4118
L.H.LALIBERTE AND B. E. CONWAY
Solute and Solvent Structure Effects in Volumes and Compressibilities
of Organic Ions in Solution by L. H. Laliberte and B. E. Conwaylanb Chemistry Department, University of Ottawa, Ottawa, Canada
(Received May 4, 1070)
Previous studies on the apparent molal volumes 6" and adiabatic compressibilities$I~(s)of organic N-containing ions have been extended to aliphatic dialkylamines, the related (cyclic) piperidines, and pyridines. In the case of tetradkylammonium salts, new data obtained down to low concentrations by means of a dilatometer are reported, and at the lowest concentration 6"begins to vary with concentration according to the DebyeHuckel limiting law. With hydrophilic alkylammonium ions, the concentration dependence of 6" is more like that of simple inorganic salts. The $K(s) behavior of di-n-alkylamine salts shows alternations of the value of $ K ( s ) with increasing number of CH2 groups, but pyridine and piperidine and their homologs give linear relations. The relative electrostrictions at the N + centers are also evaluated for various organic Nf cations. On the basis of $ ~ ( a ) measurements on tetraalkylammonium and pyridine salts an extrapolation method is proposed that enables estimation of the individual ionic contributions in the observed $K(s) values for the salts.
Introduction In previous papers2rSwe have shown how the complementary use of partial molal volume P and apparent ) on aquemolar adiabatic compressibility ~ I C ( S studies ous solutions of organic ions can give interesting information on specificities in ion-solvent interaction connected with the structure of the organic solute and on the reciprocal effects which arise in the solvent due to the reaction of the solute ions on the processes determining the lattice structure equilibrium in the solvent (solvent structure promotion and structure breaking4 and the associated changes in spin-lattice relaxation times5). Previous work has been concerned with simple inorganic ions,e tetraalkylammonium (TAA),2v7-11and alkylaminium salts2 as well as salts derived from pyridine and its homologs. l 2 l 3 In the present paper we report extensions of the previous investigation^^^^ with regard to determinations ) piperiof the apparent molar functions 4" and ~ K ( s for dine salts and the neutral bases and for a series of other selected salts chosen with regard to specificities in their interaction with the solvent, e.g., the series of dialkylammonium hydrohalides and triethyl- and triethanolammonium bromide. Additivity relations in the pyridine and TAA series have also been investigated particularly in the case of +,K(s) measurements. Here, a system for examining the results is required which will enable, by means of a suitable extrapolation procedure, individual ionic contributions in +)K(s) or (P"R(s) to be derived in an analogous way to that developed for the individual ionic partial volumes.ll n
Experimental Section 1. Preparation and PuriJication of Compounds for Study. TAA and pyridine salts were purified and reThe Journal of Physical Chemistry, Vol. 74, No. 23,1070
crystallized as described previously. 3 ~ 1 1 , 1 2 Reagent grade piperidine and 1.-methylpiperidine were distilled under a nitrogen atmosphere at reduced pressure prior to use. Piperidinium and 1-methylpiperidinium chlorides were prepared by bubbling dried HC1 gas into a cold (0") solution of the respective bases in ethanol under a nitrogen atmosphere. The products were recrystallized from ethanol, and their compositions were checked by means of a C1- analysis. 1,l-Dimethylpiperidinium iodide was prepared by slowly adding methyl iodide to a solution of 1-methylpiperidine in ethanol. The product was recrystallized from ethanol and the purity was checked by I - analysis. Piperi(1) (a) Commonwealth Visiting Professor (1969-1970) at the Universities of Southampton and Newcastle-upon-Tyne, England; (b) t o whom correspondence should be addressed at Ottawa. (2) B. E. Conway and R. E. Verrall, J . Phys. Chem., 70, 3952 (1966); see also R. E. Verrall and B. E. Conway, &id., 70, 3961 (1966). (3) B. E. Conway and L. H. Lalibertb, "Hydrogen Bonded Solvent Systems," A . K. Covington and P. Jones, Ed., Taylor and Francis Ltd., London, 1968,p 139. (4) G. NBmethy and H. A. Soheraga, J . Chem. Phys., 36, 3401 (1962). (5) G.Engel and H. G. Hertz, Ber. Bunsenges. Phys. Chem., 72, 808 (1968). (6) J. Padova, J , Chem. Phys., 40,691 (1964); see also F. Vaslow, J . Phys. Chem., 70, 2286 (1966); 71, 4585 (1967). (7) L. A. Dunn, Trans. Faraday SOC.,64, 2951 (1968). (8) F.Franks and H. T. Smith, i b s . , 64, 2962 (1968); 63, 2586 (1967). (9) W.Y.Wen and S. Saito, J . Phys. Chem., 68,2639 (1964). (10) L. G.Hepler, J. M. Stokes, and R. H. Stokes, Trans. Faraday Soc., 61, 20 (1966). (11) B. E. Conway, J. E. Desnoyers, and R. E. Verrall, ibid., 62, 2738 (1965). (12) B. E. Conway and R. G. Barradas, Electroehim. Acta, 5, 319, 349 (1961). . . (13) B. E. Conway and L. G. M. Gordon, J . Phys. Chem., 73, 3609 (1969) I
