J. Phys. Chem. 1989, 93, 8223-8226 TABLE IV: Second-Order Rate Constants for Reactions of p-X-C&14CHzCH2Brwith OH- in Water and Micelles X k,"' X los, M-I s-I k,'"lk, 8896 10.5 3.4
NO2
CI H
3.4 2.0 1.1
increase with the surfactant concentration. The application of this mass-law treatment to the reaction of hydroxide ion with para-substituted phenylethyl derivatives in (CTA)OH is illustrated in Figures 1-3. The lines are calculated taking parameters that are shown in Table 111, cmc = 9.6 X l r 3 M with no NaOH added, and cmc = 6 X lo4 M in 0.02 M NaOH. Different values of the ion-exchange parameter, gy,have been used in the literature, and there are significant differences between the values adopted by different groups.21 However, for the most widely studied systems, values of are between 10 and 20. From our results, it can be deduced that organic reagents may affect the binding of hydroxide ion to (CTA)Br micelles very stron ly and the value of is higher. We obtained the value of that is in the range calculated for pesticides: DDT, DDD,39 and p,p'-metho~ychlor,~~ Le., 32, 30, and 24, respectively. The value of KOH' = 30 M-I is similar to the reported values for DDT and DDD of 30 and 35, r e ~ p e c t i v e l y . ~It~ differs, however, from other values reported in the literature for reagents such as benzoic anh~dride:~5-nitroindoleP N-methyl-4-cyanopyridinium tetrafl~oroborate~' for which the value of 55 M-I has been found.
g, B
(46) Cipiciani, A.; Savelli, G.; Bunton, C. A. J . Phys. Chem. 1983,87, 5259. (47) Chaimovich, M.; Cuccovia, I. M.; Bunton, C. A.; Moffatt, J. R. J .
Phys. Chem. 1983,87,3584.
8223
The second-order rate constants, kM, are given with the concentrations written as a mole ratio. The constants can be converted into kzm [M-' s-I] by taking into account the volume element of the reaction in the micelles, which is assumed to be that of a Stern layer, with a molar volume of ca. 0.14 L.j4 The second-order rate constants for all the investigated reagents with hydroxide ion in (CTA)Br and (CTA)OH, kZm,given in Table IV, are about the same order of magnitude as the second-order rate constants for reaction in the aqueous phase, k,. Thus, the effect of cationic micelles on the reaction of hydroxide ion with para-substituted phenylethyl bromides is mainly due to the concentration of both reactants in the small volume of the Stem layer of the micelle. Para substituents in the benzene ring of the investigated organic reagents do not introduce any significant influence on the PIE parameters' values. Recently, D. G.Hall48 proposed a new theory based on the equilibrium formulation of transition-state theory where all attention is focused on the partition coefficients of reactants and activated complexes between micellar and aqueous environments. The present authors will consider this alternative theory to explain micellar catalysis of the elimination reaction in terms of transition-state theory in a further publication. Acknowledgment. We thank Tadeusz Wilk for his assistance in computation procedure and Marek Jon for the IH N M R measurements. This work was supported by the Polish Academy of Sciences, Research Grant No. CPBR 3.20. Registry No. p-X-C6H4CH2CH,Br (X = NO2), 5339-26-4; p-XC6H4CH2CH2Br(X = CI), 6529-53-9; p-X-C6H4CH2CH2Br(X = H), 103-63-9. (48) Hall, D. G. J . Phys. Chem. 1987, 91,4287.
