ACTIVITY COEFFICIENTS AND MOLAL VOLUMESOF TETRAETHANOLAMMONIUM HALIDES
3569
Activity Coefficients and Molal Volumes of Two Tetraethanolammonium Halides in Aqueous Solutions at 25"
by Wen-Yang Wen and Shuji Saito Chemistry Department, Clark University, Worcester, Massachusetts
(Received May 10,1966)
Two symmetrical tetraethanolammonium halides have been employed in a study of the effect of terminal hydroxyl groups on the cationic behavior in aqueous solutions. The activity and osmotic coefficients, as well as apparent and partial molal volumes of the respective fluoride and bromide in water, were measured at 25' and compared with the corresponding tetraalkylammonium halides. The results are in agreement with the notion that the substitution of terminal methyl groups with hydroxyl groups diminishes the peculiaritiesof symmetrical quaternary ammonium ions in water. In other words, quaternary ammonium cations with terminal hydroxyl groups behave more normally and affect the structure of water less than the corresponding tetraalkylammonium ions.
Introduction Studies on properties of aqueous solutions containing tetraalkylammonium salts have generally confirmed a view that some of the large symmetrical cations enhance the icelike or cagelike structure of water.'-* In contrast, the effects of tetrahydroxyalkylammonium ions on the water structure can be expected to be rather different. With a picture of water as consisting of flickering clusters of hydrogen-bonded molecules, Frank and Wen1 inferred that the solutes containing hydrogen-bonding groups like NHz or OH do not alter water structure much, if at all. In this respect it will be of considerable interest to compare physical properties of tetrahydroxyalkylammonium salt solutions with those of the corresponding tetraalkylammonium salt solutions. In this paper we are reporting the activity and osmotic coefficients as well as apparent and partial molal volumes of tetraethanolammonium fluoride and bromide in water at 25'. Experimental Section Materials. (1)Tetraethanolammonium bromide, (HOCtH&NBr, was prepared by heating a mixture of triethanolamine (in excess) and 2-bromoethanol with anhydrous methanol at 115" for 22 hr. The unreacted triethanolamine in the product was precipitated as the hydrobromide, (HOC2H&N.HBr (m.p. 185"),
by bubbling hydrogen bromide gas through the solution. Separation of the desired compound from the remaining triethanolamine hydrobromide was effected by repeated recrystallizations-five times with ethanol and twice more with ethanol-chloroform mixture. The anhydrous crystals which melt at 100-102" were very hygroscopic and were handled in a drybox. A gravimetric analysis of the bromide ion as AgBr in the compound indicated its purity to be 99.59 0.0201,. Density of the anhydrous crystal was found to be 1.600 g./ml. at 25". ( d ) Tetraethanolammonium fluoride, (HOC2H4)4NF,was prepared by the titration of a methanol solution of the mixture of (HOC2H4)4NOHand (HOCZH4)aN (Matheson Coleman and Bell) with aqueous HE' solution. By pH titration with a Beckman Model G pH meter, the neutralization point was determined to be pH 9.6. The resulting solution was and the white solid evacuated to dryness over PzOS, obtained was recrystallized several times from methanol. The same compound was also prepared by the double metathesis of ( H O C Z H ~ ) ~ Nwith B ~ Ag~S04 and BaFz. The fluoride ion content of the compound
*
(1) H. S. Frank and W. Y. Wen, Discussions Faraday Soc., 24, 133 (1957). (2) W. Y. Wen, Ph.D. Thesia, University of Pittsburgh, 1957, Microfilm no. 58-132. (3) W. Y.Wen and S. Saito, J. Phys. Chem., 6 8 , 2639 (1964).
