Transport Properties of Oil-in-Water Microemulsions - ACS Publications

18 Transport Properties of Oil-in-Water Microemulsions 1

1

1

KENNETH R. FOSTER , ERIK CHEEVER , JONATHAN B. LEONARD , FRANK D. BLUM , and RAYMOND A. MACKAY 2

2,3

1

Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104 Department of Chemistry, Drexel University, Philadelphia, PA 19104

Downloaded by UNIV OF MICHIGAN ANN ARBOR on April 2, 2013 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch018

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We have studied a variety of transport properties of several series of O/W microemulsions containing the nonionic surfactant Tween 60 (ATLAS tradename) and npentanol as cosurfactant. Measurements include dielectric relaxation (from 1 MHz to 15.4 GHz), electrical conductivity in the presence of added electrolyte, thermal conductivity, and water self-diffusion coefficient (using pulsed NMR techniques). In addition, similar transport measurements have been performed on concentrated aqueous solutions of poly(ethylene oxide) (PEO), which has the same hydrophilic group as that on the surfactant and is responsible for stabilizing the microemulsions. Some, but not all, of these transport properties are significantly lower than expected from the Maxwell or Hanai mixture theories. The dielectric relaxation at microwave frequencies (which reflects dipolar relaxation of the water) shows a relaxation time that is significantly longer than that of pure liquid water, with evidently a broad distribution of relaxation times present. It appears that these changes in large part arise from hydration phenomena, and therefore can be used to study hydration effects in these systems. The transport properties of microemulsions are of great interest both for the information they provide about the physical properties of the systems, and in industrial applications of these materials. The transport of matter or energy through oil in water (0/W) microemulsions is determined both by the volume fraction and geometry of the oil and emulsifier microdroplets (the "structure effect") and by possible modifications in the transport properties of the continuous water phase by its interaction with the hydrophilic groups in the surfactant and cosurfactant that stabilize the microemulsion (the "hydration effect"). Through the use of appropriate mixture theories, these two effects can in part be separated. 3

Current address: Chemical Research and Development Center, Aberdeen Proving Grounds, MD 21005 0097-6156/85/0272-0275$06.00/0 © 1985 American Chemical Society

In Macro- and Microemulsions; Shah, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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MACRO- AND MICROEMULSIONS

In previous studies, we examined the d i e l e c t r i c properties of several i o n i c and nonionic 0/W microemulsions at radio through microwave frequencies. The d i e l e c t r i c relaxation at microwave frequencies is due to dipolar relaxation of the water which exh i b i t s , in the pure l i q u i d , a center relaxation frequency of 20 GHz corresponding to a dipolar relaxation time of 8 ps. In the microemulsions the average d i e l e c t r i c relaxation time of the water is increased by a factor of 2-10, depending on the t o t a l water content of the system. Moreover, these systems display a d i s t r i b u t i o n of relaxation times, that suggests the presence of a f r a c t i o n of water with d i e l e c t r i c relaxation time 5-10 times that of bulk water, with the remaining water having the same d i e l e c t r i c relaxation properties as the pure l i q u i d . These changes p a r a l l e l those observed in the conductivity at audio frequencies in a variety of microemulsions with nonionic surfactant and cosurfactants containing d i l u t e e l e c t r o l y t e as the continuous phase (3). We suggested that other transport properties should show s i m i l a r changes (2). In the present study, we have examined other transport properties of 0/W microemulsions containing the nonionic surfactant Tween 60 whose d i e l e c t r i c and conductivity properties have been previously characterized. We have chosen properties (water s e l f - d i f f u s i o n , i o n i c conductivity at low frequencies, and thermal conductivity) that can be analyzed using the same mixture theory, and which therefore can be compared in a consistent way. Limited transport data are presented from other microemulsions as w e l l . Materials and Methods The transport properties were measured at 25°C using a variety of techniques. An extensive analysis of the d i e l e c t r i c relaxation measurements is provided by Epstein (4). The other methods are conventional and w i l l only be summarized b r i e f l y : Ionic Conductivity. The e l e c t r i c a l conductivity measurements were performed using a Hewlett Packard model 4192 impedance analyzer under computer control, using a conductance c e l l s i m i l a r to that described by Pauly and Schwan (5). The conductivity measurements were e s s e n t i a l l y constant between 1-100 kHz, r u l i n g out electrode polarization or other a r t i f a c t s . In 0/W microemulsions, no appreciable d i e l e c t r i c relaxation e f f e c t s are expected or observed below 1 GHz (1). Water S e l f - D i f f u s i o n . The s e l f - d i f f u s i o n c o e f f i c i e n t of the water was measured using a JE0L FX900 Fourier transform NMR spectrometer operating at 90 MHz for protons. The pulsed f i e l d gradient technique was employed using the homospoil c o i l s to establish the f i e l d gradient, s i m i l a r to that described by S t u b s (6). The s e l f - d i f f u sion c o e f f i c i e n t s of the water protons were corrected for exchange with the hydroxyl protons of the alcohols, but this correction was i n s i g n i f i c a n t except when the f r a c t i o n of water was very low. Thermal Conductivity. The thermal conductivity of the samples was measured using a thermistor probe technique s i m i l a r to that described by Balasubramaniam and Bowman (]) . A small thermistor head



