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Molecular association of pentanols in n-heptane V: Electro-dilatometric effect
j g ~ __ of MOLECULAR
LIQUIDS
ELSEVIER
Journal of Molecular Liquids 80 (1999) 65-76
MOLECULAR ASSOCIATION OF PENTANOLS IN n-HEPTANE V: ELECTRO-DILATOMETRIC EFFECT. MANIT RAPPON* and RICHARD M. JOHNS Department of Chemistry, Lakehead University, Thunder Bay, Ontario, Canada.P7B 5El Received 16 April 1998; accepted 3 July 1998
ABSTRACT Electro-dilatometry (ED) is a new technique which deals with the study of the impacts of high electric fields on the volume of liquids. It is very sensitive to hydrogen-bonded liquids and it has been used to study the changes in the volume of binary mixtures of a series of pentanols with n-heptane at 293 K. The relative volume changes are positive for all pentanols, i.e. the volume increases in the presence of the field, with the magnitude of the changes arranged in order of decreasing order from 2-pentanol, 3-pentanol, l-pentanol and tpentanol. The major differences in relative volume change are discussed in terms ofintra- and inter-chain steric interactions of the multimers. ED should have potential applications in several areas of chemistry in which hydrogen-bonding is involved, especially in supramolecular chemistry. © 1999 Elsevier Science B.V. All rights reserved. 1. INTRODUCI'ION Molecular association of alcohols, mediated by hydrogen bonding, is a topic of extensive investigations. Hydrogen bonding is a topic of much interest in both physico-chemical [1-5] and biological systems [6 ]. Hydrogen bonds are known to be important in catalyzing reaction rates [7], miscibility of polymer blends [8], microemulsions [9] , enantioselectivity of enzyme reactions [10-11], dynamics of proton transfer [12], supramolecular assemblies [1315] molecular recognition [16] etc. In some of these phenomena, alcohols are used either as solvent or co-solvent. Hence, a better understanding of hydrogen bonding should lead to our improved knowledge on some of the related chemical, physical and biological systems. Several spectroscopic techniques, physical methods and theoretical approaches have been used to study pure alcohols and their binary mixtures with another solvent. These references have been cited in our previous papers [17-20] and for brevity, they are not repeated here. The efforts of previous researchers have considerably improved our understanding of the molecular association of alcohols, however, many aspects of the problems remain unresolved. These problems were detailed in our earlier paper [20] and only important points are hereby recapitulated. In a given binary mixture of an alcohol with another solvent, several associates may be present simultaneously -multimers (both cyclic and linear) of various chain lengths; these associates have large spread of their lifetimes, thus the use of a single physical method *To whom correspondenceshould be addressed. *E-Mail: [email protected] 0167-7322/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PH S0167-7322(98) 00015-X
66 may be able to detect some species but misses out on others; finally, the system of alcohols and the solvents used in the binary mixtures are often different from one laboratory to another, making it difficult to make direct comparison. In an attempt to avoid some of these difficulties, we have chosen to study one system of alcohols (pentanols) in n- heptane by various techniques. This communication forms part V of this series. Earlier work has been reported as follows : part I : the Kerr effect [17]; part II: viscometric study [18]; part HI : IH-NMR chemical shift of OH group [19] ; and part IV : photochromic reaction probe [20]. To gain further understanding on this system of alcohols in n-heptane, we have applied electro-dilatometric effect (EDE), a newly developed technique originated in this laboratory, to study it. A brief introduction to the EDE is given in the next paragraph. Reviews of pertinent literature related to pentanois have been given [19] with more recent reports on the excess heat capacities of 1-alkanol in n-alkane [21 ] and dielectric studies of sterically hindered alcohols [22].
Electro-dilatometric Effect. In order to facilitate the discussion of the results from this investigation, a brief summary of the background theory on EDE is hereby provided. The details of EDE was first reported by our group earlier [23]. When a static electric field is applied to a liquid, it induces a pressure change in the liquid and the phenomenon is known as electrostriction, it is a classical problem which is well documented in the literature [24,25]. There is an inherent problem associated with the measurements of the field-induced pressure changes, i.e. there are disagreements among various investigators as to the choices of the reference pressure against which a pressure change is measured [26]. To circumvent this difficulty, we have proposed to measure the volume instead of the pressure change of the liquid subjected to high static electric field and named this technique "Electro-dilatometry (ED)" and the extent to which the electric field causes the relative volume change is called "Electro-dilatometric Effect (EDE)". Theoretical derivations of the effect of electric filed on the volume change of a liquid have been reported. For the ease of fluid flow into a region from the source of constant chemical potential, the volume change is shown to be [27]: • = -t-~-jt~jr.=
(1)
Where V,P,T are the volume, pressure and temperature, respectively; E is the applied electric field, and V0 is the liquid volume subjected to the field. Eqn.(1) may be integrated to give :
R = t V--7 J _
=-
,
(2)
where s is the relative permittivity o f liquid, and
AV R - VoE2
(with AV--V-Vo ), is the ratio
which represents the relative volume change per E 1 and it is called the "Electro-dilatometric
Effect (EDE). R values may also be obtained from experimental measurements. Another derivation gives [28]:
V "-8xLnE+I-nKK
(he+l-n) 2
r
(3)
67 where n is the parameter depending on the geometry of the dielectric, 1/K is the compressibility of the liquid. In another attempt, a model in which the molecular volume is allowed to change when the dielectric ellipsoid is subjected to an electric field, and the following result is obtained [29] :
1 [s+2¢o~2
¢~-¢o
1 [1_
~+2¢°
]
where R, is R evaluated with maximum semiprincipal axis (a) parallel to the field, and Lo is the depolarizing factor of the ellipsoid. I~ is the internal bulk modulus of the molecule, ~ is the internal permittivity and 6o is the permittivity of vacuum. Comparison of our earlier experimental results [23] with the theoretical calculations by the use of eqn.(1) showed that (a) the theoretical values are approximately 2 orders of magnitude lower than the experimental ones, (b) while the theory yielded negative values of R (i.e. the volume decreases in the presence of the field) on account of {---~] in eqn.(2) being positive N ~.,/A J
7
