Cationic surfactants with perfluorocarboxylates as counterion: Solubility and micelle formation

Colloids and Surfaces, 61 (1991)111-121 Elsevier Science Publishers B-V., Amsterdam

111

Cationic surfactants with perfluorocarboxylates counterion: Solubility and micelle formation

as

Gohsuke Sugihara’, Shigem’ Nagadome”,Tamami Yamashita”, Naomi Kawachi”, Hiroyuki Takagi” and Yoshikiyo Moroib*’ =Department of Chemistry, Faculty of Scrence, Fukuoka University, Jonan-ku, Fukuoka 814-01,Japan bDepartment of Chemistry, Faculty of Science, Kyushu University 33, Higashi-ku, Fukuoka 81.2, Japan (Received 3 January

1991;accepted 20 June 1991)

Abstract The aqueous solubihty and critical micelle concentration (CMC) of dodecylammonium perfluorocarboxylates (trifluoroacetate, pentafluoropropionate, and heptafluorobutyrate) were measured, and the effects of the extent of hydrophobicity of counterion on solubillty, CMC, and the Krafft point (the micelle temperature range, MTR) were examined. The aqueous solubilities were smaller than those of ionic surfactants with the same dodecyl chain, and the enthalpy changes of dissolution into aqueous media, while positive, were also reduced. The Krafft point of the butyrate was too high to be determined. From the dependence of CMC on counterion concentration, thedegrees of counter-ion association to micelle were deduced to be 0.83-0.93 for acetate ion and 0.98-l for propionate ion. These higher values resulted in their lower CMC values. Thermodynamic parameters fo,mmicelle formation indicated substantial differences in micellar structure between acetate ab*d butyrate counterions.

INTRODUCTION

The physicochemical properties of surfactants are strongly influenced by the kind of surfactant ion and counterion. In recent years interest in the dependence of micellization on the counterion has shifted from conventional metallic ions to organic ions, and the effect of hydrophobicity of counterions on micelle formation has been examined [l-7].Increasing hydrophobicity leads to lower CMC values, a higher degree of counterion association to miceiles, and decreased micellar aggregation number, followed by its increase once a minimum has been reached. In addition, the effect of the extent of charge separa“IO whom correspondence

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should be addressed.

0

1991 Elsevier Science Publishers

B.V. All rights reserved.

112

tion of divalent counterions on micellization has been examined t&9]. It was found that a certain degree of charge separation is necessary for hydrophobic association to take place between the micellar core ethylene chain between the separate charges. . nd the hydrophobic Mixed micelle formation of hydrocarbon and fluorocarbon surfactants has also been of interest for more than a decade [10-X3]. The @MC values of several mixed systems of sodium pzrfluorooctanoate with anionic hydrocarbon surfactants have shown a positive deviation from ideal mixing, suggesting a mutual hydrophobic-like interaction between hydrocarbon and fluorocarbon chains [10,15,16]. However, the mixed system of sodium perfluorooctanoate with alkyl-N-methylglucamines showed conspicuous negative deviations from ideal mixing [16--B]. These facts indicate that an interaction between hydrophilic groups plays a more important role than association between hydrophobic groups in mixed mice& formation. The simpler the system is, the more definitive the information becomes, as far as the thermodynamics of micelle formation is concerned. If a fluorocarbon ion is the counterion, aggregation of hydrocarbon surfactant ions is expected to affect differently micellization and mixed micelle formation. At the same time, micellization must throw some light on mixed micelle formation, as the degree of hydrophobicity of the fluorocarbon counterion can be compared with that of the hydrocarbon counterion. This study aims to clarify these issues. EXPERIMENTAL

Preparation

ofdodecylammonium

perfluorocarboxylates

~~decylamine was of guaranteed reagent grade (Nacalai Tesque: Inc.) and was used without further purification. Perfluorocarboxylic acids, CF,COOH, C2FsCOOH, and C3F7COOH, of the same grade, were from the same source. The concentrations of the aqueous acid solutions were determined by titration with NaOH after any contamination was removed from the solution by ultracentrifu~ation. The equivalent amount of dodecylamine was added to the acid aqueous solution at a temperature above the Krafft point (or MTR [19]),and the d~decylammonium per~uorocarbo~ylates thus formed were purified by repeated recystallization from water, and from water-ethanol mixtures for heptafluorobutyrate. DAPA, DAPP, and DAPB refer respectively to the acetate, the propionate, and the butyrate. They were dried over P,05 under reduced pressure for one week. Their purity was checked by elemental analysis; the observe& and calculated values were in satisfactory agreement (Table 1).

