Interactions between alkyl xanthates and cationic micelles

Interactions between Alkyl Xanthates and Cationic Micelles L. SEPULVEDA AND J. Pt~REZ-COTAPOS Department of Chemistry, Faculty of Sciences, University of Chile, Las Palmeras 3425, Casilla 653, Santiago, Chile Received January 3, 1985; accepted March 29, 1985 Interactions between methyl, ethyl, n-buthyl, n-amyl, and n-hexyl xanthates with monomers and micelles of cetyltrimethylammonium bromide CTAB were studied. Xanthates have a pronounced absorbance peak at 301 nm which is shifted to 306 nm when in presence of CTAB micelles. A maximum differencein absortivitiesbetween the micellar bound and free xanthates was found at 313 nm. This wavelength was used to measure the amount of xanthate associated to micelles. An insoluble ion pair formedbetween xanthates and CTA monomers which is graduallyredissolvedby increasingamounts of CTAB was detected. Association constants of xanthates to CTAB micelles as well as ionic exchange constants between xanthates and added Br- ions in presence of CTAB micelles were calculated and from their values the transfer free energies AU°xof the methyl and methylene groups of the alkyl chain of xanthates were calculated from both the association and ionic exchange constants. © 1986Academic Press, Inc.

INTRODUCTION

systematic attempts have been made in order to study the association degree of solutes to micelles as a function of the hydrophobic and hydrophilic properties of them. The available information for the association of a series of solutes (with a regular increase in hydrophobicity) to a given micelle is mainly provided by the work of Wishnia (19) and interpreted by Tanford (20) for the association ofalkanes to sodium lauryl sulfate (NaLS) micelles, Larsen and Magid (8) for a series of benzoic acids associated to eetyltrimethylammonium bromide (CTAB) micelles, Yatsimirski and Martinek (21) for nitrophenyl esters in CTAB micelles, Gitler and Ochoa-Solano (22) for pnitrophenyl alkyl carboxylates in N-myristoylL-histidine-CTAB mixed micelles, Spink and Colgan (23, 24) and Hayase and H a y a n o (25) for the association ofalkyl alcohols to bile micelles and to NaLS micelles, and the one of Sepfilveda and Hirose (7) and Bunton and Sepfilveda (26) for the interactions of phenols, phenoxide ions, aromatic carboxilic acids, and amines to anionic and cationic micelles. The interactions of an ionic solute with a miceUe are expected to increase with the length of the alkyl chain of the solute.

Solubilization by micelles is a matter of current studies (1-8), due to the importance that the problem has from both a theoretical and practical point of view. These studies have been mainly devoted to obtaining the solubilization capability of the micelles and the location of the solute either in the core or in the micellar surface (9-11). A particular interest in knowing the magnitude of the association of solutes to micelles stems from the relevance that this magnitude could have in the quantitative interpretation of micellar catalysis a n d inhibition (12-15). When the solute is a simple hydrophilic counterion, its location is assumed to be in the Stern layer of the ionic micelle and the magnitude of its association to micelle is treated in terms of the dissociation degree (a) of the ionic micelle (14, 16, 17). If more than one counterion is present, the binding capability is treated as an ionic exchange equilibrium (8, 14, 16-18). The solubilization capability of ionic micelles can then be extended from completely hydrophobic to completely hydrophilic solutes. Unfortunately, only a few 21

0021-9797/86 $3.00 Journal of Colloid and Interface Science, Vol. 109, No. 1, January 1986

Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.

22

SEPOLVEDA AND PEREZ-COTAPOS

On this basis, we have chosen for the present study, the family of ionic xanthates molecules whose structure is /S/ R--O--C

(--M + \S "

where R ranges from a single methyl group up to a very long alkyl hydrocarbon chain and M + is usually Na + or K +. On the other hand, the interactions of xanthates with micelles are by themselves a matter of interest since decomposition of xanthates in acidic media is inhibited by CTAB and enhanced by NaLS micelles (27). The interactions of xanthates with micellized CTAB will be analyzed in terms of the free energies of transference ofxanthates from water to micelles and considering the probable formation of ion pairs between xanthates and CTA monomers. The aim of the present work is to provide more experimental evidence about the hydrophobic and electrostatic contributions to the interactions of solutes with micelles. EXPERIMENTAL

Reagents Methyl xanthate (MX) and n-propyl xanthate (PX) (potassium salts) were prepared by mixing the respective potassium alcohoxides with CS2 according to reported methods (28).

