined in one molecule i.e. a polar headgroup with apolar tails and mesogenic units, compounds are obtained which can exhibit both thermotropic and lyotropic mesomorphism. This class of compounds is called amphotropic liquid crystals.This thesis deals with the study of new amphotropic compounds. The majority of the new amphotropes consist of a cationic ammonium headgroup with one, two or three hydrophobic tails containing different mesogenic units, although some compounds with anionic headgroups have also been investigated. The goal of this thesis is to correlate the molecular structure with the self- assembling behavior of these compounds. Special attentention has been given to the effect of different mesogenic units on the aggregation behavior of these compounds in water and at the water- air interface.The single chained amphotropes form micellar aggregates in water. Interestingly, these aggregates are better stabilized by azobenzene mesogens than by biphenyl or stilbene mesogens, despite the more hydrophilic character of azobenzene. This is attributed to stronger π-πstacking interactions between azobenzene mesogens. than between biphenyl and stilbene mesogens. The polarizable azo- group might give rise to extra dispersion interactions between the mesogens thus favoring self- aggregation (Chapter 2).The formation of ion-pair amphiphiles from these amphotropic ammonium compounds and sodium dodecylsulfate as second component can directly be monitoted by UV absorption spectroscopy and fluorescence spectroscopy. These ion-pair amphiphiles form bilayer vesicles which precipitate shortly after preparation (Chapter 2).When an o-hydroxyazobenzene unit is incorporated into a single chained ammonium amphiphile, a molecule is obtained which can effectively complex Cu <sup>2+</SUP>ions. This results in an extra stabilization of the formed aggregates in water and this complexation can be monitored by optical spectroscopy. This system is a very sensitive probe for the quantitative detection of minor concentrations of Cu <sup>2+</SUP>ions in water (Chapter 3).The double chained amphotropes all form bilayer vesicles in water (Chapter 4 and 5). These bilayer membranes show a phase transition from a rigid gel phase at low temperatures to a more mobile liquid crystalline phase at higher temperatures, as is also known to occur in biomembranes. The phase transition temperature is very dependent on the nature of the substituents. at the mesogenic units. The phase transition temperature is however not related to the net dipole moment of the mesogenic units. Therefore, dispersion interactions rather then electrostatic interactions beween the permanent dipoles of the mesogens determine the strength of the mesogen-mesogen interactions (Chapter 4).The 4-cyanobiphenylyl-4'-oxy mesogenic unit shows large solvatochromic shifts of its fluorescence maximum. Amphotropes carrying this mesogen are therefore good probes to monitor the micropolarity in all kinds of Iyotropic aggregates. The obtained results are in good agreement with previous studies using pyrene as a probe (Chapter 5).In the bilayer membranes of these amphotropes the mesogenic units form H-aggregates which results in a blue shift of the UV absorption maximum of the mesogenic units. The extent of this blue shift is a direct measure for the ordering of the molecules in the bilayer. This offers the possibily to study a great number of membrane processes in detail. The process of monomer transfer between bilayer vesicles of these amphotropes and bilayer vesicles of nonmesogenic double chained amphiphiles has been studied in this way (Chapter 6). This transfer process is sometimes a one-way migration of the amphotropes to the bilayer vesicles of the other amphiphile. Depending on the nature of the amphotrope the reverse process can also occur. For the solubilization of bilayers of amphotropes in micelles of other surfactants the same processes were found. Additionally, these transfer processes have also been studied by differential scanning calorimetry and titration microcalorimetry (Chapter 6).The effect of polymerization of the bilayer membrane on the ordering of the molecules in the bilayer has also been studied. The molecular ordering is clearly reduced upon polymerization. The stability of the membrane however increases (Chapter 7).The orientation of the mesogenic units along the long axis of the molecule influences the aggregation behavior in water and at the water-air interface. This is not due to contributions of the dipole moments of the mesogenic units but can rather be attributed to steric effects. (Chapter 8).Complexes of these double chained ammonium amphotropes or didodecyldimethyl-ammonium bromide with poly(acrylic acid) form tissue-like structures in water which can be regarded as model systems for biological tissues. Complexes of double chained ammonium amphotropes with poly(acrylic acid) can be considered to be ionically bound liquid crystalline side chain polymers and they exhibit very broad liquid crystalline temperature ranges (Chapter 9).The triple chained amphotropes form stable monolayers at the water-air interface. These molecules initially lie flat at the water surface and upon compression the hydrophobic chains are one by one expelled from the water surface. The compressed monolayers can be transferred onto solid substrates. Subsequent layers are transferred in a head-to-tail mode. These transferred layers are however not stable and reorganize to a head-to-head and tail-to-tail packing. Eventually a thin film of (liquid) crystalline material is obtained. In the case of the compound with azobenzene mesogens, the orientation of the molecules in the film can be influenced by irradiation with UV light. This offers the possibility to write patterns in these films; an optical data storage device (Chapter 10)