Aqueous subphase

FIG. 17 H-A isotherms for C60 on a pure aqueous subphase at 21°C (a) Monolayer prepared by spreading 50 p.L of a 0.1 mM solution in benzene (b) monolayer prepared by spreading 200 p,L and (c) a compression-expansion cycle of a monolayer prepared by spreading 100 p,L of a 0.5 mM solution. (Reproduced with permission from Ref. 232. Copyright 1993 American Chemical Society.)... 

The first study utilizing this method was reported by Schuller in 1966 [65]. Schuller used polystyrene latex beads that were spread on a salt-containing aqueous subphase in order to keep the particles at the interface. tt-A plots of the floating particles were determined, which showed several phase regions with reproducible transition points. The author determined the particle diameters from the A-value, at which a steep rise in the isotherm occurred. Moreover, Schuller also spread millimeter-sized Styropor particles and found isotherms of similar shape [66]. By taking pictures at different surface pressure, he was able to correlate the shape with different states of order in the monolayer. Shortly after that. 

FIG. 4 tt-A isotherm of carboxylated latex particles on aqueous subphase, it measured using a WU-helmy balance. (Data taken from Ref. 155.)... 

FIG. 7 Plot of the limiting area, Aq, of anionic particles la against the pH of the aqueous subphase, with the sum concentration of protons and potassium ions in the subphase being kept constant at 1 mol... 

In the first step, lipid model membranes have been generated (Fig. 15) on the air/liquid interface, on a glass micropipette (see Section VIII.A.1), and on an aperture that separates two cells filled with subphase (see Section VIII.A.2). Further, amphiphilic lipid molecules have been self-assembled in an aqueous medium surrounding unilamellar vesicles (see Section VIII.A.3). Subsequently, the S-layer protein of B. coagulans E38/vl, B. stearother-mophilus PV72/p2, or B. sphaericus CCM 2177 have been injected into the aqueous subphase (Fig. 15). As on solid supports, crystal growth of S-layer lattices on planar or vesicular lipid films is initiated simultaneously at many randomly distributed nucleation... 

For these investigations, the UME was positioned in the aqueous subphase containing 0.1 M KNO3 and held at a potential to reduce oxygen at a diffusion-controlled rate, in order to promote the transfer of O2 from air (phase 2) to the aqueous solution (phase 1), with subsequent collection at the tip (Fig. 27). Under SECMIT conditions, the flux of oxygen from air to water is given by ... 

The simplest way to predict the lipid/ water partition coefficient, Kiw, of a drug is based on measurements of the surface pressure, ttd, of the drug as a function of its concentration in the aqueous subphase (Gibbs adsorption isotherm). The Gibbs adsorption isotherm provides the air/water partition coefficient, Kaw, and the cross-sectional area, Ad of the drug and allows calculation of the lipid/water partition coefficient, K]w, according to Eq. (6) [59] ... 

However, the spreading of a surfactant monolayer from a volatile solvent leaves behind a film that may not be in thermodynamic equilibrium with its bulk crystalline form or the aqueous subphase. It has been proposed that this is a result of the relatively high energy barriers to film collapse or dissolution into the subphase as compared with lowered interfacial free energy when a stable, insoluble surfactant monolayer is formed (Gershfeld, 1976). The rate at which a whole system approaches true equilibrium in such a system is often so slow that the monolayer film can be treated for most purposes as though it were at equilibrium with the subphase. 

The few examples of deliberate investigation of dynamic processes as reflected by compression/expansion hysteresis have involved monolayers of fatty acids (Munden and Swarbrick, 1973 Munden et al., 1969), lecithins (Bienkowski and Skolnick, 1974 Cook and Webb, 1966), polymer films (Townsend and Buck, 1988) and monolayers of fatty acids and their sodium sulfate salts on aqueous subphases of alkanolamines (Rosano et al., 1971). A few of these studies determined the amount of hysteresis as a function of the rate of compression and expansion. However, no quantitative analysis of the results was attempted. Historically, dynamic surface tension has been used to study the dynamic response of lung phosphatidylcholine surfactant monolayers to a sinusoidal compression/expansion rate in order to mimic the mechanical contraction and expansion of the lungs. 

Fig. 23 Equilibrium spreading pressures of (R,S)-( +)- and(R)-( +)-stearoyltyrosine on an aqueous subphase of pH 6.86 (potassium phosphate/disodium phosphate buffer) as a function of temperature. Film type II is the film at temperatures above the transition and film type I is the film at temperatures below the transition. Reprinted with permission from Arnett et al, 1990. Copyright 1990 American Chemical Society.

Table 8 Helmholtz free energy, entropy, and internal energy of spreading and of transition for N-stearoyltyrosine on an aqueous subphase of pH = 6.86 at the transition temperature for each film."... 

Fig. 29 Surface pressure/area isotherms for compression and expansion of enantiomeric (dashed line) and racemic (solid line) stearoylcysteine methyl esters (SCME) on an aqueous subphase at various temperatures. Arrows indicate the direction of compression and expansion.

Collapsing and nucleation processes of a stearic acid monolayer have been also studied on the different aqueous subphases by a Brewster angle microscope [15]. Both of the temperature and the pH value of the aqueous subphase can influence the nucleation process and produce very different collapsing patterns. 

On the other hand, inclusion and/or adsorption of NaphSOsNa molecules from the aqueous subphase to the spread monolayer of p-CDNHC12H25 were examined by using the multicompartment trough. When the P-CDNHC12H25 mono-layer spread on the distilled water surface was compressed to the prescribed initial surface pressures of 5, 10, 20, and 30 mN/m and transferred onto the aqueous subphase containing 10 3 M NaphSCbNa, the surface pressure increased with time, air/aqueous solution interface under the suggesting the... 

Figure 15. Spectral change with time for the mixed monolayers of [MC18] [MA] [HD] and [MC18] [AA] [HD] on the aqueous subphase containg CdCfe and KHCO3 on compression at 15 mN/m. 

Figure 34. Surface pressure - area isotherms for monolayers of Ci 8TCNQ (a), the mixture of the dihydrothiophene and G 8TCNQ (b), and the complex (c), spread on distilled water, as compared with that on the aqueous subphase with 10 5M LiTCNQ (c ).

As a cationic polymer and a cationic amphiphile, poly(allyl amine hydrochloride) (PAA) and octadecylamine (ODA) shown in Fig. 6 were used, respectively. The stability of the monolayers of the anionic amphiphiles was increased by polyion-complexation with PAA added in the aqueous subphase in comparison with Ca2+ salt formation. Ion complexation (1 1) of each anionic amphiphile with ODA was also performed at the air-water interface by spreading a chloroform solution of a 1 1 surfactant mixture. 

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