J . Phys. Chem. 1986, 90, 1655-1658 TABLE VIII: Pair Interaction ( v x y ) and Triplet Interaction ( vxYy) Parameters between BHA and CTAB at 25, 35, and 45 "C"
Ox
temp, "C
OXY
GXYY
25 35 45
+6.50 f 0.50 +6.04 h 0.75 +6.38 f 1.00
-3.37 f 0.67 -4.00 h 1.5 -4.43 f 1.5
= BHA, y = CTAB
parameteres between solutes, terms to account for the shift in the monomer-micelle equilibrium of surfactant by added solute and the distribution of added solute between the aqueous and micellar phases also contribute to the magnitudes of thermodynamic transfer functions. It was not possible to apply this model in a strict quantitative manner to the volume of transfer data of BHA from aqueous to micellar phases because experimental results were available for only a few surfactant concentrations and possible structural changes in the mixed micelle reduced the number of data for analysis even more. Nevertheless, application of the following relation enabled some qualitative estimate of the sign of the mixed pair interaction parameters to be deduced. De Lisi et aI.@have shown that eq 7 can describe the volumetric behavior
1655
the surfactant head group by adsorption at the micelle-water interface and the aromatic ring, with relatively polar OCH3 and O H groups, undergoes strong interaction with the polar head group of the surfactant. It may also offer support for the argument45 that the interaction of the polar group (eg. O H ) of one molecule with the alkyl part of another may be as important as hydrophobic interactions in predicting the sign and magnitudes of pair interaction terms. The positive values of E o 2 (Table IV) are expected for hydrophobic solutes in an aqueous medium;46 however, the values reported here for BHT and BHMP are considerably larger than those reported for normal alcohols in water.47 This suggests that these solubilizates are likely to be in a hydrocarbon-like environment where there is relatively more free space than in the vicinity of a hydrophobic solute in water. The smaller E o 2values of BHA indicate that these molecules are located at the surface of the micelle where hydration effects reduce expansibility. The variation in the magnitude of Eo2 with increasing surfactant concentration does not occur in a consistent manner for all additives. Generally, micelles at high surfactant concentration are less hydrated and the larger values of E", at low surfactant concentration may be due to a decrease in the amount of electrostricted water in and around the head groups of the micelle. In conclusion, it appears from both the thermodynamic and spectroscopic studies that BHA is solubilized at the micelle-water interface at low solubilizate concentration and increasingly occupies the palisade layer as the solubilizate concentration increases. The preferential environment of BHMP is nonaqueous; however, some aqueous-like environment is also evident from some of its solution properties. This suggests that BHMP is located near the outer layer of the micelle core, the palisade layer. At high solubilizate concentration the micelle size increases and both BHA and BHMP are preferentially solubilized in the surface region of the micelle. BHT is more hydrophobic in nature and resides in the hydrocarbon-like interior of the micelle. This work is being extended to nonionic micellar systems in order to determine whether different behavior arises in the case of nonionic aggregates.
of mixed micellar systems where P is the (experimental) standard partial molar volume of the additive in the micellar solutions, Pb and Pfare the standard partial molar volumes of the additive bonded to the micelle and the free additive in a water solution containing singly dispersed surfactant molecules and micelles, respectively, Kbis the equlibrium constant for the association of an additive molecule with the micelle, and n is the aggregation number. The data obtained in this work showed that the slope of the plot of P vs. 1/1 + ( K b / n ) ( [ S -] cmc) was negative at all temperatures indicating that P b > Pf,as expected, since P of hydrophobic solutes are smaller in water than in a hydrocarbon-like medium. The estimates of P b were found to have values slightly greater than P 2 ( a q ) , the partial molar volume of Acknowledgment. We are grateful to the Natural Sciences and additive in water. If Pfis greater than P 2 ( a q ) , as has been Engineering Research Council of Canada for financial support. shown44for alcohols in surfactant systems, then ( Pf- P2(as)) Registry No. CTAB, 57-09-0; BHT, 128-37-0; BHA, 121-00-6; is a positive value. Since ( Pf- p2cas,) corresponds to the standard BHMP, 88-26-6; PBA, 3443-45-6. transfer function, A P t r for transfer of the additive from water to the aqueous surfactant system, then the positive values of A T O t r Supplementary Material Available: Tables of density and may be used to predict the sign of the pair interaction term. While volume data for various systems (9 pages). Ordering information it is difficult to make a definitive statement about the sign of uXyr, is available on any current masthead page. it is clear that, if it has a small or zero value,41 then the pair interaction terms uXyare positive. These results agree with the (45) Savage, J. J.; Wood, R. H.J . Solution Chem. 1976, 5 , 733. signs of the uXydata shown in Table VIII, which were obtained (46) Roux, G.; Perron, G.; Desnoyers, J. E. J . Solution Chem. 1978, 7 , from a less rigorous model. They may be interpreted as showing 639. that solubilization of BHA reduces the hydrophilic hydration of (47) Hoiland, H. J. Solution Chem. 1980, 9, 857.
