Arachidic acid monolayer reflectivity

Arachidic acid monolayers were prepared from a benzene solution on the water subphase of pH5.8(pure water) and 12.6(adjusted by addition of NaOH) at Tsp of 303 K below Tm(=328 K) of the monolayer [31]. The ionic dissociation state of hydrophilic group was estimated on the basis of the stretching vibrations of carbonyl and carboxylate groups by Fourier transform-infrared attenuated total reflection, FT-IR ATR measurements. 70 arachidic acid monolayers were transferred on germanium ATR prism, resulting in the formation of the multi-layered film. Transfer on the prism was carried out at surface pressures of 25 or 28 mN-nr1. Infrared absorption measurements revealed that almost carboxylic groups of arachidic acid molecules did not dissociate on the water subphase of pH5.8, whereas all carboxylic groups dissociated as carboxylate ions on the water subphase of pH 12.6. 

Figure 3.58. (a) Normalized X-ray reflectivity (R/Rj,) as a function of the scattering vector transfer for arachidic acid monolayers on pure water (T = 20°C). For better visibility the curves marked (, yand 5 are displaced upward by 0.25, 0.5 and 1.0, respectively. Here a, /3, yand 6 correspond to the surface pressures indicated in the tc[a ] isotherm of the insert, (b) Two-box density profile used to fit the data in (a). The boxes describing the tall region (T) and the polar head region (H), respectively, are smeared by a Gaussian, as indicated by the solid line. (Redrawn from J. Als-Nielsen and K. Kjaer, loc. cit.)... 

Figure 3.60. Tilt angle 0 as a function of molecular area a for arachidic acid monolayers on pure water (T = 20°C) determined from X-ray diffraction data ( ). For comparison, the tilt angle as calculated from reflectivity data (from the ratio between the thickness of the tail region and the length of the tall) is also shown (O). The drawn curve represents the function =a /cos0, with a = 19,8 nm, and the agreement between this curve and the experiment values indicates that compression does not increase the density of the tail region, but merely decreases the tilt angle. The transition from the untilted state to the tilted state appears to be continuous. (Redrawn from H. Mohwald, C. Bohm, A. Dietrich and S. Klrstein, Liq. Cryst. 14 (1993) 265.)...

In a further study, DiMasi and colleagues investigated the kinetics of amorphous CaCOs formation at a fatty acid monolayer interface using synchrotron X-ray reflectivity measurements [173]. In-situ experiments found three different parameters that control CaCOs mineralization in the presence of arachidic acid monolayers, PAA, and Mg + ions. Firstly, the crystal growth rate depends on the concentration of counterions and not on the polymer concentration in solution. Secondly, the soluble polymer only affects the lifetime of the amorphous calcium carbonate. And finally, the sole effect of Mg + is to delay the mineral film formation. These data thus suggest that competitive adsorption (e.g. Mg + vs. Ca +) is another parameter to consider in controlled mineralization processes. 

Fig, 6.1. Normalised X-ray reflectivity R/Rf vs. vertical wave vector transfor Qz, for arachidic acid monolayers on pure water (pH 5.5, T = 20 0. The measurements are displaced vertically by 0.25 units and correspond to the surface pressures indicated on the isotherm of the insert. 

Figure 2. Time dependence of enhanced light reflection at 550 nm of a mixed monolayer of the thioindigo cis-TI and arachidic acid (AA), molar ratio TI AA = 1 3, on illumination with blue light as indicated by the arrows. Subphase bidistilled water surface pressure 10 mN/m.

X-ray diffraction has been applied to spread monolayers as reviewed by Dutta [67] and Als-Nielsen et al. [68], The structure of heneicosanoic acid on Cu and Ca containing subphases as a function of pH has been reported [69], as well as a detailed study of the ordered phases of behenic acid [70], along with many other smdies. Langmuir-Blod-gett films have also been studied by x-ray diffraction. Some recent studies include LB film structure just after transfer [71], variations in the structure of cadmium stearate LB films with temperature [72], and characterization of the structure of cadmium arachidate LB films [73], X-ray [74,75] and neutron reflectivity [76,77] data on LB films can be used to model the density profile normal to the interface and to obtain values of layer thickness and roughness. 

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