Hydrogen Bonding of Simple Alcohols in Supercritical Fluids - ACS

Chapter 14

Hydrogen Bonding of Simple Alcohols in Supercritical Fluids An FTIR Study

Downloaded by PURDUE UNIVERSITY on March 11, 2013 | http://pubs.acs.org Publication Date: December 17, 1992 | doi: 10.1021/bk-1992-0514.ch014

John L. Fulton, Geary G. Yee, and Richard D. Smith Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory, Richland, WA 99352

FT-IR spectroscopy has been used to measure the degree of intermolecular hydrogen bonding between solute molecules of simple alcohols (methanol to dodecanol) in supercritical carbon dioxide and supercritical ethane. In these fluids an equilibrium is established between the free nonhydrogen bonded monomer and the various hydrogen bonded species, of which the tetrameric and pentameric species are believed to predominate at lower mole fractions of the alcohol. The fluid pressure, temperature, and the alcohol concentration significantly affect the equilibrium distribution of the monomer and oligomeric species. Both supercritical and subcritical binary solutions containing up to 0.10 mole fraction alcohol were examined under conditions ranging from 200 to 400 bar and 40°C. The spectral data support the existence of a weak complex between the alcohol monomer and carbon dioxide.

Simple alcohols such as methanol and 2-propanol are widely used as modifiers to enhance the solubility of polar solutes in the supercritical fluid phase. Such modifiers increase the solvent strength of the fluid by increasing the overall dielectric constant of the binary supercritical solution. The alcohol modifier can also readily hydrogen bond with other alcohol molecules or other polar solutes to form new bimolecular or multimolecular species. These intermolecularly hydrogenbonded species have a greatly reduced dipole moment or polarity compared to the parent solute species. This reduction in turn improves the solubility of these species in nonpolar supercritical fluids. A hydrogen-bonded aggregate represents just one of a range of different molecular assemblies that are known to form in supercritical fluids. The simplest aggregate is the van der Waals-driven solvent clusters that form around solutes when the solution is near the critical point. At the other end of the spectrum of

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In Supercritical Fluid Engineering Science; Kiran, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


176

SUPERCRITICAL FLUID ENGINEERING SCIENCE

molecular aggregates are the large multimolecular structures that form upon addition of certain surfactants to apolar supercritical fluids {1,2,3). This aggregation process is driven mainly by strong ionic interaction of the surfactant head groups within the aqueous cores as well as a host of different secondary effects. The magnitude of the forces that drive aggregation of alcohol clusters is intermediate between those that form the solvent clusters and those that form the surfactant cluster. Spectroscopic studies of hydrogen bonding of alcohols in apolar liquid solvents extend back 50 years (4,5,6). Such studies have suggested that alcohol molecules form hydrogen-bonded "clusters" in apolar liquid solvents consisting of from two to six molecules per cluster. The extent of hydrogen bonding of alcohols in apolar liquid solvents is consistent with a multiple equilibria model. At low concentrations (7) (mole fraction alcohol, x icohol< 0.003) the alcohol exists in the monomer form. Upon increasing the concentration, the aggregation progresses through dimer, trimer to tetramer, and higher oligomers at higher concentrations of the alcohol. a

In supercritical fluids the size of the alcohol aggregate is similar to that found for liquid solvents. We have reported aggregation numbers in the range of 3 to 5 for dodecanol-d in CO2 and ethane (8). This study confirmed the expectation that generally the size of an alcohol aggregate formed in supercritical fluids such as CO2 is little different from that observed in liquid and gas phases. There is, however, believed to be specific interaction between CO2 and an alcohol that is not found in the alkane/alcohol system (9). This interaction has been ascribed to an attractive C02-quadrupole/alcohol dipole interaction that greatly increases the solubility of the alcohol in CO2. This interaction also perturbs the monomeraggregate equilibria towards the monomer form of the alcohol. Recently, Yee et al. (10) reported aggregation numbers for n-butanol and a fluorinated butanol in CO2 and ethane and found that for lower concentrations of the alcohol (0.05< Xalcohol99.98% and water content of \ band of the intermolecular hydrogen-bonded alcohol species at wavenumbers below 2500 c m is partially obscured by the X>$ band of CO2, centered at 2349 cm-l. However, this CO2 solvent band does not interfere with the D i monomer band at -2680 c m , which is used in a later section to determine the amount of nonhydrogen-bonded monomer. 1

0D

0 D

-1

0D

0 Η

-1

1

0 Η

1

0 Η

0 D

0

Η

0 D

0D

-1

O D

-1


. FULTON ET AL.

