Anal. Chem. 1902, 5 4 , 2477-2487
noted. Figure 2 shows marked deviation from linearity in the 5-60 ppm range. Uranium concentrations greater than 70 ppm produced decreasing flluorescence intensity readings. Instrument response was linear for uranium concentrations less than 2.5 ppm. Values greater than 2.5 pprn showed deviations from linearity, indicating that 2.5 ppm uranium is an upper limit to linear response, Solutions with uranium concentrations in the upper ranges of this study would normally require dilution into a linear range to remain on-scale. Seven synthetic samples, ranging in concentration from 7.3 to 73.9 ppb uranium were analyzed four [separate times with an average RSD of f796. The RSD values ranged from *3 to *11%. Samples lower than 1 ppb uranium gave nonreproducible results, indicating that the lower detection limit for this particular matrix is approximately 1 ppb, although UA-3 uranium analyzer specifications show that lower quantities can be detected. Inductively coupled plasma (ICP) analyses of an extracted synthetic solution indicate that the phosphoric acid not only contains uranium but up to 13 ppm iron. This compares wiith a value of less than 0.8 ppm iron in pure 20% H3P04. '[t appears that some iron from the original solution is being carried through the extractions into the final solution. Reference 13 indicates that this concentration of iron would quench fluorescence about 78%. The fact that other cationic and anionic quenclhers may be present in the 20% H3P04 (ICP analysis also showed aluminum in the phosphoric acid) may explain why the detection limits are greater than the instrument specificationis. These detection
2477
limits would also be matrix dependent, changing with both type and quantity of interfering species. ACKNOWLEDGMENT The author thanks John Young for helpful discussions and for performing the a counting experiments. LITERATURE CITED (1) Nletzel, 0. A,; DeSesa, M. A. Anal. Chem. 1957, 29, 756. (2) Maeck, W. J.; Booman, G. L.; Elliot, M. C.; Rein, J. E. Anal. Chem. 1958, 30, 1902. (3) Eberle, A. R.; Lerner, M. W. Anal. Chem. 1957, 2 9 , 1134. (4) Palge, B. E.; Elliot, M. C.; Rein, J. E. Anal. Chem. 1957, 29, 1029. (5) Centanni, F. A.; Ross, A. M.; DeSesa, M. A. Anal. Chem. 1958, 28, 1651. (6) Johnston, M. V.; Wright, J. C. Anal. Chem. 1981, 53, 1050 (7) Perry, D. L.; Klalner, S. M.; Bowman, H. R.; Mllanovich, F. P.; Hlrshfeld, T.; Miller, S. Anal. Chem. 1981, 53, 1048. (8) Zook, A. C.; Collins, L. H.; Pletrl, C. E. Mikrochima Acta 1981, 2 , 457. (9) Morlyasu, M.; Yokoyama, Y.; Ikeda, S. J. Inorg. Nucl. Chem. 1977, 39,2211. ( I O ) Moriyasu, M.; Yokoyama, Y.; Ikeda, S. J . Inorg. Nucl. Chem. 1977, 39, 2199. (11) Morlyasu, M.; Yokoyama, Y.; Ikeda, S. J. Inorg. Nucl. Chem. 1977, 39, 2205. (12) Marcantonatos, M. D. Inorg. Chlm Acta 1977, 25, L101. (13) Robblns, J. C. CIM Bull. 1978, 793, 61 (14) Rodden, C. J. "Analysis of Essential Nuclear Reactor Materials"; U S . Atomic Energy Commlsslon, 1964; Chapter 1
RECEIVED for review June 15,1982. Accepted August 18,1982. The information contained in this article was developed during the course of work under Contract No. DE-AC09-76SR00001 with the U.S. Department of Energy.
Matrix and Solvent Effects on the Room-Temperature Phosphorescence of Nitrogen Heterocycles S. M. Ramasamy andl R. J. Hurtubiso" Department of Chemistry, The Unlversl?v of Wyoming, Laramie, Wyoming 82071
Several experimental p,arameters were studied to enhance the room-temperature phosphorescence of nitrogen heterocycles adsorbed on polyacryilc acid-salt mixtures and filter paper. The room-temperature phosphorescence was very sensitive to the salt content of the solid surface and the solvent used to adsorb the phosphor onto the surface. Sodium chloride and methlanoi were partlclularly important for inducing strong room-temperature phosphorescence. The results obtalned partially explain some of the Interactions responsible for room-temperature phosphorescence from nitrogen heterocycles.