Osmotic Properties of Some Transition-Metal Perchlorates at 30, 40, and 50 O C Judith F. Owen, Paul A. Locke, and C. Stuart Patterson* Department of Chemistry, Furman University, Greenville, South Carolina 2961 3 (Received: September 16, 1985; In Final Form: November 7 , 1985)
Osmotic coefficients for aqueous solutions of five metal perchlorates at 30, 40, and 5 0 "C, based on isopiestic ratios to NaC1, are reported for the concentration range from -0.8 to -2.5 m. Their concentration dependence is q,uite consistent with that indicated by data reported earlier at 25 "C. Our more precise measurements also demonstrate the much smaller effects of cation structure and temperature. In these more subtle effects, some significant differences from the 25 "C data are noted.
Some years ago we reported',2 development of an isopiestic vapor pressure apparatus designed to operate throughout the entire
normal liquid range of water with precision (50.1%) characteristic of the best work at 25 "C. The system has recently been improved,
0022-3654/86/2090-1655$01.5@/0 0 1986 American Chemical Society
1656 The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
Owen et al.
TABLE I: Isooiestic Molalities and Run-Average Coefficients of Variation at Three Temwratures
6.1475 5.7398 2.9273
2.3208* 2.2128* 1.3139
2.3504* 2.2337 1.3338
T = 30.0 f 0.01 “C 2.3 I45 2.207 1 * 1.323 1
2.3897 2.2721 1.3492
2.2987* 2.1871* 1.3110
0.08> 0.07, 0.03?
6.2012b 6.2050 5.7038 3.4778 3.3728 2.5038 2.8134
2.3548 2.3578 2.2126 1.8386 1.4797 1.1737 1.2843
2.3823 2.3814 2.2366 1.8612 1.5006 1.1920 1.3048
T = 40.0f 0.01 “C 2.3459 2.3476 2.2058 1.8407 1.4863 1.1825 1.293 1
2.4191 2.4209 2.2719 1.8870 1.5172 1.2023 1.3168
2.3347* 2.3350* 2,1926 1.8263 I .4729* 1.1719* 1.2811
0.044
6.2683 6.1161 4.6563 4.01 20 3.4359 2.5297 1.7221
2.391 3 2.3430 1.9083 1.7007 1.5117 1.1924 0.88 17
2.4132 2.3690* 1.9315 1.7235 1.5322 1.2095 0.8962
T = 50.0 f 0.01 2.3778 2.3353 1.9077* 1.7057 1.5170 1.1994 0.8893*
2.4502 2.4083* 1.9533 1.7395 1.5473 1.2188 0.8998
2.3646 2.3213* I .8950* I .6945* 1.5045 1.1895 0.8818
0.038 0.07, 0.06, 0.06, 0.05, 0.03, 0.07,
0.04, 0.040 0.04, 0.06, 0.03,
0.040
O C
Ocv is the relative standard deviation expressed as percent of the average. All cv‘s a r e based on three samples except those marked with an asterisk which a r e based on only two samples. bThis N a C l molality is for a single sample: the cv for this run is therefore the average for the remaining five salts
and its capabilities have been e ~ p a n d e d . ~To . ~date, our efforts have been directed primarily a t representative salts which have been most thoroughly studied at higher temperatures. We have also begun a more detailed examination of potential isopiestic reference solutions, especially at water activities below those accessible to NaCl(aq).’ Given the objectives cited above, most of our studies so far have involved only inert-gas-type ions so that the observed effects were due to first-order changes such as charge type and size. The improvements in reliability and sensitivity of the system have prompted the current project which was designed to test the feasibility of reproducibly determining more subtle differences such as the specific-ion effects of electronic structure changes. We begin in this paper with a study of the perchlorates (chosen to minimize counterion effects)h of the divalent ions of four first-row transition metals (Mn, Co, Ni, Cu) and zinc. These salts are stoichiometrically identical, and the cations are virtually identical in size and differ systematically in ground-state electronic structure (and hence in some properties of their aquo complexes).’ Previous isopiestic studies on these salts are the 25 OC data given by Robinson and Stokes* for Zn(CIO,), and similar data on the other four salts by Libus and Sadowska.’ (A Russian study’ of Zn(C10,)2 at 50 O C appears to be inconsistent with the rootntemperature data.) These and other data have been compiled and evaluated by Goldberg et al.’+’? The conclusion drawn by Libus and Sadowska was that “the osmotic coefficient of any specified concentration is the same, within experimental error, for Mn-
( I ) Humphries, W . T.; Kohrt, C. F.; Patterson, C. S. J . Chem. Eng. Data 1968, 13. 327.
(2) Moore. J. T.: Humphries, W . T.: Patterson. C. S. J . Chem. Eng. Data 1972. 17. 180. (3) Davis, T. M.; Duckett, L. M.; Owen. J. F.; Patterson. C. S.:Saleeby, R. J . Chem. Eng. Data 1985, 30, 432. (4) Davis, T . M.; Duckett, L. M.; Garvey, C. E.; Hollifield, J. M.: Patterson, C . s. J . Chem. Enp. Data 1986. 31, 54. ~
~1~
1
~~~
(5) Pitzer. K. S.: Peipei. J. C.; Busey. R . H. J . Phys. Chem. R e f Datu
(Clod),, CO(CIO,)~,Ni(CIO,), and Zn(CIO,), in the whole concentration range investigated”. (Their concentration range was 0.1-3.0 m.) The same group later ran mixturesI3 of these salts as a test of the limits of validity of their earlier conclusion. In these same papers the C$ of Cu(CIO,), was reported to be marginally but measurably lower than those of the other four. Experimental Section
Reagents. Reagent grade NaCl was dried overnight at 110 OC and stored over CaSO,. Samples of the dried salt were weighed directly into the equilibration cups for runs at the higher concentrations. For solutions below 1 m , a stock solution was prepared quantitatively by weight, and samples of this solution were then transferred into the equilibrium cups from a weight buret. Although no purification of NaCl was carried out except for drying, we did use reagents from three different suppliers, and replicate stock solutions prepared with these different reagents gave consistent results. The metal perchlorates were obtained from G. Frederick Smith Chemical Co. Stock solutions of each salt were prepared and standardized by analysis for the metal. Copper was determined by electrogravimetry according to the procedure given by Vogel.I4 Kickel, cobalt, and zinc were also determined by electrodeposition as the metals following procedures given by Erdey.” No electrogravimetric method exists for manganese so the Mn(C10,), solutions were analyzed by titration of Mn(I1) ion with standardized permanganate in neutral pyrophosphate solution. The end point was detected potentiometrically as described by Vogel. I h The electrogravimetric procedures consistently gave results comparable in precision t o that of our isopiestic data (50.1%), but results of the manganese titration were persistently poorer by almost a factor of 2, making this the weakest link in the manganese results. Since these stock solutions were not analyzed for perchlorate, we have no direct analytical confirmation of the nominal stoi-
1984, 13, 1
(6) Holmes, H. F.; Mesmer, R. E. J . Chem. Thermodyn. 1981, 13. 1035. ( 7 ) Libus, 2.; Sadowska, T. J . Phys. Chem 1969, 73, 3229. (8) Robinson, R . A.; Stokes, R. H. “Electrolyte Solutions”. 2nd ed.; Butterworths: London, 1959. (9) Lilich, L. S.:Chernykh. L. V.: Shelyapina, T . D. Zh. Fiz.Khim. 1975, 49. 1318. (10) Goldberg, R. U.;Nuttall, R. L.; Staples, B. R. J . Phjs. Chm7. Re/: Data 1975, 8, 923. (11) Goldberg. R. N. J . Phys. Chem. Ref. Data 1979. 8 , 1005 (12) Goldberg. R. N J . Phys. C h e m Re$ Data 1981, 10, I .
