Monolayers states

Prior to LB transfer, the surface pressure - molecular area (n-A) isotherms of dialkylsilane under various pH and temperature conditions were investigated. The pH condition of the subphase (water phase under the monolayer) is a crucial factor for the monolayer state. The condensed phase was formed directly without formation... 

In order to investigate the phase transition in the monolayer state, the temperature dependence of the Jt-A isotherm was measured at pH 2. The molecular area at 20 mN rn 1, which is the pressure for the LB transfer of the polymerized monolayer, is plotted as a function of temperature (Figure 2.6). Thermal expansion obviously changes at around 45 °C, indicating that the polymerized monolayer forms a disordered phase above this temperature. The observed temperature (45 °C) can be regarded as the phase transition point from the crystalline phase to the liquid crystalline phase of the polymerized organosilane monolayer. 

Given the information above, the question remains as to the nature of the monolayer states responsible for the stereo-differentiation of surface properties in racemic and enantiomeric films. Although associations in the crystalline phases are clearly differentiated by stereochemical packing, and therefore reflected in the thermodynamic and physical properties of the crystals, there is no indication that the same differentiations occur in a highly ordered, two-dimensional array of molecules on a water surface. However, it will be seen below (pp. 107-127) that conformational forces that are readily apparent in X-ray and molecular models for several diastereomeric surfactants provide a solid basis for interpreting their monolayer behavior. 

All of the experiments in pure and mixed SSME systems, as well as in the Af-stearoyltyrosine systems, have one common feature, which seems characteristic of chiral molecular recognition in enantiomeric systems and their mixtures enantiomeric discrimination as reflected by monolayer dynamic and equilibrium properties has only been detected when either the racemic or enantiomeric systems have reverted to a tightly packed, presumably quasi-crystalline surface state. Thus far it has not been possible to detect clear enantiomeric discrimination in any fluid or gaseous monolayer state. 

The stereochemically directed conformation of the FE isomer affords stronger intermolecular associations. The behavior of the molecules in a monolayer film is relatively insensitive to the temperature of the subphase over the same range of temperatures where the SE isomer is quite sensitive. It is seen from Table 14 that the FE isomer spread less spontaneously from the bulk crystal to a monolayer state. This is an indication that associations in the bulk crystal are stronger for the FE isomer than for the SE. In addition, the entropy of spreading is lower for the FE isomer, indicating a more ordered and conversely a more tightly packed film or a less ordered crystal. 

Fig. 6. Variation of Tm with alkyl chain length for (1) fatty acids in a monolayer state, (2) fatty acids in a three-dimensional crystalline state, and (3) normal paraffins in a three-dimensional crystalline state.

Since surface pressure is a free energy term, the energies and entropies of first-order phase transitions in the monolayer state may be calculated from the temperature dependence of the ir-A curve using the two-dimensional analog of the Clausius-Clapeyron equation (59), where AH is the molar enthalpy change at temperature T and AA is the net change in molar area ... 

As remarked previously, the physical state of a monolayer depends on the lateral cohesive forces between the constituent molecules. By a suitable choice of chain length and temperature, straight-chain fatty acids and alcohols, etc., can be made to exhibit the various monolayer states, one CH2 group being equivalent to a temperature change of c. 5-8 K. 

Frequency of the crystal after addition of the DNP-Ab to the dinitrospiropyran (31a)-functionalized crystal. (O) Frequency of the crystal after photoisomerization of the (31a)/DNP-Ab monolayer to the protonated dinitromerocyanine (31b), washing offof the DNP-Ab, and back isomerization to (31a) monolayer state. 

Oxides or salts in a monolayer state and in their crystalline state behave differently in many respects. Effects of monolayer dispersion show up in spectra as well as in the properties of the oxides and salts. 

Continuing to use PVAc as the canonical example of a stable and easily reproducible polymer monolayer, we show how the two quantities, the static elasticity es from 77-A the isotherm, and the corresponding ej deduced from the SLS experiment, compare and contrast with each other for the time being, we defer to later the SLS results. This is shown in Fig. 12. Agreement between the two is remarkable up to respective maximum points. The observed deviation at higher 77 is not expected since the monolayer state is no longer maintained, hence the static elastic responses in macroscopic scales are not likely to be the same as the dynamic response to spontaneous capillary waves. 

Fig. 14 A/SjC>eq vs. /s>eq for PEO, PTFiF, PMA and PVAc up to es>max for all four polymers, with the symbols identical to those in Fig. 13 (A). The same plots for PMMA and PtBMA are shown in (B), where the open symbols stand for 17 < 2 mN nr1 and the filled symbols for n > 2 mN nr1. The solid and dashed curves are the same as in Fig. 4, and the surface pressure increases counterclockwise, starting from 77 = 0, Limit I, in Fig. 4. PMMA shows a discontinuous change with can be explained by the coalescence of PMMA patches existing as a heterogeneous film prior to the monolayer state. Error bars, not shown for clarity, are 0.5% and 5% for/s>eq and A/SjC>eq, respectively...

Finally the behavior of racemic poly(c-benzyloxycarbonyl lysine) in the monolayer state is particularly straightforward the area per residue... 

Figure 27. (A) Cyclic voltammetric response of Cyt c (0.1 mM) at a (20)-pyridine-niodified electrode in (a) the neutral state (20a) and (b) the positively charged merocyanine state (20b), recorded at 50 mV s . Inset switching behaviour of the Cyt c peak current as a function of the monolayer state. (B) Cyclic voltammetric response of Cyt c (0.1 mM) with COx (1 pM) at a (20)-pyridine modified electrode in the presence of O2 in (a) in the neutral state (20a) and (b) the cationic merocyanine state (20b). Inset switching behavior of the electrode in the presence of O2. All experiments were performed in 0.1 M phosphate buffer, pH 7.0.

Malcolm, B. R., and O. Pieroni, O. The Photoresponse of an Azobenzene-Containing Poly(L-Lysine) in the Monolayer State. Biopolymers 29, 1121 (1990). 

Curves B and C in Fig. 1 are plotted for the column packings composed of silica gel covered with monolayers characterised by the surface area per molecule 0.215 and 0.245 nm. Thus, the monolayer state is intermediate between SC and LE. Only one maximum, at 81°C, occurs in these diagrams and the value of this maximum decreases with the decrease of the surface concentration of n-octadecanol molecules. This fact confirms the origin of the maximum at 81°C as well as the mechanism of the SC - LE phase transition. 

Figure 1. Upper Schematic of it-A isotherms in the transition region F-F represents region where bulk of film material is in the condensed monolayer state G-G represents the region where virtually all of the film is in the gaseous monolayer state. Area per molecule is the total area occupied by the sum of all lipid molecules in the surface.

Chemical Activities by the Surface Vapor Pressure Method. Surface pressure measurements in the transition region between the condensed and gaseous monolayer states of a single lipid component spread as a monolayer on water yield a value of ir which is independent of the surface area. This value—the surface vapor pressure, irv—is analogous to the vapor pressure of a liquid in equilibrium with its vapor. When a second lipid component is in the surface, the limits of miscibility in the condensed phase may be determined on the basis of the surface vapor pressure dependence on the mole fraction in the condensed phase (8). 

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