113 TABLE

1

Elemental

analysis

of d~decyIamm~~ium

perfhorocarboxylates

Surfactants

c (%>

H C%)

N (%)

DAPA

56.17 56.24 51.56 51.73

4.6%

48.12

9.43 9.53 8.08 7.95 7.07

48.16

7.11

3.50

DAPP DAPB

Calc. Found Calc. Found Calc. Found

4.61 4.01 4.00 3.51

Aliquots of known concentration of original surfactant solution were added stepwise to a measured volume of water, where the original concentrated solution was still turbid after sanitation for 1 h at about 50°C. The electrical conductance of a solution at a specified temperature increased linearly with concentration *upto a maximum value, then remained constant *vith further increases in concentration, accompanied by an apparent precipitate. The solubility was determined as the concentration at which the initial linearity intersected the back extrapolaiion of constant conductivity (Fig. I). Above the Krafft point (KP), this method did not give precise solubility values. A clear solution of known concentration above the CMC was cooled below the KP, and then the solution, with coexisting precipitate of the surfactants, was heated very slowly below the KP and in steps of 0.2”C above the KP, with each temperature maintained for over 10 min. The conductances were plotted against temperature. Further temperature increase after the disappearance of the precipitates induced only a linear increase of the conductance. The original concentration is the solubility at the temperature where the linear increase starts (Fig. 2). CMC measurement The CMC was determined by the usual conductivity method as the concentration at the intersection of two lines obtained by plotting the specific conductance against concentration. The CIVIC was also determined in the presence of excess counterion (C,FZn+ 1 C90) by the same conductivity method, where the solutions of C, F,,, I COONa were prepared by titration of the acids with NaOl3 solution using a pH meter. In the case of DAPB (for which the aqueous solubility was too low for micelle formation), micellization was examined in a water-

I

I 2

I

I

4

,

I

1

6 CONCENTRATION

1

8 /10m3

t

I

1

10 mol.

I

12 dm-’

Fxg. 1. Change of specific conductancea with surfxtant concentrations at different temperatures- below MTR (22 5°C); just above MTR (34°C); above MTR (35°C).

ethanol mixed solvent, where the CMC was determined as the concentration corresponding to the maximum change in gradient in the specific conductivity-concentration curve [203. RESULTS

AND DISCUSSION

Solubility changes with temperature are shown in Fig. 3, where the temperature-dependences of CMC are also given. From the figure it is clear that both the solubility and CMC decrease with increasing fluorocarbon chain length of the counterions and that micelles of DAPA are not formed over the temperature range examined up to 95 OC. From the intersection of these two lines the KP can be determined: 220°C for DAPA, 32.5”C for DAPP, and above 95°C for DAPB. The CMC value of DAPA is smaller than those of other ionic surfactants with the same hydrophobic chain. In general, a smaller CMC leads to a lower KP if the solubility is the same, while a lower solubility leads to a higher KP if the CMC is the same [Zl]. That is, the KP is determined by the balance between CMC and solubility. The CMC of DAPP is much less than that of DAPA over a wide temperature range (Table 2). This fact generally results in a lower KP of DAPP, but its smaller solubility produces a higher KP, 32.5”C. In addition, the CMC value, 7.07* 1QB3 mol dmm3 at 24°C for DAPA, is almost one half of the CMC

115

DAPA cone.=

10-'

mol

dm-'

30

20 TEMPERATURE

Fig. 2. Change of specific conductances nation of specified solubihty.

40

/ “c

with temperature

and temperature

determi-

value, 1.3. lOA2 mol dmB3 of dodecylammonium acetate [22], which clearly indicates the effect of fluorination of the acetate counterion. The solubility of DAPB is not much less than that of DAPP. However, its KP is in excess of 95°C due to its lower solubility and to the much higher CMC expected. In order to estimate the CMC of DAPB in aqueous medium, the two CMC values of DAPP and DAPB are measured as a function of ethanol content in the water-ethanol mixed solvent (Fig. 4). As for DAPB no micellization takes place below 12% ethanol content, and the surfactant molecules dissolve only in the singly dispersed state in a solution at concentrations below the solubility [23]. The CMC values of DAPB are lower than those of DAPP over a wide concentration range of ethanol examined. The solubility of DAPB at 100°C can be estimated to be 3.6 10m3 mol dm- 3 from an extrapolation of the solubility curve. Hence, if a CM@ in water exists for DAPB, its value would be higher than the above solubility at 100” C. The reason for the high CMC of DAPB may come from a repulsive interaction between hydrocarbon and fluorocarbon chains. The enthalpy change of dissolution (dh*) can l

116

4

10

Fig. 3. Changes TABLE

20

30

40

TEMPERATURE

/ ‘=C

of solubillty

l

DAPA

.

DAPP

I

DAPB

50

and CMC with temperature

of DAPA,

DAPP

and DAPB.

2

Changes

in CMC with temperature

DAPA

for DAPA

and DAPP DAPP

Temp. (“C)

CMC (mmol dmS3)

Temp. (“Cl

CMC (mmol dm-“)

22 2 24 0 30 0 35 0 40 0 45 0 50 0 55.0

7.15 7.07 7.03 7.22 7.40 7.63 7.80 8.15

33.0 34.0 35 0 40 0 45.0 50.0

4.39 4.45 4.52 4.62 4.65 4.83

be evdluated AhO=

-

from the slope of In (soluhility)

aR[a In s/a(l/T)].

vs. l/T

below the KP: (1)

where complete dissociation of surfactants and ideality are assumed. The Ah” values are 14.9 kJ mol-‘, 24.9 kJ mol-’ and 12.7 kJ mol-’ for

117

r t

?-

Temp _=35oc

0

I

.

