Insitute of Physical Chemistry

Langmuir films of four X-shaped bolaamphiphiles were studied using surface pressure and Kelvin potential measurements, Brewster angle microscopy and X-ray reflectivity. The partially fluorinated bolaamphiphiles exhibit an unusual reversibility and reproducibility of Langmuir isotherms, and create very stable and well defined single- or triple layers which can be transferred to solid substrates.

Aggregation in Langmuir films is usually understood as being a disorderly grouping of molecules turning into chaotic three-dimensional aggregates and is considered an unwanted phenomenon causing irreversible changes. In this work we present the studies of 11 compounds from the group of specific surfactants, known as bolaamphiphiles, that exhibit reversible aggregation and, in many cases, transition to well-defined multilayers, which can be considered as a layering transition. These bolaamphiphiles incorporate rigid π-conjugated aromatics as hydrophobic cores, glycerol-based polar groups and hydrophobic lateral chains. Molecules of different shapes (X-, T-, and anchor) were studied and compared. The key property of these compounds is the partial fluorination of the lateral chains linked to the rigid cores of the molecules. The most interesting feature of the compounds is that, depending on their shape and degree of fluorination, they are able to resist aggregation and preserve a monolayer structure up to relatively high surface pressures (T-shaped and some X-shaped molecules), or create well-defined trilayers (X- and anchor-shaped molecules). Experimental studies were performed using Langmuir balance, surface potential and X-ray reflectivity measurements.

We measure the frequency of collective molecular precession as a function of temperature in the ferroelectric liquid crystalline monolayer at the water-air interface. This movement is driven by the unidirectional flux of evaporating water molecules. The collective rotation in the monolayer with angular velocities ω ~ 1 s(-1) (at T = 312 K) to 10(-2) s(-1) (at T = 285.8 K) is 9 to 14 orders of magnitude slower than rotation of a single molecule (typically ω ~ 10(9) to 10(12) s(-1)). The angular velocity reaches 0 upon approach to the two dimensional liquid-to-solid transition in the monolayer at T = 285.8 K. We estimate the rotational viscosity, γ1, in the monolayer and the torque, Γ, driving this rotation. The torque per molecule equals Γ = 5.7 × 10(-8) pN nm at 310 K (γ1 = 0.081 Pa s, ω = 0.87 s(-1)). The energy generated during one turn of the molecule at the same temperature is W = 3.5 × 10(-28) J. Surprisingly, although this energy is 7 orders of magnitude smaller than the thermal energy, kBT (310 K) = 4.3 × 10(-21) J, the rotation is very stable. The potential of the studied effect lies in the collective motion of many (>10(12)) "nano-windmills" acting "in concerto" at the scale of millimetres. Therefore, such systems are candidates for construction of artificial molecular engines, despite the small energy density per molecular volume (5 orders of magnitude smaller than for a single ATPase).

ABSTRACT The article presents systematic research on Langmuir films of partially fluorinated bolaamphiphiles of different shapes. Such films exhibit a layering transition from a monolayer to a trilayer during compression on the air–water interface. Further compression gives different results depending on the shape and degree of fluorination of the molecules. Partially fluorinated compounds form well defined multilayers in a reversible process. The balance between rigidity and flexibility of the molecules, adjusted by the fluorination and shape of the molecules, seems to be the key factor in avoiding irreversible aggregation of the molecules and creating ordered multilayer structures. Anchor-shaped bolaamphiphiles form a trilayer and, subsequently, a 9-layer film due to a double roll-over mechanism. In contrast, when trilayer films of X-shaped bolaamphiphiles are compressed, 5- and 7-layer films are created according to a different mechanism. Films of thickness of up to nine layers were transferred from the water surface to solid substrates in a single step procedure without any distortion in the structure of the layers. X-ray reflectometry (XRR) was used to measure the thickness of the layers. Perfect fits of the XRR data to theoretical equations allowed for a conclusion that the multilayers are well-ordered lamellar structures. These investigations lead to an improvement in the general understanding of trilayer and multilayer formation and indicate that only in exceptional cases it happens due to a roll-over process.

By doping a nematic phase with a chiral molecule one obtains a cholesteric phase. Each chiral molecule is characterised by its helical twisting power (HTP) which is defined as HTP = q/(2πC) where q is the equilibrium pitch of the cholesteric phase and C the concentration (in wt%) of chiral molecules. In a similar way, we define the Lehmann rotatory power (LRP) as LRP = v/(2πC) where v is the thermomechanical Lehmann coefficient. By making compensated mixtures, we measured the HTP and the LRP of five chiral molecules (R811, S2011, CC, CB15 and CE4) dissolved in an eutectic mixture 8CB/8OCB. We found that, although these quantities were different, their ratio R = LRP/HTP changed little from one molecule to another. This result shows that the Lehmann effect is closely, but not completely, related to the twist of the phase.

Sensitive volumetric measurements show a classical second-order A-C transition in mixtures of 8¯S 5 with 7¯S 5 in agreement with findings of Schantz and Johnson for pure 8¯S 5, and apparently a first-order A-N transition (although an extremely weak one) for 30 and 37 mole% of 7¯S 5; thus an earlier suggestion of a (critical) end point where a second-order A-C line meets the border of a first-order region, N-A and then N-C, is corroborated.