Chemical Structure and Thermodynamics of Amphiphile Solutions. 1. Solubility of Alkylthlooligooxyethylene Glycols in Water Adam Sokolowski Institute of Organic and Polymer Technology, Technical University of Wroclaw, 50-370 Wroctaw. Poland (Received: January 9, 1989; In Final Form: May 10, 1989)
Solubilities of individual alkylthiooligooxyethylene glycols CnH~n+lSCH2CH2(OCH2CH2),0H (alkyl: n-C4H9,...,n-C9HI9; and z = 0, ..., 3) in water at 298.15 K have been determined. A quantitative correlation has been found between the standard free energy for transfer from aqueous solution to pure liquid phase, AGO,, and the structure of the studied sulfides and of oligooxyethylenated alcohols as well by using the appropriate multiple regression equation. It has been found that the thioethylene group, -SCH2CH2-, is hydrophobic. The standard free energy contribution, AGo,[-SCH2CH2-], is -3.4 kJ mol-'. The interaction between methylene groups, -CH2-, of the hydrocarbon chain and the -OCH2CH2- groups of the oligooxyethylene chain is statistically significant and has an influence on the total value of AGO,. Furthermore, the AGo,[-OCH2CH2-] and AGO,[-CH2-] are not constant. For the sulfides studied, AGO,[-OCH2CH2-] increases linearly with hydrocarbon chain length. For example, AGo,[-OCH2CH2-]for the series of butyl and nonyl derivatives is +0.24 and +0.81 kJ mol-I, respectively. The AGo,[-CH2-] for the series of alkyl-2-hydroxyethylsulfides ( z = 0) and alkylthiotetraoxyethyleneglycol ( z = 3) is -3.43 and -3.10 kJ mol-', and the AGo,[-OCH2CH2-] and AGO,[-CH2-] for the group of oxyethylenated alcohols are + O S 3 and -3.1 3 kJ mol-', respectively.
Introduction The number of scientific reports and publications on solubility, micellization, and surface activity of oligooxyethylene nonionics has rapidly increased in recent several years, mainly due to the expansion of their application field and the increased availability of well-characterized compounds for the research.'-I0 (1) Shinoda, K. Principles of Solution and Solubility; Marcel Dekker: New York, 1978. (2) Tanford, C. The Hydrophobic Effect, 2nd ed.; Wiley-Interscience: New York, 1980.
0022-3654/89/2093-8223$01.50/0
In our previous works, we have investigated the surface activity of individual w-alkylthiooligooxyethyleneglYCOls at the aqueous (3) Shinoda, K., Ed. Solvent Properties of Surfactant Solutions; Marcel Dekker: New York, 1967. (4) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley-Interscience: New York, 1978. ( 5 ) Schick, M. J., Ed. Nonionic Surfactants: Physical Chemistry; Marcel Dekker: New York, 1987. (6) Lindman, B.; Wennerstrom, H. Topics in Current Chemistry; Springer-Verlag: Berlin, 1980; Vol. 87. (7) Tadros, Th. F., Ed. surfactants; Academic Press: London, 1984.
0 1989 American Chemical Society
8224
The Journal of Physical Chemistry, Vol. 93, No. 25, 1989
