Pertanika 15(3), 251-254 (1992)

Micellization in a Mixed Alkali Soap System

HAMDAN SUHAIMI, ANUAR KASSIM and LAlLI CHE ROSEDepartment of Chemistry

Faculty of Science and Environmental StudiesUniversiti Pertanian Malaysia

43400 UPM, Serdang, SelangorMalaysia.

Keywords: Micellization, sodium octanoate, sodium dodecyl sulphate, critical micelle concentration(erne)

ABSTRAK

Keterlarutan natrium oktanoat dan natrium dodesil sulfat (SDS) di dalam larutan misel dikaji dengan mengukurketegangan permukaan larutan surfaktan campuran sebagai fungsi pecahan mol sUlfaktan tersebut. Keputusanmenunjukkan bahawa kepekatan kritikal misel untuk surfaktan campuran bemilai di antara nilai kepekatankritikal misel kedua-dua surfaktan tulen tersebut. Keputusan juga menunjukkan perubahan nilai kepekatankritikal misel berhampiran dengan nilai kritikal untuk rantai hidrokarbon yang lebih panjang. Dengan itu,dirumuskan bahawa rantai yang lebih panjang iaitu SDS lebih digemari dari segi geometri untuk pembentukanmisel campuran. Keputusan juga menunjukkan bahawa natrium oktanoat dan natrium dodesil sulfat melarutsecara campuran unggul.

ABSTRACT

The solubility ofsodium octanoate and sodium dodecyl sulphate (SDS) in the micelle was investigated by measuringthe surface tension of the aqueous solution of surfactant mixture as a function of mole fraction of surfactants.Results indicate that the critical micelle concentration (cmc) of the mixed sU1factant falls between the cmc value ofthe two surfactants. The results also indicate that the rate of change of cmc value tends to be near the longerhydrocarbon chain molecule (SDS) with lower cmc value. Therefore it was concluded that the longer hydrophobicgroup of SDS is geometrically favourable for the micelle formation. In addition, the results indicate that SDS andsodium octanoate molecules mix ideally in the micelle solution.

INTRODUCTION

A previous paper (Hamdan et al. 1992) was con­cerned with the phase equilibrium and micelleformation of the mixture of sodium octanoateand sodium dodecyl sulphate (SDS), which aredifferent in both hydrocarbon chain length andthe structure of hydrophilic group at a ratio of1:1 by weight. From the results, it was concludedthat phase equilibria construction by the methodemployed was very tedious and time-eonsuming.

In this study the cmc (Shinoda 1963) valueof various compositions of the mixed surfactantwas used to try to further clarify the behaviour ofthe mixtures and predict the best composition toconstruct the phase equilibria.

The surface tension of aqueous solution ofsodium octanoate and sodium dodecyl sulphate

has been measured as a function of mole fractionat 30°C. The various cmc values of the mixedsurfactant were then plotted against the molefraction of the surfactant to determine ideality ofthe surfactant behaviour. The surface tensionreported was only the apparent surface tension asthe study concentrated on the composition ratherthan the absolute surface tension.

MATERIALS AND METHODS

Materials

The surfactants sodium octanoate and sodiumdodecyl sulphate (SDS) were obtained from Flukaand Sigma respectively. The SDS was recrystallizedusing absolute ethanol obtained from Sigma.Water used for the preparation of surfactantsolution was doubly distilled.

HAMDAN SUHAIMI, ANUAR KASSIM & LAlLI CHE ROSE

22

21.5

22.5

o

~_o_o-20

2\

20.5

~J5 ,--------------_--,Determination of surface tension

A Fisher (Model 215) surface tension analyserwith sensitivity of 0.02 m m- l employing du Nouymethod was used for measuring the surface ten­sion. Samples of 25 ml with various concentra­tions of surfactant were prepared and run at aspeed of 0.00254 m min-l. Pre-cleaned platinum­iridium ring with a circumference of 0.06 mattached to a wire of radius 178 11m was used.The temperature was controlled at 30 ± 0.01°e.

19.5 '--'------"__--'-__-'-__-'-__....L...J

LOG(S]

Fig. 2: The suiface tension, 'Y versus logarithm of concentra­tion for sodium dodecyl sulphate

LOO(5)

Fig. 3: The suiface tension, 'Y versus logarithm ofconcentra­tion for mixture ofsodium octanoate and sodiumdodecyl sulphate (mole fraction ofsodium octanoate =0.16)

22.2\

22 0

21.8

21.6

21.4

E 21.2ZE 21ZQ 20.8enz

20.6W....W 20.4U...

