Conductivity Study of DTAB in Water and Ethanol-Water Mixture in the Presence and Absence of ZnSO4

The physicochemical behaviour of dodecyltrimethylammonium bromide (DTAB) in water and ethanol-water mixture in the presence and absence of ZnSO4 were studied by measuring the conductivity at 298.15 K. The pre-micellar (S1) and post-micellar slopes (S2) were obtained and calculated the degree of dissociation (α) and the critical micelle concentration (cmc). With an increase in ethanol content, the cmc and α of DTAB increased whereas, in the presence of ZnSO4, the cmc and α decreased. By using cmc and α, thermodynamic properties as the standard free energy of micellization (ΔGm ο ) were evaluated. With an increase in ethanol content, the negative values of ΔGm ο are decreased indicating less spontaneous in the micellization which makes micellization less favourable. The correlation of the pre-micellar and postmicellar slopes with the volume fraction of ethanol were discussed. DTAB micellization was tested in contexts of specific solvent parameters and solvophobic parameter.


Introduction:
Surfactants have a hydrophilic head and hydrophobic tail and are vigorously used in most of the areas of applications (1). In a given concentration, surfactants display dramatic differences in behaviors, named cmc, which implies the basic micelle ability of amphiphilic molecules to self-aggregate in the solutions. In contrast, the surfactant composition, the solubilizing media, the concentration of surfactants and the procedure used to create self-assemblies are all highly important for the creation of micelle (2). The cmc is primarily dependent upon its chemical composition; however, this may even influence temperature & various materials (mostly heavy electrolytes). The conductance measurement of surfactant solutions is the principal way of calculating cmc (3,4). The conductivity methods are very popular nowadays to calculate the cmc of cationic surfactant (5), anionic surfactant (6), nonionic surfactant (7) and even in zwitterionic surfactant (8).
The study on the effects of salts in cationic surfactants by conductivity methods is still useful for the researchers (9)(10)(11). Such types of research results play a very important role in a broad area of biological and physicochemical phenomena from the electrolyte effects and surfactant solutions to the transport across biological membranes and enzyme activities which are widely used for the public benefits (12). The micellar solutions of alkyltrimethylammonium bromide surfactants in the presence of KBr are widely used in various household, industrial and research applications (13).
There were some works on the effects of salts in DTAB from the different methods (13)(14)(15) study of the effect of salt on the cmc of DTAB in the aqueous medium (19) and found the cmc values of DTAB decline by adding tetra-alkylammonium bromide salts. The reason for decreasing the cmc values of DTAB on adding tetra-alkylammonium bromide salts may be due to the synergistic hydrophobic interactions between the non-polar hydrocarbon chains of DTAB and alkyl chains of tetra-alkylammonium bromide salts. The effects of sodium salts NaCl, NaH 2 PO 4 , Na 2 HPO 4 , Na 2 CO 3 , Na 2 SO 4 , Na 3 PO 4 and Na 3 Cit on the aggregation behavior of DTAB in aqueous solutions was studied and calculated the values of cmc as well as other parameters. The decrease of cmc was noticed by adding the salts and was found more pronounced for the anions with higher valency than that with lower valency (20). It was mentioned in the literature (14) that the conductance method had limitations to locate the cmc at increased NaBr concentration in comparison to the calorimetry method. Moreover, the cmc of DTAB decreased continuously with the addition of NaBr for both methods (21).
There is no conductivity study to calculate the cmc of DTAB in the presence of ZnSO 4 . Our work aims is to see the effects of ZnSO 4 on the micellisation of DTAB in the ethanol-water mixture from the conductivity methods which is the noble study to calculate the various parameters as S 1 , S 2 , α, cmc, ΔG m ο as well as the correlation of ΔG m ο with solvent parameters and solvophobic parameter.

Apparatus and chemicals
The digital conductivity meter systronics of cell constant 1.002 cm -1 was purchased from India. The digital balance was also purchased from India to get exact weights of different required experimental solutions of dodecyltrimethylammoniumbromide (DTAB) of molecular weight 308.3 g/mol. ZnSO 4 and DTAB (Figs.1 and 2) were purchased from SdFINE CHEM Ltd., India. Ethanol was obtained from E.Merck, India. The double-distilled water was taken for preparing the solvent composition of water and ethanol mixture. Double distilled water containing a fair conductivity under 10 -6 S/cm was utilized during the formulation of the mixture at 298.15 K. Double distilled water and 0.10 to 0.30 volume fraction containing ethanol were used to make 0.04 M dodecyltrimethylammonium bromide solution in the presence and absence of zinc sulphate.

