Quantitative method using differential infrared spectrometry for the

Quantitative Method Using Differential Infrared Spectrometry for the Determination of Compound Types Absorbing in the Carbonyl Region in Asphalts J. C. Petersen Laramie Energy Research Center, Bureau of Mines. U S . Department of the Interior, P.O. Box 3395, Laramie, Wyo. 82070

A method for the qualitative and quantitative determination of ketones, dicarboxylic anhydrides, carboxylic acids, and 2-quinolone types in asphalts is presented. The technique Is based on differential infrared spectrometry combined with base hydrolysis and silylation reactions. Determlnations are made using simple laboratory equipment without the necessity of reagent or reaction product separation. The method was applied to a collection of asphalts to show the effect of oxidation in road service on the production of the title compound types. Ketones were found to be the major oxidation product absorbing in the carbonyl region. Dicarboxylic anhydrides were formed on oxidation in considerably greater amounts than carboxylic acids. 2-Ouinolones were not formed on oxidation. Significant amounts of ketones and detectable amounts of carboxylic anhydrides were not found in fresh, unoxidized asphalts although carboxylic acids and 2-quinolone types were often present.

Although s m a l l a m o u n t s of compounds having f u n c t i o n a l g r o u p s that a b s o r b in the carbonyl region of the infrared s p e c t r u m are n a t u r a l l y p r e s e n t i n m a n y asphalts, oxidative aging of a s p h a l t s produces significantly g r e a t e r a m o u n t s of t h e s e functional g r o u p s (1-3). Oxygen-containing g r o u p s of t h i s t y p e a r e generally polar and c a p a b l e of p a r t i c i p a t i n g i n s t r o n g functional g r o u p interactions (4-15 ). These i n t e r a c t i o n s are believed to p l a y a m a j o r role in the h a r d e n i n g and e m b r i t t l e m e n t of asphalt in use. Because of the complex n a t u r e of asphalt, satisfactory methods for the q u a n t i t a t i v e d e t e r m i n a t i o n of t h e s e functional types in asphalt h a v e not been developed. M e t h o d s have been proposed to identify and q u a n t i f y several f u n c t i o n a l t y p e s f o r m e d i n asphalt on oxidation that a b s o r b in the carbonyl region (16).However, certain a s p e c t s of the m e t h o d s relative t o t h e i d e n t i t y of t h e functional t y p e s d e t e r m i n e d h a v e b e e n q u e s t i o n e d

(17). The p r e s e n t p a p e r describes a differential i n f r a r e d techn i q u e that m a k e s possible, w i t h o u t p r i o r s e p a r a t i o n s , the qualitative and q u a n t i t a t i v e d e t e r m i n a t i o n i n a s p h a l t s of t h e m a j o r c o m p o u n d types a b s o r b i n g i n the carbonyl region. The m e t h o d is particularly applicable t o the d e t e r m i n a t i o n of carboxylic acids, dicarboxylic a n h y d r i d e s , a n d 2quinolone t y p e s in oxidized a s p h a l t s in which the a b s o r p tion b a n d s of t h e s e f u n c t i o n a l t y p e s a r e m a s k e d b y the int e n s e b a n d of the k e t o n e s f o r m e d o n oxidation. Use of t h e m e t h o d is d e m o n s t r a t e d b y application t o a series of asp h a l t s recovered f r o m r o a d service.

EXPERIMENTAL Materials. Asphalt samples previously recovered from road cores taken from 11- to 13-year-old pavements were supplied by the Federal Highway Administration (FHWA). These were prepared by FHWA personnel by extracting the cores with a four-toone mixture of benzene and 95% ethanol. Retained asphalt samples representative of refinery production during road construction were also supplied by FHWA. Road-recovered and retained 112

