7t-A-Isotherm

Fig. 2. A schematic 7T-A isotherm for a phosphoHpid of fatty acid (see text).

FIG. 3 7t—A isotherms measured for five lecithins at 15°C. (Experimental data from Ref. 20. Reproduced from Ref. 39 with permission from The American Chemical Society.)... 

Figure 12.7t-A isotherm for 1,2-dimyristoyl lecithin at an n-heptane-aqueous NaCl solution interface. The numerals on the curves are the temperatures given in Celsius.

Potentiometric sensing based on type 3 derivatives of a-, P-, and y-cyclodextrins has been reported.122-125 We have been interested in the type 1 cyclodextrin derivatives developed by Tagaki,126 e.g., host 41. This type of cyclodextrin derivative is characteristic in that the primary hydroxyl groups at the C-6 positions are exhaustively substituted with long alkyl chains. At the surface of an oiganic membrane, this structural feature allows the secondary hydroxyl side (wider open end of the cavity) to face the aqueous solution to accommodate a guest molecule. The interfacial receptor functions of these cyclodextrin hosts have been confirmed mainly by 7t-A isotherm studies.127128... 

In this section, the derived surface equation of state [Eq. (33)] is compared with the experimental 7t - A isotherms involving the LE/LC phase transition for the following six phospholipid surfactants 1,2-dimyris-toylphosphatidylcholine (DMPC), 1,2-dipalmitoylpho-sphatidylcholine (1,2-DPPC), 1,3-dipalmitoylphosph-atidylcholine (1,3-DPPC), 1,2-dimyristoylphosphatidic acid (DMPA), 1,2-dilauroylcephalin (DLPE), and 1,2-dimyristoylphosphatidylethanolamine (DMPE). The general formula of the phospholipid surfactants can be written as R-C02- (hJ-CbC—R, where R stands... 

Monolayer measurements and film deposition A computer-controlled Langmuir type film balance (LT-102, Belarus) was used for 7t-A isotherm recording and LB film deposition by horizontal precipitation (HP) method. Silicon with native silicon dioxide layer and atomically flat mica surfaces were used for monolayer film deposition. 

Figure 12. Plateau regions of 7t-A isotherms for stearic acid at pH 10 on an expanded scale. Data represent means SD for seven isotherms (8).

Figure 5 shows the 7t-A isotherm of the mixed Langmuir monolayer composed of a] -trans-P-carotene with harium stearate (1 10) at the air/water interface. The n-A isotherm shows a steep rise at the surface pressure above 15 mN/m. This observation confirms that the molecules in the monolayer are closely packed at the surface pressure of 30 mN/m, the value which corresponds to the fabricating condition when transferring the monolayer to the glass substrate. The limiting area, which corresponds to the area occupied by one molecule at the air/water interface, can be extrapolated from the steep part of the n-A isothermtobe 32 /molecule (see broken... 

Compressibility modulus of Langmuir monolayers can be determined from the slope of the 7t-A isotherms (Vollhardt D et al. (2006) Adv Colloid Interf Sci 127 83). The compressibility modulus Eo=r(dII/dr)x, where F=1/AN, and A, N, and n are molecular area, Avogadro s number, and surface pressme, respectively. 

Phase Separation for Langmuir Monolayer in Binary System Based on a 7t-A Isotherm Measurement... 

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between CJq, the surface tension of pure water, and O, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constmcted. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stabiUty of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

Equation 3-1 implies that the 7t-r relationship becomes MN -shaped when becomes sufficiently large. Indeed, Mingins et al. have reported that at the oil/water interface the n-A isotherms of the phospholipids with Ci, C20 and C22 alkyl chains show clear first-order transitions [37],... 

Fig. 15. Compression isotherms (n vs. A) of dipaimitoyiphosphatidyichoiine (DPPC) measured at 25 °C in an atmosphere of N2 (dashed iine) and of N2 saturated with F-octyi bromide (soiid iine). insets Fiuorescence images of (a) the DPPC monoiayer compressed at 7t = 15 mN/m under N2, cieariy showing crystaiiine domains, and (b) the DPPC monoiayer in contact with F-octyi bromide, showing prevention of crystaiiization, even at high pressures (ti = 30 mN/m) [107],...

