Interaction of Alcohols and Ethoxylated Alcohols with Anionic and

Chapter 11

Interaction of Alcohols and Ethoxylated Alcohols with Anionic and Cationic Micelles D. Gerrard Marangoni, Andrew P. Rodenhiser, Jill M. Thomas, and Jan C. T. Kwak 1

Downloaded by COLUMBIA UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch011

Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4J3, Canada

Critical micelle concentrations (CMC values), distribution constants, and the aggregation numbers of the surfactant (N ) and alcohol (N ) have been determined for mixed micelles consisting of ionic surfactants with medium chain length alkoxyethanols as the cosurfactant. Distribution coefficients of the solubilizates (cosurfactants) are determined using the recently developed NMR paramagnetic relaxation experiment. The free energy of transfer of the alcohol from the aqueous to the SDS micellar phase decreases as the number of ethylene oxide (EO) groups in the alcohol is increased for a given alkyl chain length. For SDS, at a given cosurfactant concentration, CMC values and the surfactant aggregation numbers, N , decrease as the number of EO groups in the alcohol is increased. This suggests that the EO group imparts a contribution to the hydrophobic interactions. However, in DTAB/alkoxyethanol mixed micellar systems, the distribution constants, the CMC values, and N are independent of the number of EO groups in the cosurfactant, at a constant alkyl chain length. This implies that for cationic DTAB micelles, the EO group does not contribute to the hydrophobic interactions in mixed micelle formation. s

a

s

s

The determination of the interaction of organic solubilizates with ionic and nonionic surfactant micelles has been studied widely since the pioneering work of Shinoda (I) and Herzfeld et al. (2). Much of this work has focussed on the effect of the solubilizate on a single, specific property of the surfactant micelle (e.g., aggregation number, micelle shape, thermodynamics of micelle formation, and the degree of counterion binding (3-10)). It appears that relatively few investigations have appeared where a number of micellar properties have been examined as a function of the cosurfactant concentration (11). Recently, some interest has been shown in examining the interaction of alcohols other than the well studied n-alcohols, e.g., a,w-alkanediols (12,13) with ionic and nonionic micellar solutions. Although alkoxyethanols (in particular, 2-butoxyethanol)

1

Corresponding author

0097-6156/92/0501-0194$06.00/0 © 1992 American Chemical Society

In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


11.

MARANGONI ET AL.

Alcohol Interaction with Anionic & Cationic Micelles 1

are widely used cosurfactants in a number of different systems, including microemulsions (9,10,14), very little information has been found in the literature on the interaction of alcohol ethoxylates with ionic micelles (6,9,12). Interactions between small solubilizate molecules and micelles have been studied by many different techniques (15). CMC determinations as a function of added solubilizate concentration are used to estimate the change in the free energy of micellization. Yamashita et al. (16,17) have determined the mean activities of sodium decanoate (SD) in a number of alcohol solutions, using a sodium responsive glass electrode and the silver/silver decanoate electrode developed by Vikingstad (18,19). These authors observed a decrease in the CMC values with an increase in both the chain length and the concentration of alcohol. Of particular interest was the observation that the CMC values of SD/2-butoxyethanol mixed micelles are lower than those of SD/l-butanol mixed micelles at the same concentration of alcohol. Manabe et al. (12) have determined the CMC values of sodium dodecyl sulfate (SDS) conductometrically as a function of the alcohol concentration and the number of ethylene oxide (EO) groups in the alcohol, at a constant alkyl chain length. These authors observed a decrease in the CMC values of the SDS/alkoxyethanol mixed micelles as the number of EO groups in the alcohol was increased, at a constant alkyl chain length and alcohol concentration. A number of techniques (3-8,15) can be used to determine the mole fraction of solubilizate in the micellar phase, p, which is defined as follows n, P = " where n is the number of moles of alcohol in the micellar phase and n is the total number of moles of alcohol. The application of the NMR Fourier transform pulsedgradient spin echo (FT-PGSE) experiment by Stilbs and co-workers to the determination of p values of solubilizates in the micellar phase has greatly enriched our knowledge of surfactant/alcohol mixed micellar systems (20-23). Recently, an alternative NMR experiment for determining the solubilizate distribution in the micellar phase has been devised by Gao et al. (24). The method is based on the changes in the relaxation rate of the alcohol in surfactant and surfactant-free solutions, in the presence of paramagnetic ions. This experiment has been applied successfully to the determination of the micelle-water distribution coefficients for a number of organic solubilizates in some typical ionic surfactant micelles (24,25). The use of luminescence probing to determine the surfactant aggregation number is a relatively recent development (3,26-34). The main advantage of luminescence probing techniques is that they can be used to determine directly the number of micelles in solution, at any surfactant concentration, and do not require a priori a knowledge of the micellar shape. The disadvantage with these techniques is die need for solubilized probes and quenchers, which may influence the micellar concentration. The static quenching method, proposed originally by Turro and Yekta (30), has been used extensively in the literature for the determination of the aggregation number of the surfactant in the presence of additives. These authors derived a simple SternVolmer type relationship between the bulk quencher concentration and the logarithm of the fluorescence intensities as a function of the quencher concentration. Mmk

