297 1
Anal. Chem. 1985, 57, 2971-2978
ketone, 108-10-1;ethyl isobutyl ketone, 623-56-3; allylacetone, 109-49-9; 3-octanone, 106-68-3; 2-decanone, 693-54-9; cyclohexanone, 108-94-1;l,l,l-trifluoracetone, 421-50-1;hexachloroacetone, 116-16-5;acetophenone, 98-86-2; 4-fluoroacetophenone, 403-42-9; 2-aminoacetophenone,613-89-8;benzyl methyl ketone, 103-79-7: benzvlacetone, 2550-26-7: benzovhenone, 119-61-9; benzoin, 119-53-9; 2-acetylpyridine, 1122-62-9; 2-pyrrolidone, 616-45-5;N-methyl-2-pyrrolidone, 872-50-4;diacetyl, 431-03-8; acetylacetone, 123-54-6; 2,5-hexanedione, 110-13-4; 1,2-cyclohexanedione, 765-87-7; benzoylacetone, 93-91-4;benzil, 134-81-6; dibenzoylmethane,120-46-7;pyruvic acid, 127-17-3;levulinic acid, 123-76-2;ethyl acetoacetate, 141-97-9;ethyl levulinate, 539-88-8; ethyl benzoylacetate, 94-02-0; acetaldehyde, 75-07-0; propionaldehyde, 123-38-6; n-butyraldehyde, 123-72-8;crotonaldehyde, 4170-30-3;caprylaldehyde,-l24-13-0; chloral, 75-87-6;paraldehyde, 123-63-7;cyclohexane carbaldehyde, 2043-61-0; diphenyl acetaldehyde,947-91-1;benzaldehyde, 100-52-7;2-bromobenzaldehyde,
6630-33-7;salicylaldehyde, 90-02-8.
LITERATURE CITED (1) Scholz, E. “Karl Fischer Titration”; Springer-Verlag: New York, 1984; English and German edition, 140 pp. (2) Eberius, E. “Wasserbestlmmung mit Karl-Fischer-Losung”, 2nd ed.; Verlag Chemie: Weinheim, 1958. (3) Fischer, F.; Schiene, R. Z.Chem. (Leiprig)1964, 4, 69-70. (4) Kllmova, V. A.; Sherman, F. B.; L’vov. A. M. Bull. Acad. Sci. USSR, Div. Chern. Sci. Engl. Transl.) 1967, 2477-2479. (5) Mitsubishi Chemical Industries, German Patent Application DE 3040474 (1980). (6) Mitchell, J., Jr.; Smith, D. M. “Aquametry”, Part 111, 2nd ed.; Wiley: New York, 1980. (7) Hydranal Composite 5K, leaflet from Riedel de Haen AG, D-3016 Seelze.
RECEIVED for review February 4, 1985. Resubmitted May 20, 1985. Accepted July 19, 1985.
Study of Retention Processes in Reversed-Phase High-Performance Liquid Chromatography by the Use of the Solvatochromic Comparison Method Paul C. Sadek and Peter W. Carr*
Department of Chemistry, Kolthoff and Smith Halls, University of Minnesota, 207 Pleasant Street, Minneapolis, Minnesota 55455 Ruth M. Doherty and Mortimer J. Kamlet
Naval Surface Weapons Center, White Oak Laboratory, Silver Spring, Maryland 20903-5000 Robert W. Taft
Department of Chemistry, University of California, Irvine, California 92717 Michael H. Abraham
Department of Chemistry, University of Surrey, Guildford, Surrey GU2 5XH, United Kingdom
The retentlon of some 2 dozen nonslianophlllc solutes on reversed-phase bonded phase columns has been correlated by the solvatochromic comparlson method. I n accord with other related studies, the most Important solute parameters, whlch influence retentlon, are Its slze and hydrogen bond basicity but not hydrogen bond acidlty. The solute’s dlpolarlty Is a minor but stlll slgniflcant factor. Retention as a function of the nature of the bonded phase (hydrocarbon vs. fluorocarbon), mobile phase modifier (methanol, acetonltriie, tetrahydrofuran), and composltlon (70130 methanol-water to 30/70 methanol-water) has been studied using ilterature and original data as the basls set for the correlations. The resutts obtained show the best flts of any h e a r solvatlon energy relationship thus far observed In correlatlng many different data with soivatochromic parameters.
