Subphase transition

Murakami, H., Y. Watanabe, and N. Nakashima, Fullerene Lipid Chemistry Self- Organized MultibUayer Films of a CgQ-Bearing Lipid with Main and Subphase Transitions,/. . Chem. Soc. 118 4484-4485 (1996). 

The LB film prepared in the dark shows temperature dependent UV-vis absorption spectra. At lower temperatures, there is an electronic interaction between the fullerene moieties in the LB film. Upon heating over 47 C, which is the subphase transition temperature of the cast films of the fullerene lipid [28, 36], the electronic interaction of the fullerene moieties is loosened. The electronic interaction between the fullerene moieties can be controlled by the phase change of the film. The fundamental property of the self-assembled bilayer membrane film is maintained in the LB film prepared in the dark, indicating that the molecular orientations of the... 

In the following, we will discuss the aggregation of interacting semiflexible polymers by analyzing the order and hierarchy of subphase transitions that accompany the aggregation transition. 

Another very important example for a nucleation process that exhibits hierarchical subphase transitions is the aggregation of proteins. For this purpose, we return to our discussion of aggregation properties of the hydrophobic-polar peptide with the Fibonacci sequence FI AB2AB2ABAB2AB) in the previous chapter. 

Note that the picture is not always as clear as it is in the examples discussed here. The individual subphase transitions are not necessarily first-order-like, and also the complete transition itself is not necessarily of first order (at least in finitely large systems), but of second order. This typically occurs if subphase transitions are energetically close to each other [the two transitions at high energies for the 4x An system, shown in Fig. 12.3(b) are examples of such a case]. 

While for the off-lattice system, apart from the wetting transition, there is only the transition from AC2a (semi-spherical shaped) to AC2b (double-layer stmctures), on the lattice AC2 comprises a wide range of higher-layer subphase transitions (see Fig. 13,6). 

Theoretical models of the film viscosity lead to values about 10 times smaller than those often observed [113, 114]. It may be that the experimental phenomenology is not that supposed in derivations such as those of Eqs. rV-20 and IV-22. Alternatively, it may be that virtually all of the measured surface viscosity is developed in the substrate through its interactions with the film (note Fig. IV-3). Recent hydrodynamic calculations of shape transitions in lipid domains by Stone and McConnell indicate that the transition rate depends only on the subphase viscosity [115]. Brownian motion of lipid monolayer domains also follow a fluid mechanical model wherein the mobility is independent of film viscosity but depends on the viscosity of the subphase [116]. This contrasts with the supposition that there is little coupling between the monolayer and the subphase [117] complete explanation of the film viscosity remains unresolved. 

The effects of electric fields on monolayer domains graphically illustrates the repulsion between neighboring domains [236,237]. A model by Stone and McConnell for the hydrodynamic coupling between the monolayer and the subphase produces predictions of the rate of shape transitions [115,238]. 

As the barrier moves, the molecules are compressed, the intermolecular distance decreases, the surface pressure increases, and a phase transition may be observed in the isotherm. These phase transitions, characterized by a break in the isotherm, may vary with the subphase pH, and temperature. The first-phase transition, in Figure 2, is assigned to a transition from the gas to the Hquid state, also known as the Hquid-expanded, LE, state. In the Hquid... 

The last phase transition is to the soHd state, where molecules have both positional and orientational order. If further pressure is appHed on the monolayer, it collapses, owiag to mechanical iastabiHty and a sharp decrease ia the pressure is observed. This coUapse-pressure depends on the temperature, the pH of the subphase, and the speed with which the barrier is moved. 

The first study utilizing this method was reported by Schuller in 1966 [65]. Schuller used polystyrene latex beads that were spread on a salt-containing aqueous subphase in order to keep the particles at the interface. tt-A plots of the floating particles were determined, which showed several phase regions with reproducible transition points. The author determined the particle diameters from the A-value, at which a steep rise in the isotherm occurred. Moreover, Schuller also spread millimeter-sized Styropor particles and found isotherms of similar shape [66]. By taking pictures at different surface pressure, he was able to correlate the shape with different states of order in the monolayer. Shortly after that. 

Fig. 23 Equilibrium spreading pressures of (R,S)-( +)- and(R)-( +)-stearoyltyrosine on an aqueous subphase of pH 6.86 (potassium phosphate/disodium phosphate buffer) as a function of temperature. Film type II is the film at temperatures above the transition and film type I is the film at temperatures below the transition. Reprinted with permission from Arnett et al, 1990. Copyright 1990 American Chemical Society.

Table 8 Helmholtz free energy, entropy, and internal energy of spreading and of transition for N-stearoyltyrosine on an aqueous subphase of pH = 6.86 at the transition temperature for each film."... 

The fact that the polyreaction of diacetylenes is topochemically controlled is especially well documented by the polymerization behavior of the sulfolipid (22)23 . (22) forms two condensed phases when spread on an acidic subphase at elevated temperatures (Fig. 10). UV initiated polymerization can only be carried out at low surface pressures in the first condensed phase, where the molecules are less densely packed. Apparently, in the second phase at surface pressures from 20 to 50 mN/m the packing of the diyne groups is either too tight to permit a topochemical polymerization or a vertical shift of the molecules at the gas/water interface causes a transition from head packing to chain packing (Fig. 10), thus preventing the formation of polymer. 

After passing a plateau at a critical film pressure nc the liquid-condensed phase is reached via a phase transition of first order. Here, the amphiphiles exhibit a tilted phase with a decreasing tilt angle (measured against the normal to the subphase). The film is relatively stiff but there is still some water present between the headgroups. 

The phase behavior of monolayers is determined by the molecular structure of the am-phiphile and the conditions of the subphase. Phospholipids, for example, attract each other because of van der Waals interactions between the alkyl chains. The longer the alkyl chains, the more strongly the phospholipids attract each other. Thus, the LE-LC transition pressure will decrease with increasing chain length (at constant temperature). Double bonds in the alkyl chains increase this phase transition pressure. Charges and oriented dipole moments (see Chapter 6) in the headgroups, lead to a repulsion between the phopholipids and increase the pressure at which the transition occurs. Salts in the subphase, screen this repulsion and decrease the transition pressure. 

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