Volume 69,Number 10 October 1966
WEN-YANGWEN AND SHUJISAITO
3570
was found to be 99.0 f 0.5% of the theoretical value by a gravimetric analysis in which the fluoride ion was precipitated as CaF2. The cationic analysis with NaB(CsHs)4 gave results which are consistent with the formula (HOC2H4)4NFand not with (HOC2H& NaHF. The melting point of the former (our desired compound) is 195" while the latter is around 75". The density of the anhydrous crystals was measured to be 1.353 g./ml. at 25". In contrast to the bromide, the tetraeth:tnolammoniumfluoride is only very slightly hygroscopic. (3) Reagent grade KC1 was recrystallized from water and dried at 650" under nitrogen atmosphere. Doubly distilled water was used for making up the solution to be measured. Apparatus and Measurements. (1) Activity and osmotic coefficients were determined by the gravimetric isopiestic comparison method with an apparatus similar to that employed by Owen and C ~ o k e . Several ~ sets of four dishes (made of gold or gold-plated silver) . ~used on a with dimensions of 4 X 4 X 1.75 ~ mwere 3.5 cm. thick copper block having a diameter of 15 cm. All contact surfaces of the dishes and the copper block have been carefully polished to render good heat conduction. KC1 reference solutions (2 to 4 ml. each) were placed into two of the dishes and sample solutions in the other two dishes, all dishes equipped with covers. Four dishes of a set were tightly clamped onto the copper block and placed in a desiccator which was then evacuated and immersed in a 600-1. constant-tempera0.01". The desiccators in the ture bath held at 25 bath were carefully evacuated with an aspirator in several stages over a period of 24 hr. to a pressure of 25-30 mm. The equilibrium was reached after gently rocking the unit for 3 days to 2 weeks depending on the concentrations of the solutions under measurements. The dishes were then covered before admitting air into the desiccators, The solutions in duplicate dishes were considered in equilibrium when they arrived at the same molality to within 0.1%. All weighings were done with :t Sartorius single-pan balance Model 2503 having a capacity of 200 g. and a sensitivity of 0.05 mg. Our activity coefficient data are believed to be precise to within 0.50100. (2) The density measurements were made in Weldtype pycnometers of 5 and 25 ml. capacity standardized with doubly distilled water. The process of Weissberger5 was followed in the measurements, all weights being reduced to weights in vacuo. Replicate cordeterminations reproduced within 5 X responding to a precision for the apparent molal volume of 4: 0.1 ml.
*
The Journal of'Physical Chemistry
Results and Discussion A. Osmotic and Activity Coeficients. The measured molalities of the isopiestic KC1 and tetraethanolammonium halide solutions are given in Table I. In this table the values of the isopiestic ratio R, defined as the ratio of the molality of KC1 to the molality of the isopiestic sample solution, are also included. From the isopiestic ratio and the known osmotic coefficient of the reference salt (bo, the osmotic coefficient of the sample salt (b can be obtained simply by the equation (1)
(b = &o
For measurements of concentrated solutions, several kinds of saturated salt solutions were used as the reference. + of the sample salt is, in this case, given by -55.51 In a, '=
(2)
vm
Table I: Molalities of Isopiestic Solutions and Isopiestic Ratios Reference salt*
(HOCzH4)rNF
0.1042 0.1800 0.2221 0.3239 0.4535 0.6322 0.8211 0.9457 1.189 1.498 1.695 2.025 2.248 2.516 2.730 3.234 3.264 3.588 4.096 Satd. KC1 Satd. (NH4)&04 Satd. NH&1 Satd. NaCl Satd. NaNOa
0.1045 0.1800 0.2215 0.3216 0.4480 0.6199 0.7994 0.9168 I . 142 1.422 1.601 1.892 2.082 2.317 2.501 2.929 2.955 3.228 3.653 4.249 5.294 5.981 6.404 6.764
R
KC1
(HOC2Hr)rNBr
R
0.9971 0.9997 1.003 1.007 1.012 1.020 1.027 1.032 1.042 1.053 1.058 1.070 1.080 1.086 1.092 I. 104 1.105 1,112 1.121
0.1189 0.1816 0.1887 0.2972 0.3099 0.4372 0.5328 0.5560 0.7017 0.8332 0.9952 1.035 1.360 1.433 1.597 1.710 1.919 1.961 2.217 2.508 2.733 2.862 3.146 3.407 4.094 4.759
0.1239 0.1934 0.2017 0.3262 0.3410 0.4935 0.6128 0.6416 0.8259 0.9962 1.212 1.266 1.707 1.807 2.030 2.187 2.472 2.531 2.880 3.276 3.584 3.760 4.149 4.504 5.441 6.354
0.9596 0.9386 0.9355 0.9111 0.9088 0.8859 0.8695 0.8666 0.8496 0.8364 0.8214 0.8181 0.7966 0.7930 0.7870 0.7821 0.7761 0.7750 0.7697 0.7654 0.7623 0.7610 0.7582 0.7566 0.7524 0.7490
KC1 unless speczed otherwise.