Almost since its inception, there have been many attempts to relate retention in bonded phase reversed-phase chromatography to molecular properties. Horvath and his co-workers have related solute capacity factors to the nonpolar surface area of the solute ( I ) . Tomlinson and co-workers have demonstrated linear relationships between octanol/water partition
coefficients, water solubility, and reversed-phase capacity factors (2, 3). In addition, both the Horvath group and the Tomlinson groups have developed schemes for prediction of solute retention based on substituent effects on the retention of parent compounds (4,5). Karger et al. showed that in a closely related set of solutes the molecular connectivity, which is linearly correlated with molecular surface area, is a useful parameter (6). Many groups have indicated the existence of an approximate linear relationship between the logarithmic capacity factor and the carbon number in a series of homologous compounds (7). Retention in bonded phase chromatography is a complex issue due, at least in part, to two distinct problems. First, the stationary phase is intrinsically heterogeneous in that it is not possible to achieve 100% coverage of all surface silanol groups; typically, only about half of the approximately 8 wmol/m2 of surface groups can be reacted with a silane to form the bonded phase (8). Thus retention can come about via interactions both with the bonded material and with silanol groups. The second complication lies in the selective adsorption of one or more of the mobile phase constituents into the bonded phase and the concomitant variation in the phase ratio. Several groups have shown that the column dead volume depends upon the relative amounts of water and organic modifier in
0003-2700/85/0357-2971$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
the mobile phase (8,9).As a result, the composition of the stationary phase is not precisely defined, which leads to difficulties in the quantitative interpretation of retention data. Recently there has been considerable interest in establishing a retention index scale for reversed-phase chromatography (10) and in the development of techniques for comparing different stationary phases (11). The present study is aimed a t the application of the solvatochromic comparison method to the ehcidation of the chemical factors involved in retention in bonded reversed-phase chromatography. Previously one of us used the solvatochromic comparison method to explore the gas chromatographic retention of dipolar and nondipolar solutes on a series of closely related polymeric stationary phases (12). It was shown in that work that the free energy of transfer, after correction for FloryHuggins entropy of mixing, was a linear function of the r* solvatochromic parameter of the solvent. Further, we were able to demonstrate that the Si-0-Si backbone of the polymeric liquid could interact selectively with hydrogen bond donor solutes. Solvatochromism is a well-established tool of physical organic chemistry that has recently been used to study and correlate factors involved in solvation of ground states, activated complexes, and equilibrium constants of reactions. Its use in the present context is related, in a general way, to the fluorescent probe studies that Lochmuller and co-workers (13) and Callis et al. (14) used to characterize the solute’s environment via the effect of solvent on a spectroscopic property of the solute. This paper reports the first correlation of HPLC retention properties with fundamental dipolarity/polarizability and hydrogen bonding properties of the solutes and mobile phases. We use the solvatochromic parameters and the methodology associated with linear solvation energy relationships (15-1 7) to deconvolve, evaluate, and rationalize the multiple interaction effects that influence HPLC capacity factors. In recent papers we pointed out that solubility properties of organic electrolytes (18)and nonelectrolytes (19-22) in a variety of different type solvents, including water, are well correlated by equations which include terms that encompass the endoergic (disfavorable) cavity forming process and the opposing exoergic (favorable) effects of solute/solvent dipolar and hydrogen bonding interactions. Using the convention that subscript 1 applies to the solvent and subscript 2 to the solute, we pointed out that the solute property which influences the cavity term is V2,the solute molar volume, taken here as its molecular weight divided by its liquid density at 20 “C; the complementary solvent property is (6H2)1, the square of the Hildebrand solubility parameter (23). The dipolar term depends on r*land T * ~r* , being the solvatochromicparameter that scales solute and solvent dipolarity/polarizabilites (15). For hydrogen bond acceptor (HBA) solutes in hydrogen bond donor (HBD) solvents like water and the alcohols, the hydrogen bonding term depends on a1and Pz, where a and P are the solvatochromic parameters that measure HBD acidities and HBA basicities. Accordingly, for solubility properties, denoted as SP of a set of HBA solutes in a single HBD solvent, and for distributions between pairs of solvents, correlation equations were expressed in terms of solvatochromic parameters of the solutes and took the form of eq 1. We use v2/100 so that the cavity SP = SP, mV2,/100 + ST*^ bP2 (1)
+
+
term should cover roughly the same range as the terms in r*, a, and P, which facilitates the comparison of the relative influences of the contributing terms on SP. In the context of eq 1 and reversed-phase liquid chromatography, the coefficients m, s, and b are related to the chemical nature of the mobile and stationary phases, and v2,R * ~ and , PZ characterize the solute.
Thus, we reported (21) that when eq 1 is applied to solubility properties of 93 liquid aliphatic nonhydroxylic and hydroxylic solutes in water, eq 2 results. S, in eq 2 is the log S, = log (Sg/Kg,) = 0.55 - 3.36v2/100+ 0 . 4 6 ~ * 2 5-23/32 (2)
+
n = 93, r = 0.9943, std dev = 0.144 molar solubility in water, S, is the solute molar concentration in its saturated vapor (S, = P,,,/24.5), and Kgwis the gas/ water partition coefficient, all at 25 “C. In a similar vein, we reported (21,22) that octanol/water partition coefficients, KO,, of 102 aliphatic and aromatic, hydroxylic and nonhydroxylic solutes were well correlated by eq 3. To include aromatic log KO, = 0.20 + 2.74V2/1OO - 0 . 9 2 ~ * 2 3.4982 (3)
n = 102, r = 0.989, std dev = 0.17 and aliphatic solutes in the same correlation equation, it was necessary to set forth a number of “ground rules”, which will also be applied in the correlations discussed here. These “ground rules” are as follows (20): (a) An increment of 10 cm3/mol is added to for each aromatic or alicyclic ring in a molecule. (b) In place of r* we use (a* + d6) for aromatic solutes and aliphatics that have more than one chlorine on the same carbon atom. Assuming minimal polarizability contributions to the solubility properties studied, we assume d = -0.40. The 6 parameter is 0.0 for nonchlorinated aliphatic solutes, 0.5 for polychlorinated aliphatics, and 1.0 for aromatic compounds. (c) Where the main HBA site is an aromatic ring or a substituent conjugated with an aromatic ring, a known or estimated (r*- 0.406) value for the aromatic moiety is used; where the main HBA site is separated from the ring by at least one methylene group, a r* value is estimated for the aliphatic moiety. (d) A 6 value of 0.10 is assigned to aromatic rings to which electron donor substituents are attached (e.g., anisole), or which are separated from the main HBA sites by at least one methylene group. (e) For solutes with multiple HBA sites, the summation of 6 values is used. (f) For hydroxylic solutes we use r*, and p, values (i.e., a* and /3 values for the alcohols in their non-self-associated “monomer” forms); a*, values for all alkanols and arylalkanols are 0.40; 6, values are 0.40 for methanol, 0.45 for all primary alkanols, 0.51 for all secondary alkanols, and 0.57 for all tertiary alkanols. For purposes of the present discussion, we use the new amphiprotic hydrogen bonding parameters, o,and p, interchangeably (as will be discussed in a future paper).
