Apparent
and
Partial
Molal Volumes
of
Ammonium Bromides
sical into zwitterionic form with a predicted relaxation 2 frequency of about 0.6 MHz and anion + cation neutral (classical) with a predicted relaxation frequency of about 1.7 MHz. Thus, the weak absorption with a low apparent relaxation frequency which we observe in methanolic solutions of the meta and para isomers is the
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=
predicted result in the absence of a direct internal proton-transfer mechanism even in these rather concentrated solutions. D. Methanol-Water. As the fraction of water in the solvent mixture is increased the larger values of the equilibrium constants imply larger values of and thus increased absorption due to both of the low-frequency modes. As one passes from water to methanol, the solubility of the acids is much increased, and in inspecting the results in Table II it should be noted that the low values of the relaxation time in methanolwater reflect the higher concentrations rather than solvent properties.
1333
IV. Conclusions Perturbation of the internal charge-transfer equilibrium is the principal contributor to the acoustic relaxation spectrum of aqueous solutions of the isomeric aminobenzoic acids at low pH. However, it is possible to give a satisfactory account of the rate of internal transfer from the known rates of ionization and recombination reactions, and there is no need to adduce a direct unimolecular internal transfer reaction. Volume changes for zwitterion formation and related ionic reactions have been deduced from the acoustic data. In methanol the relaxation frequency (or frequencies) which characterize the acoustic absorption due to the meta and para isomers are too low to be accurately determined. In the case of the ortho isomer in methanol a discrete relaxation at 23 MHz is found at 25°. It is argued that this corresponds to internal proton transfer in a cyclic hydrogen-bonded system.
The Temperature Dependence of the Apparent and Partial Molal Volumes
of Concentrated Aqueous Electrolyte Solutions of Tetraalkylammonium
Bromides, Cetyltrimethylammonium Bromide, and Ammonium and Lithium Bromides by Antonio LoSurdo* and Henry E. Wirth Department of Chemistry, Syracuse University, Syracuse, New York
13210
{Received
July
23, 1971)
Publication costs assisted by Syracuse University
The apparent molal volume, 21, and partial molal volume, V2, of concentrated (1 m to saturation) aqueous solutions of tetraalkylammonium bromides, R4NBr (R methyl, ethyl, w-propyl, and n-butyl), cetyltrimethylammonium bromide, and ammonium and lithium bromides have been determined from density measurements in the temperature range between 5 and 65°. A modified Young’s rule for mixtures of electrolytes, obBd and V2. The ß) + ( )µß/2, has been used to explain the concentration dependences of ( )µ(1 results indicate that there are strong coulombic-hydrophobic cation-cation interactions which may give rise to multiply charged aggregates in quaternary ammonium bromide systems. =
=
—
Introduction Current interest in the influence of tetraalkylammonium halides on the structure of water,1-10 cationcation pairing,11-13 and micelle14-17 formation suggests further experimental studies of these phenomena, For this reason the behavior of concentrated aqueous electrolyte solutions of quaternary ammonium bro-
mides, cetyltrimethylammonium bromide, and ammonium and lithium bromides has been investigated in (1) W. Y. Wen and S. Saito, J. Phys. Chem., 68, 2639 (1964); ibid., 69, 3569 (1965). (2) B. J. Levien, Aust. J. Chem., 18, 1161 (1965). (3) L. B. Hepler, J. M. Stokes, and R. H. Stokes, Trans. Faraday Soc., 61, 20 (1965).
The Journal of Physical Chemistry, Vol. 76, No. 9, 197S
Antonio LoSurdo
1334
the temperature range 5-65°. Evidence18-20 from light scattering, X-rays, and conductivity experiments indicate that there are spherical micelles present at low concentrations (0.05 m) of cetyltrimethylammonium bromide solutions. These micelles transform into rod-shaped aggregates at higher concentrations.19 Some preliminary results16·17 suggest that there are strong coulombic-hydrophobic cation-cation interactions which may give rise to multiply charged aggregates in aqueous solutions of tetraalkylammonium bromides. These interactions are greatest for molecules with long hydrophobic chains. In view of this, attention has been given to the possible existence of aggregates in tetraalkylammonium bromide solutions.
