Indian 10urnal of Chemistry Vol. 38A, 1anuaryl999, pp.17-25

Energetic, surface-chemical and hydrodynamic studies on cetyltrimethylammonium bromide and sodium salicylate

B K Roy & S P Moulik*

Centre for Surface Science, Department of Chemistry, Jadavpur University, Calcutta 700032, India

Received 24 August 1998; revised 9 November 1998

Molecular interaction between cety ltrimethylammonium bromide (CTAS) and sodium salicylate (NaSal) has been in vestigated. A I: I complex is formed in solution. The mixed solution at thi s composition micellizes at 0. 12 mmol dm'> at 30DC , and the mi xed micelle binds nearly 55% counter-ions. The molecular complex formation between CTAS and NaSal combination is an exothermic process. The micellizat ion of CTAS-NaSal combination is also exothermic and a two-step process, the overall enthalpy values falling in the range of 10-15 k1 mol" at 3011 C. The micellization process is prominently entropy controlled. The surface excess of CTAB and its minimum area at the air-water interface in NaSal, NaCI and NaSr environments have been determined. The CTAS-NaSal system is non-Newtonian and viscoelas tic, and show shear thinning properties.

Cetyltrimethylammonium bromide (CTAB) is a well st udied micelle forming cationic surractant l

. 13 whose micellar charac­teristics can be influenced by the presence of addi tives. The spherical micelles ofCTAB can be converted into rod-shaped geometry by additi vesx.

,) particularly by the salicylate ion

(San. The spherical micelle grows into cylindrical form show­ing viscoelas ti c properties '4-2 1. This is a special influence which has drawn attention of researchers and prompted them to in­vestigate the mixed micellization behaviour22

.24

, micellar shape, size and mass, 19.211.25 and solution rheologylx.2 1 ofCTAB - NaSal and other combinations, viz., CTAC-NaSal, CPC - NaSa l, etc. But the interfac ial, aggregational and hydrodynamic man ifes­tations of these combi nati ons as well as energeti cs of interac­ti on between cationic surfactants (containing quaternary am­monium or pyridinium head group) and Sal-' and their mixed micelle formation have remained unexplored. There, thus, re­mains a scope for ex tending the study in the above mentioned areas. The molecular complexat ion between CrAB and NaSal , interfacial adsorpti on and mice lli zation ofCTAB, the counter­ion binding of the mice ll es formed, energeti cs of micelle for­mation and hydrodynamic behaviour of CTAB in presence of NaSal have been investigated adopting conductometri c, ten­siometric, ca lorimetri c, and viscometric methods. For contrast­ing the behaviour of NaSal , the effects of salts, viz., NaCI and NaBr on the micellization and interfacial adsorption charac­teri stics of CTAB have been also examined .

Materials

CTAB used was 99 % pure product of E. Merck, Germany (characteristics reported carli er2), Sodium salicy late was pre-

pared by add ing requisite amount of standard ised NaOH solu­tion in doubl y di st illed water to known mass of doubl y recrys­tallized salicy lic ac id (A.R., BDH, UK, pKa = 2.97 at 2511 C. The pH of the solution was around 7.00. The solution was well stirred and stored in a stoppered teflon bottle for further use. The other chemicals (NaCI and NaBr) used were of A.R. grade materials obtained from S.D. Fine Chemicals , India .

Doubly distilled conductivi ty water (spec ific conductance 2-4 ~S cm·1 at 30"C) was used for the preparation of all the samples. It 's surface tension varied between 68 and 70mNm·1 at 3011 C.

Methods

Physical measurements were made in thermostated concli­ti on in a water bath (accuracy, ± 0 . 10') C in all measurements excepting ca lorimetry where the limit was ±0.002° C).

Conductance measurements were adopted to determine the stoichiometry ofCTAB - NaSal complex with a Jenway (U K) conductometer us ing a temperature compensated celIoI' ce ll constant 1.1 cm· l . Concentration variation was made by add­ing water to concentrated mixed so lutions ofCTAB - NaSal of desired proportions. Measurements were made after thorough mixing and temperature equilibration.