78 A 60 42 " " ~ i

'~k

2,o , 200

230

260

290

320

350

FIG. 1. UV spectra of methyl (--) and propy] xanthate

(---) in water. Journal of Colloid and Interface Science, Vol. 109, No. 1, January 1986

Ethyl (EX), n-butyl (BX), n-amyl (AX), and n-hexyl (HX) potassium xanthates were Hoechts products. All xanthates were purified by recrystallization from acetone-ether mixtures and their UV spectra in aqueous solution (Fig. 1) corresponded to those reported by Rao (28) with a pronounced peak at 301 nm having an absorptivity close to 17,000 (Table I).

Solubility Products Pure cetyltrymethylammonium xanthate salts (CTAX) were prepared by mixing a concentrated aqueous xanthate solution with a diluted aqueous CTAB solution at 5°C. A precipitate was formed which was washed several times and then suspended in water at 25°C. After equilibration, the system was filtered and the absorbances of the filtrate were recorded at 301 nm. These values were used to calculate the solubility product (Kw) by means of the absorptivity values given in Table I. For CTAPX and CTAAX the removal of the excess of xanthate by the washing process, resulted in a redisolution of the precipitate. Nevertheless, pure solid CTAPX and CTAAX were obtained by using large amounts of reagents. These solids have a high solubility in water giving very viscous solutions. Their high solubility prevented the measurement of their Ksv values.

CMC of CTAB in Presence of Xanthates (CMCx) The absorptivities of xanthates in water e,, were different from those corresponding to xanthates sorbed by CTAB micelles (em), and this difference had a maximum at 313 nm (Fig. 2 and Table I). Therefore, an abrupt change in absorbance was observed at the CMC~ when the concentration of CTAB was varied at constant concentration (ca. 0.1 mM) of a given xanthate and at 25.00°C. For the soluble CTAPX, the CMCx was also measured by the usual conductimetric method (concentration of CTAPX at which the specific conductivity experiences a sharp break). Both methods were in fairly good agreement.

ALKYL XANTHATES-MICELLESINTERACTIONS

23

TABLE I Absorptivities of Xanthates in Water (~) and When Totally Associatedto Micelles (~=) Xanthate

~a (313 nm)°

~m(313 nm) a

Methyl Ethyl Propyl Butyl Amyl Hexyl

6.989 7.311 7.113 7.314 7.402 7.380

12.642 12.753 12.024 12.753 13.458 13.566

~

(301 nm)b

~*~ (306 nmy

17.019 16.017 16.285 16.836 16.903 16.801

17.441 16.635 16.118 16.890 17.754 17.891

Wavelength of maximum differencebetween (Ea)and (Em). b Xm~in water. c Xm~xin the micellar pseudophase.

Critical Micelle Concentration of CTAB in Presence of Xanthate and of Added KBr (CMCa) The absorbance method described above was used to determine the CMCs (CMC~) of a series of solutions of CTAB at a concentration close to 1.0 mM, containing a constant xanthate concentration (ca. 0.12 m M ) and an increasing concentration of KBr which varied from 1.0 to 200 mM.

Association Constants (Ka) of Xanthates to CTAB Micelles A series of solutions at constant concentration of xanthate and increasing concentrations of CTAB were prepared and their absorbances recorded at 313 nm where ea and Em present the m a x i m u m difference. Ka is defined by the equilibrium Ka

Xa + Vm ~ Xm

[1]

and given by

[Xm] K~ - [Xa]" [Dml'

E. I0-3 15

(

10

A (nm) i

i

i

i

i

280

290

300

310

320

FIG.2. Molar absorptivities of n-butylxanthate in water (e) and in CTAB micelles (l) as a function of the wavelength.

