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Wissenschaftliche Studie, 2011
The micellar properties of cationic dodecyltrimethylammonium chloride (DTAC) in aqueous media in the presence of 15-crown-5ether (CR) have been investigated by conductivity measurements over the temperature range 288.15-308.15 K. The results of the ternary DTAC/CR/W system were analysed in comparison with the reported results of binary DTAC/W system. The critical aggregation concentration (cac) and degree of ionization (Abbildung in dieser Leseprobe nicht enthalten) of the micelles were determined from the conductivity measurements at different temperatures. Thermodynamic parameters (Abbildung in dieser Leseprobe nicht enthalten, Abbildung in dieser Leseprobe nicht enthalten andAbbildung in dieser Leseprobe nicht enthalten) for the micellar system were estimated by applying the charged pseudo-phase separation model. Micellisation was found to be spontaneous and entropy-driven.
Surfactant organized assemblies have great potential applications in day to day life . Surfactants have micellar properties, which are effected by addition of small amount of electrolytes, non-polar and polar organic compounds. The critical aggregation concentration (cac) results from the hydrophobic interactions between the non- polar parts, which forms the core of the micelles and a repulsion interaction between the polar head group. During the last decades, the study of the behavior of ionic surfactants has received much attention. The effect of different kinds of additives including macrocycles, on the micellisation has also been widely studied. In some applications, macrocyclic compounds are used along with the surfactants due to counterion complexation. Crown ethers are a versatile class of macrocyclic ligands. The property of complex formation along with the selectivity shown by crown ethers towards cations distinguishes them from most non-cyclic ligands. The formation of complexes between the crown ether cavity and the counterions is expected to lead to significant alterations of the micellar properties. However, not much investigation is reported in this field [2-16].
In the present work, efforts have been made to study the effect of addition of crown ether (15-crown-5) with wider temperature variations on the cac of aqueous solutions of cationic dodecyltrimethylammonium chloride (DTAC) using conductivity technique. In addition, the thermodynamic parameters,Abbildung in dieser Leseprobe nicht enthalten, Abbildung in dieser Leseprobe nicht enthaltenand Abbildung in dieser Leseprobe nicht enthalten have also been estimated and analyzed in aqueous media.
The Crown ether (15-crown-5) from Fluka (purity greater than 99%) was used as received. Dodecyltrimethylammonium chloride (DTAC) (purity greater than 99%) was purchased from Sigma and was recrystallised from ethanol- ethyl acetate mixtures. It was dried in a vacuum oven at 60oC for two days. Water used for the preparation of samples was deionized and triply distilled (conductivity lower than 3Abbildung in dieser Leseprobe nicht enthaltens).
The conductivity of ternary 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 (Merck, purity > 99%) of different concentrations (0.001 M, 0.01M and 0.1M). 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, read on Beckman thermometer set at the working temperature. The cell constant of the cell used was 1cm-1. The measurement of conductivity was carried out with an absolute accuracy up to ±3%. The solutions were prepared by weight using an analytical balance with an accuracy of Abbildung in dieser Leseprobe nicht enthalten1.10-4 g. The conductivity measurements were made at different temperatures viz 288.15-308.15 K for ternary DTAC/CR/W systems, as a function of surfactant concentration with fixed concentration of crown ether (0.01M).
Conductivity technique has been found to be a useful tool for studying the association behavior of various systems [17-25]. Ternary DTAC/CR/W systems have been characterized through conductivity measurements at various temperatures ( 288.15–308.15 K) in order to evaluate the critical aggregation concentration, cac and the degree of counterion dissociation,Abbildung in dieser Leseprobe nicht enthalten. The intersection of the two straight lines of the conductivity-concentration plots above and below the change in the slope gives the cac while the ratio between the slopes of the postmicellar region to that in the premicellar region gives the degree of counterion dissociation,Abbildung in dieser Leseprobe nicht enthalten [20-25].Figure 1 shows conductivity, Abbildung in dieser Leseprobe nicht enthalten, as a function of DTAC concentration for DTAC/CR/W solutions at a constant concentration of CR at different temperatures.
The conductivity plots versus concentration show a gradual increase in conductivity with increase in temperature. The temperature dependence values of cac for ternary solutions along with the reported values for binary systems are also shown in table 1. The values for the ternary systems when compared with that of binary systems indicate the ease in micellisation. Figure 2 shows the variance in cac with increase of temperature. There is a gradual decrease in cac in the temperature range investigated. This observed decrease may be probably due to decrease in the degree of hydration of the hydrophilic group with increase in the thermal energy of the molecular entities which favors the micellisation. The values of degree of counterion dissociation,Abbildung in dieser Leseprobe nicht enthalten are tabulated in table 1.
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