Surface-tension ofCTAB - NaSal mixed soluti on was mea­sured with a Du Nuoy Tensiometer (Kruss, Germany). The measured tensions were corrected according to the procedure of Harkins and Jordan26

. In actual measurements, a concen­trated CTAB or CTAB - NaSal solution was progressively added in water or salt solution with the help of a Hamilton microsyringe and the tensions were ,measured after thorough

18 INDIAN J CHEM, SEC. A, JANUARY 1999

t.) aJ .!!! ~ ::J.

C C\l 13 Q)

E '+-0 Q)

(5

.§ Iii t.) ~

2 ·c OJ

C 2

0

10 -

5

0

4

3

2

0

0

Tii'ne (min)

10 20 30

"i" '·"·"·"""'···~·····r···'···· ' cmc---.:....... @) ~Hdm

•

40

A

B

•••••.•••.• •.. • ~ .•• I!' •••••.•••••.••• I

•....•.. -.•....•.. - .~. .. -...•....• -......... . .. •• c

:e-1 0 "" eme ~ 0 ~ ~-20 :0

~" @

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

reT ABlImM dm·3

Fig. 1- Microcalorillletrie evaluation of the CMC of pure CTAB at 303K. [A , heat Il ow per inj ections of surfactant at 303K. B, En­thalpy change per mole of inj ectant during the run . C, Dif­fercnti al pl ot of enthalpy pcr mole of injcctant. The CMC poiI1ls are indieated on the curves in B and C. The enthalpy of mi celli zat ion process is indicated on the vertical arrow in B.]

mix ing. The surface tension of an aqueous so luti on of CTAB - NaSal mi xture graduall y dec reased with time and allained fai rly constant value within 20 minu tes. Prior to measurement, 30 minutes of time was, therefore, allowed for equilibration.

Viscos ity measurements we re made using a calibrated Ostwald viscometer ( 130s. clearance time for 5 ml of water at 30" C) and a shear viscometer (Haake, RV-20 Gennany). The measurement procedures have been described earlie r~x.

Calorimetric measurements were made in an isothermal ti­tration mi crocalorimeter, Omega-ITC from Microcal. Inc, U.S.A. The critical micellar concentrati on (CMC) and the en­thalpy of l1lice lli zati on were evaluated following the reported Jlrocedure ~IJ-3I wherein stepwise inj ec ti on of aliqu ots in microl itres to a ce ll cont;li ning 1.3 ml of water yielded ther­mograms, i.e., a series of heat pockets (or areas) hav ing a com­mon base line (Fig. I A). In t<:gration of these areas gave the

10

9

8

'E 7 u

(/) 6 • "-

'" 5 0

x 4 ..:w: <l 3

2

2 3 4

[eTAB/ NaSal]

Fig. 2 - Difference of conductance (L'.k) versus [CTAB] / [NaSal] mole ratio profiles at a fixed overall concentration of 0.10 mmol dm·3 and at 303K .

enthalpies of dilution and these can be expressed per mole of the injectant. The calorimetric method is advantageous, for it can yield both CMC and enthalpy of micelli zation from a single run . Direct detennination of enthalpy by an y other method is not possib le and chances of error in its determ ination from the effect of temperature on CMC determined by a phys icallllethod are obviously more.

Results and Discussion

Stoichimetry of CTAB-NaSal interaction The stoichiometry of the interacti on between CTAB and

NaSal was determined conductometri call y following the Job's method of continuous variation as adopted by l ana and Moulik ' . The spec ific conductances of CTAB - NaSal mixtures at dif­ferent mole ratios but fi xed overall concentrat ion (0. 10 Illmol dm·:') were measured. The values were subtracted from the sum total of the speci fic conductances of the component s, and these differences in conductance (~k) were plotLed against the [CTAB]/[NaSal] mole ratio (Fig. 2). The maximum in ~k was observed at unit mole ratio suggestin g I: I stoichi ometry ror the interacted product. The CTAB - Bi le sa lts systems also ex hibited I: I stoichiometry' .

