fates in aqueous solutions of alkyl sulfates was determined turbidimetrically [12]. .... plete, after which there are two possibilities for the alkyl sulfate molecules: ...
LITERATURE CITED i. 2. 3. 4. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14.
The Physics of Simple Liquids [Russian translation], Mir, Moscow (1971), p. 30. J. Barker and D. Henderson, Rev. Mod. Phys., 43, 587 (1976). N. P. Kovalenko and I. Z. Fisher, Usp. Fiz. Nauk, 108, 209 (1972). V. M. Sysoev and A. V. Chalyi, Teor. Mat. Fiz., 44, 251 (1980). V. M. Sysoev and A. V. Chalyi, Physics of the Liquid State [in Russian], No. 9, Izd. Kiev. Gos. Univ., Kiev (1981), p. 102. A. Z. Golik, I. I. Adamenko, V. M. Sysoev, et al., in: The Thermophysical Properties of Liquids [in Russian], Nauka, Moscow (1976), p. 5. R. Vedam and J. Holton, J. Acoust. Soc. Am., 43, No. i, 108 (1968). J. K. Percus, in: The Equilibrium Theory of Classical Fluids, New York (1964), pp. 11-33. E. Keller and J. Keller, Phys. Rev., 142, 90 (1965). J. K. McDonald, Rev. Mod. Phys., 33, 669 (1966). E. Wilhelm, J. Chem. Phys., 63, 3379 (1975). A. Z. Golik and A. V. Chalyi, Ukr. Khim. Zh., 20, 993 (1976). V. M. Sysoev, Ukr. Khim. Zh., 25, 123 (1980). Kh. I. Mogel' , A. V. Chalyi, and Yu. I. Shimanskii, Physics of the Liquid State [in Russian], No. 4, Izd. Kiev. Gos. Univ., Kiev (1976), p. 75.
CONCENTRATION--CHAIN LENGTH DIAGRAMS AND THE STRUCTURE OF AQUEOUS SOLUTIONS OF DIPHILIC MOLECULES Yu. A. Mirgorod
UDC 541.8
The concept of the boundary of the complete hydrophobic hydration of diphilic nonelectrolytes and electrolytes has been introduced on the basis of experimental and published data. The functional dependence of the concentration at which unique points in the properties of the solutions are observed on the length of the chain of the hydrophobic radical has been used to establish the interrelationship and boundaries of separation of different aqueous solutions of diphilic molecules.
INTRODUCTION The structure and properties of aqueous solutions of diphilic molecules are determined not only by the hydrophilic group but also by the structure of the hydrophobic radical (chain), the concentration of diphilic molecules, and the structure of water. Solutions in which diphilic molecules form micelles are now well known [i], since these systems show properties of practical importance. At the same time, there are other solutions where diphilic molecules act as hydrotropes [2] (which do not form micelles, but which increase the solubility of nonpolar substances above the solubility in pure water), and form submicelles [3] (premicellar associates), and monomeric species (contact ion pairs and ion pairs separated by water molecules). The interrelationships and distinct boundaries of separation between these solutions have not yet been established, although this is important for the theory of aqueous solutions of nonelectrolytes and organic electrolytes, and also for the practical application of diphilic molecules. Thus aqueous clathrates are characteristic only of diphilic molecules with short hydrocarbon radicals [4], ion flotation is prevented by micelles and, apparently, submicelles [5], and this last feature is the reason for the errors in the analysis of surface-active substances (SAS) by two-phase titration [6]. The aim of the present work was to classify and interpret the correlations between the concentration position of various unique points in the properties of solutions and the chain Sumy Branch, V. I. Lenin Khar'kov Polytechnic Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 24, No. I, pp. 93-99, January-February, 1983. Original article submitted July 6, 1981.