Hydrogen Bonding ofSimple Alcohoh

179


1.0

Wavenumber, cm Figure 1. The C - H combination bands, the C - H symmetric stretch, and the 0~ D stretch of the methanol-^ monomer in CO2 at 400 bar and 4 0 ° C for three different mole fraction concentrations of the alcohol, XMeOD = 0.063, 0.033 and 0.011. Hydrogen bonding shifts the O-D stretch from 2700 to 2600 cnr . 1

The solution in Figure 1 which is at 0.063 mole fraction of methanol is subcritical at 40°C. Gurdial et al. (14) reported critical temperatures and pressures for the n-alkanols up to C10 in CO2. For the methanol/C02 system at 4 0 ° C , mixtures containing more than Xaicohol = 0.05 are subcritical. The transition from a subcritical to a supercritical state does not cause very much change in the solution's properties in this case because at a pressure of 400 bar the system is approaching liquid-like densities. The spectral data of experiments such as those shown in Figure 1 are summarized in Figure 2, which shows the concentration of the monomer for various total mole fractions of the alcohol at 200 bar and 4 0 ° C . Figure 2 also shows the monomer concentration for five other alcohols in CO2: ethanol-d, 2propanol-d, n-butanol-d, 2-methyl-2-propanol-d, and dodecanol-d. The dashed line in Figure 2 represents the case where essentially all added alcohol is in the monomer form, i.e., no aggregation. Deviations from this line are due to intermolecular hydrogen bonding. Below a total alcohol concentration of -0.005 mole fraction, only the monomelic form exists. Upon increasing the alcohol concentration, increasing amounts of hydrogen bonded species are formed. The data of Figure 2 show the relationship between the equilibrium concentrations of monomer and intermolecularly hydrogen-bonded aggregates. The


180



0.05

o.oo-i—•—•—•—•—ι—•—•—•—•—«—•—•—•—•—ι

0.00

0.05

0.10

0.15

χ total

Figure 2. Concentration of monomeric alcohol, Xmonomen for various total mole fractions, xtotal» of the alcohol in CO2 at 200 bar and 40°C. Data for six different alcohols are shown: methanol-d(MeOD), ethanol-d(EtOD), 2propanol-d(IPA), l-butanol-d(n-BuOD), 2-methyl-2-propanol-d(t-BuOD), dodecanol-d. four small alcohols, methanol, ethanol, 2-propanol and n-butanol show nearly the same equilibrium distribution of monomer and aggregate in CO2. For the tertiary alcohol, 2-methyl-2-propanol-d, the equilibrium is shifted strongly toward the monomer relative to the other alcohols. For this tertiary alcohol, aggregate formation is strongly sterically hindered for aggregation numbers above two, because of the close proximity of the three C H 3 groups to the O D (or OH). This is the major reason for the high monomer concentrations found in the 2-methyl-2propanol-d/C02 system. Figure 3 shows the monomer/aggregate equilibria for the six alcohols in ethane under the same conditions of temperature and pressure as in Figure 2. As was observed for CO2, the tertiary alcohol monomer concentrations are much higher than the other alcohols because of steric hindrance inhibiting formation of larger aggregates. Since the monomer concentration is so much higher for this alcohol, the solvation effects of this alcohol when used as a fluid modifier for a supercritical fluid should be significantly different from that of the other alcohols. In contrast to alcohols in the CO2 system, methanol shows the greatest extent of aggregation in ethane. Figure 4 relates the fraction of alcohol that is in the monomer form for a total alcohol mole fraction of 0.03. The effect of increasing the chain length of the


14.

FULTON ET AL.

181

Hydrogen Bonding of Simple Alcohols

0.05Τ = 40°C Ρ = 200 bar

0.04

•

MeOD

0

EtOD

•

IPA-d

•

n-BuOD

Δ

t-BuOD

Α

Dodecanol-d

ε o.03 Downloaded by PURDUE UNIVERSITY on March 11, 2013 | http://pubs.acs.org Publication Date: December 17, 1992 | doi: 10.1021/bk-1992-0514.ch014

