INTRODUCTION:

Physico-chemical studies of surface active substances in solution are important in the field of pharmaceuticals, agriculture, detergency, solubilization, enhanced oil recovery, and metallurgical processes1^-3. There have been several reports on various physico-chemical properties of surfactants in aqueous solution^4-6. However, the reports on surface and thermodynamic properties for ionic surfactant in aqueous electrolyte solutions at low temperatures are limited^7-9. In the present study we report the data for surface properties such as surface pressure at CMC, p_cmc, surface excess concentration, G_max, minimum area per molecule at the air–liquid interface, A_min , and thermodynamic parameters for micellization as well as for adsorption of cetyl pyridinium chloride (CPC) in the aqueous sodium chloride (NaCl) solutions at 288.15, 293.15 and 298.15 K.

MATERIALS AND METHODS:

Cetyl pyridinium chloride (CPC) from (SD Fine Chemicals) and Sodium Chloride (BDH) were of AR grade. Doubly distilled (specific conductance 2.0 х 10^-6Scm^-1 ) was used for preparing various solution.

Surface tension was measured using a specially designed stalagmometer, described elsewhere¹⁰. It was calibrated using standard liquids: benzene, carbon tetrachloride, n-hexane, acetophenone and water. Reproducibility of measured surface tension values was within + 0.2 mNm ^-1.

A digital conductivity meter (Model E.I. 601 E) was used for measuring conductance. The thermostatic bath (Model Tempstar, Model KW 201 A) ensured temperature control within + 0.01 K.

RESULTS AND DISCUSSION:

From the plots for surface tension (g) versus log [CPC] at different temperatures the critical micelle concentration (CMC) were obtained from the break points in these plots. CMC of CPC +H₂O solutions were also obtained from conductance measurement and are recorded in Table 1. The observed CMC for CPC+H₂O at 298.15K agrees well with the values reported in literature^11-13. CMCs of CPC for the studied systems increase with increase in temperature. This may be attributed to (a) decrease in dielectric constant of water with the increase in temperature and (b) enhanced thermal agitation at higher temperature, both these effects shift the surfactant == micelle equilibrium in favor of the monomers leading to higher CMC. These observations conform with those reported in the literature^14-15. CMC however diminishes with the successive addition of sodium chloride to surfactant solution (Table 1). This may be attributed to the contraction of electrical double layer around the micelle in the presence of electrolyte (NaCl) which results in stabilizing the micelle hence a reduction in the CMC¹⁶.

Maximum surface excess concentration (G_max) at the air-liquid interface was obtained using Gibb’s adsorption equation¹⁷-

G_max = - (1 / 2.303 nRT ) (dg/ d log C) _T (1)

Where, n is the number of particles released per surfactant molecule in the solution; R is the gas constant and C is the surfactant molar concentration. The (dg/d log C)_T represents the slope of the surface tension versus log C plot, below CMC, at constant T. In the present investigation, n =2 for the ionic surfactant (CPC). The values for G_max for the studied systems are presented in Table 1. G_max decreases with increase in temperature which may be due to the enhanced thermal agitation at higher temperature causing a partial shifting of surfactant monomers from the air-liquid interface to the bulk¹⁸. The G_max value decreases on mixing NaCl into a CPC solution. This may be due to a partial displacement of surfactant monomers by electrolyte ions from the air-liquid interface to the bulk solution.

Minimum area per molecule (A_min) at the liquid–air interface has been calculated using the equation¹⁷-

A_min = 10¹⁴/(N. G_max) (2)

Where, ‘N’ is the Avogadro's number. A_min values are also given in the Table 1. An increase in A_min with increase in temperature may be due to a more micelle forming tendency by the surfactant monomers, in the bulk at higher temperature and their depletion at the air-liquid interface.

Values of the surface pressure at CMC, p_cmc, an index of the surface tension reduction at CMC, were obtained using the relation ¹⁷-

p_cmc= g₀_- g_cmc (3)

Where, g_{0 =} surface tension of water and g_{cmc =} surface tension at CMC. p_cmc values are also presented in Table-1. p_cmc value increases both with the increasing temperature as well as with the addition of NaCl to CPC solution.

Standard Gibb’s free energy of micellization (DG⁰_mic) for pure surfactant in aqueous solution have been calculated using the equation¹⁹

DG⁰_mic= ( 2-b ) RT ln X_cmc (4)

Where, b is a measure of the fraction of micellar charge un-neutralized by the counter ions and the same was obtained from the ratio of post micellar and pre micellar slopes of specific conductance versus [surfactant] plot, X_cmc is the mole fraction of surfactant at CMC. The standard state for the surfactant is the solvated monomer at unit mole fraction referred to infinite dilute solution, and for the micelle, micelle itself is considered as the standard state. The DG⁰_micvalues are recorded in Table 2. DG⁰_{mic value} increases with increase in temperature suggesting less feasibility of micellization process at higher temperatures.

In the presence of NaCl due to common ion effect the cetyl pyridinium chloride behaves like a non-ionic surfactant therefore, standard Gibb’s free energy of micellization (DG⁰_mic) for CPC + NaCl + H₂O have been evaluated using the equation¹⁷

DG⁰_mic= RT ln X_cmc (5)

and these DG⁰_micvaluesare also recorded in Table 2 hence equation (5) have been used for calculation of DG⁰_mic. These DG⁰_micvalues are less negative than the correspondingvalues for pure surfactant suggesting that addition of sodium chloride disfavor the process of micellization.

Other thermodynamic parameters of micellization viz. DH⁰_mic and DS⁰_micfor pure surfactant (CPC) containing electrolyte have been calculated using the equations¹⁷ (6) and (7) respectively.