20.2"-a: 0::>en 20

\'_0_0-19.8

19.

19 ,

19.2-2.5- -2 • -2.3 -2.2 -2.1 -2 -1.9 -1.8 -1.7

-2.013-2.4569 -2.2222 -2.1023 -2.0768 -2,044RESULTS AND DISCUSSIONThe experimental surface tension, y versus loga­rithm of surfactant concentration curves observedat 30°C is shown in Fig. 1-6. The break point ofthe curve refers to the critical micelle concentra­tion (cmc). These cmc values were then plottedagainst the mole fraction, a for the mixtures ofsodium octanoate and sodium dodecyl sulphateas shown in Fig. 7 and are compared to thetheoretical curve obtained by using mixed micelletheory (Clint 1975) to check for ideality of themixtures.

The mole fraction, a for component 1 isdefined as described by the theory (Clint 1975).Here the components 1 and 2 are sodiumoctanoate and sodium dodecyl sulphate respec­tively. For the construction of phase equilibria, agraph of surface tension of micellar solutionversus mole fraction of the mixture is predictedto have a minimum and the value is used as thecomposition of the mixture to obtain an ex-

20

:~'~_o_o-

-1.9 -1.85 -1.8 -1.75 -17 -1.65 -16 -15!t -15'8 '---'--_-'-_.l-_-'-_.l-_.l-_L-_L-_L....J

19

'0

\\ '\ I7

o~o--o_o_35.5

'"J7

36,9

36.8

36.7

,..'.5,. ,36.3

'.2,.3.

359

358

35.

350

LOO[5]

Fig. 1: The suiface tension, 'Y versus logarithm ofconcentra­tion for sodium octanoate.

353-05375 -05277 -05179 ....049'34 -04903 -04815 LOO[S)

Fig. 4: The suiface tension, 'Y versus logmithm ofconcentra­tion for mixture ofsodium octanoate and sodiumdodecyl sulphate (mole fraction ofsodium octanoate =

0.64)

252 PERTANlKA VOL. 15 NO.3, 1992

MICELLIZATION IN A MIXED ALKALI SOAP SYSTEM

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.01

0.03

0.02

0.05

0.06

0.04

0.08

0.1 r--,-----------------~

0.09

0.07

~Ec5:::;()

22.6 ,.....-------------------,

22.4

22.2

22

21.8

21.6

21.4

21.2

21

20.8

20.6

20.4

20.2

20

19.8

19.6

19.4 L.l-_L--..-JL-------'-~:__-L--L--'--....L--'.1.5 ·1.45 -1.4 -'.35 ·1.3 -1.25 -1.2 -1.15 -1.1

LoolS}mole fraction of 50S

TABLE 1Experimental and calculated cmc values at various mole

fraction, cv. of surfactant mixtures.

0.00940.0210.0540.094

0.0100.0220.0470.085

19.3319.3019.7020.30

0.160.640.870.94

mole fraction, Ct

and 0.94 respectively. A similar method was usedto determine the cmc values of these mixtures.These values together with their correspondingsurface tension, 'Yare tabulated as shown inTable 1. With that information available the be­haviour of these mixtures becomes obvious. Thecmc values are observed to be increasing withincreasing mole fraction. The values also tend tobe near the cmc value of sodium dodecyl sul­phate. The surface tension, 'Y value of the mixedmicellar solution is observed to changeparabolically with an existence of minimum pointas predicted earlier.

A graph of cmc versus mole fraction, a ofmixtures was plotted as in Fig. 7. The theoreticalvalue from the theory was calculated and plotted(Fig. 7). The experimental points are seen to bein good agreement with the theoretical ones.The curve (Fig. 7) also shows a negative deviation

Fig. 7: Variation ofC1itical micelle concentration, cmc withmolefraction for mixtures ofsodium octanoate andsodium dodecyl sulphate; 0, experimental value and+, calculated value.

tended lamellar liquid crystalline, D region.Fig. 1 and 2 show the experimental surface

tension, 'Y versus logarithm of pure surfactant 1and 2 concentrations respectively. The break pointof the curve gives a cmc value of 0.008 and 0.31mo1e/1 for the two surfactants. These values arereferred to as C~11 and CM2 by the theory (Clint1975) and are consistent with the value obtainedby previous researchers (Ekwall et at. 1963; Evanset at. 1964; Mandell and Ekwall 1968).

