NOTES
4087
Table I : The Temperature Dependence of the Wettability of Polypropylene by Water, Glycerol, and Formamide T,
(YLV)P,
OC
dynes/cm
dynes/cm
I-
0 10 20 30 40 50 60 70 80 100
29.2 28.8 28.4 28.0 27.6 27.2 26.8 26.4 26.0 25.2
75.60 74.22 72.75 71.18 69.56 67.91 66.18 64.40 62.60 58.90
0.300
20 90
28.4 25.6
63.4 58.6
0 20 50
29.2 28.4 27.2
59.9 58.2 55.7
YLVd,
YLV.
dynes/om
COB
e
Water-Poly propylene 22.7 -0.3302 22.2 -0.3283 21.8 -0.3155 21.3 -0.3144 20.8 -0.3099 20.3 -0.3079 19.8 -0.3049 19.3 -0.2981 18.7 -0.2971 17.6 -0.2869
109 108 108 108 108 108 108 107 107 107
...
0.584
G1y cerol-Pol ypropylene 37.0 0.0221 34.2 0.0102
89 89
91-92 90-91
87
0.679
FormamidePolypropylene 40.7 0.1519 39.5 0,1512 37.8 0,1526
81 81. 81
83
...
...
... 95
...
...
...
*.. ... ,..
...
...
... ...
...
...
E. Wolfram, KolloziZ-Z., 182, 75 (1962).
sensitive to temperature. Again, as observed for the polar liquid-polyethylene systems, the experimental and calculated contact angles agree well except for the results with water. It is possible that the choice of 21.8 dynes/cm for ( Y L V ~ ) E ~ Oat 20’ needs some revision to obtain better agreement with the experimental contact angle. The apparent insensitivity of the contact angle to temperature is related to the relative constancy of [(~Lv>P/YLv]’/’ in eq 2. However, this is probably not general with respect to all liquids in contact with low energy polymers. Since the test liquid and polymer probably have different critical temperatures (To), the temperature where YLV = 0, there should be an intermediate temperature where ( ~ L V ) P = YLV. When this condition is fulfilled, 8 = 0. From the critical temperatures of low molecular weight n-hydrocarbons, it was estimated? that T, of polyethylene is 1031OK, considerably higher than any of the polar liquids included in this study. A similar value of T , is expected for polypropylene based on the work of Frisch and Rogers.8 Therefore, at some temperature below the T , of the polymer, assuming no polymer degradation, 8 = 0. However, wetta-
(7) H. Schonhorn, Makronaol. Chem., 93,262(1966). (8) H. L. Frisch and C. E. Rogers, Jr., J . Polymer Sei., C12. 297 (1966).
temperatures, well below the T, of both liquid and polymer, but, as shown in Table I, there is a general trend to a lower value of 8 as T increases.
Solvent Effects in the Proton Chemical Shifts of Acetonitrile and Malononitrile’
by Taku Matsuo and Yasushi Kodera Department of Organic Synthesis, Faculty of Engineering, Kyuahu University, Hakozaki, Fukuoka-Shi, Japan (Received July 18, 1966)
Since the original suggestion of the “reaction field theory”2 and its experimental application to the proton chemical shifts of acetonitrile in various solventsla the Buckingham formula has been widely used in the investigation of the polar effect of solvent on the chemical shift of solute molecule. Buckingham’s formula gives the change in nuclear screening constant of a proton held by an X-H bond in terms of the reaction field R of the media in the form thesis, Faculty of Engineering, Kyushu University. (2) A. D. Buckingham, Can. J . Chem., 38,300 (1960). (3) A. D. Buckingham, T. Schaefer, and W. G. Schneider, J . Chem. Phya., 32, 1227 (1960).
Volume 70, Number 12 Decembm 1966
NOTES
4088
=
-2
x
I O - 1 2 ~ COS
e -10-18~2
(1)
where 8 is the angle between the X-H bond and the reaction field vector i?. However, a number of examples have been known not to follow the above equation. Rlany criticisms have been made in this connection, as summarized in the paper by Becconsall and HampSome of the most important problems involved are (1) the point-dipole approximation for the solute molecule and (2) the assumption of homogeneous continuum of dielectric media as the model of solution. I n the present experiment, the solvent effects on the proton chemical shifts of acetonitrile and malononitrile are compared, and the appropriateness of the above two assumptions was examined.
