Pertanika J. Sci. & Techno!. 3(2): 221-232 (1995)ISSN:0128-7680

© Penerbit Universiti Pertanian Malaysia

Kinetic and Equilibrium Studies on the ExchangeProcess of n-Hexanol and Cetyl Trimethyl Ammonium

Bromide (CTAB) Micelles

Wan Ahmad Wan Badhi, Hamdan Suhaimi, Ramli Ibrahim,Senin Hassanl

, Derek M. Bloor and Evan Wyn-Jones2

Department of ChemistryFaculty of Science and Environmental Studies

Universiti Pertanian Malaysia43400 UPM, Serdang, Selangor, Malaysia

IDepartment of PhysicsFaculty of Science and Environmental Studies

Universiti Pertanian Malaysia43400 UPM, Serdang, Selangor, Malaysia

2Department of Chemistry and Applied ChemistryUniversity of Salford

Salford, England

Received 6 September 1994

ABSTRAKKajian kinetik menggunakan kaedah pengenduran ultrasonik dalam larutanmisel campuran bagi n-heksanol dan setil trimetil ammonium bromida telahmenunjukkan pengenduran tunggal yang nyata. Ini adalah berkaitan denganproses pertukaran melibatkan alkohol dan misel campuran. Pengukurankeseimbangan yang menentukan kepekatan monomer heksanol telah dilakukandengan menggunakan kaedah gas kromatografi. Gabungan data dari pengukuranpengenduran dan keseimbangan telah dianalisa menggunakan kaedahfenomenologikal di mana parameter-parameter kinetik dan termodinamikboleh didapati untuk proses pertukaran berkenaan. Kesimpulan utama darikajian ini ialah proses penyatuan monomer heksanol kepada misel campuranadalah hampir sarna dengan proses peresapan terkawal.

ABSTRACTKinetic studies using ultrasonic relaxation method in the mixed micellarsolutions of n-hexanol dan cetyl trimethyl ammonium bromide have shown awell-defined single relaxation. This is associated with the exchange processinvolving the partitioning of alcohol with the mixed micelle. The equilibriummeasurement which monitors the monomer concentration of n-hexanol wasmeasured using head space analysis involving gas chromatography. The com­bined relaxation and equilibrium data were analysed using a phenomenologicaltreatment in which the kinetics and thermodynamics parameters were evalu­ated for the exchange process. The main conclusion from this work is that theassociation of n-hexanol to the the mixed micelles is almost a diffusioncontrolled process.

Keywords: mixed micelles, exchange process

Wan Ahmad Wan Badhi, Hamdan Suhaimi. Ramli Ibrahim, Sinin Hassan, D.M. Bloor and E. Wyn-Jones

INTRODUCTIONMicroemulsions are thermodynamically stable isotropic fluid phases con­taining substantial amounts of surfactant, co-surfactant (usually alcohol),oil and water with many industrial and pharmaceutical applications (Robb1982; Mittal 1982). The key ingredient which is thought to control theproperties of microemulsions is the the mixture of surfactant and alcoholco-surfactant. As a result, much attention has been focused on the prop­erties of surfactant-alcohol mixtures.

In dilute micellar solutions, alcohols such as butanol, pentanol andhexanol usually form mixed micelles with the surfactant; the alcohol isthought to reside in the palisade layer of the micelles. Incorporation ofalcohol into the micelles results in changes of micellar properties of thesurfactan t system. Zana et al. (1981) found that addi tion of alcohol ofmedium chain length decreases the CMC of several alkyltrimethylammonium bromide surfactants followed by a strong increase inthe degree of coun terion dissociation (a) at higher concen trations ofadded alcohol. It was found that the molecular weight of the micelles andhence their aggregation number decreases when the alcohol concentra­tion is increased. These observations imply that these effects are due topartial solubilization of alcohol by the surfactant micelles. The alcohol issolubilized in the palisade layer of the micelle with the alcohol moleculesintercalated between the surfactant ions, and hence increases the averagedistance between the ionic head group of the surfactant. This results in adecrease in micelle surface charge density and in turn an increase inionization. This behaviour was supported by Bostrom et at. (1989). Theincreasing distance between the surfactant head group due to the alcoholmolecules also results in a weakening of the electrostatic repulsion. Thispromotes the association of the surfactant with the alcohol molecules at alower concentration, and hence reflects the decrease of CMC. The de­crease in aggregation number of surfactant micelles due to solubilizationof alcohols has been claimed by several authors (Birdi et al. 1978; Zana etal. 1981) as a result of destabilization of micelles. Dissociation of somesurfactant molecules occurs at the expense of replacement by the alcoholmolecules.