18.

FOSTER ET AL.

Transport Properties of Ο/ W Microemulsions

277

was immersed in the sample and subjected to a step temperature in crease of 2°C within a few milliseconds; the voltage across the thermistor was subsequently sampled at 25 Hz with 12 b i t resolution by computer. A l i n e a r regression of the power dissipated in the thermistor vs. the inverse square root of time y i e l d s the steady state power d i s s i p a t i o n whose inverse is a l i n e a r function of the inverse of the thermal conductivity of the sample. The microemulsions were i d e n t i c a l to those examined in our previous d i e l e c t r i c studies (1>2). emulsifier (Tween and npentanol) was mixed with the oil (hexadecane) in the proportion (by weight) O.594 Tween:O.306 l-pentanol:O.100 hexadecane. Water or e l e c t r o l y t e (O.1 Ν NaCl f o r the conductivity measurements) was then added to give the f i n a l composition. The physical characterization of this system is given in References 5,11,13, and 14 of our pre vious paper (JO. In the discussion to follow, the compositional phase volume ρ is the volume f r a c t i o n of oil and emulsifier, and is equal to (1 - wg), where w and g are the weight fractions of water and the s p e c i f i c gravity of the microemulsion, respectively. We w i l l also consider the apparent phase volume p which is calcu lated from the mixture theories as the t o t a l volume f r a c t i o n of the microemulsion that is excluded from the transport. Assuming that the transport property of the hydration water is n e g l i g i b l e compared to that of the bulk l i q u i d , p' would include the hydration water as well as the oil and e m u l s i f i e r . f

Theoretical The basis for i n t e r p r e t i n g the transport data is mixture theory, which relates the transport properties of the bulk suspension to those of the continuous and dispersed phases. Of the many mixture relations that have been proposed, we employ those of Maxwell and Hanai (Equations 1 and 2, r e s p e c t i v e l y ) : A

m

Λ

2*w

+ ο - 2ρ(^

-

A)

2A

+ Λ

-

Λ )

0

= A

w

W

0

+ p(A

w

(Maxwell)

(1)

(Hanai)

(2)

0

1/3 A

w

A

" o

I

= 1 - ρ A

m

In these r e l a t i o n s , Λ is the conductivity of the suspension, and the subscripts m, o, and w refer to the microemulsion, oil and emulsifier combined, and water. The Hanai expression can be con sidered to be an extension of the Maxwell theory that more con s i s t e n t l y accounts for the presence of neighboring p a r t i c l e s (8); for the 0/W microemulsions considered here, the predictions of the Maxwell and Hanai formulas (as well as various other mixture the ories) are not greatly d i f f e r e n t . Moreover, while these theories were developed for suspensions of spherical p a r t i c l e s , the predic tions of the mixture theories are not expected to vary greatly with the geometry of the dispersed p a r t i c l e s , provided that the droplets are prolate or oblate e l l i p s o i d s whose a x i a l r a t i o s are not greatly


