Langmuir 1994,10,2950-2953
2950
Surface Adsorption and Micelle Formation of Sodium Chloride-Dodecylammonium Chloride Mixtures Michio Yamanaka,*lt Hitoshi Matsuki,* Norihiro Ikeda, Makoto Aratono, and Kinsi Motomura Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashi-ku, Fukuoka 812, J a p a n Received January 25, 1994. I n Final Form: June 1, 1994@ The surface tension of an aqueous solution of sodium chloride (NaC1) and dodecylammonium chloride (DAC) mixture was measured as a function of the total molality and composition of mixture at constant temperature under atmospheric pressure. The surface densities of the solutes and the DAC compositions of the adsorbed film and micelle were then calculated. The miscibility of NaCl and DAC in the adsorbed film and micelle was discussed by drawing the composition diagrams of adsorption (CDA) and micelle formation (CDM). Because ofthe negativevalue of the surface density of sodium ion due to the electrostatic repulsive interaction with the adsorbed dodecylammonium ions, the surface DAC composition became larger than 1. In a similar way, the micelles also had a DAC composition value larger than 1, probably because of the negative value of the number of sodium ions in the micelles. Furthermore, at the critical micelle concentration, the shapes of the CDM and CDA were similar. It was finally shown that the miscibility ofNaCl and DAC in the micelle is similar to that in the adsorbed film and that our thermodynamic approach to elucidate the miscibility in adsorbed films and in micelles by use of the CDA and CDM is also applicable to mixed systems consisting of an inorganic salt and an ionic surfactant.
Introduction Self assemblies such as surface adsorbed films and micelles of ionic surfactants in aqueous inorganic salt solutions are of great theoretical and industrial interest, since ionic surfactants are usually used with inorganic salts in application. The addition of a n inorganic salt to a n aqueous ionic surfactant solution affects remarkably the adsorption and micellization behavior ofthe surfactant and the phenomena have been explained in terms of the so-called salt effect.'-* Surface tension of the aqueous solutions of such systems provides useful thermodynamic information about the adsorbed films and micelles. Because such systems have four degrees of freedom, two independent concentration variables in addition to temperature and pressure can be chosen a s the experimental variables. Ikeda et al. developed the Gibbs adsorption isotherm for plane surfaces of dilute aqueous solutions of such systems by using the concentrations of each solute as the independent thermodynamic variable^.^^^ They measured the surface tension of an aqueous solution of dodecyldimethylammonium halide and sodium halide as a function of the concentrations and evaluated the adsorption amounts of surfactant ion, counterion, and ~o-ion.~-"
* To whom correspondence should be addressed. Present address: College of General Education, Kyushu University 01,Ropponmatsu, Chuo-ku, Fukuoka 810,Japan. Present address: Department of Biological Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770, Japan. Abstract published inAdvanceACSAbstracts,August 1,1994. (1)Corrin, M. L.;Harkins, W. D. J . Am. Chem. Soc. 1947,69, 683. (2) Lange, H. Kolloid-2. 1961, 121, 66. (3) Rodakievicz-Nowak,J. J . Colloid Interface Sci. 1983, 91, 368. (4)Rosen, M. J.; Dahanayake, M.; Cohen, A. W. Colloids Surf. 1982, 5, 159. ( 5 ) Ikeda, S. Bull. Chem. Soc. Jpn. 1977, 50, 1403. (6)Ikeda, S.;Okuda, H. J. Colloid Interface Sci. 1988, 121, 440. (7) Ozeki, S.;Tsunoda, M.; Ikeda, S. J . Colloid Interface Sci. 1978, 64, 28. (8)Ozeki, S.;Ikeda, S. Bull. Chem. Soc. Jpn. 1980, 53, 1832. (9)Okuda, H.; Ozeki, S.; Ikeda, S. Bull. Chem. SOC.Jpn. 1984, 57, 1321. (10)Okuda, H.; Ozeki, S.; Ikeda, S. J. Colloid Interface Sci. 1987, 115, 155.