solution-air interface."J2 Moreover, we presented thermodynamic parameters of micelle formation for aqueous solutions of alkylthiotetraoxyethylene glycols13 and for sulfur analogues of n-hexyltetraoxyethylene glycols.14 In the first group of compounds, the bivalent sulfur atom bridged alkyl chain with oligooxyethylene chain. In the case of n-hexyl derivatives, the sulfur atom wandered along the tetraoxyethylene chain occupying a position of one oxygen atom at a time. The present paper reports the results of solubility data for alkyl-2-hydroxyethyl sulfides (I, where z = 0), alkylthiooligooxyethylene glycols (I; z = 1, 2) and some earlier published data concerning alkylthiotetraoxyethyleneglyc01s'~(I; z = 3) are included as well. From the solubility data it was further possible (i) to calculate the standard free energy for transfer from aqueous solutions to pure liquid phase, AGO,, (ii) to estimate the contribution of alkyl chain, oligooxyethylene chain, and the remaining part of a molecule (including also both thioethylene group, -SCH2CH2-, and hydroxyl group) to the free energy, and (iii) to compare the resulting quantities with those calculated from both oligooxyethylenated alcohols CnHZn+l(OCH2CH2)rOH (11) and normal alcohols CnH2,+IOH (111). Compounds I were obtained in a reaction of alkanethiols with w-chlorooligooxyethylene glycols according to the following schemeI2~l3v15
Sokolowski TABLE I: Physical Constants of AlkylthiooligooxyetbyleneGlycols R bp,' K n-C4H9 344-5 (1) n-C5H,, 345 (0.75) n-C6Hl3 349-51 (0.2) n-C7H15 383-5 (1) n-CsH17 378-81 (1) n-C9HI9 393-7 (1) R S C H 2 C H 2 0 C H 2 C H 2 0 H n-C4H9 366-8 (0.15) n-C5H,, 372-5 (0.3) n-C6H13 383-5 (0.2) n-C7Hl5 409-10 (1) n-C8H17 430-1 (2) n-C9HI9 428-9 (1) RSCH2CH2n-C4H9 402 (1) (OCH2CH&OH n - C 5 H I , 415-6 (1.2) n-C6H13 413-7 (0.35) n-C7H15 426-9 (0.45) n-CsH17 427-31 (0.2) n-C9H19 447-9 (1) RSCH2CH2n-C4H9 419-21 (0.2) (OCH2CH2)IOH tt-C5HlI 440 (0.5) n-C6H13 400-3 (0.25) n-C7H15 463-6 (1.1) n-C8HI7 455-7 (0.2) n-C9HI9 460 (1)
compd RSCH2CH20H
a Pressures
nZ9'D
4:;
1.4802 1.4790 1.4777 1.4770 1.4767 1.4761 1.4775 1.4760 1.4750 1.4756 1.4743 1.4748 1.4766 1.4748 1.4748 1.4741 1.4747 1.4739 1.4760 1.4740 1.4741 1.4738 1.4743 1.4732
0.9703 0.9543 0.9428 0.9353 0.9284 0.9218 1.0032 0.9874 0.9766 0.9646 0.9594 0.9430 1.0175 1.0062 0.9954 0.9837 0.9779 0.9714 1.0334 1.0227 1.0162 1.0023 0.9950 0.9896
(Torr) are given in parentheses.
TABLE II: Solubilities of w-AlkylthiooligooxyethyleneGlycols in C ~ H ~ ~ + I S C H ~ C H I I ( O C H ~ C H ~Water ) , ~ Hat 298.15 K compd R solubility, 1#C, mol kg-I (1) RSCH2CH2E00H" n-C4H9 10.2 where alkyl is n-C4H9, ..., n-C9HI9and z = 0, ..., 3. They were n-C5H11 2.69 isolated from the reaction mixtures by extraction with ethyl ether n-C6H13 0.603 followed by fractional distillation: their purities were checked by n-C7H15 0.158 GLC, and their structures were verified by 'H NMR.I2 The n-CSH17 0.0385 solubilities were measured interferometrically. n-C9H19 0.0100 -HP
Experimental Section Synthesis of w-Alkylthiooligooxyethylene Glycols. Methanol (0.5 dm3) and sodium hydroxide (40 g, 1 mol) were heated to reflux. Alkanethiol (1 mol) was dropped into the solution, and then w-chlorooligooxyethyleneglycol (1 mol) was added dropwise. After 2 h under reflux the cold solution was filtered by suction to remove the inorganic salt. The solvent was distilled off and the residue was washed with water and extracted with ether. Then the product was distilled under vacuum. The purity of compounds, determined by GLC (Giede G C H F apparatus equipped with FID detector, metal column (3 mm i.d. and 1 m long) packed with 15% Silicone XE-60 on Chromosorb G/AW DMCS 60/80 mesh, N2 as carrier gas) was higher than 99%. The IH N M R spectra (Tesla BS 497, 100 MHz, 10% solutions in CDC13, HMDS or T M S as external or internal standard, respectively) confirmed structures of the investigated compounds. The physical constants of the compounds are listed in Table