M

^1 - M

In

i\

(2)

m

The main advantage of the static quenching method is in its ease of implementation (it can be used routinely on any emission spectrophotometer). However, the application of the static quenching method depends on compliance with a number of


196

MIXED SURFACTANT SYSTEMS


experimental criteria (3,30). Foremost among these criteria are that both the luminescent probe and quencher are immobile and thus associated with the micelle during the luminescent lifetime of the probe, and that the quenching process is efficient so that luminescence is observed only from micelles containing a solubilized probe and no quencher. These criteria for the successful application of the static quenching method to the determination of the surfactant aggregation number have been discussed by a number of authors (3,28-34). The complete equation relating the emission intensities of the micellar solubilized probe as a function of the quencher concentration is (3,5,30)

where q is the number of quencher molecules in a given micelle, is the average quencher occupancy ( = [Q]/[M]); k is the rate constant for the quenching of thefluorescenceintensity, and r is the lifetime of the fluorescent probe. From equation 3, it is evident that the ratio of the probe intensities in the presence and absence of quencher is critically dependent on the value of the kinetic ratio, \t . In Figure 1, computer generated values of In (yi) are plotted vs. for \t values of 0.1 to oo. At the static limit, lcr = oo and equation 3 reduces to the well known Turro-Yekta equation (equation 2). For low kinetic ratios, i.e., k ^ < 10, it can be easily seen in Figure 1 that the micellar concentration, [M], is severely overestimated; hence, the surfactant aggregation number, N , is underestimated. However, for k ^ > 15, the underestimation of N is less critical, and is on the order of 5-7%. It is generally accepted that the use of the either ruthenium tris-bipyridyl chloride/9-methylanthracene (9-MA) or pyrene/cetylpyridinium ion (CP ) as the probe/quencher pair will yield reasonable estimates of the surfactant aggregation numbers by the static quenching method. Applying the known value of the quenching rate constant and probe lifetime for the pyrene/CP probe/quencher pair, 1CT = 1718 (35), we calculate that the static method used here underestimates the real micellar aggregation number in SDS micellar systems by ca. 5%. In DTAC micelles, we have estimated a kinetic ratio of ca. 15 from the lifetime of the pyrene fluorescence in DTAC micelles (36) and the quenching rate constant of pyrene by CP* in DTAC micelles, estimated from the kj of pyrene/CP in CTAC micelles (36) and the known dependence of the quenching rate constants of 1-methylpyrene fluorescence on the chain length of alkylpyridinium chloride quenchers (37). As well, we have determined the surfactant aggregation numbers of DTAC micelles as a function of the surfactant concentration. These results were in very good agreement with the results of DeSchryver et al. (37,38). For SDS, Almgren and Swarup obtain an aggregation number of 70 at an SDS molarity of 0.0346 (31), using the probe/quencher pair ruthenium tris-bipyridyl chloride/9-MA. Under the same conditions, using pyrene/CP as the probe/quencher pair, we obtain an aggregation number of 66, in very good agreement with the results of Almgren and Swarup. A number of studies in the literature have used the static quenching method to determine the effects of solubilizates on N . Almgren and Swarup determined the surfactant aggregation number of SDS micelles as a function of the concentration of both the surfactant and solubilizate for a wide variety of solubilizates, including nalcohols. A number of trends were reported in these studies, including the decrease in N brought about by the addition of alcohols. For a given concentration of surfactant, the rate of the decrease in N with an increase in the total alcohol concentration was observed to be larger for the more hydrophobic alcohols. It might be expected that this higher rate of decrease is related to the greater distribution of the alcohol in the micelle. This last parameter can be determined by the two NMR methods mentioned earlier. Almgren and Swarup used the distribution constants of q

0

q

q

0

s

s

+

+

0

+

0

+

s

8

8


11.