Experimental Section Materials. Tetramethylammonium bromide, tetraethylammonium bromide, tetra-n-propylammonium bromide, and tetra-n-butylammonium bromide (Eastman) and cetyltrimethylammonium bromide (Pfaltz and Bauer) were recrystallized once from suitable organic liquid mixtures using a modified procedure of Conway, et al.,* and were dried under vacuum at 7080° for at least 48 hr. The weight volumetric analyses for bromide using a modified Volhard’s method21 indicated their purities to be better than 99.9%. Ammonium bromide (Fisher purified) was used without further purification. Stock solutions were prepared by dissolving the recrystallized salts in doubly distilled water, and the concentrations were determined by Volhard's method. Solutions of known molality were prepared by weight dilutions of the stock solutions. Procedure. The densities were determined using the method described elsewhere.22 The dilatometer was constructed of Vycor by Corning Glass Co. and had an internal volume of approximately 66 ml. Each experimental run used a combination of mercury (about 67 g) and solution. The thermostated bath system22 was controlled to ±0.005° or better at each of the temperatures used. The coefficient of thermal expansion of the dilatometer was 2.7 X 10-6 ml/°C as compared with 2.2 X 10-6 found by the authors22 for a larger Vycor dilatometer, and 2.4 X 10-6 as calculated from the linear coefficient of expansion for Vycor.23 Results The densities of concentrated aqueous solutions (1 m to saturation) of (Me)4NBr, (Et)4NBr, (n-Pr)4NBr, (ro-Bu)4NBr, cetyl-(Me)*NBr, and NH4Br have been determined24 at 10° intervals between 5 and 65° and represented by the equation dt
=
d0
+ at +
fit*
+
2 (the slope 2/ /: is positive), Pi < Pi°. This decrease in volume could be attributed to water structure breaking by the electrolyte.18,24 With the quaternary ammonium halides usually1,11,13,15 P2 < 2 (the slope 2/ )ml/i is negative); hence Pi > Pi° and water structure making1,18,24 is implied. Values of Pi in (Me)4NBr, (Et)4NBr, and (n-Bu)4NBr solutions were calculated (Figures 4 and 5) from eq 13. The results are in excellent agreement with those of Schiavo and coworkers.36 If, however, the solutions of tetraalkylammonium bromides contain other species, as postulated here,
then (35) A. LoSurdo and .
E. Wirth, to be published. (36) S. Schiavo, B. Scrosati, and A. Tommasini, Ric. Sci., 37, 211 (1967). The Journal of Physical Chemistry, Vol. 76, No. 9, 1972
1338
S.
m4(0i
·
+
-
-
)µ
55I51—
For
an equilibrium between simple cations and cation dimers (reaction 7), eq 14 reduces to -
f''
+
|>
-
(
« -
x 5i)„ + ^ (02
-
(15)
where 0i and 5i and 02 and z?2 are the respective apparent and partial molal volumes of the monomer and dimer in solutions containing only water and these mi + 3m2; ß is the electrolytes at an ionic strength µ of association. degree =
Cabani, G. Conti,
and
L. Lepori
From this viewpoint, the effects of R4NBr on the partial molal volume of water, Fi, would be that of a mixture of “normal” electrolytes and increased water structuring would not occur.15 In summary, the results presented here suggest that the structure of concentrated aqueous solutions of tetraalkylammonium bromides is different from that of dilute solutions and that of normal electrolytes. This structure difference can be described as due to coulombic-hydrophobic cation-cation interactions leading to multiply charged aggregates arranged in a quasicrystalline lattice. Acknowledgment. This work was supported by the Office of Saline Water, Grant No. 14-01-0001-623.
Volumetric Properties of Aqueous Solutions of Organic Compounds. I.
Cyclic Ethers and Cyclic Amines by S. Cabani,* G. Conti, and L. Lepori Istituto di Chimica Física, Universitá di Pisa, Pisa, Italy
(Received October IS, 1971)
Publication costs assisted by Consiglio Nazionale delle Ricerche
Measurements of apparent molal volumes in water of some cyclic monoethers, diethers, and amines have been carried out at 25° with a differential buoyancy balance at concentrations varying from 0.02 to 0.4 M. The values of of monoethers and amines decrease linearly with increasing concentration at a rate which increases with growing size of the hydrocarbon part of the molecule. On the contrary, values of diethers are found to be independent of concentration. The volume and entropy changes associated with the transfer from the pure liquid state to aqueous dilute solution are compared. The observed phenomenology is justified in terms of local changes of solvent structure around the solute molecules.
Introduction In recent papers,1·2 data for free energy, enthalpy, and entropy changes associated with the transfer of cyclic ethers and cyclic amines from the liquid or the gas state to dilute aqueous solution were reported. In this paper the volumetric properties of the aqueous solutions of these classes of compounds at 25° will be considered. Measurements of partial molal volumes F2 in aqueous solutions of some of the ethers examined here were previously made by Franks, et al.3 Laliberté and Conway4 reported values of partial molal volumes for piperidine and 1 -methylpiperidine in 0.1 N KOH at 25°. The excess partial molar volume FE of pyrrolidine at 26.5° and in the 0.01-0.02 mole fraction range was re-
ported by Brower,
et al.&
The Journal of Physical Chemistry, Vol. 76, No. 9, 197S
The interest for the volumetric behavior of aqueous solutions as a useful tool for elucidating local changes of solvent structure brought about by neutral or charged solute molecules has already been illustrated (see, e.g., ref 6-11). (1) S. Cabani, G. Conti, and L. Lepori, Trans. Faraday Soc., 67, 1933 (1971). (2) S. Cabani, G. Conti, and L. Lepori, ibid., 67, 1943 (1971). (3) F. Franks, . 66, 582 (1970). (4) L. H.
A. Quickenden, D. S. Reid, and B. Watson, ibid.,
Laliberté and B. E. Conway, J. Phys.
Chem.., 74, 4116
(1970). (5) K. R. Brower, J. Peslak, and J. Elrod, ibid., 73 , 207 (1969). (6) F. Franks and D. J. G. Ives, Quart. Rev., Chem. Soc., 20,
1
(1966). (7)
.
(1965).
E. Friedman and .
A. Sheraga, J. Phys. Chem., 69, 3795