SOLUTE AND SOLVENT STRUCTURE EFFECTS
4117
dinium iodide was prepared in solution by titrating piperidine with aqueous HI. The H I had been purified by distillation from red phosphorus. The I- content of the H I solution was established by a volumetric analysis. Diethyl-, di-n-propyl-, and di-n-butylammonium chlorides were prepared by bubbling dry HC1 gas into an ethanolic solution of the amine. Reagent grade diethylamine, dipropylamine, and dibutylamine were each distilled under a nitrogen atmosphere, the dibutylamine a t reduced pressure. The hydrochlorides were recrystallized from an ethanol (lO%)-diethyl ether mixture, and the purity was checked by C1- analysis. Triethylammonium and triethanolammonium bromides were prepared by bubbling HBr gas into a cold solution of the amine in ethanol. The first salt was recrystallized from ethanol and the second from 3 : 1 ethanol-water mixture. The melting points agreed with literature values. Tetramethylammonium tetrafluoroborate obtained from Aldrich Chemicals was recrystallized from hot water. The tetra-n-alkylammonium bromide series from CH, to n-C4He were Eastman reagent grade materials which were recrystallized as described previously.ll 2. (bv and ( b ~ ( s ) Measurements. The previously described2 techniques were employed; a differential ultrasonic velocity apparatus described earlier2 was used to evaluate the difference of speed of ultrasound in the solvent and solution at a given temperature, from which the ( ~ K ( s )was determined at various concentrations. Special attention was given to improving the uniformity of temperature of the long bath2 by provision of multiple stirrers in the outer and inner solution baths and a special extended heater system (Figure 1). All measurements were conducted at 25 f 0.01 ", The solutions to be studied were made up directly in the test vessel by adding weighed amounts of solute to a known weight of solvent. The concentrations (molalities) were converted into molarities by means of density data which had been obtained previously in $v mea-
IIIRRCR
WATER
I \
I
l l ll \
-A"
surements. Loss of solvent by evaporation was minimked by keeping most of the long opening along the top of the test vessel covered with a Plexiglas lid. The velocity differences AU were obtained with an absolute accuracy of 10.02 m sec-l. Test runs were performed on KC1 solutions and a value of -45.5 X ml (g-mol bar)-l was obtained for the partial molar adiabatic compressibility. This compared favorably with the results of Owen and Kronick, -44 =k 1 X ml (g-mol bar)-' l4 and those of Gucker, etal.,16 -45.6 X ml (g-mol bar)-*. The C$v measurements were made by means of the differential buoyancy balance employed previously2 using the techniques referred to in an earlier paper2 for attainment of maximum accuracy. Determinations of C$v down to concentrations below 0.02 M were necessitated in the case of TAA salts for attainment of the Debye-Huckel limiting law region so that reliable extrapolations to infinite dilution could be made. For this purpose, measurements were made between 0.05 and 0.002 M by means of a special magnetically operated dilatometer described elsewhere.I0 The dilatometer was maintained in a water thermostat at 25" controlled to =kO.OOl"in a large cupboard thermostated to 10.5". Corrections for the compression caused by the hydrostatic head of liquid in the capillary of the dilatometer were negligible for most measurements. 3. Hydrolysis Corrections. Since the piperidines are strong bases, their salts did not hydrolyze ( K , ca. 7 X 10-l2), and no corrections to the volumes and compressibilities were required, as was the case with the pyridinium saltsa3 The neutral bases, on the other hand, react with water significantly to form the protonated species ( K B = 1.32 X lo-' for piperidine). If the measurements are made in alkaline solution, it can be shown that the degree of hydrolysis a is KBICOH-,where COH-is the concentration of KOH in the solvent. For piperidine in 0.1 N KOH, a = 0.0132 and since no K B value was available for l-methylpiperidine the same a value was assumed to hold. The above considerations led to corrections of 0.1 and 0.15 ml in the apparent molar volumes of piperidine and l-methylpiperidine, respectively.