278

M A C R O - A N D MICROEMULSIONS

d i f f e r e n t from 1 09). Consequently, our choice of these p a r t i c u l a r mixture theories, while somewhat a r b i t r a r y , is not c r u c i a l in the interpretation of the present r e s u l t s . The mixture theories were o r i g i n a l l y developed for d i e l e c t r i c properties, but can be applied to other properties that are governed on a macroscopic l e v e l by Laplace's equation (10). Con sequently, the generalized conductivity in the above equations can be the i o n i c conductivity σ, thermal conductivity K, or complex e l e c t r i c a l conductivity σ* defined by:


σ* = σ + ju)ee

r

where ε is the d i e l e c t r i c p e r m i t t i v i t y , ω is the frequency in radians/sec, and ε is the p e r m i t t i v i t y of free space (a constant, 8.85 χ 10"Ί4 F/cm). For the s e l f - d i f f u s i o n measurements, the generalized conductivity would be the quantity (l-p)D, where D is the s e l f - d i f f u s i o n c o e f f i c i e n t that is measured by the pulsed NMR technique, and ρ is the volume f r a c t i o n of oil and e m u l s i f i e r . The need f o r the additional factor (1-p) arises from the fact that the NMR technique measures the mean-square displacement of the water molecules in the aqueous phase, while the true s e l f - d i f f u s i o n co e f f i c i e n t is defined (by Fick's law) as the t o t a l f l u x through the entire volume of the solution (11). Γ

Results Figure 1 shows the d i e l e c t r i c relaxation properties of the Tween microemulsions plotted on the complex p e r m i t t i v i t y plane (from Foster et a l (1). The mean relaxation frequency (corresponding to the peak of each semicircle) decreases gradually from 20 GHz f o r pure water at 25°C to ca. 2 GHz for a concentrated microemulsion containing 20% water. Since the p e r m i t t i v i t y of the suspended oil/ emulsifier is 6 or less at frequencies above 1 GHz, this relaxation p r i n c i p a l l y arises from the dipolar relaxation of the water in the system. Therefore, the data shown in Figure 1 c l e a r l y show that the d i e l e c t r i c relaxation times of the water in the microemulsions are slower on the average than those of the pure l i q u i d . The depressed semicircles indicate a d i s t r i b u t i o n of relaxation times (9), and were analyzed assuming the presence of two water components (free and hydration) in our previous studies. Figures 2-4 show the thermal and i o n i c conductivity, and water s e l f - d i f f u s i o n c o e f f i c i e n t measured in these same systems. Also shown are the transport properties of PEO solutions of molecular weights ranging from 200 to 14,000 (12). The predictions of the Hanai and Maxwell relations are indicated, which were calculated on the assumption that the i o n i c conductivity or s e l f - d i f f u s i o n co e f f i c i e n t of the water or suspending e l e c t r o l y t e is equal to that of the pure l i q u i d and that of the oil and emulsifier combined is zero. Also shown are s i m i l a r r e s u l t s from the PEO solutions of various molecular weights. The thermal conductivity of the microemulsions and PEO solutions are shown in separate figures because the l i m i t i n g thermal conductivity at zero water content is s l i g h t l y d i f f e r e n t (O.27 times that of water for the microemulsion, vs. O.31 for the PEO).



FOSTER ET AL.

τ

1


1

1

1

1

1

1

1

1

1

Γ

0/W (Tween 60) 25°C

F i g u r e 1. P l o t s of the complex p e r m i t t i v i t y of the 0/W m i c r o emulsions p r e p a r e d w i t h Tween 60 on the complex d i e l e c t r i c p l a n e ( " C o l e - C o l e " p l o t s ) , showing the d e p r e s s e d s e m i c i r c l e s t h a t i n d i c a t e a d i s t r i b u t i o n of r e l a x a t i o n t i m e s . Figure lb is an expanded p o r t i o n of F i g u r e l a . A few f r e q u e n c i e s a r e i n d i c a t e d f o r r e f e r e n c e . Reproduced w i t h p e r m i s s i o n from R e f e r e n c e 1. C o p y r i g h t 1982 Academic Press.


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MACRO- A N D MICROEMULSIONS

b

ni

.