for the liquids studied, however, some R values for those liquids which can form H-bonding, are positive. Application of eqn. (4) is in qualitative agreement with our experimental data [23] in the sense that the signs of the experimental R values could be explained [29]. Quantitatively, however, the theoretical results are still lower than the experimental ones by approximately 2 orders of magnitude. One interesting aspect of El) is that it is a nonlinear effect, with relative volume change (AV/V0) varies directly as E 2 . Thus ED is similar to other techniques such as electro-optic Kerr effect [30] , other nonlinear optical phenomena [31], the nonlinear dielectric effect (NDE)-the change in relative permittivity (Ae) with the applied electric field [32,33], and other nonlinear effects [34,35]. The sign of R was found to be opposite to the sign of Kerr constant (B) and that of NDE [23]. Finally, the ED technique was shown to be very sensitive to hydrogen-bonded fiquids where the EDE, as reflected in the magnitude of the R values, are large and positive (i.e. an increase in volume in the presence of the field). Because of its large sensitivity to hydrogen-bonded liquids, ED has hereby been applied to study the molecular association of a series ofpentanols in an inert solvent. 2. EXPERIMENTAL 1-pentanol, 2-pentanol, 3-pentanol and t-pentanol were obtained form Aldrich, they were double-distilled, collecting only the middle fractions, and they were dried with molecular sieves, n-Heptane (Aldrich) was dried with sodium wires prior to use. The current cell used for the ED measurements is an improvement over the original one used in the previous investigation [23] in the sense that the changing of samples could be better facilitated. Various components of the cell are shown in Fig. 1. The cell was made from two stainless-steel cylindrical electrodes and they were sufficiently large to minimize any Joule heating of a sample during the measurements The inner electrode (EL) was a solid cylinder and the outer electrode (OE) was hollow. They were mounted concentrically. On each end of the electrode, an electrode holder ring (EH) clamped on to a KeI-F polymer ring (K) by the use e r a washer. The mechanical stability of each electrode was enhanced with a stainless-steel screw cap (SC) fastened on the top oftbe polymer ring (K) by a set of screws(S). Each end of
68
n|
Fig. 1. The electro-dilatometric cell. (See text for various components.) the electrode was capped with a Teflon cap (TC). The high tension wire (HT) was connected to the electrode (EL) and the ground wire to the outer electrode (OE). Water jacket (WJ) was made from a stainless-steel around the OE with the water 0V) flowed in and out through the water inlet 0VI) and water outlet (WO). A sample was injected by way of the sample inlet (SI) and filled up the electrode gap (of 0.105 cm) and went to the sample outlet (SO). The latter was connected to a very fine capillary with uniform pore size, equipped with a glass water jacket, so that the change in the liquid level may be monitored. The cell was orientated and clamped such that the level of the sample (between the electrodes) toward the sample outlet port was at the highest point. This positioning ensured that air bubbles could not be trapped in the sample between the electrodes. It was found that if air bubbles were trapped, they lead to erratic results. Thus the sample was carefully and gradually filled from the SI port to the electrode gap such that the highest point (toward the SO port) got filled last. Such a procedure eliminated the chance that air bubbles might be trapped in the sample. The sample level inside the capillary was monitored with a HeNe laser (Spectra Physics). The laser beam was passed through a cylindrical lens turning it to a vertical line which was focused through a narrow slit on the liquid level inside the vertical capillary. The image of the liquid level was captured by a linear charge-coupled device, CCD ( Texas Instruments, TC 104) mounted on a primed circuit board. The output of the latter was collected by an ADC (A-to-D converter) board of a PC computer. The change in the liquid level in the absence and presence of the electric field was captured and displayed with a software package. The DC voltages to the electrodes were provided by the regulated high voltage power supply (Bertan, 205A-20R). The normal range of electric field used was (0.5 - 6 ) x 106 V/m. The
69 temperature of the sample was kept constant at 293 K by a circulating water bath (Neslab, RTE-4DD ) and the temperature was regulated and controlled to +0.02 K. 3. RESULTS AND DISCUSSION A typical display of an experimental run is shown in Fig. 2 for the case of 1-pentanol in nheptane at mole fraction (F) = 0.7 and at the applied voltage of 1.0 kV. The horizontal axis represents the time scale in arbitrary unit. The vertical axis represents the arbitrary pixel numbers which are directly proportional to height of the liquid level in the capillary. Here, the picture element of the liquid level in the capillary as detected by the linear CCD is arbitrarily assigned a pixel number. These arbitrary numbers are assigned to various locations along the linear diode array of the CCD in such a way that the difference between any two numbers is directly proportional to the change in the liquid level at the two locations The net change in the pixel number (Ap) is then converted to the actual change in height of the liquid level Ah by a calibration curve which plotted the measured actual height against the corresponding pixel number at various positions of the liquid level. Fig. 3 is a typical plot of change in height in pixels against the applied voltage E2 (kV)2 for the case of 3-pentanol at F = 0.8. It may be seen that such a plot gives a straight line (within experimental errors) whose slope can be AV converted to electro-dilatometric effect ( R ) as defined in eqn.(2), i.e. R = V---~' with the known radius of the capillary, the conversion factor from pixel numbers to height, and the known volume oftbe liquid in between the electrode gap ( V0 = 5.19 x 10-6 m3). The values of R. (10 -16 m2V-2 ) thus obtained are plotted against mole fraction F and mole ratio (r) for various pentanol mixtures as shown in Figs. 4 and 5, respectively. The discussion of the results is divided into three parts : (a) general discussion, (b) specific discussion on each of the pentanols and the conclusions are drawn in part (c).
(a) General discussion. Fig.3 shows that the plot of change in pixels (Ap) which is directly proportional to change in volume (AV), against the square of the applied voltage, E 2 (kV) 2 , is linear. Such an outcome is in general agreement with theory as shown in eqn. (2). This indicates that the volume change in the presence of the field is one of the nonlinear phenomena. In this regard, electro-dilatometfic effect (I/DE) is similar to other techniques such as electro-optic Kerr effect [30], the nonlinear dielectric effect (NDE) [32], as alluded to earlier. These techniques share one common feature in that the sample is brought under the influence of external electric fields. They differ only which one of the physical parameters is being observed, i.e. the relative volume change (EDE), the change in the refractive index (Kerr effect), and the change in the relative permittivity (NDE). The sign of IL The R values for all of the pentanols investigated show a positive sign, i.e. the volume increases in the presence of the fields as shown in Figs. 4 and 5. Also our previous results of Kerr effect on B values for 1-pentanol, 2-pentanol, and 3-pentanol were negative and for t-pentanol it was positive [17]. From our previous work on EDE for a number of liquids, it was observed that the sign of R was opposite to the sign of Kerr constant (13) [23]. Thus, the correlation between the signs of R and B is in agreement with previous observation for 1,2-and 3- pentanols. However, for t-pentanol, the sign of R is positive and is the same as that of B. It should be noted that there are only two values of R for this liquid at high concentrations due to the very small volume changes compared to other pentanols. Thus the
70
1900 ":',
Field off
1880
'
-
1860 g,,ml
1840 1820 1800
i
I
0
'
I
500
,
1000
I
1500
2000
Time [arbit.] Fig. 2. A sample display of an experimental run, showing the positions of the liquid levd (in arbitrary pixel number) prior to (field off) and during (field on) the application of the electric field.
/
60 3-PENTANOL,
50
Ft0.80
40 a,_ ri m
O
t2
30 20 10 0
•
0
•
•
*
I
0.1
L
•
*
*
•
-
0.2
-
-
I
0.3
.
.
.
.
0.4
S2[kV]' Fig. 3. A sample plot of change in pixel number, Ap (arbitrary unit) against ~he square of the applied voltage, E 2 (kV)2 for 3-pentanol at F ffi0.8.