The experimental conditions used to induce room-temperature phosphorescence (RTP) from compounds adsorbed on solid surfaces are imlportant in improving the sensitivity and selectivity of RTP. In addition, impoi.tant insights about the interactions responsible for RTP can be obtained by studying chemical and physical interactions of the phosphors. Little has been reported on variation of conditions for inducing RTP. Variables such i3s the solvents used to adsorb the phosphor onto the matrix and salts mixed with the solid matrix have not been studied in detail. Parker et al. ( 1 , Z ) have discussed the physical aspects of RTP and have reviewed 0003-270O/82/0354-2477$0 1.25/0
a variety of conditions needed for RTP. Hurtubise (3) considered interactions responsible for RTP and several experimental conditions for inducing RTP. Schulman and Parker ( 4 ) studied the effects of moisture and oxygen on RTP, and McAleese et al. (5) considered the elimination of moisture and oxygen quenching in RTP. Jakovljevic (6) investigated the effect of a variety of thallium and lead salts adsorbed on filter paper. Cinoxacin was used as a model compound, and Jakovljevic found that thallous fluoride and lead tetraacetate induced strong RTP from cinoxacin. Selective heavy-atom perturbation for analysis of complex mixtures by RTP has been discussed by Vo-Dinh and Hooyman (7). De Lima and de M. Nicola (8) considered the effect of sodium hydroxide concentration, source irradiation time, and temperature during RTP measurements for 1,8-naphthyridinederivatives. Parker et al. (9) investigated the effects of moisture and support treatment on the RTP of pteridines. Sodium acetate treated filter paper was found to enhance the RTP of adsorbed pteridines. The effect of HC1 and HBr concentrations on the RTP of nitrogen heterocycles was studied (IO),and several brands of silica gel chromatoplates were tested for inducing RTP from nitrogen heterocycles (11). The comparison of conditions for RTP of nitrogen heterocycles and aromatic amines adsorbed under a variety of conditions on filter paper, a silica gel chromatoplate, and 0.5% polyacrylic acid 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Table I amt of material used for % neutralization 2 5% 50% 7 5%
-___I-
0%
wt of PAA in 3 mL of ethanol vol of NaOH (0.0766 M) wt of NaCl
21 mg 0.0 mL 4.2 g
21 mg 0.91 mL 4.2 g
(PAA)-salt mixture was presented by Ramasamy and Hurtubise (12). Several polymer-salt mixtures were examined as solid surfaces under different conditions for hydroxylsubstituted aromatics (13). The RTP behavior of several compounds adsorbed on sodium acetate was investigated by von Wandruszka and Hurtubise (141, and the conditions for the RTP of terephthalic acid on silica gel chromatoplates (15) and PAA salt mixtures have been studied (16). Niday and Seybold (17)observed the effect of added substances on the R T P lifetime of 2-naphthalenesulfonate adsorbed on filter paper. Bower and Winefordner (18) studied the effect of sample environment on the R T P of polycyclic aromatic hydrocarbons and of pharmaceutical compounds (19). In this work, the RTP of benzo[flquinoline (B[flQ) and other nitrogen heterocycles was studied with PAA-salt mixtures under a variety of conditions along with a few experiments with filter paper. The main emphasis in this work was to determine the experimental conditions for strong R T P signals and to develop a model to describe some of the interactions for inducing RTP. EXPERIMENTAL SECTION Apparatus. All RTP intensity data were obtained with a Schoeffel SD3000 spectrodensitometer equipped with a SDC 300 density computer (Schoeffel Instruments, Westwood, NJ) and a phosphoroscope accessory. Details of instrumental setup were reported previously (20). Relative RTP signals were measured with the spectrodensitometer with the inlet and exit slits set at 2 mm and 3 mm, respectively. A 150-W Xe lamp (Canrad