v2
RESULTS AND DISCUSSION Since reversed-phase high-performance liquid chromatography (RP-HPLC) involves distribution of a solute between two phases (mobile and stationary), as does octanol/water partition, it seemed a reasonable next step to ascertain whether HPLC properties such as capacity factors or retention indexes could be similarly correlated by a general equation of the form of eq 1. A convenient set of data for analysis are those recently reported by Haki and Young (24), who determined capacity factors, k’,for 68 compounds using an unmodified commercial octadecylsilane column, and a mobile phase consisting of 55% methanol and 45% aqueous ammonium phosphate buffer. The purpose of the Haki-Young paper had been to demonstrate that the HPLC capacity factors were linear with Hansch-Leo octanol/water partition coefficients (25) and could therefore be used as a tool for prediction of log Kow. Solvatochromicparameters are known or can be estimated for 29 of the solutes in the Haki-Young study. Note that the data set was not edited in any other fashion; however, the data base of Haki and Young as well as the others reported below do not contain aliphatic or aromatic amines which are strong
ANALYTICAL CHEMISTRY, VOL. 57,NO. 14, DECEMBER 1985 2973
Table I. Data Used in Correlation of Experimental Capacity Factors on Octadecylsilane Column (Mobile Phase 55/45 MeOH/H20 Buffer) no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
solute C6H6 C6H5CH3
CnHnBr
CH2=CHCH20H C6H5CH=CHCH20H
9/ looo 0.989 1.163 1.150 1.118 1.328 1.333 1.186 1.524 1.253 1.120 1.294 1.364 1.494 1.530 1.351 1.363 1.524 1.269 1.429 0.805 0.624 0.897 0.978 1.146 1.235 0.896 1.138 0.680 1.385
+b7c
0.19 0.14 0.39 0.31 0.07 0.03 0.33 (0.25) 0.75‘ 0.50 (0.45) 0.29 (0.03) 0.34 (0.36) 0.52e 0.53e 0.50 (0.48) 0.38 0.62 0.23 0.55 0.47 (0.67) (0.75) (0.45)g (0.45)g (0.45)g
p”
log k’
calcd eq 4b
log KO,
0.10 0.11 0.06 0.07 0.13 0.13 0.32d (0.32)d 0.4Sd 0.38 (0.35) 0.30d (0.13) 0.41 0.39 (0.51)d (0.54)d 0.48 0.43 0.ld 0.16 0.10 0.45 0.42 (0.50) (0.25) (0.55)d8 (0.45)s (0.55)dJ
0.83 1.16 1.22 1.14 1.48 1.48 0.80 1.44 0.32 0.36 0.64 1.06 1.78 1.07 0.79 0.48 0.75 0.45 0.75 0.56 0.27 (1.05)h 0.09 0.42 0.29 0.09 0.15 -0.35 0.53
0.94 1.22 1.16 1.13 1.47 1.50 0.73 1.29 0.29 0.42 0.77 1.07 1.74 1.08 0.83 0.53 0.71 0.45 0.81 0.56 0.15 (0.79)h 0.04 0.40 0.27 0.20 0.14 -0.36 0.52
2.01 2.74 2.99 2.49 3.20 3.15 2.08 3.18 1.56 1.56 1.95 2.51 3.62 2.64 2.18 1.49 1.96 1.66 2.20 1.94 1.25 2.29 0.34 1.50 1.38 0.69 1.16 0.17 1.95
0.10 is added to V/lOO of aromatic compounds. Values are for (n* - 0.406). Values in parentheses are estimated from corresponding values for closely related compounds. dSummationof p values at multiple sites (Le., 0.10 added to p for hydrogen bonding to aromatic ring) (18). ‘The ?r* value is that estimated for the aliphatic moiety. fWhile we assign values of 0.10 w chloroaliphatic solutes in aqueous solvents, we continue to use p = 0.00 for chloroaliphatic solvents. gPrn values are used for alcohols. hObviouslyout-of-linepoint excluded from correlation leading to ea 4b. ‘Data of Haki and Young (24).
silanophiles. The V2/100, T * ~and , P2 (or T * ~and , ~ values for these solutes are assembled in Table I, together with the log k values and corresponding log octanol/ water partition coefficients. Through the use of the appropriate parameter estimation rules and eq 1,the correlation for these 29 solutes is given by eq 4a. If the single obvious outlier, trichloroethylene (see Table I and Figure l),is excluded, the correlation is given by eq 4b. log lz’ = (-0.16 f 0.11) (1.47 f O.O8)V2;/100 (0.61 f 0.12)~*2- (1.97 f O.14)Pz
+
n = 29, r = 0.9877, std dev = 0.083 log k ’ = (-0.27 f 0.09)
(44
+
(1.53 f 0.07)~2/100(0.54 f 0.10)~*2- (1.97 f 0.12)/32
n = 28, r = 0.9921, std dev = 0.067 (4b) It is seen in eq 4a and 4b, as might be expected from a priori
v2)
considerations, that increasing solute size ( causes an increase in retention @’), i.e., free energy concepts favor solute transfer from the more cohesive mobile phase to the less cohesive stationary phase. Opposing this effect, increases in both solute dipolarity/polarizability ( T * ~and ) HBA basicity (&) lead to lower b’ values because the solutes have increased affinities for the more highly dipolar and hydrogen bond donating aqueous mobile phase. A plot of eq 4b is shown in Figure 1. Thus the equations show that, as with solubility in water and partitioning between octanol and water, the major factors influencing the current (and, as will be shown, most) reversed-phase HPLC properties are the opposing cavity (mv2/100) and hydrogen bonding (b&) terms, with the dipolar term ( s ~ * exerting ~) only a minor order influence. It should
-0 5
1.5
0.5
2.5
Log ( kbxJ Figure 1. Plot of calculated capacity factor vs. experimental capacity factor. Experimental data are from ref 24. Calculated values were obtained from eq 4b. The circled data point corresponds to trichloroethylene.
