3952
B. E. CONWAY AND R. E. VERRALL
an intermediate structure. Hockey' and others have noted that the surface of a crystalline silica will be covered exclusively by free hydroxyls, with a separation between adjacent hydroxyls of 5.0 A (as in the P-tridymite structure). With perturbation of an initially crystalline structure and randomization of the positions of surface hydroxyls, the average spacing between nearest hydroxyl neighbors must tend to decrease, and an increasing number of hydroxyls will be sufficiently close to permit hydrogen bonding. Reactive hydroxyls appear to comprise the closer, more tightly bound surface hydroxyls, and their concentration should increase regularly with decreasing silica crystallinity and de-
creasing concentration of free hydroxyls. The parallelism of surface crystallinity and average pore diameter probably reflects a dependence of each of these properties on some basic aspect of the original silica synthesis. Thus it seems likely that those factors which promote silica crystallinity during its synthesis will likewise favor large crystallite size and increased silica pore diameter.
Acknowledgment. The authors are grateful to W. P. Cummings of W. R. Grace and Company for making available pore size distribution data on samples I1 and VIII.
Partial Molar Volumes and Adiabatic Compressibilities of Tetraalkylammonium and Aminium Salts in Water. I.
Compressibility Behavior
by B. E. Conway and R. E. Verrall' Department of Chemistry, Unicersity of Ottawa, Ottawa, Canada
(Received J u l y 6 , 1966)
Differential ultrasonic velocity measurements have been carried out on a series of aqueous solutions of symmetrical tetra-n-alkylammonium salts, corresponding salts of protonated methylamines, and the neutral methylamines themselves. The apparent molal adiabatic compressibilities q5K(s) have been derived and the values a t infinite dilution estimated. The dependence of upon the coordination of the N + center by H 2 0 molecules and by Me groups has been considered in relation to the variation of with molecular weight in the homologous series of symmetrical R4N+ salts from R = Me to R = n-Bu. Effects due to electrostriction and structure promotion are considered as a function of alkyl substitution a t the N center.
Introduction In recent years, considerable interest2-11 has arisen concerning the behavior of tetraalkylammonium salts in aqueous solution, particularly with regard to their behavior, and in water* A apparent structure-promoting review has also been given.12 E l s e ~ h e r e ~we~ 'have ~ examined the additivity of partial ionic volumes
v
The Journal of Physical Chemistry
of symmetrical homologous ions in this series, the concentration dependence2* of for various corresponding salts, and deduced the individual ionic contributions
v
(1) Work carried out in partial fulfillment of the requirements for the Ph.D. degree in the University of Ottawa, Ottawa, Canada. (2) W. Y. Wen and S. Saito, J . P h y s . Chem., 68, 2639 (1964). (3) w. y. wen and s. Saito, &id., 69, 3569 (1965). (4) B. J. Levien, Australian J . Chem., 18, 1161 (1965).
PARTIAL MOLARVOLUMES AND ADIABATIC COMPRESSIBILITIES
with a very small thermodynamic uncertainty. Also attention has been directed'o to the role of nonelectrostatic relative size effects in the thermodynamics of solutions of these salts. I n the present two papers (see part I1 following), we report data on the compressibility and density behavior of a series of symmetrical R4N+ salts and of salts derived from corresponding primary, secondary, and tertiary aminium acids R,H4-,N+ where n, an integer, is 1 n 6 3. Measurements have also been made on the corresponding neutral amines R,HI-,N, so that the compressibility and volume changes on ionization13 can be derived. The relation between ionic compressibilities, partial molal volumes, and electrostriction in relation to hydration has been dealt with theoretically in previously published papers. 14-17 The choice of the compounds studied in the present work was dictated by the desire to examine effects associated with changing coordination about the IT+ center in the aminium and tetraalkylammonium salt series. I n the compressibility measurements reported here in part I, the differential adiabatic method'* was used as this gives satisfactory results down to concentrations of salt which are lower than can be studied by the static method or the direct interferometric method.16 A slight disadvantage is that only the adiabatic compressibility P is obtained, but for aqueous solutions this quantity can be given meaningful interpretations (cf. ref 15, 16, 17, and also 19 where the isothermal compressibility a was calculated from p with the necessary partial molar heat capacity data) and is not too different (ca. 7-100jO)1g from a for alkali halide salts.
solutions, e.g., KC1, KBr indicates that the cation must cause this effect; for example, +K(s)KCl = -40 X whereas t $ ° K ( B ) ~ a= ~ ~~ 16.6 X cc (mole bar) -l. It should be noted that this higher compressibility is not limited to the alkyl-substituted ammonium ions but that the NH4+ itself has a higher (ie., less negative) apparent compre~sibility~~ than that of other inorganic ions, e.g., K+. It appears that the ability of the NH4+ ion to form H bonds results in an influence on water structure which is less than that for the K+ ion which has a similar radius. The same applies to H30+. Previous studies2!3,6;9, lvZ5 on tetraalkylammonium salts seem to indicate a strong structural influence of the large cations upon water. Interpretations of the present results may be made tentatively in terms of changes of the local compressibility of the solvent near the ions. The bulkiness of the large hydrophobic cations suggests that their inB. Corey, Phys. Rev., 64, 350 (1943). (25) H. S. Frank and W. Y. Wen, Discussions Faraday SOC, 24, 133 (1957).
