90
Anal. Chem. 1987, 59, 90-94
applied pressures on the reagent side without reaching the plateau level. In practice, both the membrane reactors and the screen-tee mixer gave reproducible peak areas and good peak shapes and operated reliably over eluent flow rates of 0.5-2.0 mlsrnin-l.
LITERATURE CITED (1) Spackman, D. H.; Stein, W. H.; Moore, S. Anal. Chem. 1958, 30,
1190- 1206. (2) Reaction Defectors in Liquid Chromatography; Krull, I. S., Ed.; Academic: New York, in press. (3) Knight, C. H.; Cassidy, R. M.; Recoskie, 8. M.; Green, L. W Anal. Chem. 1984, 56, 474-478. (4) Cassidy, e. M.; Elchuk, S.; Elliot, N. L.; Green, L. W.; Knight, C. H.; Recoskii, 0. M. Anal. Chem. 1986, 58, 1181-1186. (5) Frei, R. W. Chfomtrographla 1082, 75, 161-166. (6)Jansen, H.; Brinkman, U. A. Th.; Frei, R . W. J. Chromatogr Sci. 1985, 23, 279-284. (7) Huber. J. F. K.; Jonker, K. M.; Poppe, H. Anal. Chem. 1980, 52, 2-9. (8) Malavoltl, N. L.; Piiosof, D.; Neiman, T. A. Anal. Chem. 1984, 56, 2 191-2 195. (9) Pllosof, D.; Neiman, T. A. Anal. Chem. 1982, 54, 1698-1701. (10) Apffel, J. A.; Brinkman, U. A. T.; Frei, R. W. Chromafographia 1984, 78, 5-10. (11) Davis, J. C.; Peterson, D. P. Anal. Chem. 1985, 57, 768-771.
(12) Dasgupta, P. K. Anal. Chem. 1984, 56, 103-105. (13) Dasgupta, P. K. Anal. Chem. 1984. 56, 769-772. (14) Dasgupta, P. K.; Gupta, V. K. Environ. Sci. Techno/. 1988, 20, 524-526. (15) Hwang, H.; Dasgupta, P. K. Anal. Chem. 1986, 58, 1521-1524. (16) Grushka, Eli Anal. Chem. 1972, 44, 1733-1738. (17) Elchuk, S.; Cassidy, R. M. Anal. Chem. 1979, 51, 1434-1438. (18) Dasgupta, P. K., unpublished results. (19) Riviello, J. M.; Pohl, C. A. 35th Pittsburgh Conference on Analytical Chemlstry and Applied Spectroscopy, Atlanta City, NJ, March 1984; paper 506. (20) Kirkhnd, J. J.; Yu, W. W.; Stoklosa, H. J.; Dilks, C. H., Jr. J. Chromatogr. Sci. 1977, 75,303-316. (21) Dasgupta, P. K. J. Liq. Chromafogr. 1984, 7, 2367-2382. (22) Dasgupta, P. K.; Yang, H. C. Anal. Chem. 1986, 58, 2839-2844.
RECEIVED for review May 14, 1986. Accepted September 16, 1986. The membrane reactor work at Texas Tech University is supported by the State of Texas Advanced Technology Research Program and by the U.S. Department of Energy, Office of Basic Energy Sciences, through Grant No. DEFG05-ER-13281. However, this report has not been subjected to review by the agency and no endorsements should be inferred.
Operational Variables for the Separation of Styrene-Methyl Methacrylate Copolymers According to Chemical Composition by Liquid Adsorption Chromatography Sadao Mori* and Yoshitaka Uno Department of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie 514, Japan
The copolymers of a large range of cmposnlon were separated wlth a mlxture of chloroform (or 1,P-dkhloroethane (DCE)) and ethanol on a sillca gel column by linear gradient elutlon. Chloroform (and DCE) wlthout ethanol retatned the copolymers in the column. By the addltlon of ethanol to chloroform, copolymers having less methyl methacrylate (MMA) started to elute, and wlth lncreaslng ethanol content In chlorofann, those having more MMA could be eluted. The copolymers tend to adsorb on the column at hlgher column temperature, and thaw having more MMA require a lower column temperature for elutlon. Ethanol content or column temperature dld not affect peak retentlon volume for the copolymers. The effects of both ethanol concentratlon and column temperature were attrlbuted to the change of populatlon of free sUanol groups on the surface of sillca gel, because the hydrogen bonding of carbonyl groups in the copolymers to the sllanol groups was the main mechanlsm of thls separation.
