poly(propylene glycol) mixed micelles - American Chemical


Química-Física, Universidad de Alcalá de Henares, Alcalá de Henares, 28871 Madrid, Spain. Received: May 4, 1993; In Final Form: August 24, 1993®...
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J . Phys. Chem. 1993,97, 12387-12392

12387

CTAB/Poly (propylene glycol) Mixed Micelles: Characterization and Its Properties as a Reaction Medium M. L. Sierra and E. Rodenas’ Dpto. Quimica- Fisica, Universidad de Alcalb de Henares, Alcalh de Henares, 28871 Madrid, Spain Received: May 4, 1993; In Final Form: August 24, 1993”

ThesystemCTAB/poly(propyleneglycol)(PPG) ( M W = 425,1000) at low concentration has beencharacterized by conductivity, steady-state fluorescence quenching, and surface tension measurements. Its influence in the basic hydrolysis of the crystal violet has also been studied. The results show that PPG ( M W = 425, 1000) behaves as a medium chain alcohol.

Introduction The effect of macromolecules in micellar systems’s2and the importance of surfactant/polymer mixed micelles in the oil recovery industry, pharmacy, medicine, and even in cell fusion protocols3 is well-known. Most of the studies in the literature are related to systems formed by anionic surfactants and nonionic polymers, such as SDS and PEO (poly(ethy1ene oxide)) with molecular weights larger than 10 000. These systems have been characterized by different techniques, such as surface tension: conductivity,5q6 relaxation T - j ~ m p viscosity? ,~ light-~attering,’-~ NMR paramagnetic relaxation,I0time-resolve fluorescence quenching,’l . I 2 electrophoresis,sand ESR.I3 From the experimental results it can be deduced that PEO wraps around the micelle while most of the polymer molecules form loops in the surrounding ~ a t e r ; ~ and v ~ *in~ the case of surfactants with small head groups, such as SDS, the formation of aggregates at a concentration lower than the cmc is induced by the presence of p o l ~ m e r . ~ . ~ Studies of cationic micelles’ interactionswith nonionic polymers are now increasing, and the effect of polymers such as ethyl(hydroxyethyl)cellul~se,~~ hydroxypropylcellulose,ls poly(viny1methyl ether) (PVME),I6 poly(propy1ene oxide) (PPO),I7 poly(ethylene oxide) (PE0),l8 and poly(vinylpyrro1idone) (PVP)16v19 has been studied. From experienceit seems that cationic and nonionic surfactants weakly interact with nonionic polymers, although recent publications also show strong interaction between the cationic surfactant and the nonionic polymer,I6even though the aggregation is not induced by the polymer, such as with DTAC.l* In this paper we present the results obtained with the system CTAB/PPG (MW = 425, 1000) at low concentration, 1:l molar ratio. The system has been characterized by steady-state fluorescence quenching measurements, conductivity, surface tension, and the system influence on the basic hydrolysis of crystal violet have been studied. The kinetic results have been analyzed taking into account ion distribution around micelles according to the nonlinearized Poisson-Boltzmann equation.

Experimental Section CTAB (Sigma),poly(propyleneglycol) (MW = 425 and 1000) (Aldrich), crystal violet (Merck), KBr (Merck, 99.5%), and NaOH (volumetric standard, Carlo Erba) were used without further purification. Pyrene (Merck) and cetylpyridinium chloride (Merck) were recrystallized several times, pyrene from methanol (Scharlau, gradient HPLC grade) and cetylpyridinium chloride from MeOH/Et,O, cetylpyridinium was first dissolved

* To whom correspondence should be addressed.

* Abstract published in Aduonce ACS Abstracts.

November 1 , 1993.

in the minimum amount of MeOH and then precipitated adding Et20 (Scharlau, purissimo). Specific conductivities were carried out in a Crison 525 conductimeter, cell constant 0.969 cm-I. Surface tension was measured in a Lauda TE-IC tensiometer connected to a Epson HX-20 computer. The kinetics were run in cuvettes using a Hewlett-Packard 8452 diode array spectrophotometer. The reaction was followed at 590 nm, A,,, of substrate absorbance in water. The crystal violet concentration was 2.67 X mol dm-3 in all the experiments. The hydroxide ion concentration was always kept in large excess with respect to the substrate. The CTAB/PPG molar ratio was always kept as 1:l. The rate constants were calculated with an incorporated program in the spectrophotometer through a Marquardt algorithm. Steady-state fluorescence measurements were carried out in a Perkin-Elmer LS-5B spectrofluorometer. Pyrene was excited at 336 nm, and its emission was monitored at 375 and 386 nm, corresponding to the first and third vibronic peaks of pyrene. All theexperiments were carried out at a constant temperature of 25 f 0.1 OC. Rb?SdtS

