3077
IZFFECTS OF hfANNITOL AND SORBITOL ON WATER
anmitol and Sorbitol on Water at 25”
.Stern* and M. E. O’Connor Department of Chemistry, California State College at Long Beach, Long Beach, California 90801 (Received February 16, 297%’)
Significant differences were observed, via enthalpies of transfer of NaCl, in the effect of small quantities of the two stereoisomer alcohols mannitol and sorbitol on the solvent and structural properties of water. Enthalpies of transfer from pure water to dilute mannitol are at first positive while those to sorbitol are negative, indicating opposing effects for the two alcohols.
~~trQductio~ The structuring effect of small quantities of low molecular weight alcohols and glycols on water has been demonstrated by a variety of physicochemical metho d ~-3. ~This contribution reports on the remarkable difference in the effects on water by two biologically important4 stereoisomer polyalcohols mannitol and sorbitol
c HzOE
CHzOH
HO--6--H I
H-A-OH
EO-C-HI
HO-L-H
I
I
H-C-OH I H-C-OH
EI-$--OH I
N-C--OH
I
I
~ ~ H ~ O H sorbitol
CJ-12OH manr i to1
They were investigated via enthalpies of transfer of NaCl from pure water t o the two aqueous alcohols, respectively, in a manner analogous t o that of several other electrolyte-nonelectrolyte-water systems, 1,2,5,6
(An2)
described e l ~ e w h e r e . ~ The , ~ materials were sodium chloride (Baker, AR), mannitol (Mallinckrodt, AR), sorbitol (Mallinckrodt, monohydrate), and water (distilled and deionized).
Results and Discussion Table I and Figure 1 show a summary of enthalpies of transfer of NaCl from pure water to aqueous mannitol and sorbitol, based on a total of 35 runs. T h e enthalpies of transfer in mannitol begin with a n endothermic peak and are followed by exothermic enthalpies a t higher concentrations of mannitol. This behavior is typical of enthalpies of transfer of electrolytes in other alcohols.1~2 However, the endothermic peak is very low and narrow, indicating that the effect of structuring induced by alkyl groups is soon overcome by the
Table I : Enthalpies of Transfer ARz of NaCl from Pure Water to Aqueous Mannitol or Sorbitol a t 26” Alcohol, rnol/kg of HzO
Experimental ~ e c t ~ o ~
0.10 0.20 0.30 0.60
The calorimeter’ and calorimetric method have been
0.70 1.00
170 i40 140 i:20 -10 Zt 40
- 180 i30 -230 & 40
Arfjz(Sorb.), cal/mol
-80 It 40
--220 4 30
- 260 .f40 --310 i 20
i
Uncertainty intervals are standard deviations of the means. NaCl concentration range is 0.005--0.01 mol/kg of H20.
I-
(1) J. €3. Stern and J. Nobilione, J. Phys. Chem., 72, 3937 (1968); 73, 928 (1969). (2) J. H. Stern and S. L. Hansen, J. Chem. Eng. Data, 16, 360 (1971). (3) F. Franks and D. J. G. Ives, Quart. Rev. Chem. Soc., 20, 1 (1966). (4) E. S. West, W. R. Todd, H. S. Mason, and J. T . Van Bruggen, “Textbook of Biochemistry,” 4th ed, Macmillan, New York, N. Y., 1966, p 194. (5) J. €I. Stern and W. R. Bottenberg, Jr., J . Phys. Chmn., 75, 2229 (1971). (6) J. €1. Stern and J. D. Kulluk, ihid., 73, 2795 (1969). (7) J. H. Stern and C. W. Anderson, ibid., 68, 2528 (1964).
Q
* Overall
IO0 -
-4OG
A81(Mann.), cal/molu j b
1-i-U0
0.2
0.4 0.6 0.8 Alcohol, mollkg of H,G.
Figure 1. Enthalpy of transfer
1.0
AR2 of NaCl to aqueous mannitol ( A ) and aqueous sorbitol (B).