Figure 8-8 (A) The Langmuir-Adam film balance. Tension on the moveable barrier is recorded for different areas of the surface between the barriers. This gives the surface pressure jt, which is the difference between the surface tension (y0) of a clean aqueous surface and that of a spread monolayer (y) K = ya-y. Courtesy of Jones and Chapman.81 (B) Surface pressure (7t)-area per molecule isotherm for a typical fatty acid (e.g., pentadecanoic acid C14H29C02H) at the aqueous-air interface. From Knobler.81a...

This deformation can be followed by UV-vis spectroscopy directly at the air/water interface. Upon compression of the monolayer, the absorbance changes not very much around the point where the first increase in surface pressure is recorded (see Figure 6.17), This is caused by the disappearance of the voids between the domains. Above the kink in the isotherm, howevei the K-n band (-323 nm) decreases while the band at 248 nm increases. This increase is due to the increase in chromophore concentration upon compression of the monolayer. The decrease of the 7t-ic band is caused by a preferential orientation of the chromophorra perpendicular to the surface. Chromophores oriented perpendicularly are not detected because the transition dipole of the n-n band is parallel to the probing light. The band at 248 nm, however is not sensitive to the orientation of the chromophore, because the transition dipole moment has a component perpendicular to the long axis of the molecule (see Figure 6.17). 

The chemical compositions of the samples were determined by atomic adsorption spectroscopy (AAS). X-ray powder diffraction patterns were recorded after synthesis and template removal on a Siemens D5000 diffractometer using CuKa radiation. After calcination, nitrogen adsorption and desorption isotherms were measured on a Micromeritics ASAP 2010 sorption analyzer. Al MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer using single pulse excitation with standard 4 mm rotors The resonance frequency was (Oo/271 = 104.31 MHz for Al using a 7t/20 pulse and a 0.5 s recycle delay. A 0.1 M solution of aluminum nitrate in water was employed as the chemical shift reference... 

For spreading solvent I (Figure 1), all the isotherms have a surface pressure plateau above 10 dynes/cm thus, the curves are composed of two parts. For pressures lower than the plateau, we derived from ir—A experimental values the corresponding equations of the two-dimensional state ttA—7T. From their coefficients, using an accepted procedure (8, 9, 10,11), we derived the following parameters ... 

Equation (II. 16) is the state equation of the ideal two-dimensional gas, which is represented by the two-dimensional pressure isotherm, 7t(sm), shown in Fig. II-8. To analyze the behavior of real systems, and to study the origin of deviations of such systems from ideal two-dimensional state, the two-dimensional pressure isotherm is plotted in nsM - 7r coordinates. The isotherm of an ideal two-dimensional gas in these coordinates is a straight line, parallel to the x axis (Fig. II-9). If the two-dimensional pressure and area per molecule... 

Thus, for a system consisting of N components wih specified partial pressure in the gas phase, there are 2N + 1 independent variables to be solved x(/), P and 7t. Equations (l)-(4) provide the 2A + 1 necessary equations. Since the equations are nonlinear and the integrals in Eq. (2) cannot be solved analytically for most of the pure component isotherm equations, the 2N + 1 equations have to be solved numerically. 

On the basis of the analysis of the IR spectra and the information gathered from the surface pressure isotherms, a model (see Fig. la) is proposed for the monolayer arrangement at the air/water interface [40] which imphes two sets of dimers of water per molecule of Chi a. One pair of dimers constitutes the water of the first kind and is composed of vapor-like dimers. The two water molecules are held together by a hydrogen bond, and on one side by a weak coordinate bond with the Mg atom of one Chl a molecule and on the other side by a weak hydrogen bonds with the 7t-electron network of another Chl a molecule. This kind of water is situated between the porphyrin planes of Chl a molecules and is easily removed from the monolayer. The second pair of dimers is composed of water of the second and third kinds situated between the... 

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