0

• 1.0

VOLUME

FRACTION

(p)

Figure 2. Thermal conductivity of the Tween microemulsions (Figure 2a) and of a series of PEO solutions of d i f f e r e n t molecular weights (2b). The predictions of Equations 1 and 2 are shown f o r reference. The thermal conductivity has been normalized to that of water.



FOSTER ET AL.


VOLUME

FRACTION

(ρ)

Figure 3, Ionic conductivity of the Tween microemulsions and PEO solutions, compared with Equations 1 and 2. For these experiments, the aqueous phase was O.1 Ν NaCl or O.1 N KC1, and the measured conductivity values were normalized to that of the suspending e l e c t r o l y t e .

VOLUME

FRACTION

(p)

Figure 4. Water s e l f - d i f f u s i o n c o e f f i c i e n t D of the micro emulsions and PEO solutions, normalized to that of the pure l i q u i d water. The need for the additional factor (1-p) is described in the text. Also shown are predictions of the Maxwell and Hanai equations.


282

MACRO- AND MICROEMULSIONS

The s t r i k i n g observation is that the i o n i c conductivity and water s e l f - d i f f u s i o n c o e f f i c i e n t , but not the thermal conductivity, deviate s i g n i f i c a n t l y from the predictions of the mixture theories. This could arise from s t r u c t u r a l e f f e c t s , such as a gradual t r a n s i tion from 0/W to W/0 structure with decreasing water content. We argue instead that these deviations p r i n c i p a l l y result from hydra tion e f f e c t s , and not from s t r u c t u r a l properties of the microemul sions. This would be expected because of the s i m i l a r i t y of the data from the microemulsions and PEO, in which structure effects would be quite d i f f e r e n t .


Discussion A simple analysis, based on the mixture theory, supports this in terpretation. In the following discussion we w i l l assume that the hydration water can be included with a volume f r a c t i o n p which is excluded from transport. The difference between the apparent ex cluded volume f r a c t i o n hydration p and ρ corresponds to values that are in the range expected for the hydrophilic moieties in the surfactant and cosurfactant. F i n a l l y , we w i l l suggest a physical interpretation for the observations. f

1

Apparent Hydration of the Suspended Microdroplets. The i o n i c con d u c t i v i t y and water s e l f - d i f f u s i o n data, divided by the respective values for the bulk l i q u i d , are summarized in Table I, together with the apparent volume fractions p that are calculated from the Maxwell and Hanai mixture theories. The s i m i l a r i t y in the i o n i c conductivity and water s e l f - d i f f u s i o n data is surprising, in view of the greatly d i f f e r e n t underlying mechanisms f o r these phenomena. By hypothesis, the difference between the compositional phase volume ρ and the t o t a l excluded volume p represents the volume f r a c t i o n of hydration water. The calculated hydration, expressed as a r a t i o of (moles hydration water) : (moles EO plus moles OH) is presented in Table I I . From thermodynamic studies, the expected hydration of the EO and OH groups are 2 and 3 water molecules, respectively (13,14). While the "hydration" as obtained from the present transport measurements does not r e f l e c t stoichiometric binding but rather k i n e t i c e f f e c t s , the hydration values obtained are quite reasonable. f

f

Table I.

Summary of Transport Properties of the Tween 60 Microemulsions

Compositional phase volume ρ O.80 O.60 O.40 O.25 O.20

o/o

w

O.002 O.070 O.230

P' (Eq. 1) (Eq. 2) O.99 O.90 O.69

-

-

O.540

O.36

O.96 O.84 O.63

O.34

(l-p)D/D

O.016 O.097 O.255 O.413 —

w

0/W

Ρ (Eq. 1) (Eq. 2) O.98 O.86 O.66 O.49 —


O.94 O.79 O.60 O.44 —

18.

FOSTER ET AL. Table I I .

compositional phase volume ρ O.80 O.60 O.40 O.25 O.20


(Note:

Transport Properties of Ο/ W Microemulsions Apparent Hydration

283

of the Microemulsions

(conductivity) (Eq. 1) (Eq. 2) 1.06 O.89 2.23 1.78 3.24 2.57 3.58 3.13

(diffusion) (Eq. 1) (Eq. 2) 1.00 O.70 1.93 1.41 2.91 2.24 4.31 3.41

Hydration numbers expressed as moles water per moles EO plus moles OH.)

