PH, subphase

The quantity [k T] is approximately 4 10-14 erg at ordinary room temperature (25°C), and [k T/e] = 25 mV. The magnitude of nel can be estimated from monolayer studies at varying pH. At the isoelectric pH, the magnitude of nel will be zero (Birdi, 1989). These IT versus A isotherms data at varying pH subphase have been used to estimate nel in different monolayers. 

Figure 8. Surface pressure vs. surface concentration. Influence of oligomer length. Fsubphase pH subphase 8 2 (fraction CB).

Fig. IV-21. Surface pressure versus area for monolayers of immiscible components a monolayer of pure cadmium arachidate (curve 1) and monolayers of mixed merocyanine dye, MC2, and cadmium arachidate of molar ratio r = 1 10 (curve 2) 1 5 (curve 3), 1 2 (curve 4), and pure MC2 (curve 5). The subphase is 2.5 x 0 M CdC, pH = 5.5 at 20°C. Curve 3a (O) was calculated from curves 1 and 5 using Eq. IV-44. (From Ref. [116].)...

Most of the Langmuir films we have discussed are made up of charged amphiphiles such as the fatty acids in Chapter IV and the lipids in Sections XV-4 and 5. Depending on the pH and ionic strength of the subphase, electrostatic effects can become quite important. Here we develop the theoretical foundation for charged films with the Donnan relationship. Then we mention the influence of subphase pH on film behavior. 

The most often used subphase is water. Mercury and otlier liquids [12], such as glycerol, have also occasionally been used [13,14]. The water has to be of ultrapure quality. The pH value of tire subphase has to be adjusted and must be controlled, as well as tire ion concentration. Different amphiphiles are differently sensitive to tliese parameters. In general it takes some time until tire whole system is in equilibrium and tire final values of pressure and otlier variables are reached. Organic contaminants cannot always be removed completely. Such contaminants, as well as ions, can have a hannful influence on tire film preparation. In general, all chemicals and materials used in tire film preparation have to be extremely pure and clean. 

Apart from fatty acids, straight-chain molecules containing other hydrophilic end groups have been employed in numerous studies. In order to stabilize LB films chemical entities such as tlie alcohol group and tlie metliyl ester group have been introduced, botli of which are less hydrophilic tlian carboxylic acids and are largely unaffected by tlie pH of tlie subphase. 

New factors for tlie establislmient of multilayer stmctures are, for example, tire replacement of tire hydrocarbon chain by a perfluorinated chain and tire use of a subphase containing multivalent ions [29]. The latter can become incoriDorated into an LB film during deposition. The amount depends on tire pH of tire subphase and tire individual ion. The replacement of tire hydrocarbon by a rodlike fluorocarbon chain is one way to increase van der Waals interaction and tlierefore enlrance order and stability in molecular assemblies [431. 

If these materials are deposited as LB multilayers, polymerization can be induced either by thennal or optical means. This subject has been intensively studied [95, 96, 92, 98 and 99]- Since parameters such as m, subphase components, pH and polymerization before and after dipping, as well as temperature and wavelength employed for polymerization can be varied, the literature on diacetylenes is extensive and the reader is referred for example to the book of Tredgold [1001. 

As the barrier moves, the molecules are compressed, the intermolecular distance decreases, the surface pressure increases, and a phase transition may be observed in the isotherm. These phase transitions, characterized by a break in the isotherm, may vary with the subphase pH, and temperature. The first-phase transition, in Figure 2, is assigned to a transition from the gas to the Hquid state, also known as the Hquid-expanded, LE, state. In the Hquid... 

The last phase transition is to the soHd state, where molecules have both positional and orientational order. If further pressure is appHed on the monolayer, it collapses, owiag to mechanical iastabiHty and a sharp decrease ia the pressure is observed. This coUapse-pressure depends on the temperature, the pH of the subphase, and the speed with which the barrier is moved. 

FIG. 11 Force profiles between poly(glutamic acid), 2C18PLGA(44), brushes in water (a) at pH = 3.0 (HNO3), (b) at pH 10 (KOH) 1/k represents the decay length of the double-layer force. The brush layers were deposited at tt = 40 mN/m from the water subphase at pH = 3.0 and 10, respectively. 