DS⁰_mic= -d(DG⁰_mic)/ dT (6)

DH⁰_{mic =}DG⁰_mic+ TDS⁰_mic(7)

Thermodynamic parameters of micellization are presented in Table 2. DG⁰_mic are negative indicating the spontaneity of micellization process in aqueous medium. The values of DG⁰_mic decreases (becomes more negative) with the increase in temperature. This may be due to the desolvation of the surfactant hydrophilic group in the presence of electrolyte at higher temperature²⁰ leading to more entropy gain. A further decrease in DG⁰_micon mixing NaCl to CPC is due to disruption of water structure in the presence of NaCl as well as decreased hydration of the surfactant head group both leading to higher entropy.The process of micellization is favored both by entropy gain and exothermic enthalpy change.

The thermodynamic parameters of adsorption viz . DG⁰_ad, DH⁰_ad and DS⁰_ad at constant pressure have been evaluated using the relations²¹ 8 to 10

DG⁰_{ad =} RT ln X_cmc– 6.023 x 10 ^–1 p_{cmc .}A_min(8)

DS⁰_ad= - d(DG⁰_ad) / dT (9)

DH⁰_ad= DG⁰_ad+ T DS⁰_ad (10)

The values of DG⁰_ad, DH⁰_ad and DS⁰_adare presented in Table 2. The lower value of DG⁰_adcompared to corresponding DG⁰_micvaluesindicated that the process of adsorption of surfactant molecules at the air-liquid interface was preferred over their aggregating to form micelle in the bulk. The DS⁰_advalues are positive and higher than DS⁰_mic. It may be due to more degree of freedom of the surfactant molecules at the air-liquid interface compared to that in the confined interior of micelle ^{22, 23}.

The observed endothermic DH⁰_admay be due to the hydrophobic-hydrophobic interaction of surfactant hydrocarbon chain at the surface. However, adsorption becomes feasible due to predominate entropy gain at the air- liquid interface.

Table – 1. Critical micelle concentration (CMC), surface excess concentration (G_max), minimum area per molecule (A_min) and the surface pressure at the CMC (p_cmc) of CPC in water and water + NaCl system.

[NaCl]

moldm^-3

Temp.

CMC x 10⁴

(mol dm^-3)

G_max x 10¹⁰

(mol m^-2)

A_minx 10²

(nm²)

p_cmc

(mNm^-1)

288.15

293.15

298.15

6.2 (6.2)

6.7 (6.8)

7.0 (7.2)

2.18

2.06

1.92

76.2

80.6

86.5

24.5

25.4

26.4

0.025

288.15

293.15

298.15

5.9

6.4

6.8

2.05

1.93

1.82

81.0

86.0

91.2

30.7

32.8

34.9

0.050

288.15

293.15

298.15

5.5

6.0

6.4

1.81

1.69

1.58

91.7

98.2

105.1

31.7

33.4

35.4

0.075

288.15

293.15

298.15

5.2

5.7

6.0

1.63

1.51

1.40

101.9

110.0

118.6

32.1

34.0

36.4

0.100

288.15

293.15

298.15

4.8

5.3

5.6

1.50

1.34

1.24

110.7

123.9

133.9

33.2

35.1

37.2

0.125

288.15

293.15

298.15

4.4

4.9

5.4

1.37

1.20

1.11

121.2

138.4

149.6

34.3

36.8

38.4

CMC values in parenthesis obtained from conductance method.

Standard deviation + 0.1.

Table 2- Thermodynamic parameters of micellization / adsorption of CPC in water and water + NaCl system.

[NaCl]

moldm^-3

Temp.

-DG⁰_mic /-DG⁰_ad

(kJ mol^-1)

DH⁰_mic/ DH⁰_ad

(kJ mol^-1)

DS⁰_mic/DS⁰_ad

(kJK^-1 mol^-1)

0.00

288.15

293.15

298.15

37.7 / 38.8

36.4 / 37.7

35.2 / 36.6

-109.7 / -102.2

-0.25 / -0.22

0.025

288.15

293.15

298.15

27.4 / 28.9

27.7 / 29.4

28.1 / 29.9

-7.20 / -0.10

0.07 /0.10

0.050

288.15

293.15

298.15

27.6 / 29.4

27.9 / 29.8

28.2 / 30.5

-10.30 / + 2.40

0.06 / 0.11

0.075

288.15

293.15

298.15

27.8 / 29.6

28.0 / 30.3

28.3 / 30.8

-13.40 / + 4.90

0.05 / 0.12

0.100

288.15

293.15

298.15

28.1 / 30.2

28.2 / 30.8

28.5 / 31.5

-16.50 / + 7.30

0.04 / 0.13

0.125

288.15

293.15

298.15

28.3 / 30.7

28.4 / 31.5

28.6 / 32.1

-19.60 / + 9.50

0.03 / 0.14

CONCLUSION:

Critical micelle concentration (CMC), maximum surface excess concentration (G_max), minimum area per molecule (A_min) at air-liquid interface of cetyl pyridinium chloride (CPC) in aqueous sodium chloride solutions have been determined at 288.15, 293.15 and 298.15 K by surface tension measurements. CMC values have also been determined by conductance method. Thermodynamic parameters of micellization as well as of adsorption at air-liquid interface have been evaluated. Micellization process is exothermic whereas adsorption of surfactant at the air-liquid interface is endothermic in nature and becomes feasible owing to the predominant entropy gain.

ACKNOWLEDGEMENT:

One of the authors (RP) is grateful to Principal Govt. College Hisar for providing basic research facilities and The Secretary UGC New Delhi, for financial assistance in the form of a minor research project.

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Received on 10.11.2011 Modified on 04.12.2011

Asian J. Research Chem. 5(1): January 2012; Page 34-37