asphalts are identified by the numbers used to represent the original asphalts in earlier publications (18, 19). The Wilmington (California) asphaltenes prepared by pentane precipitation have been described ( 4 , 6, 7, 20, 21 ). Model compounds used in determining apparent integrated absorption intensities were laboratory chemicals with better than 95% purity. Benzene was analyzed reagent grade from J. T. Baker Chemical Co. Tetrahydrofuran (THF) was from Eastman Chemical Co. and was passed through a column of dry, activated alumina to remove possible interfering amounts of oxidation products and water, stabilized with 0.025% butylated hydroxytoluene, and protected from moisture by storing over activated, type 4A molecular sieve from J. T. Baker and Co. Hexamethyldisilazane (HMDS) and trirnethylchlorosilane (TMCS) were “specifically purified grade” from Pierce Chemical Company. Sodium hydroxide and potassium bicarbonate were analytical reagent grade; the latter was free of carbonates. Sample Preparation. Duplicate 0.250-gram samples of asphalt were weighed into 25-ml Erlenmeyer flasks. One flask was stoppered with a septum cap for later treatment. The sample in the other flask was treated as follows. Four-tenths ml of 0.5 N sodium hydroxide, 10 ml of benzene, and boiling chips were added. The benzene was evaporated on a hot plate at a temperature sufficient to cause evaporation in about 15 minutes, leaving the residue and a small amount of water. An additional 0.1 ml water and 2 ml benzene were added, and the evaporation was repeated. Residual water on the sides of the flask was evaporated by inserting a hypodermic needle attached to a vacuum line into the neck of the flask. Complete sample drying was accomplished by evaporation of an additional 3 to 4 ml of benzene from the residue. The flask was removed from the hot plate and immediately stoppered with a rubber septum cap to prevent moisture pickup by the residual sodium hydroxide. The samples in the two flasks were each dissolved in 5.00 ml dry T H F which was added using a hypodermic syringe while venting with a hypodermic needle to release pressure buildup. A 3-ml portion of each sample was transferred to 6-ml septum-stoppered vials and silylated by the addition of 0.02 ml HMDS and 0.01 ml TMCS followed by heating for one-half hour at about 40 “C. (The top of the source compartment of the spectrophotometer provided adequate heat.) At this point, four samples had been prepared(1) untreated; (2) NaOH-treated; (3) untreated, silylated: and (4) NaOH-treated, silylated-and were ready for infrared analysis. To assist in the differentiation between carboxylic acids and dicarboxylic anhydrides, an independent asphalt sample (0.250 gram) was treated with 0.4 ml of 0.5N potassium bicarbonate (which selectively reacts with carboxylic acids) in the same manner as for the sodium hydroxide treatment except that an additional step of the addition of 0.1 ml water and 2 ml benzene was required. The potassium bicarbonate-treated sample was then used in the same manner as the sodium hydroxide-treated sample in the analysis. Determination of I n f r a r e d Spectra. Infrared spectra on absorbance scale chart paper were obtained using a Perkin-Elmer Model 621 infrared spectrophotometer. When differential spectra were obtained with asphalt solutions in both sample and reference beams, an attenuator was placed in the sample beam and adjusted to produce a “zero” base line at about 0.25 absorbance. Spectra were obtained from 1900 to 1500 cm-’ using 2X slits (slit width range 260 to 310 1 during the scan); source current, 0.8 A; attenuator, 1100: slit program, 1000: suppression, 6; scan time, 32; and gam just under the point of pen oscillation. Attention must be given to proper machine settings to overcome strong solvent and sample absorption during the differential scans, which may result in insufficient transmitted energy for proper pen response. The set of four sample solutions previously prepared were used without

ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975

Table I. Sequence for Obtaining Differential Spectra and Their Use Sample beam S ectra isentification

Examples, Figure No.

Reference beam Cell no.

Asphalt solution

~

Asphalt solution

Cell no.

Principal use

(1)Reference, (2) Determination of total carbonyl Reference Determination of a c i d s and anhydrides A s s i s t in differentiating a c i d s f r o m anhydrides Determination of acids, acid s a l t s , 2-quinolone types Assist in differentiatinq a c i d s f r o m anhydrides Check on uniformity of solution concentrations

A

1

Untreated

2

Solvent only

variable c e l l

B" C

1 1,2

NaOH -treated NaOH - t r e a t ed

2 2

Solvent only Untreated

variable cell 1

D

2

NaOH - t r eat ed

2

Untreated, silylated

3

E

3

Untreated, silylated

3

Untreated

1

F

2

NaOH -t r eat e d

2

4

G

not shown

NaOH-treated, silylated

4

NaOH-treated, silylated Untreated, silylated

3

a This spectrum is recorded superimposed on Spectrum No. A, using a pen of a different color. b NaOH is replaced with KHCOa when obtaining sequence based on potassium bicarbonate. ~

Table 11. Values of Apparent Integrated Absorption Intensity ( B )Used to Estimate Compound-Type Concentrations in Asphalts E , 1. mol-'

Functional type

cm-2

10-4

1.2 3.1 0.7 3.2

Carboxylic a c i d s Dicarboxylic anhydrides Ketones 2-Quinolones

further treatment. Spectra were obtained on the sample solutions according t o the sequence A through G described in Table I, using four numbered 0.100-cm sodium chloride sealed cells matched in thickness to within 0.001 cm and a variable cell set a t 0.095 cm containing solvent only. With proper spacing, all the spectra of a sequence for one asphalt can be placed on the same sheet of recording paper. Organic insolubles and ammonium chloride which may slowly deposit on the cell windows with prolonged usage can usually be removed with pyridine followed by glacial acetic acid. If several asphalts are analyzed concurrently, the average time for sample preparation and spectrometry is about 1%hours per asphalt. Calculations. The apparent integrated absorption intensity (I3), in units of 1. mol-' cm-I, of an infrared absorption band obtained on a spectrophotometer having a nonmonochromatic energy source and finite slit width is defined (22 j as

