Alcohols pressure-area isotherms

Langmuir mono/bilayers of mono/multidendrons based on 3, 5 - dihydroxyben-zyl alcohol at the air - water interface were studied using pressure - area isotherms and neutron reflectivity by Frechet et al. [108],... 

Saville et al.1411 studied the U.—A (surface pressure-area) isotherms at the water-air interface at 25 °C for monomolecular films of the ethereal dendrimer series (i.e., the benzylic alcohol series represented by structures 50 in Scheme 5.12). They reported a strong dependence of the isotherms on molecular weight, which compared well with that observed for hydroxyl-terminated polystyrene. 

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. 

Studies of monolayer films have included alcohols and carboxylic acids with hydrocarbon chains of varying length in the range C10-C26 [20, 21]. In the liquid condensed and solid states the 11- isotherms have similar shapes at high surface pressures. These data show that the monolayer in the self-assembled state occupies an area of 0.205 nm per monomer. This result is independent of chain length and provides evidence that the monolayer consists of a close-packed structure with all molecular units oriented with their hydrocarbon chains perpendicular to the interface. Under these circumstances, strong attractive van der Waals forces are present between the hydrocarbon tails. As a result, formation of the solid-state film can be irreversible, so that the film does not break up when the surface pressure is decreased. 

Figure 4. Surface pressure and change in substrate height vs. time using the distillation isotherme superficielle technique. Trough area—201 cm2. Bridge—four glass rods, ody = 4 mm, length = 6.94 cm. OA (oleic acid) spread on 0.01N HCly pH == 1.9, 25°C COH (decyl alcohol) spread on water, pH 6.45> 22°C ME (methyl laurate) spread on water, pH 6.45, 22°C BSA (bovine serum albumin) spread on water, pH 6.45, 22°C.

It is seen from these dependencies that, with the increase of m, the onset of surface tension decrease (or the surface pressure increase) corresponds to lower surfactant concentrations at the same time, the decrease of the isotherm slope at high concentrations takes place. The increase of surface activity with the increase of m for low pressure in the framework of reorientation model can be qualitatively explained by strong increase of the molar area of C EO, molecule in the state 1 (coj), while the decrease of the isotherm slope (and also the decrease of surface activity) at high concentrations can be ascribed to slight increase of the CO2 value. Therefore, the Intersection of the isotherms which is observed for the oxyethylated alcohols with different m values is the consequence of the fact that these two molar areas are increased with the increase of m, but the rate of this increase is different for CO2 and coj values. 

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