Solution properties of mixed fluorocarbon—hydrocarbon surfactant systems

Colloitls trntl Surjilcc.;. 67 ( 1992) 29-35 Elscvicr Science Publishers B.V., Amsterdam

Solution properties of mixed fluorocarbonhydrocarbon surfactant systems Masahiko Abe”sb, Tomohiro Yamaguchi”, Yasuhiro Shibata”, Hirotaka Uchiyama”, Norio Yoshino’, Keizo Ogino”Ob and Sherril D. Christiand aFact&y ofScience and Techology, Scierm University of Tokyo, 2641 Yantazaki. Noda, Chiba 278,

Japan

‘Institute of Colloid ar:J interface Science, Sciertce University af Tokyo, 1-3 Kagurazaka, Shinjttktr, Tokyo 162, Jaym ‘:F‘,‘,&y @l.L1I~‘I‘CC, r;.. - :.. ^ I”; r#r&, ..,I “L,CI.CC c,.;,a..*.,mV.I.“_. r I..im~ol.ritrr 1-3 _K_O__q _‘_.. ___,S!!i!%ir!kr?. Tnkyc, !67, Japatt mwm-nkn d.., of Tc+, ’ Jnstittrtc_fi~r Apphed Surfactwt Resmrch, T&z University of Oklahoma, Norman, c3K 73019. USA

Abstract Solution properties of mixed fluorocarbon-hydrocarbon surfactant systems have beer. investigated with dynamic and static light sca:lcring and with lluorcsccncc-probe mcasurcmcnts. The systems arc mixtures of hcxadecylpolyoxycthylcnc ether (C,(, POEZO) with ammonium perlluorooctanoatc (AI”-0) and/or with ammonium pcrfluorodccanoatc (APFDc). When the concentration of fluorocarbon surfactant increases. the mutual diffusion cocllicicnt and the second virial cocficicnt of mixed micelles increase, but the micellc molecular weight and the micropolarity decrease. The mutual diffusion coefficient and the second vi~ial cocficicnt of r.Ged miccllcs in C,,,POtt,,/.4PFDc arc lorgcr than those in the C,,POEID/APFO system. but the micelle molecular w?ight ol ;,‘\PFDc is smr?llcr than that ofC,,POE,,/APFO. C:,,POE,,, It isl~oi!~C: !?:I! r,,:P\3E.,. L a hydrocarbon non-ionic surfactant, mixes well with APFO and/or AI”CT>e. a fluorocarbon anionic surfactant. and :Ix u:ixcd miccllc is rormcd more easily with the surfactnnt containing a longer !lu;:ro:rlkyl chain (APFDc) than \v~th ii::~: b;,ring :: c:!i.?-?: tcr fluor~)alkyl chain (APFO). Kqrr~ords: cocflicicnl.

Fluorocarbon

surractanl:

b!~!!roc::rbon

surfactant:

micropoiarity:

Introduction

It is well known that the solution properties of mixtures of surfztants are often superior to those of the individual surfactants involved [1,2]. Therefore, mixed surfactant systems are used in many practical applications, and they are also of great theoretical interest. Furthermore, fluorocarbon surfactants containing a fluoroalkyl chain as the hydrophobic group -Corrcspotrderlce IO: Dr. Masahiko Abe, Faculty of Scier.cz and Tcchnoiogy, Science University of Tokyo. 26.:; pamazaki. Noda. Chiba 278, Japan. 316G-6622;92/%C5.C0

8

!WZ -

Else-Gier S~~CXC Pub!ishcrs

mixed micclle; mutual

difiusion

co:ficicnt:

second

virial

have been synthesized with the development of fluorine chemistry, and the solution properties of mixed fluorocarbon-hydrocarbon surfactant systems have been studied [3- 1 I]. For example, Funasaki and co-workers [ 12- 161 and Carlfors and Stilbs [ 171 have reported that in aqueous solutions of fluorocarbon-hydrocarbon mixed surfactant systems at certain compositions, two types of mixed micelles (a fluorocarbon-rich type and a hydrocarbon-rich type) ere present simultaneously. In this iYork, we report a study of mixed micelle formation between anionic fluorocarbon surfactants ammonium perfluorooctanoate (APFO) and/ or ammonium perfluorodecanoate (APFDe) and a E.V. Xl rights

reserved.

