Monolayer phases

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

Monolayer phase sequences are temperature dependent. For chain lengths greater than that of pentadecanoic acid, the LE phase will not be observed at room temperature. For chain lengths shorter than myristic acid (C14), condensed phases will not be observed at... 

The surface viscosity varies significantly along the isotherm and across monolayer phase boundaries. Addition of subphase metal ions increases the surface viscosity drastically, as was recently reinvestigated [36]. Recently, microscopy methods have been used to image velocity profiles of different monolayer phases flowing through a narrow channel, such as used in the canal viscometer [37], The two main methods used to study monolayer viscosity are the canal viscometer and the oscillating disc method [8,9]. 

The difference between the static or equilibrium and dynamic surface tension is often observed in the compression/expansion hysteresis present in most monolayer Yl/A isotherms (Fig. 8). In such cases, the compression isotherm is not coincident with the expansion one. For an insoluble monolayer, hysteresis may result from very rapid compression, collapse of the film to a surfactant bulk phase during compression, or compression of the film through a first or second order monolayer phase transition. In addition, any combination of these effects may be responsible for the observed hysteresis. Perhaps understandably, there has been no firm quantitative model for time-dependent relaxation effects in monolayers. However, if the basic monolayer properties such as ESP, stability limit, and composition are known, a qualitative description of the dynamic surface tension, or hysteresis, may be obtained. 

When a reversible transition from one monolayer phase to another can be observed in the 11/A isotherm (usually evidenced by a sharp discontinuity or plateau in the phase diagram), a two-dimensional version of the Gibbs phase rule (Gibbs, 1948) may be applied. The transition pressure for a phase change in one or both of the film components can be monitored as a function of film composition, with an ideally miscible system following the relation (12). A completely immiscible system will not follow this ideal law, but will... 

The phase rule has been applied more conveniently to ESP measurements taken as a function of temperature. Again, Gershfeld (1982) has shown that a plateau or discontinuity in the ESP versus temperature plot may be indicative of a three-way equilibrium between the floating crystal and the separate monolayer phases that have spread from this crystal. This treatment has been used to argue for the existence of surface bilayers of phosphatidylcholine derivatives (Gershfeld, 1986, 1988). 

Unlike electron and scanning tunneling microscopy, the use of fluorescent dyes in monolayers at the air-water interface allows the use of contrast imaging to view the monolayer in situ during compression and expansion of the film. Under ideal circumstances, one may observe the changes in monolayer phase and the formation of specific aggregate domains as the film is compressed. This technique has been used to visualize phase changes in monolayers of chiral phospholipids (McConnell et al, 1984, 1986 Weis and McConnell, 1984 Keller et al., 1986 McConnell and Moy, 1988) and achiral fatty acids (Moore et al., 1986). 

Single crystals of the 1223 thallium monolayer phase were grown from a copper-rich melt with molar composition 1 2 2 4 (Tl Ba Ca Cu) the mixture was heated to 925°C, soaked 6 h, and cooled at l°C/min (57). Magnetic flux exclusion experiments indicated a sharp Tc onset of 110 K. 

Syntheses of near-single phases of the lead-substituted thallium monolayer phases with up to 6 Cu-O layers i.e., Pb-doped 1212, 1223, 1234, 1245, and 1256, have been recently reported (21). Reactant mixtures of various proportions of Tl2Os, PbO, CaO, Ba02, and CuO were pelletized, wrapped in gold foil, and sintered at 860-900°C under flowing oxygen for 10-30 h. The Tc value reached a maximum of 121 K for the 1234 compound and declined with further increase in the number of Cu-O layers. X-ray powder diffraction data for the different phases were refined using the Rietveld method and a consistent increase in the c-axis accompanied the increase in number of Cu-O layers. 

V. Molinier, P. J. J. Kouwer, Y. Queneau, J. Fitremann, G. Mackenzie, and J. W. Goodby, A bilayer to monolayer phase transition in liquid crystal glycolipids, J. Chem. Soc., Chem. Commun. (2003) 2860-2861. 

Schwartz, D.K. Rnobler, M. Direct observation of transitions between condensed Langmuir monolayer phases by polarized fluorescence microscopy. J. Phys. Chem. 1993, 97, 8849. 

Mitchell, M. L. and DIuhy, R. A. (1988). In situ FTIR investigations of phospholipid monolayer phase transitions at the air-water interface. J. Am. Chem. Soc. 770 712-718. 

Weis, R. M. (1991). Fluorescence microscopy of phospholipid monolayer phase transitions. Chemistry and Physics of Lipids 57 227-239. 

G.M. Bell, L.L. Combs and L.J. Dunne, Theory of Cooperative Phenomena in Lipid Systems, Chem. Revs. 81 (1981) 15-48. (Review of lipid monolayers, phase transitions and statistical interpretations. Because of extended referencing (202 entries) good coverage of literature up to 1980.)... 

The next jump in the isotherm, which shows hysteresis, is to the complete monolayer phase. The hysteresis arises from the existence of two distinct phases, seven-stripe and eight-stripe, which have nearly equal free energies these are seen in the density plot. Above the monolayer regime there occurs a step, of height AN == 9, in the isotherm that is clearly seen in the density profile as a second-layer groove phase. Even this feature was seen in the experimental isotherm data of Migone s group [82, 87]. 

At TT > TTg the relaxation phenomena for insoluble monolayers are caused by the transformation of a homogeneous monolayer phase into a heterogeneous monolayer-collapse phase system. However, some differences exist between saturated-LMWE and unsaturated-LMWE monolayers (Eigure 14.6b). Relaxation phenomena in saturated-LMWE monolayer are controlled predominantly by the collapse mechanism because the surface pressure relaxes to TTg. Eor these systems the monolayer collapses by nucleation and growth of critical nuclei. Unsaturated-LMWE monolayers behave differently to saturated-LMWE monolayers. As the surface pressure relaxes from the collapse value, which is close to TTg, towards values lower than TTg at longer times, the collapse competes with a desorption mechanism (Patino and Nino, 1999). 

Figure 11.4 STM micrograph of the (5 x 5) monolayer phase of perylene on Cu(l 10) (a) together with corresponding line profiles along the rim (I) and the bottom (II) of the troughs, and perpendicular to the rows (III) (b). The inset in (a) shows the molecular stmcture of perylene (C20H12).

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