Interfacial tension sodium oleate

As the interfacial tension is directly related to the concentration of surfactant molecules at the interface [46], the rhythmic changes in the interfacial tension suggest that the concentration of surfactant, sodium oleate in Figure 22A and DPPE in Figure 22B, at the oil/water interface changes repeatedly between high and low values. 

Precision of Measurements. Aliquots from a stock solution of 0.1 M sodium oleate (five months old) were used to prepare aqueous test solutions that were 0.01 M in sodium oleate and 0.1 M in sodium chloride pH 9 5 Interfacial tensions were measured against n-undecane without pre-equilibration. The second solution was made and measured one week after the first and the third solution two weeks after the first. The results in Table I... 

Precision of Interfacial Tension Measurements 0.01 M Sodium Oleate, 0.1 M Sodium Chloride pH 9 5 vs n-Undecane... 

Effect of Surfactant Concentration. Figure 3 compares results of alkane scans for three concentrations of sodium oleate at constant sodium chloride concentration and pH. The 0.002 M solution is derived from 95% oleic acid, the 0.01 M and the 0.1 M solutions are derived from 99% oleic acid. Both the magnitude and the alkane position of minimum interfacial tension (r = 11) are essentially concentration independent under these conditions. Wade, et al (12) reported a similar invariance in nm with a pure alkyl benzene sulfonate, although there was more change in the minimum value of interfacial tension with the sulfonate concentration than is observed with the carboxylate. The interfacial tension at for 0.01 M sodium oleate is in the range of the values of Table I. Very high interfacial tension (> 10 dynes/cm) was found at 0.0001 M sodium oleate in 0.1 M sodium chloride. 

Effect of Sodium Chloride Concentration. Figure b compares interfacial tensions of several different surfactant concentrations verses n-undecane in the presence of 0.1 M sodium chloride with values obtained without salt. Salt reduces the interfacial tension at all surfactant concentrations. Aqueous potassium oleate has a critical micelle concentration of 0.001 M (13). It could be inferred from Figure b that 0.001 M sodium oleate with no added salt is below the cmc, because of the high interfacial tension. If so, the much lower interfacial tension in the presence of 0.1 M sodium chloride stems from reduction of the cmc expected in the presence of added salt (lb). 

In Figure b9 interfacial tensions at 0.01 and 0.1 M sodium oleate and 0.1 M sodium chloride are higher than in Figure 3. However, the desired surfactant concentration in Figure U was obtained by dissolving the required quantity of sodium oleate and pH was not controlled. Data from Figure b are not directly comparable to Figure 3 for this reason. 

Figure 5 summarizes the results for two concentrations of sodium oleate of varying the pH on interfacial tension. One trend is that the lower the solution pH the lower the interfacial tension. Harkins et al (l6 ) have demonstrated that the interfacial tension between benzene and aqueous sodium oleate is lower when the benzene layer contains oleic acid. Since the partition coefficient of oleic acid between alkane and aqueous sodium oleate greatly favors the alkane phase (17), any oleic acid from hydrolysis would tend to partition into the alkane phase. Thus lowering the solution pH would consequently increase the concentration of oleic acid in the alkane phase and bring about lower interfacial tension. 

In Table III the results of adding excess acid to sodium oleate solutions are given. These data are not directly comparable to Figure 5 since the aqueous phase contains cosolvent and the solutions were preequilibrated before testing. However, the trend to lower interfacial tension with increasing additions of oleic acid (and thus lower pH) is parallel to Figure 5 ... 

Effect of Unsaturation. Alkane scans for C g carboxylic acid salts are compared in Figure 6. The alkane position of minimum interfacial tension (nm) increases with the degree of unsaturation. Increasing interfacial tensions are found in the order oleate, linolenate, linoleate. Sodium stereate, the corresponding... 

Determination of the critical micellar concentration (c.m.c.) of sodium oleate Use a suitable available method for determining the surface tension of the oleate solutions already prepared. Some modem instruments have facilities for the determination of surface tension, interfacial tension and c.m.c. Plot the surface tension against molar oleate concentration and locate the concentration beyond which the surface tension does not change significantly. 

The cmc of sodium oleate was lO M at 25 C using surface tension/concentration plots where there is an intersection between a rapidly decreasing nearly linear part and the slightly decreasing linear part. The ring method was used for the measurements. Modem Tensiometers are automated to measure surface or interfacial tension as well as the cmc. See also NSRDS, NBS.36, National Bureau of Standards, U.S. Government Office, Washington, 1971. 

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