0022-4766/83/2401- 0081507.50
9 1983 Plenum Publishing Corporation
81
length of the nonpolar radicals, and not to obtain details of the structure of aqueous solutions of diphilic molecules, which in many respects is not yet clear. This is particularly true of the structures of nonelectrolytes and electrolytes with short chain lengths, and the structure of submicelles, which is gradually being investigated in the studies of many research workers. The classification of aqueous solutions of diphilic molecules will be carried out using concentration--chain length diagrams, that is the functional dependence of the concentration at which unique points are observed in the properties of the solutions on the chain length of the hydrophobic radical. SAS which are diphilic molecules are traditionally classified as ionogenic (cation- and anion-active) and nonionogenic. In accordance with this classification principle, the materials chosen for study in the present work were S-alkylisothiuronium bromides (ATB) and sodium alkyl sulfates, for which experimental data are given in the present work, and also sodium alkylsulfonates and aliphatic alcohols, for which published data were used [7, 8, 14, 23]. The ATB were synthesized and purified by the published methods [9]. The critical micelle concentration (CMCI) was determined from hydrolysis using a pH-340 instrument [I0], and CCM= was determined by dye solubilization [ii]. The pH = f(C) relationship for ATB with low molecular weight (n = 4-6) was obtained using a pH-340 instrument. The solubility was determined by titrating a saturated solution with silver nitrate. The alkyl sulfates were purified by recrystallizing from ethanol. The solubility of the alkylammonium alkyl sulfates in aqueous solutions of alkyl sulfates was determined turbidimetrically [12]. The surface tension of aqueous solutions of the alkyl sulfates was measured by Wilhelmy's method. TOTAL HYDROPHOBIC HYDRATION OF DIPHILIC MOLECULES In aqueous solutions, diphilic molecules undergo hydrophobic and hydrophilic hydration [13]. The hydrophobic hydration increases with increase in the concentration and chain length of the diphilic molecules, so that, for example, for each alcohol there exists a concentration at which its hydrophobic group undergoes hydrophobic hydration to the maximum extent. This can be called the limit of complete hydrophobic hydration (LCHH). The determination of the LCHH for higher alcohols, which are sparingly soluble in water, presents no difficulty. It can be determined from the solubility [8] of the alcohols (straight line bf in Fig. i). The concentrations of the alcohols and other diphilic molecules (Figs. 2-4) are given in M throughout. The alcohols of low molecular weight, methanol, ethanol, and propanol, mix with water in all proportions, so that the LCHH cannot be determined from the solubility. In [14], the isothermal compressibility of aqueous solutions of alcohols with low molecular weights was studied, and it was found that it passes through a minimum corresponding to the maximum stabilization of the structure (a synonym for limiting hydrophobic hydration), and then increases. The LCHH of alcohols of low molecular weight (ab in Fig. i) can be determined from the concentration corresponding to the minimum isothermal compressibility. It was assumed that the concentration of the minimum isothermal compressibility of aqueous solutions of tert-butanol is equal to that for aqueous solutions of propanol, from the fact that the behavior of the tert-butyl radical in hydrophobic hydration and hydrophobic interaction is approximately equivalent to that of the propyl radical, because of the intramolecular hydrophobic interaction of the methyl groups [15]. An interesting feature is that the LCHH of alcohols with low molecular weight lies below the LCHH determined by extrapolating the LCHH of higher alcohols to low values of n, that is, the lower alcohols as it were increase the hydrophobic hydration. The LCHH of lower alcohols might also be estimated from the minima of the partial molar volume, the inflections on the light-scattering curve, and other properties of solutions. The LCHH determined by these methods naturally is not so accurate as the LCHH obtained from the isothermal compressibility minimum, as in the case of the CMC of colloidal electrolytes and nonionogenic colloids, obtained by different methods which give information on different properties of solutions. These differences are not significant, however, on the concentration scale used to construct the diagram in Fig. I for the complete set of alcohols. There arises the question of whether it is possible to identify the LCHH with the solubility limit. For hydrocarbons, there is no doubt that the solubility limit coincides
82
~gs 2I-
0
.4 I
8
~,
12
I4-
I
I
_]_l "
o-
-2j -2-
r -4-
-5-
6
~gC
Fig. 1
Fig. 2
