Condensed Phases—Liquids

The three general states of monolayers are illustrated in the pressure-area isotherm in Fig. IV-16. A low-pressure gas phase, G, condenses to a liquid phase termed the /i uid-expanded (LE or L ) phase by Adam [183] and Harkins [9]. One or more of several more dense, liquid-condensed phase (LC) exist at higher pressures and lower temperatures. A solid phase (S) exists at high pressures and densities. We briefly describe these phases and their characteristic features and transitions several useful articles provide a more detailed description [184-187]. 

At lower temperatures a gaseous film may compress indefinitely to a liquid-condensed phase without a discemable discontinuity in the v-a plot. 

Initially, the compression does not result in surface pressure variations. Molecnles at the air/water interface are rather far from each other and do not interact. This state is referred to as a two-dimensional gas. Farther compression results in an increase in snrface pressure. Molecules begin to interact. This state of the monolayer is referred as two-dimensional liquid. For some compounds it is also possible to distingnish liqnid-expanded and liquid-condensed phases. Continnation of the compression resnlts in the appearance of a two-dimensional solid-state phase, characterized by a sharp increase in snrface pressure, even with small decreases in area per molecule. Dense packing of molecnles in the mono-layer is reached. Further compression results in the collapse of the monolayer. Two-dimensional structure does not exist anymore, and the mnltilayers form themselves in a non-con trollable way. 

S-layer proteins adsorb preferentially at lipid films in the liquid-expanded phase [138] Crystalization is observed only at the liquid-condensed phase [138]... 

It has been shown by FM that the phase state of the lipid exerted a marked influence on S-layer protein crystallization [138]. When the l,2-dimyristoyl-OT-glycero-3-phospho-ethanolamine (DMPE) surface monolayer was in the phase-separated state between hquid-expanded and ordered, liquid-condensed phase, the S-layer protein of B. coagulans E38/vl was preferentially adsorbed at the boundary line between the two coexisting phases. The adsorption was dominated by hydrophobic and van der Waals interactions. The two-dimensional crystallization proceeded predominately underneath the liquid-condensed phase. Crystal growth was much slower under the liquid-expanded monolayer, and the entire interface was overgrown only after prolonged protein incubation. 

The combustion wave of HMX is divided into three zones crystallized solid phase (zone 1), solid and/or liquid condensed phase (zone 11), and gas phase (zone 111). A schematic representation of the heat transfer process in the combustion wave is shown in Fig. 5.5. In zone 1, the temperature increases from the initial value Tq to the decomposition temperature T without reaction. In zone 11, the temperature increases from T to the burning surface temperature Tj (interface of the condensed phase and the gas phase). In zone 111, the temperature increases rapidly from to the luminous flame temperature (that of the flame sheet shown in Fig. 5.4). Since the condensed-phase reaction zone is very thin (-0.1 mm), is approximately equal to T . 

Figure 3.4. Isotherm for pentadecanoic acid at 25°C. (Taken from Pallas, N.R. and Pethica, B.A. 1985 Langmuir 1 509-13. Published by permission of The American Chemical Society and the authors.) The curve follows the experimental points (not shown) closely. The two-phase region in which the liquid expanded and liquid condensed phases coexist corresponds to the plateau.

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. 

Figure 13.4 shows three different pressure-area isotherms measured by DPPC at different temperatures. Real isotherms deviate in two aspects from the ideal, schematic ones. First, the phase transition between the liquid expanded and liquid condensed phase at a a = 50...52 mN/m is not sharp but smooth. This is due to the fact that the liquid condensed phase is still relatively soft and can still easily be compressed. Thus, the pressure increase during the phase transition not only causes more molecules to condense but also compresses the already formed liquid condensed phase. In addition, contaminations contribute to this effect. 

Contaminations are also responsible for the second difference between real and ideal isotherms. At 7rc the isotherm is not perfectly horizontal but slightly tilted, in particular at elevated temperatures. Contaminations are expelled from the liquid condensed phase. Thus, when more and more of the monolayer goes into the liquid condensed phase, contaminations are enriched in the remaining liquid expanded phase. This reduces the two-dimensional... 

A general thermodynamic theory and a statistical thermodynamic approach are presented, which describe the phase transitions in insoluble monolayers, particularly the inclined transition from a liquid-expanded to a liquid-condensed phase. 

Originally cage effect meant an effect of the liquid (condensed) phase on a reaction studied in the gas phase. Strictly speaking, there is no sense to refer to any effect when one deals with reactions in the liquids. 

Chemical Interferences. Chemical effects originate either in the flame or in the sample solution. The interference mechanisms may be divided into the two groups (i) The atomization of the analyte element is not completed either in the solid phase or in the liquid (condensed) phase (ii) The vaporized atoms react with other atoms or radicals present in the gas phase. 

The molecular tilt angle depends on the surfactant concentration at the interface. In the regime of infinitely small surfactant concentrations, the molecules tend to lay almost flat on the surface. This corresponds to tilt angles of about 90° measured with respect to the surface normal vector. During film compression, the phase transitions towards liquid-expanded and liquid-condensed phases can be characterized by decreasing tilt angles. 

Fig. 3. A representative pressure vs. area isotherm. The cartoons represent idealized gas-analogous, liquid-expanded, and liquid-condensed phases of the monolayer. Two phases often coexist at a given temperature and surface pressure.

Liquid-condensed phase (surface) A monolayer phase of high surface viscosity in which there is a substantial degree of translational ordering and also possibly orientational... 

Plateau (in 71-A isotherms) The flat or close to flat region upon compression of a Langmuir monolayer exhibiting a liquid expanded phase which is progressively converted into a liquid condensed phase. 

Why do drops evaporate at all A liquid (condensed phase) with a planar surface evaporates only when its vapor pressure Po is higher than the pressure of its vapor (gas phase) in its surroundings. Thus, if the surroundings are saturated with its vapor the liquid does not evaporate. It is in equilibrium, because at any time the number of molecules evaporating from and condensing to the surface is similar. However, drops have a slightly higher vapor pressure in comparison to a planar surface due to their curvature. For this reason, they evaporate also in a saturated atmosphere. 

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