Surfactant-laden Liquid Crystal Interfaces

Surfactant-laden Liquid Crystal Interfaces

Surfactant-laden interfaces between thermotropic liquid crystals and aqueous phases enable studies of the surface-induced liquid crystal order under a systematic variation of the strength of the ordering surface field. The surface field is controlled via the molecular structure of the surfactant and the magnitude of the surfactant coverage of the interface. Our group performs ellipsometric studies in order to elucidate the liquid crystal surface order near nematic - isotropic and smectic - isotropic transitions.

Nematic wetting of isotropic liquid crystal/aqueous interfaces

Surfactants possessing the usual structure (polar head, nonpolar tail) induce a strong homeotropic anchoring at liquid crystal/aqueous interfaces, provided the coverage of the interface is large enough. At the interface, an ordering surface field exists which leads to the formation of a thin nematic film even at temperatures where bulk liquid crystal is in the isotropic state. When the temperature approches the bulk transition to the nematic phase, the thickness of the nematic film diverges, i. e., the isotropic liquid crystal/aqueous interface is completely wetted by the nematic phase. Decreasing the surfactant coverage results in a decrease of the ordering surface field, and the wetting behavior changes first from complete to partial and finally to a nonwetting situation. This behavior is demonstrated in the following figure showing ellipsometric results for the 8CB/CTAB/water system.

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Surfactant-induced nematic wetting layer at a thermotropic liquid crystal/water interface
Ch. Bahr, Phys. Rev. E 73, 030702(R) (2006).
DOI: 10.1103/PhysRevE.73.030702 

Cross-over in the wetting behavior at surfactant-laden liquid-crystal/water interfaces: experiment and theory
E. Kadivar, Ch. Bahr, and H. Stark, Phys. Rev. E 75, 061711 (2007). 
DOI: 10.1103/PhysRevE.75.061711

Smectic layering transitions at isotropic liquid crystal/aqueous interfaces

Whereas the nematic surface order increases continuously on approaching the transition to the bulk nematic phase, a smectic surface film increases via a series of layering transitions. Usually, the thickness of the smectic film increases at each layering transition by the formation of an additional single smectic layer. The precise control of the surface field enables the confirmation of long-standing theoretical predictions concerning the occurrence of multiple-layer transitions in the regime of small surface fields.

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Surface triple points and multiple-layer transitions observed by tuning the surface field at smectic liquid crystal/water interfaces
Ch. Bahr, Phys. Rev. Lett. 99, 057801 (2007).
DOI: 10.1103/PhysRevLett.99.057801

Nematic prewetting transitions

Prewetting transitions are generally expected for all systems showing a first-order wetting transition. A prewetting transition should be observable as a discontinuous change of the wetting layer thickness occurring before the bulk transition is reached. For most experimental systems, such a behavior is difficult to observe because the ordering surface field must be tuned such that the system is in close vicinity to the wetting transition. The precise control of the surface field at surfactant-laden isotropic liquid crystal/aqueous interfaces enables the experimental observation of a prewetting transition in a nematic wetting layer.

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Experimental study of prewetting transitions by systematic variation of the surface field at nematic liquid crystal/water interfaces
Ch. Bahr, EPL 88, 46001 (2009).
DOI: 10.1209/0295-5075/88/46001

Interfaces to liquid phases with different polarities

The surfactant coverage at the interface between two immiscible liquids is controlled not only by the surfactant concentration in the bulk phase(s) but also by the difference between the polarities of the two liquids. The polarity difference determines the affinity of the surfactant molecules, which usually possess a polar and a nonpolar part, to the interface. Thus, the variation of the polarity of a liquid phase at constant surfactant concentration, should have the same effect as the variation of the surfactant bulk concentration. We have experimentally demonstrated this behavior by studying the smectic layering transitions and nematic prewetting transitions at interfaces between isotropic liquid crystals, doped with a small constant amount of the surfactant monoolein, and various water/glycerol mixtures with different glycerol concentration. 