[2]

where [Xm] is the total analytical molar concentration of xanthate incorporated to micelles, [Xa] is the molar concentration of xanthate in water, and Dm the concentration of micellized monomers of CTAB. Ka was obtained from the slope of the straight lines obtained by plotting f/(1 - f ) vs ([Dr] - [Xt]) (7, 26, 29), wherefis the fraction of total xanthate concentration ([Xt]) associated to micelles and [Dr] is the total concentration of surfactant. From absorbance measurements, f i s given by (26, 29) [Arm] _ A - A, [3] f = [Xtl Am - A a ' where A, Aa, and Am are the absorbances of xanthates in a surfactant solution, in water, and when the xanthate is completely micellar bound, respectively. Journal of Colloid and Interface Science, Vol. 109, No. 1, January 1986

24

SEPIJLVEDA AND PI~REZ-COTAPOS

Ionic Exchange Constants (Ki) between Xanthates and Bromide Counterions in CTAB Micelles A series of solutions containing a constant concentration of both xanthate (0.12 m M ) and CTAB (2.0 m M ) and increasing concentrations of KBr going from 3.0 to 6.0 m M were prepared and their absorbances recorded at 313 nm. As expected, the addition of KBr produces an increase in the absorbances of the solutions and these changes were used to calculate Ki according to the treatment previously reported (17, 18).

.62 J A

,56

,50

,44~~C M C ×

CMC

RESULTS AND DISCUSSION For all xanthates, Ar,ax in water is 301 nm and is shifted to 306 nm in presence of CTAB. The absorptivities and Xmaxat 301, 306, and 313 n m are shown in Table I. The spectral changes of BX in the presence of increasing concentrations of CTAB are shown in Fig. 3. Below the CMCx of CTAB, the spectra of xanthates are practically the same as in water, with only a decrease in the absorptivities. However, at high concentrations of CTAB, An~axis shifted from 301 to 306 nm.

"\ '.

.78 A

.60,42

3

6

9

CTABx 104 i 12

FIG. 4. Absorbancesof n-butylxanthate solutions (0.05 raM) as a function of CTAB concentrations at 313 rim. The CMCs of CTAB in presence (CMCx) and absence (CMC) of xanthates are shown.

A typical behavior of the absorbances (A) of xanthates at 313 n m is shown in Fig. 4 for the system BX plus CTAB. In Fig. 4 it is observed that at very low concentrations of CTAB (zone I) A, decreases, reaches a minim u m , and then linearly and abruptly increases (zone II) up to a point where the slope levels off(zone III) and finally reaches a plateau. For methyl xanthate it was not possible to detect a decrease in absorbances at low concentrations of CTAB (zone I). In what follows we shall analyze separately each zone of Fig. 4.

~I.~ Zone L Precipitation of CTA Xanthate Salts

.24 .06 , . 274

.

. . 286

.

. . 298

310

¢

'/' ,"-I 322

FIG. 3. Spectra of n-butyl xanthate (0.05 raM) in water (--) and in CTAB 0.015 mM (---), 0.15 mM (- • • ), and 1.5 raM( .... ). Journal of Colloid and Interface Science, Vo]. 109, No. 1, January 1986

In Fig. 4 and Table I1 the CMCs values of pure CTAB (30) and the CMCx of CTAB in presence ofxanthates are shown. With the exception of MX, the decrease in absorbances in zone I ends at the CMCx. This behavior can be interpreted as due to the formation of an insoluble ion pair (CTAX) between the xanthates and CTA m o n o m e r with a solubility product Kspgiven by Ksp = [X~]. [CTA +] = [X~] 2,

[4]

25

ALKYL XANTHATES-MICELLES INTERACTIONS TABLE II CMCx Values of CTAB in Presence of Xanthates (ca. 0. l raM)

Xantato

CMC~X 10"

.%x IO"

A,.°(K~.)

K.

A.°.(K.)

(mole/liter)

(mole/liter)2

(kcal/rnole)

(mole/liter)-~

(kcal/mole)

--

8.0

MX EX PX BX AX HX

a

.

--

4.5 3.8 2.0 1.1 0.40

7.8 1.9 -0.16 -0.0066

--

.

7.2 -7.6 --8.4 --9.3

. 768 675 904 4108 19318 .

. -6.3 -6.2 -6.4 -7.3 -8.2 .

a,~ (K.) /t?' X 102

.

.

(keN/mole)

. 5.9 3.1 1.6 --0.46 0.23 .

. -5.82 -6.20 -6.50 -7.33 --7.74 . .

"a . . . .