Hanovia Inc., Newark, NJ) and R928 photomultiplier tube (Hamamatsu Corp., Middlesex, NJ) were employed in the spectrodensitometa. The following excitation and emission wavelengths were used: benzo[flquinoline, 370 nm, 510 nm; 4-azafluorene, 330 nm, 465 nm; phenanthridine, 360 nm, 510 nm; 13H-dibenzo[a,i]carbazole, 315 nm, 505 nm; isoquinoline, 340 nm, 510 nm. Reagents. Methanol, ethanol, and isoquinoline were purified by distillation. Benzolflquinoline and 13H-dibenzo[a,i]carbazole (Gold Label, Aldrich Chemical Co., Milwaukee, WI) and polyacrylic acid (PAA) (Secondary Standard, Scientific Polymer Products, Ontario, NY) were used as received. Phenanthridine and 4-azafluorene were recrystallized from ethanol. Aliphatic carboxylic acids, inorganic salts, and organic solvents were reagent grade and used without further purification. Salt mixtures of polyacrylic and carboxylic acids were prepared by grinding to a homogeneous powder in a ball-mill. Filter paper (Whatman No. 1)was developed in ethanol to move impurities to the upper part of the paper prior to use. Procedures, Phosphorescence Measurements. Solutions for phosphorescence measurements were prepared in neutral and acidic (0.1M HBr) solvents. One-microliter aliquots containing 100 ng of phosphor were spotted onto filter paper and dried at 80 OC for 20 min unless noted otherwise (20). Sal-PAA mixtures (20 mg) were added to 20 pL of solvent and 1 pL of phosphor solution. The mixtures were handled as were sodium acetate samples (21). Background signals were subtracted for the appropriate sample-substrate system. Solubility of Salts. Excess salts samples (NaCl, KCl, and LiCl) were added separately to 200 mL of distilled ethanol and stirred for 2 h. The supernatant was filtered and three 50-mL aliquots of a given saturated salt solution were evaporated at 100 OC in a vacuum oven for 24 h. The dried residues were weighed, and the average weight of triplicate runs was used to calculate solubility. A similar procedure was used for other solvents. Reaction of P A A with NaOH. It was determined by titration that 21.0 mg of PAA was neutralized by 3.65 mL of 0.07660 M
21 mg 1.83 mL 4.2 g
21 mg 2.74 mL 4.2 g
100%
21 mg 3.65 mL 4.2 g
Table 11. Relative RTP Intensities of B[f] Q Adsorbed on Aliphatic Carboxylic Acid-NaC1 Mixtures ' re1 re1 acid intens acid intens b crotonic stearic fumaric maleic D-tartaric
1.0 1.0 1.1
1.2 1.2
succinic citric oxalic D-malic polyacrylic
1.3 1.4 1.5 1.6 4.6
a One microliter of 0.1 M HBr ethanol solution containing the phosphor was added to 0.5% carboxylic acidNaCl mixture to give 100 ng of adsorbed phosphor. SamAverage of dupliples were dried at 80 "C for 20 min. cate experiments,
NaOH using phenolphthalein to detect the end point. With this information several 0.5% PAA-NaC1 mixtures were prepared and reacted with different amounts of 0.07660 M NaOH. The various amounts of materials used are given in Table I. After allowing the reaction to occur, the above mixtures were dried at 110 "C for 30 min and then ground to form a powder. Samples of 20 mg of powder were used in the phosphorescence work. RESULTS AND DISCUSSION Aliphatic Carboxylic Acids. Because PAA-NaC1 mixtures induced RTP from several classes of compounds (10-13, 15, 16), it was of interest to investigate various aliphatic carboxylic acid-NaC1 mixtures for their potential to induce RTP from B[flQ. Benzo[flquinoline was shown previously to yield strong RTP signals from PAA-NaC1 mixtures (11). Table I1 lists the relative R T P signals obtained from the mixtures including a PAA-NaC1 mixture. Even though the simple aliphatic carboxylic acid-NaC1 mixtures induced RTP from B[flQ, the PAA-NaCl mixture induced the largest signals. Because there are more carboxyl groups per molecule of PAA relative to an aliphatic carboxylic acid molecule, the potential exists for more of the carboxyl groups from a PAA molecule to interact with