Experimental Section Acetonitrile and other various solvents of reagent grade were purchased from Wako Pure Chemical Industry Ltd. and were purified by the standard procedure for each. Extreme care was taken to prevent contaminal ion by any impurities that would disturb the experiment. n-Hexane, carbon tetrachloride, chloroform, and benzene were easily purified to various spectroscopic qualifications. Acetonitrile, dioxane, acetone, and dimethyl sulfoxide (DATSO) were also free from any impurities except a trace of water (much less than the solute used in the experiment). However, since the data in these polar solvents were hardly affected by Ihe further addition of a small amount of water, the initial values were concluded to represent the true status in the respective solvents. Malononitrile was either purchased from Tokyo Kasei Chemical Industry Ltd. or synthesized from ethyl cyanoethanoate and were purified by vacuum distillation before use. The chemical shifts were measured by the use of a Varian A-60 spectrometer with TATS signals as the internal reference. The concentrations of the solutions were approximately 0.5 M , except for malononitrile in carbon tetrachloride and benzene, where the solute was hardly soluble and the saturated solutions (less than 0.1 M ) were used in the experiments. Results and Discussion The chemical shifts of acetonitrile and malononitrile thus obtained are summarized in Table I where the dielectric constants of the solvents are also included. On the basis of dipole moments of acetonitrile (3.94 D.) and malononitrile (3.56 D.), the reaction field for the proton of the former is expected to be almost the same as that for the latter when the value is estimated from the Onsager model (2e - l)(n' - 1) R = 3(2e n') a
+
The Journal of Physical Chemistry
Table I : Tne Chemical Shifts of Acetonitrile and Malononitrile in Various Solvents (cps)" Dieleotrio constant Solvent
%-Hexane Carbon tetrachloride Chloroform Acetonitrile Dioxane Acetone DMSO Benzene
Aoetonitrile
Malononitrile
(1.89) (2.24)
109.5 119.6
209.2
(4.81) (38.8) (2.21) (21.4) (45) (2.28)
119.7 118.1 118.2 123.1 126.3 47.3
215.5 226.7 228.1 253.8 264.4 85.5
(ZOO)
...
a The shifts are measured from the internal TMS signal taking the low-field shift as the positive value. The accuracy of measurement is 0.5 cps.
In this expression, n, M , a, and E represent the refractive index, the electric dipole moment, the polarizability of the solute, and the dielectric constant of medium, respectively. Then the point-dipole approximation combined with eq 1 indicates that the proton chemical shift of malononitrile should be affected by the use of polar solvent to almost the same extent as that of acetonitrile. The data in Table I show, however, that the chemical shift of malononitrile is much more strongly dependent on the polarity of solvent than that of acetonitrile. The difference in the chemical shift of malononitrile between the solutions in carbon tetrachloride and in DMSO amounts to 55 cps in comparison with 7 cps in the case of acetonitrile. Clearly, the situation cannot be explained by the simple reaction field theory as described above. The largest difference between acetonitrile and malononitrile is the number of cyano groups which are highly electronegative in nature. Then a better understanding may be made by considering the specific solvation in the close neighborhood of the solute molecule. In a microscopic model, the lone-pair electrons of polar solvent molecules are considered to be weakly associated with the effectively positive carbon atom of the cyano group to avoid the electron-rich nitrogen atom. Such specific solvation brings about a considerable change in the electromagnetic environment of the proton closer to the cyano group than otherwise. On the other hand, however, neither preferential association nor repulsion is expected between any parts of a polar solvent and TMS molecules. Then the data in Table I should be taken as the difference in the solvent shifts between TMS and the solute molecule caused by (4) J. K. Becconsall and P. Hampson, Mol. Phya., 10, 21 (1965).