RSCH2CH2E,0H
n-C4H9 n-CsH1 I n-C6H13 n-C7H15 n-CS
17
n-C9H19
RSCH2CH,E20H
n-C4H9 n-C5Hll
n-C6H13 n-C7H15 n-C8H17 n-C9H19
RSCH2CH2E30H
n-C4H9 n-CJ-4 I n-C6H13
n-C7H15 n-CsH17 n-C9H19
11.0 3.09 0.776 0.209 0.0562 0.0145 13.3 3.31 0.912 0.260 0.07OS 0.0195 14.5b 3.9,b 1.10b 0.28: 0.095b 0.0282b
"Abbreviation in this paper: E = oxyethylene unit -OCH2CH2-. The values of cmc from surface tension measurements.I3
I. Solubility Measurements. The solubilities were determined refractometrically according to the procedure described by (8) Chattoraj, D. K.; Birdi, K. S. Adsorption and the Gibbs Surface Excess; Plenum Press: New York, 1984. (9) Mittal, K. L., Lindman, B., Eds. Surfactants in Solution; Plenum Press: New York, 1984. ( I O ) Mittal, K. L.; Fendler, E. K., Eds. Solution Behavior of Surfactants: Theoretical and Applied Aspects; Plenum Press: New York, 1982. ( I 1) Sokobwski, A,; Burczyk, B.; Beger, J. Colloids Surf. 1989,36, 373. (12) Sokobwski, A.; Burczyk, B.; Beger, J. Colloids Surf., in press. ( I 3) Sokobwski, A,; Burczyk, Beger, J. Proceedings of the International Conference on Surface Active Substances, Bad Suer (DDR) 1985; Akademie Verlag: Berlin, 1987; pp 419-422. (14) Sokobwski, A,; Burczyk, B.; Beger, J. Proceedings of the IXth European Chemistry of Interfaces Conference, Zakopane (Poland);Materials Science Forum; Trans. Tech. Publications: Switzerland; 1988; Vol. 25-26, pp 343-346. (15) Szymanowski, J.; Voelkel, A.; Beger, J. Piischmann, C. Tenside Detergents 1983, 20, 173.
Meeussen and Huyskens.16 The measurements were made at 298.15 f 0.1 K in a LI-3 interferometer (C. Zeiss, GDR). The saturated solutions were obtained by intensive stirring of water with the organic phase in a thermostat for 24 h. Two-phase mixtures were then left to stand for further 24 h or centrifugated. Finally, the clarified aqueous layers were transferred into the glass measuring cell. Results and Discussion Solubilities in Water. A large number of amphiphilic compounds from the group of oligooxyethylene derivatives of alkanethiols (I) were synthesized. The compounds were coded as CnSCH2CH2E,0H, where n = 4, ..., 9 and z = 0, ..., 3, while oligooxyethylene derivatives of aliphatic alcohols were denoted (16) Meeussen, E.; Huyskens, P. J. Chim. Phys. Phys.-Chim. Biol. 1966, 63, 845.
The Journal of Physical Chemistry, Vol. 93, No. 25, 1989 8225
Solubility of AlkylthiooligooxyethyleneGlycols
TABLE IV: Multiple Regressions of Values of the Standard Free Energy for Transfer of Alkylthiooligooxyethylene Glycols (I) from Aaueous Solution to Pure Liauid Phase
TABLE III: Standard Free Energy for Transfer from Aqueous Solution to Pure Liouid State at 298.15 K
R
compd RSCH2CHZEoOH
detd w C ~ H ~-15.62 n-C5HII -18.92 n-C6HII -22.63 n-C7HI5 -25.94 n-C8HI7 -29.45 n-C9Hi9 -32.79 RSCH2CH2EIOH n-C4H9 -15.44 n-C5Hll -18.57 -22.00 n-C7H15 -25.25 n-C8H17 -28.51 t~C9H19 -31.88 RSCH2CH2E2OH n-C4H9 -14.95 n-C5H11 -18.40 fl-C&13 -21.60 n-C7H15 -24.71 n-C8HI7 -27.94 n-C9HI9 -31.13 RSCHzCH,E,OH n-C4H9 -14.74 n-CSHII -17.98 n-C&i, -21.14 n-C,HiS -24.53 n-CBH1, -27.21 ~ C 9 H 1 9 -30.22
" From eq 6. IV.