71 correlation of sign between R and B is not available for comparison at low concentrations. Explanations for the signs of R for a dielectric ellipsoid [29] and in relation to the signs of B [23] have been attempted. One of the possible sources of errors to obtain R , is the effect of Joule heating during the measurements of the volume changes. However, such effect, if any, is very small for the following reasons: (I) If the Joule heating were to contribute to the measured volume change, we would have observed the volume to increase with time over the period during which the field is turned on. This, however, was not observed as shown in Fig.2 in which the Ap versus time attained a constant after the initial rise due to molecular reorientation. (II) If the Joule heating were the major component of AV, the liquid volume should have increased in the presence of the field, consequently we should have observed the sign of R to be positive for non-hydrogen bonded liquids. However, the sign of R for these liquids was found to be negative [23]. The errors of R have been estimated to be approximately + 12-15 % . (b) Specific pentanols
R/
R/
R/ Scheme
]
(i) 1-Pentanol. Figs 4 and 5 show that the R values are very small up to F = 0.7 or at r (the mole ratio of 1-pentanol to n-heptane) up to 2.3. The reasons for this small change may be attributed to relatively small amounts of alcohol presence in the mixtures and/or that the multimers formed are less affected by the fields. Above this concentration, R value increases more, reaching the maximum at F = 0.8. At F= 1, R shows a small drop from the maximum value, however, within the errors of this experiment, this value of R is not significantly different from the maximum R value. Such a trend in the R values might suggest that linear multimers are dominant and that the number of n-reefs increases with increasing concentration of 1-pentanol in the mixtures. The reasons for this are provided in the next paragraph. From our previous Kerr effect measurements on the this alcohol, the magnitude of Kerr constant (B) changes more up to F=0.6, beyond which B remains virtually unchanged [17]. The detailed reasons for this were provided previously [17] and only salient features are hereby given. In general, for dipolar molecules, the magnitude of B is a function of permanent dipole moment, the polarizability and the hyperpolarizabilities[30]. At low pentanol concentrations, linear multimers with short chain length are formed which reflected in the small magnitude of B values. At higher concentrations up to F= 0.6, the chain length of the multimers increases with increasing concentration as indicated by the larger magnitude of B values. The formation
72
6 • 1-PENTAN(:X.
5
r~ E
x 2-PENTAN(].
4
D 3-PENTANOL
3
+ t-~NTN~Ot.
2 Iv"
w
1 0 -1
1
!
I
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
MOLE FRACTION, F
Fig. 4. The plot of R ( l0 -]6 m2 W 2 ) against the concentrations in mole fi'action (F) for various pentanols in n-heptane.
6 • I-PENTANOL
5
p
x 2-PENTANOL
/ ¢qE
3
0 -1
0
I
I
,
I
2
4
6
8
10
MOLE~RATIO
Fig. 5. The plot of R ( 10-16 m 2 V "-2 ) against the concentrations in x~ole ratio (r) for various pentanols in n-heptane.
73 of multimers with longer chain length as the concentration increases is also supported by our polarity measurements as probed by the substituted Reichardt dye which shows the polarity increases with increasing concentration [36]. Another evidence to further substantiate it may be forwarded from our results of the viscometric investigation of this pentanol in a low concentration range [18]. It shows that ~ (the activation energy of viscous flow ) is increasing with increasing concentration. The increase in E~ is associated with the increase in chain length of multimers. Insofar as the EDE is concerned, the increase in relative volume change as reflected in the positive value of R may be attributed to two factors: (I) H-bonding destruction. The appfication of the electric field leads to molecular reorientation which may cause the destruction of some H-bonds. This situation is caused by the applied electric field forcing the molecules of alcohol such that their resultant dipole moment vectors are aligned with the direction of the applied electric field. By this realignment, some H-Bonds which are formed when the molecules are not subjected to the external field, may be destroyed. Consequently, the intermolecular forces due to H-bonding are lessened, resulting in the dilation of the bulk volume. (II) Steric interactions. The application of the electric field forces the molecules to realign with the field and the reofientation leads to an increase in steric interactions which manifest themselves in the dilation of the liquid volume. This factor may be further explained with the aid of Scheme I, which shows a segment of a multimers. Each "monomer" (of alcohol molecule ) is held together by H-bonding to form a long chain. It is most likely that in the absence of the field, the resultant dipole moment vectors ( as shown by the arrows) are arranged in random fashion (with the exception that some cooperative motions among a number of monomers may be present, in which case some local order may be found). Under this condition, the multimers should arrange themselves such that the alkyl groups (R/-) experience minimum steric interactions. Upon the application of an external field, these dipole moment vectors are forced to realign along the direction of the field. The latter process leads to steric interactions between the R/-(alkyl) groups within the same chain of the multimers (intra-chain interactions) as well as between different chains of the multimers (interchain interactions) depending upon the structure of the R/- group of the alcohol. For the case of 1-pentanol, both factors discussed above may contribute to the observed EDE being positive. Due to the R/- being n-pentyl group, thus the steric interactions are not significant until the molecular ratio of pentanol/heptane is greater than 2, as shown in Fig. 5, in which case the R values show definite volume increase. (ii) 2-Pentanol. The F_DE for this alcohol is small and is similar to that of 1-pentanol at low concentrations up to F=0.6. However, above F=0.6, R values increase much more rapidly than the corresponding R values for all other pentanols, at the same concentrations. The largest R value is at F=0.8 or at r---4. No R values are obtained at F > 0.8, due to difficulties in obtaining concordant measurements. While multimers of increasing pentanol units are likely to form with increasing concentration, steric hindrance of the pentyl group ( R ~-) may hinder the formation of long-chain linear multimers and the presence of cyclic multimers with small dipole moments cannot be ruled out, as implicated by our Kerr effect measurements in the range F=0.6 to 0.8 [17]. However, the dominant multimers are likely to be of the linear type. The largest R value at F = 0.8 (Fig.4) or r--4 (Fig. 5), merits further discussion. While the contribution to R value at F=0.8 may be due to the combination of three factors, i.e. the dielectric compression as shown in eqn (2) (normally, small negative contribution),