(13) Libus, 2 . : Sadowska, T. J . Phys. Chem. 1970, 74, 3674. (14) Bassett, J.; Denny, R. C.; Jeffrey, G. H.; Mendham, J. “Vogel’s Textbook of Quantitative Inorganic Analysis”, 4th ed.; Academic Press: New York. 1978: p 532. (15) Erdeq, L. I n “International Series of Monographs in Analytical Chemistry”; Pergamon Press: New York, 1965; pp 390, 406, 456. (16) Bassett, J.; Denny, R. C.; Jeffrey, G. H.; Mendham, J. “Vogel’s Textbook of Quantitative Inorganic Analysis”, 4th ed.: Academic Press: New York. 1978: p 606.
Transition-Metal Perchlorates
The Journal of Physical Chemistry, Vol. 90, No. 8, 1986 1657
L R L
101
100
0 99
1-
__
- 0 -
x--
- - .-- . 2
I - -
LI
- - - - - _-.
-~
a
.---
2.2
0 98-
0 97
2.0
O
' " I O
I 5
m
2 0
2 5
Figure 1. Isopiestic ratios at 50 O C of four transition-metal perchlorates to zinc perchlorate vs. molality: 0 , Mn(CIO,),; A, Ni(CIO,),; 0, Co(CIO,),; 0,C U ( C I O ~ )R~ .for Zn(ClO,), is unity, by definition.
chiometry of the reagents. Therefore, we recrystallized portions of three of the salts from hot water and prepared and analyzed new stock solutions. In no case was a discernible difference found between the isopiestic ratios by using these solutions and those using the previously prepared stock solutions. Freshly boiled deionized water was used in preparing all solutions and for dilutions to run concentrations. Apparatus and Procedures. The apparatus design and operation are described by Humphries et al.' and Moore et al.,z and recent improvements are described by Davis et aL3
Results A total of 17 successful isopiestic equilibrations were carried out at three temperatures (3 at 30 OC, 7 at 40 OC, and 7 at 50 "C). In each experiment triplicate samples of a NaCl solution (isopiestic reference) were equilibrated simultaneously with triplicate samples of each of the five metal perchlorate solutions. Results, in the form of average isopiestic molalities, are compiled in Table I. All averages are for triplicates except those marked with an asterisk; in these cases one sample was lost or had to be discarded so that the average is that of the remaining pair. (Inexplicably, a disproportionately large number of sample casualities occurred with Zn(ClO,),.) Precision of the data is indicated by the run-average relative standard deviations or coefficients of variation (cv) shown in the last column of the table. The overall average cv, indicating reproducibility of the isopiestic molalities, is 0.05,%. For convenience in recovering values at any given molality and as an interpolation tool for smoothing the data, we have fit the for the 40 and 50 OC runs isopiestic ratios ( R 2mYac1/3msalt) to simple polynomials of the form Y = Cla,xlwhere Y is R ( m ) and X is either m or m l / , for the best fits." Inclusion of a Debye-Huckel like term to accommodate some of the curvature does not improve these fits as had been found in some previous cases3 Osmotic coefficients calculated from these ratios and Pitzer's5 NaCl data as reference were fitted to the same form. Since the equations used are purely empirical fits to the data with no model-dependent term, the coefficients have no theoretical significance in themselves. Therefore, given the short concentration range and small number of data points defining each curve, we have not reported the best-fit coefficients here. Discussion Examination of the data in Table I, or plots of the isopiestic ratios which may be calculated from them, shows that the osmotic properties of this series of salts are indeed very similar. The variations among the salts are so small relative to variations with m that we have resorted to a normalized display of the data in Figure 1 in order to accentuate their differences. The values (17)