ο c ο

ε

Δ—

0.021

I

0.01 .• ί

0.00 0.00

0.05

0.10

0.15

total

Figure 3. Concentration of monomeric alcohol, Xmonomer* for various total mole fractions, xtotal* of the alcohol in ethane at 200 bar and 40°C. Data for six different alcohols are shown: methanol-d (MeOD), ethanol-d (EtOD), 2propanol-d(IPA), 1-butanol-d (n-BuOD), 2-methyl-2-propanol-d (t-BuOD), dodecanol-dJ. alcohol and the differences between the two supercritical solvents, C O 2 and ethane, can be clearly seen. In C O 2 , the monomer/aggregate equilibria of all the alcohols is shifted more towards the monomer relative to the equilibria established in ethane. In a related study (9), we have shown that because of a weak chemical interaction, presumably involving the C O 2 quadrupole and the O H dipole, the free monomer is more soluble in C O 2 . This occurs even though the solvent dielectric constant, of C O 2 ( ε = 1.50), which is a simple measure of solvent strength, is less than that of ethane (8 = 1.61) under these conditions. As shown in Table I, methanol has a slighdy higher dipole moment than that of the other alcohols. Because of the stronger electrostatic effects of the dipole/dipole interactions (or hydrogen bonding), the methanol equilibria should be shifted towards greater aggregation relative to the higher alcohols. This shift is in fact observed for methanol/ethane system, where we see a significantly greater extent of aggregation for this small alcohol. However, for C O 2 , this effect was not observed. In C O 2 , we have the competing "reaction" of the monomer dipole with the C O 2 quadrupole of one or several local C O 2 solvent molecules. The higher dipole moment of the methanol increases the C02/alcohol interaction, which offsets


182


1.0

0.8

Τ = 40°C Ρ = 200 bar

i


0.61

0.4 H

1

*-

0.2 H Ο • —ι

0.0 MeOD

EtOD

1

in CO 2 in ethane

1—^

1

IPA n-BuOD

t-BuOD

Dodecanol

Figure 4. The fraction of the total alcohol that is in the monomer form for the six different alcohols in C O 2 and in ethane. In all cases the total alcohol concentration is x tal = 0.03, Τ = 40 °C, and Ρ = 200 bar. The line connects the n-alkanols data points. to

Table I.

a

Alcohol

Boiling Point (°C)

methanol

Properties of alkanols

3

Melting Point (°Q

Dipole Moment (debyes)

Polarizability (10-24 cm3)

65

-98

1.70

3.3

ethanol

78

-130

1.68

5.4

2-propanol

82

-90

1.66

7.6

2-methyl-2propanol

83

25

1-Dutanol

118

-89

1.66

8.8

dodecanol

260

26

-1.66

-16

8.9

Gas phase values



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FULTON ET AL.

Hydrogen Bonding ofSimple Akohok

183

the expected trend of higher aggregation of this alcohol. We also note in Figure 4 an increase in the extent of aggregation of dodecanol in C O 2 relative to the smaller alcohols, but the extent of aggregation of dodecanol in ethane is about the same as that observed for the lower alcohols. The dipole moment of dodecanol is the same as for the lower alcohols, indicating that the hydrogen-bonding energies of the alcohols are about the same. A factor that may be affecting this monomer/aggregate equilibria is the attractive interaction of the long hydrocarbon tails of this alcohol. The polarizability of dodecanol (see Table I) is much larger than that of the lower alcohols. The strong tail-tail interactions will help drive the equilibria towards aggregation in C O 2 . Ethane is a much better solvent (it has an appreciably higher dielectric constant) for solvating the long hydrocarbon tail of the alcohol so that the tail-tail attractive interactions are substantially reduced for this alcohol, and the monomer/aggregate equilibria is little perturbed from that of the lower alcohols. The vapor-liquid equilibria curve for the methanol/C02 system in the solvent-rich region of the phase diagram is shown in Figure 5. The dashed line in Figure 5 represents the phase boundaries between a single-phase region above the line and a two-phase region below the line. Schematically illustrated in Figure 5 is the fluid structure at different regions of the vapor-liquid equilibrium curve. These structures have been deduced from FT-IR measurements of hydrogen bonding of single phase systems that are just slightly above the phase boundary. Below an alcohol concentration of x icohol = 0.02 the alcohol is mostly in the nonintermolecularly hydrogen-bonded monomer form. At a pressure just slightly below the critical pressure (at the critical composition), the two coexisting phases will be structurally and compositionally identical with about 40% of the alcohol molecules in monomer form and about 60% in aggregates. The liquid phase at higher mole fractions of the alcohol will contain a large proportion of intermolecularly hydrogen-bonded aggregates. For two coexisting vapor-liquid phases at about 79 bar (represented by the tie line in Figure 5), the upper vapor phase contains a small percentage of alcohol that is mainly in the monomer form. This upper phase is in equilibrium with an alcohol-rich liquid phase that is mostly in the aggregate form. a