Conductivity Study. Specific conductivity of the system CTAB/ PPG (MW = 425, lOOO), molar ratio 1:1, and at a fixed PPG concentration are represented in Figures 1 and 2, respectively. From the figures it can be deduced that in the CTAB/PPG system, molar ratio 1:1, the cmc is the same as in CTAB micelles in aqueous solution (cmc = 9 X 1 V mol dm-’), while in the system with a [PPG] = mol dm”, the cmc slightly decreases, 8.8 X 1Vmol dm-j for MW = 425 and 7.0 X 10-4 mol dm-’ for MW = 1000. The fraction of micellar head groups neutralized, 8, was obtained from the micellar surface ionization degree, a,given as the ratio of the slopes above and below the cmc, /3 = 1 - a.j3 = 0.661 f 0.025 for the system CTAB/PPG (MW = 425) and j3 =0.323&0.130forthesystemCTAB/PPG(MW = 1000),both molar ratio 1:l. For the systems with a [PPG] = mol dm-3, /3 = 0.648 f 0.077 for MW = 425 and j3 = 0.367 & 0.104 for MW = 1000. Surface Tension Measurements. The surface tension of the system CTAB/PPG, molar ratio 1:1, is not influenced by the presence of the polymer and the obtained cmc value agrees with the conductivity value. The surface tension of the system at a fixed polymer concentration, 1 W mol dm-’, and with varying surfactant concentrations, does not indicate the presence of the critical aggregation concentration (cac) which should appear at a lower concentration than the cmc. The cac is related to the polymer’s influence in the micellization process, and other such systems, such as with SDS, have been previously d i s c ~ s s e d . ~ . ~ , ~

0022-365419312097-12387$04.00/0 0 1993 American Chemical Society

12388 The Journal of Physical Chemistry, Vol. 97, No. 47, I993

251

04

Q

1

I

3

I

5

lo3 [CTAB/PPG] (M)

Figure 1. Specific conductivities of CTAB/PPG system, ratio 1:1, compared to specific conductivity of CTAB. 0 denotes CTAB only, 0 CTAB/PPG (MW = 425),and A CTAB/PPG (MW = 1000).

Sierra and Rodenas

TABLE I: Micellar Aggregation Numbers at Different [CLWPPCI, Molar Ratio 1:1, Salt Concentration, and at a Fixed CTAB Concentration Varying PPC Concentration Compared to Aggregation Numbers of Other CTAB/Additive Systems [CTAB] [additive] [KBr] system (M) (MI (M) N CTAB" 0.011 52 0.061 CTAB' 65 0.011 0.25 CTAB/ 1-ButOH' 37 54 0.061 0.25 CTAB/ 1-ButOH' 0.004 0.004 CTAB/ 1-HexOHb 55 CTAB/ 1-HexOHb 0.008 0.008 55 0.012 CTAB/ 1-HexOHb 0.012 55 0.020 CTAB/ 1 -HexOHb 0.020 55 CTAB/ 1-HexOHb 0.040 0.040 55 0.060 0.060 CTAB/ 1 -HexOHb 55 CTAB/PPG (MW = 425) 0.005 0.005 24 3 0.010 CTAB/PPG (MW = 425) 0.010 36 f 20 CTAB/PPG (MW = 425) 0.020 48 18 0.020 0.050 0.050 CTAB/PPG (MW = 425) 52* 12 0.020 CTAB/PPG (MW = 425) 0.010 40 f 5 0.020 CTAB/PPG (MW = 425) 0.040 35 16 0.020 0.060 CTAB/PPG (MW = 425) 31 14 0.020 0.080 CTAB/PPG (MW = 425) 26* 15 0.010 CTAB/PPG/KBr 0.010 0.020 39f6 0.010 CTAB/PPG/KBr 0.060 38 f 5 0.010 0.005 14&2 CTAB/PPG (MW = 1000) 0.005 0.010 CTAB/PPG (MW = 1000) 0.010 18f2 0.020 CTAB/PPG (MW = 1000) 0.020 18f7 0.050 CTAB/PPG (MW = 1000) 0.050 20f6 CTAB/PPG (MW = 1000) 0.020 0.010 20 5 0.040 CTABIPPG (MW = 1000) 0.020 19h7 0.060 CTAB/PPG (MW = 1000) 0.020 18k 1 1 0.010 CTAB/PPG/KBr 0.010 0.060 2 8 h 4 0.010 CTAB/PPG/KBr 0.010 0.100 28 f 1 a PCrez-Benito,E.; Rodenas,E. An. Qulm. 1990,86,126-31. Valiente, M.; Rodenas, E. Longmuir 1990,6,115-82.