30

non-ionic hydrocarbon su:‘factant hexadccylpolyusing dynamic and 0xycU~ylcnc cthcr (C 1(, PCE,,), and fluorescence-probe static light scattering mcasurcments.

Experimental

Pcrfluorooctanoic acid and perfluorodecanoic acid, obtained from PCR Inc. (Gainsville, FL) wcrc used as received. APFO (C7FlsCOONHs) and APFDc (CgF,,,COONHJ) were prepared by ncutralizing thcsc acids, as rcccivcd, with ammonia solution. The hydrocarbon surfactant Clh POE,, (C,~,H,,O(CIH,O),,H), was supplied by Nihon Surfactant Industries Co., Ltd.. Tokyo. This has a narrow molecular wei~hl disl iibution.

C,,POE,o

: 1x10-’

mol/L

; APFO Q ; APFDe (p ; NaCl

0 1.6

1.2

0.8

0.4 7 0-O

1 o-5

concentration Fig.

I. Mu~unl

tration solutions

diiTusion

or APFO. (I.

APFDe,

10m2 molt-‘).

cocllicicnt and/or

1 o-*

1o-3 (mol/L)

IIS a function NrrCl

or the concen-

in C,,, POE?,,

micclk~r

The purities of thcsc sur&ctants were ascertained by surface tension measurements. Sodium chloride (NaCl), used as an added salt, was obtained from Wako Pure Chemical Industries Co., Ltd., Tokyo. It was of reagent grade and used as rcceivcd. Pyrene for use as a fluorescence probe was from the same source. It was recrystallized three times from ethyl alcohol, dissolved in cyclohcxanc, and passed through silica gel. Water used in this experiment was distilled water for injection, JP (Japanese Pharmacopoeia), obtained from Ohtsuka Pharmacy Co., Ltd., Tokyo.

All experiments were performed at 30°C. The mutual diffusion ccefflcicnt of miccllcs, micelle molecular weight, and second virial coefficient of micellcs were meastired by a submicron particle analyzer (4700-type, Malvcrn Instruments Ltd., Worces!crshire, IJK). The optical source of the light-scattering apparatus was an argon-ion laser operating at 488.0 nm with an output power of 5 W maximum (Inova-90, Coherent Co., Palo Alto, CA). Measurement of the time-dependent correlation function of the scattered intensity at a scattering angle of 90” was made to determine the mutual diffusion coefficient of micelles (dynamic lightscattering method). The data analysis was performed by the combined use of the cumulant method and the model-free algorithm [18]. The average scattered intensity at a scattering angle of 90” was measured to determine the micelle molecular weight and second virial coefficient for micelles (static light-scatlcring method). Refore measurement, the aqueous solution of surfactant being used a: sample was passed three times through a membrane filter of pore size 0.1 pm (cellulose nitrate, Toyo Roshi Co., Ltd., Tokyo) for optical purification. The rcfraclive index measurements required to determine the micelle molecular weight and second virial coelficien~ of micelles were obtained with a

4.0

4.0

3.0

3.0 C,,POE,,:APFO

2.0

2.0

C,,POE,,:APFDI

1 .o

1.0

0

0 0

1.0

total Fig. 2. Reduced mixed solutions concentration.

concentration

2.0

0

3.0

: C, (g cme3) xl O2

intensity Tar miccllcs of C,,POE,,/APFO at different molar ratios with increasing total

differential refractometer (RM-102, Ohtsuka Electronics Co., Ltd., Osaka, Japan). The mutual diffusion coefficient, D, for a diiute micellar solution at a given concentration, can be expressed as: D=Do[l+kD(C-CC,)]

(1)

where D, is the transla:ional diffusion coeficient and kD is the hydrodynamic virial coefficient. The reduced intensity of scattered light, R, is given by:

(2) where I, and I are incident and scattered refractive indices of tively. The calibration for benzene, &, was

the measured intensities of light, and 11~and !?b are the water and benzene, respecconstant of the apparatus determined by the value of

1.0

total Fig. 3. Reduced mixed solutions concentration.