How does the fluid structure at the vapor-liquid equilibrium change at different temperatures and, in particular, how does the structure at the mixture critical point change? Previously, we have examined the effects of both pressure and temperature on the monomer/aggregate equilibria of methanol in C O 2 and ethane (9). The main factors that affect the hydrogen-bonding equilibrium are, in order of their importance: (i)the amount of alcohol, (ii) the type of solvent, (iii) the temperature of the system, and finally (iv) the system pressure. For C O 2 , increasing the mole fraction of alcohol moves the critical temperature to higher and higher values. At moderate temperatures, - 2 5 ° to 75°C, the extent of aggregation at the critical point increases at higher critical temperatures because of the increase in amount of alcohol in the systems. At higher temperatures, >100°C, the monomer may begin to dominate because of the aggregate dissociation at higher temperatures (as related through the van't Hoff relationship (9)). The formation of intermolecularly hydrogen-bonded alcohol follows a multiple equilibria model of aggregation. At low concentrations, the alcohol exists only as free monomer. Upon increasing the alcohol concentration, aggregation proceeds through dimer, trimer, and finally to higher oligomers. Because of certain structural features, the methanol dimeric and especially the tetrameric species have a



184

special stability. One expects, then, that above a certain concentration of the alcohol, further additions of alcohol are likely to form mainly the tetrameric aggregate. A representation of the reaction of monomelic species to form an aggregate is shown in Equation 1. n(CH OD)

(CH OD)


3

3

n

(1)

In a solution of an alcohol in an apolar solvent, one finds a relatively narrow distribution of oligomers (n = 2,3,4 and higher) that is centered around approximately η = 4 (21). The equilibrium constant, K, for this reaction, Equation 2, K=

X n

m g r

f Λ V ( monomer) J

(2)

x

can be used in a mass action-type of relationship that yields an approximate value for the size of the alcohol aggregate. Equation 3 has been used by Aveyard et al. (7) for simple alcohols and by Pacynko et al. (22) for nonionic surfactants to obtain aggregation numbers. In (xtotal - Xmonomer) = NA«( In Xmonomer) + In ( Ν Α · Κ )

(3)

where N A = average aggregation number. The value obtained for N A is only approximate since it is assumed that only two different alcohol species are in solution: monomer and the monodisperse-aggregate species. The actual aggregation process most likely involves a polydisperse system containing dimers, trimer, tetramers and higher oligomers in equilibrium; however, Equation 3 does give an approximate measure of the dominant species in solution. A plot of In ( x i - x onomer) vs. In Xmonomer yields a line whose slope is equal to the average aggregation number, N A - Figure 6 shows a typical plot for methanol-d, 1-butanol-d, and dodecanol-d at 4 0 ° C and 200 bar. Using this technique, the aggregation numbers for these three alcohols in C O 2 and ethane were determined and are shown in Table II. Aggregation numbers vary from about 3.5 up to 5.0 for the three alcohols, indicating that only small aggregates are forming in these apolar solvents. There is no statistically significant difference in the measured aggregate size between C O 2 and ethane. Similarly, there is little or no difference in the aggregation numbers of the various alcohols. Carbon dioxide does not appear to significantly perturb the types of species forming in the fluid, but does alter the monomer-aggregate equilibria. In liquids, Aveyard et al. (7) reported an aggregation number of 4 for dodecanol in η-octane at 30°C, whereas Pacynko et al. (22) reported an aggregation number of 5 ± 1 for dodecanol in heptane at 30°C. Aveyard et al. found the aggregation number of dodecanol in η-octane increased slightly at higher concentrations of the alcohol. Our measurements of aggregation numbers at slightly higher temperatures, 4 0 ° C vs. 30°C, are in general agreement with these previous studies of aggregation of alcohols in liquid alkanes. tota

m


14.

FULTON ET AL.

185

Hydrogen Bonding of Simple Alcohols

100

90


80

3 CO

70

60 Liquid

Phase

Vapor Phase 50 0.00

0.05

X

0.20

0.15

0.10 MeOD

Figure 5. The phase behavior of methanol-^ in C O 2 at 40°C. In the regions above the solid lines, the systems are single-phase; in the regions below the lines the systems are two-phase. The fluid structure in each region of the vapor-liquid equilibria curve is illustrated. Table II. Alcohol aggregation numbers, N A , at 40°C and the range of concentrations, xtotai (mole fraction), used for the determination of N A

a

N (±1.0)a

Alcohol/Fluid

xtotai Range, ΧΙΟ"

methanol-d/C02

2.0-12.0

4.9

methanol-d/ethane

2.0-12.0

4.4

n-butanol-

Hydrogen Bonding of Simple Alcohols in Supercritical Fluids - ACS

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