where c = concentration of functional type, mol 1 = cell path length, cm; v = absorption frequency, cm-'; 7'0 = incident radiation; T = transmitted radiation. The area under the absorbance L'S. absorption-frequency curve for the absorption bands of interest is represented by the term In (TO/?"),, dv. The term In ( T O I T )is equivalent to the commonly used term, absorbance, designated by the notation A . In our work, the band area, S A , du, was estimated by counting the squares on the recording chart paper. The concentrations of functional types in the neat asphalt were then estimated using the equation

c =

J'AA,dw Bl

5.25 0.25

x-

~~~~

~~~~~

T h e volumes of asphalt and solvent were assumed to be additive; therefore the term 5.25/0.25 corrects for sample dilution by the solvent. Values of B used to estimate the concentrations of functional group types in asphalts are shown in Table I1 and were obtained by averaging values determined for sets of model compounds in T H F solutions (Table 111). When absorption bands were narrow, base-line expansion was used to increase the accuracy of the peak area determination. Identification a n d Measurement of Absorption Bands. The spectra cited in this section are those obtained in the sequence described in Table I and are identified by the corresponding letter designation shown in the table. To assist the reader, examples of representative spectra may be found in Figures 1 through :3. For convenience, the spectra in the figures are identified with the same letter designation used to identify the spectra cited in the table. Acids a n d Anhydrides. The combined absorption of the carboxylic acids and dicarboxylic anhydrides are represented by the area of the band in spectrum C (described in Table I) bounded by the null line of the differential spectrum ( e . g . , dashed base line in spectrum C, Figure 1)and the frequencies from about 1850 to 1680 cm-'. Carboxylic acid derivatives hydrolyzable with NaOH ( e . g . , esters, some amides, lactones, etc.), if present, would appear in this band and be determined as anhydrides. If KHC03 is used instead of NaOH in the initial sample preparation, absorption in this region results from carboxylic acids only. Thus, in the absence of esters, lactones, etc. (these have not been detected in any sample tested to date), the difference in the areas of the bands obtained by NaOH and KHC03 treatments represent absorption by dicarboxylic anhydrides. An estimate of the carboxylic acid absorption may also be made by measuring the area under the band at about 1725 cm-' in spectrum E ( e . g . , spectra E and E', Figure 3) after adjusting this area for band cancellation. The measured area of this band is always too small and the frequency of the band maximum too low because of band cancellation from the corresponding silyl ester band of the acids; the silyl ester band appears a t about 1715 cm-I on the opposite side of the null line in the differential spectrum. If a symmetrical band is synthesized beginning with the high frequency wing of the 1725 cm-I band in spectrum E, and made to center with a peak maximum at 1730 cm-' (the frequency for free acids), then the area under this synthesized band (bounded at the base by the null line in the differential spectrum) is usually a good approximation of the area of the carboxylic acid absorption. This area may usually be used in place of the area obtained from the KHCO:j treatment for determining free acid concentration. Spectra D and F in Table I (examples in Figure 2) are used to assist in the confirmation and differentiation of acids and anhyd-

ANALYTICAL CHEMISTRY, VOL 47, NO 1 JANUARY 1975

113

I

,

I ,930

IBCC

#JOG

) / , 150:

I

I , 5 2 0 1533

I800

I

I720

I520

l

l

150:

500

Figure 1. Examples of infrared spectra used in analysis of carbonyl region (H) Unoxidized-untreated;(A) untreated: (B) NaOH-treated: (C') differential spectrum of NaOH-treated vs. untreated using a deficiency of NaOH: (C) differential spectrum of B ws. A. All spectra obtained using road-oxidized asphalt 30, except spectrum H was obtained using unoxidized asphalt 30

I 900

1 , I803

l 1100

600

I 5 3 0 1900

l , 1800

/

1730

,

VI l , ,600

l I500

h4VENbMBER.

Figure 2. Examples of infrared spectra used in analysis of carbonyl

region (C) NaOH-treated vs. untreated: (D) NaOH-treated vs. siiylated: (F) NaOHtreated vs. NaOH-treated. silyiated. All spectra obtained using road-oxidized asphalt 30

rides. Spectrum G (no example spectrum shown) is used in detecting differences in concentration between untreated and NaOH- or KHCOS-treated samples initially prepared. An absorption band about 1600 cm-I is present in the spectrum when concentrations

