• In order to determine the salt effect on the helical pitch in the chiral nematic phase the electrolyte effect on the phase behavior has to be accounted for in a suitable manner. This separation of effects had been neglected in earlier studies and is the potential reason for the contradicting findings in these studies.
• The chiral induction represented by the helical pitch is inhibited by the added ions. However, the effect of small and medium electrolyte concentrations is in the first approximation negligible. Only high salt concentrations close to the salting-out limit lead to an essential increase of the helical pitch by about 30 %.
• The strong influence of added salt on the helical pitch observed in earlier studies is shown to be mainly caused by the thermodynamic electrolyte effects on the chiral-nematic phase.
• The salt effect on the chiral-induction mechanism consists in a weakening of the here predominately steric chiral interactions. The influence of low to medium salt contents is in the first approximation negligible and becomes important only at high electrolyte concentrations. The phenomenon is in accordance with the salt induced increase of the pitch.
• Several reasons for the salt induced inhibition of the chiral induction at high electrolyte concentrations can be discussed. These are salt effects on, first, the elastic twist constant of the phase, second, on the reorientational dynamics and reorientational anisotropy of the chiral dopant molecules and, third, on the effective concentration of dopant molecules which are solubilized within the micelles. To judge the relevance of these aspects will require further studies.
In conclusion, electrostatic interactions are found to play a certain role within the chiral induction mechanism. However, this role is apparently of subordinate importance.
The second part of this work deals with the potential relationship of the chiral induction with the actual location of a dopant molecule within the chiral-nematic phase and with the reorientational dynamics of the dopant molecules. In earlier experimental works the dopants’ solubilization location was considered to be of importance for the chiral induction. However, in experiment, the solubilization location could not be determined clearly. The aspect of the dopant dynamics as a parameter influencing the chiral induction was considered here for the first time. It was motivated by molecular models about the chiral induction in thermotropic liquid crystalline phases.
For the investigations a homologous series of dopants of terminally phenyl substituted α-hydroxyalkanoic acids, R-MA, R-PLA and R-HPBA, was used. Within this series the number of methylene units between the phenyl ring and the chiral and polar head group
8 Summary 131
increase systematically from zero to two. Despite their chemical similarity these dopants exhibit an alternating helical twisting power HTP when added to the same nematic host phase of the system CDEA/DOH/H2O. R-MA and R-HPBA possess a high HTP while the middle homologue R-PLA shows almost no twisting power in this phase.
The magnetic resonance method avoided-level-crossing muon spin resonance (ALC-µSR) was identified as a suitable method for the unambiguous and precise determination of the dopant location. In the present work, ALC-µSR was applied for the investigation of lyotropic liquid crystals for the first time. Using this method the exact location of dopant solubilization is determined via the relative polarity of the dopant’s environment.
Furthermore, ALC-µSR allows an estimation of the reorientational dynamics and the reorientational anisotropy of the dopants. The aspect of the dopant dynamics was considered in this work for the first time.
The following main results were obtained.
• Within the chiral nematic phase all three dopants are solubilized in the micellar surface. The polar head groups of the dopants reside between the ionic head groups of the micelle building amphiphiles. With increasing length of the methylene unit between the dopant head group and the phenyl ring the latter penetrates more deeply into the apolar inner core of the micelles.
• The solubilization location of the three dopants does not correlate with their alternating HTP values. This implies that the role of the dopant’s solubilization location for its chiral induction is of subordinate importance and was rather overvalued in earlier works.
• The dynamics and anisotropy of the dopant reorientation alternates throughout the homologous series. This can be related to varying interactions of the dopant molecules with the micelle building amphiphiles due to the alternating molecular shape of the dopants.
• The alternation of the dopant dynamics mirrors qualitatively the alternating HTP of the three homologues. Here, relatively slow and partly anisotropic reorientation dynamics are obviously connected with a strong chiral induction. In the same manner the dopant dynamics correlate with the pitch dependence on temperature and on dopant concentration, which was evaluated for R-MA as a proof of concept.
• From these observations the dynamics and anisotropy of the dopant reorientation apparently are of central importance for the chiral induction.
This finding is essentially new and of high relevance for future studies in this field.
In summary, the dopant location is revealed to be of minor relevance for the chiral induction while the dopant dynamics plays a major role for the transfer of chiral interactions within the chiral nematic phase. Especially with the results concerning the dopant dynamics, this work contributes significant new insights into the process of chiral induction in lyotropic chiral nematic liquid crystals.