B[f]Q compared to a molecule of an aliphatic carboxylic acid. For example, the molecular weight of the PAA is 2.0 X lo6 (22). Because of the long polymer chains in a mixture of PAA-NaC1, it appears B[flQ is entangled in the polymer-salt matrix and thus is held very rigidly which allows strong RTP to be observed. It is also possible that B[flQ is buried in the matrix and thus collisions with oxygen are minimized. Effect of Different Salts. Sodium chloride was found to be important for inducing RTP from adsorbed compounds (11,13). Sodium chloride has to be mixed with PAA for strong RTP to be observed. Very weak or no RTP is observed for phosphors adsorbed separately on either PAA or NaC1. Several other inorganic salts were mixed separately with PAA to study the effects of the salts on the RTP of B[flQ and the variation in background RTP of the PAA-salt mixtures. Table I11 shows the results obtained. Interestingly the PAA-NaBr mixture yielded the weakest RTP from B[flQ indicating the heavy atom effect with excess NaBr is not an important factor. As seen in Table 111, the PAA-NaC1 mixture induced the largest RTP from B[flQ. The solubilities of NaC1, LiC1, and KC1 in ethanol were determined as described in the Experimental Section and found to be 1.1 X 10" M, 1.6 M, and 3.0 X M, respectively. There seems to be a rough correlation
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Table 111. Relative RT6 Intensities of B[f]Q and of Background with PAA-Salt Mixturesa re1 intens'
salt NaCl Licl KCI
sample 8.8 6.8 3.3
&SO,
6.0 4.6 2.8
Na,SO, NaBr
background 3.9 1.0 4.7 1.5 3.2 4.8
a One microliter of 0.1 M HBr ethanol solution containing the phosphor was added to 0.5%PAA-salt mixture to give 100 ng of adsorbed phosphor. Samples were Average of duplicate experidried at 80 "C for 20 miin. ments.
between the RTP values for B[flQ and the solubilities of NaCl and KC1. However, LiCl does not fit the correlation. Because LiCl is strongly solvated by ethanol, it tolok about 3 h to dry the sample. Even after the extensive drying period, the RTP signal of B[flQ was not as large as the one from the PAA-NaC1 mixture. Various correlations were investigated for the RTP of B[flQ and certain properties of the alkali halides, namely, ionic radii, lattice energies, and Madelung constants. However, no correlation was found. As indicateld in Table 111, the PAA-K2S04 mixture induced a stronger signal than the PAA-Na2S04 mixture for B[flQ. Because Na2S04 is very hygroscopic, adsorbed water may not have been completely removed by heating or anhydrous Na2S04may have absorbed moisture during the R'I'P measurement step. The data in Table I11 are important ,analyticallyin that it shows that NaCl mixed with PAA will induce the largest RT'P signal from B[flQ from the salt mixtures studied. Also, Table I11 shows that the RTP background signals vary in a different fashion compared to B[flQ. The PAA-LiC1 mixture gave the lowest background signal of the series, while the PAA-NaBr mixture gave the highest background signal of the series. Some additional considerations should be mentioned. Deanin (23)has discussed the effects of inorganic filler on the mechanical and thermal properties of polymers. In polymers without a filler, the polymer molecule has a certain freedom to rotate and migrate. I h a polymer mixed with an inorganic salt, some polymer molecules lie directly a.djacent to inorganic particles which have essentially no mobility. A polymer molecule lying near suich a rigid species) is restricted in its ability to rotate and migrate. Many inorganic fillers produce a significant increase in the modulus of a filled polymer. Modulus is a measure of the rigidity or flexibility of a solid material. In this work, an excess of sodium chloride was mixed with PAA. Only 0.5% PAA was present in the mixture. Nielsen (24) has given data showing the increase of the modulus of polyurethane filed with