calcd" -15.60 -19.03 -22.47 -25.90 -29.34 -32.77 -15.36 -18.68 -22.00 -25.32 -28.64 -31.96 -15.1 1 -18.32 -21.53 -24.74 -27.95 -31.16 -14.87 -17.97 -21.06 -24.16 -27.26 -30.35
regression coeff bo bl
kJ mol-' -3.43
W,)
b2 S(b2) -3.32
TABLE V Multiple Regressions of Values of the Standard Free Energy for Transfer of Alkyloligooxyethylene Glycols (11) from Aqueous Solution to Pure Liquid Phase regression coeff eq 6b" eq 6c"
-3.21
or
pb
= Fm
(1)
The solubilities of the compounds I, except perhaps for the n-butyl derivative, are very low. Therefore, it seem justified to consider the saturated solutions as ideally diluted ones, so that the thermodynamic activity equals the mole fraction xb pb = pb,O
+ R T In xb
0.1405 (0.2748) -1.1998 (0.0383) 0.5749 (0.1958) -0.04844' (0.02597) 8 0.9996
-3.10
by C,E,OH. Each of the compounds studied contained a straight hydrocarbon chain and a primary hydroxyl group and differed by the presence or absence of the thioethylene group, -SCH2CH2-. The solubilities of studied individual RSCH2CH2E,0H are listed in Table 11. It can be seen from the results shown that solubilities in water increase with the length of oligooxyethylene chain (for corresponding size of the substituent R). The relations between the solubility and the number of carbon atoms in the alkyl chains are shown in Figure 1. The linear variations have been obtained for all series of homologues. The critical micelle concentration (cmc) values for alkylthiotetraoligoyethylene glycols,13 as determined from the surface tension (y) plots vs log c, are also listed in Table I1 and shown in Figure 1. The solubilities of I compared with those of both oligooxyethylenated alcohols" (11) and of normal alcohols18-20(111) indicate that the sulfides (I) are more hydrophobic than compounds I1 and 111 (for corresponding size of both oligooxyethylene and hydrocarbon chain length). Thus, the thioethylene group -SCH2CH2indicates hydrophobic character. Standard Free Energies for Transfer. If pband p1 (or pm) are chemical potentials of the amphiphile in the saturated solution (bulk) and pure liquid phase (or micellar state), respectively, then since they are in equilibrium = pi
eq 6a 0.04544 (0.00545) 24 0.9998
" N = total number of data points; R = multiple correlation coefficients.
bAGo,[-CH2-]l = [(b,), + (b,)p]RT. (b,)], see Table
pb
regression coeff b, S(b,) N" R
eq 6a -0.7519 -1.3853 (0.0204) -0.08369 (0.0367 1)
(2)
Since the pure liquid phase (or micellar material) is in its standard state, = pl,O or pm = p . 0 (3) (17) Kucharski, S.;Sokolowski, A,; Burczyk, B. Rocrniki Chem. (Ann. SOC.Chim., Polonorum) 1973, 47, 2045. ( I 8) Huyskens, P.;Mellens, J.; Gomez,A.; Tack, J. Bull. Soc. Chim.Belg. 1975, 84, 253. (19) Vochten, R.; Petre, G. J . Colloid Interface Sei. 1973, 42, 320. (20) Sokolowski, A.; Burczyk, B.; OleS, J. J. Phys. Chem. 1984,88, 807.
0.6175 (0.13 15) -1.2645 (0.0210) 0.2125 (0.03 18) 8 0.9994
" Calculations based on the values of solubility reported by Kucharski et aLi7 measured at 292.15 K. bt(b,) = b,/S(b,) = 1.869 < t(a = 0.05) = 2.776. TABLE VI: Regressions of Values of the Standard Free Energy for Transfer of Aliphatic Alcohols from Aqueous Solution to Pure Liquid Phase regression coeff
eq 6d'
eq 6ea
bo Who) bl S(bl) N R
1.781 (0.119) -1.4193 (0.0164) 7 0.9990
1.517 (0.126) -1.3842 (0.0190) 15 0.9994
LI Calculations based on the values of solubility reported by Vochten and Petrel9 measured at 288.15 K and Huyskens et al.18 measured at 298.15 K, respectively.