Critical micellar collcelltratioll (CMC) The CMCs of CTAS and CTAB - NaS al mixtures in the

absence and presence of sal t (NaCi and NaBr) we re deter­mined by tensiometry and micro-calorimetry. The conductance method was not used because or its insensiti veness in sa lt so­luti on to obtain CMC hy comparing the slopes of pre- and post- CMC strai ght lines . The grap hica l rc pre ·entati ons o r CMC yielding plots are omitted to save space ; their types are we ll doc umented in Iiterat ure l

. The CMC va lues reali7.ed by

ROY el af. : STUDIES ON CETYLTRIMETHYLAMMONIUM BROMIDE & SODI UM SALICYCLATE INTE~,8.?~-7-~::::-.... / ~, ., - , """~

/ <-<. \. ~ , .

f O'~ I.e i l' Table I - The effect of addi ti ves on cri tical micell ar concentration (CMC), rmax , Amin and L\G"" fo (pu re . ~1.'T ........

CTAB at different ionic strength at 303 K \ ' . . _, "-..,) . . • ~j;:::..-,,"

Added salt! CMCxlO5/

System mol dm-3 mol dm·3

crAB 0 100 0.01 43.60 0.02 14.40

crABlNaBr 0.05 4.80 0.20 2.10 0.50 1.60 1.00 1.00

0.05 11.00 0.25 3.20

CTABlNaCI 0.50 1.40 1.00 0.50

0.02 12.50 CTABINaSal/ 0.05 9.60 NaCl

a 0.25 5.50 0.50 2.40 1.00 1.00

0.02 11.00 CT ABINaSal/ 0.05 3.20 NaBr

a

0 .25 2.40 0.50 1.40

CTABlNaSal 4xlO-s 40.70 8xl0-5 26.90 Ix 10"'" 20.80 2xlO"'" 19.00 4xlO"'" 10.00

, [CTAB I NaSal] = I : I

the two methods (surface tension and micro calorimetry) are presented in Tables I and 2. Table I contains CMC values of pure CTAB and CTAB - NaSal at different added salt concen­trations in presence and absence of addi ti ves and realized by surface tension method. Table 2 presents CMC values of pure CTAB and CTAB - NaSal at different mole ratios obtained from both micro-calorimetry a nd te ns iom e try. Good aggreeme nt has been observed between the CMC va lues ob­tained by both the methods (Table 2), although method- de­pe'rioent CMC values of surfac tants are not uncommon in lit­erature lJ2. Large differences have been reported for bile saltsl. The results of surface tension method are used for estimating the interfacial para.meters (max imum adsorption and minimum area per molecu le) at the CMC points.

The CMC of CTAB decreases in presence of NaSal and salts, the reduction is more pronounced in presence of NaSal.

,\ I 1 "- . /-\~ '1

A.nin /nm~ r ma.' X lO6/ !l.Go ad / MJ .;; "", , . ' -~.'~~---=::~ mol m·2 kJmor ' kJ mol"

1.95 0,85 -35.30 -31.3 1.53 1.08 -62.20 -35.10 1.38 0.83 -70.10 -40.10 1.72 1.04 -69.20 -45.10

2.07(4.14) 1.24(0.62) -69.3 -48.80 [38(2.76) 0.83(0.42) -80.8 -50.10 1.43(2.86) 0.86(0.43) -80.8 -52 .20

1.37 1.21 -69.80 -42.70 I. 91(3 .82) 0.87(0.43) -69.00 -48.50 2.73(5 .46) 0.61(0.30) -68.10 -54.90 3.01(6.02) 0.55(0.27) -70.9 -59.70