*

*

- I

00

05

15

10

2.0

lo3 [CTAB] (M)

Figure 2. Specific conductivity of the CTAB/PPG system when [PPG] = lo-* mol d w 3 is fixed and only [CTAB] is varied. 0 PPG (MW = 425) and A PPG (MW = 1000). Cmc value for CTAB/PPG (MW = 425) is 8.8 X l e mol d m 3 and 7.0 X l e mol dm-3 for CTAB/PPG (MW = 1000).

Fluorescence Measurements. The aggregation numbers were obtained from steady-state fluorescence quenching measurements using pyrene as a probe and cetylpyridinium chloride as a static quencher,20in accordance with the treatment in the literature.21*22 We can consider that probe and quencher are located in the same environment of the micelle than in simple CTAB micelles in aqueous solution, as cetylpyridinium chloride acts as a quencher of the probe pyrene. Both probe and quencher distribute between the aqueous and micellar pseudophase according to Poisson statistics. Therefore, pyrene fluorescence intensity in the absence and in the presence of quencher is related to the average aggregation number by ( l ) , where ft is the average quencher ln(Zo/Z) = ft = [Q]N/[Dn] occupation number and N denotes the aggregation number. ZO represents the pyrene emission intensity in the absence of quencher, and I the pyrene emission intensity in the presence of quencher. [Dn] is the micellized surfactant concentration ([Dn] = [D] cmc, [D] is the total surfactant concentration, and cmc is the critical micelle concentration obtained by conductivity measurements for both systems). The experimental results fit the theoretical treatment and the aggregation numbers obtained at different CTAB/PPG concentrations are given in Table I. In the table are also included aggregation numbers for the system in the presence of bromide (KBr), the micelle counterion, CTAB aqueous micelles, CTAB/

l - B ~ t o H , and ~ ' CTAB/ 1-HexOH mixed micelles.24 From the results it can be deduced that the presence of polymer decreases the micellar aggregation number of CTAB micelles, such as seen with the incorporation of 1-ButOH. Other authors have also obtained smaller aggregation numbers for surfactants in the presence of polymers, using specific ion-electrode,*s fluorescence.26 and neutron-s~attering~~ techniques. From the pyrene fluorescence spectrum it is possible to estimate the dielectric constant of the micellar interface where the probe is supposed to be located. It is well-known that the intensity ratio Z ~ / I I I Iof the pyrene spectrum is affected by the polarity of the medium.28 Theintensityratioof thevibronicpeaks I-II1,obtained in the literature for solvents with different dielectric constant, shows that thereis a decrease in the ratiolI/ZrII when thedielectric constant decreases, i.e., the polarity decreases. The ratio II/IIII obtained for the CTAB/PPG system was 1.10-1.12 for M W = 425, and 1.06-1 -08for M W = 1000 in a surfactant concentration range (5-50) X lo-' mol dm-', so that the effective micellar interface dielectric constant is approximately 17 for M W = 425 and 13 for M W = 1000. The effective dielectric constant for the CTAB aqueous micelles is 35,23 noticeably larger than that obtained in the presence of polymer. Kinetic Results. The pseudo-first-order rate constants for the basic hydrolysis of crystal violet a t different CTAB/PPG (MW = 425, lOOO), molar ratio 1:1, and hydroxide ion concentrations are given in Figure 3. The effect of an additional amount of KBr is shown in Figure 4. The mixed micelles produce a catalytic effect on the reaction, there is a first increase of the pseudofirst order rate constant with surfactant concentration, reaching a maximum and after that the constant decreases. This is the typical behavior of micelles whosecounterions aredifferent from the reactiveions. Thekinetic constants obtained in these systems are lower than the ones obtained for CTAB midelles in aqueous solution,29which indicates