--0.37 -0.35 -0.50 -0.43 -0.16

b"

--10.6 -10.7 - 11.4 -12.1 -10.8

Note. Solubility products (K~p) of CTAX compounds, transfer free energies of xanthates from water to the solid (CTAX) (Aux°(go))and from water to CTAB micelles obtained from the association constants K, (A~x°(Ka))and from the ionic exchange constants Ki (A#°(Ki)), and the a and b parameters of Eq. [26]. a CMC of pure CTAB (38).

where [Xa] and [CTAa+] are the molar concentrations of aqueous xanthate and CTA monomers, respectively. The existence of a CTAX insoluble compound, previously reported by Scowen and Leja (31) for nonyl xanthate and CTAB, was confirmed by the appearance of a precipitate after mixing a diluted solution ofxanthate with a concentrated solution of CTAB.

8 i

9 i

10 ,

11 w

12 ,

13 ,nc

14

The Ksp values in molarity concentrations for CTAMX, CTAEX, CTABX, and CTAHX are shown in Table II and they follow the linear relationship (Fig. 5) In Ksp = -1.4n x - 14.9,

[5]

where n x is the number of carbon atoms in the alkyl xanthate chain. It is very interesting to note that a linear relationship between In Ksp and n~ is also outlined from the data reported by Mukhayer and Davis (32) for salts formed by benzyltriphenylphosphonium and alkyl sulfates (Fig. 5). From these data we obtained the equation

- i n KSD

In Kp~ = - 1.3n~c - 0.8, 22

2C

14

18

12

16

10

FIG. 5. Logarithms of the solubility products (Ksp) as a function of the number of carbon atoms of the alkyl chain of xanthates (n x) in CTAX compounds (e) and ng of the alkylsulfates in benzyltriphenylphosphonium alkyl sulfates

(B) (Ref. (32)).

[6]

16

where n~ corresponds now to the number of carbon atoms in the alkyl chain of the sulfates. The free energies of precipitation (AU°p) in the unitary system, can be obtained from the expressions Auto = R T ln(Yx,) 2,

[7]

where (Yx,) is the aqueous molar fraction of xanthate ion in equilibrium with the solid CTAX and given by [Xa] Y~" = [X21 + [CTAa+] + 55.5

_ [S~] 55.5

[8]

Finally, by using Eqs. [4], [7], and [8], it is obtained Journal of Colloid and Interface Science, Vol, 109, No. 1, January 1986

26

S E P U L V E D A A N D PI~REZ-COTAPOS

AU°p = - R T I n

55.5 + R T l n

Kps.

[9]

The values of Au°po are presented in Table II and they can be analyzed according to the following considerations. All xanthates compounds can be represented by the general structure

9,1

_~#ox (pp)

//~

(

8,5

n~ - (R-O-CSB). • • N(CH3)~ - n~ = n x - X - D + - n~,

where n~ and n~ represent the number of carbon atoms of the alkyl chain of the xanthate and CTA, respectively. By using the principle of additivities of the free energies (20, 33), ~xu°p can be expressed as A U O p = AUpp(CX o ) ° n cx -~ m/~p(D+X- )

o ~ ° ncD Av ~Upp(Cn')

[10]

In this work, n~ is contant and equal to 16 and the contributions due to the interactions of ionic xanthate groups with CTA + can also be considered constants. Therefore, it is possible to conclude that Au~p might be a linear function of n~ as it is shown in Fig. 6 and represented by the equation Au°p = -414n~ - 6795

[11]

Au°p = --414n~-- (414 X 16) - 171.

[12]

or

In Eq. [12] is is assumed that the contributions to Au°p of the -CH2 and -CH3 groups in the alkyl chain of xanthates are the same as those in the alkyl chain of CTAB ( - 4 1 4 cal/mole). The residual - 171 cal/mole can be attributed to the second term in the right-hand side of Eq. [10]. This low negative residual value could indicate that the precipitation is mainly promoted by the hydrophobicity of the alkyl moieties of both xanthate and CTA ions. However, in spite of the small contribution of Lxu°pw+x -) to the total Au°p, the existence of an ion pair seems to be essential for the obtention of an insoluble CTAX compound, since CTAB and xanthates, taken separately are soluble at the concentrations corresponding to zone I and before mixing them to obtain the solid CTAX. According to Eq. [10], the Journal of Colloidand InterfaceScience, Vol. 109, No. 1, January 1986

7,9

7,3

~¢/

.~

1

2

.

Z. 3

.