4089
NOTES
the respective mode of solvation as above. The larger the number of cyano groups or the amount of positive charge around the nearby proton, the larger the effect of the solvation on the chemical shift of the proton. The relation between acetonitrile and malononitrile in Table I is in good agreement with this expectation. As to the solvent molecules, DMSO is found to give the highest degree of solvation effect among those in Table I. To be compared with this observation, DMSO is known to afford a very powerful hydrogen bonding site to the hydroxyl group of organo~ilanols.~Formation of charge-transfer complex with iodine WLLS also reported on DMSO as a strong n donor.6 Furthermore, Stewart, et al., found the presence of strong charge-transfer interaction between DMSO and tetracyanoethylene, a typical ?r accept0r.I All of these works ascribe the strong basicity to the lone-pair electrons of oxygen of the sulfinyl group. Then it may be reasonable to suggest a kind of specific interaction between the oxygen lone-pair electrons of DMSO and the positive part of the solute molecule. This association introduces a new electric field and magnetic anisotropy due to the sulfinyl group and also causes a change in the polarization of the cyano group. I n addition to that, the dispersion interaction between the proton and the surrounding solvent becomes larger under such circumstances than otherwise. Quantitative estimation of each effect cannot be made easily. However, all of these factors result in a low-field shift of the proton signals as shown in Table I. The fact that the effect of acetone is a little less than that of DMSO is interpreted as due to the less polar nature of the former in comparison with the latter. Drago and his co-workers also came to a similar conclusion from the complex behavior of the same compounds with iodine and phenoL6 I n this connection, much less solvent shift in acetonitrile solution seems odd, since both the dipole moment and the dielectric constant of this solvent are larger than those of acetone. The situation may be that the diamagnetic field along the axis of the cyano group has some compensating effect on the low-field shift caused by the association. The effect of dioxane, carbon tetrachloride, and chloroform is not explained by the simple reaction field theory, as is obvious from the inspection of the data in Table I. The compounds of this group are characterized by the presence of easily polarizable lone-pair electrons and the absence of the unsaturated, polar functional group in the molecular structure. Then the solvent shift may be caused by the weak intermolecular interactions due to the dispersion force and others of similar nature. Attention should also be paid, in the case of dioxane, to the small but noticeable
amounts of the basicity due to the lone-pair electrons. As to the effect of polar solvent on the proton chemical shift, the most important factor may be thus concluded to be the microscopic nature of polar functional groups that participated in the solvation rather than the dipole moment or the dielectric constant of the molecule as a whole. The large diamagnetic shifts of acetonitrile and malononitrile in the benzene solutions are interpreted as the result of complex formation between the solute and the solvent molecules, in analogy to the similar study on the proton chemical shift of acetonitrile in toluene solutions.* Carbon tetrachloride may be chosen as the reference solvent because of both its inertness and the closeness of its dielectric constant to that of benzene. The trends for complex formation are evaluated by the differences between the shifts in benzene and in carbon tetrachloride solutions: acetonitrile (72.3 cps) and malonitrile (123.7 cps). Then the ease for malononitrile to form such a complex is found to be much higher than that for acetonitrile. Thus the cause of complex formation between benzene and the polar solutes under investigation also is anticipated to be correlated with the amounts of positive charge induced by the cyano group. ( 5 ) J. F. Hampton, C. W. Lacefield, and J. F. Hyde, Inorg. Chem., 4, 1659 (1965). (6) R. S. Drago, B. Wayland, and R. L. Carlson, J . Am. Chem. Soc., 8 5 , 3125 (1963). (7) F. E. Stewart, M. Eisner, and W. R. Carper, J. Chem. Phys., 44, 2866 (1966). (8) J. V. Hatton and W. G . Schneider, Can. J . Chem., 40, 1285 (1962).
Diffusion of the Solutes at Trace Concentrations in the Ternary System Water-Sucrose-Mannitol at 25 O
by R. Mills and H. David Ellerton' DiffusionResearch Unit, Research School of Physical Sciences, Australian National Universitu, Canberra, Australia (ReceCed July 18, 1966)
Diffusion data have been obtained recently2 using a Gouy diff usiometer for the ternary system water(1) Visiting Fellow from Department of Physical and Inorganic Chemistry, University of Adelaide, Adelaide, South Australia. (2) H. D. Ellerton, Ph.D. Thesis, University of Adelaide, Adelaide, South Australia. 1966.
Volume 70, Number 18 December 1966
