2724
D. Balasubramanian and P. Mitra
The Journal of Physical Chemistry, Vol. 83, No. 21, 1979
Critical Solution Temperatures of 'Liquid Mixtures and the Hydrophobic Effect D. Balasubramanian' and P. Mitra School of Chemistry, University of Hyderabad, Hyderabad 50000 1, India (Received September 21, 1978; Revised Manuscript Received April 16, 1979)
Two important generalizations applicable to liquid pairs that exhibit critical solution temperatures (CST) are the Prigogine rule (CST will occur at equimolar component compositions) and the Timmermans rule (an additive that is preferentially soluble in one component of the liquid pair will reduce the mutual solubility). We show that, while these rules are obeyed by organic liquid pairs, they are violated by aqueous binary systems. Contrary to the Prigogine rule, the CSTs of aqueous binaries occur at high water mole fractions, and contrary to the Timmermans rule, globular protein denaturants and Hofmeister ions solubilize the second liquid in water. We trace these violations to the structure of water and the hydrophobic effect.
Introduction Binary liquid mixtures that display partial miscibility exhibit critical solution temperatures (CST) or consolute temperatures. CSTs are of two kinds: upper critical solution temperatures (UCST), above which the liquid pair is completely miscible and below which phase separation occurs (e.g., nitrobenzene-hexane), and lower critical solution temperatures (LCST) when the inverse occurs (water-triethylamine). Francis1 lists over 6000 such conjugate binaries in his monograph. Detailed thermodynamic analysis of conjugate binaries has been done by Prigogine and Defay2i3using the concepts of regular solution theory and excess functions, which explains the empirical observations reported by Tirnmerman~.~ Two generalizations that have emerged from these studies2are (i) the CST of a conjugate liquid pair will occur a t equimolar composition of the two components (the Prigogine rule), and (ii) an additive that is preferentially soluble in one of the components will decrease the mutual solubility, i.e., increase UCST, while an additive that is about equally soluble in both components will increase mutual solubility (decrease UCST or increase LCST). This may be referred to as the Timmermans rule. Our interest in this paper is to highlight the contrasts between organic conjugate liquid pairs and aqueous binaries. The latter offer a convenient model for a study of hydrophobic interactions, as suggested by K a u ~ m a n n . ~ Copp and Everett6 had hinted a t the important roles of the structure of water and hydrocarbon-water interactions in the LCST behavior of amine-water mixtures. We explore these points further in this paper, and look at the effects of additives that alter liquid water structure on the CST of both aqueous and nonaqueous conjugate solutions. The specific additives of interest are water-structure breakers such as urea, thiourea, and guanidinium chloride (GuCl), and water-structure enhancers such as sugars, and also the lyotropic or Hofmeister series of inorganic salts. We present results to show that while the Prigogine and the Timmermans rules are obeyed in general by nonaqueous liquid pairs, these rules are violated in watercontaining binaries. Agents that are thought to disrupt water structure solubilize the organic component into water, and water-structure makers decrease the mutual solubility. Experimental Section All chemicals used were the best analytical grade available. The experimental details of the measurements of CST were as described by Hales, Bertrand, and H e ~ l e r . ~ Typical denaturants chosen were urea, thiourea, GuC1, and glycol. The lyotropic series of ions chosen were those 0022-365417912083-2724$0 1,0010
suggested in the review by von Hippel and Schleich.g Experiments on methanol-containing systems were conducted with sealed tubes to avoid moisture contamination. Data exist in the literature on the effects of added salts on the CST of some aqueous and organic binaries, but not on the effects of protein structural perturbants. Since these compounds are sparingly soluble in several organic solvents, we have had to choose liquids in which these dissolve to significant extents. Accordingly, we chose the systems benzyl alcohol-hexane, ethylene glycol-methyl acetate, and methanol-hexane,