Thus, if AGO, is the standard free energy for transfer of compounds from bulk solution to the pure liquid phase,20 then AGO, =
plqo - pbvo = p1- pb
+ RT In xb = RT In xb
(4)
and standard free energy for transfer of 1 mol of amphiphile from the monomolecular state to the micellar standard state is
The results of the calculations are shown in Table 111. The relationship between the chemical structure and solubility for the group of studied sulfides C,SCH2CH2(OCH2CH2),0H can be described by the following multiple regression equation
In x = bo + bln + b2z
+ b3nz
(6)
where In x is equal to AGO (in RT units), n is the number of carbon atoms in the hydrocarbon chain, z is the number of oxyethylene groups, and bo, ..., b, are the regression coefficients. The coefficients bi are presented in Table IV, together with their standard errors S(bi). The mean error estimate for In x in eq 6a (see Table IV) equals 0.40%. For the sake of comparison the regression coefficients bi, for oligooxyethylenated alcohols (11) and alcohols (HI),as well as respective standard errors S(bi),are also presented in Tables V and VI. By using the t statistics"*12 it has been shown that, on the confidence level of a = 0.05, the product b3nz can be neglected in eq 6 for oxyethylenated alcohols I1 (see Table V).
8226 The Journal of Physical Chemistry, Vol. 93, No. 25, 1989
-- I 54
2
Sokolowski group between the alkyl chain and oligooxyethylene group of the oxyethylenated alcohol molecule. It means that the bivalent sulfur atom located at the distance of two carbon atoms from oligooxyethylene group has a weakly hydrophilic character compensating merely ca. one CH2 group in the hydrocarbon chain. This finding is in accord with the results obtained in studies on adsorption of sulfides (I) at the aqueous solution-air interface.], For the group of sulfides studied AGo,[-OCH2CH2-] increases linearly with hydrocarbon chain length. AGO,[-OCH2CH2-] = (b, 6,n)RT = (-0.207 + 0.1126n) kJ mol-] (12)
P
+
L ,
I
I
1
1
1
I
I
L
5
6
7
8
9 n
Figure 1. Solubility (log c) vs number of carbon atoms ( n ) in the alkyl chain of alkylthiooligooxyethylene glycols: (0) alkyl-2-hydroxyethyl
sulfides; (*) alkylthiodioxyethylene glycols; (A)alkylthiotrioxyethylene glycols; (0)alkylthiotetraoxyethylene glycols. The value of AGO, for individual compound can be regarded as a sum of increments AGo,[Xi] of changes coming from each part X, of compound. For compounds I such a sum has the following form AGO, [C,SCH2CH2EZOH] = nAG0,[-CH2-], + Z A G ~ , [ - O C H , C H ~ - + ] ~ AG0,[X,II (7)
where AG0,[Xw],denotes the sum of increments of the standard free energy of transfer due to all fragments of compounds I other than -CH2- or -OCH2CH2-, Le., according to the equation AGo,[X,]I AGo,[C,SCH2CH2E,0H]~,z-.~= AGo,[CH3-]I - AGo,[-CH2-]I + AGo[-SCH2CH2-]I AGo[-OH]I (8)
+
By analogy to the monoethers 11, the sum of increments can be written as AGO,[C,E,OH] = ~IAGO,[-CH,-]II
+ zAG0,[-OCH2CH2-]II + AG",[X,]II
(9)
where AGo,[X,],~ AGo,[C,E,OH],,, = AGo,[CH3-]11 - AGo,[-CH2-]11
+ AGo[-OH]I1
(10)
If one assumes that ACof[CH3-] - ACo,[-CH2-] + A G O , [ OH] is the same for both types of compounds, the difference AGo,[X,]I - AGo,[X,]II may be treated as the increment AGO,[-SCH2CH2-] measuring the hydrophobic character of the -SCH2CH2- group. Thus AGo,[-SCH2CH2-] = [(bo),
- (bO),l]RT = -3.4 kJ mol-' (11)
where (b& and (6o)lI are the regression coefficients from Table IV and V, respectively. The thioethylene group exhibits a weak hydrophobic character similar to that induced by the insertion of additional ca. one