MARANGONI ET AL.


the solubilizates, p, obtained by Stilbs (20,21) using the FT-PGSE experiment, the concentration of added alcohol, C , and the micellar concentration from the static quenching method, to calculate the alcohol aggregation number, N . a

a

N = a

(4)

m

N was observed to increase slowly for the less hydrophobic alcohols; for the more hydrophobic alcohols, N increased more quickly, so that the total aggregation number, N = N + N , remained relatively constant or increased slowly. Similar results for 0.0400 molal SDS/alcohol mixed micelles were found by Malliaris (32), using the static quenching method with pyrene/CP ion as the probe/quencher pair. In this study, we present CMC values, surfactant aggregation and alcohol aggregation numbers in SDS/alkoxyethanol and dodecyltrimethylammonium bromide (DTAB)/alkoxyethanol mixed micellar systems as a function of the concentration of alcohol and its ethylene oxide (EO) chain length, in order to investigate the role of the EO group in the formation of mixed micelles. The surfactant aggregation numbers, N , of the mixed micelles are determined using the static fluorescence quenching method, with pyrene/CP as the probe/quencher pair. The alcohol aggregation number, N , and the total micelle aggregation number, N , are estimated from the micelle concentrations determined from the static quenching method, and the distribution constants of alkoxyethanols obtained from the paramagnetic relaxation experiment. As well, the pyrene VI3 ratios (indicative of the micropolarity sensed by the luminescent probe) have been determined. All these results will be discussed in terms of the decrease in the free energy of transfer from H 0 to the micellar phase as a function of the number of EO groups in the alcohol, and the significant differences in the interactions of ethoxylated alcohols with anionic and cationic micelles. a

a

t

s

a


+

s

+

a

t

2

Experimental Sodium dodecylsulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) were obtained from Sigma and purified by repeated recrystallizations from ethanol and an acetone/ethanol mixture, respectively. The deionized water had a resistivity of 2.5xl0 ohm • cm. 1-butanol (C Eo) and ethylene glycol mono-n-butyl ether (QEj) were reagent grade solvents from the Fisher Chemical Company; they were purified by two distillations and stored over molecular sieves. Diethylene glycol mono-n-butyl ether (C4E2, Aldrich, Spectroscopic Grade) was used as received. Triethylene glycol monon-butyl ether (QE3) was obtained from Tokyo Kasei and used without further purification. All alcohol/water mixed solvent systems were prepared on a molality basis; the surfactant solutions made up directly in the mixed solvent. The CMC values of SDS/alcohol mixed micellar systems were obtained from conductivity measurements, using an Industrial Instruments resistance bridge, operating at 1000 Hz, and a dip cell (K = 1.0 cm ). The CMC values in DTAB/alcohol mixed micellar systems were determined from EMF titrations using a PVC membrane electrode responsive to the cationic amphiphile (39,40), in combination with a calomel reference electrode. The EMF titrations were carried out on an automatic titration system consisting of a Dosimat automatic buret (Metrohm) and a Keithley high impedance electrometer (model A614), controlled by an IBM PC. All spin-lattice relaxation times (T^s) were measured on freshly prepared solutions. The NMR experiments and the solution preparation have been described previously (25). 6

4

1

cell


198


Solution preparation for the luminescence quenching experiments was as follows. A small amount of a 0.0050 molal pyrene/ethanol solution was placed in a small flask and the solvent allowed to evaporate, usually overnight, depositing the pyrene as a thin film on the bottom of the vessel. The stock solution of the surfactant/mixed solvent system was prepared directly in the flask containing the pyrene, and it was stirred for at least four hours to ensure complete dissolution of die pyrene in the surfactant solution. This method of solution preparation was found previously (5,28,52) to be a very effective means of dissolving hydrophobic fluorescent probes (i.e., pyrene) into a surfactant solution. The quencher solutions were prepared from one-half of the stock surfactant/probe solutions by adding the desired amount of CP to the solution, and stirring for 1-2 hours. Mixtures at specific quencher concentrations were prepared by mixing portions of the two stock solutions. Steadystate pyrene fluorescence emission spectra were recorded at room temperature ( « 23°C) on a Perkin-Elmer MPF-66 spectrophotometer, using an excitation wavelength of 338 nm and scanning the emission from 350 to 500 nm. The Ii/I ratios were measured directly from the spectra. The intensity of the emission at 373 nm was used in the plots of \n(ljl) versus the quencher concentration.