2.0

concentration

3.0

: C, (g cmw3) xl O2

intensity for micelles of C16POE2,/APFDe at different molar ratios with increasing total

the reduced intensity of light scattered from benzene [ 191, 3.259 - 10e5 cm-‘. Light scattered from a dilute miceilar solution at the given concentration, C, is described by Debye’s equation: K(C-CC,)

R-R0

I

=M+2&(C-cCo)

(3)

where M is the average molecular weight ar micelles and B2 is the second viriai coefficient. RO is the reduced scattering intensity for the solution at the CMC, C,,, which is practic&y equal to the reduced scattering intensity of water. However, Co is so small that the total concentration (C,) could be used instead of (C - C,). K is the optical constant derived from Eqn (4): K=

4n2n;(d,t/dc)2 NA1.4

where

(dn/dc)

is the

(4) specific

refractive

index

32

increment of a solution, IV,, is Avogadro’s number, and i_ is the wavelength of incident light. The ffuorescence emission spectra of pyrene monomers in the surfactant solution were measured with a fluorescence spectrophotometer (Shimadzu Co., Tokyo, model RF-540; the excitation wavelength was 335 nm). Each spectrum has five predominant vibronic peaks, numbered 1-5 from the shorter to the longer wavetength. As 1,/13, which is the ratio of the intensities of the first (375 nm) to the third (386 nm) peak, is known to be almost proportional to the polarity in the region near the pyrene molecule solubilized in the micelles, the micropolarity in the miccllc was monitored by measuring this ratio [5,10,1 1,20,21]. The preparation of mixed surfactant solutions solubilizcd with pyrcne was as follows. Replicate 0.1 ml pyrene-ethyl alcohol solutions (I .O - IO -3 mol I- ’ ) were placed in several 5 ml test tubes. After ethyl alcohol was evaporated o!T, C1(,POEZO miceilar solutions (1.0. lo-” mol 1-l)

were added to these samples, and then fluorocarbon surfactant (APFO and/or APFDe) solutions of the given concentration wcrc added. The mixtures were placed in a thermostat for 24 h at 3C”C in order to establish equilibria. Results

and discussion

Figure I shows the mutual diffusion coefficient of non-ionic hydrocarbon surfactant (Cl6 POE,,) micellar solution (1 - lo-’ mall-‘) as a function of concentration of anionic fluorocarbon surfactant (APFO or APFDc) and/or NaCI. As the scattered intensity of the solutions dccrcases remarkably above I - IO-‘mol I’-’ for APFO and above 1 - IO --’ mol1- ’ for APFDe, the measurements were performed below these concentrations. As can be seen in Fig. 1, the diffusion coemcient of non-ionic hydrocarbon surfactant systems

1.2

1.2 C,,POEp,:APFO

1 .c

“0 F

0

; loo:o

c)

; 1OO:l

C,,POE,,:APFDn

t.0

0.8

*

0

d

0.8

/

a

ri: r Y

;

0

; 1OO:l

Q 8

; 1OO:lO

loo:o

; 100:5 100:50

e

x

-2

0

0.6

0.2

0 0

t.0

total concentration Fig.4. Dcbyc plots lor C,,l?OE,,/APFO difkrcnt molar ratios.

2.0

3.0

: C, (g cmm3) xf mixed

solutions

0

O2

total at

1.0 concentration

Fig. 5. Dcbyc plots for C,,POE,,/APFDc difkrcrcnt molar ratios.

2.0

3.0

: C, (g cmS3) xl O2 mixed

solutions

31

M. Ahe et ol./Colloitls Swjoccs 67 (1992) 79-35

increases remarkably above a certain concentration of anionic fluorocarbon surfactant (APFO, 2. 10-smol 1-l; APFDe, 5. 10e5 mall-‘). At a given concentration, the diffusion coefficient of the C16POEIO/APFDe mixed surfactant system is larger than that of the CI, POE,,/APFO one. This may be attributed to the fact that the penetration of anionic fluorocarbon surfactant molecules (APFO or APFDe) into the palisade layers of the micelle may result in intermicellar C,, POE,, (micelle-mice!le) interactions of the electrostatic type. As a result, the apparent diffusion coefficient of the solution becomes larger. However, when NaCl is added to the C,,POE,, micellar solution, the diffusion coefficient of the non-ionic surfactant solutions is nearly independent of the concentration of NaCl, showing the lack of an ionic strength effect on the diffusion coefficient of C,6 POEzo micelles. Scco~d virial coeficient rttolecrtlar weight

