HEXAMETHYLENETETRAMINE AQUEOUSSOLUTIONS
2313
Hexamethylenetetramine Aqueous Solutions.
Isopiestic Data at
25" and Density and Viscosity Data in the Range 3-34"
by V. Crescenzi, F. Quadrifoglio, and V. Vitagliano Centro Nazionale di Chimica delle Maeromolecole, (CNR) Sez. 111, Istituto Chimico, Uniuersitb di Napoli, Naples, Italy Accepted and Transmitted by The Faraday Society
(November 28,1966)
~
~
~~~
~~
~
~~~
The results of isopiestic measurements at 25" and of density and viscosity measurements performed in the range 3-34' on hexamethylenetetramine (HMT) aqueous solutions are reported. The isopiestic data show that deviation from ideal behavior exhibited by HMT aqueous solutions is strong and suggest that solute-solvent interactions should play a dominant role. It is proposed that in HMT aqueous solution immobilization of water molecules in solvation cages around the hydrophobic solute takes place, and an evaluation of HMT average hydration number is attempted in terms of a simplified treatment of the isopiestic data. Increasing the temperature, an extensive breakdown of HMT hydration structure occurs as indicated by our viscosity and density data.
Recently, a number of indications have been discussed according to which hexamethylenetetramine (MMT) in water would noticeably perturb the structural organization of the solvent.' Indirect evidence in favor of this hypothesis was provided by the marked influence exerted by HMT on the occurrence, in aqueous solution, of a few phenomena in which hydrophobic interactions are of critical importance. These interesting feahres have prompted an investigation directed toward a mcre detailed description of some basic physic >chemicalproperties of HMT aqueous solutions. Knowledge of these properties is, in fact, poor at present. We wish tr, report here the results of isopiestic measurements carried out at 25' and of density and viscosity measurements, performed in the range 3-34' on HMT aqueous solutions. Our data confirm the hypothesis that HMT noticeably influences water structural organization and behaves in aqueous solution in a way typical of "structure-forming" substances. Experimental Section Materials. Hexamethylenetetramine was obtained from Carlo Erba (Milan) and the reagent grade product recrystallized from absolute ethanol. Potassium chloride was a Merck product, analytical
grade. Solutions of the carefully dried hexamine and KCl samples were prepared gravimetrically using doubly distilled demineralized water. Apparatus. (a) Isopiestic Measurements. About 3 ml of the solution whose solvent activity we wished to determine was placed in each of two or three weighing bottles. A similar amount of the reference solution was placed in another set of weighing bottles. All bottles were placed on a flat aluminum block contained in a stainless steel vacuum desiccator and evacuated to a pressure of approximately 10 mm. The evacuation was carried out very slowly in order to avoid splattering of the solutions. The solutions were then allowed to equilibrate under vacuum for 3 or 4 days in a large thermostat bath at 2 5 O , controlled to approximately *0.01 O , before being reweighed. Each weighing bottle contained a few glass beads to assist equilibration while the desiccator was being rocked in the thermostat bath. The concentrations of unknown and reference solutions were prepared initially so as to be fairly close to the anticipated equilibrium value. The solutions were accepted as being at equilibrium if the molalities of a particular set differed by not more (1) G. Barone, V. Creesoenzi, A. M. Liquori, and F. Quadrifoglio, J. Phys. Chem., 71, 984 (1967).
