Investigation of the interaction of alkali ions with surfactant head groups for the formation of lyotropic biaxial nematic phase via optical birefringence measurements

Abstract

Lyotropic liquid crystals exhibiting nematic phases were obtained from the mixtures potassium laurate/alkali sulfate salts (M 2 SO 4 )/1-undecanol (UndeOH)/water and sodium dodecyl sulfate (SDS)/M 2 SO 4 /1-dodecanol (DDeOH)/water, where M 2 SO 4 represents the alkali sulfate salts being Li 2 SO 4 , Na 2 SO 4 , K 2 SO 4 , Rb 2 SO 4 or Cs 2 SO 4 . The birefringences measurements were performed via laser conoscopy. Our results indicated that cosmotropic and chaotropic behaviors of both ions and head groups are very important to obtain lyotropic biaxial nematic phase. To obtain the biaxial nematic phase, surfactant head group and ion present in lyotropic mixture have relatively opposite behavior, e.g. one more cosmotropic (more chaotropic) other less cosmotropic (less chaotropic) or vice versa.

Effect of Hofmeister anions on the existence of the biaxial nematic phase in lyotropic mixtures of dodecyltrimethylammonium bromide/sodium salt/1-dodecanol/water

“…Recent studies have shown that the choice of components of lyotropic mixtures is a key issue for the stabilisation of both uniaxial and biaxial nematic phases [10,11] and also cholesteric [12] phases. For instance, alcohols having long and straight alkyl chain were used as co-surfactants, and their alkyl chain lengths, n a , play an important role on the preparation of mixtures that give the nematic phases.…”

Experimental Conditions for the Stabilization of the Lyotropic Biaxial Nematic Mesophase

“…The alkali salts (sodium salts) were Li2SO4, Na2SO4, K2SO4, Rb2SO4 and Cs2SO4 (Na2SO4, NaF, NaCl, NaBr, NaNO3, NaClO4 and NaSCN). The results [14] indicated that when the surfactant head groups and the ions bound to them at the micelle's surface exhibit the same property in terms of the chaotropic or kosmotropic The results discussed in [88] may be considered as preliminary. For this reason, complementary investigations were needed to understand the effect of the specific interactions between the ionic species at the micelle surfaces on the stabilization of the lyotropic nematic phases, especially the biaxial.…”

“…Consequently, it would be better to conclude that the larger value of the birefringences is attributed to the formation of tightly bound ion-pairs at the micelle surfaces. The results discussed in [88] may be considered as preliminary. For this reason, complementary investigations were needed to understand the effect of the specific interactions between the ionic species at the micelle surfaces on the stabilization of the lyotropic nematic phases, especially the biaxial.…”

“…The interaction of alkali ions with the head groups of the surfactant molecules on obtaining the different nematic phases, especially biaxial one, was investigated [88]. In this study, two different lyotropic mixtures were examined, KL/alkali sulfate/undecanol/water, as shown in Figure 6, and sodium dodecylsulfate (SDS)/alkali sulfate/dodecanol/water, as shown in Figure 7, where alkali sulfates are Li 2 SO 4 , Na 2 SO 4 , K 2 SO 4 , Rb 2 SO 4 and Cs 2 SO 4 .…”

“…Temperature dependences of the birefringences of the mixtures KL/alkali sulfate/UndeOH/water, including (a) Na2SO4, (b) K2SO4, (c)Rb2SO4 and (d) Cs2SO4, separately. (○) and (•) represent δn and Δn, respectively[88].…”