1. INTRODUCTION:
Surfactants place a special role in modern day to day life and technological applications1,2. The properties of surfactants solutions largely depend upon the nature and amount of additives. Although the solubilization of additives in micelles makes the micellar system more complicated than the binary system, it provides an additional opportunity to explore micelle structure and micellar solution properties in terms of the interaction between the micelles and the additives 3. Among the various properties of crown ethers, their unique chemical architecture plays a prominent role and opens the way to practical design of host molecules for selective complexation of various metal ions 4. However the rational design of new formulations requires a good knowledge of the encapsulation process. Structural information, such as the stoichiometry and the geometry of the complex, and thermodynamic information of binding, are necessary to draw a complete picture of the driving forces governing the crown ether- surfactant interactions.
We report in the present paper the results of UV, surface tension, conductance, density and ultrasonic velocity studies of the solution of 15-crown-5-ether (CE) solubilized in aqueous Sodium dodecyl sulphate (SDS) solutions as a function of SDS concentration.
The main aim is to study the host- guest encapsulation, if any, and the resultant change in the apparent behavior of the inclusion complexes.
2. EXPERIMENTAL:
2.1. Materials:
15-crown-5-ether (CE) and Sodium dodecyl sulfate (SDS) having purity greater than 98%
were purchased from Merck. All the chemicals
were of analytical grade and were used without further purification. Water used
for the preparation of samples was deionized and triply distilled (conductivity
lower than 3
S).
2.2. Methods:
The solutions were prepared by weight using an electronic balance with an accuracy of ±1×10-4 g. For binary SDS/W system and the ternary SDS/CE/W systems, the concentration of the surfactant was varied from 3-15mM and CE concentration was varied from 1-6mM. In each measurement concentration of crown ether was kept constant.
The UV-visible spectra were recorded with JASCO V-530 spectrophotometer using quartz cells. Surface tension measurements were done at 15.0 ± 0.002°C using Kruss processor tensiometer with an accuracy of 0.01 mNm-1. The conductivity of mixtures was measured in a thermostatic glass cell with two platinum electrodes and Pico conductivity meter from Lab India. The conductivity meter was calibrated by measuring the conductivity of the solutions of potassium chloride of different concentrations (0.001, 0.01 and 0.1 M). Electrodes were inserted in a double walled glass cell containing the solution. The glass cell was connected to the thermostat controlled to better than ±0.01K temperature variation. The cell constant of the cell used was 1.01 cm-1. The measurement of conductivity was carried out with an absolute accuracy up to ±3%. The solution densities and ultrasound velocities were determined at 15.0 ± 0.002 °C with an oscillating-tube densimeter (Anton Paar, DSA 5000).
3. RESULTS AND DISCUSSION:
The understanding of surfactant-crown ether interactions is important not only for its own sake but also to grasp the inter-moiety interactions. While dealing with the macrocycle-surfactant solution, a certain type of association can be observed between the micelles and the macrocyclic cavity in water (4, 5). The nature of such association results in the formation of inclusion complexes. In order to study such novel systems, aqueous solutions of the binary SDS/W system and the ternary SDS/CE/W systems have been characterized through Spectroscopic and physico-chemical measurements.
3.1. UV-Visible Spectroscopic measurements:
Spectroscopic analysis of inclusion complexes of 15-crown-5-ether (CE) with Sodium dodecyl sulphate (SDS) has been investigated as a function of concentration of surfactants and CE at room temperature. It can be observed from Fig.1a that the spectra for binary SDS/W system shows only one characteristic peak centered at lmax = 260 nm, with absorbance values increasing with surfactant concentration, following a typical Lambert – Beer behavior. The spectra were then analyzed in presence of crown ether. Fig. 2 depicts the UV-Vis spectra of ternary SDS/CE/W system. There are no significant shifts in the characteristic peaks of surfactant by addition of CE indicating the absence of any strong bonding between surfactant molecule and CE. The molar absorptivity coefficients ‘e’ for all the systems, were estimated from the linear regression of the values of the absorbance at the lmax as a function of surfactant concentration and the values are tabulated in Table 1. There is an increase in the magnitude of molar absorptivity coefficient ‘e’ in all ternary SDS /CE/W systems with increase of concentration of CE (fig.3).
Table. 1. Estimated molar absorptivity coefficients
|
[CE] (M) |
|
|
0 |
4.27 |
|
0.001 |
4.69 |
|
0.002 |
5.04 |
|
0.004 |
5.15 |
|
0.006 |
5.30 |
Table.2. Concentration variation of β, and DG°m at 15oC
|
[CE] (M) |
CMC(mol kg-1) |
b |
DG°m (kJ mol-1) |
|
0 |
0.00854 |
0.52 |
-32.76 |
|
0.001 |
0.00779 |
0.49 |
-33.77 |
|
0.002 |
0.00742 |
0.46 |
-34.63 |
|
0.004 |
0.00714 |
0.44 |
-35.23 |
|
0.006 |
0.00681 |
0.42 |
-35.87 |
Fig.1: UV-visible spectra of binary SDS/W system.
The above observations confirm the interaction between surfactant and CE so as to affect the micellization process. This aspect has been investigated using the physico-chemical measurements for the studied systems.
3.2 Surface/interfacial tension studies:
The CMC values were obtained through a conventional plot of the surface/interfacial tension versus the surfactant concentration. The CMC concentration corresponds to the point where the surfactant first shows the lowest surface/interfacial tension. The surface/interfacial tension remains relatively constant after this point 6-8. The plots of surface tension (g) vs. log mSDS of aqueous solutions of the studied systems at 15oC are shown in Figure 4b. As shown in Figures 4a and 4b, surface/interfacial tension is concentration dependent. As the surfactant concentration increases, surface/interfacial tension decreases until the surfactant CMC value is reached and remains relatively constant there afterwards. It was found that the presence of CE in SDS/W systems can depress the surface tension. This phenomenon shows that there are host- guest interactions with the hydrophilic portion of the surfactants.
3.3 Conductivity measurements:
Conductivity technique has been
found to be highly useful for studying the association behavior of various
systems 8-14. In order to gain insight into the micellar systems,
the conductivity was measured as a function of [surfactant] at 15oC.
Fig. 5 depicts the behavior of conductivity 
for both the binary SDS/W
system and the ternary SDS/CE/W systems. On comparing the conductivity of
surfactant in water and in the presence of CE (Fig. 5), it is found that there
is decrease in the conductivity when CE is added which points to the
interaction of surfactant with CE moiety, because the mobility of associated
surfactant is expected to be less than that of the free surfactant.