Volume 71. Number 7 June 1967
2314
than 0.1%. Osmotic coefficients, cp, and water activity, a,, at 25' of HMT solutions were obtained by the equations cp = (2rn~cl/m)cp~~l and In a, = -(cpm/ 55.51). The ~ ~ K Cvalues I were obtained by interpolation of data reported by Robinson and Stokes.2 The very small degree of protonation3 of HMT was not taken into consideration. Molecular weights of 74.553 and 140.19 for potassium chloride and hexamethylenetetramine, respectively, were used. (b) Density Measurements. Densities were measured with pycnometers, the total volume of which was approximately 25 ml. The volumes were computed at each working temperature from measurements on doubly distilled deionized water and density data from the l i t e r a t ~ r e . ~The temperature of the constanttemperature water bath was controlled to +O.0lo. Solutions in the pycnometers were allowed to equilibrate in the water bath at the highest temperature considered 1:33.46') for a few hours. After temperature equilibrium had been achieved, a few drops of liquid were carefully added in the pycnometers until the meniscus reached the calibration mark on the capillary tube of the pycnometers. The pycnometers were then dried with 11 lint-free cloth and weighed on a hlettler balance. After weighing, the pycnometers were returned to the water bath for reequilibration a t the next-lowest temperature. Apparent molal volumes of HMT were calculated from the experimental data by the equation 4" = ( M / d ) - [1000(d - dH,o)/md. ~ H ~ where o ] m is the molality of HMT and d and d H t O are the density of the solution and of pure water, respectively. Replicate determinations of the density reproduced within 5 X low5,corresponding to a precision for the apparent molal volume of *O.l ml. (c) Viscosity Measurements. All solutions for the vjscosity measurements were prepared on a weight basis. Ubbelohde-type suspended-level viscometers with flow times for water at 25' of about 300 sec were employed. I n no case was a kinetic energy correction found to be required. Runs were repeated until determinations within 0.2 sec were obtained.
Results ( a ) Isopiestic Data. Osmotic coefficient (cp) data, at 25', of HMT aqueous solutions are reported in Table I. From these data the linear correlation cp = 1 0.220m where m is HMT molality is derived (standard deviation =t0.0052). Although our isopiestic measurements have not been extended to solutions less concentrated than 0.4 m (owing to experimental difficulties), it appears TeaSonable to assume that the linear cp-m relationship is
+
The Journal of Phgsieal Chemietry
V. CRESCENZI, F. QUADRIFOGLIO, AND V. VITAGLIANO
Table I : Isopiestic Solutions of Hexamethylenetetramine and Potassium Chloride nl
cp
mKCl
0.4368 1,028 1.399 1,864 2,323 2,330 2.991 3.363 3.810 4.586 4.683
1.095 1.228 1.302 1.410 1.504 1.518 1.663 1.746 1.844 2.017 2.027
0.2641 0.704 1.015 1.436 1.919 1.941 2.679 3.125 3.678 4.689
also valid a t higher dilutions. general relationship In y
= (cp
- 1)
+
Saturated
On the basis of the
c(cp
- l)d l n m
tlhe equation for the activity coefficient of HMT in aqueous solution would then be In y = 0.440m. These results indicate that deviation from ideal behavior exhibited by HMT aqueous solutions is noticeable indeed. The type of deviation suggests that solute-solvent interactions should play a dominant role. (b) Density and Viscosity Data. Density and viscosity of HMT aqueous solutions have been determined for a range of HMT concentrations and at different temperatures in the interval 3-34'. Density data are reported in Table I1 in which, for each temperature considered, the parameters of the d-m relations, as obtained by a least-square analysis of the experimental values, are also reported. I n Figure 1 the apparent molal volume, 4", of HMT is plotted as a function of molality, m. It is interesting to point out that the 4" values decrease with increasing m. The effect, however, is smoothed out with increasing temperature. For each given molality, Q V is larger the higher the temperature. Of course the same features are found for the partial molal volume, Vz, of HMT (dotted curves of Figure 1). The VZ"value is seen to increase from 108.9 to 3.66' to 110.9 a t 33.46'. The results of the viscosity measurements are reported in Figure 2 and in Table 111. The parameters B and Q of the equation5 In 7/70 = (2) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions," Butterworth and Co. Ltd., London, 1959, p 476. (3) H. Tada, J. Am. Chcm. sot,, 82, 255 (1960). (4) "International Critical Tables," Vol. 3, P 24. (5) Reference 2, p 305.
HEXAMETHYLENETETRAMINE AQUEOUSSOLUTIONS
2315
Table 11: Relative Densities of HMT Solutions a t Various Temperatures d/& 3.66'
16.07'
= 1 $ am
+ bm* + em* + dm4
28.73'
Ab X
33.46'--
AX
AX
AX
m
d/da
1os
m
d/do
10s
m
d/do
10'
m
0.2405 0,3956 1.2367 1.7354 2.6955
1.00733 1.01199 1.03427 1.04643 1.06665
$3 -7 +16 -17 $4
0.477 0.998 1.950 2.720 3.687
1.01380 1.02742 1.04934 1.06466 1.08098
0 $1 -2 4-4 -1
0.615 1.178 1.516 1.950 2.720 3.687
1.01703 1.03132 1.03920 1.04834 1.06340 1.08006
$1 -5 -7 $14 -10 +3
0.2405 0.3956 1.2367 1.7354 2.6955
-
t,
Parameters of equation
oc
a X
3.66 16.07 28.73 33.46 (1
10:
$3.1342 $3,0481 $2.8788 +2.9789
See lower part of table for parameters of equation.