Results 1. Extrapolation Procedures. a. Partial Molar Volumes. Extrapolations of the 4, data were carried out by means of the following equation2I1l
DLTAILS OP T E I T V K S I E l
+
$v - 1.868~"~ Uz" jc (1) using the limiting slope given by Redlich and Meyer.l7 RUBDER " 0 " R I N G
Owen and P. L. Kronick, J . Phys. Chem., 6 5 , 84 (1961). (15) F. T. Guoker, C. L. Cherniok, and P. Roy-Chowdjury, Proc. Nat. Acad. Sei. U.S., 55, 12 (1966). (16) B. E. Conway and L. H. LalibertB, Trans. Faraday SOC.,in (14) B. B.
END V I E W
C E N T R A L END SECTION
Figure 1. Schematic representation of mechanical assembly used in differentialvelocity measurements, and details on the test vessel used.
press. (17) 0. Radlioh and D. M. Meyer, Chem. Rev.,
64,
221 (1964).
The Journal of Physical Chemistry, Vol. 74, N o . $3, 1970
41 18
L. H. LALIBERTB AND B. E. CONWAY
Extrapolations of rpy as a function of cl/' were also carried out. b. Partial Molar Adiabatic Compressibilities. An equation for apparent molar adiabatic compressibility can be obtained by differentiating eq 1 with respect to pressure and neglecting thejc term, i.e.
$1579
-IN
o 125 2
L -0
or - - 2 . 60 l
where
I
0
[If the termjcis included and differentiated,an equation similar to eq 2 results but with the additional term [ ( b j / b P ) j P ] c . I n the case of neutral solutes both the 4, and ~#JK(s) data were plotted empirically as a function of c for lack of any theoretically justified extrapolation equation. ] Since PS is a function of concentration, the slope SK(S) should not be constant; however, this variation is less than the uncertainty in the slope for the concentration range used in this work, so that eq 3 can be regarded as linear in cl/'. 2. Partial Molar Volumes. a. Protonated N + Centers. The & data for the piperidinium salts were plotted according to eq 1 and p2° values were obtained. These data are given in Table I. The slope of the plots
.O5
.IO .I5 C (g.rnolsa.litrc-')
1
.20
Figure 2. Plot of - 1.868c1/' as a function of concentration for 1,1-dimethylpiperidinium iodide, 1-methylpiperidinium chloride, and piperidinium chloride a t 25'.
+
10921 0
Table I: Values of
vzoand j in Eq 1 * 0.05,
j f 0.02
Salt
ml (g-mol) -1
ml 1. g-mol-'
1,l-Dimebhylpiperidinium iodide 1-Methylpiperidinium chloride Piperidinium chloride
158.68
0.0 up to c = 0.40
125.51
0 . 0 u p to c = 0.55
106.67
0 . 0 u p to c = 0.75
Compound
ml g-mol-1
(ddv/dc)c+ot ml 1. g-mol-'
Piperidine in 0.1 N KOH 1-Methylpiperidine in 0 . 1 N KOH
91.10 109.91
0.0 0.0
t20,
of the apparent molar volume vs. c'/' for all three salts approaches the limiting law value in the concentration range studied as shown in Figure 2. Also given in Table I are the V20 values for the two corresponding neutral bases obtained from the plot shown in Figure 3. The partial molar volumes of selected n-alkylammonium halide salts were examined to study further the specific structural effects associated with coordination of N + centers by alkyl groups and H atoms and thus to The Journal of Physical Chemistry, Vol. 74,No. $3,1970
I
I
.05
.IO
I
.I5 c lgmolar~litra~")
I
I
.20
Figure 3. Plot of apparent molar volume as a function of concentration for piperidine and 1-methylpiperidine in 0.1 N KOH at 25'.
complement the volume data already gathered in this 1aboratory2J~" and by 0thers7-~0,'8 for related compounds. The apparent molar volumes of the salts in the series R2NH2C1,where R = C2H~--,n-CaHv-, and n-C4Hg-, were determined and are shown in Figure 4. As may be seen from Figure 4,the deviations from the (g-mol)-*/' increase limiting law slope of 1.868 ml with increasing size of the R group. I n Figure 5 the plot of & against c1I2 for (HOC2H4)aNH+Br-shows that the introduction of a polar-OH group into the alkyl residues of the molecule causes it to behave like a "normal" 1: 1 electrolyte as opposed to the behavior of (C2Hs)3NHBr (Figure 5 ) which exhibits a large negative deviation from the Debye-Huckel limiting law for 4". The data for the compounds shown in Figures 4 and 5 are listed in Table 11. The VZodatum for Me4NBF4 is also included in Table 11.
vZo
(18) J. E. Desnoyers and M. Arel, Can. J . Chem., 45, 359 (1967).