increasing amounts of sodium chloride. Deanin (23) commented that the most important effect of fiiers on thermal properties of polymers was to reduce the coefficient of thermal expansion of the polymer. This means the mobility and motion of the polymer will be less. It appears then with added rigidity and a lower coefficient of thermal expansion for the polymewdt matrix, the nitrogen heterocycle is in an environment with less relative motion and this would favor enhanced RTP. No attempt was made to measure the moduli or (coefficientsof thermal expansion for the mixtures in Table 111. However, different salts will affect the dry polymer conformation differently, and NaCl apparently gives the most favorable conformation for strong RTP and B[flQ. In contrast to the PAA results for B[flQ in Table I11 with NaBr and NaC1, it wasi found that B[flQ spotted on filter paper from 0.1 M HBr-ethanol solution containing NaBr
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Table IV. Relative RTP Intensities of B[f]Q Adsorbed on Filter Paper from Ethanol-NaBr and Ethanol-NaC1 Solutions re1 intens b salt solution Na Br NaCl 0 1.4 1.1 25 1.3 1.0 50 1.2 1.0 75 1.2 1.0 100 1.1 1.0 a One microliter of 0.1 M HBr enthanol-salt solution containing the phosphor was added to filter paper to give 100 ng of adsorbed phosphor. Samples were dried at 90 "C for 30 min. Average of dudicate experiments. % Saturated
25 %
i
1
50
75
NEUTRALIZATION
OF
0
PAP,
Figure 1. Graph of RTP of B[f]Qvs. percent neutralization of PAA. Samples of 100 ng of B[f]Q were adsorbed onto 0.5% PAA-NaCI from 0.1 M HBr ethanol solutions. yielded greater RTP than from 0.1 M HBr-ethanol solution containing NaCl (Table IV). Sodium chloride has a solubility in ethanol of 0.011 M and NaBr has a solubility of 0.17 M in ethanol; thus a relatively larger amount of NaBr is deposited on the filter paper compared to NaCl on filter paper. It appears from the data on Table IV that the heavy atom effect is important with the NaBr samples; however, the increase in RTP with the NaBr solutions is relatively small. In similar experiments, neutral NaCl solutions of B[flQ spotted on filter paper showed a small increase in RTP with increasing amounts of NaC1. The 100% saturated NaCl solution yielded the largest signal. With neutral NaBr solutions of B[flQ, the RTP signal increased with the NaBr solution content up to 75% saturated salt solution, but at 100% saturated salt solution the RTP signal was the same as at 25% saturated salt solution. The neutral solution with no salt present gave the weakest RTP signal for B[flQ. For the systems tested with fiiter paper, a 0.1 M HBr ethanol solution with no salt present gave the strongest RTP for B[flQ. This result is important analytically because it indicates an acidic HBr solution is needed for strong RTP from B[flQ. Neutralization of PAA. Samples of PAA were reacted with different amounts of NaOH, as described in the Experimental Section, to give 25%, 50%,75%, and 100% neutralized samples of PAA (Figure 1). As seen in Figure 1, the RTP signal of H[flQ almost drops by a factor >10 at 100% neutralization, and a distinct break in the curve is indicated at 50% neutralization. The break in the curve at 50% neutralization suggests a change in the conformation of the polymer. This could result from electrostatic repulsion of
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Table V. Effect of Solvents on the Relative RTP Intensity of B[f]Q on 0.5% PAA-NaCI a re1 intens
solvent
sample
bkgd
methanol acetonitrile acetone 2-propanol ethanol water ethyl acetate cyclohexane 1,2-dichloroethane ni tromethane chloroform
19.6 16.2 10.2 8.2
4.4 1.4 4.0 4.2
5.8
13.0
5.6 14.4 1.8 1.4 1.8 1.0 1.6 1.2 1.2 1.4 1.8 1.0 a One microliter of 0.1 M HBr solution containing the phosphor was added t o 0.5% PAA-NaCl mixture to give 100 ng of adsorbed phosphor. Samples were dried at 80 "C for 1 h. Cloudy solution.