Fig. i. Concentration-chain length diagram for aliphatic alcohols at 25~ abf -- LCHH, ed -- boundary of the existence of nonhydrated alcohol molecules, i -- solubility of hydrocarbons. Fig. 2. Dependence of the solubility of hydrocarbons and alcohols, CSMC, and CMC for sodium alkyl sulfates on the chain length at 25~ i) solubility of hydrocarbons, 2) solubility of alcohols, 3) CSMC (open points -determined from the maximum adsorption, filled points -determined using the ion-hydrophobic probe), and also CMC, with the addition of 0.45 M NaCI, 4) CMCI, 5) CMCI with the addition of 0.05 M NaCI, 6) CMCI with the addition of 0.i M NaCI. with the LCHH. For other nonelectrolytes it has been shown [16] that the free energies of solution represent the additive contributions of different functional groups. This is also characteristic of hydrophobic hydration [17]. The fact that the straight lines i and bf for hydrocarbons and alcohols are parallel, as shown by Fig. i, therefore means that the change in the solubility of the higher alcohols with change in the chain length is due exclusively to the hydration of nonpolar groups. The determination of the LCHH for diphilic electrolytes represents a complex experimental problem. Among the unique points in the properties of solutions, only CMC~, when the colloidal electrolytes form spherical micelles, and CMC2, when the spherical micelles are converted into rod-shaped micelles, have been effectively determined. We have proposed the determination of the LCHH for colloidal electrolytes from the partial molal heat capacities of organic electrolytes with low molecular weights [3] and by the ion-hydrophobic probe method [12]o In the last method, dilute solutions of sodium alkyl sulfates close to the LCHH are titrated with concentrated solutions of alkylammonium chlorides with the same chain length. The two hydrophobic counterlons react to give an insoluble salt. The titration is stopped when the appearance of crystals is detected turbidimetrically. For reasons which will be discussed below, the LCHH is identified by us with the critical submicelle concentration (CSMC) of the diphilic colloidal electrolytes. Information about submicelle formation can be obtained from the solubility of the salt in the solution. The dependence of the apparent solubility product ~ on concentration, log ~ = f(log C), is divided by unique points into three sections: monomeric ions, submicelles, and micelles [12]. In [18] it was suggested that the CSMC be determined from the point of attainment of maximum adsorption of SAS in the surface layer. The CSMC for sodium alkyl sulfates, determined by the ion-hydrophobic probe methods and from the attainment of maximum adsorption in the surface layer coincide (Fig. 2). It can be seen from Fig. 2 that log CSMC = f(n) is parallel to the straight lines for the solubility of hydrocarbons and alcohols, and intersects the CMC~ straight line at sodium heptyi sulfate. This suggests that at the CSMC, the formation of contact ion pairs is com-
83
plete, after which there are two possibilities for the alkyl sulfate molecules: either to associate by means of hydrocarbon groups or to leave the solution like nonelectrolytes. Association by hydrocarbon groups should be accompanied by cation--anion disso$iation. The experimental data show that on bf (see Fig. i), formation of the triplet RSO,NaSO,R begins. It must be assumed that at the CMC,, contact ion pairs and ion pairs separated by water undergo association. Some of the ions are situated in micelles, and the others in the diffuse layer. The addition of an inorganic electrolyte, by increasing the concentration of contact ion pairs in the micelles, lowers CMC~. log CMCI = f(n) and log CCSM = f(n) approach one another when an electrolyte is added, log CMC, = f(n) with the addition of 0.I M NaCI approaches log CSMC = f(n) (see Fig. 2), and at 0.45 M they coincide (not shown in Fig. 2, because of the superposition of the points). With increase in the NaCI concentration above 0.45 M, the spherical micelles are converted into rod-shaped micelles, as shown by light scattering for sodium dodecyl sulfate [19]. Thus the CSMC is the concentration at which, with the addition of a definite quantity of an inorganic electrolyte, rearrangement of the micelles takes place, depending on the nature of the colloidal electrolyte. At the CSMC or LCHH with the addition of 0.45 M NaCI, the micelles apparently consist only of contact ion pairs. For ionogenic (anion- and cation-active) SAS, the limit of true solubility is not the CMC (as is usually assumed at present [20]), but the CSMC or LCHH. Thus the LCHH for colloidal electrolytes is the CSMC and at the same time the limit of the solubility of the colloidal electrolyte without association by hydrocarbon groups. It can be seen from Fig. 2 that the LCHH of colloidal electrolytes is easily determined. It is necessary to know which of the lower members of the homologous series still form micelles (in the present case n = 8), after which a straight line parallel to log s for hydrocarbons is drawn through log CMC~ for the next representative in the homologous series (n = 7). CONCENTRATION-CHAIN LENGTH DIAGRAM Solutions of S-Alkylisothiuronium
Bromides (Fig. 3).
to determine the unique points on the diagram.
In this case, hydrolysis was used
The hydrolysis
ff, NH2 ] + R--S"_-'C~ NH~ i \
yNH,] + R--S"--"Ck