+

3

Results and Discussion Anionic Surfactant/Alkoxyethanol Mixed Micelles. CMC values for SDS/alkoxyethanol mixed micelles, obtained from the breaks in the conductance vs. Qurf.t pl° > are presented in Table 1 and plotted in Figure 2 as a function of the total concentration of alcohol. As expected, the CMC values of SDS/alkoxyethanol mixed micelles decrease as the concentration of the alcohol is increased. More importantly, it is evident from Figure 2 that for a given alcohol concentration, there is a steady decrease in the CMC values from SDS/C Eo mixed micelles to SDS/C4E3 mixed micelles. This effect of added ethoxylate groups is in agreement with the previous results of Manabe at al. (12) and Yamashita et al. (16). Shinoda (1) has noted that the decrease in the CMC of alcohol/surfactant mixed micelles is linear with the total concentration of alcohol, c . From the data in Table 1, the rates of change of the CMC of SDS/alkoxyethanol mixed micelles with the total concentration of alkoxyethanol, d CMC/d c, values, are calculated to be 0.020 for C Eo, 0.042 for C E 0.065 for Q E ^ and 0.101 for QE3/SDS mixed micelles. The rates of change can be related to the free energy of transfer of the alcohol hydrophobic group from the aqueous phase to the micellar phase (7) dCMC mw (5) ts

4

t

4

4

l9

where w is the transfer free energy of the alcohol hydrophobic group from the aqueous to the micellar phase, and c is a constant related to the free energy of micellization of the pure surfactant micelles. For the series C E C A , we can apply equation 5 to the d CMC/d c values for the series of alkoxyethanols in SDS micelles and obtain an estimate of the transfer free energy of the EO group, W . From the d CMC/d c, values reported above, we estimate w to be 0.53 ± 0.10 kT or 1.3 ± 0.3 kJ mol . The degree of solubilization, p, of the cosurfactant in surfactant micelles, defined by equation 1, is obtained from NMR relaxation experiments as described previously (24,25). These values are presented in Table 2 for alkoxyethanols in SDS and DTAB micelles. From the degree of solubilization, we can calculate the mole fraction based distribution coefficient, K , defined as follows p

4

0

a

EO

EO

1

x


11.

MARANGONI E T A L .


1n (l /l)

1

0

/is/ 00

/ Ay

0.8 /

//

0.6 Downloaded by COLUMBIA UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch011

///

/

5.0

0.4

0.2 0.1 0 0

0.2

I 0.4

I 0.6

I 0.8

1

Figure 1. Calculated values of Ln Q.JI) vs. at different kinetic ratios,

Table 1. CMC values (± 0.5 mmolal) for SDS/Alkoxyethanol Mixed Micelles as a Function of the Total Concentration of Added Alcohol c /molal

CA

C E,

C E,

C E,

0.0000

8.10

8.10

8.10

8.10

0.0025

— —— — — —

— — —

7.82

7.54

7.08

—

—

0.0200

— — — — — —

0.0250

7.58

7.06

—

0.0300

—

—

6.24

0.0500

6.21

0.0750

7.18 6.64

0.1000

6.02

a

0.0050 0.0075 0.0100 0.0150

4

4

4

7.55 7.34

6.85

5.09 4.56


200



CMC / mmolal

0

0.02

0.04

0.06

0.08

0.1

C / molal a

Figure 2. CMC values/mmolal for SDS/alkoxyethanol mixed micelles as a function of c (est'd error ± 0 . 2 mmolal). O C Eo, O C E„ • C A , V QE3. a

4

4

Table 2. Distribution Coefficients and Free Energies of Transfer for Several Alcohols in SDS and DTAB Micellar Solutions 1

Alcohol

2

K

P

AG° / kJ mol t

x

SDS C4E0

0.42 ± 0.04

134 ± 24

-12.1 ± 0.4

QE

0.54 ± 0.03

-13.4 ± 0.3 -15.3 ± 0.4

CA

0.66 ± 0.03

219 ± 29 364 ± 47

QE3

0.72 ± 0.03

487 ± 71

1

-14.6 ± 0.3

DTAB C4E0

0.32 ± 0.04

128 ± 32

-12.4 ± 0.4

C E,

0.37 ± 0.07

-13.1 ± 0.7

QE2

0.32 ± 0.07

163 ± 33 141 ± 42

QE3

0.31 ± 0.07

131 ± 41

4

1.

C

S D S

= 0.243 m, T = 298 K, C

PRO

XYL

= 0.010 m,

C

A

-12.7 ± 0.7 -12.7 ± 0.7 « 0.050 m.


1

11.

MARANGONI ET AL.


K =^

(6)

A

aq

X and X are the mole fractions of solubilizate in the micellar and aqueous phases, respectively, and are calculated as follows mic

a q

Y

_

Interaction of Alcohols and Ethoxylated Alcohols with Anionic and

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