LC-air interface

Freely suspended films provide a perfect homeotropic alignment of smectic LCs since the layers always orient parallel to the LC/air interface.33 The director structure in such films can then be determined by analyzing the optical properties of plane-polarized light reflected from the surface of the films at a slightly oblique angle.34 The technique gains additional power when electrodes are added to the setup, allowing observation of the behavior of the films in the presence of an electric field parallel to the plane of the film.35... 

Interestingly, MHOBOW and other B7 materials prefer to form freely suspended fibers instead of films.53 Such fibers, occurring at the LC-air interface,... 

Fig. 8.7. Sketch of different systems characterized by hybrid frustration (a) Confining substrates repared in a way, so that one induces homeotropic anchoring and the other homogeneous planar anchoring, (b) Hybrid frustration due to opposing geometry induced direction of order and the direction induced by anchoring, (c) Liquid crystal in a contact with a solid substrate and with a free LC-air interface.

The major source of error for thin LC layers is due to depolarization from reflection at glass-liquid crystal interfaces and to intensity modulation caused by the reflection at glass-air interfaces. The latter source of error can be corrected relatively easily, while correcting for the former is impractical. Measurements done on thin layers are most severely affected since the thinner the layer, the weaker the variation of the transmission versus angle curve. 

Surface SHG has also been used to study monolayers of molecules at a liquid/air Interface.In a recent experiment, we have used it to probe the so-called liquid expanded (LE) - liquid compressed (LC) transition of a Langmuir film floating on a water surface.The system is usually prepared by first spreading a monolayer of molecules on a water surface. The molecules can then be swept together by a moving barrier. 

The phenomenon of phase transitions in two dimensions is of great fundamental interest and has therefore drawn a considerable amount of attention.i 2 insoluble monomolecular layers at a water-air interface provide a quite ideal two-dimensional model system with an isotropic substrate and an easily controllable density of molecules. At low densities they often exhibit a two-dimensional gas behavior,3 whereas at higher densities transitions to liquid and solid states can be found. In many systems, the liquid phase is further divided into the so-called liquid-expanded (LE) and liquid-condensed (LC) phases.4 Though observed and intensively studied, the nature of the LE-LC phase transition is still controversial. 

In this paper we will demonstrate how we can use ellipsometry and optical second harmonic generation (SH) to study the structure of a monolayer of pentadecanoic acid (PDA) [CH3(CH2)i3 COOH] near its LE-LC transition at a water-air interface. By simultaneously measuring the surface pressure versus surface molecular area we can show that this LE-LC transition is accompanied by a reorientation of the molecules and that the two phases are separated by an inhomogeneous coexistence phase. 

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

Figure 13.16 LC separation of urban air particulate exrtact (a), along with the GC/FID cliro-matogram (b) of an oxy-PAC fraction (transfeired via a loop-type interface). Reprinted from Environmental Science and Technology, 29, A. C. Lewis et al., On-line coupled LC-GC-ITD/MS for the identification of alkylated, oxygenated and nirtated polycyclic aromatic compounds in urban air particulate exti acts , pp. 1977-1981, copyright 1995, with permission from the American Chemical Society.

But there are signs that simpler, less expensive LC/MS systems designed and priced for the general laboratory bench chemist, production facilities, and quality control laboratories may soon be possible. It remains to seen whether manufacturers will decide to produce these systems. Older MS systems have been purchased, attached to HPLC systems equipped with relatively inexpensive interfaces, and pressed into service for molecular weight determination as a 30,000 detector, indicating that the desire and need exists for general laboratory LC/MS systems. As prices continue to drop and technology advances work their way out of the research laboratories, the LC/MS will become a major tool for the forensic chemist whose separations must stand up in court, for the clinical chemist whose separations impact life and death, and for the food and environmental chemist whose efforts affect the food we eat, the water we drink, and the air we breathe. 

The first concerns the temperature effect for a monolayer of the phospholipid DMPA (I -dimyristoyl phosphatidic acid) at an air/water interface (see fig. 3.8). As the temperature is lowered the pressure at the onset of the LE-LC transition,, decreases and the corresponding area per molecule, increases. Hence, the... 

Table 3.1. Heat and entropy changes for the LC- LE transition in monolayers of myristlc acid at an air-water interface ).

DPPC is prominent in the lipid bilayer making up the cell membrane and is also a major constituent of lung surfactant ( pulmonary surfactant). The lung membrane resembles a mixed surfactant monolayer at the air/water interface. Since the temperature in a lung is below the critical temperature for DPPC monolayers, the LE-LC transition may be of significance in the continuous compression and expansion loops that this membrane undergoes during respiration. We will say more about this in sec. 3.9. 

The quality of the solvents used in the mobile phase is important in LC-MS. Phthalates and other solvent contaminants can cause problems [7]. Appropriate filtering of the solvents over a 0.2-0.4-pm filter is required. Degassing of the mobile phase is required to prevent air bubble formation in the pump heads, but also in interface capillaries. 

---