be noted that the data set does not include any stronger HBD solutes than the alkanols (a, = ca. 0.3), so that the dependence on HBD acidity remains uncertain. Since Haki and Young correlated log k’with log KO,,it is of interest to compare the coefficients of v2/100, T * ~and , PZ in eq 3 and 4b. In eq 3 and m, -s, and -b values are in the ratio 1.00/0.30/1.29; in eq 4b the ratio is 1.00/0.35/1.29. It is as a consequence of these very similar ratios of the contributing terms that the correlation of log k’with log KO,for the same data set as was used for eq 4a (but excluding an
2974
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
obviously out-of-line point for ethyl acetate) has a highly acceptable correlation coefficient of 0.985 and a standard deviation of 0.14 log KO,units. These results lend credence to the suggestion that HPLC capacity factors might be used to estimate log KO,values, at least for solutes of the classes described herein. One should note that in liquid/liquid or bonded phase chromatographic studies, the m, s, and b coefficients in eq 1 are related to differences in properties of the mobile and stationary phases, whereas in gas-solid chromatographic and gas-liquid partitioning studies these coefficients will characterize the stationary phase per se. Characterization of Stationary Phases by Means of the Solvatochromic Equations. The success of the above correlations led us next to consider whether total solvatochromic equations for capacity factors, of the form of eq 1, might not serve to characterize those stationary phases in terms of the parameters that most strongly influence retention properties. This necessarily requires the test solutes and the mobile phase to be the same, while only the stationary phase is changed. A useful set of data to test this possibility are capacity factors reported by Smith (10) (see Tables I1 and 111) for nine test solutes and three mobile phases on eight octadecyl bonded stationary phases. Considering first the experimental data in Table 11, it is seen that there are few changes in elution sequence for this set of eight columns. n-Butyl phenyl ketone is the most strongly retained and 2-phenylethanol the least strongly retained on all columns. The number in parentheses next to each log Iz’value indicates the numerical order of elution (1 is last, 9 is first). Overall, there are some 16 changes in sequence compared to the elution order on column HI. The data in Table I1 are for a 70/30 methanol/water mobile phase. Also assembled in the table are the input solvatochromic parameters, and the SPo,m, s, and b terms in the correlations by eq 1, as well as the r (correlation coefficient) and std dev (standard deviation) measures of statistical goodness of fit. It is seen that, despite the far from optimal set of solutes ( p values in this set range from 0.10 to 0.55; in an optimal data set the range would be from 0.00 to 1.05), the measures of statistical goodness of fit are even better than for the Haki-Young results (eq 4b); r values range from 0.9872 to 0.9950, and standard deviation values from 0.030 to 0.053 log units. In terms of standard deviations, these are among the most precise correlations which we have yet encountered in our linear solvation energy relationship studies. It is also seen that the coefficients of the independent variables all have chemically reasonable signs and that the major factors influencing log k ’are again the endoergic cavity terms and the exoergic hydrogen bonding terms. Although the excellent fit is encouraging, it deserves mention that it could be related to the fact that all of the solutes in Smith’s data set (and in our data set discussed below) are nonsilanophilicaromatic compounds. In contrast, most previous studies had involved data sets containing aliphatic, polyhaloaliphatic, and aromatic solutes. Thus, the improved validity of the fit relative to other studies may be a consequence of the relative uniformity of the soluted employed. As with the Haki and Young 55/45 MeOH/H20 data set, the results for Smith‘s 70/30 MeOH/H20 data set indicate that the s / m and b / m ratios are approximately -0.3 and -1.2, respectively. Recent work by Snyder et al. (11,261supports the idea that the stationary phase strength of reversed-phase columns may be adequately characterized by one or perhaps two parameters, In this work, we find that two major parameters ( m and b) and a distinctly less important parameter (s) are needed to characterize the retention of solutes in re-
i
m
8
m
m
8 i
m
8 rm
8 hhhhhhhhh
mw m w w t - m m m w i m t-wmmmmclrio
4 cl (0 u5 t- 00 w vvvw w w w v
000000000
ol“
Q
+
ri
m
8
+
5
m
8
E
+
a“ m II
a
0
8 0
M 3
m
8
i i i i i r i r i r l i
Id
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
2975
Table 111. Correlation Equations for Capacity Factors Reported by Smith (IO) log k' = SPo
+ m ~ z / l O O+ ST*^ + boz
mobile phase
column"
SPO
m
S
70130 MeOH/HzO
Hl HZ H3 L
-0.46 -0.53 -0.56 -0.43 -0.54 -0.18 -0.44 -0.78 -0.49 (0.17) -0.20 -0.31 -0.44 -0.40 -0.48 -0.41 -0.36 -0.47 -0.38 (0.09) -0.31 -0.12 -0.24 0.00 -0.17 -0.16 -0.15 -0.21 -0.17 (0.10)
1.03 0.94 1.06 1.13 1.25 0.83 1.21 0.92 1.05 (0.15) 1.31 1.21 1.34 1.28 1.20 1.20 1.40 0.78 1.21 (0.19) 1.32 1.07 1.20 1.11 1.15 1.12 1.32 0.95 1.15 (0.13)
-0.37 -0.31 -0.45 -0.31 -0.12 -0.17 -0.28 -0.03 -0.27 (0.15) -0.02 0.04 -0.03 -0.02 -0.56 0.05 -0.03 0.11 -0.002 (0.04) -0.15 -0.03 -0.02 0.00 0.08 0.00 -0.02 0.20 -0.008 (0.10)
S
T Z
P av (A)
50/50 MeCN/H,O
HI HZ H3 L S
T Z
P
av f
S T Z P av f