~~
(4) B. B. Owen and T. F. Cooke, Jr., J . Am. Chem. Sac., 59, 2273
(1937). ( 5 ) A. Weissberger, "Physical Methods of Organic Chemistry," Vol. 1, 2nd Ed., Part I, Interscience Publshers, Inc., New York, N. Y., 1949.
ACTIVITY COEFFICIENTS AND MOLAL VOLUMES OF TETRAETHANOLAMMONIUM HALIDES
3571
where a, is the activity of water in the reference solution and v is the number of moles of ions formed by the ionization of one mole of salt (Y = 2 for 1-1 electrolytes). The mean molal activity coefficients, y*, of the salts have been calculated by the equation
The integral in the right-hand side of eq. 3 was evaluated by plotting 1 - 4 against y/m, but the calculation involves an extrapolation to infinite dilution. Since the isopiestic method cannot be employed t o solutions with concentrations appreciably less than 0.1 m, the values of 1 - Q, at m less than 0.1 were determined by a graphical extrapolation to 1 - 4 = 0 at m = 0 applying the DebyeHuckel limiting law, 1 - 4 = 0.3908. at 25". In the absence of low-concentration data, this extrapolation method is likely to introduce some error, and, consequently, In y* values so determined may be subject to a small constant correction when data on more dilute solutions become available, perhaps from concentration cells with transference. The osmotic and activity coefficients of the two tetraethanolamrnonium halides so obtained are given in Table 11. Mean molal activity coefficients of the two salts are 1 for the entire dotted against molalities in Figure concentration range studied. In Figure 2, values of
d%,
0 0
.
m
2
2
Figure 1. Mean molal activity coefficients, T*, of two tetraethanolammonium halides in aqueous solutions at various molal concentrations, m, at 25': A, (HOC2H&NF; B, (HOC2H4hNBr.
4
-0.31
I
0.5
0
1.0
i
1.5
45 Figure 2. Plots of log -r* vs. l/m for ( ~ & H T ) ~ N(CH,),NF, F, CsF, KF, (HOCsH&NF, and NaF in aqueous solutions a t 25".
