I. J. LINAND A. METZER
3000
Effect of Dissolved Paraffinic Gases on the Surface Tension and Critical Micelle Concentration (cmc) of Aqueous Solutions of Dodecylamine Hydrochloride (DACl) by I. J. Lin Technion, Israel Institute of Technology, Haifa, rsrael
and A. Metzer* Israel Mining Industries, Institute for Research and Development, Haifa, Israel
(Received December 89, 1970)
Publication costs borne completely b y The Journal of Physical Chemistry
The solubilities of ethane and propane in aqueous solutions of DACl have been measured at 25' and atmospheric pressure. Their solubility reaches a minimum for a concentration of DAC1 of 2 x mol/l. For air, the lowest sohbility was determined at a concentration of 5 X mol/]. DACl. The decrease in surface tensions and in the cmc of solutions of DACl saturated with ethane and propane have been determined by two independent methods as equivalent to a lengthening of the hydrocarbon chain of the DACl of approximately 0.35-0.46 unit of -CH2- for ethane and 0.72-0.80 unit of -CHT for propane.
Introduction The solubilities in water of the homologous series of paraffinic gases from methane to butane have been reported in the literature, and a number of hypotheses on the mechanism of solubilization have been advanced, but practically no data are available on the surface and bulk properties of these solutions. lloreover, although there are many publications on the solubilization by surface-active agents, nothing has been reported on the effect of these agents on the solubility of paraffinic gases in water. The effect of some paraffinic gases on the froth flotation of quartz with dodecylamine hydrochloride (DAC1) was studied by Gratch,l who showed that these gases greatly enhanced the floatability as a result of increased contact angles as compared with those obtained when using air as the flotation gas. As a first step of a more detailed study, the effect of ethane and propane on the surface tension and cmc of solutions of DACl was investigated. I n addition, the sohbilities of these gases in aqueous solutions of DACl as a function of concentration were determined. The solubility of inert gases in water is closely dependent on the structure of water. Ben Kaim and Baer2have shown that the solubility of argon in waterethanol mixtures shows a minimum at about 0.10 mol fraction ethanol. This anomaly was attributed to the influence of alcohol on the structure of water, a small amount of alcohol probably increasing the degree of crystallinity of water. Likewise, ionic salts decrease the solubility of gases in water by a partial disruption of the structure of water similar to an increase in temperature, which also decreases the gas solubility. This The Journal of Physical Chemistry, Vol. 76, N o . 19?1971
effect can also be explained by the fact that part of the water molecules become associated t o the ions in the solvated layer, being unavailable for dissolving gas. Frank and Evans3 proposed a theory of solubility of inert gases in water, which assumes that the introduction of the gas causes the ordering of the water molecules to a state having lower energy and entropy (iceberg). They assumed that the water structure contains vacancies or holes, which are filled with the gas molecules. The solubilities of the homolog series of normal paraffinic gases in water as determined by various authors are given in Table I.4-7
Experimental Section and Results Solubilities. The solubilities of ethane, propane, and air in aqueous solutions of DACl were determined with the apparatus developed by Ben Xaim and Baer.2 The method is quite simple, rapid, and accurate to 0.2%. The solutions of DACl were prepared with bidistilled, deaerated water and introduced into the apparatus under vacuum. All the determinations were carried out at 2 5 " . The results are shown in Figure 1 as cubic centimeters of gas absorbed in 1000 em3 of solution at 750 mm and 25 h 0.1'. (1) E. Gratch, M.Sc. Thesis, Israel Institute of Technology, Haifa, Israel, 1966. (2) A. Ben Naim and S. Baer, Trans. Faraday SOC.,59, 2735 (1963). (3) H. S. Frank and M. M .Evans, J . Chem. Phys., 13, 509 (1954). (4) C . McAuliffe, J . Amer. Chem. SOC.,86, 275 (1964). (5) W. Y. Wen and J. H. Hung, J . Phys. Chem., 74, 170 (1970). (6) W. F. Claussen and M. F. Polglase, J . Amer. Chem. Sac., 74, 4817 (1952). (7) T . J. Morrison and F. Billett, J . Chem. Sac., 3819 (1952).