The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12389

CTAB/Poly(propylene glycol) Mixed Micelles

‘1

01 0

20

40

Bo

I

80

1OS[CTAB/PPC] (M) Figure 3. Pseudo-first-order rate constants for the basic hydrolysis of crystal violet at different surfactant/polymer, ratio 1:1, and hydroxide ion concentrations. Solid lines are calculated values with the electrostatic model. 0 [NaOH] = 10 X l t 3 mol dm-’ and 0 [NaOH] = 6 X 16’ mol dm-’ for CTAB/PPG (MW = 425), and 0 [NaOH] = 10 X lO-’ mol dm-3 for CTAB/PPG (MW = 1000). that the PPG incorporation produces an apparent inhibitory effect on the reaction. Figure 4 represents the typical effect of the presence of B r , the surfactant’s counterion, in the system. They produce a decrease in the pseudo-first-order rateconstant which is explained assuming that the reactive ions, OH-, are substituted by the counterions in the Stern layer, where the reaction is supposed to occur.

Theoretical Description of the Electrostatic Model We have used the electrostatic model reported in ref 33 to fit the kinetic results in CTAB/PPG micelles. This theoretical treatment is based on the pseudophase kinetic model given by Menger30 and developed by Buntonj’ and Romsted,j* but it considers ion distribution around micelles according to the Poisson-Boltzmann nonlinearized equation, and specific interaction between the bromide and hydroxide ions with the micellar surface have been included. The rate equation can be written in terms of effective concentrations in aqueous and micellar phases, expressed in eq 2, where kw and k, are the second-order rate constants for the

(2) reaction in the aqueous and micellar phases, respectively, being s-I for the basic hydrolysis of crystal violet,j4 [SM] and [OH,] are the effective substrate and hydroxide concentration in the micellar phase, respectively, [S,] and [OH,] are the effective substrate and hydroxide concentration in the aqueous phase, and V is the volume element of the micellar phase per mole of micellized surfactant where the reaction occurs. The substrate distribution between both phases can be explained by considering an ideal substrate behavior. The electrochemical substrate potential in the micellar and in the aqueous phases are the same at equilibrium:

kw = 0.201 M-l -

-

poSM

+ RTln[SM] + ~$4,

= NOS,

-

+ RTln[Sw] +

zsF4w (3) Here pas, and p0sw denote the standard chemical potential of the substrate in the micellar and aqueous phases, respectively, 4, corresponds to the potential in the layer a t Ar and 4, the potential in the bulk solution, 4~ = 0. The substrate partition

d0 do [KB~](M) Figure 4. Pseudo-first-order rate constants for the reaction at different mol dm-’ for both KBr concentrations for [CTAB/PPG] = 10 X molecular weight. Solid lines are calculated values with the electrostatic model. 0 [NaOH] = 10 X l e 3 mol dm-’ and 0 [NaOH] = 6 X mol dm-’ for M W = 425, and 0 [NaOH] = 10 X mol dml-’ for MW = 1000. -0

$0

d0

40

10’

coefficient, Ps,between both phases is given by

--

ps = [ s ~ l / [ s w l= Po, ~XP[+ZS(~M - 4w)/kTl (4) where Pas = exp(posw - posM)/RT and ( 4 -~)4, denotes the potential barrier the substrate must overcome to dissolve in the micellar pseudophase. In this study we have considered this value to be given by the Poisson-Boltzmann equation at thepoint Ar = 4 A, where the reaction is supposed to occur, 4 4 ~ . [s,] and [S,] are related to the analytical concentrations by

[sMl

= ([SMl/[DnlV);