2.15 0.77 -57.20 -37.6 1.51 1.10 -63.40 -38.80

1.89(3 .80) 0.88(0.44) -62.20 -41.10 1.95(3.90) 0.85(0.43). -65.10 -44.50 1.92(3.80) 0.86(0.43) -69.50 -48.10

1.58 1.05 -62.50 -36.30 2.07 0.80 -62.40 -41.20

1.72(3.44) 0.96(0.48) -67.00 -42 .30 1.94(3.88) 0.86(0.43) -66.90 -44.50

1.17 1.41 -66.30 -30.70 1.23 1.35 -64.20 -32.30

1.35(2.70) 1.23(0.62) -63.10 -33.30 0.72(1.44) 2.31(1.15) -90.20 -33.70 1.17(2.34) 1.4ll0.1n -71.90 -36.20

While micell ar charge shield ing in salt environment becomes the reason for lowering of CMC, the significant lowering of CMC by lower concen trat ion of NaSal is a consequence of efficient charge neutra li zation between the complexing ions CTA+ and SaJ'.

Counter-ion binding of lIIicelles The binding of counter-i ons to ionic micelles is the main

reason for the observed break in conductance-concentration plot. Charge neutrali zation between CTA + and Sal' in the mi­ce lle may lead to decreased counter-ion binding by the formed mixed micelle. To decide upon this, frac ti on of counter-i on bound (f) was obtai ned11 according to Eq. ( I) by tensiometric determination ofCMC at different sa lt (SaSa l, NaCI and NaBr) concentrati ons. We reiterate that CMC determination by the conductance method was not done for its insens iti veness in

20 INDIAN J CHEM, SEC. A, JANUARY 1999

Table 2 - Critical micellar concentration, inte rfaci al properties and thermodynamics of CTAB in presence of

additives at different mutual mole ratios at 303K.

System CMCxIO~ I mol dm'} /),(J""m I ~G" od I f"maxl Amini Mf'ml j;)" m I Umor l kJmor l moIm·2 : nm2

kJmor ' JK·1mor l

Calor] Tcnsio

1.00 1.00 -31.30 -35.30 1.95 0 .85 -14.73 8.80 CTAD

CTAB/ NaSal

· 40:1 0.87 -31.90 -10.70 23.20

20:1 0 .64 -33.30 -3.55 49.4 (I.i 0) (-5.61) (42 .9)

\0: 1 0.42 -30.50 -7.06 41.3 ( 1.(0) (-3.76) (52 .2)

4:1 0 .33 0.32 -31.50 -47.70 2.12 0 .78 -9.69 34.6 (1.0) (-2.17) (59.4)

3:1 0.30 0 .27 -31.80 -46.70 2.53 0 .66 -12.20 27.1

1:1 0 .11 (-1.76) (6 \.6)

0.12 -35.60 -49.80 2.79 0 .59 -15.29 24.9 (3.58) (0.30)

1:3 0.08 -37.00 -52.00 2.76 0 .60 (5 .52) (0.30)

1:4 0.06 -38.20 -51.90 2.85 0.58 (5.70) (0.29)

CTAB/ NaCi

1:1 0.73 -18.19 -7.98 -33.7

1:2 0.73 -18.19 -8.83 30.9

a, where CMC values from both measurements available, ACO m values calculated from their aritlunatic mean.

ACOm values for CTABlNaSal ratios 40:1 and 20 :1 have been calculated using fvalue of pure CTAB.

presence of moderate sa lt co ncentrati on.

log CMC = cons tan t - f log [salt] ... ( I )

The plots o f log CMC vs. log [salt] are presen ted in Fig. 3.