74 H-bonding destruction, and the steric interactions (with the latter two factors providing positive contributions), the major contributor to R value is likely to be the steric interactions. The exact reasons for the large values of R are not known and only a plausible explanation is hereby offered. Assuming that the linear multimers are dominant at this concentration, part of their chain may be depicted as shown in Scheme 1 [with the R j- = -CH (CH3)(CH2CH2CH3) ]. Prior to the application of the external field , the molecules may associate such that the pentyl groups are further apart. In the presence of the field, however, the forced realignment of the dipole moment vectors within the same chain, generates more intra-chain steric interactions. Furthermore, the mismatch between the sizes of the -CH3 and -CH2CH2CH3 groups in the R/- should also create the inter-chain interactions. It is expected that such interactions should be increasing with increasing concentrations of 2-pentanol due to the presence of relatively small number of n-heptane molecules to keep the multimers further apart. And this is indeed observed experimentally, i.e. R increases rapidly at high concentrations ofpentanols, F=0.7 - 0.8 (Fig.4) or r=2.3 - 4.0 (Fig. 5). Thus the combination of these factors could lead to the relatively large values of R. (iii) 3-Pentanol. The R values up to F =0.8 for this pentanol are very similar to those of 1-pentanol , thus the formation of linear multimers are likely to dominate at high concentrations. At F = 0.9 and 1, R values are greater than the corresponding values e r r for 1-pentanol at the same concentrations, but less than R at F--0.8 for 2-pentanol. The reasons for this may be attributed to the branching nature of the R/- group, i.e. R / = -CH (CH2CH3)2. Again, if it is assumed that the linear multimers are dominant, then Scheme 1 may be used as part of the chain for the linear muitimers. Upon the application of the field, the forced realignment of the dipole moment vectors of the monomeric pentanols in the multimers should bring about the intra-and inter-chain steric interactions. Since the R/-group for 3pentanol is branched compared to the n-pentyl of 1-pentanol, the resulting steric interactions should be more for 3-pentanol than 1-pentanol and this is in general observed in the electrodilatometric effect R values in Fig 4, except at F=0.8 where their R values are comparable. However, the R/-group for 3-pentanoi has two equal size ethyl moieties (at the C-atom to which -OH is attached), compared to R/- of 2-pentanol which is consisted of two mismatched groups of -CH3 and -CH2CH2CH3. It is expected that the steric interactions for 3-pentanol multimers are less than those of 2-pentanol, especially the inter-chain interactions. This is reflected in the values e r r observed in Fig. 4. (iv) t-Pentanol. Due to a very small electro-dilatometric effect at low concentrations, no R values were obtained at low concentrations and only two values were obtained at high concentrations (F--0.9 and F=I) as shown in Fig.4. These two values are strikingly small. This may be explained by realizing that the R / = -C(CH3)2(C2Hs) in the tert-pentanol is the most sterically hindered pentyl group among all the pentyl groups of the pentanols in this series. Thus, the formation of long linear chain multimers, as suggested for other pentanols, is somewhat restricted on steric grounds. Instead, the formation of cyclic multimers of various ring sizes may be plausible. Among one of the possible candidates for this is the formation of cyclic trimers. We have some evidences from our Kerr effect studies of the temperature dependence of the Kerr constant (13) in the range of F = 0.4 - 0.6 that the multimers formed are of non-dipolar type [16]. If it is assumed that the cyclic trimers are dominant (this is by no means excluding other species such as those with ring sizes larger than 3 and short chain linear ones), the t-pentanols of a trimers are associated such that they form a structure
75 similar to a chair conformation of a cyclohexane with O and H atoms occupying alternating positions on the ring. Such a cyclic multimers would possess low dipole moment ( if any) and is little affected by the external field. We did observe that the Kerr constant (13) were small and that they changed very little in the concentration range F = 0.5 - 0.8. The formation of such cyclic trimers would also facilitate the formation of hydrogen bonding between rings, i.e. one above and one below each ring in the ring-stacking mode of association. This may be better understood with the aid of a model. Each of the ring is arranged such that each of the three t-pentyl groups is not directly above and below other pentyl groups of the two nearest rings, but is staggered with respect to the nearest pentyl groups. Such an arrangement would facilitate the free rotations of the 2(-CH3) and -CH2CH3 belonging to the pentyl group while allowing for the formation of H-bonds between the rings. This spatial arrangement requires that bifurcated H-bonds are formed between -OH groups ,i.e. each of the O and H atoms of the -OH participating in the formation oftbe cyclic trimers, may form up to 2 H-bonds. This stacking model is consistent with our NMR results for this alcohol, i.e., the magnitude of the AH of H-bond formation is almost twice the magnitude of the corresponding AH for other pentanols whose mode of association is the linear mummers [19]. It is likely that the stacking of several rings to form cylinder-like associates may be important at very high concentrations (F= 0.9 - 1 ). Upon the application of the external field, the associates are not likely to be broken to individual rings due firstly to a very small (if any) resultant dipole moment of each cyclic trimers, and secondly to the formation of H-bonds with the neighbouring rings. Thus, the field is likely to reorientate the whole stack offings as a unit, thereby causing little change in the volume as reflected in the rather small values of g in Fig.4. (c) Conclusions. The application of the new technique called electro-dilatometry to study a series of pentanols in n-heptane has shown that for 1-,2- and 3- pentanols, the molecular associates formed are likely to be dominated by linear mummers at high concentrations. The observed changes in R values with increasing concentrations of pentanols are accounted for by the intra- as well as inter-chain steric interactions of the alkyl groups. For tert-pentanol, while the formation of cyclic multimers (e.g., a trimers) may be important, it does not exclude the formation of other muitimers. At high concentrations, these trimers rings are stacked on top of each other by the use of bifurcated H-bonds. The ring-stacking model is used to account for the very small values of observed R at high concentrations. These findings are consistent with our previous results using various techniques to study the same series of alcohols. This work has shown that electro-dilatometry is a promising technique from which unique information may be extracted. It should be very useful in several areas of chemistry in which H-bonding is involved, especially in the studies of supramolecular chemistry. Acknowledgment : This work was funded in part by the Senate Research Committee of Lakehead University. Professors I. Brevik and J.S. Hoye are thanked for the reprint. E. Drotar, B.K. Morgan and G. Crichlow are thanked for their technical assistance. REFERENCES: 1. W.A.P. Luck, Angew. Chem. Int. Ed. Engl., 19 (1980) 28. 2. P. Schuster, G. Zund¢l and C. Sandorfy (Editors), The Hydrogen Bond, Vols. 1- 3, North-Holland Publishing Co., Amsterdam, 1976.