Rard, J. A.; Miller, D.G.J . Chem. Eng. Data
1982, 27, 169
I .8
I .6
.,
Figure 2. Osmotic coefficients for three transition-metal perchlorates at two temperatures: A, Ni(C10,J2 at 40 O C ; A, Ni(CIO,), at 50 O C ; 0 , CO(CIO,)~at 40 O C ; 0, Co(CI0,) at 50 O C ; CU(CIO,)~ at 40 O C ; 0, Cu(ClO,), at 50 O C . Points are rounded values from Table 111. TABLE 11: Comparison of Our 25 OC Extrapolated Values with Literature Data
ma Mn(CIO,),
CO(ClO,), Ni(ClO,), Cu(CIO,), Zn(CIO,),
1.3139 2.2128 2.3208 1.3339 2.2337 2.3504 1.3231 2.2071 2.3145 1.3492 2.2721 2.3897 1.3110 2.1871 2.2987
@e$
1.557 2.165 2.286 1.535 2.152 2.253 1.546 2.173 2.292 1.514 2.114 2.209 1.559 2.203 2.312
41,: 1.544 2.161 2.241 1.546 2.175 2.263 1.531 2.151 2.232 1.510 2.106 2.186 1.519 2.130 2.213
A,d
%
+0.8,
+0.1, +2.0 -0.7, -1.1 -0.4, +0.9, +1.0 +2.6 +0.2, +0.3, +1.0 +2.6 +3.4 +4.4
Experimental molalities for 30 O C equilibrations. bOsmotic coefficients extrapolated to 25 OC from experimental data at 30, 40, and 50 O C . COsmoticcoefficients from ref 10-12 interpolated to our experimental molalities. A = @ex plotted are isopiestic ratios to Zn(C104)2,RZn= mz,/m,lt. (Recall that v I = u2 for salts of the same valence type.) By virtue of the equilibrium condition vlml4l = v2m242
RZnvalues are also the ratios of the osmotic coefficients of the respective salts relative to that of Zn(C104),, Le., RZn= These ratios all fall within -4% of unity and hence may be displayed over the entire experimental range on a highly expanded scale as in Figure 1. The following are noteworthy features of these results: (a) The curves, except that for Mn(C104)2,are clearly distinct with no
1658 The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
Owen et al.
TABLE 111: Smoothed Osmotic Coefficients (d)
T = 40 1.1
1.2 1.3 1.4 I .5 1.6 1.7 1.8 I .9 2.0 2.1 2.2 2.3 2.4 2.5 2.6
(1.41 2,) 1.471, 1.530, 1.591, 1.653, 1.717, 1.783, 1.850, 1.919, 1.990, 2.062, 2.136, 2.212, 2.290, ( 2.3 695)
(1.396,) 1.4548
1.555, 1.616, 1.680, 1 ,745, 1.812, 1.880, 1.950, 2.022, 2.096, 2.171, 2.247, 2.325, I:2.406J
1.577, 1.641, 1.708, 1.776, 1.846, 1.9183 1.992, 2.068, 2.147, 2.227, 2.310, 2.3950 (2.480,)
1.515,
T = 50 0.7
0.8 0.9
1 .o
1.1 1.2 1.3 1.4 1.5 I .6 1.7 1.8 1.9 2.0
2.1 2.2 2.3 2.4 2.5 2.6
(1.230,)
1.285, 1.3420 1.399, 1.457, 1.516, 1.577, I ,638, 1.7018 1.7660 1.831, 1.898, 1.966, 2.036, 2.107, 2.179, 2.253, 2.3289 (2.4038)
OC
1 .3806 1.4370 1.495,
( I . 140,) 1.199, 1.256, 1.313, 1.371, 1.428, 1.486, 1.545, 1.604, 1.6650 1.726, 1.789; 1.854, 1.921, 1.990, 2.061, 2.136, 2.2 14, 2.295, (2.377,)
crossovers, typical separations being several times the precision of the measurements. This result is in contrast to that of ref 7 and 13 which reported the 25 O C data for all except copper perchlorate to be indistinguishable. (b) The curve for CU(CIO,)~ is indeed the lowest of the group, consistent with the previous report.' (c) The curve for MII(CIO,)~,whose course is indicated by the dashed line, runs counter to the behavior reported by Libus and also seems anomalous in comparison to the rest of our data. Although we encountered greater difficulty with standardization of the stock solutions