The ex tents of coun ter-i on binding for different combinati ons

are presented in Table 3 . A llowing fo r the inaccuracy of the

sa lt vari atio n me thod (Eq . I , F ig.3), it is observed that the

cou nter-i on binu ing ofCTAB mice ll es in the absence of NaSal

fa ll s in the range of 80 - 90% . The resul ts agree fa irly well

w ith the lit eratu re reports,·9. The f values are much lower (0.56

- 0.66) in presence of NaSa l but are not zero. T he mixed mi ­

celle ofClA + - Sal - has got res idua l charges on it. The int e rac­

tion o f added counter-ions (d ifferen t from that contr ibuted hy

the surfactant itse lf) with the ion ic mece ll e m ay not be eq ui va­

lent to that of the parent counter-i on. Th is problem has heen

seldom attended to. Among the s tud ied counte r-i ons, the com­

plex forming Sal- ion has the maximum affinity fo r CTA + ion.

whi ch is followed by CI- and Br- according to the ionic sizes

o f the latter two. The added Sa l- becomes a part of the mi xed

micelle, the effecti ve charge on the micelle gets reduced mak­

ing the Br- ion binding considcrably weaker. In the case of

NaCI , both CI- and Br- ions compete for neutra l iza ti on of the

mice ll ar charge, the overall effect is manifested in r. But f va l­

ues with NaCI and NaBr add ition are comparabl e . Altholl!!h

CTA +and Sal- form I: I compl ex at concentratio ns lower th;n

CMC (Fig . I), the mi xed mice ll e gets g reater share ofCTA + to

m anifest counter-ion binding. Unequal sharing o f componen ts

ROY el at . ; STUDIES ON CETYLTRIMETHYLAMMONIUM BROMIDE & SODIUM SALICYCLATE INTERACTION 21

V IT III I ,ll 7 8

6 7 B

5 6 7 8

4 5 6 u ~ 3 4 5 v u en

2 0 3 4 cr

2 3 III

2 II I

2

2 3 4 5

tog [Satt]

Fig. 3 - Effects of added salt (mol dm·» on the CMC (mol dm·3) of CTAB at 303 K.[ ~ , CTAB - NaSal ; 1'1, CTAB - NaSal -NaCI D, CTAB - NaCl; 0 , CTAB-NaSal-NaBr; . , CTAB­NaBr)

in the mi xed mice lles compared to the stochiometri c compos i­tion of surfac tants in so lution has been reported in litera­ture)4-)X . Lower (but not zero) counter-ion binding by CTAB -bi le salt micelles in I: I rati o has been demonstrated 1 •

/ntelfac ial adso rption The maximum interfac ial adsorpti on f m'" of CTAS

in presence of NaSal , NaC I and NaSr at CMC has been evalu­ated according to a suitab le form of Gibbs adsorption equa­tionu7•411

fma

• = lim (dlt/dlogc) 4.61 R T c-1CMC ... . (2)

where 7t is the surface pressure at the interface (the difference o f su rface tens ion between the so lven t (wate r) surfac tant (CTAS) so lut ion at an y concentration c); Rand Thave their usual significance.

The mini mum area (Ami ,) occupied by the interfac ial mol­ec ules has been esti mated from the relation,

A . = 10'x / N rl1l ax rmn

... (3)

where N is the Avogadro number. The rmax and Amin values in differe nt environments are

presented in Tables I and 2. The results indicate higher rmax (in terfac i:lI adsorption at CMC) in presence of sa lt s as add i­tive· in so lution. The trend in Ami" is ohviously in the direct ion oppos it e to rmax . The factor 4.61 in the den ominator of Eq . (2) depends on the charge of the adsorbcd amphiphile at the ai r / liq uid in te rface. In presence of excess salt , the alll phi phile ions behave like neutra l spec i e ~ and the facto r reaches its lim iting val ue of2.3. In the presence of added NaCI and NaBr

Table 3 - The effect of additi ves on counter-i on association of CTAB

System Counter-ion binding a Literature, .5·9 vatue (f) (I)

CTABlNaSaI 0.56 0.7-<l .8

CTABlNaBr 0.80 0.9

CTABlNaCI 0.86

CT ABlNaSallNaBr 0.58

CT ABlNaSallNaCI 0.66

a Reported counter-ion binding of IT AB in absence of additives can be found in ref. 1 and 5.