*
*
carboxylate groups. Because the RTP intensity decreases with percent neutralization, this suggests that some carboxyl groups participate in hydrogen bonding with B[flQ to anchor the molecules so RTP can be observed. Also the data imply that the conformation of the polymer may be important for RTP. The weak R T P signal at 100% neutralization most likely results from the small amount of HBr from the ethanol solution added to neutralized PAA. With the small amount HBr, carboxylate groups are protonated and can interact with
WflQ.
On the basis of the molecular weight of PAA (2.0 X lo6) and the molecular weight of the repeating group in PAA H
I
6I
- C H ~ - COOH
I one can calculate the approximate number of carboxyl groups in a given amount of unneutralized PAA. The number of carboxyl groups in 5.00 X pmol (amount used in RTP experiment) of PAA was 8.45 X The micromoles of B[flQ used in the experiments was 5.58 X or 3.36 X 1014 molecules of B[flQ. At about 99.96% neutralization, there are the same number of carboxyl groups as B[flQ molecules. These calculations indicate that a large number of the carboxyl groups do not interact with B[flQ. For example, there are 2.51 x lo3 (8.45 x 101'/3.36 X 1014)carboxyl groups in an unneutralized sample of PAA for one B[flQ molecule. Also, because the R T P relative intensity decreases with percent neutralization and there is an excess of carboxyl groups up to about 99.96% neutralization, then it appears that some specific geometric requirements in the matrix are needed to achieve the optimal environment for strong RTP. Effect of Solvent. Very little consideration has been given to solvents used to adsorb phosphors onto solid surfaces that induce RTP. Table V shows the RTP results for B[flQ with 11different solvents. As seen, a considerable range of relative intensity values were obtained. Ethanol has been used by us and others in RTP work, and it is placed about midpoint in Table V. The use of methanol gave a substantial increase in the RTP signal for B[flQ. The observed differences for B[flQ in Table V are most likely related to the initial wet chemistry of the system. For example, PAA, NaC1, and B[dQ are soluble to different degrees in the various solvents tested. It is possible that methanol, for example, interacts with the carboxyl groups of PAA more strongly than does ethanol. This would help in breaking intra- and intermolecular hydrogen bonds between the carboxyl groups in PAA and make these groups more
0
0.2
0'4
% PAA
0'6
I N NaCl
0'8
PO
MIXTURE
Figure 2. Graphs of RTP for nitrogen heterocycles vs. percent PAA In NaCl mlxtures. One hundred nanograms of each phosphor was adsorbed from 0.1 M HBr ethanol solutlons: (0)benzo[f]quinoline;
(V)4-arafluorene; (0)phenanthridlne; role; (0)Isoquinollne.
(0) 13Hdibenzo[a ,i]carba-
readily available to interact with B[flQ. The results in Table V indicate the importance of solvents and the initial wet chemical and physical interactions in RTP work. These aspects are in need of additional investigations. As Table V shows, the background relative intensity is a function of the solvent employed. Water yielded the largest background signal and cyclohexane the lowest background signal. In related work, several ethanol-water solvents were studied and 50% ethanol/water (v/v) gave the largest RTP signal from B[flQ with a signal 2.3 greater than the signal from using ethanol. For methanol-water solvents a 70% methanol water (v/v) solvent resulted in the largest RTP signal with B[f]Q. The signal was 3.1 times greater than a sample adsorbed from methanol. These samples were dried for 1 h at 100 "C. Variation of PAA Content. The PAA content in the PAA-NaCl mixtures used to induce RTP from nitrogen heterocycles was varied over a wide range. Relatively large signals were obtained for the nitrogen heterocycles between 0.5 and 1.0% PAA. Beyond 1% PAA, the RTP of B[flQ gradually decreased nonlinearly so that at 90% PAA-NaC1 practically no RTP was observed. Figure 2 shows