76 3. S.N. Vinogradov and Linell, Hydrogen Bonding,Van Nostrand-Reinhold, NewYork, 1971. 4. P. Huyskens, J. Mol. Struct., 100 (1983) 403. 5. K. Marsh and F. Kohler, J. Mol. Liquids, 30 (1985) 13. 6. G.A. Jeffrey and W. Saenger, Hydrogen Bonding in Biological Structures, Springer-Verlag, Berlin, 1991. 7. Jpn. Kokai Tokkyo Koho, JP 03 81,241 (1991). 8. L.W. Kelts, C.J.T. Christine, D.M. Teegarden, Macromolecules, 26 (1993) 2941. 9. H.D. Doerfler, E. Borrmeister, Tenside, Surfactants, Deterg., 29 (1992) 154. 10. F. Secundo, S. Riva, G. Carrea, Tetrahedron: Asymmetry, 3 (1992) 267. 11. C.S. Chen and C.J. Sih, Angew. Chem. Int. Ed. Engl., 28 (1989) 695. 12. M. Chachisvilis,T. Fiebig, A. Douhal and A.H. Zewail, J.Phys.Chem. A:102 (1998) 669. 13. M.D. Ward, Chem. Soc. Rev. 26 (1997) 365. 14. J.-M. Lehn, Angew. Chem. Int. Edit Engl., 29 (1990) 1304. 15. J.-M. Lehn, Supramolecular Chemistry, VCH, Weinhein,1995. 16. A. Laschewsky, E Wischerhoff, S. Denzinger, H. Ringsdorf, A. Delcorte, P. Bertrand, Chem Eur. J., 3 (1997) 34. 17. M. Rappon and J.M. Greer, J. Mol. Liquids, 33 (1987) 227. 18. M. Rappon and J.A. Kaukinen, J. Mol. Liquids, 38 (1988) 107. 19. M. Rappon and R.M. Johns, J. Mol. Liquids, 40 (1989) 155. 20. M. Rappon, R.T. Syvitski and K.M. Ghazalli, J. Mol. Liquids, 62 (1994) 159. 21. T. Hofman and H. Buchowski, J. Chem. Soc. Faraday Trans. 1, 88 (1992) 689. 22. J. Malecki, J. Nowak, J. Liszi and Z. Varga-Puchony, Pol. J.Chem., 65 (1991) 1773. 23. M. Rappon, Chem. Phys. Lett., 118 (1985) 340. 24. D.R. Corson and P. Lorrain, Introduction to Electromagnetic Fields and Waves, Freeman, Sanfrancisco, 1962, p. 117. 25. L.D. Landau and E.M. Lifshitz, Electrodynamics of Continuous Media, Pergamon Press, Oxford, 1960, p.55. 26. EA. Guggenheim, Thermodynamics, 5th Ed., North-Holland, Amsterdam, 1967, p.335. 27. A.D. Buckingham, The Laws and Applications of Thermodynamics, Pergamon Press, Oxford, 1964, p. 168. 28. L.D. Landau, E.M. Lifshitz and UP. Pitaevskii, Electrodynamics of Continuous Media, 2 Ed., Pergamon Press, Oxford, 1982. 29. I. Brevik and J.S. I-Igye,Physica 149A (1988) 206. 30. A.D. Buckingham, in : Molecular Electro-optics, vol 1, Pt., ed. C.T. O'Konski, Dekker, New York, 1976, p.27. 31. N. Bloembergen, Rev. Mod. Phys., 54 (1982) 685. 32. J, Malecki, J. Chem. Phys., 36 (1962) 2144. 33. L. Hellemans amd M. DeMaeyer, J.Chem. Faraday Trans.II 78 (1982) 401. 34. M.P. Bogard and B.J. Orr, in:MTP International Review of Science, Physical Chemistry, Series two, Molecular Structure and Properties, Vol.2, ed.A.D. Buckingham, Butterwonhs: London, 1975, p149. 35. C.J.F. B Ottcher, Theory of Electric Polarization, Vol. 1, Elsevier, Amsterdam, 1973, p.289. 36. M. Rappon, S. Gillson and C. Reichardt, ( To be submitted for publication).
TM
LIQUIDS
ELSEVIER
Journal of Molecular Liquids 80 (1999) 65-76
MOLECULAR ASSOCIATION OF PENTANOLS IN n-HEPTANE V: ELECTRO-DILATOMETRIC EFFECT. MANIT RAPPON* and RICHARD M. JOHNS Department of Chemistry, Lakehead University, Thunder Bay, Ontario, Canada.P7B 5El Received 16 April 1998; accepted 3 July 1998
ABSTRACT Electro-dilatometry (ED) is a new technique which deals with the study of the impacts of high electric fields on the volume of liquids. It is very sensitive to hydrogen-bonded liquids and it has been used to study the changes in the volume of binary mixtures of a series of pentanols with n-heptane at 293 K. The relative volume changes are positive for all pentanols, i.e. the volume increases in the presence of the field, with the magnitude of the changes arranged in order of decreasing order from 2-pentanol, 3-pentanol, l-pentanol and tpentanol. The major differences in relative volume change are discussed in terms ofintra- and inter-chain steric interactions of the multimers. ED should have potential applications in several areas of chemistry in which hydrogen-bonding is involved, especially in supramolecular chemistry. © 1999 Elsevier Science B.V. All rights reserved. 1. INTRODUCI'ION Molecular association of alcohols, mediated by hydrogen bonding, is a topic of extensive investigations. Hydrogen bonding is a topic of much interest in both physico-chemical [1-5] and biological systems [6 ]. Hydrogen bonds are known to be important in catalyzing reaction rates [7], miscibility of polymer blends [8], microemulsions [9] , enantioselectivity of enzyme reactions [10-11], dynamics of proton transfer [12], supramolecular assemblies [1315] molecular recognition [16] etc. In some of these phenomena, alcohols are used either as solvent or co-solvent. Hence, a better understanding of hydrogen bonding should lead to our improved knowledge on some of the related chemical, physical and biological systems. Several spectroscopic techniques, physical methods and theoretical approaches have been used to study pure alcohols and their binary mixtures with another solvent. These references have been cited in our previous papers [17-20] and for brevity, they are not repeated here. The efforts of previous researchers have considerably improved our understanding of the molecular association of alcohols, however, many aspects of the problems remain unresolved. These problems were detailed in our earlier paper [20] and only important points are hereby recapitulated. In a given binary mixture of an alcohol with another solvent, several associates may be present simultaneously -multimers (both cyclic and linear) of various chain lengths; these associates have large spread of their lifetimes, thus the use of a single physical method *To whom correspondenceshould be addressed. *E-Mail: [email protected] 0167-7322/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PH S0167-7322(98) 00015-X
66 may be able to detect some species but misses out on others; finally, the system of alcohols and the solvents used in the binary mixtures are often different from one laboratory to another, making it difficult to make direct comparison. In an attempt to avoid some of these difficulties, we have chosen to study one system of alcohols (pentanols) in n- heptane by various techniques. This communication forms part V of this series. Earlier work has been reported as follows : part I : the Kerr effect [17]; part II: viscometric study [18]; part HI : IH-NMR chemical shift of OH group [19] ; and part IV : photochromic reaction probe [20]. To gain further understanding on this system of alcohols in n-heptane, we have applied electro-dilatometric effect (EDE), a newly developed technique originated in this laboratory, to study it. A brief introduction to the EDE is given in the next paragraph. Reviews of pertinent literature related to pentanois have been given [19] with more recent reports on the excess heat capacities of 1-alkanol in n-alkane [21 ] and dielectric studies of sterically hindered alcohols [22].