[sWl

=

[sWl/l

- iDnIv

(5)

in our experimental conditions 1 - [Dn]V 1. From eq 2 the pseudo-first-order rate constant can be easily deduced, giving expression6

k ~ is~the0 first-order rate constant for the reaction in H20 with a value of 1.94 X l t 5s-],j4 [OH] is the total hydroxide ion concentration. To fit the experimental results, it is necessary to know mOH, the fraction of micellar head groups neutralized by the hydroxide ion, and the potential the substrate should overcome in order to solubilize inside the micelles, 4 4 ~ . Both parameters can be obtained from the electrostatic treatment which considers ion distribution around micelles based on the Poisson-Boltzmann nonlinearized equation and takes into account specific interactions between bromide and hydroxide ions in solution and the micellar surface. The theoretical treatment is based on the cell model. The total volume of the micellar solution is divided into cells, each containing one micelle, and the amount of water and electrolyte given by the whole concentration of the particular system. Under our experimental conditions, CTAB/PPG mixed micelles have been considered spherical as with CTAB,35and the ions are distributed in the region r, < r < r,, where r, denotes the micelle radius and r, the cell radius. The nonlinearized Poisson-Boltzmann equation for spherical symmetry and if only monovalent ions are present in the solution, is expressed as7 ereo( 1/ r 2 )d / d r (r2 d#/dr) = -p = - F [ C + exp(-e$/kT) ~

-

(CAI exp(e4lkT))I (7) where er is the relative permittivity, assumed to be constant in the

Sierra and Rodenas

12390 The Journal of Physical Chemistry, Vol. 97, No. 47, 1993

cell with a value of 78.54, €0 is the vacuum permittivity, T i s in kelvin, k is the Boltzmann constant, F is the Faraday number, 4 is the potential, p is the charge density, e the electron charge, and C+O and C_O are the concentration (mol m-3) of positive and negative ions, respectively at the point of the solution where 4 = 0. This equation can be solved with the boundary and electroneutrality conditions +(r=rc) = 0

(8)

([email protected]/dr),=, = q5’(r=rc) = 0

(9)

(d&/dr),=,m = &(r=r,,,)

= -u/crco

(10)

where u is the micellar surface charge/m2. A first calculation which neglected specific interaction between hydroxide ion and micellar surface was used to explain kinetic data in CTAB micelles,29 CTAB/alkanols mixed micelles,36 CTAB/oils mixed micelles,37CTAB/ 1-hexanol reverse micelles,38 and SDS/1-hexanol reverse micelles.39 But this treatment was unable to explain the kinetic results for very hydrophobic substrates; therefore, we corrected the former treatment so that hydroxide ion specific adsorption was ~ o n s i d e r e d . ~ ~ In thecaseof the CTAB/PPG mixed micelles and in theabsence of ions in solution, we have considered the following equilibrium for the bromide and hydroxide ion contact adsorption: mBrCA/([BrNCAl(l - mBrCA - mOHCA)) = KBrCA

mOHCA/([oHNCA](l

- mBrCA

exp(e&/kg

(

- MOHCA)) = KoHCA e x ~ ( e $ o l k T ) (12)

[B ~ N C A and ] [O&cA] are the nonspecifically adsorbed bromide and hydroxide ions concentrations in the bulk solution, (1 - mgrcA - mOHCA) represents the fraction of free micellar surface, KOHCA and KB~CA denote the hydroxide and bromide ion specific adsorption equilibrium constant, and 40 represents the micellar surface potential. Thus, the micellar surface charge density is given by (7

= Ne( 1 - mBrCA - mo~c~)/47Tr,,,’

(13)

where mBrCA and moHCA are the fraction of micellar head groups neutralized with specifically adsorbed bromide and hydroxide ions. The solution to the Poisson-Boltzmann equation gives the average concentration of bromide and hydroxide ions that distribute in a layer thickness Ar around the micelle, which is assumed to be constant, Ar = 4 A. The fraction of micellar head groups neutralized with statistically distributed bromide and hydroxide ions is expressed as

(14) where N A is Avogadro’s number. It can be considered that this amount of ions statistically distributed in the layer rm + Ar moves with the micelle and neutralizes it, so that the total fraction of micellar head groups neutralized can be given by

B = mBrCA + mOHCA + mBrNCA + lllOHNCA

(

and mOHNC.4 can be easily deduced and given by

where [OHcA] is the specifically adsorbed hydroxide ion concentration in solution.