The accurate determination of the chemical composition distribution (CCD) for copolymers is very important for the characterization of copolymers. Among several techniques to measure CCD, high-performance liquid chromatography (HPLC) holds great promise because of its high efficiency. There are a number of published papers in this area, e.g., separations of styrene-methyl acrylate copolymers on a silica gel column ( I ) , styrene-acrylonitrile copolymers by precipitation liquid chromatography ( 2 ) , styrene-butadiene copolymers on a polyacrylonitrile gel column (3),styrenemethyl
methacrylate copolymers on a silica gel column ( 4 ) ,styrenemethyl methacrylate block copolymers by column adsorption chromatography using a 50-mm-i.d. cylindrical column ( 5 ) , and styrene-n-butyl methacrylate copolymers by orthogonal chromatography (6). In a previous paper (7), separation of styrene-methyl methacrylate random copolymers (P(S-MMA)) according to chemical composition by liquid adsorption chromatography (LAC) was reported. Silica gel was used as an adsorbent. The copolymers were separated by stepwise gradient elution using chloroform and 1,2-dichloroethane (DCE) as mobile phases. When DCE was used as the mobile phase, the copolymers adsorbed on the surface of silica gel, though polystyrene (PS) eluted from the column. When chloroform was used as the mobile phase instead of DCE, the copolymers having a methyl methacrylate (MMA) component less than 45% could be eluted as well as PS. With increasing chloroform content in a mixture of chloroform and DCE, the copolymers have been separated as a function of their compositions. In the paper, the effects of ethanol content in the mobile phase on the elution behavior of these copolymers were suggested. In the present work, the role of ethanol in the mobile phase in the separation of the copolymers was investigated first in addition to column temperature effects. Then, optimal separation conditions of copolymer components were investigated by linear gradient elution.
EXPERIMENTAL SECTION Apparatus. LAC measurements were performed on a Jasco TRIROTAR-VI high-performance liquid chromatograph (Japan Spectroscopic Co., Ltd., Hachioji, Tokyo 192, Japan) with a
variable-wavelength ultraviolet absorption detector Model
0003-2700/87/0359-0090$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
1 il
0
I ol-
0
.
0.5
91
1.0 V
W
w
0.5
Flgure 1. Effect of column temperature on the elution of copolymer 111: mobile phase, chloroform/ethand (991, v/v); column temperature (a) 10 "C, (b) 20 "C, (c) 30 "C, (d) 40 "C, (e) 50 "C.
UVIDEC-100IV at a wavelength of 254 nm. Attenuation of the detector was 0.64 AUFS. Silica gel with a pore size of 30 A and a mean particle diameter of 5 pm (Nomura Chemical Co., Seto, Aichi 489, Japan) was packed in 4.6-mm4.d. X 50-mm-length stainless-steeltubing. This column was thermostatd at a specified temperature to a precision of 0.1 "C by using a column jacket in which constant-temperature water was circulated. Samples. P(S-MMA) copolymers used in this experiment were the same as those in the previous report (7). Samples were prepared by solution polymerization at a low degree of conversion, and the composition of the samples are shown as styrene content in percent as follows: I (85.5%), I1 (73.4%)) I11 (66.3%)) IV (57.4%), V (48.7%),VI (42.1%))VI1 (41.5%), VI11 (26.5%),and IX (15.2%). The sample copolymers were dissolved in the mobile phase in a concentration of 0.05%. The injection volume was 0.05 mL. Elution. The mobile phase was a mixture of chloroform and ethanol or DCE and ethanol. First, elution was performed by an isocratic elution mode. The flow rate of the mobile phase was 0.5 mL/min. Ethanol contained in chloroform as a stabilizer was removed before use by washing chloroform with water and drying it with anhydrous calcium chloride, followed by distillation. Henceforth, chloroform in this report means it does not contains any ethanol unless otherwise specified. Linear gradient elution was performed as follows: the initial mobile phase (A) was a mixture of chloroform and ethanol (99.01.0, v/v); the composition of the final mobile phase (B) was 95.5:4.5 (v/v) chloroform/ethanol; and the composition of the mobile phase was changed from 100% A to 100% B in 15 min linearly after injecting a sample solution. Samples were dissolved in the initial mobile phase.