ofmicek

and ,nicelle

Figures 2 (APFO) and 3 (APFDe) are plots of the reduced intensities as a function of total concentration of mixed surfactants with different molar ratios. Here, the measurements were perforri J below the range of individual CMC values of the anionic fluorocarbon surfactants, respecttvely, because the scattered intensity of the solution markedly decreases for ratios of C16POEzO/ anionic fluorocarbon surfactant >100:50. Although the plots are not shown in Figs 2 and 3, in the case of the C,,POEzo system containing NaCl, the reduced intensity is nearly equal to that of C,,POE,, alone. As can be seen in Figs 2 and 3, the reduced intensity increases with increasing total concentration of mixed surfactants. At a constant concentration, the reduced intensity decreases with increasing mole fraction of anionic fluorocarbon surfactant. This may be attributed to the fact that the molecules of the anionic fluorocarbon surfactant are inco:*porated into the C 16POEzO micelle. Figures 4 (APFO) and 5 (APFDe) show the

33 2.0

1.5

1.0

0.5 ”

I

0

1 0

0

e

; C16POE,,-APFO

mixed

; C,6POE,,-APFDe

mixed

I

I

I

0.1

0.2

0.3

mole fraction

of fluorocarbon

Fig. 6. Second virial coeflkicnt surfactant in mixed systems.

vs mole fraction

system system

1 0.4

surfactant of tluorocarbon

Debye plots, which conform to Eqn (3). This equation predicts that a plot of C, vs KC,/(R - R,) will be linear and that measurement of the slope and intercept leads to values of the micelle molecular weight (M), and second virial coefficient (B,). Figure 6 shows the variation of the second virial coefficient of micelles in mixed solution as a function of the mole fraction of fluorocarbon surfactant. As can be seen in Fig. 6, the second virial coefficients are positive, and increase with increasing mole fraction of fluorocarbon surfactant in both systems. It is well known that the second virial coefficient reflects intermicellar (micelle-micelIe) interactions and that a positive value indicates repulsion between t?e mixed micelles in solution. It can be postulated, therefore, that ionic fluorocarbon surfactant molecules penetrate into the C16 POE,, micelle, with increasing mole fraction of fluorocarbon surfactant, causing an increase in the number of surface charges and in the repulsion between micelles. Figure 7 shows the micelle molecular weight in

34

12.0 0 v

0 7 x

l ; C,,POE,,-APFDe

10.0

E .c”

; C,,POE,,-APFO

mixed

APFO. APFDc more easily than

system’

mixed

into

the

micellc

system

8.0

s E z

6.0

5 E z0)

4.0

.o E 2.0

0 0.4 mole fraction

of fluorocarbon

Fig. 7. Micclle molecular weight vs molt carbon surhctnnt in mixed systems.

surfactant

fraction

or fluoro-

as a function of mole fraction of surfactant. The micclle molecular weight decreases with increasing mole fraction of fluorocarbon surfactant. According to the Einstein-Stokes equation, the diffusion coefficient is inversely proportional to the size of the micelle. As can be seen in Fig. 1, micellar size decreases with increasing concentration of fluorocarbon surfactant. When the lower molecular weight fluorocarbon surfactant penetrates into the micclle of the higher molecular weight non-ionic surfactant, release of the non-ionic surfactant molecules can take place, and this effect increases with increasing concentration of fluorocarbon surfactant. At a constant mole fraction, the second virial coefficient of the C16POE20/APFDe mixed surfactant system is larger than that of C16POEZ0./ APIYU, but the micelle moIecular weight and the micellar size of the C,, POE,,/APFDe mixed surfactant system is smaller than that of Cl(,POE.20/

mixed

may penetrate APFO.