Conductance measurement:
The specific conductance of dodecyltrimethylammonium bromide solution in the presence and absence of ZnSO4 was measured on the conductivity meter. The cell was calibrated with the help of aqueous KCl solution (0.1 Demal and 0.01 Demal) (22). Such type of calibration with the help of aqueous potassium chloride was also found in the literature (23).

Results and Discussion:
The specific conductance versus concentration can be shown if a molar conductance is replaced by the specific conductance (λ = κ/c). The cmc is the interaction point of two linear straight lines and can be calculated by solving two linear equations described simultaneously. The degree of dissociation (α) in the plot of the specific conductivity with the concentration of surfactant is derived from slopes over and under the cmc proportion: Where, S 1 is the slope of pre-cmc and S 2 is the slope of post-cmc (24)(25). In the ethanol-water mixture, both slopes (S 1 and S 2 ) decline along with the rise of ethanol.

Determination of cmc of DTAB in the absence of salt:
The conductance of dodecyltrimethylammonium bromide in pure water as well as in three separate mixtures of ethanol-water (comprising 0.10, 0.20 and 0.30 proportions of the volume of ethanol) for measuring the cmc at 298.15 K was determined (Fig. 3).  (Table 1), cmc and α increases with the addition of ethanol ( Table 2).

Determination of cmc of DTAB in the presence of 0.005 M salt:
The conductance of dodecyltrimethylammonium bromide in pure water as well as in three separate mixtures of ethanol-water in the presence of 0.005 M salt for measuring the critical micelle concentration at 298.15 K was determined (Fig. 4).  (Table 1) but higher values of S 1 and S 2 in comparison with the DTAB system.

Determination of cmc of DTAB in the presence of 0.01 M salt:
The conductance of dodecyltrimethylammonium bromide in pure water as well as in three separate mixtures of ethanol-water in the presence of 0.01M salt for measuring the critical micelle concentration at 298.15 K was determined (Fig. 5).  (Table 1) but higher values of S 1 and S 2 in comparison with DTAB in 0.005 M ZnSO4 system. The cmc and α increase with the addition of ethanol but lesser values of cmc and α in comparison with DTAB in the presence of 0.005M ZnSO4 and 0.01M ZnSO4 system ( Table 2).
The value of DTAB's degree of ionization 'α' is found to be higher in the presence of ethanol than in its absence. With a rise in the concentration of ethanol 'α' value continues to rise as a result of the rise in the surface area of the ionic head group facilitating counter ionizations (26). The evidence indicates that the cmc, as well as α, is clarified by the volume fraction of ethanol is firstly due to the binding counter ions of alcohol molecule between surfactant ions, which for steric reasons raises the average length among ionic head groups and the dielectric constant of the palisade layer is the second consequence. Here the cmc obtained in the

Calculations of thermodynamic parameters:
The micelle-forming free energies are determined using the following equation (28): (2) where R= gas constant and T= temperature This is clear through Table 3; ΔG m ο is negative in all cases but is less negative as the ethanol concentration in the mixture increases, suggesting that micelle development at a greater ethanol concentration is less spontaneous. It shows the accumulation of greater ethanol output is less favorable. Koya et. al (29) and many other study groupings have also identified related trends (30). It becomes less negative in 0.005 M and 0.01M respectively in the presence of salt ZnSO 4 .   (Figs. 9-13) gives not only the stability of the solution but also relate micellization of molecular association, the fluidity, polarity, and solvent structure (31). The values of D, G, η, E T and S P were taken from Table 4.   (Fig.9).   (Fig. 11).   (Fig. 13).
Besides it can be seen from Fig. 13

Conclusions:
The conductivity of dodecyltrimethylammonium bromide was found to increase in pure water including in three separate mixtures of ethanolwater in the presence of zinc sulphate (0.005 and   0.01) M with concentration. All pre-cmc and postcmc slopes decline with the rise in ethanol. The cmc of dodecyltrimethylammonium bromide increases with the addition of ethanol but decreases simultaneously in 0.005M and 0.01M ZnSO 4 . The degree of dissociation was obtained for pure water including in three separate mixtures of ethanolwater as well as in presence of zinc sulphate (0.005 and 0.01) M. It was apparent that the standard free energy of micelle formation (ΔG m ο ) in most situations is negative as the ethanol concentration in the mixture rise, it appears slightly negative, suggesting that micellization development gets less spontaneous at greater rates ethanol levels. This behaviour suggests that accumulation on low ethanol content becomes less desirable. The physical quantities like a dielectric constant(D), viscosity(η), Reichardt's parameter, Gordon parameter relation with the free energy change(ΔG m ο ) give not only the stability of the solution but also relate micellization of molecular association, fluidity, polarity and solvent structure.

Data Availability:
The authors share the data underlying the findings of the manuscript.