RESULTS AND DISCUSSION Column Temperature Effects. Elution behavior of the copolymers was investigated by changing column temperature from 10 "C to 80 "C every 5 "C. The mobile phase used here was a mixture of chloroform and ethanol at concentration of 99:l by volume. A typical example is shown in Figure 1 for copolymer 111. Peaks were sharp and their heights were unchanged between 10 "C and 25 "C. Peak height at 30 "C decreased to 85% of that at 10 "C and that at 40 "C to 48%. Moreover, peak shape at 40 "C was rather broad. At 50 "C column temperature, peak height decreased to less than 6%, and no peaks eluted even after 1 h. Similar experiments were carried out for other copolymers. The results are illustrated by plotting % peak height vs. temperature and are shown in Figure 2. Peak height when a sample copolymer elutes completely is expressed as 100% and other peak heights are shown by percentage. Estimation of whether or not the copolymer eluted a t 100% was based on the peak height of PS and the compcsition of the copolymer injected. As the peak width at base line was almost unchanged
Column Temperature I
OC
Flgure 2. Column temperature effect on peak height: composltlon of the mobile phase, chloroform/ethanol (99: 1, v/v).
* p j,J& ! A&
05
VRlrnI
K, 05
0
05
0
05
w '10,
'
10
w
10
VRiml-
'
&-
Flgure 3. Effect of ethanol content In chloroform on the elution of copolymer V: column temperature, 10 "C; mobile phase, chloroformkthanol (a) 100:0, v/v, (b) 9950.5, (c) 99.0:1.0, (d) 9851.5, (e) 98.0:2.0, (f) 9 7 5 2 . 5 , (9) 9 7 . 0 3 0 .
by column temperature except in some cases in which the peak was broad, percentage of peak height expresses the proportion of a sample eluted from the column. Peak height of copolymer I was unchanged at a column temperature between 10 "C and 80 "C, indicating 100% of the sample eluted from the column. Copolymer I1 eluted 100% from the column at column temperatures between 10 "C and 55 "C, and adsorption of the copolymer in the column gradually increased above 55 "C. Copolymer IV eluted 100% below 15 "C, and it adsorbed in the column over 30 "C. Copolymer V eluted at only 35% even at 10 "C. Copolymers VI-IX did not elute at all under these experimental conditions. These results can be summarized as follows: (1)the copolym%.rstend to adsorb in the column a t higher column temperature; (2) the copolymers having more MMA component require lower column temperature to elute from the column; and (3) peak retention volume was unchanged and it was about 0.5 mL, which corresponds to the interstitial volume of the column system. Effects of Ethanol Concentration in the Mobile Phase. At constant column temperature, elution behavior of the copolymers was examined by changing ethanol content in chloroform or DCE used as the mobile phases. Examples for chromatograms are shown in Figure 3 for copolymer V. Column temperature in this case was 10 "C, and ethanol content in chloroform was increased every 0.5% from 0% to 3.0%. Copolymer V did not elute from the column when the ethanol content in the mobile phase was less than 0.5%. At 1% ethanol concentration in the mobile phase, the copolymer appeared from the column, and the peak height increased as
92
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
0
05
10
15
20 26 30 35 40 45 Elhanoi Content in DCE / v/v%
50
5.5
Figure 5. Change of peak helght by increasing ethanol content in WE: column temperature, 10 OC and 30 OC. Number in parentheses indicates samples measured at 30 OC. Figure 4. Change of peak height by increasing ethanol content In chloroform: column temperature, 10 OC and 30 OC. Number in parentheses indicates samples measured at 30 OC.