~ , 4

5

6

FIG.6. Freeenergiesoftransference(Lxu°(pp))ofxanthates from waterto the solidCTAXas a functionof the number of carbon atoms of the alkyl chain of xanthates (n~).

similarity in the slopes of Eqs. [5] and [6] suggests that the contribution to Aup°pdue to any -CHz or -CH3 group is independent of the nature of the solid. Nevertheless, the importance of the nature or structure of the solid is reflected in the constant terms of Eqs. [5] and [6]. For CTAPX and CTAAX it was not possible to measure their Ksp because at 25 °C they are highly soluble in water and the resulting solutions also show high viscosities. This unexpected behavior can be rationalized in terms of the odd/even alternation rule that is usually found in homologous alkyl series when a solid phase is involved (34, 35, 36). The specific conductivities of CTAPX solutions were also used to determine the degree of dissociation (a) by using the methods of Evans (37) which gave a a value of0.15 (much less than the 0.2 value reported for CTAB (14) which indicates that xanthates counterions are strongly bound to CTAX micelles. Z o n e II. R e d i s s o l u t i o n o f the C T A X Insoluble Compounds

The increase in absorbances observed in zone II of Fig. 4 can be interpreted as a reso-

27

ALKYL XANTHATES-MICELLES INTERACTIONS

lubilization of the precipitate as soon as micelles are formed in presence of the given xanthate. Assuming that in zone II a solid CTAX is always present, it is possible to obtain the relationships [Xt] = [Xa] q-

[Xml + CTAXsolid

ofA S

o6o

0.5(

0'541A

,/

¢.

, , b , 1.5 2.1 2.7 3.3

5 6 7 8 9 iO

[131

and

Q . 5 4 1 ~ 0.48

Q

0.711A 0.65

[Dr] = [XI] + [Din]

0.42 + CMCx + CTAXso~ia [141 combining Eqs. [3], [4], [131, and [14], the fraction f b e c o m e s f =

Ka" K~p

. [Dt] +

CMCx. Kt

K a • K~p

CMC - Xt)

[15]

In terms of total absorbances (At) and ~m and Ca,f c a n also be expressed as f = At - eaKsp"CMCx

[16]

~m°Xt

Combining Eqs. [ 15] and [ 16] it is finally obtained Ksp" ~m CMCx

At = K a - Ksp" ~ m [Dr] + K a -

CMCx

{ Kso

X \CMCx

C M C - Xt

) *.. Ksp +

0.9 i.i 1.3

5

5 8

Ii

FIG. 7. Absorbances of xanthate solutions as a function of the total concentration of CTAB. (0) EX, 0.056 mM; (E]) PX, 0.050 raM; (A) BX, 0.058 mM; and (O) AX, 0.72

0 In Table II are shown the Au~(/~) values for the different xanthates and in Fig. 8 these values are plotted as a function of the number of carbon atoms of the alkyl residue of xanthates (n~). Transfer free energies (Au°(K,)) can also be obtained from the ionic exchange equilibrium existing between xanthates and added bromide counterions (Br~,d) ( 17, 18, 38, 39). The ionic exchange between (Br~aa)) and xanthate counterions in micellar CTA solutions might obey the equilibrium Ki

X~ + Br2 ~ X2 + Brm " [17]

When solid CTAX is present in the solution, Eq. [ 18] would be valid and a plot of At against [Dr] would result in a straight line from whose slope the value of Ka can be calculated, provided that the values of Ksp, era, and CMCx are known. Figure 7 shows that Eq. [18] is consistent with the experimental data and the Ka values obtained from it are shown in Table II. K,, in the unitary system, is related to the transfer free energy (AU°x(K.))of a solute from water to micelles through Eq. [ 18] (26, 27) AU°x(r~)= - R T l n 55.5 - R T l n K~.

0 l/oQ

0.36

raM.

CMCx. Xt

X \( C Ksp MG

0.591

[18]

[19]

and Ki is given by [X21. [Brm] Ki - [Xm]" [Br2] "

[20]

Subindexes a and m refer to aqueous and micellar bound counterions. The concentration terms occurring in Eq. [20] were calculated by assuming that the change in absorbances at 313 nm after addition of KBr to solutions of CTAB containing xanthates were due to the expulsion of the xanthate from the micelle as a consequence of the equilibrium competition predicted by Eqs. [19] and [20]. Ki values for the different xanthates were calculcated at CTAB concentrations where all xanthate (beJournal of Colloid and Interface Science,

Vol. 109,No. l, January 1986

28

SEPI~ILVEDA AND Pt~REZ-COTAPOS

8,0

(kcallmole)

teresting to note that the Coffin and Harkins correlation (40, 41).

