984
G. BARONE, V. CRESCENZI, A. LIQUORI, AND F. QUADRIFOQLIO
Physicochemical Properties of Hexamethylenetetramine Aqueous Solutions
by G.Barone, V. Crescenzi, A. M.Liquori, and F. Quadrifoglio Centro Nazwnale d i Chimica de& Macromolecole (CNR) Sezwne 111, Instituto Chimico, Univeraitd di Napoli, Naples, Italy (Received November 8, 1066)
Some experimental results concerning physicochemical properties of the hexamethylenetetramine (HMT)-water system are discussed. These results indicate that HMT behaves as a typical water structure reinforcing agent. As an interesting aspect of this property, the influence exerted by HMT on hydrophobic interactions in simple systems is illustrated.
Hexamethylenetetramine (HMT) solubility in water decreases with increasing temperature. This property, which is shared by some other tertiary amines, is explicable in terms of increasing association with water at low temperatures. Furthermore, by decreasing the temperature of a saturated HMT aqueous solution toward 0" an HMT hexahydrate is easily obtained. For this hydrate, X-ray data have revealed a novel type of clathrate structure in which each HMT molecule is located in a cage within a clathrating framework of hydrogen-bonded water molecules.2 HMT also forms a large series of complexes with different inorganic salts, linking a high number of water molecules, the crystal structure of some of which has been recently determined in detai1.3*4 All of these findings clearly suggest that HMT should exert a marked influence on water structural organization also at room temperature. HMT might thus also influence the occurrence of certain phenomena in aqueous systems in which hydrophobic interactions are of critical importance. The interesting possibility has prompted an investigation, which is being carried out in this laboratory, on some physicochemical properties of HMT aqueous solutions. Properties studied include, besides density, viscosity and near-infrared spectra of HMT-water systems, their solubilizing power toward some hydrocarbons, and the effect on micellization of colloidal electrolytes. We wish to report here some preliminary results of this investigation. For comparison purposes, data regarding analogous properties of urea and methylurea are also reported. In Figure 1, the relative density and the relative The J o u r d of Physical Chemktry
viscosity at 25" of HMT aqueous solutions are plotted as functions of HMT molality. In the same plot, similar data for urea and methylurea are also reported. All HMT, urea, and methylurea (carefully purified samples) solutions were prepared shortly before use at room temperature to avoid hydrolysis of the compounds. The apparent potentiometric pH of the more concentrated solutions of HMT is below 9. The density and particularly the viscosity of HMTwater solutions suggest that HMT would exert an influence on solvent organization. An indication of this is also obtained by considering the near-infrared spectra of water in 2.5 M HMT solution reported in Figure 2 together with those of water in 8 M methylurea and urea solutions. With reference also to the spectra reported by Buijs and Choppin,6 HMT appears to exhibit a unique structure-forming power. In Figure 3, data are reported on the solubility at 25" of n-pentane in HMT, urea, and methylurea aqueous solutions, respectively, determined by means of a gas chromatographic procedure.6 S, is the increment in n-pentane solubility in the aqueous solutions relative to the solubility in water at 25". Higher solubility of n-pentane is thus seen to occur in the HMT solutions. A few data on the solubility at 25" of pyrene and 3,4benzpyrene in HMT, urea, and methylurea solutions (1) J. F. Walker, "Formaldehyde," 3rd ed, Reinhold Publishing Corp., New York, N. Y., 1964,p 524. (2) T. C. W. Mak, J . Chem. Phys., 43, 2799 (1965). (3) P. DeSantis, A. L. Kovaos, A. M. Liquori, and L. Marearella, J . Am. Chem. SOC..87, 4965 (1965); L. Mazzarella, A. L. Kovacs, P. DeSantis, and A. M. Liquori, Acta Crust., in press. (4) A. L.Kovacs and L. Mazzarella, Ric. Sci., in press. (5) K. Buijs and G. R. Choppin, J . Chem. Phys., 39, 3025 (1963). (6) G.Barone, V. Crescenei, B. Pispisa, and F. Quadrifoglio, to be published.
PHYSICOCHEMICAL PROPERTIES OF HEXAMETHYLENETETRAMINE AQUEOUS SOLUTIONS
fj*) . I
1
1.100
_.___! 1
1
1
1
I ' 0.6
985
I
-
1.050
/A 1000 5
2
1
a4
I 4
3
-
-
Figure 2. Near-infrared spectra at 25" of: -, water, and of water in: - - -, 8 M methylurea; , 8 M urea; and -, 2.5 M hexamethylenetetramine. Optical path = 1.000 cm.