of Mn(CIO,),, the results of the isopiestic equilibrations were of comparable precision to those for the other salts even when new stock solutions prepared from recrystallized reagent were used. The unique position, and especially the slope, of the Mn(C10,)2 curve is therefore surprising, and the Mn(CIO,), data must be considered tentative pending further investigation. Figure 2 shows a typical plot illustrating the sensitivity of the data to variation in T. Obviously, the effect of a 10 O C temperature change is barely measurable at most molalities and is comparable to the differences between the salts. We have extrapolated our data (for the three molalities measured at 30 "C) to 25 O C and compared the results with the literature 25 "C values in Table 11. An obvious feature of these results is that, in spite of the additional uncertainty produced by the extrapolation, the values for the various salts remain clearly distinguishable at 25 OC. While our extrapolations agree at some points within the uncertainty of the literature data, there are significant discrepancies. Surprisingly, the largest differences are with what we assumed were
(1.365,) 1.419, 1.4746 1.531, 1.5896 1.649, 1.710, 1.773, 1.8372 1.9030 1.970, 2.039, 2.1 10, 2.182, 2.256, (2.331 ,)
( I ,4140) 1.474, 1 ..536, 1.600, 1.665, 1.7321 1.801, 1.871, 1 .9435 2.017, 2.0928 2.170, 2.249, 2.330, 2.41 3, (2.4968)
(1.198,) 1.2500 1.3023 1.3559 1.410, 1.466, 1.523, 1.581, 1.641, 1.701, 1.763, 1 .8258 1.889, 1.953, 2.019,
(1.228,) 1 ,286, 1.344, 1.402, 1.4627 1.523, 1.585, 1.649, 1.714, 1.780, 1.848, 1.918, 1.990, 2.064, 2.140, 2.218, 2.298, (2.380,)
oc ( I ,2130) 1.269, 1.328, 1.386, 1.444, 1.504, l.5660 1.6289 1 ,693, 1.759, 1.827, 1.897, 1.969, 2.042, 2.1 17, 2.195, 2.274, 2.354,
2.085, 2.152, 2.220, (2.2 8 9,)
the more firmly established Zn(C10,), data. These discrepancies cannot be extrapolation error since our Zn(CIO,), values at 50 O C are already higher than the corresponding literature 25 O C values, and 6's for all five of these salts increase as T decreases. Given the importance of 25 "C as a reference temperature, more precise measurements at 25 "C are clearly in order. The data were smoothed by two p r o c e d ~ r e s ,one ~ . ~using R ( m ) and the other 6 ( m ) ,in order to assess the error introduced by the polynomial ripple inherent in the fitting equations. The smoothed values reported in Table I11 are the average of the two sets which, with the exception of the highest and lowest points, differed by less than 0.1,%. This smoothing error, combined with the average cv of the experimental data, suggests that the results in Table III should be reliable to approximately f0.2%, barring the presence of undetected systematic error. These data extend to the limit that can be reached by using NaCl as isopiestic reference. We have just completed a study of some prospective reference data for lower water activities and are therefore extending the concentration range of these studies at 50 OC using LiCl as reference. The results of that investigation and similar studies at higher temperatures will be published shortly.
Acknowledgment. We thank Lisa Duckett for assistance with the computer analyses and in preparation of the manuscript. This work was supported in part by a grant from Research Corporation. Registry No. Mn(C10,)2, 13770-16-6; C O ( C I O ~ )13455-31-7; ~, Ni(CIO,),, 13637-71-3; CU(CIO,)~,13770-18-8; Zn(C104)2,13637-61-1.