Solubilization of alcohol by the surfactant micelles causes abruptchanges in the microscopic properties of the system (Bucklund et at. 1986;·Vikholm et at. 1987). This has been interpreted as a change in the alcohol'solubilization pattern in which two solubilization sites for the alcohol areinvolved, i.e. the palisade layer and the interior of the micelle; in the lattercase swollen micelles are formed. On the other hand, Stilbs (1982) foundthat solubilized short chain alcohols cause a breakdown of surfactantmicelles above a certain total surfactant-alcohol concentration ratio whichis in contrast with the formation of swollen micelles. The interpretation of

222 Pcrtanika J. Sci. & Technol. Vol. 3 No.2. 1995

Kinetic and Equilibrium Studies on the Exchange Process of n-Hexanol and Cetyl Trimethyl

the solubilization behaviour of alcohol by the surfactant micelles asreported above has been the subject of some controversy and therefore itseems desirable to carry out more systematic and independent studies onthese systems. In the present work, our strategy was to combined equilib­rium data from the head space analysis (which involved gas chromatogra­phy with ultrasonic relaxation experiments) so that new thermodynamicand kinetics parameters for the partitioning process could be evaluated.

MATERIALS AND METHODSCetyl trimethylammonium bromide (CTAB) and n-hexanol were obtainedfrom BDH and used directly as received. The hexanol-CTAB solutionmixtures were prepared in a series of hexanol concentrations at twoconstant concentrations of CTAB, i.e. 0.05 and 0.075 mol dm':\.

The kinetic measurements on the solution mixture were made usingthe Eggers ultrasonic resonance technique which has a frequency range of0.4 to 17 MHz. All measurements were made at the constant temperatureof 25°C.

Equilibrium measurements concerning the partition of hexanol be­tween the aqueous and micellar CTAB micelles were carried out usinghead space analysis involving a Shimadzu model G.G.8A gas chromatogra­phy fitted with a CRIB integrator and flame ionization detector. Theprocedure used has been described elsewhere (Smith et at. 1989). Thepartitioning data were also measured at two concentrations of CTAB atvarying concentrations of hexanol up to 0.18 mol dm':]. All measurementswere taken at 25°C.

RESULTS AND DISCUSSIONThe results fj-om equilibrium measurements are displayed in Fif!:. I; theamoun t of solubilized hexanol in the mixed micelle (en is plotted as afunction of hexanol monomer concentration (m). The total hexanol

. (' em I b h em. I' t' . f' Fconcentration '~= 2 + m~. n ot cases 2 IS a Inear unctIon 0 m~. orpurposes of the present work the partition coefficient, K, for hexanol inthe two phases is defined as follows:

(1)

where C1

represents the CTAB concentration. Using this equation, thevalue of K can be obtained from the slope of the plot in Fif!:. 1 as follows:

K(O.05 mol dm':] CTAB)= 43.0 mol om':]

K(0.075 mol dn1':\ CTAB)= 50.0 mol dm': l

Pertanika J. Sci. & Techno!' Vo!. 3 No.2, 1995 223

Wan Ahmad Wan Badhi. Hamdan Suhaimi. Ramli Ihrahim. Sinin Hassan. D.NI. Bloor and E. Wyo-Jones

0.05

•

•

0.04

• CTAB ~ 0.05 mol dm,'--'j

• CTAB = 0.075 mol dm·3

0.03

0.16

0.14

0.12

0.10

•"'e." 0.08-0e

uM 0.06

•0.04

0.02 •

0.000.00 0.01 0.02

m2

( mol dm,3 )

Fig:. I. l'ln! nF (/II/nil II ! nF S,illlhili:"r/ 11".\"(/1/111 (C;') (/I' fI IlIlIt!ioli of (fll"I'O/IS

11I'.\'(/lItJ/ (II/,J

In the ultrasonic measurements, typical relaxation cur\'es showing theplot of air against I' arc given in Fig:. 2 and 3 for both CTAB concentra­tions where a is the sound absorption coefficient and.! is the frequency.These results can be fitted within experimental accuracy using the follow­

ing equation:

