Water molecules phase diagram

Another interesting class of phase transitions is that of internal transitions within amphiphilic monolayers or bilayers. In particular, monolayers of amphiphiles at the air/water interface (Langmuir monolayers) have been intensively studied in the past as experimentally fairly accessible model systems [16,17]. A schematic phase diagram for long chain fatty acids, alcohols, or lipids is shown in Fig. 4. On increasing the area per molecule, one observes two distinct coexistence regions between fluid phases a transition from a highly diluted, gas -like phase into a more condensed liquid expanded phase, and a second transition into an even denser... 

With increasing water content the reversed micelles change via swollen micelles 62) into a lamellar crystalline phase, because only a limited number of water molecules may be entrapped in a reversed micelle at a distinct surfactant concentration. Tama-mushi and Watanabe 62) have studied the formation of reversed micelles and the transition into liquid crystalline structures under thermodynamic and kinetic aspects for AOT/isooctane/water at 25 °C. According to the phase-diagram, liquid crystalline phases occur above 50—60% H20. The temperature dependence of these phase transitions have been studied by Kunieda and Shinoda 63). 

Note that in the above diagram there are still vibrational states but that the rotational states are "smeeired" one into the other. There is little translational motion for the water molecules within the interior of the liquid unless they escape from the liquid phase. If they do so, we call this "evaporation (This may be contrasted to the escape of molecules from a solid which we call "sublimation"). 

The melting point of carbon dioxide increases with increasing pressure, since the solid-liquid equilibrium line on its phase diagram slopes up and to the right. If the pressure on a sample of liquid carbon dioxide is increased at constant temperature, causing the molecules to get closer together, the liquid will solidify. This indicates that solid carbon dioxide has a higher density than the liquid phase. This is true for most substances. The notable exception is water. 

Water is the only form of matter occurring abundantly in all three phases (or states) solid, liquid, and gas (or vapor) (Fennema, 1996). Temperature and pressure determine the phase of water, as well as the type(s) and velocity(ies) of water molecule motion. A basic phase diagram (moderate pressure-temperature range) for pure water is shown in Figure 7. Given the... 

One of the major differences among the phases of water at the molecular level is the motions of the water molecules. Using the phase diagram (Figure 7), we can follow the effects of temperature and pressure on the molecular mobility of water. For example, if we hold pressure constant (say at 1 atm) and increase temperature, molecular mobility increases as we move from the solid to the liquid to the gas phase regions. Conversely, if we hold temperature constant (say at 100°Q and increase pressure, molecular mobility decreases as we move from the gas to the liquid phase region. 

FIG. 9 Diagram illustrating the three vibrational modes (31V— 6) of water in the gas phase. (A) The first mode is called bending, in which the water molecule moves in a scissors-like manner. (B) The second is the symmetric stretch, where the hydrogen atoms move away from (or toward) the central oxygen atom simultaneously—i.e., in-phase motion. (C) The third is the asymmetric stretch, in which one hydrogen atom approaches the central oxygen atom, while the other moves away—i.e., out-of-phase motion. 

Recently, new ordered mesoporous silicas have also been synthesized by using self-organization of amphiphilic molecules, surfactants and polymers either in acidic or basic condition. A schematic phase diagram of water-surfactant is shown in the figure. 

Fig.1 Phase behavior types of surfactant-oil-water systems as Winsor Diagrams for difer-ent cases of the ratio R of interactions between the surfactant adsorbed at interface and the oil and water molecules...

For mixtures of lecithin plus Na cholate it appears possible to infer the molecular arrangement in the dispersed micelles from the most likely structure of the liquid crystalline phase suggested by x-ray analysis. However, there are cases where dispersion is not possible because neither component is sufficiently hydrophilic to be dispersed even when alone in water. This is shown by the association of cholesterol and lecithin in the presence of water. The ternary diagram of Figure 4 is relative to these systems. Here only the lamellar liquid crystalline phase is obtained (region 1< in Figure 4). This phase is already given by lecithin alone, which can absorb up to 55% water. Cholesterol can be incorporated within this lamellar phase up to the proportion of one molecule of choles-... 

The leftmost diagram below shows two phases of a single substance. In the middle box, draw what these particles would look like if heat were taken away. In the box on the right, show what they would look like if heat were added. If each particle represents a water molecule, what is the temperature of the box on the left ... 

Figure 3.5. Schematic arrangement of the various phases of behenic acid (docosanoic acid) at the air/water interface corresponding to the phase diagram shown in Figure 3.6. (Taken from Kenn, R.M., Bohm, C., Bibo, A.M., Peterson, I.R., MOhwald, H., Als-Nielsen, J. and Kjaer, K. 1991 J. Phys. Chem. 95 2092-7. Published by permission of the American Chemical Society and the authors.) All the phase structures are distorted forms of hexagonal packing. denotes an end view of a molecule which stands vertically but does not rotate about its axis and is in the phase denoted by CS. is similar to the above but is in the S phase and librates. O denotes a molecule the axis of which is vertical and which rotates about its axis. denotes a tilted molecule. In the liquid expanded, L2, phase the tilt is towards the nearest neighbour and in the liquid condensed, L, phase the tilt is towards the next nearest neighbour.

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