at a concentration of 0 .2 to 1.5 mol dnl"\ the fac tor 2.3 is real­isti c. The calculated values o f rm ax and A . on such a bas is mm

are presented in the parentheses in Table I . The results in the parentheses of the last set of Table I and the first se t of Tab le 2 are also based on the same rati onale because their CTAS : NaSal mole ratios are ~ I: I (the molar rati o o f complex for­Illati on). The [salt] dependent trend in the minimum area (A . ) is oppos ite to that of the bulk phenomenon (i.e. micell e fOfl~~­ti on) ; the mice lle formation is fa voured whereas the A . is lowered. Compared to pure CTAB, the drop in A . in ;';:~s­ence of salt is apprec iab le. The observed trends i~~"~he mea­sured parameters in the studied salt environments are consid­ered to be in herently controlled by of interfac ial charge neutralisation in presence ofNaCI, NaBr and NaSal. For quan­tifi cation of the observations on a li rm bas is, further study in thi s direction is worthwhile.

Enthalpy ofCTAB-NaSal interactioll The enthalpy of interact ion of NaS al wi th CTAB has heen

obtained by in teractin g CTAS at a concentrati on below CMC with excess (more than 100 times) of NaSal. The enthalpy value correc ted for the d iluti on of CTAB is 3 1. 8 kJ mol·' at 30° C. Energetics of l1Iicellizatioll and adsorption

The values of standard free energy of micell ization of CTAS-NaSal mixtures have been ca lcul ated from the CMC va lues (Table I) and by Eq. 4 ( Tab le 2) using f val ues given in Table 3.

flG"m = ( 1+1) R T in CMC ... (4)

where the term s have their usual signi ficance. The 1'10'", va lues are plotted in Fig. 4a in terms of bo th

X ['JaS,"- (for the binary mix tures) and square root of ionic strength (-V I-') 111 presence of NaCI and NaSr. The plots are nonlinear ; fiG"", va lues decrease with increasing [sa lt! or X NaSal . The e f­fec t of salt on I: I mi xtu re of CTAS and NaSal is al so similar as on pure CTAS .

22 INDIAN J C HEM. SEC. A. JANUARY 1999

XNaSal XNasal

0.0 0.4 0.8 12 02 0 6 1.0 1.4

a • b

-40

-30

bO CO

0

00 . 0 -50 0

00 E -:, ~:

~ ... '1:, til ·

0

-60 C) ., ... ~

0 0 , 00

..-- -40 ~ . D •

0 I E 6

-:l 0 • ~

E 06 0

l? -50 0 ~

6

o

... • ~ 6 6

6 ... -70 -60

0.0 0.4 0.8 1.2 0.2 . 0.6 1.0 1.4

112 ~

112 . Il

Fig. 4 - f.Gi\,,(a) and f.G"," (b) against mole fraction of NaSal ( XN,S,' ) and square root o f ionic strength ("l/p ) of salt at 303K.

[O.CTAB/NaSal (Table 2); O . CTAB/NaBr (Table I); f. .CTAB/NaCI (Table I); .... . CTAB/NaSaI/NaCI (Table I); • . CTAB/

NaSal/NaBr (Table I))

The standard free energy of adsorption has been also evaluated from f>G"m on the basis of the equation 1.37.

... (5)

The results arc given in Tables I and 2 and plotted in Fig. 4b. Nonlinearity with respect to -YJ.1 and XNaSal is also observed as for f>G"m (Fig. 4A). The addition of salt has also made the interfacial adsorption process more spontaneous. of course with minor variations among the salts. The phenomenon can be rec­ognized as the salting out e ffect.