Electro-dilatometric Effect. In order to facilitate the discussion of the results from this investigation, a brief summary of the background theory on EDE is hereby provided. The details of EDE was first reported by our group earlier [23]. When a static electric field is applied to a liquid, it induces a pressure change in the liquid and the phenomenon is known as electrostriction, it is a classical problem which is well documented in the literature [24,25]. There is an inherent problem associated with the measurements of the field-induced pressure changes, i.e. there are disagreements among various investigators as to the choices of the reference pressure against which a pressure change is measured [26]. To circumvent this difficulty, we have proposed to measure the volume instead of the pressure change of the liquid subjected to high static electric field and named this technique "Electro-dilatometry (ED)" and the extent to which the electric field causes the relative volume change is called "Electro-dilatometric Effect (EDE)". Theoretical derivations of the effect of electric filed on the volume change of a liquid have been reported. For the ease of fluid flow into a region from the source of constant chemical potential, the volume change is shown to be [27]: • = -t-~-jt~jr.=
(1)
Where V,P,T are the volume, pressure and temperature, respectively; E is the applied electric field, and V0 is the liquid volume subjected to the field. Eqn.(1) may be integrated to give :
R = t V--7 J _
=-
,
(2)
where s is the relative permittivity o f liquid, and
AV R - VoE2
(with AV--V-Vo ), is the ratio
which represents the relative volume change per E 1 and it is called the "Electro-dilatometric
Effect (EDE). R values may also be obtained from experimental measurements. Another derivation gives [28]:
V "-8xLnE+I-nKK
(he+l-n) 2
r
(3)
67 where n is the parameter depending on the geometry of the dielectric, 1/K is the compressibility of the liquid. In another attempt, a model in which the molecular volume is allowed to change when the dielectric ellipsoid is subjected to an electric field, and the following result is obtained [29] :
1 [s+2¢o~2
¢~-¢o
1 [1_
~+2¢°
]
where R, is R evaluated with maximum semiprincipal axis (a) parallel to the field, and Lo is the depolarizing factor of the ellipsoid. I~ is the internal bulk modulus of the molecule, ~ is the internal permittivity and 6o is the permittivity of vacuum. Comparison of our earlier experimental results [23] with the theoretical calculations by the use of eqn.(1) showed that (a) the theoretical values are approximately 2 orders of magnitude lower than the experimental ones, (b) while the theory yielded negative values of R (i.e. the volume decreases in the presence of the field) on account of {---~] in eqn.(2) being positive N ~.,/A J
7
for the liquids studied, however, some R values for those liquids which can form H-bonding, are positive. Application of eqn. (4) is in qualitative agreement with our experimental data [23] in the sense that the signs of the experimental R values could be explained [29]. Quantitatively, however, the theoretical results are still lower than the experimental ones by approximately 2 orders of magnitude. One interesting aspect of El) is that it is a nonlinear effect, with relative volume change (AV/V0) varies directly as E 2 . Thus ED is similar to other techniques such as electro-optic Kerr effect [30] , other nonlinear optical phenomena [31], the nonlinear dielectric effect (NDE)-the change in relative permittivity (Ae) with the applied electric field [32,33], and other nonlinear effects [34,35]. The sign of R was found to be opposite to the sign of Kerr constant (B) and that of NDE [23]. Finally, the ED technique was shown to be very sensitive to hydrogen-bonded fiquids where the EDE, as reflected in the magnitude of the R values, are large and positive (i.e. an increase in volume in the presence of the field). Because of its large sensitivity to hydrogen-bonded liquids, ED has hereby been applied to study the molecular association of a series ofpentanols in an inert solvent. 2. EXPERIMENTAL 1-pentanol, 2-pentanol, 3-pentanol and t-pentanol were obtained form Aldrich, they were double-distilled, collecting only the middle fractions, and they were dried with molecular sieves, n-Heptane (Aldrich) was dried with sodium wires prior to use. The current cell used for the ED measurements is an improvement over the original one used in the previous investigation [23] in the sense that the changing of samples could be better facilitated. Various components of the cell are shown in Fig. 1. The cell was made from two stainless-steel cylindrical electrodes and they were sufficiently large to minimize any Joule heating of a sample during the measurements The inner electrode (EL) was a solid cylinder and the outer electrode (OE) was hollow. They were mounted concentrically. On each end of the electrode, an electrode holder ring (EH) clamped on to a KeI-F polymer ring (K) by the use e r a washer. The mechanical stability of each electrode was enhanced with a stainless-steel screw cap (SC) fastened on the top oftbe polymer ring (K) by a set of screws(S). Each end of
68
n|
Fig. 1. The electro-dilatometric cell. (See text for various components.) the electrode was capped with a Teflon cap (TC). The high tension wire (HT) was connected to the electrode (EL) and the ground wire to the outer electrode (OE). Water jacket (WJ) was made from a stainless-steel around the OE with the water 0V) flowed in and out through the water inlet 0VI) and water outlet (WO). A sample was injected by way of the sample inlet (SI) and filled up the electrode gap (of 0.105 cm) and went to the sample outlet (SO). The latter was connected to a very fine capillary with uniform pore size, equipped with a glass water jacket, so that the change in the liquid level may be monitored. The cell was orientated and clamped such that the level of the sample (between the electrodes) toward the sample outlet port was at the highest point. This positioning ensured that air bubbles could not be trapped in the sample between the electrodes. It was found that if air bubbles were trapped, they lead to erratic results. Thus the sample was carefully and gradually filled from the SI port to the electrode gap such that the highest point (toward the SO port) got filled last. Such a procedure eliminated the chance that air bubbles might be trapped in the sample. The sample level inside the capillary was monitored with a HeNe laser (Spectra Physics). The laser beam was passed through a cylindrical lens turning it to a vertical line which was focused through a narrow slit on the liquid level inside the vertical capillary. The image of the liquid level was captured by a linear charge-coupled device, CCD ( Texas Instruments, TC 104) mounted on a primed circuit board. The output of the latter was collected by an ADC (A-to-D converter) board of a PC computer. The change in the liquid level in the absence and presence of the electric field was captured and displayed with a software package. The DC voltages to the electrodes were provided by the regulated high voltage power supply (Bertan, 205A-20R). The normal range of electric field used was (0.5 - 6 ) x 106 V/m. The
69 temperature of the sample was kept constant at 293 K by a circulating water bath (Neslab, RTE-4DD ) and the temperature was regulated and controlled to +0.02 K. 3. RESULTS AND DISCUSSION A typical display of an experimental run is shown in Fig. 2 for the case of 1-pentanol in nheptane at mole fraction (F) = 0.7 and at the applied voltage of 1.0 kV. The horizontal axis represents the time scale in arbitrary unit. The vertical axis represents the arbitrary pixel numbers which are directly proportional to height of the liquid level in the capillary. Here, the picture element of the liquid level in the capillary as detected by the linear CCD is arbitrarily assigned a pixel number. These arbitrary numbers are assigned to various locations along the linear diode array of the CCD in such a way that the difference between any two numbers is directly proportional to the change in the liquid level at the two locations The net change in the pixel number (Ap) is then converted to the actual change in height of the liquid level Ah by a calibration curve which plotted the measured actual height against the corresponding pixel number at various positions of the liquid level. Fig. 3 is a typical plot of change in height in pixels against the applied voltage E2 (kV)2 for the case of 3-pentanol at F = 0.8. It may be seen that such a plot gives a straight line (within experimental errors) whose slope can be AV converted to electro-dilatometric effect ( R ) as defined in eqn.(2), i.e. R = V---~' with the known radius of the capillary, the conversion factor from pixel numbers to height, and the known volume oftbe liquid in between the electrode gap ( V0 = 5.19 x 10-6 m3). The values of R. (10 -16 m2V-2 ) thus obtained are plotted against mole fraction F and mole ratio (r) for various pentanol mixtures as shown in Figs. 4 and 5, respectively. The discussion of the results is divided into three parts : (a) general discussion, (b) specific discussion on each of the pentanols and the conclusions are drawn in part (c).