between treated and untreated samples are not identical; when this occurs, samples must be discarded. 2-Quinolone Types. The absorption of the 2-quinolone types is estimated from the area of the peak above the null line and centering at 1668 cm-I in spectrum E ( e . g . , spectrum E', Figure 3). Spectrum E in Figure 3 is void of 2-quinolone types, and the small broad band centering at about 1640 cm-I results from trace amounts of water. Ketones. The absorption of the ketones is obtained by difference, using spectrum A, and is equivalent to the total absorption in the carbonyl region between 1850 and 1640 cm-' and bounded by a horizontal base line ( e . g . , area bounded by the dashed lines and spectrum A in Figure 1) minus the absorption of the acids, anhydrides, 2-quinolone types, and a small correction for background absorption. The background absorption is defined as the absorption between 1850 and 1640 cm-' that is not accounted for by carboxylic acids and 2-quinolone types in unoxidized asphalts and largely results from the use of a horizontal base line. ( A significant ketone band in the 1700 cm-I region or detectable amounts of dicarboxylic anhydrides have not been found in fresh, unoxidized asphalts.) When applying the analysis to oxidized asphalts and no spectra of unoxidized asphalt are available or to asphalts having a significant carbonyl band at about 1700 cm-' that is not affected by NaOH, a correction of 6.4 S A d Y units is usually a satisfactory estimate of the background absorption.

RESULTS AND DISCUSSION The present method for the analysis of compound types absorbing in the carbonyl region overcomes two major problems that have thwarted successful detailed analysis of 114

I

I

'BOO

I?K

,

, 150:

, IMC

I ,PO:

I

iaao

17::

, 160:

$53

I A * E Y L M B F R , cm.'

W l l i Y U H B E R , t6

Flgure 3. Examples of infrared spectra used in analysis of carbonyl

region (E') silylated Wilmington asphaltenes vs. Wilmington asphaltenes; (E) silylated, road-oxidized aspahlt 30 vs. road-oxidized Asphalt 30; (C") NaOH-treated Wilmington asphaltenes vs. Wilmington asphakenes

the carbonyl region of asphalts. These problems result from intermolecular association of carbonyl types in asphalts and from severe overlap of the infrared absorption bands. In fact, intermolecular association severely complicates the band overlap problem, making resolution and identification of the individual bands of the compound types virtually impossible without special techniques. Carboxylic acids and 2-quinolone types in asphalts have been shown to form cyclic dimers and mixed dimers in neat asphalt and in nonpolar solvents such as carbon tetrachloride (5, 6 ) ; the associated species are in equilibrium with their monomers, thus giving rise to a number of overlapping absorption bands. Success of the present method depends largely on the use of THF as a spectral solvent. Because of its electron-donating properties, THF associates with the acidic hydrogen of the carboxylic acids and 2-quinolone types, thus freeing the carbonyl group and preventing dimer and mixed dimer formation (6). With intermolecular association eliminated, only the carbonyl absorption of the unassociated species need be considered. Typically, oxidized asphalts have a broad absorption band centering a t about 1700 cm-l that is composed of the overlapping bands of ketones ( 2 3 ) ,dicarboxylic anhydrides ( 2 4 ) , carboxylic acids, and 2-quinolone types. These compound types will be considered subsequently in detail, but suffice it to say that the anhydrides, acids, and 2-quinolone types are usually unrecognizable shoulders on the much more intense ketone band centering a t about 1695 cm-l. Resolution problems resulting from band overlap in the carbonyl region are circumvented using selective chemical reactions with the asphalt components followed by analysis using differential infrared spectrometry. General Description of Method. The underlying principle of the present method is to react the anhydrides, acids, and 2-quinolone types with selective chemical reagents to alter or shift their absorption bands so that the bands may be identified and measured. The identification and measurement is facilitated by the use of differential spectra which null or cancel out the interfering bands produced by the unreactive components, such as the ketones, and thus show only net changes produced by the chemical treatment. In the method, asphalt is reacted with sodium hydroxide to convert the carboxylic acids, dicarboxylic anhydrides, and other carboxylic acid derivatives to carboxylate ions, thus shifting their carbonyl absorption bands from the 1800 to 1700 cm-l region to a broad band centering at about 1580 cm-l. This shift is illustrated in Figure 1 by comparing spectra A and B. The shift is more vividly illustrated by spectrum C, which is the differential spectrum of

ANALYTICAL C H E M I S T R Y , VOL. 47, NO. 1 , J A N U A R Y 1975

B us. A. The area of the band above the null line (dashed base line) in curve C from about 1850 to 1680 cm-l represents the loss of the absorption of the carboxylic acids and their derivatives and is used in their quantitative determination. The band centering at about 1580 cm-l (below the null line) represents the absorption of the corresponding carboxylate ions. Carboxylic acids are differentiated from dicarboxylic anhydrides by their selective silylation and their selective reactivity with potassium bicarbonate. Effects of these reactions on the infrared spectra are discussed in later sections. 2-Quinolone-type compounds are determined by silylation (6) which destroys their carbonyl group. The 1688 cm-' band (spectrum E', Figure 3) results from the silylation of the hydroxy form as illustrated below for 2-quinolone. Under equilibrium conditions prior to silylation, however, 2-quinolone is virtually all in the carbonyl form.