s/m -0.36 -0.33 -0.42 -0.27 -0.10 -0.20 -0.23 -0.03 -0.015 0.033 -0.022 -0.016 -0.470 0.042 -0.021 0.141 -0.11 -0.028 -0.017 0.00 0.069 0.00 -0.015 0.21
b
blm
R
std dev
-1.30 -1.19 -1.41 -1.56 -1.42 -1.18 -1.62 -0.99 -1.33 (0.21) -1.92 -1.76 -1.98 -1.87 -1.12 -1.77 -2.03 -1.07 -1.69 (0.38) -2.34 -1.79 -2.09 -2.09 -1.98 -1.93 -2.39 -1.71 -2.04 (0.24)
-1.26 -1.27 -1.33 -1.38 -1.14 -1.42 -1.34 -1.08
0.9934 0.9933 0.9936 0.9950 0.9927 0.9907 0.9872 0.9900
0.033 0.030 0.035 0.031 0.037 0.031 0.053 0.031
-1.47 -1.45 -1.48 -1.46 -0.93 -1.48 -1.45 -1.37
0.9886b 0.9882b 0.9875* 0.9885b 0.9808b 0.9882b 0.9829* 0.9866
0.052 0.048 0.056 0.051 0.063 0.048 0.068 0.031
-1.77 -1.67 -1.74 -1.88 -1.72 -1.72 -1.81 -1.80
0.9910 0.9911 0.9914 0.9923 0.9908 0.9923 0.9913 0.9876
0.053 0.040 0.046 0.042 0.044 0.040 0.052 0.042
" See Table I1 for composition of columns. These r values would typically be 0.992-0.996 if we used a 0value of 0.46 for acetophenone, which is not inconsistent with the 0 values which we have used for the higher alkyl phenyl ketones; the correlations for the other mobile phases would also be improved, albeit to a lesser extent. The results could also accommodate a slightly higher /3 value of 0.30 for nitrobenzene. versed-phase chromatography. By the use of the soluatochromic comparison method, we can state with some certainty that the chief factors involved in solute retention, when aqueous mobile phases are used, are the solute size and hydrogen bond acceptor strength. Solute dipolarity/polarizability is considerably less important, but sometimes still significant. In contrast, in GLC, solute polarizability and related dispersion characteristics are extremely important to retention (12). We infer that the dispersive interactions which the solute undergoes with both the mobile phase liquid and the stationary bonded phase, or in the case of octanol/water partition coefficients, the second liquid phase, are to some extent self-compensating (see below). Smith also examined solute retention on the eight reversed-phase test columns with two additional mobile phases, 50/50 MeCN/H20 and 40/60 THF/H20, and the correlations of these data are compared with the 70/30 MeOH/H20 results in Table 111, where we can examine the effects of changing mobile phase on the parameters of eq 1. The three mobile phases are, according to Smith, virtually equieluotropic for a homologous series of alkyl phenyl ketones (Le., the slopes in plots of log k' vs. C, on the various columns are very similar). This is reflected in the reasonably similar average m values in the solvatochromic equations for the three mobile phases, which suggests that the three mixed solvents have reasonably similar bH values; i.e., the cohesive energy densities of the solvents, which govern the cavity terms, are similar. Interestingly, on the assumption that this parameter varies linearly with solvent composition for mixed solvents, bH values are calculated to be 17.2 for 70/30 MeOH/H,O, 17.5 for 50/50 MeCN/H20, and 17.7 for 40/60 THF/H,O. The net HBD acidities (solvent a values) do differ, however,
-2.4
-2.0
Smith Data Set
-
0 Methanol / H20 0 Acetonitrile / H,O A Tetrahydrofuran /H20
-
n
I
1
0.8
I
I
1.o
I
I
1.2
I
1
1.4
1
1
1.6
m
Correlation of b values against m values: 0 , methanol/ water; 0, acetonitriWwater; A, tetrahydrofuranlwater. The data are given in Table 111 and are based upon the capacity factor from ref Figure 2.
10.
with increasing water content of the mobile phases, and it is in the hydrogen bonding term that we do see significant changes in the solvatochromic equations. Average values of b (exoergic dependencies on solute HBA basicities), as expected, increase from -1.33 in 30% aqueous methanol to -1.67 in 50% aqueous acetonitrile to -2.04 in 60% aqueous tetrahydrofuran. Careful inspection of the data in Table I11 also reveals the existence of a relationship between the m and b coefficients
2976
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
70/30 MeOH/H,O
Table IV. Solvatochromic Parameters Used in Comparison of Hydrodecyl and Heptadecafluorodecyl Bonded Phase Columns no.
solute
V/lOO”
a*b,cg
pc”
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
benzene toluene ethylbenzene n-propylbenzene n-butylbenzene tert-butylbenzene p-xylene fluorobenzene chlorobenzene bromobenzene benzonitrile phenylacetonitrile benzaldehyde acetophenone nitrobenzene o-nitrotoluene benzyl alcohol
0.989 1.163 1.324 1.494 1.661 1.649 1.333 1.039 1.118 1.150 1.120 1.253 1.119 1.269 1.123 1.279 1.138
0.19 0.14 (0.08) (0.04) (0.02) (0.02) 0.03 0.22 0.31 0.39 0.50 0.75d 0.52 0.50 0.61 (0.50) 0.46
0.10 0.11 (0.12) (0.12) (0.12) (0.12) (0.13) (0.07) 0.07 0.06 0.38 0.48e 0.44 0.48 0.25 (0.25) 0.55ef
m = (0.33 f 0.16) - (0.52 f 0.12)b n = 7 , r = 0.893, std dev = 0.064
(5a)
50/50 MeOH/H,O m = (0.15 f 0.04) - (0.60 f 0.02)b n = 7 , r = 0.996, std dev = 0.02 40/60 THF/H,O
(5b)
m = (0.13 f 0.12) - (0.50 f 0.06)b n = 8, r = 0.956, std dev = 0.04
(54
Clearly, these correlations are quite strong and might be taken to suggest that some linear combination of m and b could be used in an operational way to reduce the number of parameters required to describe the systems. This could lead to a single dominant master parameter for reversed-phase chromatography, bonded phase selectivity, i.e., the j factor described by Snyder et al. (11, 26), if, in the light of the correlations discussed below, highly fluorinated nonpolar phases are excluded. Comparisons Involving Decyl and Heptadecafluorodecyl Stationary Phases and Methanol/ Water Mobile Phases. Two of us recently reported the retention characteristics of a series of fluorinated bonded phase columns (27). In that study we compared the retention of some 40 aromatic solutes on a per“hydro”decy1bonded phase column to retention on a highly fluorinated analogue, using 60/40 methanol/water as the mobile phase. We have since expanded the data base to include four additional methanol/water compositions, which gives us the opportunity to compare effects of systematic variations in the mobile phases. The T*,, &, and Vz/lOO values are known for 17 of the solutes and are assembled in Table IV. The log k’values and the results of a series of correlations by eq 1are given in Table V. The same