the ethanol content in the mobile phase was increased to 1.5% where the copolymer eluted 100%. Similar experiments have been performed at column temperatures of 10, 20, and 30 "C, and the results at 10 O C and 30 "C are illustrated in Figure 4. Similarly to Figure 2, peaks eluted completely from the column are expressed as 100%and the peak height is indicated as percentage. When chloroform without ethanol was used as the mobile phase, none of the copolymers eluted, except for polystyrene which was not retained. At a column temperature of 10 O C , copolymer I eluted 100% when 0.5% ethanol was added to chloroform, but only 85% of copolymer I1 came out from the column. Copolymer I11 and other copolymers did not elute. With increasing ethanol content in the mobile phase up to l.O%, all copolymers I-IV eluted from the column. At 1.5% ethanol content, 60polymers I-VI eluted with 100% recovery; at 2.0% ethanol, copolymers I-VI11 completely eluted; and at 2.5%, all copolymers were recoverd. With increasing column temperature from 10 to 20 and 30 "C, even the mobile phase that could elute the copolymers at 10 "C could not elute them. For example, at 1.5%ethanol content in the mobile phase, copolymers I-VI eluted completely from the column at a column temperature of 10 "C, but only copolymers I-IV a t 20 "C and copolymers 1-111 at 30 "C eluted completely. Figure 4 tells us that the copolymers having more MMA component can be eluted with increasing ethanol content in chloroform or with decreasing column temperature. Elution volumes of these copolymers were all about 0.5 mL as in Figure 2. Similarly to chloroform, DCE can also elute the copolymers of the entire range of composition (copolymers I-IX) with increasing ethanol content in DCE. Figure 5 summarizes the results. The difference was in the ethanol content in chloroform. DCE requires more ethanol than chloroform in order to elute the copolymer of the same composition. For example, in order to elute copolymer V at a column temperature of 10 "C, chloroform required 1.5% ethanol and DCE needed 2.5%. Similarly, the 95.5% DCE/ethanol mobile phase could elute copolymers I-IX. Linear Gradient Elution for Separation of the Copolymers. In the present report, it becomes clear that the elution of the copolymer from the column could be controlled by regulating column temperature and ethanol content in the mobile phase. However, the elution position was unchanged and all the copolymers eluted at the same retention volume. As a next step, linear gradient elution has been attempted for separating the copolymers according to composition. The examples for the separation are shown in Figure 6. Four
V
VI
3
4
5
6
7 , VRlrnl
Figure 8. LAC chromatograms of P(S-MMA) copolymers obtained by the linear gradient elution method: column temperature (a) 80 O C , (b) 30 OC; sample (a) 0.013% each, (b) 0.017% each; injection volume, 100 pL; for other conditions, see text.
copolymers (111-VI) could be separated into four peaks, the first peak being copolymer 111 and the last being copolymer VI (Figure 6a). By decreasing column temperature from 80 " C to 30 OC, copolymers containing more MMA such as copolymers VI1 and VI11 could be separated under the same gradient condition (Figure 6b). This method is more improved than the previous method for separation of the copolymers. By controlling the content of ethanol in chloroform used as the mobile phase and the column temperature, improved and optimal chromatography of copolymer mixtures could be obtained. Discussion of Operational Variables. In adsorption chromatography with many eluents, an increase in column temperature results in a decrease of retention. Thermodynamics can predict the retention of a solute from the enthalpy change (AZf) and the entropy change (AS) as In K = -AH/RT
+ AS/R
where K is a partition coefficient and the sign of AH is usually
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
Table I. Composition and Molecular Weight Averages of Fractions of Copolymer V
%
styrene content, mol %
fiw
fi"
1 2
63 37
50.5 47.7
1.13 x 105 1.60 x 105
4.5 x 104 7.0 x 104
native
100
49.1
1.21
lo5
6.8 X lo4
fraction no.