(a) General discussion. Fig.3 shows that the plot of change in pixels (Ap) which is directly proportional to change in volume (AV), against the square of the applied voltage, E 2 (kV) 2 , is linear. Such an outcome is in general agreement with theory as shown in eqn. (2). This indicates that the volume change in the presence of the field is one of the nonlinear phenomena. In this regard, electro-dilatometfic effect (I/DE) is similar to other techniques such as electro-optic Kerr effect [30], the nonlinear dielectric effect (NDE) [32], as alluded to earlier. These techniques share one common feature in that the sample is brought under the influence of external electric fields. They differ only which one of the physical parameters is being observed, i.e. the relative volume change (EDE), the change in the refractive index (Kerr effect), and the change in the relative permittivity (NDE). The sign of IL The R values for all of the pentanols investigated show a positive sign, i.e. the volume increases in the presence of the fields as shown in Figs. 4 and 5. Also our previous results of Kerr effect on B values for 1-pentanol, 2-pentanol, and 3-pentanol were negative and for t-pentanol it was positive [17]. From our previous work on EDE for a number of liquids, it was observed that the sign of R was opposite to the sign of Kerr constant (13) [23]. Thus, the correlation between the signs of R and B is in agreement with previous observation for 1,2-and 3- pentanols. However, for t-pentanol, the sign of R is positive and is the same as that of B. It should be noted that there are only two values of R for this liquid at high concentrations due to the very small volume changes compared to other pentanols. Thus the
70
1900 ":',
Field off
1880
'
-
1860 g,,ml
1840 1820 1800
i
I
0
'
I
500
,
1000
I
1500
2000
Time [arbit.] Fig. 2. A sample display of an experimental run, showing the positions of the liquid levd (in arbitrary pixel number) prior to (field off) and during (field on) the application of the electric field.
/
60 3-PENTANOL,
50
Ft0.80
40 a,_ ri m
O
t2
30 20 10 0
•
0
•
•
*
I
0.1
L
•
*
*
•
-
0.2
-
-
I
0.3
.
.
.
.
0.4
S2[kV]' Fig. 3. A sample plot of change in pixel number, Ap (arbitrary unit) against ~he square of the applied voltage, E 2 (kV)2 for 3-pentanol at F ffi0.8.
71 correlation of sign between R and B is not available for comparison at low concentrations. Explanations for the signs of R for a dielectric ellipsoid [29] and in relation to the signs of B [23] have been attempted. One of the possible sources of errors to obtain R , is the effect of Joule heating during the measurements of the volume changes. However, such effect, if any, is very small for the following reasons: (I) If the Joule heating were to contribute to the measured volume change, we would have observed the volume to increase with time over the period during which the field is turned on. This, however, was not observed as shown in Fig.2 in which the Ap versus time attained a constant after the initial rise due to molecular reorientation. (II) If the Joule heating were the major component of AV, the liquid volume should have increased in the presence of the field, consequently we should have observed the sign of R to be positive for non-hydrogen bonded liquids. However, the sign of R for these liquids was found to be negative [23]. The errors of R have been estimated to be approximately + 12-15 % . (b) Specific pentanols
R/
R/
R/ Scheme
]
(i) 1-Pentanol. Figs 4 and 5 show that the R values are very small up to F = 0.7 or at r (the mole ratio of 1-pentanol to n-heptane) up to 2.3. The reasons for this small change may be attributed to relatively small amounts of alcohol presence in the mixtures and/or that the multimers formed are less affected by the fields. Above this concentration, R value increases more, reaching the maximum at F = 0.8. At F= 1, R shows a small drop from the maximum value, however, within the errors of this experiment, this value of R is not significantly different from the maximum R value. Such a trend in the R values might suggest that linear multimers are dominant and that the number of n-reefs increases with increasing concentration of 1-pentanol in the mixtures. The reasons for this are provided in the next paragraph. From our previous Kerr effect measurements on the this alcohol, the magnitude of Kerr constant (B) changes more up to F=0.6, beyond which B remains virtually unchanged [17]. The detailed reasons for this were provided previously [17] and only salient features are hereby given. In general, for dipolar molecules, the magnitude of B is a function of permanent dipole moment, the polarizability and the hyperpolarizabilities[30]. At low pentanol concentrations, linear multimers with short chain length are formed which reflected in the small magnitude of B values. At higher concentrations up to F= 0.6, the chain length of the multimers increases with increasing concentration as indicated by the larger magnitude of B values. The formation
72
6 • 1-PENTAN(:X.
5
r~ E
x 2-PENTAN(].
4
D 3-PENTANOL
3
+ t-~NTN~Ot.
2 Iv"
w
1 0 -1
1
!
I
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
MOLE FRACTION, F
Fig. 4. The plot of R ( l0 -]6 m2 W 2 ) against the concentrations in mole fi'action (F) for various pentanols in n-heptane.
6 • I-PENTANOL
5
p
x 2-PENTANOL
/ ¢qE
3
0 -1
0
I
I
,
I
2
4
6
8
10
MOLE~RATIO
Fig. 5. The plot of R ( 10-16 m 2 V "-2 ) against the concentrations in x~ole ratio (r) for various pentanols in n-heptane.
73 of multimers with longer chain length as the concentration increases is also supported by our polarity measurements as probed by the substituted Reichardt dye which shows the polarity increases with increasing concentration [36]. Another evidence to further substantiate it may be forwarded from our results of the viscometric investigation of this pentanol in a low concentration range [18]. It shows that ~ (the activation energy of viscous flow ) is increasing with increasing concentration. The increase in E~ is associated with the increase in chain length of multimers. Insofar as the EDE is concerned, the increase in relative volume change as reflected in the positive value of R may be attributed to two factors: (I) H-bonding destruction. The appfication of the electric field leads to molecular reorientation which may cause the destruction of some H-bonds. This situation is caused by the applied electric field forcing the molecules of alcohol such that their resultant dipole moment vectors are aligned with the direction of the applied electric field. By this realignment, some H-Bonds which are formed when the molecules are not subjected to the external field, may be destroyed. Consequently, the intermolecular forces due to H-bonding are lessened, resulting in the dilation of the bulk volume. (II) Steric interactions. The application of the electric field forces the molecules to realign with the field and the reofientation leads to an increase in steric interactions which manifest themselves in the dilation of the liquid volume. This factor may be further explained with the aid of Scheme I, which shows a segment of a multimers. Each "monomer" (of alcohol molecule ) is held together by H-bonding to form a long chain. It is most likely that in the absence of the field, the resultant dipole moment vectors ( as shown by the arrows) are arranged in random fashion (with the exception that some cooperative motions among a number of monomers may be present, in which case some local order may be found). Under this condition, the multimers should arrange themselves such that the alkyl groups (R/-) experience minimum steric interactions. Upon the application of an external field, these dipole moment vectors are forced to realign along the direction of the field. The latter process leads to steric interactions between the R/-(alkyl) groups within the same chain of the multimers (intra-chain interactions) as well as between different chains of the multimers (interchain interactions) depending upon the structure of the R/- group of the alcohol. For the case of 1-pentanol, both factors discussed above may contribute to the observed EDE being positive. Due to the R/- being n-pentyl group, thus the steric interactions are not significant until the molecular ratio of pentanol/heptane is greater than 2, as shown in Fig. 5, in which case the R values show definite volume increase. (ii) 2-Pentanol. The F_DE for this alcohol is small and is similar to that of 1-pentanol at low concentrations up to F=0.6. However, above F=0.6, R values increase much more rapidly than the corresponding R values for all other pentanols, at the same concentrations. The largest R value is at F=0.8 or at r---4. No R values are obtained at F > 0.8, due to difficulties in obtaining concordant measurements. While multimers of increasing pentanol units are likely to form with increasing concentration, steric hindrance of the pentyl group ( R ~-) may hinder the formation of long-chain linear multimers and the presence of cyclic multimers with small dipole moments cannot be ruled out, as implicated by our Kerr effect measurements in the range F=0.6 to 0.8 [17]. However, the dominant multimers are likely to be of the linear type. The largest R value at F = 0.8 (Fig.4) or r--4 (Fig. 5), merits further discussion. While the contribution to R value at F=0.8 may be due to the combination of three factors, i.e. the dielectric compression as shown in eqn (2) (normally, small negative contribution),