EFFECT OF DISSOLVED PARAFFINIC GASESON SURFACE TENSION OF DACl 70
Table I : Solubilities of Paraffinic Gases in Water
Gas
CH, CzHe CBH~ n-CdHlo Q
Molar vol at
Mol wt
ZOO,
16 30 44 58
Reference 4.
ml
39a 55" 88.1" 100.4' b
Solubility, ml of gas/1000 g of water, at 2 5 O , 760 mm
31.3jb 41.20b 32.31b 26.34'
Reference 5.
31.8OC 43.0OC 32.7lC 23.40' (30')
Reference 6.
3001
30.14d 41.18d 33,30d 25.88d
-
60
I
5 50
u
s
40
Reference 7. 30 .. 10-5
10'6
10-4
lo-'
C,,,, , mol x lit - 1
(for 0 ) 44.0 43,6
Figure 2. Surface tensions of DACl saturated with air, ethane, and propane at 25'.
43.2 42.8 42.4
7
.-
42,O
35.6
41.6
35.2
41.2
34.8
40.8
34.4
40.4
34.0
40.0
33.6
c
"E u
l
e
X
m
33.2
>"
32.8
j
32.4 (for
32.0
*I
17.6
31.6
17.4
17.2 17.0 16.8 IO-#
10-7
10-8
C,,
10-8
lo-'
IO"
, moixtit"
Figure 1. Solubility of gas in cubic centimeters per liter of DACl solution at 25'.
Surface Tensions. Measurements were performed by the ring method with a De Nouy tensiometer with an accuracy of &0.1 dyn/cm. The DACl solutions were saturated with the respective gas and maintained in a thermostatic bath at 25" under an atmosphere of circulating gas. The results are shorn in Figure 2. Critical Micelle Concentration. Lawrence8 determined the cmc of dodecylamine hydrochloride by measurements of pH values vs. Concentration and confirmed the results of this method with conductivity measurements. He found that the cmc corresponded to a solution with 0.275 g of DACl per 100 g of solution (1.25 X 10+ mol/l.) at 25". This value is in quite good agreement with the previous determinations of Hoyer and Greenfie1djgwho found 1.38 X mol/l., Corrin mol/l., and Ralston and and Harkins,'" 1.31 X Hoerr,ll 1.3 X mol/l. (All these values are at 25".) The method used in this work was that of Lawrence,8
based on the change of pH with concentration of solutions of DL4C1. Solutions of high-purity DACl in distilled, degassed water were prepared, and the pH's were measured after saturation with air, ethane, or propane. The results are given in Figure 3. The points of inflection of the curves indicate the cmc: for air, 1.3 X lo+; for ethane, 1.0 X lo+; for propane, 7.5 X
Discussion The results show that the bulk as well as the surface properties of solutions of DACl are strongly affected by the presence of paraffinic gases. With reference to Figure 1, the solubilities of ethane and propane decrease up to a concentration of DACl of 2 X M and that of air decreases up to 5 X lo-" M DAC1. These decreases in solubility are followed by a n increase for higher concentrations of DAC1. A simple interpretation of this phenomenon is that for very dilute solutions of DACl this compound acts as an electrolyte, salting out the gas from solution. For higher concentrations the electrolyte character of DACl is balanced by the effect of the hydrocarbon chain, to which the gas molecules become attached by van der Waals forces, and therefore the solubility curve passes through a minimum. For still higher concentrations of DACl, but below the cmc, the heteropolar ions may associate progressively to form dimers, as suggested by Mukerjee,I2 on which the gas may become absorbed. It is interesting to note that the concentration of DACl at which the gas solubility is a minimum is lower for the paraffinic gases than for air. This should be connected with the lowering of the cmc of DACl in the presence of the gases, which will be discussed later. (8) A. S. C.Lawrence, et al., Proc. Int. Congr. Surface Activ., h d , 1, 385 (1957). (9) H . W.Hoyer and A. Greenfield, J. Phys. Chem., 61, 818 (1957). (10) M. L . Corrin and W. C. Harkins, J . Amer. Chem. Sac., 69, 638 (1947). (11) A.W.Ralston and C. W. Hoerr, ibid.,64,772 (1942). (12) P.Mukerjee, Advan. Colloid Interface Sci., 1, 241 (1967). The Journal of Physical Chemistry, Vol. 76, No. 19, 1971
I. J. LIN AND A. METZER
3002
5.6
5.4
5.2
5.0
I
n
4‘8
4.6
4.4
4.2 10-3
Concl
, mol x l i t - ’
Figure 3. Cmc of solutions of DACl saturated with air, ethane, and propane at 25’.