The energetic parameters f>G"m • f>H"m and l1S"m (presented in Table 2) for pure CTAB, CTAB-NaSal and CTAB-NaCI systems suggest exothermic micellization of both pure CTAB and CTAB in presence of salt; in compari son, micelli zation of CTAB-NaSal system is more exothermic than that of CTAB­NaCI. At very low proportion of NaSal (CTAB : NaSal = 40: I) micellization process is similar to that of pure CTAB. With increasing proportion of NaSal (from a NaSal = 0 .048) , the rnicelli zati on process occurs in two steps (Fig . 5) , which are di stincLly vis ible in the differential plots shown in Fig. 6 . The f>H" values corresponding to the higher CMC (which is al­way~ around 1.0 mmol dm·3) are shown in parentheses in col­umn e ight in the Table 2. The f>H"m' value correspond ing to lower CMC which declines with increasing proportion of NaSal in the mixture, passes through a minimum at arou nd a NaSal = 0.05 ; the observation is closely similar to that of CPC-NaSal system". Microcalorimetri c measurements at higher aNaSal (>

3000

2000

1000

C D -O

co '000 ti 1.0 1.' Ol 'E' 4000

'0 Q)

'0 3000

E (j; 21lOO

a. ro 1000 -

0

0. ' 1.0 1.5 2.0 '000

3000

2000

1000

0 0.0 0.5 u

[eTAS] / mmol dm-·3

Fig. 5 - Enthalpy change per mole of injectan t vs [CTAB) in mmol dm-3 fordiffercnt [CTAB J/[NaSal) mole rat ios. [0. CTAB/NaSal (20: I );f.. CTAB/NaSal ( 10: I); O. CTAB/ ~aSal (3: I) 1

ROY et al.: STUDIES ON CETYLTRIM ETHYLAMMONIUM BROMIDE & SODIUM SALICYCLATE INTERACTION 23

z- 1.5 -

'c :J .ri ~ (,)

:!2 :c-<l :0

2.0

·5000

· 10000

- j5000 -+_~.,..., ----r--.,----,--~-,...~~ _ _ .,........j 0.0 0-2 0 .• 0.6 0.' 1.0 1.2 1..

[eT AS] I mmol dm-3

Fig. 6 - DifTerenti al plot of enthalpy per mole of injectant ; the CMC points are indicated on the curves. [0, CTAB/NaSal (20:1) ; 6 , CTAB/NaSal ( 10:1) ; 0 , CTAB/NaSal(3: 1) ]

2

0

-2 •

-4

"7 . -6 • 0 E -, -8 o ~ . 0

:r: -10 <l

- 12

-14

- 16

- 18

-20

0 10 20 30 40 50 60 7 0

t:;.S/ JK-1 mol-1

Fig. 7 - Enthalpy - entropy compensation curve fo r the micell ization of CTAB - NaSal system at 303K.

a. u

"'-r='

4·0

3 ·6

3 ·2

2·B

2 ·4

1·2

O·B

0.4

0

•

0·20·40·6 0 ·8 , ·0 ' ·2 '·4 1·6 1·8 2.0

[CTAB] x 103 /moi dm3

Fig. 8 - Dependence of vi scosity coeffi cient (11) on [CTAB] for CTAB

- NaSal system ([CTAB]/[NaSal] = I ).

0 .5) could not be fo llowed because o f thickening of the solu­

tion causing inequaliti es in the heat fl ow. The M-i"m and f1S"m va lues are found to fa irl y compensate each other (Fig. 7) with a compensation temperature of 281 K.

Hydrodynamic studies The hydrodynamic properties o fCTAB - NaSa l com­

binations are interesting. Viscoelastic effects o f such mi xtures were studied in the past I 4

•1fi

• Herein the viscosity behav iour at low concentrati ons at various levels of NaSal is presented.