” game “ground rules” apply as in previous tables; 0.10 is added to V/lOO of aromatic compounds. bValues are for (a* - 0.406). Values in parentheses are estimated from corresponding values for closely related compounds. a* is for aliphatic moiety. e Value is for CP;0.10 added for hydrogen bonding to ring. ’Values are for and p,. 8The greatest covariance among the independent variables is between a* and p. The r value for this covariance is 0.733. For solutes 1-16, the r value is 0.763. for each mobile phase as the stationary phases are varied. These results are shown in Figure 2. Column S appears to be distinctly out of line when methanol or acetonitrile are used as the organic modifiers, which may reflect either a special column characteristic such as substantially different densities of silanol groups or some experimental error. When this datum is eliminated, the following correlations are obtained:
Table V. Capacity Factors and Correlation Equations for Comparison of Hydrodecyl and Heptadecafluorodecyl Bonded Phase Columns log k’
solute 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
70130
hydrodecyl column, MeOH/H,O 60/40 50/50
0.000 0.216 0.395 0.601 0.810 0.686 0.428 0.007 0.201 0.262 -0.386 -0.545 -0.388 -0.325 -0.161 -0.012 -0.565
0.331 0.596 0.831 1.097 1.362 1.199 0.852 0.342 0.592 0.662 -0.076 -0.105 -0.071 -0.006 0.168 0.518 -0.288
0.621 0.940 1.225 1.548 1.860 1.662 1.253 0.661 0.952 1.037 0.210 0.183 0.178 0.267 0.449 0.693 -0.024
40160 0.863 1.213 1.566
1.582 0.904 1.261 1.373 0.484 0.497 0.432 0.556 0.765 1.008 0.200
70130 -0.337 -0.195 -0.069 0.079 0.217 0.178 -0.054 -0.259 -0.244 -0.270 -0.521 -0.650 -0.600 -0.521 -0.450 -0.323 -0.826
log k’= SPo+ mV,/100 -0.81 (0.13) 1.06 (0.09) -0.35 (0.11) -1.49 (0.13) -0.33 -1.41 0.9927 0.057
-0.92 (0.13) 1.48 (0.10) -0.16 (0.12) -1.81 (0.14) -0.11 -1.22 0.9937 0.061
-0.77 (0.15) 1.69 (0.20) -0.29 (0.13) -1.94 (0.15) -0.17 -1.15 0.9942 0.061
-0.95 (0.24) 2.10 (0.21) -0.16 (0.14) -2.16 (0.17) -0.08 -1.03 0.9894 0.073
heptadecafluorodecyl column, MeOH/H,O 60140 50/50 40160 -0.002 0.187 0.360 0.564 0.759 0.685 0.360 0.086 0.135 0.122 -0.223 -0.344 -0.292 -0.182 -0.108 0.053 -0.572
0.255 0.493 0.713 0.968 1.213 1.119 0.719 0.363 0.456 0.447 0.066 -0.037 -0.008 0.084 0.160 0.375 -0.301
0.485 0.773 1.043 1.362 1.672 1.543 1.051 0.637 0.772 0.780 0.367 0.286 0.283 0.430 0.448 0.719 -0.068
-0.90 (0.10) 1.35 (0.08) -0.20 (0.09) -1.36 (0.11) -0.15 -1.01 0.9949 0.047
-1.00 (0.13) 1.69 (0.10) -0.09 (0.11) -1.49 (0.14) -0.05 -0.88 0.9933 0.061
30170 0.728 1.076 1.404 1.610 2.165 2.036 1.467 0.923 1.041 1.093 0.703 0.642 0.581 0.774 0.731 1.070 0.209
+ s a * 2 + b&
-0.98 (0.09) 0.79 (0.07) -0.20 (0.08) -1.01 (0.10) -0.25 -1.28 0.9918 0.042
-0.92 (0.11) 1.09 (0.07) -0.20 (0.09) -1.25 (0.11) -0.18 -1.15 0.9929 0.049
-1.07 (0.18) 2.00 (0.13) -0.06 (0.15) -1.50 (0.19) 0.03 -0.75 0.9906 0.081
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
parameter selection rules ("ground rules") apply as in the earlier correlations. It is possible to compare the results in 70/30 methanol/ water mixtures directly with Smith's results. Despite the use of a rather different data set, the m and b values for the hydrodecyl bonded phase are well within the uncertainty limits of Smith's average values (Table 111) for octadecyl bonded phases. Thus, to a first approximation, the chain length of the bonded hydrocarbon does not have a major effect on the m and b coefficients (the cavity and hydrogen bonding terms) in eq 1. In contrast, the SP, value (intercept) is quite small in comparison to the values for the eight columns studied by Smith (his column P being the only exception). Thus, it is likely that the intercept value (SP,) to a limited extent reflects the amount of bonded phase material on the column. The carbon load of even a highly loaded decyl phase should be substantially less than that of an octadecyl bonded phase. (It may also reflect differences in properties due to variation in surface silanol groups.) Note also that the SPovalues do not differ markedly between the hydrodecyl and heptadecafluorodecyl bonded phases. It is also seen in Table V that the m and b terms for both columns increase regularly with increasing water content between 70j30 and 40160 MeOH/HzO, but that on the heptadecafluorodecylcolumn there is a leveling off of the b value (but not m) between 60 and 70% water content. Indeed, between 30 and 60% water content, the rates of increase are fairly linear with one another as is shown in eq 6 (subscript D indicates decyl, subscript FD heptadecafluorodecyl). rnFD = (-0.16 f 0.09) + (0.88 0.06)rnD (6a) n = 4, r = 0.9960, std dev = 0.042 bFD = (0.06 f 0.06) (0.73 f 0.03)bD (6b)
*
+