With reference to Figure 2 the paraffinic gases appear to have a definite effect on the surface tension of DACl solutions. Since ethane and propane dissolved in pure water do not change appreciably the surface tension, it must be concluded that they do not concentrate at the surface. This is also true for very dilute solutions of DACl (lob6mol/l.), for which it has been shown that the gas solubility reaches a minimum. However, the effect of ethane and propane begins to show a t concentrations of mol/l. DAC1, for which a small decrease in surface tension is noted. For still higher concentrations of DACl the decrease in surface tension reaches more than 10 dyn/cm for propane and about 7 dyn/cm for ethane as compared with air. This behavior of the solution in the presence of the paraffinic gases indicates that these are adsorbed on the surface in the presence of the DAC1. Surface adsorption is represented by the Gibbs equation
where I’is the surface excess in mo1/cm2, y is the surface tension in dyn/cm, and C D A C ~is the molar concentration, which in dilute solution can be used instead of the more rigorous activity. The concentration of gas is considered constant, since the relative change in gas solubility is less than 10% in the range of concentration of DACl studied here. Equation 1 states that the slope of the surface tension y vs. In c is a measure of the surface adsorption of the solute. Referring t o the linear part of the curves of Figure 2, The Journal of Physical Chemistry, Vol. 76, N o . 10,1971
it can be seen that the maximum surface excess (calculated as DAC1) decreases from air to propane, since the absolute value of the slope of the lines decreases in that order. Therefore, the effect of the gases is manifested as an absolute decrease in the magnitude of the surface tension-evidence of their adsorption on the surfaceand also in decreasing the adsorption of DAC1. Calculations based on eq l and results from Figure 2 (for the linear part of the curves) give the following values of adsorption of DACl on the surface: for air, 4.7 X 10-lO mol/cm2; for ethane, 4.0 X 10-lo mol/cm2; for propane, 3.5 X 10-lo mol/cm2. For the purpose of this discussion it has been assumed that only the amine ion or the undissociated amine salt (or hydroxide) is adsorbed on the surface layer. A rigorous treatment of the Gibbs equation would require the introduction of a coefficient 2 in the denominator of eq 1 to take care of the fact that the DACl is an electrolyte which dissociates into DA+ and C1- ions.l3,l4 However, the question of whether both cation and anion contribute to the lowering of the surface tension has been the subject of considerable controversy. Substantial experimental evidence available15 indicates that eq 1 as it stands is applicable to dilute solutions of ionic surface-active reagents. This is also supported by the fact that the area per adsorbed molecule in the presence of ajr calculated from the value of rmax given above is 35.4 A2 per molecule, which is what might be expected, instead of double this value if the coefficient 2 is introduced.16 The contribution of the adsorbed gas to the decrease in surface tension can be calculated in terms of equivalent lengthening of the hydrocarbon chain of DACl. For homologous series of aliphatic substances in water, Traube observed that the surface activity increased by a factor of 3 for each additional methylene group in the alkyl chain. This relationship is known as Traube’s rule. According to Langmuir’s interpretation of this rule,16the work W to transfer 1 mol of solute from the bulk to the surface is
W
=
RT In (cs/c)
=
RT In (F/rc)
(2)
where cs = r / r is the surface concentration, r is the surface excess in mol/cm2, and r is the thickness of the surface region. For a difference in chain length of one -CH2- group, the difference in work is (3)
(13) P. L. de Bruyn and G. E. Agar, “Froth Flotation,” 50th Anniversary Vol., D. W. Fuerstenau, E d . , AIME, 1962, pp 91-138. (14) K. Durham, “Surface Activity and Detergency,” Macmillan, London, 1961. (15) J. K. Dixon, C. M. Judson, and D. J. Salley, “,Monomolecular Layers,’’ American Association for the Advancement of Science, Washington, D. C., 1954, p 63. (16) A. W. Adamson, “The Physical Chemistry of Surfaces,” 2nd ed, Wiley, New York, N. Y., 1967.