(24) V.
PARTIAL MOLARVOLUMESAND ADIABATICCOMPRESS~BILITIES
T E T R A A L K Y L AMMONIUM IODIDES TETRAALKYL AMMONIUM BROMIDES
4.0
L
0
n
-4.0
- 24.0
A L K Y L A M I N E HYDROCHLORIDES
t
I
I
I
4
8
12
I
16
20
NUMBER of C ATOMS
Figure 5. Values of +OK(., as a function of the number of carbon atoms in a series of tetraalkylammonium bromides and iodides, and in the methylamine hydrochloride series including MeBCl. (See also part 11.)
trinsic molecular compressibilities might account for some of the apparent increase of compressibility in relation to that for small inorganic ions. However, in general, the intrinsic compressibility of ions themselves is expected to be much less than that of the solvent water since in the latter case, it is the free volume that is principally diminished with increasing pressure. The compressibility of the ion itself will presumably be similar to that for a substance, e.g., a hydrocarbon, a t very high pressures or to that for a close-packed metal. Previously it has been argued,14 when the process of taking inorganic ions from the crystal lattice into the solution is considered, that the compression of the solute appears negligible in comparison with that of the solvent. The intrinsic compressibility of the tetraalkylammonium ions may, however, be somewhat greater than that of small monatomic ions since the former are, in effect,,microscopic droplets of a different phase in the water, probably with some free space between the CH2 groups in R particularly when R is large. However, in the present case, if the intrinsic molecular compressibility of the cation made a significant contribution to the observed compression, the effect ’
3959
would be expected to be greatest for the largest cation, since it is reasonable to assume that any “free within the intrinsic volume of n-BurN+ is greater than that in Me3NH+ or R!Ie4N+. If this were the case, then the plots of 4°K(8) as a function of the number of C atoms in Figure 5 for the R4NX salts should show a trend in the opposite direction to that observed, Le., c $ ~ ( ~should ) become less negative with increasing cation size for a given halide anion. A different possibility arises if the R4N+ ions are, in fact, relatively incompressible in comparison with water. Then with increasing size of R the apparent compressibility would tend to be more negative. However, since the volume of the ions increases almost in exact proportion to the number of carbon atom^,^^'^ it would be expected that the effect referred to above would lead to a linear decrease of 4 K ( swith ) molecular weight, which is not observed (Figure 5 and see part 11). Therefore we must consider how the observed effects might arise from structure promotion or electrostriction in addition to the last possibility considered. Qualitatively, it may be useful to distinguish, limitingly, three types2’ of local solvent water near the ions as shown in Figure 6; (1) is an ice-like configuration257 28 *, (2) is “free” water (cf. NBmethy and S c h e r a g a ’ ~“unbonded” ~~ liquid state) ; and (3) is electrostricted water. l 4 Structure 1 in Figure 6 is regarded as being less compressible than bulk solvent water because of a stronger intermolecular framework of H bonds. This is supported by the fact that Pice Br- > C1-. This trend is consistent with a decreasing electrostrictive effect with increasing anion size (primary hydration effect). Some estimate of the differences of individual +OK(s) values deduced from the data for R4NX salts with a common cation or anion are shown in Table VI. The difference in the values of A+°K(s) and ARzofor Cl--Br- l 9 in Table VI is in part due to the fact that the adiabatic and isothermal compressibilities differ according to eq 4. However, in the case of NaCl or IICl,lg neglect of the correction to the adiabatic quantities causes an error of only ea. i’.5Y0 in R20. Although a complete series of salts of alkyl-substituted ammonium cations ranging from ;LIeNH3X t o n-BuJX having a common anion have not been studied, it may be assumed (using the thermodynamic differences for A4°K(S,(Br--C1-) from Table VI that the values for such a series, vz’x., the bromide series, tend to go through a discontinuous maximum (see part 11, Figure 8). The maximum occurs at Rle4NBr, and in the series of cations NH,+, MeNHB+, etc., there is evidently an almost linear decrease in the amount of electrostricted water of type 3 (and possibly an increase in type 2). This would cause an apparent increase in 4°K(s). However, in comparison with the file&+ ion, EtJf appears to promote structure slightly, ie., it increases type 1 water at the expense of type 3, thus decreasing the compressibility. This structure promotion is evidently greatly increased The Journal of Physical Chemistry
-9.5 -19.9 -10.4 +2.4b (3Z0.3)‘ $3.76 ( 3 Z O . l ) C +10.6b (fO.9)C
x 104 cc (mole bar) -1
W20a
-8.6
‘From ref 19 for sodium and potassium salts. Average values from the bromide and iodide series. Mean deviation. A different value would follow from the recent data of Allam and Lee.32 However, their measuremeBits were not made to a sufficiently high dilution for extrapolation to be satisfactory and their data for NaC1, NaBr, and KCl are not in good agreement with those of Owen and Kronick for these salts (see Table 10 in ref 12).