It was e a r l i e r shown (12) that the hydration values of the PEO solutions that are calculated in the same manner also agree with expected values. Since the EO group is the moiety in the surfactant that is responsible for s t a b i l i z i n g the microemulsion, a comparison of the transport data of the microemulsions with those of the PEO solutions is of i n t e r e s t . The s l i g h t l y higher i o n i c conductivity and water s e l f - d i f f u s i o n c o e f f i c i e n t s of the microemulsions can be attributed to the f r a c t i o n of oil that is not hydrated and conse quently can only contribute to the obstruction e f f e c t . While structural changes might be expected in the microemulsions at high phase volumes, they evidently produce no large changes in the trans port properties presently reported. Physical Mechanisms. The simplest interpretation of these results is that the transport c o e f f i c i e n t s , other than the thermal conductiv i t y , of the water are decreased by the hydration i n t e r a c t i o n . The changes in these transport properties are correlated: the micro emulsion with compositional phase volume O.4 ( i . e . 60% water) exhibits a mean d i e l e c t r i c relaxation frequency one-half that of the pure l i q u i d water, and i o n i c conductivity and water s e l f d i f f u s i o n c o e f f i c i e n t one half that of the bulk l i q u i d . In bulk solutions, the d i e l e c t r i c relaxation frequency, i o n i c conductivity, and s e l f - d i f f u s i o n c o e f f i c i e n t are a l l inversely proportional to the v i s c o s i t y ; there is no such r e l a t i o n for the thermal conduc t i v i t y . The transport properties of the microemulsions thus vary as expected from simple changes in " v i s c o s i t y " of the aqueous phase. (This is quite d i f f e r e n t from the bulk v i s c o s i t y of the microemulsion.) This is, however, a macroscopic explanation of changes that occur on a molecular l e v e l , and is rather s u p e r f i c i a l . There is c l e a r l y a d i s t r i b u t i o n of d i e l e c t r i c relaxation times in the micro emulsion. The timescale of the d i e l e c t r i c relaxation measurement (tens of picoseconds) is too short for the phenomenon of f a s t ex change. It would appear, therefore, that the motional r e s t r i c t i o n of the water must vary throughout the microemulsion. Perhaps a more defensible hypothesis is that the r o t a t i o n a l and translational c o r r e l a t i o n times are both increased, by s i m i l a r factors of ten or l e s s , when a water molecule is s u f f i c i e n t l y close or hydrogen bonded to an EO or OH group in the surfactant.


284

MACRO- AND

MICROEMULSIONS


Nevertheless, the concept of "average v i s c o s i t y " , has predictive value. A more extensive discussion of this problem is presented elsewhere (12). Effect of Microemulsion Structure on the Transport Properties. It appears from the discussion above that the reduction in the i o n i c conductivity and water s e l f - d i f f u s i o n c o e f f i c i e n t is primarily attributable to hydration e f f e c t s , not p r i n c i p a l l y to changes in the structure of the microemulsion with higher phase volume. Either no pronounced changes in structure occur with increased phase volume (which seems u n l i k e l y ) or they are of such a nature as not to greatly a f f e c t the transport properties. Since the mixture theories are not extremely sensitive to the exact shape of the suspended p a r t i c l e s the second p o s s i b i l i t y seems more l i k e l y . Water s e l f - d i f f u s i o n data from other microemulsions suggest an effect of s t r u c t u r a l changes on the transport properties. Figure 5 shows the water s e l f - d i f f u s i o n c o e f f i c i e n t in several i o n i c and noni o n i c systems (15). The data, while remarkably s i m i l a r , do show more v a r i a t i o n than would be expected from hydration e f f e c t s alone. In p a r t i c u l a r , the water s e l f d i f f u s i o n c o e f f i c i e n t in the microemulsion prepared with the i o n i c surfactant SCS appears to be anomalously low at one composition (p = O.42). However, that composition corresponded to a point in the phase diagram close to a region of phase separation (16). The sample exhibited unusually high bulk v i s c o s i t y which presumably arose from long range s t r u c ture. Further NMR studies of the s e l f - d i f f u s i o n properties of each species in these systems w i l l be reported (15). Lindman and coworkers (17) in s i m i l a r studies have shown how s e l f - d i f f u s i o n properties of each species in a microemulsion can y i e l d information about changes in structure with composition that is d i f f i c u l t to obtain from measurements of the sort reported here. Conclusions To our knowledge, this is the f i r s t report of such a wide variety of transport measurements in a single series of microemulsions. Some of the properties (water s e l f - d i f f u s i o n , i o n i c conductivity, d i e l e c t r i c relaxtion) are s u b s t a n t i a l l y d i f f e r e n t from predictions of mixture theories; another property (thermal conductivity) is in much better agreement. The discrepancies between the i o n i c cond u c t i v i t y and water s e l f - d i f f u s i o n c o e f f i c i e n t and predictions of the mixture theories y i e l d hydration values for the microemulsion that agree with anticipated values. It appears that a l l of these changes can be correlated with v a r i a t i o n in one property - the v i s cosity of the suspending water. While it appears that s t r u c t u r a l changes with varying composition are also r e f l e c t e d in the transport properties in some cases, hydration effects appear to play a s i g n i f i c a n t and perhaps dominant role in determining the o v e r a l l transport properties. Our r e s u l t s suggest that the usefulness of d i e l e c t r i c measurements at microwave frequencies, together with the other transport measurements described here, in studying hydration phenomena in these complex systems.