The balance of the carbonyl absorption remaining in oxidized asphalts after the carboxylic acids, dicarboxylic anhydrides, 2-quinolone types, and the background (see Experimental section) have been accounted for, results primarily from ketones, as described in another publication (23). Although part of the background absorption in unoxidized asphalts may result from compounds containing the C=O group, the amount is small when compared with the ketone absorption in oxidized asphalts, as can be seen by comparing spectra H and A, Figure 1, of an unoxidized and an oxidized asphalt, respectively. The indications of bands a t 1765 and 1700 cm-l in the unoxidized asphalt result primarily from small amounts of oxidation which occurred in the asphalt during and/or after manufacture. A well defined band attributable to ketones (1695 cm-l) or anhydrides (1765 cm-I) has not been found in fresh, unoxidized asphalts. Discussion of Experimental Procedures. The experimental procedure used is not complicated; however, because differential spectra are used to make quantitative measurements and the spectra vary with different asphalts, careful attention must he given to experimental details and interpretation of the spectra. Therefore, several aspects of the method will be discussed in detail. Neutralization or Hydrolysis of Carboxylic Acids a n d Deriuatiues. The reaction of asphalt with sodium hydroxide is heterogeneous with asphalt in the benzene phase and sodium hydroxide in the aqueous phase. Benzene was chosen as the asphalt solvent for its good solvent properties, its inertness, and its azeotrope with water. T o ensure complete hydrolysis of the dicarboxylic anhydrides, it was necessary in most instances that some water be present in the flask following evaporation of the first addition of benzene. This apparently provides intimate contact between the residual aqueous base and the asphalt as the benzene is evaporated. Slight traces of benzene left in the residue produce a small, noninterfering band at about 1820 cm-l which may be ignored. The sodium hydroxide treatment converts carboxylic acids, dicarboxylic anhydrides, and added model esters to carboxylate ions. Naturally occurring carboxylic acids produce a sharp infrared band at about 1730 cm-1 (e.g., spectrum C", Figure 3), and dicarboxylic anhydrides produce a broad doublet band with peaks a t about 1765 and 1725 cm-l (e.g., spectrum C, Figure 2). Verification that the dou. blet band results from anhydrides and evidence that oxidized asphalts tested to date are void of significant

amounts of esters and closely related functional types are reported in another publication ( 2 4 ) .Although the anhydrides are resistant to silylation, they are readily converted to silyl esters if first hydrolyzed by treatment with sodium hydroxide, as shown by the sharp silyl ester band in the differential spectra of the silylated, sodium hydroxidetreated sample (spectrum F, Figure 2). The absorption band of carboxylic acids (about 1730 cm-l) that are naturally present or produced on oxidation is superimposed on the 1725 cm-l band of the anhydride doublet. The presence of both free acids and acid anhydrides in an oxidized asphalt is illustrated by comparing the differential spectra of a sodium hydroxide-treated us. an untreated sample (spectrum C, Figure 2) with the differential spectra of a sodium hydroxide-treated us. a silylated sample (spectrum D, Figure 2). Although most of the absorption in the 1725 to 1730 cm-' region of these spectra results from anhydrides, conversion of the small amount of free acids to silyl esters shifts the 1725-1730 cm-' band maximum (spectrum C) to about 1715 cm-' (spectrum D). The anhydride band at 1765 cm-l is unaffected. The difference between the two spectra cited is equivalent to spectrum E, Figure 3. Reaction with aqueous potassium bicarbonate was often found useful in distinguishing between free acids and acid anhydrides. Under the conditions noted in the Experimental section, dicarboxylic anhydrides produced on oxidation react with sodium hydroxide but not with potassium bicarbonate. The free acids react with both reagents. Some asphalts appear to have trace amounts of anhydrides that are sufficiently reactive to be hydrolyzed by potassium bicarbonate; the amounts of these reactive anhydrides can be judged by noting the presence of a 1765 cm-' band in the differential spectrum of the sodium bicarbonate-treated us. the untreated samples and also by comparing this differential spectrum with the differential spectrum of the silylated us. unsilylated samples (spectrum E , Figure 3). Because anhydrides do not silylate, the latter spectrum has only free acid bands in the 1700-1800 cm-l region. Excess sodium hydroxide produces an increase in background absorption a t frequencies below about 1700 cm-' with some asphalts. This is illustrated in Figure 1, spectra C and C'. Spectrum C' was obtained from a sample reacted with 0.8 ml of 0.1N sodium hydroxide, just sufficient to cause hydrolysis of nearly all the anhydrides. Spectrum C was obtained from a sample that was reacted with 0.4 ml of 0.5N sodium hydroxide, as outlined in the Experimental section. The use of 0.8 ml of 0.5N sodium hydroxide produced no further change in the spectra. The increase in background from excess sodium hydroxide has little effect on the band area between 1850 and 1680 cm-1, which is used to calculate the concentration of carboxylic acids and anhydrides and, at most, causes only a slight slope in the base line used to determine band area. The increase in background must be taken into account, however, if one wishes to quantitatively assess the carboxylate band at 1580 cm-'. One final note on the sodium hydroxide reaction. If the sample contains 2-quinolones, these may partially react with sodium hydroxide uia their OH form and produce a negative peak in the differential spectra (spectrum C, Figure 1) a t about 1668 cm-l that may interfere with the shoulder at 1680 cm-l used to draw the base line for quantitative measurements. For the most part, however, this is not troublesome, and a judgment of the correct position of the base line is easily made. Silylation. In addition to the determination of 2-quinolone-type compounds, the silylation reaction is used to as-

ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975 * 115

Table 111. Infrared Spectral Data in the Carbonyl Region for Model Compounds in THF Solution Apparent

Molar

integrated

absorptivity, absorption € 8

Compound

Carboxylic acids Benzoic (Tr i met hy lsi1y 1 e st e r ) 4-Cyclohexylbutanoic (Trimethylsilyl ester) Cyclohexane carboxylic (Trimethylsilyl ester) I-Naphthoic (Trimethylsilyl ester) Nicotinic Anhydrides Benzol (glzi)]perylene-l, 2 -dicarboxylic (Trimethylsilyl ester) 3,3’, 4,4’-Benzophenonetetracarboxylic (Trimethylsilyl ester)

Frequency, -1 cm

1. mol-’

cm-1

B, 1.mol-l cm-2x10-4

1722 (1702) 1736 (1718) 1732 (1715) 1715

1.11

(1696) 1727

1. 1 6

1.21 1.29

1.23

1768 plus shoulder > 1768 (1705) 1857,1785 (1693 plus shoulder > 1693) 1786,1751 (1710, 1693) 1780 1745 (1702)=

Diphenic (Trimethylsilyl ester) 1,8-Naphthalic (Trimethylsilyl e s t e r ) 3 , 4 , 9 , 10-Perylenetetracarboxylic 1775,1756 1744, 1730d 2 -Quinolones 2-Hydroxy-4-methylquinoline 1677 2-Quinolone 1680 5 , 6 , 7,8-Tetrahydro-2quinolone 1674 Ketones Acetophenone 1690 3,4-Dihydro-l[ ( 2 H ) I naphthalenone 1688 Vale r ophenone 1690

3.18‘

3. O l b

3.08*

3.32 3.21

600

0.64

523

0.77

395

0.65

a Accuracy better than k2 cm-1 using polystyrene film calibration standard. 0 Calculated from combined area under doublet peak. c Trimethylsilyl ester was prepared from the sodium salt of the acid. d Not soluble. Spectrum obtained from solid suspension

in THF.

sist in the determination of carboxylic acids and in differentiating between carboxylic acids, dicarboxylic anhydrides, and possible esters. Under the conditions used, added model esters and the dicarboxylic anhydrides produced in asphalt on oxidation are not affected by the silylation reagent. Silylation of naturally occurring carboxylic acids in asphalt to form trimethyl silyl esters is illustrated using Wilmington asphaltenes by differential spectrum E’, Figure 3. The band above the null line a t 1733 cm-l represents the carbonyl vibration of the free acids and the band below the null line a t 1715 cm-’ represents the corresponding silyl esters. Methods for dealing with band overlap and thus partial cancellation of the free acid band in the analysis of car116

boxylic acids are presented in the Experimental section. Barid overlap generally reduces the free acid peak height about 20%, as illustrated by comparing the free acid carbony1 peaks in spectrum E’, Figure 3, and spectrum C”, Figure 3. The larger peak in the latter differential spectrum of NaOH-treated asphaltenes represents all the free carboxylic acids present. Precautions When Using THF. Several precautions must be taken for successfuluse of T H F in the analysis. The T H F must be free of hydroperoxides (OH stretching frequencies of 3280 and 3430 cm-l), which produce spurious carbonyl bands a t 1720 and 1775 cm-1. Water, which gives a broad interfering band centering a t about 1640 cm-I, must be removed as completely as possible from the THF. Because water reacts readily with the silylation reagent, differential spectra obtained when one of the cells contains a silylated sample will show the water band if water is present. Without elaborate precautions, it is virtually impossible to eliminate all water, and the amount normally present when following the experimental procedure as outlined is illustrated by the broad band above the null line centering a t 1640 cm-l in spectrum E, Figure 3. Care must be taken not to confuse the water band with a sharper band centering a t 1668 cm-l resulting from 2-quinolones. (Although water may contribute to the nonsymmetry of the 2-quinolone band-spectrum E’, Figure 3-nonsymmetry is characteristic of this band.) Water pickup from glassware may be troublesome and, in humid environments, predrying the glassware may be helpful in reducing the water band. Finally, T H F slowly permeates and swells most septum materials. Therefore, it is essential that liquid contact with the septum of stoppered sample flasks be avoided. Because T H F vapors permeate the septum, analysis should be completed within 2 to 3 hours after sample preparation to avoid significant concentration changes. I t is essential t o the differential spectral technique that sample concentrations in both the sample and reference cells be identical. Estimation of Apparent Integrated Adsorption Intensities. Spectral data on the model compounds used to estimate the apparent integrated adsorption intensities of functional groups in asphalts reported in Table I1 are shown in Table 111. Compounds were selected to provide variations in structure within each class. Although molar absorptivities ( t , based on peak height) varied widely for compounds within each class, apparent integrated adsorption intensities (B, based on peak area) were relatively constant. Therefore, because the precise molecular structure of functional types in asphalts was not known, B seemed the logical constant to use in estimating concentrations of functional types. Another important consideration influenced the use of B rather than t. Adsorption bands of asphalt are much broader than those of pure compounds because the band in asphalt for each class of compounds is a composite band of many compounds within the class which have different structural features and thus slightly different adsorption frequencies. Estimated concentrations of a functional type in asphalt based on an assumed t derived from pure compounds would therefore give low results. A value for B for naturally occurring acids in asphalts was independtly determined by stepwise neutralization of the acids in separate samples of Wilmington asphaltenes. The absorption a t 1733 cm-l in the differential spectra from each neutralization step was plotted us. the sodium hydroxide consumed, and the end point was determined from the plot. The value obtained of 1.16 1. mol-l cm-2 X lo4 agrees well with the average value of 1.2 for pure compounds in Table 11.