Fig. 3-6 show the experimental surface ten­sion, 'Y versus logarithm of mixture concentra­tion curves at mole fraction, a of 0.16,0.64, 0.87

Loo[S]

Fig. 6: The swface tension, y versus logarithm ofconcentm­tion for mixture ofsodium octanoate and sodiumdodecyl sulphate (mole fi'action ofsodium octanoate =

0.94)

23.6

23.40

23.2

23

22.8

22.6

.§ 22.4

z 22.2EZ 22Q

21.8V>zw 21.6>-w 21.4()

~ 21.2a:::> 21V>

20.8

20.6

20.4 0---0---0-20.2

-1.13 -1.102 ·1.071 -1.041 -1.013

Fig. 5: The surface tension, y versus logmithm ofconcentra­tion for mixture ofsodium octanoate and sodiumdodecyl sulphate (mole fraction ofsodium octanoate =0.87)

PERTANIKA VOL. 15 0.3, 1992 253

HMvIDAN SUHAIMI, ANUAR KASSIM & LAlLI CHE ROSE

from ideality indicating a strong attraction to­wards the sodium dodecyl sulphate ions. Thissimple comparison of the mixtures of thesesurfactants is evidence for the assumption thatthe mixed alkali soap micelles behave as idealmixLUres.

The existence of a minimum in the surfacetension of the micellar solution (Table 1) formsa striking interest and a graph of micellar surfacetension versus mole fraction, a is plotted in Fig.8. From the curve in Fig. 8, a value of 0.41 isselected as the composition of the mixture and isspeculated to give a stable mixed alkali soapsystem with a large lamellar liquid crystallineregion upon addition of an appropriatecosurfactant. This is however too early to predictas the stability of the system depends on factorssuch as:

1. The cmc value of the surfactant or mixtureof surfactants

2. The slope of the adsorbed monolayer3. The reduction in surface of interface tension

after addition of surfactants.

for instance the thermodynamics of themicellization (Norihiro et al. 1992; Clint andWalker 1975).

It is however important to note that themixtures behave ideally in accordance to theMixed Micelle Theory and that the longer hydro­phobic group of sodium dodecyl sulphate ion isgeometrically favourable for the micelle forma­tion.

ACKNOWLEDGEMENTS

The authors wish to thank Universiti PertanianMalaysia for supporting this work (Grant No.50205). Special gratitude also goes to Dr. HjSalleh Harun for allowing us to use the surfacetension analyser and Mr. Rani Zaman for lendinghis technical support.

REFERENCES

CLINT, c.H. 1975. Micellization of Mixed NonionicSurface Active Agents. J. Chern. Soc., Faraday

Trans. 171(6): 1327-1334.

CLINT, C.H. and T. WALKER. 1975. Thermodynamicsof Micellization of Homologous Series of N­alkyl Methyl Sulphoxides and N-alkyl(dimethyl)Phosphine Oxides. J. Chern. Soc., Faraday Trans.

I, 71 (4): 946-954.

MOLE FRACTION OF SODIUM OCTANOATE

20.4

20.3

20,2

20.1

~ 20E<5 19.9::;uI-

19.8'"z~ 19.7

~I- 19.6wu~ 19.5a:::>

'" 19.4

19.3

19.2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

EKwAu., P., H. EIKREM and L. MANDELL. 1963. Proper­ties and Structure of Na Caprylate Solution.Acta. Chern. Scand. 17: 111.

EVANS, H.B. and J. CLEVER. 1964. Surface Tension ofBinary Mixtures. J. Phys. Chern. 68(11): 3433­3435.

HA.VlDAN, S., K ANUAR and C.R. LAlLI. 1992. PhaseEquilibria of a Mixed Alkali Soap/CarboxylicAcid/Water System. Pertanika (submitted forpublication) .

MANDELL, L. and P. EKWALL. 1968. The Three-compo­nent System Sodium Caprylate-decanol-water:The Phase Equilibria at 20·C. Acta Polytechnica

Scand. 74: 1-116.

Fig. 8: The micellar su1ace tension, y plotted against molefraction for mixtures ofsodium octanoate and sodiumdodecyl sulphate.

It appears the third factor is to be accounted forin this case because of the lowest reduction insurface tension. More information should beobtained by further examining the mixture ofsodium octanoate and sodium dodecyl sulphate,

NORlHlRO, 1., S. NORlHIKO and A. Ko]I. 1992. SurfaceAdsorption and Micelle Formation ofDodecyltrimethylammonium Chloride andDodecylammonium Chloride Mixture. Bull.

Chern. Soc. lpn. 65: 858-863.

SHINODA, K. (1963). Colloidal Surfactant. New York:Academic Press.

(Received 20 August 1992)

254 PERTAl lKA VOL. 15 10.3,1992