| The adsorption behavior of a functional material, i. e. sodium nervonic acid at the water-air interface was investigated in terms of both thermodynamics and kinetics. In addition, the interaction between the sodium nervonic acid and -cyclodextrin CD( -CD) was studied in this paper.By the way of the ring method, the equilibrium surface tension was measured for the aqueous solution of sodium nervonic acid at different bulk concentrations. The curve of -lgC between the equilibrium surface tension and the solution concentration of sodium nervonic acid was the typical one of the ordinary surfactants. From this curve, we could find its cmc, which is used to describe for sodium nervonic acid having the ability to decrease the interfacial tension as low as 26.6mN/m. At the same time the other two important parameters could be obtained, including the value of pC20 and the value of CMC, the former represented that the efficiency of decreasing interfacial tension for sodium nervonic acid is 4.96, and the latter, i. e. the critical micelle concentration of sodium nervonic acid, is 6.76 10-5mol/L. All these data show that the sodium nervonic acid has a distinct surface activity. Its Y -lgC curve was also analyzed by the the classic Gibbs adsorption equation. It was shown in the experimental result that the extreme adsorptive capacity ( ) is 4.16 10-10mol/cm2 and the molecule area of extreme adsorption (Am) is 24.0 nm2. It also was found that the Kraffte temperature of sodium nervonic acid is in the temperature range from 55C to 58C. At the certain range of concentration of Na+, the increase of its concentration will decrease the value of CMC of sodium nervonic acid in water and increase its ability and efficiency ofdecreasing surface tension.The theoretical base for the drop volume method was studied for the case of the liquid/liquid interface. First of all, we should identify clearly the difference between the real radius of the liquid drop and that of the capillary tube used in experiments. In order to measure accurately the surface tension for the aqueous solution of a surfactant, it is necessary to make the equality of both radii.By the drop volume method and the Wilhelmy plate, the curve between the dynamic surface tension and the adsorptive time at the water-air interface ( t-t curve) was obtained experimentally. In the later stage of adsorption, by the approximate expression of the traditional equation, i. e. Word-Tordai adsorption equation, which means that there is a liner relationship between y and t-1.2, the adsorption kinetics of sodium nervonic acid at the water-air interface is measured. It could reduce the other important parameter, i. e. the diffusion coefficient, which is used to describe the adsorption kinetics for molecules of sodium nervonic acid at the water-air interface. The diffusion coefficient for that case was in the range from 10~7 to 1CT8 (cm2/s) . Through further studies on the relationship between the diffusion coefficient and the concentration for the aqueous solution of sodium nervonic acid, it was concluded that there was a liner relationship between the diffusion coefficient and the concentration in the log-log plot.The influence of -CD on the adsorption behavior at the water-air interface was also studied in this work. With increasing the concentration of -CD in the aqueous solution of sodium nervonic acid, the value of the equilibrium surface tension increase until achieving the interfacial tension of pure water at last. With increasing the concentration of -CD in the aqueous solution of sodium nervonic acid, the necessary time for the dynamic surface tension achieving the equilibrium becomes shorter and the diffusion coefficient at the stage of later adsorption larger, which means that - CD could increase the adsorption rate of sodium nervonic acid at the water-air interface.
|
PDF Full Text Request