EFFECT OF DISSOLVED PARAFFINIC GASESON SURFACE TENSION OF DACl
3003
Table 11" 7,
dyn
x
---r
e,, mol 1.-'
1010, mol/cml-
om-'
Air
Ethane
Propane
45 50 55
4.7
4.0
3.5
---
'--rme*/rncm--
e,, mol el-
Ethane
Air
x 10-4 6 . 2 x 10-4 2 . 0 x 10-4 9.5
5 . 5 x 10-4 3 . 3 x 10-4 1.3 x 10-4
Propane
Ethane
x 10-4 2 . 2 x 10-4 1.3 x 10-4
1.5
1.9
1.6 1.7
2.1 2.3 2.1
3.8
Propane
Av 1.6 a From these values the effect of the two paraffinic gases in terms of - C H r units can be calculated: for ethane, m In 2.8 = 0.46; for propane, m - n = In 2.1/ln 2.8 = 0.72.
1
where n is the number of -CH2- groups in the hydrocarbon chain. By Traube's rule, for values of ?% = ~ ~ - 1C,-I/C, , = 3, and if we assume r n / P n - 1 as approximately constant, then, at 25"
W , - W,-I
=
RT In 3
= 640 cal/mol
1
I
I
I
-
n = In 1.6/
I
I
'
I
A
~dton
0
This work (CMC from flg.3)
I2
13
a' H o w l
I
(4)
This 640 cal/mol should be regarded as the work necessary to bring one -CH2- group from the bulk to the surface. Fuerstenau" determined experimentally this value as l.OkT, which is equivalent to 600 cal/ mol, very close to the values derived from Traube's rule. If this value is introduced in eq 4
W,
- W,-l
=
RT In 2.76
(5)
Indicating with subscript m the apparent hydrocarbon chain length of DACl in the presence of the paraffinic gases (n = 12), we have
W,
- W , = RT In x
(6)
and eq 5. From eq 2 and 3 we obtain, on the assurnption that r is constant 10
m-n=
W m - Wn 600 - In($) In 2.76
I1
I4
IS
I6
I7
I8
Hydrocarbon chain length (7)
where rmand c, are the surface excess and the concentration of DAC1, respectively, in the presence of paraffinic gases. Calculation of m - n for the system studied can be only approximate, since it varies with the concentration of DACl. Therefore, three values of the concentration of DACl were determined from Figure 2 for three values of the surface tension in the linear region of the curves, and an average value for the last term of eq 7 was calculated. The results are given in Table 11. I n other words, the effect of ethane on surface tension in presence of DACl is equivalent to a lengthening of the hydrocarbon chain of 0.46 unit of -CH2- and for propane 0.72 of -CH2-. Expressed in another form, it appears as if approximately 1 mol of gas is adsorbed on the surface for every 4 mol of DAC1. This apparent lengthening of the hydrocarbon chain of DACl in presence of ethane and propane also results
Figure 4. Cmc us. hydrocarbon chain length of homologous amine hydrochlorides a t 25".
in a corresponding decrease of the critical micelle concentration, as shown in Figure 4. It has been shownI8 that for each homologous series of normal hydrocarbon chain surface-active reagents the value of the cmc is doubled for each decrease in a -CH2- group. For longchain amine salts the following relation was p r o p ~ s e d ' ~ log cmc = A
- Bn
(8) where n is the number of -CH2- groups in the alkyl chain and A and B are constants. The results of the cmc of (17) D. W. Fuerstenau, T. W. Healy, and P. Somasundaran, Trans. A I M E , 229, 321 (1964). (18) A. B. Scott and H. V . Tartar, J. Amer. Chem. Soc., 6 5 , 692 (1943). (19) L . I. Osipow, "Surface Chemistry," Chapman and Hall, London, 1963, p 473.