4119
SOLUTE AND SOLVENT STRUCTURE EFFECTS ~-
~~
Table 11: Values of
~
S
~~
for O a Series of Alkylammonium Salts
vao
-
0.06ml (g-mol) -1
d
IO6 5
I
I
I
I
13851
I
I
I
I
36.27 106.73 138.66 170.68 146,17 147.28 193.65 132.60
I
J
Derived from the data of Conway and VerralP for the Cland I- salts. Q
IO7
I
.20
O
30
3 .40
50 l
0 C+ (grnoles ~ i , , e - t ~ t
Figure 4. Plot of apparent molar volume +" against c'/l for (CzH&NH2Cl, (C8H7)aNH&l,and (C4Hs)2NHzClat 25".
rate +v data in the region 0.10 < c < 0.5 M . Therefore the existing data in this high concentration region were extended and augmented where needed. The first member in the series, (CH&NBr, exhibits a positive slope (d+y/dcl/') at quite high (50.1 M ) concentrations and has been shown to follow the DebyeHuckel limiting law lo quite well. Dilatometric measurements were carried out on (C2H6)4r\TBr, (n-CaH7)4NBr, and ( w C ~ H B ) ~ NinBHzO ~ and in one case in DzO a t 25". The results for (CzH&NBr in DzO have also been reported elsewhere.16 Figure 6 shows the behavior of dv plotted against cl/' for (CzH5)4NBrin HzO and DzO. The relation is linear up to c = 0.04 M and has a slope of 1.65 f 0.2 ml l.'/' (g-mol)-'/', the +v plots for D20 being parallel to those of HzO in this concentration range. The present results for (n-CsH7)dNBr
"I 1730
172.5
+,
Figure 5. Plot of apparent molar volume against cl/pfor triethanolammonium and a plot of apparent molar volume +" against c'/l for triethylammoniumbromide a t 25 '.
b. Quaternized N + Centers. The partial molar volumes of tetraalkylammonium (TAA) salts have been studied extensively, both in this laboratory2S1l and el~ewhere.~-'O*'~The present measurements were primarily confined to the high dilution region, i.e., 0.002 < c < 0.01 M ; however, because of the t,ype of dilatometer used,Ie it was also necessary to have accu-
0 IO
om
OIO O S c42 ( 9 M O L E LlTRE"1ln
0.S
0.60
om
-7 la$!-
l
7
0.103
ck ( 9 MOLE
.
ow
5
LITRE-~I~
Figure 6. A plot of apparent molar volume against c'/lfor tetraethylammonium bromide in HzO and DzOa t 25'. (19) H. E.Wirth, J. Phys. Chem., 71, 2922 (1967). The Journal of Physical Chemistry, VoL 74, No. $8,1970
~
L. H. LALIBERT~~ AND B. E. CONWAY
4120
L
I
0.10
I
I
0.20
1
0.30 C'2
0.40
1
0.50
I
0.60
I
0.70
( q. MOLE, I-')'2
Figure 7. Plot of apparent' molar volume aga,iiistcl/, for t'etra-n-propylammol~il~m bromide in 1120. Present result,s: dilatometer, A; buoyancy balance,A; Wen and Srtito I , 9 Franks and Smith,8
---.
1
0
10
20
30
I
40 Ct(QflW108 IdrO-')'
1
I
50
60
L-____ 70
Figure 8. Plot of apparent molar volume+, against cl/, for tetra-n-butylammonium bromide in HzO at 25". Present results: dilatometer, A ; buoyancy balance, 0; Conway and Verrall," a; Dunn,' 0 ; Franks and Smith,S -.
--
have been plotted in Figure 7 together with those of Wen and Saitog and Franks and Smith.s The plot of against cl" goes through a maximum with (b+v/dcl") being positive below ca. 0.015 M . The limiting law slope is not reached even at the high dilutions used, the highest positive slope being ca. +0.7 k 0.2 ml 1.' (g-mol) The concentration range over which a straight line of positive slope can be drawn is so small The Journal of Phgsical Chemistry, Vol. 74, No. ,$3,1970
that a scatter of A0.02 ml (g-mol)-' in +v introduces an appreciable uncertainty in the value of the slope. Dilatometric measurements on (n-CdH9)dNBrhave recently been reported20 and are in agreement with the results of the present determinations on this compound (Figure8). (20) L. A. Dunn, Trans. Faraday soc., 64, 189s (196s).
SOLUTEAND SOLVENT STRUCTURE EFFECTS
4121
rzofor the Tetran-alkylammonium
Table 111: Values of Bromides in HzO at 25"
a
ml (g-mol) b
114.26d 173.70 =k 0.05 239.38 =!= 0.05 300.41 =k 0.10
114.40 173.65 239.15 300.35
--------Vz0,
Salt
(CHdrNBr (C2H5),NBr (n-CsH7)dNBr (n-C4Hs)rNJ3r
Table IV: Values of 104+(1