n = 4, r = 0.9980, std dev = 0.016 b D = (-0.83 f 0.07) - (0.64 f 0.04)rnD (6~) n = 4, r = 0.9953, std dev = 0.033 bFD = (-0.77 f 0.11) - (0.40 f 0.07)rnF~ (6d) n = 5, r = 0.9466, std dev = 0.075 bFD = (-0.64 f 0.09) - (0.052 f 0.07)rnFD (6e) n = 4, r = 0.9811, std dev = 0.048 This is as expected since (6H2)1, the solvent parameter that influences the cavity term, and al,the solvent parameter that influences the hydrogen bonding term, would also be expected to increase in a regular (but not necessarily linear) manner with increasing water content in the mixed solvent. The latter regular progressions are evidenced by the following free energies reported by Abraham et al. for transfer of chloride ion from methanol to mixed MeOH/HzO solvents (28): v/v%MeOH/H,O
100
aG,"(Cl-), kcal/mol aaG,"(Cl-), kcal/mol
0
70
60
50
40
30
-1.76 -2.11 -2.41 -2.65 -2.82 -0.35 -0.30 -0.24 -0.17
0 -3.15
AG,(Cl-) is another property which would be expected to show an endoergic dependence on solvent and an exoergic dependence on al.It follows from the above that the m and b terms should show good linear regressions with AG,"(Cl-), and this is indeed the case as is seen in eq 7. rnFD
= (-1.23 f 0.25) - (1.11 f O.ll)AG,"(Cl-)
(7a)
n = 5, r = 0.9867, std dev = 0.090 (-0.22 f 0.12) + (0.47 f 0.05)AGt"(C1-) (7b) n = 5, r = 0.9830, std dev = 0.043 The situation is less clear-cut, however, when we compare the relative m and b values for the hydrodecyl and fluorodecyl stationary phases, with the mobile phases being the same. It bFD =
2977
is seen in Table V that the cavity and hydrogen bonding terms are in all instances larger for the hydrodecyl than for the fluorodecyl column. Values of mD/mFD are 1.34 for 30% HzO and 1.24 for 60% HzO. Values of bD/bFD are 1.47 for 30% HzO and 1.45 for 60% HzO. In previous studies we hypothesized that the cavity term depended on the differences between the squares of the Hildebrand solubility parameters of the two phases, eq 8; the hydrogen bonding terms depend on the differences between their HBD acidities, eq 9 (we use subscript M for the mobile phase, S for the stationary phase). As a general rule, fluoro cavity term = K [ ( 6 H 2 ) M hydrogen bonding term = K'(CYM - as)
(9)
compounds have lower cohesive energy densities than corresponding hydrogen compounds; compare, for example, the following 8H values: polyethylene, 7.7-8.2, vs. polytetrafluoroethylene, 5.8-6.4; CH2CIZ,9.7, vs. CF2ClZ,5.5. On this basis we were led to believe that the endoergicity of cavity formation should be greater on the hydrodecyl than on the fluorodecyl bonded phase. Taken with eq 7, this would suggest that the mD values should be greater than the corresponding mFD values, the opposite of which is observed. Our identification of m with the difference in the square of the solubility parameters of the two phases must be considered ad hoc. Indeed, it is quite unrealistic to treat a bonded phase as a bulk liquid. It is likely that rn also encompasses a favorable (exoergic) dispersive interaction between the solute and the two phases. The dispersive interaction between a hydrocarbonaceous solute and hydrocarbonaceous phase must be stronger than with a fluorocarbonaceous phase. These dispersive effects are a priori in such a direction to cause mHC to be smaller than mFc. We have mentioned, however, that in the equilibrium situations, significant amounts of the mobile phase constituents will have been imbibed by the stationary phases, and the relative m values could be rationalized if the heptadecafluorodecyl stationary phases contained more methanol and/or water than did the corresponding decyl phases. Although such an effect could also explain the relative b values, it does not automatically resolve the problem, but rather introduces another dilemma. Chemical intuition, even if relatively sophisticated, suggests that water and methanol should be more soluble in hydrocarbon solvents than in fluorocarbon solvents. We know of two possible reasons why the converse might be the case for fluorocarbon relative to hydrocarbon bonded phases, one of which explains why chemical intuition might be wrong, and the other which demonstrates, without explaining, what might be an analogous solubility phenomena. The first possibility is that the fluorodecyl bonded phase contains more unreacted silanol groups than the hydrodecyl bonded phase (see below for a discussion of how the unreacted silanol groups affect the retention of pyridine). It might be that these free silanol groups act as hydrogen bond donors to methanol and methanollwater clusters and that this is the dominant influence on the relative amounts of solvent imbibed by the two stationary phases. Unreacted silanol groups are known to cause instability of bonded phases in alkaline media, and other data from our laboratory (Universityof Minnesota) indicate that the FC-10 material is much less stable to alkali than the corresponding HC phase. Further, the retention of amines on the FC phase indicates poorer peak shape and stronger retention, relative to the HC phase, which makes sense if we accept that there are more silanol groups on the FC phase than on the HC phase. The other possibility involves some observations collected (at University of Surrey) regarding the solubilities of non-
2978
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
T a b l e VI. G i b b s E n e r g i e s o f S o l u t i o n , in k c a l / m o l at 25 "C, and G i b b s E n e r g i e s of T r a n s f e r f r o m W a t e r
perfluoroalkanea solute He
Ar
52 67 83 101 116 132 147 164 196
methane ethane propane butane pentane hexane heptane octane decane
hexane
AG,"
AG,"
AG,"
AGto
4.16 3.10 2.84 2.29 1.76
-2.87 -3.17 -3.44 -3.82 -4.47
-0.05
-6.87
-1.17 -2.28
-8.33 -9.72
4.89 3.54 3.14 2.06 1.30 0.55 -0.20 -0.96 -1.66 -2.36
-2.14 -2.73 -3.14 -4.05 -4.93 -5.80 -6.81 -7.78 -8.58 -9.52
"Perfluoroheptane except f o r Cs, Derfluorohexane.