3
.
6
6
~
c
+2.612 +5.357 -3.862 -1.872
moles/l.
7/70
c, moles/l.
0.2344 0.3793 0.5764 1.0903 1.4604 2.0867
1.107 1.182 1.291 1.671 2.049 2.987
0.0513 0.1984 0.4102 1.4273 1.6678 2.0441
t,
oc
3.66 16.07 28.73 33.46
d
10'
x
d/do
106
0
1.00701 1.01138 1.03274 1,04376 1.06246
-1 +1 -2 0
10'
... -5.93 $7.83 3-8.62
Difference between the calculated and experimental value.
(q/qo) =
16.07°
c,
x
b X 10:
-3.153 -3.480 -1.557 -2.577
Table III: Relative Viscosities of HMT a t Various Temperatures;' In ~
5
BC/(1 - QC)"
28.73O
~
c,
3
3
.
4
6
~
c,
9/70
moles/l.
7/m
moles/l.
1.021 1.082 1.186 1.918 2.182 2.689
0.0511 0,1977 0.4088 1.4222 1.6616 2.0356 2.6149
1.020 1.077 1.174 1.838 2.072 2.509 3.483
0.2330 0.3770 0,5728 1.0826 1.4490 2.0672
Parameters of equation B
0
0.423 0.398 0.379 0.374
0.093 0.088 0.080 0.076
7/m
1 093 1.156 1.250 1.553 1.840 2.504
Results reported in Figure 3. All solutions were prepared by weight and the concentrations, C, in moles of HMT/I. were calculated using the equations for the density reported in Table 11. See lower part of table for parameters of equation.
BC/(1
- QC) according to which our viscosity data
may be reproduced, at each temperature considered, within the limits of the experimental accuracy (* 0.1%) are also listed in Table 111. The values of both parameters are seen to increase with decreasing temperature.
Discussion Considering the osmotic coefficient data it may be argued that deviation from ideality exhibited by HMT aqueous solutions is to be attributed in large part to solute-solvent interactions. Of course other important factors such as difference in dimensions between solute
and solvent molecules and the structured nature of water contribute to the observed effects. An even qualitative evaluation of such factors, however, does not appear feasible at present. It is nevertheless of some interest to consider the conclusions to which a very simplified theoretical approach to nonionic compound aqueous solutions as applied to the HMTHzOsystem may lead. The HMT-H20 system is here considered to belong to the class of "semiideal" mixtures. The semiideality approximation, introduced by Scatchard,6 (6) G . Scatchard, J . Am. Chem. SOC.,43, 2387 (1921).
Volume 71,Number 7 June 1967
V. CRESCENLI, F. QUADRIFOGILIO, AND V. VITAGILIANO
2316
of the solute. The latter is conceivably an important phenomenon for HMT in water. As an application of the Stokes and Robinson' treatment let us suppose that each HMT molecule binds n water molecules in n successive steps characterized by a common value of the equilibrium constant K . Average hydration number, 5, of HMT is accordingly defined as -
u
h=-
z
-.-
I
1
I
I
1
X
I
where
I
3
m
Z=1
4
Figure 1. Apparent molal volume, +, and partial molal volume, 72,at. different temperatures, as functions of molality of HMT aqueous solutions, m. & values are calculated from experimental density data. The vrm lines were drawn considering the + k m relations to be linear and with the aid of the equation 74 = QY m(&pv/h).