/,~.

In CMCa = a. in Br(ad) + b 7,C

6,(

5.0

nX

FIG. 8. Transfer free energies of xanthates from water to CTAB micelles obtained from the association constants (Au°(~a) (©), and from the ionic exchange constants (AU°xxi)(A) as a function of the number of carbon atoms of the alkyl chain of the xanthates (n~).

[26]

is also observed in these systems as it is shown in Fig. 9. The a and b values were calculated from the slopes and intercepts of the straight lines of Fig. 9 and are presented in Table II. According to Eq. [25], a plot of the left-hand member against the first right-hand member would result in a straight line with slope Ki. Figure 10 shows these plots from where the Ki values were calculated and are inserted in Table II. These Ki values can now be related to the transfer free energy of counterions from water to micelles through the relation (17, 18) U0m - - U0 a = ~Orm -- U0Bfa -~- R T

In K i

or 0 o Aux(xi) = AUBr0c0 + R T I n Ki.

fore addition of KBr) is micellar bound. Under these conditions, it is possible to write [Xt] = [Xa] + [Xm].

[21]

The total concentration of Br-([Brt]) is given by the total concentration of CTAB([Dt]) and [Br~aa)], so that

[27]

[28]

The AU°x(~) values are presented in Table II and in Fig. 8 are shown the Aux°(r~) and Au°~KI)as a function of the number of carbon atoms of the alkyl chain of xanthates. -in CMC o

[Bq] = [Bra] + [BrCa,~)]

+ [Br~] = [Dt] + [Brf~d)] [22] and

11

fOr] = [Dm] - C M C a .

[23]

Assuming now that the micellar dissociation degree (a) is constant (14, 17, 18, 26), one obtains [Xm] ~- [Brm] = (1

-

o0 • [Om].

1o

[24]

Combining Eqs. [20]-[23] and [3] it is finally obtained f_._.~). (I {(i - a)([Dt] - CMCa)

- f" [Art]}

f 8

= ~{[BrCad)] + (1 -- a)CMCa +f[Xt]} + Ki[Dt],

[25]

where the actual CMC(CMCa) in presence of xanthate and added KBr were used. It is inJournal of Colloid and Interface Science, Vol. 109, No. 1, January 1986

L 2

I 3

i 4

i 5

I 6

I 7

FIG. 9. Logarithms of the CMC, values of CTAB in presence of 0.12 m M xanthate as a function of added KBr.

(0) MX, (A) EX, (15])PX, (×) BX, (O) AX.

ALKYL XANTHATES-MICELLESINTERACTIONS

/S;

2.o"_, _"2" T

1,C

,,

÷

o

0.5

~ - -

{ [cadl+(I-°OCMCx+f[St]} x

a

i

i

2

3

4

10 2

~

FiG. 10. Plot of parameters of Eq. [25] for M X (X), EX ([]), PX (N), BX (A), 0.13 mM, with 2.0 m M CTAB and

for AX (©) 0.3 rnM with 0.01 mM CTAB. It is seen in the figure that A/~°x(K0is a lineal function of the number of carbon atoms for all the homologs studied with a constant term o f - 5 . 3 kcal/mole and a slope that amounts to -0.5 kcal/mole for the transfer of methylene or methyl group from water to micelles. This value is rather small but it is close to the values reported by Sepfilveda and Hirose (7) for the transfer of p-alkyl-substituted benzene derivatives from water to micelles. The A/~°x(Ki)values obtained from the ionic exchange constants between Br- and xanthate counterions might contain some specific effects of the added Br- ions. On the other hand, the 0 values reflect the hydrophobic forces A~tx(Ki) that oppose to the removal of the micellarbound xanthates by the added Br- ions. In contrast with AlZx(Ki), 0 A~t°(r~ is a lineal function of the number of carbon atoms only for the highest member of the series (PX, BX, and AX) with an slope o f - 0 . 9 kcal/mole and an intercept of -4.2 kcal/mole. The lowest members (MX and EX) present A~°(r~)values which are far from the linear trend followed by the highest homologs and remain practically constants up to EX. This behavior can be rationalized considering that the association constants of MX and EX would be essentially electrostatic in nature and the hydrophobic contributions of the methyl and ethyl moieties can be neglected. It must be noted that the slope of -0.9 kcal/mole for the highest ho-

