A. A. ZAVITSAS, M. COFFINER, T. WISEMAN,AND L. R. ZAVITSAS
2746
stearate on fused quartz by measurements at Brewster’s angle. 3. In these experiments on silica, and in similar ones on glass, it is clear that the adsorbed water film is not modified to the degree cited by Deryagin and Churaev as maximum. There is, indeed, no indication that it is different from ordinary bulk water despite the fact that the appearance of anomalies should be favored by a t least two factors due to the films being extremely thin: (1) the proximity of the substrate; ( 2 ) a surface to volume ratio much larger than in the narrowest capillaries used to date. This by no means proves that anomalous water does not exist. In the first place, whether or not it forms is supposed to depend upon minute details of surface preparation. Moreover, even under favorable condi-
tions its genesis seems to be quite unpredictable.32 These results serve to narrow the scope of the search for the material. 4. Finally, the results illustrate a discrepancy between adsorption data obtained by ellipsometry and by conventional methods. Since these techniques are applied to optically smooth, planar surfaces, and to small particles, respectively, it may be of interest to discover the reasons for the substantial differences that are found. Acknowledgment. This paper is published by permission of the management of Mobil Research and Development Corp. (32) V. I. Anisimova, B. V. Deryagin, I. G. Ershova, D. S. Lichnikov, Ya. I. Rabinovitch, V. Kh. Simonova, and N. V. Churaev, Russ. J . Phys. Chem., 41, 1282 (1967).
The Reversible Hydration of Formaldehyde. Thermodynamic Parameters by Andreas A. Zavitsas, Mark Coffiner,l Thomas Wiseman,‘ and Lourdes R. Zavitsas Department of Chemistry, The Brooklgn Center of Long Island University, Brooklyn, New York (Receiued January 27, 1970)
11801
The t,hermodynamic parameters for the reversible hydration of formaldehyde have been evaluated by ultraviolet spectrophotometry, with appropriate corrections for polymerization and for nonspecific absorption contributions; AH = -8.4 f. 0.5 kcal/mol, A 8 = -12.6 f 1.7 eu. No significant shift of Am,, with temperature was found.
Our investigations of the kinetics and mechanisms
of the reactions of formaldehyde with phenol have led us to an examination of formaldehyde equilibria.2 We found no reliable measurements of the temperature dependence of the equilibrium constant for the hydration of f~rmaldehgde,~ eq 1. Although the mechanism
HCHO
+ HzO
HOCHzOH
(1)
of the reaction may involve more than one molecule of water, eq 1 is considered adequate for thermodynamic considerations. Spectrophotometric studies of the n-r* carbonyl absorption near 288 mp4have been hampered by several, occasionally unrecognized, factors : (a) the concentration of the unhydrated species is very small, for equilibrium 1 lies far to the right; (b) the extinction coefficient of the aldehyde is small and it cannot be determined without previous knowledge of the hydration equilibrium constant; (e) high total concentrations of formaldehyde cannot be used with impunity because of extensive reversible polymerization reactions6 of the type shown in eq 2, which become sigThe Journal of Physical Chemistry, Vol. 74,No.
i4,I070
nHOCHzOH
HO(CHZO).H
+ (?z- 1)HzO
(2)
nificant above 1 M total formaldehyde; and (d) concentrated aqueous solutions of formaldehyde exhibit substantial nonspecific absorption well into the region of the n--A* transition, thus making it difficult to assign a base line for the reading of the specific absorption. Calorimetric measurements of AH for reaction 1 on dissolving HCHO gas in water are complicated by the simultaneous release of a substantial heat of dilution.8 Values for the equilibrium constant’ are available (1)
Undergraduate research participant.
(2) A. A. Zavitsas, J . Polymer Sci., Part A-I, 6, 2533 (1968); A. A . Zavitsas, R. D. Beaulieu, and J. R. LeBlanc, ibid., 6, 2541 (1968); A. A. Zavitsas, J . Chem. Eng. Data, 12, 94 (1966). (3) For a review see: R. P. Bell, Adu. Phys. Org. Chem., 4, 1 (1966). (4) D. E. Freeman and D. Klemperer, J . Chem. Phgs., 40, 604 (1964). (5) Equation 2 is not meant to imply that HCHO is not the species
involved in the polymerisations. (6) A. Iliceto, Gazs. Chim. Ital., 84, 536 (1954). (7) Usually the equilibrium constant has been expressed as the ratio of the anhydrous to the hydrated species.