+
+ K h + . . . + (Ku,)"
(2)
and
dz d In a,
g=--
- Ka,
+ 2K2aW2+ . . . + ~(Ku,)" (3)
For an aqueous semiideal solution it is 55.51 ---
m
aw
1-a,
=&
(4)
Then from eq 1 and 4 is obtained
55.51 - _ - - - a, m 1-a,
0
1
2
3 r n
Figure 2. Reduced viscosity at different temperatures as a function of molality of HMT aqueous solutions, m. In the insert the coefficient B (see text) is plotted against the temperature.
has been recently utilized by Stokes and Robinson' to interpret osmotic coefficientdata of sucrose, glucose, and glycerol aqueous solutions. The approximation consists in considering that solute molecules interact with the solvent in a succession of solvation equilibrium steps to yield species which mix conforming to ideal laws. Deviations from ideality as experimentally observed would thus be entirely ascribed to hydration
-
U
z
(5)
Manipulations carried out according to relation 5 on osmotic coefficient data of the HMT-H20 system indicate that a very good agreement with the experimental data (pairs of aw-m values for a, values ranging from 0.84 to 0.96) has been obtained putting n = 18. The corresponding value of the hydration equilibrium constant is K = 1.06 f 0.01 in the indicated range of a, values. The value of E increases from 6.5 to 8.8 in the dilution range 4.7 to 0.96 m HMT. For n values greater or less than 18, eq 5 does not correctly fit the experimental m-a, data with a constant value for K . For n = 9, furthermore, no acceptable K value was found to reproduce the data. Quite naturally the n and % values reported above are merely indicative. In the case of sucrose the value n = 11 used by Stokes and Robinson,' besides giving a best fit of the experimental data ( K = 0.994), was a reasonable guess of the possible number of waterbinding sites on a sucrose molecule. In the case of HMT there is not, of course, such a high number of sites per molecule to which water molecules may be directly bound. Relying entirely on the validity of the treatment outlined above, it might be argued that the high hydration number obtained for HMT would (7) R. H. Stokes and R. A. Robinson, J. Phya. Chem., 70, 2126 (1966).
HEXAMETHYLENETETRAMINE AQUEOUS SOLUTIONS
actually reflect the immobilization of water molecules in a solvation cage around the hydrophobic solute. Considering the structure of the clathrate hydrate of HMT8 (easily formed upon cooling an HMT solution toward 0') in which three nitrogen atoms out of four of an HMT molecule are hydrogen bonded with three water molecules, it is logical to assume that also in solution and a t t > Oo, hydrogen-bond formation of water with HMT nitrogens takes place. Water molecules engaged in direct interaction with HMT may be also linked with other water molecules to give rise to a kind of flickering clusters of which HMT molecules constitute the cores. A justification of this hypothesis may be also found in the negative temperature coefficient of HMT solubility in water, a phenomenon which is common to other nonpolar structureforming solutes. Qualitative indications about the nature and extent of solute-solvent interactions in HMT aqueous solutions are also provided by the viscosity and density data reported in Figures 1 and 2 (Tables I1 and 111). The parameters B and Q of the viscosity equation In q / v o = B[C/(l - QC)], which gives a very good representation of the viscosity of HMT solutions a t each temperature considered (see Table 111), may be related to the so-called "effective rigid molar volume," Vz,of the According to theory it is B = 2.5V2, while Q is a function of V n and of the interactions between solute particles. The value of V z may be assumed to represent the molar volume of the solute including the hydration shell which is held too firmly to participate in the viscous shearing process. If the value 104 cc mole-' is taken for the molar = Of HR4T(d = 1*34dcCs Of "lid HMT and 140.19) then the B values reported in Table I11 suggest that an average of three water molecules is firmly attached per HMT molecule in solution. According to the interpretation of our isopiestic data presented above, an "equilibrium" hydration of HMT is derived which is about twice the hydration evaluated from the viscosity data. I n our opinion, however, a crilical comparison of these hydration numbers would be highly questionable. For what concerns the dependence of HMT solvation upon temperature, the negative temperature coeficient of parameter B suggests that hydration of HMT in dilutcl solution decreases with increasing temperature.1° Knowledge of B values a t different temperatures also permits evaluation of activation energy for the viscous flow of the HMT solution. It is easily shown in fact
2317
1 - 0 0 4
Figure 3. Difference between activation energy (cal mole-*) for the viscous flow of HMT aqueous solution (in the limit form -* 0) and water as a function of the temperature.