-
....
Figure 1. (A) Relative density a t 25" of urea (0,methylurea and . , hexamethylenetetramine aqueous solutions) as a function of molality. The line for urea has been drawn using the data reported by F. T. Gucker, Jr., F. W. Gage, and C. E. Moser, J. Am. Chem. SOC.,60, 2582 (1938). (B) Relative viscosity at 25' of 0, urea, 0, methylurea, and . , hexamethylenetetramine aqueous solutions as a function of molality, m.
4~ 3
obtained by means of experimental procedures fully illustrated elsewhere' are reported in Table I.* Table I
--
Solubility at 2 5 O , in moles/l.
Hydrocarbon
Pyrene 3,4-Benzpyrene
HnO
8 M urea
2.9 M
HMT
3
8 M metbylurea
4.4 X 10-6 1.4 X lo-' 7 . 7 X lo-'" 6 . 6 X 1.9 X 10-Sb 1 . 8 X 10-7 2 . 8 X 10-6 1 . 3 X lod6
See ref 8. 'See ref 7.
Concentrations of solubilizing agent were chosen in order to obtain figures for the solubility of the two polycyclic hydrocarbons studied, of the maximum attainable accuracy in each case. Studies carried out by different authorss-" have shown that urea increases the critical micelle concentration (cmc) of typical colloidal electrolytes, such as sodium lauryl sulfate (NaLS). Our results indicate
Figure 3. Relative increment in solubility, S, at 25" of n-pentane in: 0, urea, 0 , meththylurea, and 0, hexamethylenetetramine aqueous solutions (with respect to n-pentane solubility in water a t 25°)6 as a function of the concentration, M , of the latter three compounds.
that methylurea, and more so HMT, are capable of exerting a similar action, to a higher degree. This is illustrated in Figure 4, in which the specific conductivity of NaLS in H20, 2 M methylurea, 1 M and 2 M HMT, respectively, is plotted against NaLS concentration. Critical micelle concentration is revealed in (7) G. Barone, V. Crescenzi, A. M. Liquori, and F. Quadrifoglio,
J. Phya. Chem., in press. (8) H. B. Klevens, J. Phya. Chem., 54, 283 (1950); W.W.Davis,
M. E. Krahl. and G. H. A. Clowes, J . Am. C h a . Soc., 64, 108 (1942). (9) P.Mukerjee and A. Ray, J. Phy8. Chem., 67, 190 (1963). (10) W. Bruning and A. Holtzer, J. Am. Chem. Soc., 8 3 , 4866 (1961). (11) M. J. Schick, J. Phys. Chem., 68, 3585 (1964).
Volume 71, Number 4
March 1967
986
G. BARONE,V. CRESCENZI, A. LIQUORI,AND F. QUADRIFOGLIO
10
-
I
I
I
I
,
8-
6+=
Q h-
I
0'
0.5.10-'
I
1
I
1.0
1.5
2.0
CN~LS
I
2.5
.,
Figure 4. Specific conductivity of sodium lauryl sulfate (NaLS) in: 0,water; at 2 M methylurea; ut 1 M hexamethylenetetramine; and 2 M hexamethylenetetramine at 25". Concentration of colloidal electrolyte is given as equivalents per liter of solution.
the plots of Figure 4 by the breaks in the dependence of specific conductivity upon colloida.1 electrolyte con-
The J o u Tof~Physical ~ ~ Chistry
centration. Details on the experimental procedure and other results will be soon reported and discussed elsewhere.l 2 It is relevant to point out here that the cmc for NaLS in 2 M methylurea is shifted to 1.15 X N , while in 2 M HMT it is 1.35 X N as compared to the cmc N . Similar but less value in water, 0.81 X marked effects have been found by Schick" in his investigation on the influence of urea on the crnc of NaLS, by means of surface tension measurements. The set of data reported here appears to afford, in our opinion, evidence that HMT behavior in aqueous solution has many points of similarity with those typical of strong water-structure perturbing agents. These observations deserve further detailed investigations: work is being carried out along these lines in our laboratory. The study of the possible influence of HMT on the stability of biological macromolecules secondary structures in water appears a logical extension of the present research.
(12) HMT is a very weak base, and its limited ionization should not affect appreciably, by itself, the cmc of NaLS. See H. Tada, J. Am, Chem. SOC.,82, 255 (1960).