The parameters C+O and c4 are related to the whole number of positive and negative ions in the cell, n+ and n-, by the normalization conditions n+ = c+,JVA~exp(-e4/kT)(4~r2) m dr

(17)

n- = cJVAS,r’exp(e4/kT)(4~r2) m dr

(18)

The micellar surface potential depends on the amount of ions statistically distributed, which depends on the amount of specifically adsorbed ions, which also depends on the micellar surface potential. Thus to solve the Poisson-Boltzmann equation, we have used an iterative calculation method with the fourth-order Runge-Kutta method, giving initial values to the micellar surface potential, 40, C+O, and c-0 and optimizing them to a complete solution using the boundary (eq 9 and 10) and normalization (eqs 17 and 18) conditions. The calculated results were checked using eq 13. The numerical integral values, n+ and n-, were obtained by Simpson’s rule. In the calculation, cells have been considered spherical with a cell radius, r,, well defined by 4

/gvC3 = N/~o~N,([D] - cmc)

(19)

Theoretical Treatment Application to the Kinetic Results in CTAB/PPC Mixed Micelles To fit the kinetic results to the theoretical treatment, it is necessary to know the micelle volume, which is difficult as it depends on the polymer location, how many monomeric units are inside the micelle and the polymer concentration solubilized in the micelle. At the surfactant concentration used in this paper an increase in PPG (MW = 425) concentration produces a cloudy solution which does not separate into phases even after several months. Therefore we have considered that the polymer incorporation in CTAB micelles is of such an extent to be able to suggest that all the polymer is solubilized in the CTAB micelles. In the case of PPG (MW = 1000) an increase in polymer concentration produces phase separation. Polymer incorporation constant in CTAB micelles can be obtained from solubility measurements, according to a treatment for medium-chain alcohols,4° which considers alcohol incorporation according to the expression KS = [ S ~ l / ( [ S w j ( [ D n ]+ [SW])). The KS value obtained from solubility measurements is 18.2 M-I, higher than the value 10 M-l given for 1-HexOH. However, it has been recently observed that for pure cationic surfactants it is extremely difficult to determine the phase boundary by visual inspection: solutions which are in the two-phase region looked transparent and isotropic.41 Therefore, taking intoaccount that incorporation constant values in micelles are higher than those obtained from solubility measurements, we have considered the same proportion, 1:1, of polymer inside the micelle. If the incorporation constant for the PPG incorporation into micelles, is taken into account, the results remain unaffected. The polymer/surfactant ratio has been taken as 1:1, and the complete solubility of the polymer in the micelle has been considered, so that according to Tanford’s expression4*the micellar radius, rm,including the volume of the PPG molecules, is given by 4

/3“7,3

= N[27.4

+ 26.9nC+ 4/37r X 2.33 X a]

(20)

where n, = 15 for CTAB. The third term corresponds to the monomer volume, with a monomeric radius of rp = 2.3 0.2 A. a represents the number of monomeric units of each molecular weight: a = 6 for M W = 425 and a = 15 for M W = 1000. The cmc value used in the calculation takes into account the ion’s influence and was calculated by an empirical equation.43

CTAB/Poly(propylene glycol) Mixed Micelles

The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12391

TABLE II: Calculated Values of Micellar Surface Potential, 40 and 44~,Fraction of Micellar Head Groups Neutralized with Specifically and Nonspecifically Bromide and Hydroxide Ions, SA, ~ O H C A , ID&NCA, and ~ O H N C A , Fraction of Micellar Head Groups Neutralized by the Specifically and Nonspecifically Adsorbed Hydroxide Ion, OH and the Fraction of Head Group Neutralized, 8, for the System (TAB/PPG (MW = 425) [CTAB/PPG] (M)

N

*O

(mv)

*4A

(mv)