.(

A-----=-+81+ (f/.t.,)2

(2)

where r\ is the relaxation amplitude, B is the background absorption whichis independent of frequency and/r; is the relaxation frequenc\'. In all cases,the experimental data were described bv a single relaxation spectrum inthe experimental frequency range. This was associated \\'ith the exchangeprocess of hexanol between the aqueous and micellar phases since theexchange process in\'oh'ing the CTAB monomer and the mixed micellesis too slow and ''fro/,en'' out within t.he experimental range as reportedby Hall I}I (II. (1977), Gettings III (II. (197H) and Gormally I'! (If. (1980).Furthermore, pure alcohol solut.ions up to the solubility limit. did notshow any relaxat.ion; a relaxation was only observed when t.he alcohol wasadded to micellar surfactan t sol ution. The dynamic equili brium existi ngin solution can be represented as follows. In this case only st.ep I inScheme I is observed.

224 Perlanika J. Sci. & Techno!. Vo!. -' No.2. 1<)<)5

Kinelic and Equil ihriul1l Siudic" on the Exchange Proce" oj" n-Hexano) and Cetyl Trimclhy I

1400

1200

1000

N;;'1;\.,

800ab

2- 600

""tJ

400

200

900

800

700

600NO

1;\

"s 500

b2- 400'<l..tl 300

200

100

0

-.- 0 02 moldm'] Hexano1 I-e- 0 08 moldm·3 Hexanol I-A- 014 moldm'] Hexanol

L.- _

10' 10'

Frequency (Hz)

Fig:. 2. 1'/0101 a/I" l'I'J"I//1 /11'11111'111\' liJi U()'; 1110/ dill (.T\Iiwilh (II/r/I'I/ JI-lw\,(l/lo/

-.- 0.02 moldm'] hexanol

-e- 0.06 moldm'] hexanol

-A- 0.12 moldm'] hexanol

-,,- 0.18 moldm'] hexanol

10'

Frequency (Hz)

Fi.~. 3, /'/01 0/ IY/I" 7'I'}:I//1 li('I/III'IIIT /01 U()/5 1110/ r/III ;CFI/i with ((r/r/1'I1 //-hl'.\'111/II/

Pertanika J. Sci. & Technol. Vol. -' No.2, )lJlJ5 225

Wan Ahmad Wan Badhi. Hamdan Suhuimi. Ramli Ibrahim. Sinin Hassan. D.M. Bloor alld E. WYIl-JOIlCS

hexanolmonomer

Scheme 1

CTABmonomer

The pertinent relaxation parameter required for the phenomenological

treatment are T, = 1/27ifc, which is the relaxation time, and f-l11l,\X =1/2 Acf,which is the maximum absorption per wavelength; (is the velocity ofsound through the sample. The relaxation data associated with step 1 inthe scheme above were analysed using the phenomenological approachbecause it has been successfully used in determining the thermodynamicand kinetic parameters of a number of alcohol-surfactant mixtures (Gettingsat ai. 1978; Gormally at al. 1985; Smi th el ai. 1989; Kelly pl al. 1989). Forthe relaxation process in question, which essentially occurs at constantmicelle concentration, ~Il' and constant micellar concentration, C

1, the

maximum absorption per wavelength (f-llll,) has been shown by Gormallypl ai. (\985) to have the following concen tration dependence:

where t,V is the reaction volume change, K, the adiabatic compressibility,c., the total hexanol concentration; Rand T have their usual meaning.According to Eq. 3, a plot of f-l

llJasagainst (m~C;")/C~ should be a straight

line passing through the origin with the slope equal to the thermodynamicterm in brackets from which t,V can be evaluated. For the two CTABconcentrations, these plots (shown in Fig. 4) display the linearity predictedby Eq. 3 above. The following t,V values have been obtained £i'om theplots:

0.05 mol elm-\ CTAB;0.075 mol dm-:I CTAB;

t,V =5.6 cm:\ mol-It,V =S.4 cm:\ mol-I

116 PcrIanika J. Sci. & Tcchnol. Vol. 3 No.1. 1995

Kinclic and Eljuilibriul1l Studies on thc Exchange Process of n-Hexanol and Celyl Tril1lethyl

• CTAB =0.05 moldm·3

o CTAB =0.075 moldm"

0.0400.0350.0300.025

( m, c,m )I C, ( mol dm-l )

Fig. --I. ,H([xillllllll ([bso/I/lio/l !JPf lI!mwlmgl!l (Ct",,) ([.1

([ jilll(liOIl o! ((111 1 C;,,)/C~.