10

20

ETHANOL

Fig. 4. Change

CONCENTRATION

rn CMC with ethanol

-.I

30 /

Wd

content

in ethanol-water

mixed solvent.

DAPA, DAPP, and DAPB, respectively. These values are much less ‘?lan those of other ionic surfactants with a dodecyl chain; typically, e.g. 50 kJ mol-l for sodium dodecyl sulfate. The lower solubility and smaller enthalpy change of dissolution of these surfactants indicate that they are energetically stable in the solid state and that a smaller positive entropy change is accompanied by dissolution into the aqueous medium. The degree of counterion association to micelles can be evaluated by the CMC change with counterion concentration [24]: In CMC = - (m/n) In [G-l + constant

(2)

where n, and m are respectively the average aggregation number of the surfactant ion and the average number of counterions attached to a micel2e, and Grepresents the counterions. The value of m/n is the degree of counterion “binding” to micelles. The changes of m/n values with temperature are shown in Fig. 5. A striking finding is high njn values which are almost equal to, or more than, those of divalent

,

L

-9)

’

20

30

Fig. 5. Changes

5c

40 TEllPERATUi7E

DAPP

/ OC

of degree of counterion

association

to micelIe with temperature

counterions [25,28]: 0.83-0.93 for DAPA and 0.98-1.0 for DAPP, although the larger n2lr2 values of DAPP are easily anticipated from its larger propensity to remove hydrophobic counterions from the aqueous environment. Organic counterions with more than three met> iene groups are known to interact with the hydrophobic micellar ‘core, resulting in smaller CMC values 121. From the higher CMC of DAPB the usual hydrophobic interaction, as above, would not take place between the perfluorobutyl chain and the hydrophobic micellar interior. However, the position of perfiuorocarboxylate anions in the micelle is still unresolved. Assuming Phillips’ definition of CMC [20], the micellization constant K,, of an ionic surfactant can be expressed by l/K,, = 2n(;z + m)(CMC)“+” Hence, follows

(3)

the standard free energy change per mole of surfactant ions from the equilibrium constant, and IS given from Eqn (3) by

dG” = (I + m/n)RT

In CMC + @T/n)

In [2n(n + m)]

(4)

where the second term can be neglected for large aggregation numbers n, which can be easily judged from a steep increase of solubility at temperatures above the KP, as in the present cases [21]. The corre-

119

sponding

enthalpy

AH”/RT2

= - (I+

As”=

and entropy

changes

m/n)(d In CMC/3T)P

- (I.+ mjn>(aRT

In CMC/aT),

are given respectively

as

- [L?(mln)/3T]p In CMC

(5)

- RT In CMC [a(m/n)/aT],

(6)

The results of AGO, AH’, and TAS” for DAPA and DAPP are shown respectively in Figs 6 and 7, where the m/n values of DAPA are fitted to the third-power curve of temperature and the derivatives with respect to temperature are used to obtain these thermodynamic variables. The AGO values are dependent mainly upon the CMC values and remain almost constant irrespective of the counterions. In addition, they are similar to those of other ionic surfactants of the same hydrocarbon chain size 127,281. The values of AH” and TAS” of DAPA are quite temperature dependent, which means that the micellar structure changes greatly with temperature. However, those of DAPP decrease monotonically and only slightly with temperature. As for DAPA, the enthalpy term plays a more important role than the entropy term at lower temperatures but the entropy term dominates at higher temperatures, while for DAPP the entropv term is predo nlinant over the whole temperature range examined. Tht: difference in tnese thermodynamic DAPA

. 25

.

. 45

35 TEMPERATURE

/

55

‘C

Fig. 6. Changes of AG”, AHo and TdS” with temperature for DAPA.

120

DAPP

50

-50

3s

40 TEtlPERATURE

Fig. 7. Changes

45 /

50 OC

of AG”, AH" and TAS”

with temperature

for DAPP.

parameters between DAPA and DAPP arises from the difference in position and in arrangement of the two counterions in the micellized state. This will be made clear in a following paper. Further, it is quite natural that the enthalpy change of micelle formation is temperature dependent, since the change of CMC with temperature determines a larger part of the enthalpy change. This fact has been verified by calorimetric measurements [29,3G]. In summary, substantial differences in the thermodynamic parameters of dissolution and micellization have been observed among the three kinds of counterions. This behaviour cannot be expected with ionic hydrocarbon surfactants having a hydrocarbon counterion. Surfactants composed of both hydrocarbon and fluorocarbon portions should be investigated in more detail for development of functional amphiphiles. ACKNOWLEDGMENT

This work was supported by a Grant-in-Aid No. 03453013 from the itiinistry of Educ-

for Scientific Research Science and Culture,

121

which is gratefully acknowledged, and by a fund from the Research Institute of Fukuoka University.

Central

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