Results and Discussion Prigogine and Timmermans Rules. It has been shown2 that, in a conjugate binary system, the condition with respect to phase separation is apl/ax2 e 0 apz/axl < 0 (1) where ki and xi are the chemical potential and mole fractionof component i. Application of regular solution theory leads to a restatement of the condition for phase separation as
where a12 is the net interaction free energy between components 1 and 2, R, the gas constant, and T , the temperature. If the aI2is large and positive, this inequality cannot be satisfied for all x 2 , since x,(l - x 2 ) has a maximum value of 0.25. Hence, when (2a12/RT)> 4, there will be a range of x2 where the system will separate into two phases. At the temperature CST = T, ( x J , = 0.5 T, = ~ ~ 1 2 / 2 R (3) This is the equimolar composition rule of Prigogine, and T, above is the UCST; if we write the alternate possibility in eq 2, the corresponding T , would be the LCST. The effect of the addition of a third component to the binary system can be expressed in terms of the intercomponent interaction free energies a+ There are two possibilities: (a) if component 3 is more soluble in one of the liquids than the other, a23 >> a 1 2 and a23 >> a13 (so that 2 and 3 are much less soluble than they are in l),then >O
or the alternate case of
a13
0 1979 American Chemical Society
>> a 2 3 and
a13
>> a12gives
The Journal of Physical Chernistty, Vol. 83,No. 21, 1979 2725
Critical Solution Temperatures of Liquid Mixtures
TABLE I : Data on Some Consolute Liquid Pairs item
lliauid A
a b
nitrobenzene an dine be:nzyl alcohol nit roethane dirnethylf'ormamide phenol phenol methanol met h an ol methanol ethanol ethylene glycol fo.rmamide water water water water
C
d e f g
h i j
k 1 m n 0
liauid B
~a'
n-hexane n-hexane n-hexane
0.50 0.50 0.50
19.0 59.6 50.5
3-meth ylpentane
0.50 0.50
20.0 50.5
n-heptane n-hexane cyclohexane carbon disulfide n-hexane paraffin (decane) methyl acetate
0.50 0.55 0.49 0.63
52.9 52.6 49.1 40.5 33.7 33.5 21.4
nitrobenzene phenol triethylamine diethylamine 1-butanol
0.42 0.08
cyclohexane
P q a Mole fraction of B.
0.60 0.69 0.47
0.08 0.30 0.10
118
CST,"C
J r14 w
g t10 0 W 0
- 16 c 6 tZ
0
-*
01
0.5
02 0.3 M O L A R I T Y OF ADDITIVE
t3
108.2 65.9 18.5 140.0 124.4
W W K
0
EG +I
-
U, also GL
Q
$
-
0 C6H5 CHZO H . n - C6H14 , T, 50.5 ' C
-
and (b) if' 3 is about equally soluble in 1and in 2, we have a13 ~ 3 and , then
( 6 T / 6 ~ , )=, - c x I ~ / R< 0
(5)
Thus, an additive that is preferentially soluble in one component of the liquid mixture will decrease the mutual solubility of the pair (raise UCST or lower LCST), and one that is about equally soluble in 1and 2 will increase their mutual solubility (lower UCST or raise LCST). This is the Timinermans rule. The behavior of the excess functions GE, HE,and SEmay be written, if these functions have the same sign for all x 2 a t a given T,, as a t UCST
0.01 M
y
M O L A R I T Y OF A D D I T I V E -
Figure I. Upper curve: Effect of additives on the CST of cyclohexane-DMF G = glucose; TU = thiourea; U = urea; and GI = glycerol. The additives are preferentially soluble in DMF. Lower curve: Change in the CST of equimolar benzyl alcohol-n-hexane brought about by TU (thiourea), EG (ethylene glycol), U (urea), and GI (glycerol). The additives are preferentially soluble in benzyl alcohol. WETHANOL : n-HEXANE T,
25.5OC
6.0 1
cn
GE > 0
HE> 0
SE> or < 0
(6)
w
$
4.0
C Y
w
at LCST
n
GE>O
HEIO
SE K'. In other words, the greater the capacity of an ion to disrupt the structure of a solvent, the greater is its salting-in ability and vice versa. The Hofmeister series can thus be seen to be a sequence of agents that are increasingly effective in disrupting water structure and weakening hydrophobic interactions, a fact that has been intuitively accepted,21J2 and which is illustrated by the present studies. It would be interesting to study whether the Hofmeister sequence would hold in other solvents that possess high degrees of intermolecular structure.
The Journal of Physical Chemistry, Vol. 83, No. 21, 1979 2727
Acknowledgment. P.M. is a Research Fellow of the U.G.C., India. We are grateful to the referees for suggestions and criticism.