obtained,
mol w t av
X
negative. In the case where the retention volume of a solute increases with an increase in column temperature, AH should be positive. Adsorption of several types of polymers (e.g., poly(ethy1ene glycol), poly(viny1 acetate), and poly(ethy1ene terephthalate)) on the surface of silica gel has been shown to increase with increasing column temperature (8). in these cases, AH must be positive. The conformation of polymers in solution also changes with temperature, and the entropy change may contribute to the increase of K with increasing column temperature to some extent. However, in these cases, the change of K with temperature results in the change of retention volume. In our results, retention volumes of the sample copolymers were unchanged with temperature. The elution of the copolymers was very simple. The copolymers either eluted from the column a t the exclusion limit, which is proportional to the interstitial volume in the column, or they did not; that is, the adsorption isotherm is on the Y axis when the copolymers eluted or is on the X axis when they were retained in the column. Therefore, we have to consider not the change of enthalpy or entropy, but other mechanisms as will be discussed later in detail. In the present work, copolymers I-IX could be eluted from the column by controlling the ethanol content in chloroform or DCE used as the mobile phases. In the previous report (7), we showed that copolymers I-IV could elute from the column with chloroform, but not with DCE. In the report, chloroform included 1%ethanol. In the present report, no elution of copolymers with pure chloroform that did not include ethanol was observed. Therefore, it would be clear that the elution of the same copolymers with chloroform in the previous report was mainly due to the existence of ethanol in chloroform. In Figure 3, it can be seen that only 63% of copolymer V eluted from the column with the 99.01.0 chloroform/ethanol mobile phase. In order to know the difference of composition between fractions eluted from (fraction 1) and adsorbed in (fraction 2) the column, compositions and molecular weight averages of both fractions were determined and the results are listed in Table I. Molecular weight averages, A?&., (weight average) and (number average), were determined by injecting fractions into a size exclusion chromatograph and by using a calibration curve obtained with polystyrene. Therefore, these molecular weight averages are polystyrene equivalent molecular weight. The adsorbed part was collected with the 98.0-2.0 chloroform/ethanol mobile phase. Though the copolymers used here are supposed to have narrow distributions of compositions, the results indicate that there is somewhat compositional heterogeneity in copolymer V and that the partial elution with the specified mobile phase results from composition difference. The fraction having the larger MMA content has higher molecular weight averages as in the previous paper (7). Ethanol content in the environment of the copolymers in solution seems to play an important role in the elution behavior. Solvents in which samples were dissolved were all identical to the mobile phases in the present work, and data listed in Figures 2, 4, and 5 were thus obtained. If samples were dissolved in pure DCE or chloroform and the mobile phase at first was the same solvent, then the ethanol content
93
in the mobile phase must be increased to a value other than that listed in the table in order to obtain the same separation. Both chloroform and DCE are good solvents for PS, PMMA, and P(S-MMA), but ethanol is a nonsolvent for them. Therefore, elution of the copolymers from the column with increasing ethanol content in the mobile phase is not because of the increase in solubility of the copolymer in the mobile phase, but is due to other mechanisms. Carbonyl groups in MMA will hydrogen bond to silanol groups on the silica gel surface, and consequently, both PMMA and P(S-MMA) will be adsorbed on the surface of silica gel and neither chloroform nor DCE can displace the solutes. Glockner pointed out that adsorbed polymers cannot be removed from the surface by dilution, though the adsorption is reversible, and that the desorption of polymers will easily advance if a competing substance with a higher level of adsorption energy is added (9). Ethanol might fill the role of the competing substance. He also stated that polymers are adsorbed with no more than 1%of their repeating units. The