1.6xl0·3

1.4xlO"3

1.2xI0"

1.0x10·l

~8.0xI0'·

::..E

6.0xl0'·

4.0xl0"

2.0xI0'·

0.00.000 0.005 0.010 0.015 0.020

The kinetic parameters of the partitioning behaviour of alcohol can beinvestigated using the phenomenological equation

)1111"' =[Jr(~V)2]Rrl 2RTK I

(4)

where R represents either the forward rate or backward rate of theequilibrium (step 1) being perturbed by the sound wave, i.e. alcoholassociating to or dissociating from the mixed micelle. In dilute hexanol­CTAB solutions it is expected that the solubilized hexanol resides in thepalisade layer of the micelle. In these cirumstances the kinetics of thedissociation of hexanol molecule from the mixed micelle is expected to bea first-order process whose rate is directly proportional to the amount ofsolubilized hexanol.Thus,

(5)

Penanika J. Sci. & Techno!. Vo!. 3 NO.2. 1995 227

Wan Ahmad Wan Badhi. Hamdan Suhaimi, Ramli Ibrahim. Sinin Hassan, D.M. Bloor and E. Wyn-Jones

where k is the backward rate constant. Combination of Eqs, 4 and 5rearrangement gives

(6)

In Eq. 6, Ulllax

and 1'[ are known from the relaxation experiments, andthe thermodynamic terms on the left-hand side are the inverse of the slopeof plots in Fig. 4 while c~" follows from the head space data. Thus for afirst order dissociation, a plot of the left-hand side of Eq. 6 against C;'should be a straight line passing through the origin. The plots for the twoconcentrations of CTAB studies in this work are shown in Fig. 5. In thedilute hexanol region, the term on the left-hand side is directly propor­tional to C~" giving good straight lines passing through the origin. In thisregion it is clear that the backward rate is a first-order process whose rateis directly proportional to the amount of solubilized hexanol. However,above a certain hexanol concentration the reaction rate appears to levelotf to a constant value. The levelling-off of the reaction rate takes placeonly in the concentrated alcohol solutions.

•

• CTAB = 0.05 moldm"

• CTAB = 0.075 moldm·3

5.0xl0'

4.0xl0'

'~ 3.0xl0'

''6<38

r:I:2.0x1O'

LOxlO'

0.020 40

•

60 80

C2

m (mol m")

100 120 140 160

Fig. 5. B(/I'kwmd mil' (R,) liS ({ jilllClioll oj solllbilizl'Il 1I-!lI'x({//o!concmtration (C;")

We turn our attention to discuss the experimental data describedabove in relation to the solubility behaviour of hexanol in CTAB micelleswith special attention to changes in the solubility pattern.

228 Pertanika J. Sci. & Techno!. Vo!. 3 No.2. 1995

Kinetic and Equilibrium Studies on the Exchange Process of n-Hexanol and Cetyl Trimethyl

a) The partition coefficient, K as defined by Eq. 1 is constant over thehexanol concentration range 0.02-0.18 dm -:J as is demonstrated bythe linearity of the plots in Fig. 1.

b) The maximum absorption per wavelength, ILIll<lx measured from theultrasonic experiment monitors the thermodynamic parameter, ~V,

which is the volume change between the "free" and "solubilized"hexanol. The linearity of plots shown in Fig. 4 for both 0.05 and

0.075 mol dm-'! of CTAB solution shows that .6.V is constant at allhexanol concentrations with a value of 5.5 ± 0.1 cm~ mol-to

c) A more specific indicator for kinetic behaviour, J-l max / TI which

according to the phenomenological approach monitors the back­ward rate associated with the partitioning process, shows deviationfrom linearity at the higher hexanol concentrations.