by n-PrdS+ and n-Bu4N+ with a consequent marked decrease in compressibility despite the lower electrostrictive field at the larger ions. Analogous explanations have been to explain the nonlinear decrease from i\le4?u’+ to n-Bu4N+ of the ratio of the Walden product for DzO to that for HzO as a function of ion size. The nonlinearity of the plots of 4°x(b)as a function of cation size precludes the possibility of using such for individual ana graph for estimating the 4°Or,K(s) * ’ ~the case of the partial ions i as was p o s ~ i b l e ~in molal volumes. No interpretation of the slopes SK(s)for the R4N+Xsalts is attempted here on account of the complexity of the factors that determine X K ( * ) and also Xv to which it is relatedg (eq 6). The values of 4K(s)for the uncharged alkylamines CH3NH,, (CH&KH, and (CH3)3Nwere also measured and show a nonlinear dependence on concentration (Figure 4); the 4K(s)values are also positive, while for the corresponding protonated amine salts the values are negative. The order of 4K(s)values is (CH3)2NH > CH3(NHz) > (CH3)3N.
Acknowledgments. Grateful acknowledgment is made to the National Research Council, Canada, for support of this work. R. E. V. acknowledges the award of Province of Ontario Graduate Scholarships ~~
(32) D. S. Allam and N. H. Lee, J . Chem. Soc., 6049 (1964); ibid., Sect. A, 5 , 426 (1966). (33) R. L. Kay and D. F. Evans, J . P h y s . Chem., 69, 4216 (1965).
3961
PARTIAL MOLARVOLUMES AND ADIABATIC COMPRESSIBILITIES
in 1964 and 1965. We are also indebted to Mr. A. Couture of the engineering staff of the Pure Chemistry Division of the National Research Council for fabrica-
tion of the ultrasonic velocity bath and racking mechanism, and to Dr. E. W. Carstensen for discussion on the design of the electrical circuit.
Partial Molar Volumes and Adiabatic Compressibilities of Tetraalkylammonium and Aminium Salts in Water.
11.
Volume and Volume Change Relationships
by R. E. Verrall' and B. E. Conway Department of Chemistry, University of Ottawa, Ottawa, Canada
(Received J u l y 6 , 1966)
Partial molal volumes of primary, secondary, and tertiary methylamine hydrohalides have been determined by a differential buoyancy method and compared with similar data for tetraalkylammonium salts in an aqueous medium. The volumes of acid ionization H+ have been determined, and values of in reactions such as R,HL,N+ e R,NHI-, the partial molar volume have been related to the apparent molal compressibilities and the partial specific compressibilities. The results are interpreted in terms of a change from electrostrictive effects to structure-promotion effects as the extent of coordination of the N+ center by alkyl groups is increased.
+
Introduction I n previous papers2*3the partial molar volumes P of tetraalkylammonium ions have been considered in relation to the additivity2"v3of alkyl function contributions, the effects these ions have on the water struct ~ r e , ~and ~ ,to~ the , ~ concentration dependence of ?.za I n this paper we present data on ?for primary, secondary, and tertiary alkylamine hydrohalides in relation to the 7 data for tetraalkylammonium salts published previously.2 Xeasurements on the volumes of the neutral alkylamines together with the data for the corresponding hydrohalides lead to estimates of the volume change A T for ionization. Relations data presented in between the P data and the 4x(8) part I will be examined in terms of electrostriction and structure promotion effects. (See preceding paper.)
Experimental Section (1) Partial Molar Volume Measurements. The determination of
B values was made by evaluating the
apparent molar volumes Qv from density measurements. Densities were determined to six decimal places by the differential Archimedian balance method described by Wirth.6 The details and accuracy of an improved procedure based on this method have been described and discussed previously2"~3in relation to determinations of 7 for a series of tetra-nalkylammonium salts. (2) Compounds Studied. Data for a series of tetra(1) Work carried out in partial fulfillment of the requirements for Ph.D. degree in the University of Ottawa, Ottawa, Canada. (2) (a) B. E. Conway, R. E. Verrall, and J. E. Desnoyers, Trans. Faraday Soc., 62,2738 (1966) ; (b) W. Y. Wen and 5. Saito, J. Phys. Chem., 68, 2639 (1964). (3) B. E. Conway, R. E. Verrall, and J. E. Desnoyers, 2. Phgsik. Chem. (Leipzig), Falkenhagen Anniversary Papers, 230, 157 (1965). (4) R. M. Diamond, J . Phys. Chem., 67, 2513 (1963). (5) J. E. Desnoyers, G . E. Pelletier, and C. Jolicoeur, Can. J . Chem., 43, 3232 (1965). (6) H. E. Wirth, J. Am. Chem. Soc., 59, 2549 (1937); see also F. Vaslow, J. Phys. Chem., 70, 2286 (1966).
Volume 70,Number 1.9 December I966