sequence of styrene and MMA in the copolymer P(S-MMA) used in this work was random. Therefore, it might be possible to assume that the population of carbonyl groups in the segment that contacts the surface of silica gel is proportional to the MMA content in the copolymers. Consequently, the copolymers having a larger MMA content tend to adsorb on the surface of silica gel and be retained in the column. The ratio of ethanol in the mobile and the stationary phases is fmed by specifying column temperature. Free silanol groups on the surface of silica gel decrease in porportion to ethanol content in the mobile phase, since ethanol in the stationary phase is regarded as forming hydrogen bonds to silanol groups on the surface of silica gel. Consequently, with increasing ethanol content in the mobile phase, silanol groups that can form hydrogen bonds to carbonyl groups of MMA decrease and the copolymers having a larger MMA content tend to elute from the column. As shown in Figures 4 and 5, the copolymer of a given composition elutes from the column with the chloroform (or DCE)/ethanol mobile phase above a certain amount of ethanol and is retained in the column below that value. The less MMA content in the copolymers, the less ethanol in the mobile phase is required in order to elute the copolymers from the column. Adsorption will occur above a certain value of either the population of free silanol groups on the surface of silica gel or the population of carbonyl groups of the segment of the copolymers contacting the surface of silica gel. A critical content of ethanol in the mobile phase will exist for a copolymer of a certain content at the specified temperature, and a critical composition of the copolymer will also exist for the mobile phase of a certain composition. Figures 4 and 5 display these critical points. At elevated temperature, ethanol included as a moderator in chloroform (or DCE) is desorbed from the silica gel stationary phase and removed from the column (IO). Consequently, the ratio of ethanol in the stationary phase to that in the mobile phase decreases with elevating column temperature, and free silanol groups on the surface of silica gel will increase. As a result, the stationary phase is apt to adsorb more MMA. Therefore, an increase in adsorption of the copolymers with increasing column temperature results from the change of ethanol content in the stationary phase. Registry No. (S)(MMA) (copolymer), 25034-86-0.
LITERATURE CITED (1) Teramachi. S.; Hasegawa. A.; Shima, Y.; Akatsuka, M.; Nakajima, M. Macromokcufss 1979. 12, 992-996. (2) Glockner, 0.;van den Berg, J. H. M.; MeiJerink, N. L. J.; Schoke, 1.0.; Koningsveld, R. Macromolecules 1984. 17, 962-967. (3) Sato. H.; Takeuchi, H.; Suzuki, S.; Tanaka, Y. Mkromol. Chem., RapM Commun. 1984, 5 , 719-722.
94
Anal. Chem. 1987, 5 9 , 94-97
(4) Danielewicz, M.; Kubin, M. J . Appl. Polym. Scl. 1981, 26, 951-956. (5) Tanaka, T.; Omoto, M.; Donkai, N.; Inagakl, H. J . Macromol. Scl. PhyS. 1980, 617,211-228. (6) Bake, S. T. Sep. Purif. Methods 1962, I f , 1-28. (7) Morl, S.; Uno, Y.; Suzuki, M. Anal. Chem. 1986, 58, 303-307. (8)Glockner, G. Polymercharakterislerung durch Fluss~~schromatographle ; VEB Deutscher Verlag der Wisssenschaften: Berlin, 1981; pp 91-92.
(9) Glockner. G. I n New Approaches in LlquM Chromatography; H., Kalasz, Ed.; Elsevier: Amsterdam, 1984; pp 23-33. (10) Engeihardt, H.; Elgass, H. In Hlgh-PerformanceLquM Chromatography, Advances and Perspectlve; Horvbth, C., Ed.; Academic Press: New York, 1980; Vol. 2, pp 57-1 11.
RECEIVED for review April 15,1986. Accepted August 1,1986.
Prediction of Kovats Retention Index of Saturated Alcohols on Stationary Phases of Different Polarity Jenaro Bermejo* and Maria D. Guill6n
Znstituto Nacional del CarbBn y sus Deriuados, C.S.I.C., A p . 73, 33080 Oviedo, Spain
Gas chromatographic retenth Indexes of saturated alcohols on statlonary phases of Merent polarity are related to m e of thek topological and physlcochemlcal properties by means of multiilnear regression analysis. The solutes are linear, branched, and cyclic alcohols with thelr functlonal group on a primary, secondary, or tertiary carbon atom. Very close retatlonshlps are found, which can be utiHzed for the predlctlon of retention Indexes of saturated alcohols on Stationary phases of any polarity. Other equatbm obtained with a m a l number of representatlve alcohols are also suitable for calculating accurate retenth Indexes of the 26 tested alcohols.