*
@
However, their equations to calculate the surface excesses of ions are somewhat complicated. This complexity seems to cause some errors in the calculations. Therefore, it is necessary to develop a direct and simple thermodynamic treatment of the adsorption and micelle formation of the systems of inorganic salt and ionic surfactant. Recently, we developed thermodynamic treatments of adsorption and micelle formation of aqueous two-surfactant m i x t u r e ~ . l ~In - ~the ~ treatments, we adopted total molality and composition of the mixtures, instead of the concentrations of each solute components, as the independent experimental variables besides temperature and pressure. Further, the two-dividing surface convention was used in the treatment of adsorption and the micellar surface excess thermodynamic quantity was introduced in the treatment of micelle formation. Applying the treatments to the surface tension data, calculating the surface and micellar compositionof the mixtures, and then drawing the composition diagrams of adsorption (CDA) and micelle formation (CDM),we elucidated the miscibility of the surfactants in the adsorbed films and mi~e1les.l~ These treatments are also applicable to the systems of aqueous solutions of ionic surfactant and inorganic salt. In the present work, the surface tension of aqueous solutions of sodium chloride (NaC1)and dodecylammonium chloride (DAC)mixture is measured as a function of total molality and composition of the mixture a t constant temperature and pressure. With the surface tension data analyzed in a similar way as the two-surfactant systems, the CDA and CDM are drawn. It is expected that the useful information about the miscibilityof DAC and NaCl in the mixed adsorbed film and mixed micelles and, moreover, the distribution of DA+, C1-, and Na+ ions in (11)Okuda, H.; Ikeda, S. J. Colloid Interface Sci. 1989, 131, 333. (12) Motomura, IC;Matsukiyo,H.; Aratono, M. Phenomena in Mixed Surfuctant Systems; Scamehorn, J. F., Ed.; ACS Symposium 311; American Chemical Society: Washington, DC, 1986,p 163. (13)Motomura, K.;Ando, N.; Matsuki, H.; Aratono, M. J . Colloid Interface Sci. 1990, 139, 188. (14)Motomura, K.;Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984,262, 948. (15) Motomura, K.; Aratono, M. Mixed Surfactant Systems; Ogino, K., Abe, M., Ed.; Marcel Dekker: New York-Basel-Hong Kong, 1993; p 99.
Q743-7463l94l241Q-295Q$O4.5QlQ 0 1994 American Chemical Society
Adsorption and Micelle Formation of Mixtures
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Langmuir, Vol. 10, No. 9, 1994 2951 40 I
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0
1
I
1
30.
20 -
10 -
30 40 50 60 70 80 90 m / mmol kg-' Figure 1. Surfacetension vs total molality curves at constant composition: XZ= (1)0, (2) 0.080,(3) 0.100,(4) 0.140,(5)0.200, ( 6 )0.280,(7) 0.400,(8)0.580,(9) 0.799, and (10) 1. 0
10 20
0.0
0.2
0.4
0.6
x,,x;
0.8
1.0
1.2
the vicinity of the surface and the micelles, are obtained from the CDA and CDM.
Figure 2. Total molality vs composition curves at constant surface tension: (0)m vs Xz; (- - -) m vs XzH,y/mN m-l = (1)60,(2) 50,and (3) 40.