Figure 11a shows a force-distance profile measnred for poly(L-glutamic acid) brushes (2C18PLGA(44)) in water (pH = 3.0, 10 M HNO3) deposited at 40 mN/m from the water subphase at pH = 3.0. The majority of peptides are in the forms of an a-helix (38% determined from the amide I band) and a random coil. Two major regions are clearly seen in... 

C, 0.25 nm molecule in the coexistence region between the liquid-expanded and the liquid-condensed (L2) phases (b) BAM image of stearic acid at 22°C, 0.60 nm molecule in the coexistence region between the gas (G) and the hquid-condensed (L2) phases. In each of these images, the polarizer angle has been set to 60°. The subphase is milh-Q water acidified to pH 1.8 with HCl. The scale bar in the lower left of each image is 450 p,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. 

FIG. 9 Silver nanoparticles capped by 4-carboxythiophenol electrostatically adsorbed to positively charged octadecylamine monolayers, (a) Mass uptake versus number of layers at subphase pH 12 and pH 9 the inset shows the contact angle of water versus the number of layers, (b) Absorbance spectra as a function of the number of layers transferred (left), with the inset showing the plasmon absorbance at 460 nm versus the number of layers. Thickness versus number of layers as determined by optical interferometry is shown on the right. (Reprinted with permission from Ref. 103. Copyright 1996 American Chemical Society.)... 

In the same year, Fulda and Tieke [75] reported on Langmuir films of monodisperse, 0.5-pm spherical polymer particles with hydrophobic polystyrene cores and hydrophilic shells containing polyacrylic acid or polyacrylamide. Measurement of ir-A curves and scanning electron microscopy (SEM) were used to determine the structure of the monolayers. In subsequent work, Fulda et al. [76] studied a variety of particles with different hydrophilic shells for their ability to form Langmuir films. Fulda and Tieke [77] investigated the influence of subphase conditions (pH, ionic strength) on monolayer formation of cationic and anionic particles as well as the structure of films made from bidisperse mixtures of anionic latex particles. 

In 1997, a Chinese research group [78] used the colloidal solution of 70-nm-sized carboxylated latex particles as a subphase and spread mixtures of cationic and other surfactants at the air-solution interface. If the pH was sufficiently low (1.5-3.0), the electrostatic interaction between the polar headgroups of the monolayer and the surface groups of the latex particles was strong enough to attract the latex to the surface. A fairly densely packed array of particles could be obtained if a 2 1 mixture of octadecylamine and stearic acid was spread at the interface. The particle films could be transferred onto solid substrates using the LB technique. The structure was studied using transmission electron microscopy. 

Salt addition to the subphase has a strong influence on monolayer formation, too. The effect of salt was studied by spreading particles la on an aqueous KCl solution of different salt concentration, with the pH of the subphase always being 5. If no salt is present at pH 5, the particles simply disappear into the subphase, as discussed earlier. However, the presence of salt causes the metal ions to penetrate the particle shell and shield the ionic groups electrostatically. Consequently, the particles become less hydrophilic and monolayer formation is improved, as indicated by the larger value of Aq. As shown in Figure 6a, a KCl concentration of 10 moles is sufficient to cause formation of a stable particle layer even at pH 5. 

One may conclude that a particle monolayer is actually formed at each pH value of the subphase, only if care is taken that the sum concentration of protons and metal (e.g., potassium) ions in the subphase, Ch + Ck, is constant. This is actually true, as shown in Figure 7, where the Aq-value of a monolayer of particles la is plotted against the pH of the subphase at constant sum concentration of H and of 1 mol 1 [77]. 

Monolayers of cationic particles 2 show an analogous dependence on the salt concentration of the subphase (Fig. 6b). If particles 2 are spread on a neutral subphase without any salt present, they mainly disappear into the subphase due to the large hydrophiUcity of the shell. However, if KCl is added, electrostatic shielding of the alkylammonium groups by the chloride ions sets in, the hydrophilicity of the particle shell is diminished, and a stable monolayer is obtained. Different from particles la, the pH of the subphase has no direct... 

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... 

Generally, the recrystaUization of S-layer protein into coherent monolayer on phospholipid films was demonstrated to depend on (1) the phase state of the hpid film, (2) the nature of the lipid head group (size, polarity, and charge), and (3) the ionic content and pH of the subphase [122,138] (Table 6). 

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