RTP intensity as a function of percent PAA from 0 to 1%PAA in PAA-NaC1 mixtures for five nitrogen heterocycles. In all cases, the RTP signal increased with PAA content and then the R T P intensity reached a maximum signal and stayed relatively constant over a range of percent PAA values as indicated in Figure 2. Benzo[flquinoline, isoquinoline, and 13H-dibenzo[a,i]carbazoledid show a tendency toward lower RTP signals beyond 0.5% PAA. As Figure 2 shows, a certain optimal percent of PAA was needed to obtain maximum RTP signals. By use of the point of intersection of two lines drawn from the flat portion of the curve beyond 0.5% PAA and the rising portion of the curve the optimal percent values were obtained: B[flQ, 0.31 %; 4-azafluorene, 0.42%; phenanthridine, 0.29%; 13H-dibenzo[a,i]carbazole,0.37%; isoquinoline, 0.45%. By use of the preceding optimal percent values, the molecular weight of PAA, the molecular weight of the repeating group in PAA, the molecular weight of a given nitrogen heterocycle, and a weight of 100 ng for the nitrogen heterocycle, the ratio of number of repeating groups (I) in PAA to one phosphor molecule was calculated. These calculations should be considered as approximate because it is assumed
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
that the reported molecular weight of PAA is an average molecular weight valuia (22). For the five compounds the following values were obtained: B[flQ, 1.54 X lo3; 4-azafluorene, 1.98 X lo3; phenanthridine, 1.45 X lo3; 13H-dibenzo[a,i]carbazole, 2.76 X lo3; isoquinoline, 1.62 X lo3. Obviously many of the carboxyl groups do not interact with the phosphor moleculen. A similar series of experiments was run with 50 ng of B[flQ and the optimal percent of PAA was almost the same as for 100 ng of B[flQ, namely, 0.30% PAA. The ratio of the number of repeating groups in PAA to one B[flQ molecule was callculated as 2.99 2: lo3. The previous value of 2.99 X lo3 is ahmost twice the value obtained for 100 ng of B[flQ indicating a proportional number of carboxyl groups are not interacting with the 50 ng of B[flQ. The large ratio values obtained for the number of repeating groups in PAA to one phosphor molecule implies that a given nitrogen heterocycle molecule would be far from its nearest neighbor. This would minimize the interaction of nitrogen heterocycle molecules with each other and allow the PAA-NaC1 matrix to interact effectively with the nitrogen heterocycle molecules. In addition to the above calculations, the ratio was calculated of the number of repeating groups in PAA at the optimal PAA concentration to one molecule of dissolved NaC1. The solubility of NaCl in ethanol was determined as described in the Experimental Section and used in the calculation. The following values were obtained for the five nitrogen heterocycles: B[flQ, 3.8; 4-a:zafluorene75.2; phenanthridine, 3.6; 13H-dibenzo[u,i]carbazole,4.5; isoquinoline, 5.5. From these data, it is concluded that there are more repeating groups than NaCl molecules at the optimal concentratilon of PAA. At 0.1% PAA the ratio is 1.23 which indicates that there are approximately the same number of NaCl molecules as repeating groups. The role of NaCI in RTP of the nitrogen heterocycles in this work is an important consideration because strong RTP is not induced without NaC1. It is speculated that dissolved NaCl initially on the wet surface breakai some of the intraand/or intermolecular hydrogen bonds of the PAA dissolved in ethanol and allows nitrogen heterocycles to compete for hydrogen bonding with the carboxyl groups. In other words, NaCl can be solvated not only by ethanol but also by dissolved PAA. In a related experiment, the RTP of B[flQ was determined as a function of percent PAA; however, methanol was used as a solvent. The solubility of NaCl in methanol is much higher than in ethanol. The solubility of NaCl in methanol has been reported as 11.1 g/L (25). The RTP vs. % PAA graph for the methanol system was similar to the one obtained for B[QQ in Figure 2. The maximum RTI' signal was reached a t 0.40% PAA and remained almost conetant to about 1.0% PAA. The ratio of repeating groups in PAA to a B[flQ molecule a t 0.40% PAA was calculated ,as 1.99 X lo3. The ratio of repeating groups in PAA to one NaCl molecule at 0.40% PAA was calculated to be 0.28 which is smaller than the ratio obtained with ethanol (3.8). The ratio of 0.28 obtained with methanol indicates that there are an excess of sodium and chloride ions in solution at 0.4% PAA. This