mOHNCA4A

~ B I N C A ~ ~ mOHCA

MBrCA

mOH

B

[NaOH] = 0.006 M 0.004 0.005 0.010 0.020 0.030 0.040 0.050 0.080

23 26 36 47 44 45 44 45

86.7 86.3 83.8 77.7 70.8 65.8 62.0 54.2

60.6 60.9 58.8 55.3 48.6 45.3 41.7 35.2

0.004 0.005 0.010 0.020 0.030 0.040 0.050 0.080

23 26 36 47 44 45 44 45

83.6 82.8 80.8 75.8 68.8 64.0 60.2 53.2

56.2 56.8 55.1 51.9 46.4 43.5 40.3 34.2

0.0376 0.0343 0.0257 0.0186 0.0138 0.01 18 0.0101 0.0077

0.0165 0.0175 0.0209 0.0248 0.0257 0.0280 0.0291 0.0329

0.0220 0.0202 0.0150 0.0100 0.0072 0.0057 0.0048 0.0033

0.449 0.483 0.577 0.642 0.660 0.672 0.682 0.703

0.060 0.055 0.041 0.029 0.021 0.018 0.015 0.01 1

0.525 0.555 0.639 0.695 0.707 0.7 18 0.726 0.747

0.0148 0.0161 0.0197 0.023 1 0.0253 0.0277 0.0292 0.0328

0.0334 0.0306 0.0230 0.0158 0.01 15 0.0091 0.0076 0.0054

0.423 0.455 0.554 0.629 0.647 0.661 0.671 0.697

0.088 0.08 1 0.062 0.044 0.033 0.028 0.024 0.018

0.525 0.552 0.636 0.695 0.706 0.7 17 0.725 0.747

[NaOH] = 0.01 M

W r l (MI

N

*O

(mv)

* 4 ~(mv)

0.0544 0.0506 0.0390 0.0280 0.0219 0.0189 0.0165 0.0126 mOHNCA4A

mBrCA

mOH

B

0.0035 0.0020 0.0014

0.694 0.732 0.749

0.014 0.009 0.007

0.749 0.792 0.816

0.0058 0.0033

0.690 0.725

0.022 0.015

0.752 0.790

~ B I N C A ~ ~ mOHCA

[NaOH] = 0.006 M, [CTAB/PPG] = 0.01 M 0.020 0.040 0.060

43 43 43

54.0 43.0 35.5

35.9 26.2 21.1

0.020 0.040

43 43

53.8 42.2

34.3 25.5

0.0104 0.0070 0.0057

0.0415 0.051 1 0.0603

[NaOH] = 0.010 M, [CTAB/PPG] = 0.01 M 0.0164 0.0115

0.0392 0.0501

TABLE III: Calculated Values of Micellar Surface Potential, 40and 4 4 ~ Fraction , of Micellar Head Groups Neutralized with Specifically and Nonspecifically Bromide and Hydroxide Ions, ~ I C A ,~ O H C A , ~ , N C A ,and DIOHNCA, Fraction of Micellar Head Groups Neutralized by the Specifically and Nonspecifically Adsorbed Hydroxide Ion, OH and the Fraction of Head Groups Neutralized, B, for the System (TAB/PPC ( M W = 1000) ~

[CTAB/PPG] (M)

N

*O

(mv)

*4A

(mv)

mOHNCA4A

~ B I N C A ~ ~ mOHCA

mBrCA

mOH

B

0.151 0.242 0.294 0.361

0.079 0.067 0.045 0.027

0.262 0.355 0.399 0.470

[NaOH] = 0.01 M 14 19 19 20

0.005 0.010 0.020 0.050

78 78 69 56

52.3 52.6 45.3 34.5

0.0721 0.0604 0.0413 0.0250

0.03 17 0.0474 0.0598 0.08 16

0.0070 0.0062 0.0041 0.0022

[NaOH] = 0.01 M, [CTAB/PPG] = 0.01 M 0.020 0.060 0.100

24 29 32

61.5 48.0 40.0

39.1 27.0 21.1

0.0348 0.0203 0.0156

The aggregation numbers used were the ones determined by fluorescence quenching measurements and given in Table I1 for PPG (MW = 425) and Table I11 for PPG (MW = 1000). As a first step it is necessary to obtain the value of bromide ion specific adsorption constant, Ker, which explains the experimental fraction of micellar head groups neutralized in the absence of hydroxide ions. It was necessary to decrease the bromide specific adsorption equilibrium constant, compared to CTAB micelles, Ker = 30. The values that best fit the results in CTAB/ PPG mixed micelles are K B = ~ 12 mol-' dm3 for PPG (MW = 425) and Ker = 2 mol-' dm3 for PPG (MW = 1000). TheratioKBr/KOH = 50 has beenmaintained throughout, such as in CTAB micelles. This value corresponds to the ion-exchange equilibrium constant being bromide and hydroxide ions, according to the ion-exchange equilibrium model given by R ~ m s t e d , and ~* explains plenty of kinetic results for ionic surfactants with different nonionic substrate^.^^ In Tables I1 and I11 are given the calculated values for 40,44~, the fraction of neutralized micellar head groups with specifically adsorbed bromide and hydroxide ions (mBrCA and ~ O H C A ) ,the fraction of neutralized micellar head groups with nonspecifically adsorbed OH and Br ions (mOHNC.4 and m g l N C A ) , the neutralized