Table II : Osmotic and Activity Coefficients of Tetraethanolammonium Fluoride and Bromide in Aqueous Solutions a t 25' -(HOCHzCHz)rNF--
-(HOCHzCH2)4NBr-
Conen., m
Q
Y*
Q
Y*
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
0.925 0.912 0.911 0.911 0,912 0.914 0.917 0.921 0.925 0.929 0.938 0.948 0.960 0.972 0.985 0.997 1.010 1.022 1.034 1.046 I . 076 1.105 1.132 1.158 1.183 1.208 1.234
0.758 0.710 0.685 0.671 0.664 0.657 0.653 0.650 0.648 0.645 0.644 0.645 0.648 0.652 0.660 0.667 0.676 0.684 0.695 0.705 0.732 0.762 0.798 0.832 0.868 0 * 900 0.940
0.902 0.860 0.835 0.814 0.798 0.784 0.773 0.764 0.757 0.750 0.737 0.728 0.720 0.715 0.711 0.708 0.706 0.706 0.706 0.706 0.709 0.712 0.717 0.723 0.728 0.734 0.740
0.730 0.641 0.588 0.545 0.515 0.488 0.465 0.448 0.432 0.418 0.395 0.376 0.360 0.345 0.333 0.324 0.316 0.308 0.300 0.296 0.283 0,275 0.266 0.258 0.252 0.250 0.248
Volume 69,Number 10 October 1966
3572
log y* are plotted against m < for (HOC2H4)4NF as well as some tetraalkylammonium fluorides and alkali fluorides for comparison. As can be seen from the figure, the activity coefficients of (HOC2H4)4NF are very different from those of either (n-C3H7)4NF or (C2Hs)4NFbut rather close to those of KF. This clearly demonstrates large differences which exist between quaternary ammonium ions with terminal methyl groups and those with terminal hydroxyl groups. The strikingly high activity coefficients of tetraalkylammonium fluorides in water have been attributed to the different structural effects of cations and anions upon water structure.6 In contrast to (CnH2,+1)4N+ ions, the (HOC2H4)4N+ion would be expected to affect the structure of water rather little since it should be able to enter into the flickering clusters of water and participate readily in the formation and disruption of the clusters. In this respect the structural effect of the F- ion upon water is much closer to that of the (HOC2H4)4N+ ion than to that of the (C,H2,+d4N+ ions, and, therefore, (HOC2H4)4NFin water should be more stable and have lower free energy than the tetraalkylammonium fluorides. Incidentally, K+ ion is believed t,o affect the structure of water very little,l and in this regard it appears similar to the (HOC2H4)4N+ion although the size and type of these two ions are quite different. In Figure 3, values of log y* are plotted against m < for (HOC2H4)4NBrand other bromides for a similar comparison. From Figure 3 one sees that the activity coefficients of (HOC2H4)4NBr are higher than those of either (C2Hs)4NBror ( C ~ H T ) ~ but N Bvery ~ much lower than those of KBr. Since tetraalkylammonium bromides and iodides have been known to possess abnormally low activity coefficient^,^!^ the substitution of terminal methyl groups of these cations by hydroxyl groups should result in raising the low values. This expectation is borne out by our results, at least for the compound under discussion, though to considerably lesser extent than we expected. As mentioned above, the y* values of (HOC2H4)4NBr are much lower than those of KBr in distinct contrast to the case of the corresponding fluorides. Although the reasons for this observation are not yet clear, it is probably due to the “structural salting-in”* of the cations by the bromide ions. At concentrations above 1.3 m the T* values of (HOC2H4)4NBrbecome smaller than those of (C3H7)4NBr k. opposition to the situation at concentrations below 1.3 m. This may be attributed to the associationof (HOC2H4)4NBrthrough hydrogen bonding of the cations. Similar inference can be drawn from the data of the molal volumes of this salt which will be discussed in the following section. T h e Journal of Physical Chemistry
WEN-YANGWEN AND SHUJISAITO
\.
’. 1 .
’.
‘.‘.
y* 0s. 4% for KBr, (HOC*H&NBr, (CsH&NBr, and (n-CsH,)dNBr in aqueous solutions a t 25’.
Figure 3. Plots of log
B. Apparent and Partial Molal Volumes. The apparent molal volumes, cp2, were calculated from the density data by the equation cp2
= -
m
Po
where po is the density of pure water; M2, the molecular weight of the salt; p , the density of the solution. The partial molal volumes, V2, were computed from cpz by the equation
L
-
2
where c is the molar concentration. These values for the two salts at 25’ are listed in Table 111. In Figures 4 and 5 the values cp2 and V2 are plotted against 6 for (HOC2H4)4NF and (HOC2HJ4NBr, respectively. For both salts p2 seems to vary linearly with di in the low concentration range (c, 0.1-0.6) following the equation (6) W. Y. Wen, 8.Saito, and C. M. Lee, Abstracts of Papers, Division of Physical Chemistry, 148th National Meeting of the American Chemical Society, Chicago, Ill., Sept. 1964, p. 35V. (Details will be published shortly.) (7) S. Lindenbaum and G. E. Boyd, J . Phys. Chem., 68, 911 (1964). (8) H. S. Frank, &id., 67, 1554 (1963).
