Interface heptane-water

Calculate 7wh for the heptane-water interface using Eq. IV-7. Compare the result with the experimental value and comment. 

Stigter and Dill [98] studied phospholipid monolayers at the n-heptane-water interface and were able to treat the second and third virial coefficients (see Eq. XV-1) in terms of electrostatic, including dipole, interactions. At higher film pressures, Pethica and co-workers [99] observed quasi-first-order phase transitions, that is, a much flatter plateau region than shown in Fig. XV-6. 

FIG. 3 MD simulation of the heptane-water interface, (a) Configuration of solvent molecules in a two-phase cell (b) density vs. distance profile along the axis from aqueous phase to heptane phase. 

FIG. 5 MD simulation of 2-hydroxy-5-nonylbenzophenone oxime (LIX65N) at the heptane-water interface after 120 ps. Heptane molecules are not shown. 

FIG. 7 MD simulation of the adsorption of 5-Br-PADAP at the heptane-water interface after 100 ps. 

Crawford et al.24 explored the possibility of SHG-CD and SHG-ORD to study biological samples. They studied the dipeptide (tert-butyloxycarbonyl)tryptophanyltryptophan adsorbed at an air-water and a heptane-water interface. A dye laser that operated in the wavelength region 550-580 nm was used to record SHG-CD and SHG-ORD spectra. The SHG spectra from the LL and DD enantiomers showed equal but opposite dependences on the handedness of the fundamental laser beam and the DL diastereoisomer gave rise to different SHG spectra.24... 

The combination of resonance Raman microscope spectrometry and the CLM method allowed us to directly observe the Raman spectra of the liquid-liquid interface and the bulk phases by shifting the focal point of an objective lens. A schematic diagram of the measurement system is shown in Fig. 6. CLM/ Raman microscope spectrometry was applied in order to measure the rate of complex formation between Pd(II) and 5-Br-PADAP (HL) at the heptane-water interface and it was demonstrated that this method was highly useful for the kinetic measurement of the interfacial reaction [37],... 

Fig. 11. (a) s-Polarization analyzed SHG spectrum of adsorbed TPPS at the heptane-water interface, lcp left-circularly polarized fundamental beam, rep right-circularly polarized fundamental beam, (b) SHG/CD spectrum of adsorbed TPPS at the heptane-water interface. Aqueous phase was containing 5.35 x 10 M TPPS, 3.36 x 10"7M CTAB, 0.001 M HC1 (pH 3.0), and 0.009 M NaCl. 

Fig. 17. Solvation energy of LIX65N around the heptane-water interface calculated by the molecular dynamics simulation.

The complex formation proceeded almost completely at the interface. The rate constant of k=5.3xl02M 1 s 1 was determined by a stopped-flow spectrometry in the region where the formation rate was independent of pH. The conditional interfacial rate constants represented by k[ = k k2 [HL] / (k2 + k i[H + ]) were larger in the heptane-water interface than the toluene-water interface, regardless of metal ions. The molecular dynamics simulation of the adsorptivities of 5-Br-PADAP in heptane-water and toluene-water interfaces suggested that 5-Br-PADAP could be absorbed at the interfacial region more closely to the aqueous phase, but 5-Br-PADAP in the toluene-water... 

Considerable care needs to be taken in extracting the interfacial concentration from the SHG intensities because of the interaction between surface density and surface order on the SHG process [49]. Table 1 shows a comparison of the values of Aahydrocarbon/water interfaces determined by SHG methods. The different results obtained at the dodecane/water interface where different isotherms were used to fit the SHG data suggest that the determination of—AadsG° at the heptane/water interface using only a Langmuir isotherm gives a value that is too high and thus this value should be re-examined. 

FIGURE 1.12. The SHG optical rotation from boc-Trp-Trp at the air/water interface (upper) and at the heptane/water interface (lower). The solid line is the UV absorption in water. Error bars are probably overestimated in this work. 