The Journal of Physical Chemistry, Vol. 76,No. 21, 197%
3078
M. I. CRUZ,W. E. E. STONE,AND
six polar hydroxy sites. Sorbitol, in contrast, appears to be a s t r u c h r e breaker, as is indicated by t h e monoronic negatiw erilhalpies over the entire concentration
This behavior of ART,is similar for XaCE in acjil~aoixshydrogen peroxideJ5 ureaJ6 and
range investigated,
%,hi& differ
in position of one of their six hy-
dro~ides.~
Acknowledgmerti. The financial assistance of the National Science Foundation is gratefully aeknowledged. J. K. S. wishes to thank Professor Y, Marcus at>t h e Hebrew University for his hospitality.
bility of Inorganic and Organic Compounds,'' Supplement t o 3rd ed, D. Van Nostrand, New York, N. Y . , 1952, p 677; F. J. Kelly, R. A. Robinson, and R. H. Stokes, J . Phys. Chem., 65, 1958 (1961).
The Methanol-Silica Gel System. 11. The Molecular Diffusion and Proton Exchange from Pulse Proton Magnetic Resonance Data by M. I. Cruz,la W. E. E. Stone,lband J. J. Fripiat*lb Laboratoire de Physico-Chimie MinBrale, Institut des Sciences de la Terre, de Croylaan, 48, $030 Heverlee, Louvaia, Belgium (Receiued January 10, 1978) Publication costs assisted by the Muage Royal de E'Afrigue Centrale
The proton motions in the methanol-silica gel (Xerogel) systems have been deduced from the measurement of the spin-lattice (TI) and spin-spin (T,) proton relaxation times in the systems C€LOD-XOD, CDIOHXOK, and CH,OH-XOH, where XOH and XOD stand for the Xerogel-OH or Xerogel-OD surfaces, respectively. In CH30D-XOD, only one TI or T 2 is observed. The experimental data may be accounted for by a laandational motion of CH,OD, the methyl. group rotating very rapidly around the triad axis. In CD30HXOH, one TI aiid two Tt are measured. TI is the superimposition of two functions. The rate of exchange of nuclei between the two populations is fast with respect to T1-l but slow with respect to T2-I. The long T Zrepresents the diffusion motion observed in the first system, while the short T 2is assumed to be due to a proton exchange process between the hydroxylic protons of C&OH and XOH or between CDIOI~molecules. Moreover, it has been possible to compute the observed Ti for the CHIOH-XOH system from the data obtained for the two simpler cases. At 25", the surface diffusion coefficient of adsorbed methanol is reduced by two orders of magnitude with respect to that in the liquid for a degree of coverage e ^I 0.6. For 1 < e < 2, the lowering is by a factor of 10. For e 2 0.7, the surface diffusion coefficient a t 25' is the same as that in the liquid state at -85". More than three layers should be probably required to find a diffusion coefficient in the adsorbed state close to that in the liquid state. In the assumed exchange process a proton is transferred from the surface into a cluster of adsorbed species where it jumps from a molecule to another before CB301-I 3 1 0CH10H2+; CH30H2+ CH80J1 -+ CH30H coming back on the surface: [-OH C€InOIh+; CH3OH2" -0- [ ] -OH CHaOH. The ratio of the exchange to the molecular diffusion jump frequency ( v e / u d ) is 6 x 10-3 for e = 1.7 and 2.5 x 10-2 for e = 0.8. This means that the number of times a methanol molecule is protonated during its stay on an adsorption site increases at decreasing degree of coverage. For 0 = 0.8, u./ud is very close to the value observed in the liquid protonated methanol al, 25" The activation energies of both the diffusion and the proton exchange processes are dose to each other (-6 kcal a t 8 = l),and there are approximately equal to half the isosteric heat of adsorption. Both these motions are characterized by a broad distribution of activation energies.
+
+
-+
+
been discussed elSeWhere2a and the following model proposed. Iluring t h e first step of t h e adsorption proT h e Journal of Phssical Chemiatry, Vol. 7'6, N o . 21, 1.978
+
+
+
( 2 ) (a) IM, 1. Cruz, K. Verdinne, and J. J. Fripiat, submitted for publication; (b) G. Mertens and J. J. Fripiat, J . Colloid Tnterfuce Sci., in press.