74 H-bonding destruction, and the steric interactions (with the latter two factors providing positive contributions), the major contributor to R value is likely to be the steric interactions. The exact reasons for the large values of R are not known and only a plausible explanation is hereby offered. Assuming that the linear multimers are dominant at this concentration, part of their chain may be depicted as shown in Scheme 1 [with the R j- = -CH (CH3)(CH2CH2CH3) ]. Prior to the application of the external field , the molecules may associate such that the pentyl groups are further apart. In the presence of the field, however, the forced realignment of the dipole moment vectors within the same chain, generates more intra-chain steric interactions. Furthermore, the mismatch between the sizes of the -CH3 and -CH2CH2CH3 groups in the R/- should also create the inter-chain interactions. It is expected that such interactions should be increasing with increasing concentrations of 2-pentanol due to the presence of relatively small number of n-heptane molecules to keep the multimers further apart. And this is indeed observed experimentally, i.e. R increases rapidly at high concentrations ofpentanols, F=0.7 - 0.8 (Fig.4) or r=2.3 - 4.0 (Fig. 5). Thus the combination of these factors could lead to the relatively large values of R. (iii) 3-Pentanol. The R values up to F =0.8 for this pentanol are very similar to those of 1-pentanol , thus the formation of linear multimers are likely to dominate at high concentrations. At F = 0.9 and 1, R values are greater than the corresponding values e r r for 1-pentanol at the same concentrations, but less than R at F--0.8 for 2-pentanol. The reasons for this may be attributed to the branching nature of the R/- group, i.e. R / = -CH (CH2CH3)2. Again, if it is assumed that the linear multimers are dominant, then Scheme 1 may be used as part of the chain for the linear muitimers. Upon the application of the field, the forced realignment of the dipole moment vectors of the monomeric pentanols in the multimers should bring about the intra-and inter-chain steric interactions. Since the R/-group for 3pentanol is branched compared to the n-pentyl of 1-pentanol, the resulting steric interactions should be more for 3-pentanol than 1-pentanol and this is in general observed in the electrodilatometric effect R values in Fig 4, except at F=0.8 where their R values are comparable. However, the R/-group for 3-pentanoi has two equal size ethyl moieties (at the C-atom to which -OH is attached), compared to R/- of 2-pentanol which is consisted of two mismatched groups of -CH3 and -CH2CH2CH3. It is expected that the steric interactions for 3-pentanol multimers are less than those of 2-pentanol, especially the inter-chain interactions. This is reflected in the values e r r observed in Fig. 4. (iv) t-Pentanol. Due to a very small electro-dilatometric effect at low concentrations, no R values were obtained at low concentrations and only two values were obtained at high concentrations (F--0.9 and F=I) as shown in Fig.4. These two values are strikingly small. This may be explained by realizing that the R / = -C(CH3)2(C2Hs) in the tert-pentanol is the most sterically hindered pentyl group among all the pentyl groups of the pentanols in this series. Thus, the formation of long linear chain multimers, as suggested for other pentanols, is somewhat restricted on steric grounds. Instead, the formation of cyclic multimers of various ring sizes may be plausible. Among one of the possible candidates for this is the formation of cyclic trimers. We have some evidences from our Kerr effect studies of the temperature dependence of the Kerr constant (13) in the range of F = 0.4 - 0.6 that the multimers formed are of non-dipolar type [16]. If it is assumed that the cyclic trimers are dominant (this is by no means excluding other species such as those with ring sizes larger than 3 and short chain linear ones), the t-pentanols of a trimers are associated such that they form a structure
75 similar to a chair conformation of a cyclohexane with O and H atoms occupying alternating positions on the ring. Such a cyclic multimers would possess low dipole moment ( if any) and is little affected by the external field. We did observe that the Kerr constant (13) were small and that they changed very little in the concentration range F = 0.5 - 0.8. The formation of such cyclic trimers would also facilitate the formation of hydrogen bonding between rings, i.e. one above and one below each ring in the ring-stacking mode of association. This may be better understood with the aid of a model. Each of the ring is arranged such that each of the three t-pentyl groups is not directly above and below other pentyl groups of the two nearest rings, but is staggered with respect to the nearest pentyl groups. Such an arrangement would facilitate the free rotations of the 2(-CH3) and -CH2CH3 belonging to the pentyl group while allowing for the formation of H-bonds between the rings. This spatial arrangement requires that bifurcated H-bonds are formed between -OH groups ,i.e. each of the O and H atoms of the -OH participating in the formation oftbe cyclic trimers, may form up to 2 H-bonds. This stacking model is consistent with our NMR results for this alcohol, i.e., the magnitude of the AH of H-bond formation is almost twice the magnitude of the corresponding AH for other pentanols whose mode of association is the linear mummers [19]. It is likely that the stacking of several rings to form cylinder-like associates may be important at very high concentrations (F= 0.9 - 1 ). Upon the application of the external field, the associates are not likely to be broken to individual rings due firstly to a very small (if any) resultant dipole moment of each cyclic trimers, and secondly to the formation of H-bonds with the neighbouring rings. Thus, the field is likely to reorientate the whole stack offings as a unit, thereby causing little change in the volume as reflected in the rather small values of g in Fig.4. (c) Conclusions. The application of the new technique called electro-dilatometry to study a series of pentanols in n-heptane has shown that for 1-,2- and 3- pentanols, the molecular associates formed are likely to be dominated by linear mummers at high concentrations. The observed changes in R values with increasing concentrations of pentanols are accounted for by the intra- as well as inter-chain steric interactions of the alkyl groups. For tert-pentanol, while the formation of cyclic multimers (e.g., a trimers) may be important, it does not exclude the formation of other muitimers. At high concentrations, these trimers rings are stacked on top of each other by the use of bifurcated H-bonds. The ring-stacking model is used to account for the very small values of observed R at high concentrations. These findings are consistent with our previous results using various techniques to study the same series of alcohols. This work has shown that electro-dilatometry is a promising technique from which unique information may be extracted. It should be very useful in several areas of chemistry in which H-bonding is involved, especially in the studies of supramolecular chemistry. Acknowledgment : This work was funded in part by the Senate Research Committee of Lakehead University. Professors I. Brevik and J.S. Hoye are thanked for the reprint. E. Drotar, B.K. Morgan and G. Crichlow are thanked for their technical assistance. REFERENCES: 1. W.A.P. Luck, Angew. Chem. Int. Ed. Engl., 19 (1980) 28. 2. P. Schuster, G. Zund¢l and C. Sandorfy (Editors), The Hydrogen Bond, Vols. 1- 3, North-Holland Publishing Co., Amsterdam, 1976.
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