The dynamic behaviour of the 2-hydroxy oxime and its adsorptivity at the interface were well depicted by the molecular dynamics (MD) simulations [34]. It was revealed that the polar groups of —OH and =N—OH of the adsorbed 2-hydroxy oxime molecule were accommodated in the aqueous phase side so as to react with the Ni(II) ion in the aqueous phase [35]. This was thought to explain that the magnitude of the reaction rate constants of Ni(II) at the heptane/water interface and that in the aqueous phase were similar to each other. The diffusive and adsorptive behaviour of LIX65N around the interface was also simulated for 1 ns. The molecule was active around the interfacial region, moving... 

FIGURE 10.10. Change in resonance Raman spectra upon complexation of Pd(II) with 5-Br-PADAP at the heptane/water interface. Aqueous phase PdCh, 8.0 x 10 M HCl, 0.1 M pH 1.0 heptane phase 5-Br-PADAP, 7.8 X 10" M. 5-Br-PADAP was injected at 4 seconds. Final volumes of the aqueous and heptane phases were 0.250 and 0.150 cm, respectively. Laser power (514.5 nm) was 40mW and the integration time for each spectrum was 5 seconds. 

Similar interfacial aggregation phenomena were observed in the complexation of Cu(II) with octadecylthiazolylazophenol (CigTAR, HR) at the heptane/water interface [54], The CuR+ complex was formed at the interface with the absorption maximum at 560 nm. The increase in pH promoted the formation of an aggregate of Cu2R shifting the absorption maximum from 560 to 510 nm. 

Figure 11.5a shows SHG spectra of rhodamine B (RB) adsorbed at the heptane/water interface. A single major peak assigned to a resonance with the Sq-S i electronic transition of RB is recognized in each spectrum. The peak positions in these spectra are determined by fitting data points to a combination of Lorentzian functions based on the two-state model [60]. 

FIGURE 11.6. Adsorption isotherms of RB (filled circles) and RllO (open circles) for the heptane/water interface fitted by a Langmuir model. 

FIGURE 11.7. Schematic illustiation of the in-plane associate of RB at the heptane/water interface... 

From the results of SHG spectroscopy, the rhodamine dyes adsorb and form the in-plane associates at the heptane/water interface, pointing the xanthane moiety towards the heptane phase with a tilt angle as shown schematically in Figure 11.7. The structure of the associates at the interface differs from that in a bulk aqueous phase. In an aqueous solution, RB makes a sandwich dimmer when the concentration is above 1.0 x 10 M [66], and the molar fraction of the dimmer is 0.56 at 1.0 x 10 M. The absorption spectra of the sandwich dimmer show blue-shifted peak around 525 nm. In contrast, red-shifted peaks found in SHG spectra indicate that the in-plane associates are predominantly formed at the interface despite the presence of the sandwich dimmer in the water phase. The hydrophobic property arising from heptane molecules at the h tane/water interface would be crucial for the predominant formation of in-plane associates. 

Alkali Metal Recognition at the Heptane/Water Interface... 

Figure 11.8 shows the SHG spectra of [2-hydroxy-5-(4-nitrophenylazo)phenyl]-methyl-15-crown-5 (azoprobe 1) upon addition of alkali metal salts in the aqueous phase. The wavelength of the peak maximum of each spectrum is ca. 540 nm, which indicates the red-shift of ca. 45 nm from the UV-vis absorption peaks in bulk water. The red-shift is ascribed to a negative solvatochromism induced by different polarities between the heptane/water interface and bulk water, since the polarity of the interface is lower than that of bulk water [9,13]. On the other hand, the peak intensity of azoprobe 1 is fbund to significantly increase upon addition of alkali metal salts. The salt-dependent increase... 

FIGURE 11.8. SHG spectra of azoprobe 1 at the heptane/water interface containing alkali metal and tetram-ethylammonium (TMA) ions (as chloride salts). The concentration of azoprobe 1 in bulk aqueous phase is 1.0 X 10- M. 

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