ACTIVITY COEFFICIENTS AND MOLAL VOLUMES OF TETRAETHANOLAMMONIUM HALIDES
y2 =
920
+ S,Z/c
(6)
where y20 (= V2O) is the pz at infinite dilution and S, is the limiting slope. Values of Vz0 obtained from these plots art? 150.6 &./mole for (HOCzH4)4NFand 176.9 ml./mole for (HOC2H4)4NBr,respectively. S, values are 3.0 for the fluoride and 1.4 for the bromide, respectively, in.units of (IO&./mole) a'z.
5!O l O-
'
0.5
1.5
1.0
2.0
I
d5
Figure 4. Apparent and partial molal volumes of (HOC2H4)4NFin aqueous solutions a t 25" plotted against .\/E where c is the molar concentration.
' * 5 1 - - i
0
0.5
1.0
1.5
2.0
Figure 5. Apparent and partial molal volumes of (HOC2H4)iNBrin aqueous solutions at 25' plotted against .\/E where c is the molar concentration.
According to the Debye-Huckel theory, the limiting slope of apparent molal volume for any 1-1 electrolytes should be 1.868 at 25" as emphasized recently by Redlich and Meyer.g Since this value of S, has also been confirmed for (CH&NBr by Hepler, Stokes, and Stokes,IO there is no reason why the same value of
3573
Table 111: Apparent and Partial Molal Volumes of Two Tetraethanolammonium Halides in Aqueous Solutions a t 25' (Unit: ml./mole) -(HOCzH4)tNF-Concn., m
0.1 0.2 0.4 0.6 0.8 1.0 1.5 2.0 2.5 4.0 10.0
PZ
151.5 151.9 152.4 152.8 153.2 153.5 154.1 154.7 155.3 156.5
...
vz 151.9 152.6 153.4 154.0 154.5 155.0 156.0 157.0 157.8 159.5
...
----(HOCzH4)aNBf9 2
VP
177.3 177.5 177.7 177.9 178.0 178.1 178.4 178.6 178.8 179.4 180.5
177.5 177.8 178.1 178.3 178.6 178.8 179.1 179.4 179.7 180.4 181.8
S, would not apply to (HOGH4)dNF and (HOC2H4)dNBr. The S, values of 3.0 and 1.4 obtained for the two saIts indicate that the range of our measurements has not extended to a dilute enough region to conform to the theoretical value. It also shows that there are at least two ranges of concentration in which the plot of p2vs. 4;can be linear with different slopes. Similar remarks can be made about the S, values of several tetraalkylammonium bromides reported by us previ~usly.~ In view of this, the Vzovalues found above may be slightly in error, but not too much because of the fact ion, obtained by subthat Vzo for the (HOC2H4)4N+ tracting anionic volume, is 152.2 ml./mole from the fluoride,ll which is in a reasonable agreement with a value of 151.2 &./mole obtained from the bromide.11 The values of V2O obtained by the above-mentioned procedure for (C2H5)4NBr and (C3H7)4NBr3are included in Table IV for comparison. S , values for these salts in the concentration range of 0.1 to 0.6 M are also given in this table. Figure 6 gives a composite plot of Vzoagainst 6for the three bromides. It is noteworthy that Vzo of (HOC2H,)4NBr is rather small, and its S, is positive when compared with the respective quantities of (C3H7)4NBr. The value of V20 for (HOCZHJ4Nf is peculiarly small, as a matter of fact, almost as small as that for (CzHb)qN+. It appears as if the OH groups were occupying hardly any volume, an observation which we are not ready to interpret at present. (9) 0. Redlich and D. M. iMeyer, Chenz. Bev., 64, 221 (1964). (10) L. G. Hepler, J. M. Stokes, and R. H. Stokes, Trans. Faraday Soc., 61, 20 (1965). (11) 7 2 0 values for F - and Br- ions are taken as -1.6 and 25.7 ml./mole, respectively. See J. Padova, J . Chem. Phys., 39, 1552
(1963).