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THEREVERSIBLE HYDRATION OF FORMALDEHYDE or can be calculated from some existing data at various temperatures and concentrations of formaldehyde in aqueous solutions;8-10 there is rather poor agreement among the various values of the equilibrium constant and particularly among the slopes of plots of the points from each investigation us. l / T ° K . An excellent critical review of available data has been published.s An apparently reliable value for the equilibrium constant is that of Valenta,ll obtained by pulse polarography in dilute phosphate buffer of pH 7, reported as [HCHO]/ at 20”; the method is [HOCH20H] = 4.37 X based on the fact that HCHO is reducible whereas the hydrates are not,1zand on the assumption that dehydration is slow relative to the duration of the polarographic pulse. To be better applicable to systems containing appreciable amounts of formaldehyde, the equilibrium conO tent should be defined as in eq 3, in which N K ~ indi-
K
[HOCHzOH]/( [HCHOINH~O) (3) cates molar fraction of free water. Thus defined the polarographic value becomes K = 2.33 X los. =
Experimental Section Reagents. Formaldehyde solutions were prepared by mixing 20 g of paraformaldehyde powder (Mallinckrodt), 100 ml of distilledHzO andO.5 g of reagent grade magnesium oxide powder and distilling at a very rapid rate with a short head and a condenser cooled by lukewarm water; the first 25% of the distillate was discarded and the next 50% was collected. The p H of the solutions was usually between 3.8 and 4.6; batches of lower pH were discarded. Magnesium oxide greatly facilitates the solution of paraformaldehyde. The formaldehyde content of the collected distillate was approximately 17%, as HCHO, by the hydroxylamine hydrochloride titration method.I3 Solutions above 22 wt % were found to develop a haze on standing at room temperature. The pH was adjusted with 0.5 M sodium hydroxide immediately before use; it was rechecked a t the end of the measurements and it was found to have remained unchanged within the accuracy of the pH meter. Measurements. Uv measurements were obtained on freshly prepared formaldehyde solutions in thermostated cell compartments, either with 10.00-cm cells in a Bausch and Lomb Spectronic 505 operated in the absorbance mode, or with 5.00-cm cells in a PerkinElmer 450 operated in the transmittance mode with subsequent conversion of the readings to absorbance, We employed concentrations between 14 and 18 wt % over the pH range of 5.60-8.90. An average of ten observations a t various temperatures were taken per sample with the highest temperature reading taken first, the following six readings at progressively lower temperatures, and the last three readings at progressively higher temperatures again. In this manner, any ther-
mally irreversible changes that might have occurred during the measurements would have become apparent; this procedure also verified that the equilibria were established at each temperature. We found it imperative to limit the time of exposure of the samples to heat and to the uv beam of the instrument. The samples were removed from the thermostated compartments during most of the substantial time required for the attainment of a steady new temperature at the circulating bath; prolonged exposure led to an increase in the nonspecific absorbance (see below) and to the appearance of a new absorption at 326 mp.
Results In principle the concentration of HOCHzOH can be calculated if the polymerization equilibrium constants for each value of n in eq 2 are known; such calculations, using the same value of equilibrium constant for all values of n greater than 2, have been performed with the appropriate computer programs. l 4 We employed a different and simpler approximation which describes the polymerization equilibria quite accurately; the generalized eq 2 was found to be equivalent, on the average and over a wide range, to eq 4. The average K=26
2.6[HOCH20H]E [HO(CHzO)z.aHI
+ 1.6[HzOl
(4)
values of n and of the pseudoequilibrium constant given in eq 4 were found to give the best fit over the widest range to the available experimental data as shown in Figure l.15 We attempted to fit the bisulfite titration datal6 rather than that of the other sources which was obtained by nmr in DzOsolutions with unspecified deuterium isotope effects on the equilibrium constant for polymerization. In the hydration equilibria of higher aldehydes, isotope effects in D20 are known to amount to over 15%.1° Cyclic structures such as trioxane amount to less than 1%up to 30 wt yoformaldehyde and they can be ignored for our purposes.14 The per cent of formaldehyde in the form of monomeric hydrate is affected only slightly by temperature and pH and the same approximation, eq 4, can be used over the range of these reaction variables used in our measurements. (8) R.Bieber and G. Trumpler, Helv. Chim. Acta, 30, 1860 (1947). (9) J. F. Walker, “Formaldehyde,” 3rd ed, Reinhold, New York, N . Y . , 1964, p 61. (10) L. C. Gruen and P. T. McTigue, J. Chem. Xoc., 5217 (1963). (11) P. Valenta, Collect. Czech. Chem. Commun., 25, 853 (1960). (12) R.Brdicka, ibid., 20, 387 (1956); Chem, Listy, 48, 1458 (1954); 2. Elektroehem., 59, 787 (1955). (13) See ref 9, p 493. (14) K. Moedritaer and J. R. Van Waaer, J. Phys. Chem., 70, 2025 (1966). (15) For a description of the solution of the empirical equation see the Appendix. (16) A . Ilioeto and S. Beaei, Ric. Sci., 19, 999 (1949). The Journal of Phgsical Chemistry, Vol. r4, No. 14, 1970
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6 0.2
a fv
0.1
A. A. ZAVITSAS, M. COFFINER, T.WISEMAN,AND L. R. ZAVITBAS
-
260
270
280
290
300
310
M I L L I M I
330
320 C R 0
N
340
350
S
Figure 2. Absorbance us. wavelength for run no. 1 of Table I. The presence of some nonspecific absorbance under Amax is apparent.
The small temperature dependence is evident from the data in Figure 1, and from reported values for the average degree of polymerization, P,, excluding monomer: at 13.76 wt % formaldehyde in D20, P, = 2.28 at 34") 2.27 at 60°, and 2.25 at (30"; at 22.61 wt %, P, = 2.42 at 34", 2.44 at 60°, and 2.43 at QO"." These concentrations bracket our range. I n agreement with the above, the enthalpy change for the polymerization has been reported to be very mall.'^,'^ The negligible effect of pH has been demonstrated by cryoscopic measurements of the average molecular weight of a 25.52 wt % solution; the value increased by 0.4% in going from a neutral solution to p H 10.7.19 The extinction coefficient of the n-n" absorption of HCHO was assumed to remain constant over our temperature range, by analogy from similar reports on acetonelnconfirmed by us. Typical uv curves from our work are shown in Figure 2. Since the hydration equilibrium lies far to the right, eq 1, semilog plots of absorbance vs. l/T°K should be linear. The relation between the equilibrium constant for hydratioh, I