0.0929 0.1341 0.1646

0.0028 0.0014 0.0009

0.366 0.459 0.499

0.038 0.022 0.016

0.497 0.614 0.680

micellar head groups with specifically and nonspecifically adsorbed OH ion, mOH, and the total fraction of neutralized micellar head groups, 0,for different surfactant/polymer concentrations and both molecular weights, respectively. According to the data given in the tables, polymer incorporation in CTAB micelles produces a decrease in the micellar surface potential although the mOH values are nearly the same, as plotted in Figure 5, where the data for CTAB micelles in aqueous solution were taken from ref 33. The micellar surface potential and m O H decrease with surfactant concentration which agree with the results given in the l i t e r a t ~ r e . ~ ~ To fit the experimental kinetic results to eq 6, we have used m O H and 4 4 as ~given in Tables 11and 111, and P0s and k, have been taken as adjustable parameters. The volume element, V, is considered as the volume of the Stern layer, Le., the volume of the layer between r, and r, + Ar, as we must take into account that crystal violet is a very hydrophilic substrate, and according to our experimental evidence in which 1-butanol incorporation in the CTAB micelle displaces the substrate from the micellar to the aqueous we can safely assume that the substrate is located in the Stern layer of the CTAB/PPG mixed micelles, as in the simple CTAB micelles.

Sierra and Rodenas

The Journal of Physical Chemistry, Vol. 97, No. 47, 1993

0252 and PB 92-1080, and the Ministeriode Educacidn y Ciencia

100-

-

; > E

of the Spanish Government (M. L. Sierra) is gratefully acknowledged.

-Bo

Bo-

3 B

60-

40-

20-

0

do

iOa[CTAB~PPG] (Y)

do

e%0

Figure 5. Micellar surface potential, %, and mOH values for CTAB and CTAB/PPG system, molar ratio 1:l. 0 denotes the micellar surface potential for CTAB, 0 for the surfactant/polymer M W = 425 and 0 for M W = 1OOO. 0 denotes the mOH of CTAB, for M W = 425, and for M W = 1000.

The values that best fit these results for CTAB/PPG (MW = k, = 0.247 M-1 s-1 and P0s = 11 000, nearly the same values as for CTAB micelle^,)^ but in the case of CTAB/PPG (MW = 1000) mixed micelles it is necessary to decrease the rate constant in order to fit the experimental values. The parameters that best fit the experimental data are k, = 0.100 M-I s-I and P0s = 7500. The rate constant decrease in the CTAB/PPG (MW = 1000) mixed micelles cannot be explained by taking into account the effect of the dielectric constant in the micelle, and it should be considered that the reaction medium CTAB/PPG (MW = 1000) is different to theCTAB/PPG (MW = 425) mixed micelles or CTAB micelles in aqueous solution medium. The calculated values for the pseudo-first-order rate constants are represent by solid lines in Figures 3 and 4. We repeated thecalculation considering that thePPG molecule was not completely solubilized in the micelle, but the results obtained were not different enough to predict the PPG location in the micelle. 425) mixed micelles are

Conclusions From all the experimental results we can conclude that poly(propylene glycol) (MW = 425, 1000) strongly interacts with CTABmicellesto form CTAB/PPG mixed micelles. The polymer incorporation in CTAB micelles produces an increase in the micellar ionization degree and a decrease in the surfactant aggregation number and the cmc value. These are the same effects found with the medium-chain alcohols. The polymer incorporation in CTAB micelles also produces an apparent inhibitory effect on the basic hydrolysis of the crystal violet, compared to the effect found in CTAB micelles. This effect is related to (1) the increase in the micellar ionization degree, related to the micellar surface potential the substrate should overcome to solubilize in the micelle, (2) the increase of the volume element where the reaction is supposed to occur, which produces a dilution of the reactants, and (3) there is a change of the apparent dielectric constant of the reaction medium of CTAB/PPG (MW = 425) compared to CTAB/PPG (MW = 1000).

Acknowledgment. Financial support of this work by Comisi6n Interministerial de Ciencias y Tecnologia (CICYT), MAT90-

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