Monolayers long-chain alcohols

As mentioned in Section IX-2A, binary systems are more complicated since the composition of the nuclei differ from that of the bulk. In the case of sulfuric acid and water vapor mixtures only some 10 ° molecules of sulfuric acid are needed for water oplet nucleation that may occur at less than 100% relative humidity [38]. A rather different effect is that of passivation of water nuclei by long-chain alcohols [66] (which would inhibit condensation note Section IV-6). A recent theoretical treatment by Bar-Ziv and Safran [67] of the effect of surface active monolayers, such as alcohols, on surface nucleation of ice shows the link between the inhibition of subcooling (enhanced nucleation) and the strength of the interaction between the monolayer and water. 

Above a plane surface, the resistance J i of the gas phase may be quite large, of the order 100 sec. cm., if there is a stagnant film (e.g., of air) overlying the water. Compared with such a value of Ri, the resistance Rj is rather small, even if a monolayer of a long-chain alcohol or acid is present. But if the air pressure is reduced below atmospheric, or if the air is stirred (e.g., by wind or by a fan), Ri can be reduced to only a few sec. cm. S and the resistance Ri (up to 20 sec. cm. with suitable films) may become controlling. 

From Eq. (5) it is clear that for very small droplets (a small) the rate of evaporation will become extremely great unless Rj is appreciable. Conversely, a small amount of surface impurity may have a large effect on the rate of evaporation of very small drops. Thus, a monolayer of a long-chain alcohol or acid, which at room temperature can increase Ri (from 0.002 sec. cm. for the clean water surface) to 10 sec. cm. , should be able to reduce the rate of evaporation of a very small drop by 5000 times. The life-times in dry air of water drops of 1 /.i radius are correspondingly increased from a few milliseconds to about a minute by... 

The adsorption of amphiphilic molecules at the surface of a liquid can be so strong that a compact monomolecular film, abbreviated as monolayer, is formed. There are amphiphiles which, practically, do not dissolve in the liquid. This leads to insoluble monolayers. In this case the surface excess T is equal to the added amount of material divided by the surface area. Examples of monolayer forming amphiphiles are fatty acids (CH3(CH2) c 2COOH) and long chain alcohols (CH3(CH2)nc iOH) (see section 12.1). 

The analogy between three- and two-dimensional phase diagrams can be carried much further. Monomolecular amphiphilic films show ordered phases similar to three-dimensional systems [579], The phases of an amphiphilic monolayer can be detected most conveniently in pressure-area (7r-versus-OA) isotherms. These may look different for different substances. The behavior of simple amphiphilic molecules, like long-chain alcohols, amines, or acids, was extensively investigated (reviews Refs. [580,581]). In monolayers so-called mesophases can occur. In a mesophase the tail groups are ordered over relatively large areas, while the order in the hydrophilic head groups is only over a much smaller distances. 

The study of surface films of the Langmuir type covers an extremely diverse group of phenomena. Measurements of film viscosity, diffusion on the surface, diffusion through the surface film, surface potentials, the spreading of monolayers, and chemical reactions in monolayers are just a few of the topics that have been studied. One interesting application is the use of long-chain alcohols to retard evaporation from reservoirs and thus conserve water. The phrase to pour oil on the troubled waters reflects the ability of a mono-molecular film to damp out ripples, apparently by distributing the force of the wind more evenly. There are also several different types of surface films only the simplest was discussed in this section. 

Multilayer formation can also be observed in many other lipids such as fatty acids, long-chain alcohols and esters of short-chain alcohols. Lundqvist (1971) has reported monolayer formation in fatty acid ethyl esters, where it is possible to observe from the H-A isotherms how multifilms consisting of two, three, four, five and six molecular layers are formed. In this case the forces between the polar groups are so weak that the molecules can be arranged head-to-tail as observed in the crystal forms of ethyl stearate (see Section 8.5). 

As a consequence of stronger chain-chain interactions between long chain alkanes and surfactants, the hydrophobic chains are straightened, while in the case of long chain alcohols the hydrophobic effect plays a major role compared to the chain-chain interaction, which are minor when the alcohol is diluted in a nonpolar liquid. Even if literature reports data in contrast to the weak environmental effects on compact, self-assembled monolayers, the influence of the liquid environment on the structure of self-assembled monolayers is relevant in the wetting properties of these monolayers. Wetting depends on how the liquid is in contact with it, because the surrounding liquid molecules influence the chain conformation. ... 

FIGURE 13.11 Schematic illustration (not to scale) of the SECM-induced transfer of oxygen across a long-chain alcohol monolayer at the air-water interface. 

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... 

We Umit this section to a discussion of stereochemical studies that sought to demonstrate discriminating enantiomeric interactions in monolayers of simple surfactants having one hydrophobic chain of methylenes and, generally, a single chiral center. Work in this area includes derivatives of long chain fatty acids, alcohols, or esters whose chiral center is included in the methylene chain. 

G.T. Bames and V.K. La Mer, The evaporation resistances of monolayers of long-chain acids and alcohols and their mixtures, in V.K. La Mer (Ed.), Retardation of Evaporation by Monolayers Transport Processes, Academic Press, New York, 1962, pp. 9-33. 

Techniques for spreading monolayers of polar long chain compounds on mercury in a Langmuir type film balance, and for measuring their surface area-pressure properties, have been described by one of the present authors (3). Using these techniques, it has proved possible to measure continuously the change in contact angle of a water droplet superposed on the monolayer, as the film pressure is controllably varied. This has now been done for monolayers of the normal C12-C20 fatty acids and the normal primary Ci4-Ci8 alcohols on the mercury substrate. 

Figure 3.47. Rheology of collapsed monolayers of long-chain fatty alcohols. Comparison between measured (—) moduli (fig. a) and loss tangents (fig. b) and those computed by a...

Table 3.7a. Normal long chain fatty acid and fatty alcohols, used in monolayer studies.

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