Alcohols constitute a family of compounds usually present in liquids with biological interest. The analysis of mixtures of these compounds is mainly carried out by gas chromatography, making use of their specific retention parameters which are related to their physicochemical and topological properties. The search for these relationships with the aim of predicting the chromatographic retention of alcohols is an interesting subject to study. Accurate prediction of Kovlta retention index, Z, of aliphatic and aromatic hydrocarbons on low-polarity stationary phases has been achieved by using empirical equations that relate the retention index of the solutes to their boiling point and molar refraction (1-4).recently, Rohrbaugh and Jurs (5) have studied the relationships between I and different properties of olefins, obtaining the best correlations when the aforementioned empirical equations (1,4) were used. The molar refraction and the molecular polarizability are related to London's dispersive forces (6) and are a quantification of the ease with which a dipolar system undergoes electronic distortion in the presence of an external field to give an induced dipole moment (7). Therefore, in these equations the molar refraction accounts for the dispersive solute-stationary-phae interactions, which are the main interactions when low-polarity solutes are chromatographed on low-polarity stationary phases. Obviously, interactions between alcohols and any stationary phase have to be much more complex because alcohols have higher dipole moments than hydrocarbons and present a clearly defined autoassociation. Therefore, it is necessary to find out the properties of the alcohols that are able to account for all their chromatographic interactions. In this paper, we report multiparameter equations obtained by linear regression analysis, which closely relate the I values of 26 alcohols on
five stationary phases of different polarity to some of their physicochemical and topological properties. Linear, branched, and cyclic alcohols containing the hydroxyl group on primary, secondary, and tertiary carbon atoms are used. The validity of these equations for predicting Z of alcohols is also studied.
EXPERIMENTAL SECTION Retention Indexes. The I data at 120 "C of the 26 alcohols on the five stationary phases utilized in this work were those reported by Mcbynolds (8). The alcohols used,which range from one to eight carbon atoms, are shown in Table I. The polarity of the five stationary phases ranges from 217 to 2587 in the McReynolds polarity scale (9), and they are shown in Table 11. Physicochemicaland Topological Properties. Some of the physicochemical properties used in this study, such as boiling refractive index, n ~ and , density, d, were taken from point, Tb, literature (IO). Other properties were obtained as follows. The molar refraction, RM, was calculated from the LorentzLorentz expression
where M is the molecular weight of the compound. The molar refraction is also related to molecular polarizability, a,by the expression RM = 4 ~ N * c ~ / 3 (2) where N A is Avogadro's number. Also,the two factors of eq 1,the function of the refractive index D = (nD2- l)/(nD2+ 2), and the molar volume V , = M / d , were utilized separately in the regression analysis. The van der Waals volume, V,, is the volume occupied by the molecule when the atoms are considered to be hard spheres with van der Waals radii. They were calculated by Bondi's method (11).
The first-order connectivity is defined as the sum over all the edges in the graph weighted by the reciprocal square root valencies (12) as follows: Ne
lX
=
E(&.)-112 I J 3
(3)
8=1
where Ne is the number of edges in the graph, and di and 6, are the valencies on vertices v i and uj, respectively. The valencies di and 6, are the number of non-hydrogen atoms connected to the carbon atom. The hydroxyl functionality was assigned the valency 1 (13),and the first-order connectivity of the cyclic alcohols was calculated by subtracting a constant of 0.5 for the presence of the ring (12). The Gasteiger-Hutchingsconnectivity number, N , ( 1 4 , 1 5 ) ,was calculated by counting bonds starting from the atom of interest 0 1986 American Chemical Society