LITERATURE CITED
C8, a n d Clo, w h i c h refer t o
dipolar solutes in n-hexane vs. perfluoro-n-hexane and perfluoro-n-heptane. The data are assembled in Table VI in the form of Gibbs free energies of solution; the standard states are, as usual, 1 atm gas and unit mole fraction solution. It is seen that the larger solutes are, as expected, more soluble in the hydrocarbon, but that the small solutes are more soluble in the fluorocarbon. This is predicted by regular solution theory in that the solubility parameters of the small solutes, especially the inert gases, are closer to the solubility parameter of fluorocarbons than to hydrocarbons. The AG,' values can be combined with AG,' (gas water) values to yield AG,O (water solvent) and then log K values for the two partitions, water hydrocarbon and water fluorocarbon. If these log K values are plotted against for the alkanes, there result good straight lines in each case (the value for CH4 in the fluorocarbon seems somewhat in error) log K(water hexane) = 0.131 4.19v2/100 (10)
-
--
log K(water
--
v2
-
+
perfluorocarbon) = 0.303
high relative to the predicted retention. This observation was anticipated based on the very strong silanophilic interaction of aliphatic and aromatic amines in reversed-phase chromatography in the absence of mobile phase additives which will act to block such interactions. We believe that the silanophilic effect may involve proton transfer rather than hydrogen bonding (proton sharing) interactions of the SiOH groups with the stronger proton transfer bases. R e g i s t r y No. M e O H , 67-56-1; M e C N , 75-05-8; THF, 109-99-9.
+ 3.39v2/100
(11) To whatever extent the solubility properties of the bonded phases resemble the solubility properties of the pure hydrocarbon and fluorocarbon solvents, the coefficients of v2;/100 in eq 10 and 11 correspond to the m values in Table V. On this basis, the numerical m values of 4.19 and 3.39 are very near to those expected for partition from water if we extrapolate the m values in Table V to zero methanol content. Although the chromatographic data and solvatochromic parameters are available for pyridine and other nitrogen bases, the actual retention for these solutes is always systematically
(1) Horvath, C.: Melander, W.: Molnar. I. J. Chromatour. 1976. 125. 129. Hafkenscheld, T. L.; Tomlinson, E. J. Chromato&. 1984,' 292,' 305. Hafkenscheid, T. L.; Tomllnson, E. J. Chromatogr. 1981, 218, 409. Melander, W. R.; Horvath, C. Chromatographla 1982, 15, 66. Tomlinson, E.; Poppe, H.; Kraak, J. C. I n t . J. fharm. 1981, 7 , 225. Karger. B. L.; Gant, J. R.; Hartkopf, A.; Weiner, P. H. J. Chromatogr. 1976, 128, 65. Tchalpa, A.; Colin, H.; Guiochon, G. Anal. Chem. 1984, 56, 621. Berendsen, G. E.; Pikaart, K. A.; de Gaian, L. J. Liq. Chromatogr. 1980, 3 , 1437. McCormick, R. M.; Karger, B. L. Anal. Chem. 1980, 52, 2249. Smith, R. M. Anal. Chem. 1984, 56, 256. Antle, P. E.; Snyder, L. R. LC Mag. 1984, 2 , 841. Brady, J. E.; Bjorkman, D.; Herter, C. D.; Carr, P. W. Anal. Chem. 1984, 56,278. Lochmuller, C. H.; Marshall, D. B.; Wilder, D. R. Anal. Chim. Acta 1981, 130, 31. Bogar, R. G.; Thomas, J. C.; Callls, J. B. Anal. Chem. 1984, 56, 1080. Kamlet, M. J.: Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 49, 2877. Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. frog. fhys. Org. Chem. 1981, 13, 485. Taft, R. W.; Abboud, J. L. M.; Kamlet, M. J.; Abraham, M. H. J. Solution Chem. 1985, 14, 153. Taft, R. W.; Abraham, M. H.; Doherty, R . M.; Kamlet, M. J. J. Am. Chem. SOC. 1985, 107, 3105. Kamlet, M. J.; Abraham, M. H.; Doherty, R. M.; Taft, R. W. J. Am. Chem. SOC. 1984, 106, 464. Taft, R. W.; Abraham, M. H.; Faminl, G. R.; Doherty, R. M.; Kamlet, M. J. J. Pharm. Sci. 1985, 74, 807. Taft, R. W.; Abraham, M. H.; Doherty, R. M.; Kamlet, M. J. Nature (London) 1985, 313, 384. Kamlet, M. J.; Doherty, R. M.; Abraham, M. H.; Taft, R. W., submitted for publlcation In J. fharm. Sc/. Kamlet, M. J.; Carr, P. W.; Taft, R. W.: Abraham, M. H. J. Am. Chem. SOC. 1981, 103, 6062. Haki, J . E.; Young, A. M. J. Liq. Chromatogr. 1984, 7 , 675. Hansch, C.; Leo, A. "Substituent Constants for Correlation Analysis in Chemistry and Blology"; Wiley: New York, 1979. Antle, P. E.: Goldberg, A. P.; Snyder, L. R. J. Chromatogr. 1985, 321, 1. Sadek, P. C.; Carr, P. W. J. Chromatogr. 1984, 288, 24. Unpublished results, University of Surrey.
RECEIVEDfor review April 17, 1985. Accepted July 19, 1985. Work at the University of Minnesota was supported in part by a grant from the National Science Foundation. The work by R. W. Taft was supported in part by a grant from the Public Health Service. The work by M.J.K. and R.M.D. was done under Naval Surface Weapons Center Task IR-060.
