Mixed Monolayer Of Macromolecule And Small Molecule At Air/Water Interface

Adsorption monolayer, spreading monolayer and mixed monolayer of macromolecule and small molecule at air/water interface were studied. The model includes two kinds of amphiphilic macromolecules and low molecular weight substances. Amphiphilic macromolecules include sericin and polyvinyl pyrrolidone(PVP), while low molecular weight substances include fatty acid (stearic acid, nervonic acid), lecithin, lycopene and OP-20. The dynamic adsorption of sericin solution and desorption of spreading OP-20 monolayer have been researched. On the other hand, the mixed monolayer of lycopene + stearic acid, nervonic + PVP and nervonic acid + lecithin have been studied to interpret their interaction.Firstly, the dynamic surface tension of sericin solutions at a given concentration was measured by Wilhelmy plate method at 16℃. Some curves were be plotted according to the dynamic surface tension (γ_t) data in different pH, such as the curve of surface pressureÏ€versus adsorption time t, the curve about relationship between surface pressure and time(π～logt), and the curve of ln(dÏ€/dt)～π. The curves were analyzed by using some experiential expressions to elucidate the dynamic adsorption behavior, through which the adsorption parameters were deduced of adsorption half-time (t_1/2, the time required for surface tension decreasing to the half of the equilibrium value), the constant controlling adsorption mode (n), the mean molecule area of the molecule occupied at the interface both in initial penetrated and anchored state (â–³A₁) and in rearranged and reoriented state (â–³A₂). The results indicated that the adsorption process of sericin protein is attributed to a model of diffusion-controlled adsorption kinetics. The conformational transformation of sericin protein molecules adsorbed at the surface undergoes two dynamic steps. The first is initial penetration and anchoring of the molecule from aqueous solution to the surface, and the other is the rearrangement and reorientation of the adsorbed molecule at the interface. The pH of sericin solution has influences on the dynamic surface tension and the adsorbed molecule area. When pH of sericin solution is smaller than the isoelectric point of sericin protein,â–³A₁ andâ–³A₂ are smaller along with higher the dynamic surface tension and slower the adsorption rate, in comparison with those in the pHs higher than the isoelectric point.Second, the dynamic desorption process of poly(ethylene oxide)alkyl ether spreading monolayer was measured using Wilhelmy plate technique at air-water interface. The calculated desorption diffusion coefficients is about 10^-3～10^-4cm²/sec, which are much higher than the diffusion coefficients of adsorption at the solution surface. The desorption rate constants show that the desorption for spreading monolayer of C₁₈H₃₇O(CH₂CH₂O)₂₀H is controlled by diffusion step at the initial time, and it is controlled by a mixed mechanism including diffusion controlling and surface energy barrier controlling.Third, theÏ€-A isotherm of lycopene at air-water interface did not show characteristics of solid-film even for the molecular area up to 0.1nm2, for lack of obvious polar group in its molecule. After mixed with stearic acid, a kind of stable monolayers could be available, but appeared an unusual phase-separation phenomenon under surface pressures over 45 mN m^-1. For the mixed film, it was shown experimentally that above the collapse point of the surface pressure a plateau region was appeared in theÏ€- A isotherms, and then the surface pressure went up with the decrease of the molecular area. The result was suggested that lycopene molecules were pushed out from the mixed monolayer over a higher surface pressure, gradually forming a multilayer with a pattern that lycopene molecules tended to randomly lie down on the stearic acid monolayer. On the other hand, the characterization of their mixing behaviour has been assessed by thermodynamic additivity rule. The additivity of the molecule area of mixed monolayers at different pressures showed a positive deviation in low molar fraction of lycopene in mixture, and a negative deviation in high ratio to its ideal valules. The mixing energies and interaction parameters at different surface pressures were calculated.Fourth, surface pressure - mean molecule area curves of nervonic and polyvinyl pyrrolidone (PVP) at different molar fraction (0～1.0) were measured and analyzed. When the molar fraction of nervonic acid in mixed monolayer was at the range of 0.1～0.5, the position ofÏ€-A curve gradually transfered from the cuve of pure PVP