A N A L Y T I C A L CHEMISTRY, V O L . 47, NO. 1, J A N U A R Y 1975

Table IV. Estimated Concentration of Functional Group Types Absorbing in the Carbonyl Region from 1850 to 1640 cm-1 in Various Asphalts Concentration, mol 1.-'

As-

phalt'

19 25 30 61 67 71 72 73 74 166

Ketone t m e s Or

BA d

0 0 0 0 0 0 0 0 0 0

52.5 52.5 64.2

44.1 31.5 51.0 67.5 35.3 42.6 22.5

x IO*

Carboxylic acid types Anhydrides Free acids Or

0 0 0 0 0 0 0 0 0 0

BA

1.8 2.2 3.8 2.0 1.0 2.2 2.9 1.1 1.7 0.9

Or

1.0 1.6 0 0 0.4 0 0 0.4 0.5 0.7 FHWA

BA

2.5 1.6 1.6 0.7 0.5 0.4 0.7 0.4 0.7 1.0

2-Quinolone Woes Or

0.1 0.3 0 0.2 Tr 0 0 Tr

BA

Tr e 0 0 0.2 Tr 0 0 Tr

Tr

Tr

0

0

a Sample numbers correspond t o numbers in references (18) a n d (19). "ackground absorption of about 6.4 JA,du u n i t s in unoxidized asphalts n o t a t t r i b u t a b l e t o carboxylic acids a n d 2quinolones was n o t i n c l u d e d in analyses. c O r i g i n a l asphalt. Asphalt recovered from r o a d cores by extraction w i t h benzene95% ethanol (4: 1, v / v ) . e Trace.

Absorption frequencies for the bands in the carbonyl region for the model compounds are also shown in Table I11 for use in comparing with asphalt spectra. The frequencies of the corresponding silyl esters are also included for the acids and anhydrides. Application of Method. The analytical method was applied to a collection of asphalts, and the concentrations of functional group types absorbing in the carbonyl region from 1850 to 1640 cm-' were determined and are reported in Table IV. Asphalts 19 through 166 are representative of highway asphalts that have been studied by the Bureau of Public Roads (18, 19) (now Federal Highway Administration, FHWA) and others (25-27). Determinations were made on the original asphalts (columns Or) and on the corresponding asphalts recovered by FHWA from 11- to 13year-old highway pavement cores with a solvent mixture of 4:l benzene-95% ethanol (columns BA). Ketones were found to be the major product formed in asphalts on oxidation that absorb in the carbonyl region. Their concentration ranged from 0.23 mol 1.-l in asphalt 166 to 0.68 mol l.-I in asphalt 72. T h e levels of oxidation found in the asphalts taken from road service cannot be compared with each other to assess the relative susceptibility of the different asphalts toward oxidation because of the variations in environmental exposure in service. Examination of the data in Table IV indicates that dicarboxylic anhydrides are not naturally occurring in asphalts but are artifacts of oxidation. Dicarboxylic anhydrides and carboxylic acids were found in all oxidized asphalts although some unoxidized asphalts were void of detectable amounts of carboxylic acids. Only small amounts of free carboxylic acids were formed in any of the asphalts on oxidation. The dicarboxylic anhydrides accounted for a major part of the carboxylic functional group formed. This is unusual. Apparently, most oxidation not associated with anhydride formation, under the oxidation conditions imposed, terminates with the ketones. The unusually large amount of free acids found in the road-recovered asphalt 19 may have resulted from antistripping additives that