References and Notes A. W. Francis, Adv. Chem. Ser., No. 31 (1961). I. Prigogine and R. Defay, "Chemical Thermodynamics", Translated by D. H. Everett, Wiley, New York, N.Y., 1954. Y. Marcus, "Introduction to Liquid State Chemistry", Wiiey, London, 1977. J. Timmermans, Z. Phys. Chem., 58, 129 (1907). W. Kauzmann, Adv. Protein Chem., 14, l(1959). J. L. Copp and D. H. Everett, Discuss. Faraday Soc., 15, 174 (1953). B. J. Hales, G. L. Bertrand, and L. G. Hepler, J . Phys. Chem., 70, 3970 (1966). P. von Hippei and T. Schieich in "Structure and Stability of Biological Macromolecules", S.N. Timasheff and G. D. Fasman, Ed., Marcel Dekker, New York, 1969, pp 417-574. D. H. Everett, Discuss. Faraday Soc., 15, 267 (1953). N. Nishinoand M. Nakamura, Bull. Chem. Soc.Jpn., 51, 1617 (1978). D. Balasubramanianand C. Ramachandran, Proc. Indian Acad. Sci., Ser. B , 87, 53 (1978). T. S. Lakshmi and P. K. Nandi, J. Phys. Chem., 80, 249 (1976). D. 9. Wetlaufer, S.K. Maiik, L. Stoiler, and R. L. Coffin, J. Am. Chem. SOC.,86, 508 (1964). W. Bruning and A. Holtzer, J. Am. Chem. Soc., 83, 4865 (1961). L. J. Howard and W. H. Patterson, J . Chem. Soc., 2787 (1926). E. L. Eckfeidt and W. W. Lucasse, J . Phys. Chem., 47, 164 (1973). P. P. Kosakewitsch, Z . Phys. Chem., 143, 216 (1929). H. Schneider in "Solute-Solvent Interactions", J. F. Coetzee and C. D. Ritchie, Ed., Marcel Dekker, New York, 1969. W. F. McDevitt and F. A. Long J. Am. Chem. Soc., 74, 1773 (1952); Chem. Rev., 51, 128 (1952); N. C. Den0 and C. H. Spink, J . Phys. Chem., 67, 1347 (1963). E. L. Eckfeldt and W. W. Lucasse, J . Phys. Chem., 47, 183 (1943). J. W. Larsen and L. J. Magid, J . Am. Chem. Soc., 96, 5774 (1974). A. Hamabata and P. von Hippel, Biochemistry, 12, 1264 (1973).
Relationship of Structure to Properties in Surfactants. 8. Synthesis and Properties of Sodium 3-Alkyltetrahydropyranyl 4-Sulfates Milton J. Rosen* and Chang-chin Kwant Department of Chemistry, Brooklyn College, City University of New York, Brooklyn, New York 11210 (Received March 8, 1979)
A series of 3-alkyltetrahydropyranyl 4-sulfates, containing 9-15 carbon atoms in the alkyl group, has been synthesized,purified, and characterized. These materials have unusually high solubility in water. Surface tension measurements on aqueous solutions of these compounds have been used to calculate surface excess concentration, area/molecule at the liquid-air interface,critical micelle concentration,and efficiency and effectiveness of surface tension reduction. Values are compared with those for sodium n-alkyl sulfates. These data indicate that the tetrahydropyran group in these compounds is lying in the interface with the alkyl group oriented away from the aqueous phase. Using a new method for calculating standard free energies of adsorption and micellization, we calculated the standard free energy of adsorption of the tetrahydropyran group at the aqueous solution-air interface at 25 "C to be -1.7 kJ mol-', somewhat less than that for a methylene group (-3.03 kJ mol-l) in the alkyl chain of the molecule, while the standard free energy of micellization of the tetrahydropyran group is shown to be -3.0 kJ mol-', somewhat more than that for a methylene group (-2.58 kJ mol-') in the alkyl chain.
Introduction In a continuation of our investigations into the relationship between the molecular structure of surfactants and their surface properties,1.2 a series of sodium 3-alkyltetrahydropyranyl4-sulfateshas been synthesized and their surface and related properties measured in order to determine the effect of the tetrahydropyran nucleus on Onyx Chemical Company, Jersey City, N.J. 07302. 0022-3654/79/2083-2727$0 1.OO/O
these properties. The presence of an acyclic ether linkage in the hydrophobic group of a surfactant is known to affect significantly its surface proper tie^,^^^ but data are not available on the effect of cyclic ether functions. Interest in the tetrahydropyran group stems from a recently developed process of preparing long-chain alkyltetrahydropyranols by the Prins reaction of a-olefins and paraf~rmaldehyde.~ The sodium alkyltetrahydropyranyl sulfates were synthesized by the following reaction sequence: 0 1979 American
Chemical Society