Figure 4. Near-infrared spectra a t 25" of water (- - -) and of water in 2.5 M HMT aqueous solution (). Spectra were recorded with a Beckman DK-2 spectrophotometer equipped with thermosbted cell holders. Matched 1.000-cm cells were used.
-
that R a / d ( l / T )
= lim[(E* m+O
-
Eo*)/m],where E *
and Eo* are the activation energies for the viscous flow of an infinitely dilute HMT solution and water, respectively. Values of RdB/d(l/T), plotted in Figure 3 against T, are seen to steadly increase with decreasing temperature. This may be interpreted to reflect a buildup of structure in the solutions promoted by the hydrophobic solute, which is obviously facilitated by decreasing the temperature. The density data (Table I1 and Figure 1) show that a t all temperatures the limiting (m --+ 0) partial molal volume of HMT (see Figure 1) is a t least 5% higher than the molar volume of HMT (Vzo = 104 cc). (8) T.C. W.Mak, J. Chem. Phys., 43, 2799 (1965). (9) G.W.Smith, ibid., 36, 3081 (1962). (10) R. L.Kay, T. Vituccio, C. Zawoyski, and D. F. Evans, J. Phy8. Chem., 70,2336 (1966).
Volume 71, Number 7 June 1967
JAN SANDSTR~M
2318
Calculation of the latter is based on the density of solid HMT and although the comparison is in this case a qualitative one, it appears plausible to assume that an “excess” increase in volume takes place when 1 mole of HMT is dissolved in a large amount of liquid water. The negative slopes of the approximate VZ m relations (Figure 2) lead us to think that an increased ice-likeness of water in the HMT solutions is promoted by the solute. This view appears to be in agreement also with some features exhibited by the near-infrared spectrum of water in HMT solution (see Figure 4) which suggest
that HMT affects water structure in a way formally equivalent to a decrease in temperature.” The attempted description of the HMT-H20 system is of course still highly speculative, but one according to which the results reported here find a consistent though qualitative interpretation.
Aclcnowkdgments. The authors wish to thank Professor Alfonso M. Liquori for his helpful advices during the course of this work and Dr. Donald G. Miller for critically reading the manuscript. (11) K.Buije and G. R. Choppin, J. Chem.Phys., 39, 2035 (1963)
Barriers to Internal Rotation in Thioamides. Experimental Results and Molecular Orbital Calculations
by Jan Sandstrom Department of Chemietry, u n ~rrsita, . of h n Lun., Sweden Accepted and Transmitted by The Faraday Society (November 50,1966)
The influence of the substituent R in RCSN(CH& on the barrier to the rotation of the dimethylamino group has been studied by nuclear magnetic resonance. The Arrhenius parameters have been determined together with the AF*, A H f , and AS* values and the effects of solvent and concentration on these have been studied. The AF* values show a very qualitative correlation with C-N ?r-bond orders calculated by a modified w method and a better correlation with the loss in 1-electron energy, AE,, which occurs when the dimethylamino group is rotated out of conjugation. This quantity also gives correct relations between the barriers of thioamides, amides, and amidinium ions, and it allows a crude prediction of the barriers in amidines and enamines.
Introduction It has been observed by Luttringhaus,
et al.,1 that
the carbonyl group interacts more strongly than the thiocarbonyl group with weakly electron-donating groups, whereas the reverse is tJ” with strongly ekcThis is in agreement with the tron-donating- _groups. opinion expressed by several authors2 that thioamides are more than the corresponding amides. As has been shown by Loewenstein, et The Journal of Physicol Chemtktw
one consequence of this is a considerably higher barrier to internal rotation in N,N-dialkylthioformamides (1) (a) A. Lattringhaus and J. Grohmann, 2. Naturforsch., lob, 365
(1955); (b) A. Luttringhaus, R. Mecke, R. Mecke, and J. Grohmann in “Elektronentheorie den Homiipolaren Bindung,” AkademieVerlag, Berlin, 1956, p 152. (2) (a) C. M. Lee and W. D. R u d e r , J. Ow.Chem.,27,2052 (1962); (b) K.A. Jensen, Acta Chem. Scand., 17, 551 (1963). (3) A. Loewenstein, A. Melera, P. Rigny, and W. Walter, J . Phys.
chem.,
a,1697 (ISM).