to the curve of pure nervonic acid at the middle place. When the molar fraction of nervonic acid at mixed monolayers was from 0.6 to 0.9, the position ofÏ€-A curve gradually transfered from top to middle position. According to theseÏ€-A curves, with the increase of the molar fraction of nervonic acid, the limitted molecule area of mixed monolayers gradually decreased, and the flat of curve varied from short to long, all this can explain that the phases of thermodynamic transit would reduce.Based on the addition rule about mean molecule area in a mixed monolayer, the molecular interaction of nervonic acid and PVP with different molar fraction in mixed monolayers were analysed. The experimental curve appeared positive deviation comparing to ideal curve calculated by the formula. The positive deviation indicated that there was a repulsion force among the different molecules. The repulsion force was little in the range of 0.1～0.5, is with a maximum at 0.5, and is moderate in the molar ratio of 0.6～0.9, which leading the mixed monolayer to expand. On the other hand, in the range of 0.1-0.9 molar fraction, the surface pressure at collapse appeared positive deviation, which can explain that the repulsion in the mixed monolayer is very greater. Moreover, the curve of mixing energies AG,, at different surface compressures vs molar fraction of nervonic acid appeared positive deviation comparing to ideal mixing surplus energies AG_M¹ curve. The interaction parameter (a) and interaction power (â–³h) can account for above phenomena.Static elasticity of nervonic monolayer, PVP and their mixed monolayer were measured by varying molar fraction of nervonic acid in mixed monolayer at air /water interface. There were some special state transformations for nervonic acid monolayer, including that from gas state to liquid expanding state, and from liquid condensed state to solid state, which showed a long fiat region for theÏ€-A curve with a maximum of compressibility and static elasticity in the liquid expanding state. However, the region of gas state atÏ€-A curve for the PVP monolayer is the longest, and compressibility and static elasticity of solid state is the largest in these states. Moreover, two kinds of mixed monolayers with nervonic acid molar fraction respectively 0.2 and 0.8 were measured. TheÏ€-A curve of mixed monolayer with 0.8 molar fraction was similar to that of pure nervonic, representing an additive property for nervonic acid and PVP, and the flat region is much longer than pure nervonic acid. The compressible degree of liquid condensed state was the largest at these states including gas state and liquid expanding state. The mixed monolayer from liquid expanding state to solid state have the best static elasticity. However, theÏ€-A curve of mixed monolayer with 0.2 molar fraction was similar to pure PVP, which have obvious additive property, longer gas state region, and the best compress degree and static elasticity at solid state.Fifth, surface pressure-mean molecule area curves of nervonic and lecthin at different molar fractions (0～1.0) were measured and analyzed. As the surface pressure increased and approached collapse pressure, nervonic acid molecules were extruded from mixed monolayers, floating over the lecthin monolayer and forming a multilayer. The mean molecular area of mixed monolayers in the experiment is smaller than that of idealÏ€-A curve at the range of 0.2～0.8 molar ratio. However, the experimental data at 0.1 or 0.9 molar ratio is bigger than that of ideal data. The positive deviation indicated that repulsion exists among different molecules in mixed monolayer, and negative deviation indicated that an attraction exists among molecules. The rule could explain that attraction of hydrophobic groups among molecules resulring in a condensed state, and that the mean molecular area may be smaller than ideal mixed area in the range from 0.2 to 0.8. But at 0.1 or 0.9 molar ratio, the repulsion between nervonic acid and lecthin lead the mixed monolayer expanding state, and mean molecular area in the mixed monolayer was larger than that of ideal data. On the other hand, collapse pressure of the mixed monolayer was positive deviation from theideal data at all the range of molar fraction.Moreover, the curve of mixing energiesâ–³G_m at different surface compressure vs molar fraction of nervonic acid appeared a positive deviation comparing to the ideal mixing surplus energiesâ–³G_M¹ curve, which indicated a phase separation phenomena in the mixed monolayer. The interaction parameter (a) and interaction power (â–³h) could be used for accounting for this phenomena with the separated phases.