Therefore, on balance, the two equilibrium measurements whichmonitor different aspects of the partitioning described in (a) and (b)above do not show any evidence of a change in solubilization pattern. Onthe other hand, the kinetic information indicates clear deviation at high~exanol concentrations as shown in Fig. 5. The constant R

hat this high

concentration of hexanol may be associated with the solubilization behav­iour of the alcohol. A previous report by Vikholm et ai. (1987), who alsoobserved a break in their experimental data using other techniques at thishigh concentration, claimed that the breakpoint was associated with thesolubilization of alcohol in the core of the CTAB micelles. If this is trueand on assumption that the hexanol in the interior does not take part inthe exchange process, then Rb will depend only on the concentration ofhexanol in the palisade layer. If we assume that the palisade layer is"saturated" and has a constant concentration after the breakpoint, then itis expected from Eq. 5 that Rb will also be constant. However, this is onlya crude assumption since the solubilization of alcohol into the interior ofthe micelle is still a matter of controversy and the assumption that alcoholin the micellar core does not take part in the exchange process isquestionable. With the lack of any obvious deviation in tI'\.e equilibriumexperiments we conclude that the evidence from the ultrasonic measure­ments is not convincing enough to lend support to a change in solubilizationof hexanol. The observation in (c) above which is thought to beassociated with a change in solubilization behaviour is debatable since asimilar behaviour is also observed in a pure surfactant system involvingmonomer-micelle exchange process at high surfactant concentrations.Furthermore, the ultrasonic measurements are made close to the phaseboundary where possible additional contribution (e.g. from concentrationfluctuation or phase changes) to the relaxation data may occur. In

Pertanika J. Sci. & Technol. Vol. 3 No.2. 1995 229

Wan Ahmad Wan Badhi. Hamdan Suhaimi. Ramli Ibrahim. Sinin Hassan. D.M. Bloor and E. Wyn·Jones

addition, the solution mixtures at high hexanol concentrations appearedto turn cloudy with time, suggesting that their thermodynamic stability isquestionable.

The phenomenological treatment as illustrated in Fig. 4 and 5 allowsus to determine the forward rate constant (k+) from the relaxation data.As mentioned previously, in dilute hexanol solutions, the backward ratedescribing the dissociation of hexanol from the mixed micelle is directlyproportional to the solubilized hexanol, confirming that it is a first-orderrate process. On similar grounds one expects the forward rate, R" describ­ing the association of hexanol to the mixed micelle to be a bimolecularprocess proportional to the product of the concentrations of free hexanol,m~, and the mixed micelle. Thus,

Rj

= k+m~ [mixed micelle] (7)

where k+ is the forward rate constant, at equilibrium R, =: ~)' so that fromEq. 5 and 7 we obtain

(8)

or

If we consider the limit of this equation as C~' -70) then

[mixed micelle] = Ct

11(9)

where n is the aggregation number of CTAB micelles. From Eq. I,

Cm

K = J - and upon rearranging Eg. 8 and 9 the aggregation number (n)fi2Ct

takes the following form

(10)

The backward rate constant (k) can be evaluated from the linear partof Fig. 5. For 0.05 mol dm-:\ CTAB the value is 4.39 x 106 S·l with K =: 43.0mol dm-:\. The aggregation number of CTAB is about 80 (Smith et at.1989). Under these circumstances, the fonvard rate constant (k+) for the

230 Pertanika J. Sci. & Techno!. Va!. 3 No.2. 1995

Kinetic and Equilibrium Studies on the Exchange Process of n-Hexanol and Cetyl Trimethyl

association of n-hexanol with CTAB micelles can be calculated, and hasthe value of

!t = 1.51 x 10 111 mol dm-:1S-I

This value is close to the diffusion control value of kd

= 1.74 X 10 111 moldm-:1 S-I which is calculated from Eq. 11

(II)

Here N is the Avogadro number, J?, and Dill are the respective diffusioncoeHicient of n-hexanol and the mixed micelle, R, is the effective reactiondistance and f~lctor 10-:1 arises because the concen trations are expressed inmol dm-:1

• In the present circumstances, H, is effectively equal to themicellar radius and U?, + D,) = I?, = 9 x 10-(; Cln~s-t according to Glasstone(1941) .

ACKNOWLEDGEMENTS

Wan Ahmad W.B. is grateful to Universiti Pertanian Malaysia and theGovernment of Malaysia for study leave and also to the Department ofChemistry and Applied Chemistry University of Salford, Salford, U.K. forits hospitality.

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232 Pcrtanika J. Sci. & Tcchnol. Vol. 3 No.2. 1995