FOSTER ET AL.


F i g u r e 5. Water s e l f - d i f f u s i o n c o e f f i c i e n t s in a v a r i e t y of i o n i c and n o n i o n i c m i c r o e m u l s i o n s . The c o m p o s i t i o n s of these m i c r o e m u l s i o n s a r e g i v e n in R e f e r e n c e 2.


M A C R O - A N D MICROEMULSIONS

286

Acknowledgments This work was supported in part by National Science Foundation Grant CPE 82-04911, Drexel University Graduate School, Drexel University Research Corporation and the Donors of the Petroleum Research Fund,

Literature Cited 1.


2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Foster, K. R., Epstein, B. R., Jenin, P. C. and Mackay, R. A. J. Colloid Interface Sci. 1982, 88, 233. Epstein, B. R., Foster, K. R. and Mackay, R. A. J. Colloid Interface Sci. 1983, 95, 218. Mackay, R. A. and Agarwal, R. J. Colloid Interface Sci. 1978, 65, 225. Epstein, B. R. Ph.D. Thesis, University of Pennsylvania, Philadelphia, Pennsylvania, 1982. Pauly, H. and Schwan, H. P. Biophys. J. 1966, 6, 621. Stilbs, P. J. Colloid Interface Sci. 1982, 87, 385. Balasubramaniam, T. A. and Bowman, H. F. J. Biomech. Eng. Trans. ASME (Series K), 1979, 99, 148. Hanai, T. "Electrical Properties of Emulsions"; In Emulsion Science, Sherman, P. Ed., Academic Press, NY, 1968, Chap. 5 p. 353. Hasted, J. B. "Aqueous Dielectrics", Chapman and Hall, Londdon, 1973. Chiew, Y. C. and Glandt, E. D. J. Colloid Interface Sci. 1983, 94, 90. Clark, M.E., Burnell, Ε. E. Chapman, N. R. and Hinke, J. A. M. Biophys. J. 1982, 39, 289. Foster, K. R., Cheever, E . , Leonard, J. B. and Blum, F. D. Biophys. J. (in press). Franks, F. In " Water, a Comprehensive Treatise"; Franks, F. Ed.; Plenum Press: New York, 1973, Vol. II, pp. 1-54. Molyneux, P. In "Water, a Comprehensive Treatise"; Franks, F. Ed.; Plenum Press: New York, 1975, Vol. IV, pp. 617-633. Cheever, E . , manuscript in preparation Mackay, R. Α., Letts, K. and Jones, C. In "Micellization, Solubilization, and Microemulsions"; Mittal, K. L. Ed.; Plenum Press: New York, 1977, Vol. II, pp. 801-815. Lindman, B., Stilbs, P. and Mosely, M. E. J. Colloid Inter face Sci. 1981, 83, 569.

R E C E I V E D June 8, 1984


Transport Properties of Oil-in-Water Microemulsions - ACS Publications

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