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situation should favor breaking of intermolecular hydrogen bonds of the carboxyl groups in the initial wet state of the solid surface. Also, the ratio of 1.99 X lo3 for the repeating groups in PAA to a B[flQ molecule is higher than the corresponding ratio (1.54 X lo3) for ethanol. As previously discussed, B[QQ gives a greater RTP signal when adsorbed onto 0.5% PAA-NaCl with methanol compared to ethanol (Table V). Possibly with methanol as a solvent and the relatively large amount of NaCl dissolved in methanol, more intraand/or intermolecular carboxyl hydrogen bonds are broken in PAA, and B[flQ can interact with a larger number of carboxyl groups which results in the greater RTP. There is also the possibility of a different structural change for the polymer with methanol compared to ethanol which could favor stronger RTP. Allerhand and Schleyer (26) reported a very large spectral shift to lower cm-l values for the OH stretching frequency of methanol with halide ions present in solution. They attributed this to anion hydrogen bonding with the OH of the methanol, and the magnitude of the shift was in the order C1- > P> Br- > I-. Their results support the hypothesis that C1- can interact in solution with the carboxyl goups of PAA in addition to OH groups of methanol. LITERATURE CITED Parker, R. T.; Freedlander, R. S.; Dunlap, R. B. Anal. Chlm. Acta 1980, 719, 189. Parker, R. T.; Freedlander, R. S.; Dunlap, R. B. Anal. Chlm. Acta 1980, 720, I . Hurtubise, R. J. "Solid Surface Luminescence Analysis: Theory, Instrumentation, Apllcations"; Marcel Dekker: New York, 1981; Chapters 5 and 7. Schulman, E. M.; Parker, R. T. J . Phys. Chem. 1977, 87, 1932. McAleese, D. L.; Freedlander, R. S.; Dunlap, R. B. Anal. Chem. 1980, 52, 2443. JakovlJevic,I . M. Anal. Chem. 1977, 4 9 , 2048. Vo-Dinh, T.; Hooyman, J. R. Anal. Chem. 1979, 5 7 , 1915. DeLima, C. G.; de M. Nicola, E. M. Anal. Chem. 1978, 5 0 , 1658. Parker, R. T.; Freedlander, R. S.; Schulman, E. M.; Dunlap, R. B. Anal. Chem. 1979, 57, 1921. Hurtubise, R. J. Talanta 1981, 28, 145. Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1980, 5 2 , 656. Ramasamy, S. M.; Hurtubise, R. J. Anal. Chem. 1982, 5 4 , 1642. Dalterio, R. A.; Hurtublse, R. J. Anal. Chem. 1982, 5 4 , 224. Von Wandruszka, R. M. A.; Hurtubise, R. J. Anal. Chem. 1977, 4 9 , 2184. Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1978, 5 0 , 610. Hurtubise, R. J.; Smith, G. A. Anal. Chlm. Acta 1982, 739, 315. Niday, G. J.: Seybold, P. G. Anal. Chem. 1978, 5 0 , 1577. Bower, E. L.; Winefordner, J. D. Anal. Chlm. Acta 1978, 102, 1. Bower, E. L. Y.; Wlnefordner, J. D. Anal. Chlm. Acta 1978, 107, 319. Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1979, 57, 659. Von Wandruszka, R. M. A,; Hurtublse, R. J. Anal. Chem. 1976, 4 8 , 1784. Scientific Polymer Products, Catalog 801, Ontario, NY, p 20. Deanln, R. D. "Polymer Structure, Properties and Applications"; Cahner Books: Boston, MA, 1972; pp 387-389. Nlelsen, L. E. "Mechanical Properties of Polymers and Composites"; Marcel Dekker: New York, 1974; Vol. 2, p 423. Harner, R. E.;Sydnor, J. B.; Gilreath, G. E. J . Chem. f n g . Data 1963, 8 , 411. Allerhand, A.; Schleyer, P. von R. J . Am. Chem. Soc. 1963, 85, 1233.
RECEIVED for review June 18, 1982. Accepted September 7, 1982. Financial support for this project was provided by the Department of Energy, Division of Basic Energy Sciences, Contract No. DE-AC02-80ER10624.