Volume 69, N u m b e r 10
October 1966
3574
WEN-YANG WENAND SHUJISAITO
Table IV : Partial Molal Volumes a t In6nite Dilution and the Limiting Slopes" a t 25" Compd.
VzO, d . / m o l e
( HOC2H4)4NBr
176.9 175.3 240.8
(CJ%)4NHr3 (C3H7)4NHr3
s,, (10
I
&./mole)
1.4 -3.3 -6.0
a Obtained from an extrapolation of experimental data on the concentration range of 0.1 to 0.6 M .
The positive S, value for (HOC2H4)4NBr in the indicated concentration range can be taken as a manifestation of its hydrophilic nature in contrast to the negative values for (CzH&NBr and ( T L - C ~ H ~ ) ~ N B ~ which have been interpreted as indicative of their hydrophobic ~ h a r a c t e r . ~As far as S, values are concerned, alkali halides also give positive values in the concentration range under discussion, but owing to its 0 b.5 I.o I.5 large size, the (HOCzH&N+ ion should show little 45 electrostrictive effect in contrast to the much smaller Figure 6. Partial molal volumes of (HOCZH&NBr, alkali metal ions. We think, therefore, that perhaps (C2H6)aNBr,and (n-CaHT)DBr in aqueous solutions a t some interionic hydrogen bonds as well as hydrophobic 25" plotted against d Jc where c is the bonds among (HOCZH~)~N+ ions may be formed at molar concentration. higher concentrations and result in poorer packing with water niolecules and cause the cp2 to increase with measurements that, for (HOC2H4)4NBr in aqueous concentration. However, since the electrostrictive solutions, the B coefficient in the Jones-Dole equation effect of Br- ions will also contribute to Sv, the above and also dB/dT, are quite small when compared with speculation about the cations is made plausible only those of (n-C3H&NBr.la Price and Agar observed in view of the low activity coefficients of this salt, and low heats of transport for the (HOCzH4)4N+14 particularly above 1.3 m. As reported above, (HOG(CH3)3NC2H40H+ ions in distinct contrast to the H4),NF has, higher activity coefficients indicating high values of the corresponding tetraalkylammonium relatively little association in aqueous solutions. ions. All these results support the idea that the solutes The large value of S, observed for the fluoride may, containing hydrogen-bonding groups like OH do not therefore, be taken to indicate a strong electrostrictive alter water structure much, if at all. effect of F- ions on water.I2 In conclusion, present studies offer support to the Acknowledgments. We wish to thank Professor contention that the substitution of terminal methyl Benton B. Owen for the technical assistance at the groups with hydroxyl groups will diminish the peculiariinitial stage of our isopiestic investigation. This ties of quaternary ammonium ions in water. In other research is supported by the U. S. Department of the words, cations with terminal hydroxyl groups will Interior, Office of Saline Water, through Grant No. behave more normally and affect the structure of 14-01-WOl-306 and 14-01-0001-456. water less than the corresponding tetraalkylammonium ions. From heat capacity measurements, Frank and (12) 8, values for EF, KC1, KBr, and KI are 3.35, 2.33, 1.94, and Wen have found that (HOC2H4)NBrin water gives 1.56,respectively, in the concentration range under discussion. almost no extra apparent molal heat capacity,13 as (13) H. 8. Frank and W. Y. Wen, unpubhhed results. opposed to the large excess observed for (n-CdH&(14) C. D. Price, Ph.D. Thesis, Cambridge University, 1961. NBr.1 In :tddition, they have found by viscosity (15) J. N. Agar, Advan. Eledrochem. Eledrochem. En+, 3 , 96 (1963).
The J o u r d o j Physical Chemistry