Figure 8 shows the c!langcs of the I I /I3 ratio of C,,POEzo miccllar solutions (1 - 10-’ mol 1-l) as a function of concentration of fluorocarbon surfactant. In a dilute soIution, the I, /I3 ratio of the mixed surfactant system is nearly equal to that of the pure Cl,POE,, micellcs. However, the Ii /I, ratio decreases with increasing concentration of fluorocarbon surfactant (I, /I3 ratios of C,, POE,,, (I - lo-” mo! I-‘), APFO (I * IO-’ ritol I-‘), and APFDc (I -IO-” mol 1-l) arc t.13, 1.32, and 1.33 rcspcctivcly); thcsc arc smaller than for the single component systems. In a conccntratcd solution, the dccrcasc of the 1,/I, ratio may be attributed to compact packing

solution

fluorocarbon

0 ‘3 2 =, = ,’

C&POE,,

: 1~10~~ mol/L

1.05

1o-5

10

1 o‘3

concentration Fig. 8. I, iI3

ratio plotted against surl’actant in C,,POE,, IO-” mol I-:).

tluorocitrbon (I

*

TC2

10-l

(mol/L) the

concr!.:iatlon or miccllar soluticns

hf. /lb<, er crl./Colloids

35

Sltrjiice.~ 67 (1992) 29-35

in the miceliar core, assuming formation of mixed micelles. The I 1/Z3 ratio of the C16 POE,,/APFDe is nearly equal to that of mixed system C,G PQE2,/APF0. In a dilute solution, the fluorocarbon surfactant (APFO and/or APFDe) is readily miscible with C,, POE,,, and the mixed micelle with C16POEz0 is formed more easily by a fluorocarbon surfactant having longer fluoroalkyl chains (APFDe) than by one having shorter fluoroalkyl chains (APFO). References K. Ogino and M. Abe, Mixed Surfactant Systems. Marcel Dckkcr, New York. in press. J.F. Scamchorn. Phcnomcna in Mixed Surractant Systems. ACS Symp. Ser. 3 I I, American Chemical Society. Washington. DC. 1986. K. Shimx!a and T. Nomura. J. Phys. Chcm., 84 (1980) 365. K. Mcguro, M. Ucno and T. Suzuki, J. Jpn. Oil Chcm. sot.. 3 I ( 1982) 909. Y. MUIO. K. Esumi. K. Mcguro and R. Zana, J. Colloid Intcrfacc Sci., 120 (1987) 162.

K. Yoda, K. Tamori, K. Esumi and K. Meguro, J. Colloid Interfscc Sci., 13 I (1989) 282. 7 G. Sugihara. D. Nakamura, M. Okawauchi. S. Sakai. K. Kuriyama and M. Tanaka. Fukuoka Univ. Sci. Repts., I7 (1987) 31. 8 S.J. Burkitt, B.T. Ingram and R.H. Ottcwill, Prog. Colloid Polym. Sci., 76 (1988) 247. K. Johtcn. S. Miyagishi and M. Nishida, 9 T. Asakawa. Langmuir. 4 (198X) 136. Langmuir. 4 (1988) 942. IO K. Kalyanasundaram. T. Asakawa. M. Mouri. S. Miylgishi and M. Nishida, II Langmuir, 5 (1989) 343. I2 N. Funasaki and S. Hada, Chcm. Lett.. 1979 (1979) 717. 13 N. Funasaki and S. Hada, J. Phys. Chcm., 84 (1980) 736. I4 N. Funasaki and S. Hada, J. Phys. Chem., 84 (1983) IS68. 15 N. Funasaki and S. Hada. J. Phys. Chcm.. 87 (1983) 347. 16 N. Funasaki, S. Hada and S. Neya. Bull Chem. Sot. Jpn.. 56 (1983) 3839. 17 J. Carlfors and P. Stilbs. J. Phys. Chem.. 88 (1984) 4410. Dynamic Light Scattering. I8 B.!. Bcrnc and R. Pecora, Wiley-Intcrsciencc, New York, 1976. I95 pp. I8 (1979) 2943. I9 E. Gulari and B. Chu. Biopolymcrs, and J.K. Thomas. J. Am. Chcm. 20 K. Kalyanasundaram sot.. 99 (1977) 2039. and K. Wong, 21 N.J. Turro. P.L. Kuo. P. Somasundaran J. Phys. Chcm., 90 (1986) 288. 6