F igure 8 depic ts vi scosity - [CTAB] profiles of equimolar com­positions of CTAB - NaSal at vari ous concentrati on levels at 3011 C. A sharp increase in '1 is observed at [NaSal ] > 0 .22 mmol dm') ; struc tura l trans ition (o ther than mice ll e forma­ti on) is presumed to occur in soluti on. Surface tens ion and ca lorimetri c l1Ieasurement s y ie lded CMC values of 0. 12 and 0.11 mmol dm') res pec ti ve ly fo r I: I CTAB-NaSal mi xtu re; their average, 0.115 mmol dm') is certa inly lower than the above mentioned change in vi scos ity at 0 .22 m mol dIll'·' . Shear vis­cos ity measurements o f I : I CTAB - NaSal at differe nt con­cent rat ions and at di ffe rent shear rates at 30nC are dep ic ted in F ig. 9 . A ft er an initi a l small increase upt o 8.2 mmol d nl') , the viscos ity increases sharpl y with concentrati on and is inverse ly dependent on shear rate. Shear thinning o f the aggregated (v is­coe lasti c) form is envi saged . E ither aggregates break down to small er entiti es or the e longated product gets a li gned in the direc ti on o f fl ow increas ing the so luti on fluidit y. The CTAB -NaSal aggregates appear 10 reta in norm al micell ar archi tec ­ture (with Newtonian flui d behav iour) upto a concentrati on of 8.2 mmol dnl'1, thereafte r, s truc tura l changes occur ; the vi s­cos ity depende nce on log 0 (shear rate) a t d ifferent equimolar l CTAB - NaSa l] is an apprec iable fun cti on of concentrati on (Fig. 10). At 10 and 12 mIllo l dIll 1, the dependence is linear with onward shear thinning. A t hi gher shear rate log 0 > 3.4,

24 INDIAN J CHEM, SEC. A, JANUARY 1999

0-U

"-c-'

13

12

11

10

9

S

7

6

5

4-

3

2

o 2 4 6 8 10 12 14 16 18 20

[CTAB] x 103/ mol dm-3

Fig. 9 - Dependence of viscosity coefficient (11 ) on CTAB - NaSal system at I: I mole ratio at 303K at different shear rates (D). [e, 0 = 2700 sec l ; 0 , 0 = 1617 sec l ; 0 , 0 = 969 sec ' ]

100r--------------------------------.

90

0-

80

70

60

Z 50

1='40

30

20

10

1·0 1·2

tog D

Fi g. 10- Shear rate (D) dependent viscosity coefficient (11) of CTAB - NaSal at I: I mole ratio at 303 K with varying [CTAB]. [CTAB1=20mmoldm') [O] , 15mmol dny' [e] , 12m mol dm') [0], 10m mol dm') [ill

the studied sol uti on appears to culmi nate to shear independent stages. The complex thus appears to withstand considerable shearing st ress. Earlier reports l ~.20 have conclusively supported the viscoe lastic ity and shearthinning of the 'worm orrod-like' CTAS - NaSal aggregates that may form at hi gher concentra­ti ons through transi tion from spherica l micelles. The soluti on may also show a power law behaviour'~. It may be added that a shear thickening ofCTAS - NaSal has been also repor\ed~n. ~,

due to cont inuous kinetic coagulation forming large micell es

upto CTAB - NaSal ratio of unity; thereafter, the structure breaks down. We have not observed the shear thickneing behaviour of the system.

Conclusions The fi ndings of the present study lead to the followi ng con­

clusions : (I) The CTAS - NaSal form a I: I molecular complex in

solution and the mixed system micellizes at 0.1 2. mmol dm ') at 30°C.

(2) The micellization of mixed CTAB - NaSal system and its in terfacial adsorption are both more spontaneous (i.e. hav­ing lower free energy change) than those in the pure CTAB.

(3) The enthalpy of micellization of CTAS and CTAS­NaSal combi nat ions is exothermic. For the CTAB-NaSalmo­lar ratios from 20: I to 3: I the microcalorimetric method has evidenced a two-step micellization process.

(4) The I : I CTAS - NaSal mixture above its CMC is vis­coelastic and exhibits shear thinning properties.

Acknowledgement B.K. Roy thanks Jadavpur Universi ty for laboratory facili­

ties and DST, (New Delhi), for microca lorimetric measure­ments facilities provided to the Centre for Surface Science.

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