29

mologs agrees with that reported by Tanford (20) for the transfer of methyl or methylene groups from water to micelles or to n-alkanes. It is interesting to compare that A~t° of transfer of bromide ions from water to CTAB mieelles (-4.14 kcal/mole) (18) with the A/z° of the smallest xanthate counterion MX which accounts to -5.8 kcal/mole (Table II). Since the charge of MX- and Br- is the same, the difference o f - 1.7 kcal/mole in favor of the interactions of MX- with CTA micelles, cannot be attributed to an electrostatic effect but rather to specific interactions between either the methyl of xanthate group with the surface ofCTA miceUes. The small -CH3 group poorly contribute to any specific electrostatic or hydrophobic interaction with micelles and, therefore, the specificity might be attributed to the xanthate moiety. These specific interactions can be related to the collector capability that xanthates have in the process of mineral flotation, in the sense that their interactions with the surface of the mineral could be similar in magnitude and origin to those found in presence of cationic micelles. On the other hand, xanthates decompose rapidly in acidic solutions (27, 28, 42) and the decomposition is inhibited by CTAB micelles (27). The inhibition starts at different concentrations of CTAB which are lower as the alkyl chain of the xanthate increases. These results can now be easily rationalized considering that the CMC of CTAB decreases as the xanthate alkyl chain increases. Finally, Bunton et al. (27, 42), found that the rate of decomposition of BX in presence of CTAB and HC10.1 m M increases about 28 times by the addition of 0.5 MNaCI. This behavior can be explained from the results of the present work on the basis that C1- counterions could displace xanthate ions from the micelles where they have a lower rate of decomposition than in the water phase.

Zone IIL Micellar Association of Remaining Aqueous Xanthate In this zone, all the xanthate still existing in the aqueous phase is being absorbed by the Journal of Colloid and Interface Science, Vol. 109,No. 1, January 1986