might have been added to the asphalt prior to road construction but were not present in the original asphalt. It is reported that the original asphalts were not all collected from the refineries a t the actual time of road construction. Little or no 2-quinolone types were found in the asphalts. Unlike the other functional types determined, the 2-quinolone types are not formed on oxidation but are only naturally occurring. T o provide a physical concept of the amounts of polar functional groups absorbing in the carbonyl region which are introduced in asphalt molecules on oxidation, the following hypothetical calculation is made. If one assumes for asphalt a conservative average molecular weight of 700, a density of 1, and monofunctionality, then 45% of the molecules in asphalt 30 were altered on oxidation by the introduction of polar groups absorbing in the carbonyl region. Because of the ability of functional groups absorbing in the carbonyl region to form strong association complexes, this could have a significant effect on asphalt physical properties.

ACKNOWLEDGMENT The personal interest and assistance given by Woodrow J. Halstead, Edward Oglio, and others of the Office of Research, Materials Division, FHWA are greatly appreciated. LITERATURE CITED (1) 8. D. Beitchman, J. Res. Nat. Bur. Stand., Sect. A, 63, 189 (1959).

(2) J. R. Wright and P. G. Campbell, J. Appl. Chem., 12, 256 (1962). (3) C. D. Smith, c. C. Schuetz, and R. S. Hodgson, lnd. Eng. Chem., Prod. Res. Develop., 5, 163 (1966). (4) J. C. Petersen, Fuel, 46, 295 (1967). (5) J. C. Petersen, J. Phys. Chem., 75, 1129 (1971). (6) J. C. Petersen, R. V. Barbour, S. M. Dorrence, F. A . Barbour. and R. V. Helm, Anal. Chem., 43, 1491 (1971). (7) R. V. Barbour and J. C. Petersen, Anal. Chem., 46, 273 (1974). (8) L. H. Thomas and R. Meatyard, J. Chem. SOC.A, 92 (1966). (9) L. L. Blyler, Jr., and T. W. Haas. Polym Prepr., Amer. Chem. SOC.Div. Polym. Chem., 10 (l), 72 (1969). (10) T. Gramstand, Spectrochim. Acta, 19, 497 (1963). (1 1) L. T. Boucher and J. J. Katz, J. Amer. Chem. SOC..89, 4703 (1967). (12) J. H. Lady and K. B. Whetsel, J. Phys. Chem., 71, 1421 (1967). (13) T. D. Epley and R. S. Drago, J. Amer. Chem. Soc., 89, 5770 (1967). (14) L. J. Bellamy and R. J. Pace, Spectrochim. Acta., Part A, 27, 705 (1971). (15) P. A. Hopkins, R. J. W. Le Fe'vre, L. Radon, and G. L. D. Ritchie, J. Chem. SOC.6, 574 (1971). (16) J. Knotnerus, J. lnst. Petrol., 42, 355 (1956). (17) P. G. Campbell and J. R. Wright, J. Res. Nat. Bur. Stand., Sect. C, 68, 115 (1. (18) J. Y. Welborn and W. J. Halstead, Public Roads, 30, 197 (1959).Also, Proc. Ass. Asphalt Paving Technol., 28, 242 (1959). (19) J. Y. Welborn, W. J. Halstead and J. G. Boone, Public Roads, 31, 73 (1960). Also, Proc. Ass. Asphaltpaving Technol., 29, 216 (1960). (20) T. C. Davis, J. C. Petersen, and W. E. Haines, Anal. Chem., 38, 241 (1966). (21) J. W. Ramsey, F. R. McDonald, and J. C. Petersen, lnd. Eng. Chem., Prod. Res. Develop., 6, 231 (1967). (22) R. N. Jones, D. A. Ramsay, D. S. Keir, and K. Dubriner, J. Amer. Chem. Soc., 74, 80 (1952). (23) S. M. Dorrence, F. A. Barbour, and J. C. Petersen, Anal. Chem., 46, 2242 (1974). (24) J. C. Petersen, F. A. Barbour, and S. M. Dorrence, Anal. Chem., 47, 107 (1975). (25) F. S. Rostler and R . M. White, Proc. Ass. Asphalt Paving Technol., 31, 35 (1962). (26) T. C. Davis and J. C. Petersen, Anal. Chem., 39, 1852 (1967). (27) B. A. Vallerga and W. J. Halstead, Highw. Res. Rec., 361, 71-91 (1971).

RECEIVEDfor review March 5, 1974. Accepted September 6, 1974.The author gratefully acknowledges partial financial support of this work by the Federal Highway Administration in an interagency cooperative program with the Bureau of Mines. Mention of specific brand names or models of equipment is made for information only and does not imply endorsement by the Bureau of Mines.

ANALYTICAL CHEMISTRY, VOL. 47. NO. 1, J A N U A R Y 1975

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