The Journal of Physical Chemistry, Vol. 76,No. 19,1971
M. C.V. SAUERAND JOHN 0. EDWARDS
3004 the homologous series of straight-chain amine hydrochlorides, as determined by conductivity measurements by Ralston and Hoerr,l' are plotted in Figure 4, and a straight-line relationship, in accordance with eq 8, was obtained log cmc = 1.252
- 0.265n
(9)
If the values obtained for the cmc of dodecylamine hydrochloride from Figure 3 are introduced in eq 9,
the values of n calculated for the paraffinic gases result: ethane, n = 12.35; propane, n = 12.80. I.e., this is 0.35 unit of -CH2- for ethane and 0.80 for propane, in close agreement with the values calculated previously from surface tension considerations.
Acknowledgment. This work is part of I. J. Lin's D. Sc. (Tech) Thesis to the Technion, Israel Institute of Technology, Haifa.
The Reactions of Acetone and Hydrogen Peroxide. I.
The Primary Adduct1
by M. C. V. Sauer and John 0. Edwards* Metcalf Chemical Laboratories, Brown University, Providence, Rhode Island
03912
(Received M a y 6 , 1971)
Publication costs assisted b y the U.S. Air Force Ofice of Scientific Research
The reaction between acetone and hydrogen peroxide leads to 2-hydroxy-2-hydroperoxypropane as a first product. The equilibrium constant of formation of this compound was found to be K = 0.086 M-1 a t 25". Thermodynamic parameters of the reaction are AH = - 7.0 kea1 mol-' and AS = - 28 cal mol-' deg-1. The kinetics of formation and dissociation of 2-hydroxy-2-hydroperoxypropane were studied by both uv spectroscopy and nmr line-broadening techniques. The reaction exhibits general acid and base catalysis. Values of the Br$nsted parameters oc and p were obtained. The variation of the base-catalyzed rate constants koH with temperature were studied by the nmr line-broadening technique. Values of activation enthalpy (1.7 kea1 mol-1) and entropy of activation (- 18 cal mol-' deg-1) were obtained for the forward reaction; correspondiiig values for the reverse reaction were found to be 8.7 kcal mol-' and 10 cal mol-' deg-'.
+
Introduction The reaction between acetone and hydrogen peroxide in aqueous solution was studied by several investigators and different products were isolated and identifieda2 However, the product that results from the addition of 1 mol of hydrogen peroxide to acetone, that is 2-hydroxy-2-hydroperoxypropane (compound I), was not detected until recently. lgB This compound has, how-
0
II
CH,-C-CH,
OOH
I + HZOz J7CH,-C-CHs kf
kr
I
(1)
OH
I ever, been isolated from the photosensitized oxidation of isopropyl d c ~ h o l . ~ We have found considerable evidence for the existence of this compound in solutions of acetone and hydrogen peroxide. Studies of the thermodynamics and kinetics of its formation determine the experimental conditions under which detection and isolation of this compound The Journal of Physical Chemistry, Val. 76, No. 19, 1971
may be possible. The quantitative solution data are presented in this article.
Experimental Section Materials. Reagent grade acetone was used without further purification. Hydrogen peroxide [50% (w/w) and 90% (w/m)donated by the FMC Corp. and 30% (w/w) from Allied Chemical] was used in making peroxide solutions. Reagent grade inorganic salts were used as obtained. Organic acids and their salts were either distilled or recrystallized before use. Analytical. The total peroxide was determined. A known volume of dilute sample was added to 10 ml of water, 1 ml of sulfuric acid (1/1),and 2 g of KI. I n the analysis of acetone-hydrogen peroxide mixtures, acetic acid was used instead of water. Ammonium molybdate was added as a catalyst. The solution was allowed to (1) Abstracted from part of the Ph.D. Thesis of M. C. V. Sauer at Brown University, June 1970. (2) A. Rieche, Angew. Chem., Int. Ed. E d . , 70, 251 (1958); (b) N. A. Milas and A. Golubovic, J . Amer. Chem. Soe., 81, 6461 (1959). (3) J. Hine and R. W. Redding, J . Org. Chem., 35,2769 (1970). (4) G. 0. Schenk and €1. D. Becker, Angew. Chem., Int. Ed. Enol., 70, 504 (1958).