30

SEPULVEDA AND PI~REZ-COTAPOS

13. Bunton, C. A., Rivera, F., and Sepfilveda, L., J. Org. Chem. 43, 1166 (1978). 14. Romsted, L. S., Ph.D. thesis. Indiana University, 1975. 15. Bunton, C. A., "Progress in Solid State Chemistry" (J. O. McCaldin and G. Somorjai, Eds.), Vol. 8. CONCLUSIONS Pergamon, Oxford/New York, 1973. Alkyl x a n t h a t e s strongly interact with b o t h 16. Stigter, D., Prog. ColloidPolym. Sci. 65, 45 (1978). m o n o m e r s a n d micelles o f C T A B a n d the 17. Barter, D., Gamboa, C., and Sepfilveda, L., J. Phys. Chem. 84, 272 (1980). m a g n i t u d e o f the i n t e r a c t i o n increases as the 18. Gamboa, C., Sep61veda, L., and Soto, R., Z Phys. alkyl chain o f the x a n t h a t e s also increases. Chem. 85, 1429 (1981). Specific i n t e r a c t i o n s between the x a n t h a t e 19. Wishnia, A., J. Phys. Chem. 67, 2079 (1963). group with the positive charged surface o f C T A 20. Tanford, C., "The Hydrophobic Effect." Wiley-Inmicelles are detected. A n insoluble ion pair terscience, New York, 1973. ( C T A X ) which is g r a d u a l l y solubilized b y the 21. Yatsimirski, A. K., Martinek, K., and Berezin, I. V., Tetrahedron. 27, 2855 ( 1971). increasing concentration o f C T A micelles gives 22. Gitler, C., and Ochoa-Solano, A., J. Amer. Chem. Soc. a c c o u n t for the p r e s e n t e d results. T h e transfer 90, 5004 (1968). free energies o f x a n t h a t e s from water to C T A 23. Spink, C. H., and Colgan, S. C., J. Phys. Chem. 87, micelles, c a l c u l a t e d f r o m an ionic exchange 888 (1983). m o d e l (2x#°(Ki))is a lineal f u n c t i o n o f the n u m 24. Spink, C. H., and Colgan, S. C., J. Colloid Interface ber o f c a r b o n a t o m s o f the alkyl c h a i n o f the Sci. 97, 11 (1984). xanthates. H o w e v e r , the transfer free energies 25. Hayase, K., and Hayano, S., Bull. Chem. Soc. Jpn. 50, 83 (1977). calculated from an association m o d e l 26. Bunton, C. A., and Sepfilveda, L., J. Phys. Chem. 83, (A~t°~)) is only lineal for the highest homologs. 680 (1979). 27. Bunton, C. A., Salam6, J. E., and Sepfilveda, L., J. ACKNOWLEDGMENTS Org~ Chem. 39, 3128 (1974). Support of this work by the Departamento de Investi28. Ramachandra Rao, S., "Xanthates and Related gaci6n y Bibliotecas de la Universidad de Chile and by Compounds." Decker, New York, 1971. the Comisi6n Cientifica y Technol6gica de la Repfiblica 29. Supfilveda, L., J. Colloid Interface Sci. 46, 372 (1974). de Chile is gratefully acknowledged. The authors also thank Ms. Eliana Villagra and Maria Luz Pefia for their 30. Mukerjee, P., and Mysels, K. J., N.S.R.D.A.-NBS 36, Washington, D.C., 1971. collaboration in typing and drawing. 31. Scowen, R. V., and Leja, L., Canad. J. Chem. 45, 2821 (1967). REFERENCES 32. Mukhayer, G. I., and Davis, S. S., Z Colloidlnterface 1. McBain, M. E. L., and Hutchinson, E., "Solubilization Sci. 59, 350 (1977). and Related Phenomena." Academic Press, New 33. Leo, A., Hansch, C., and Elkins, D., Chem. Rev. 71, York, 1955. 525 (1971). 2. Klevens, H. B., Chem. Rev. 47, 1 (1950). 34. Mukerjee, P., KolloidZ. Z. Polym. 236, 76 (1970). 3. Mukerjee, P., J. Phys. Chem. 60, 1528 (1971). 35. Birdi,K. S., Dalsager,S. V., and Basklund, S., J. Chem. 4. Wan, L. S. C., J. Pharm. Sci. 55, 1395 (1966). Soc. Faraday Trans. 1 76, 2035 (1980). 5. Mukerjee, P., in "Solution Chemistry of Surfactants" 36. Hamann, S. D., J. Phys. Chem. 66, 359 (1962). (K. L. Mittal, Ed.), Vol. 1, p. 153. Plenum, New 37. Evans, H. C., J. Chem. Soc. 1, 579 (1956). York, 1979. 38. Quina, F. H., and Chaimovich, H., J. Phys.Chem. 83, 6. Treiner, C., J. Colloidlnterface Sci. 93, 33 (1983). 1844 (1979). 7. Hirose, C., and Sepfilveda,L., J. Phys. Chem. 85, 3689 39. Quina, F. H., Politi, M. J., Cuccovia, I. M., Martins(1981). Franchetti, S. M., and Chaimovich, H., in "Solu8. Larsen, J. W., and Magid, L. J., J. Phys. Chem. 78, tion Behaviour of Surfactants" (K. L. Minal and 834 (1974). E. J. Fendler, Eds.), Plenum, New York, 1982. 9. Wennerstr6m, H., and Lindman, B., Phys. Rep. 52, 40. Corrin, M. L., and Harkins, W. D., J. Amer. Chem. 1 (1979). Soc. 69, 683 (1947). 10. Yiv, S., Zana, R., Ulbricht, W., and Hoffmann, H., J. Colloid Interface Sci. 80, 224 (1981). 41. Shinoda, K., Nakagawa, T., Tamamushi, B., and Is1I. Nagarajan, R., Chaiko, M. A., and Ruckenstein, E., emura, T., "Colloidal Surfactants." Academic J. Phys. Chem. 88, 2916 (1984). Press, New York, 1963. 12. Bunton, C. A., Cerichelfi,G., Ihara, Y., and Sepfilveda, 42. Bunton, C. A., Ng, P., and Sepfilveda, L., a~ Org. L., J. Amer. Chem. Soc. 101, 2429 (1979). Chem. 39, 1130 (1974). increasing a m o u n t o f micelles until reaching a plateau region where it is c o m p l e t e l y micellar bound.

Journal of Colloid and Interface Science, Vol. 109, No. 1, January 1986