With increasing glycerol concentration, the static permittivity of the water/glycerol mixture decreases, resulting in a decrease of the monoolein coverage of the interface. Thus, we observe the same effect as we would have decreased the monoolein bulk concentration:

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Surface order at surfactant-laden interfaces between isotropic liquid crystals and liquid phases with different polarities
X. Feng and Ch. Bahr, Phys. Rev. E 84, 031701 (2011).
DOI: 10.1103/PhysRevE.84.031701

Interfaces laden with semifluorinated alkanes

All studies mentioned above concern liquid crystal/aqueous interfaces, at which surfactants, possessing the common polar head/nonpolar tail structure, readily form Gibbs films by adsorption from the bulk phase. Semifluorinated alkanes (SFAs) are known to be surface active for oil/air interfaces. Thus, they should form Gibbs films on LC/air interfaces and it should be possible to extend  the studies of the surfactant-laden LC/aqueous interfaces to liquid crystal/air interfaces.

There are several differences between these two types of interfaces: One important difference is the anchoring condition of the bare (without surfactant) interfaces which is planar for water but homeotropic for air. So far, no study has addressed the question how the intrinsic anchoring and the accompanying surface ordering at LC/air interfaces is influenced by surfactants (since usual surfactants do not adsorb at oil/air interfaces). A second difference concerns the relevant interactions: Whereas at aqueous interfaces, laden with ionic surfactants, electrostatic interactions are important, interactions at LC/SFA/air interfaces can be expected to be mainly steric or of the van der Waals type. Another important point is that SFA monolayers can show structural phase transitions: For SFA Gibbs films on the surface of alkanes, a first-order transition with decreasing temperature from a dilute to a condensed state is observed [P. Marczuk, P. Lang, G. H. Findenegg, S. K. Mehta, and M. Möller, Langmuir 18, 6830 (2002)].

The following Figures show results for the nematic surface order of 8CB and the smectic surface order of 12CB at air interfaces laden with the semifluorinated alkane C18H37-C12F25(abbreviated as H18F12). We find that SFA films on LC surfaces show a similar transition as observed on alkane surfaces and that it has a striking effect on the anchoring and surface ordering behaviour of nematic and smectic LCs.

The behavior of the ellipticity coefficent ρ and the Brewster angle θB (not shown in the Figures) show that formation of the dense H18F12 film causes an anchoring transition of the LC phase from homeotropic at the dilute H18F12 film to planar at the dense H18F12 film.

The change of the anchoring condition from homeotropic to planar has a striking effect on the nematic or smectic order which is present at the surface of the isotropic LC bulk phase. For 8CB, which shows nematic surface order, the ellipsometric data suggest that a thin planar nematic film remains when the dense H18F12 film forms. The planar nematic film grows in thickness as the bulk transition to the nematic phase is approached from above (cf. inset in Figure 7).

In the case of 12CB, which shows smectic surface order, our data suggest that the few (essentially one or two) smectic layers, which exist on the surface of the isotropic bulk phase, vanish when the dense H18F12 film forms, i.e., the smectic surface order is destroyed when the LC molecules change their alignment from homeotropic to planar. 

In addition to the ellipsometry measurements, we have conducted AFM studies of the free surface of 8CB and 12CB droplets doped with H18F12 in order to clarify the structure of the dense film of the semifluorinated alkane. The AFM images show clearly that the dense film consists of a crystalline-like hexagonal packing of surface micelles, similar to those observed in transferred Langmuir films and spin-coated films of semifluorinated alkanes (see, e.g., M. P. Krafft, Acc. Chem. Res. 45, 514 (2012)). In addition to the hexagonal pattern of the surface micelles, we observe in some cases linear steps in the surface (arrows in Figure 10b) the height of which is clearly smaller than the smectic layer thickness. So far, we do not have an explanation for these steps. The formation of the surface micelles might be the reason for the observed anchoring transition from homeotropic to planar (see Figure 12).

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Surface ordering and anchoring behaviour at liquid crystal surfaces laden with semifluorinated alkane molecules
X. Feng, A. Mourran, M. Möller, and Ch. Bahr, Soft Matter 8, 9661 (2012).
DOI: 10.1039/c2sm26177d

AFM study of Gibbs films of semifluorinated alkanes at liquid crystal/air interfaces 
X. Feng, A. Mourran, M. Möller, and Ch. Bahr, ChemPhysChem 14, 1801 (2013).
DOI: 10.1002/cphc.201300173

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