Experimental Section Sodium chloride (Matsunaga Chemical Co., Ltd., Standard Reagent, 99.99%)was used without further purification. Dodecylammonium chloride (DAC) was synthesized by the method described elsewhere16 and was recrystallized from an ethanol solution of a small amount of hydrochloric acid. The water was distilled 3 times; in the second and third steps, small amounts of potassium permanganate and sodium hydroxide were added. On the surface tension vs concentration curve of the aqueous DAC solution, no minimum was observed in a concentration range around the cmc. Therefore, we may say that the amount of dodecylamine produced by the hydrolysis of DAC is negligibly small. Surface tension y of the aqueous solution of sodium chloride (NaC1, component 1) and dodecylammonium chloride (DAC, component 2) mixture was measured as a function oftotalmolality m and solution compositionX2 by use ofthe dropvolume method." m and XZare defined respectively by
a t constant surface tensions (40, 50,and 60 mN m-l) by interpolation of the y vs m curves in Figure 1and plotted them against Xz in Figure 2. The m vs Xz curves a t constant y are convex downward and decrease steeply in the lower XZregion. According to the thermodynamic treatment of adsorption of m i ~ t u r e s ,the ~ ~ composition J~ of the adsorbed film XzHcan be calculated directly from the m vs XZcurve a t constant y by using
m=m,fm,
(1)
X2= mz/m
(2)
and
where mi is the molality of component i. For DAC and mixtures, the surface tension was measured at the molalities larger than that of the break on the y vs m curves which is due to the phase transition,from a gaseous filmto an expanded one,of the adsorbed film." The experimental error was within 0.05 mN m-*. All the measurements were carried out at 298.15 f 0.01 K under atmospheric pressure.
Results and Discussion Experimental results are shown in the form of y vs m curve a t constant Xz in Figure 1. It is easy to find that the curve of NaCl (XZ= 0) is almost linear and has a slight positive slope. On the other hand, the curves of DAC (XZ = 1.0) and mixtures measured in this study are different in shape from that of NaC1: y decreasing rapidly with increasing m up t o the critical micelle concentration (cmc) and very slightly in the concentration range above the cmc. With decreasingxz, the y vs m curve of the mixture is observed to move gradually to the right. In order to examine the composition dependence of the experimental results more closely, we obtained the total molality values (16) Aratono, M.; Yamanaka, M.; Matsubayasi, N.; Motomura, K.; Matuura, R. J . Colloid Interface Sci. 1980,74, 489. (17) Motomura, K.; Iwanaga, S.; Hayami, Y,.; Uryu, S.; Matuura, R. J . Colloid Interface Sci. 1981,80,32.
XzH is defined by
xZH= rZH/rH
(4)
+
where TH(=TIH TzH)is the total surface density and TIH and TzH are the surface densities of NaCl and DAC, respectively. In eq 3, we assumed that DAC is a strong 1:l electrolyte. The m vsXzHcurves are drawn in Figure 2. Since the m vs Xz and m vs XZHcurves a t same surface tension give the relationship between the compositions of mixtures in the adsorbed film and bulk solution which are in equilibrium with each other, we refer the figure composed of these curves as the composition diagram of adsorption (CDA). Figure 2 shows that CDS's of the NaC1-DAC mixture have a very different feature from those of two-surfactant m i x t u r e ~ ; l ~ -is~greater ~ X ~ ~than 1and increases with decreasingxz (increasing m )in such a way that it approaches asymptotically to its value a t a high total molality. Further, it is found that the magnitude of the deviation of the m vs XzH curve from the vertical line atXzH= 1decreases with decreasing surface tension. In order to consider this point more closely, let us estimate the total surface density and surface densities of NaCl and DAC. First, let us calculate TH by applying
rH= - ( " u + ) ( a ~ / a m ) ~ ~ ~ ~
(5)
to the y vs m curves in Figure 1. On deriving eq 5, we have assumed ideal solutions. In Figure 3, the TH vs m curves a t constant Xz are shown. Because of the slight and linear increase of y of the aqueous NaCl solution with increasing m (see curve 1in Figure 11, TH of NaCl (XZ = 0) decreases slightly and linearly with increasing m. On the other hand, TH of DAC (Xz = 1.0)and the mixtures increase with increasing m;the curves are convex upward
2952 Langmuir, Vol. 10,No.9,1994
Yamanaka et al. 70
60
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50
M Y
E 6
2
1
0
40
30
i
20
m / mmol kg-' Figure 3. Total surface density vs total molality curves at constant composition: X Z= (1)0;(2) 0.080,(3) 0.100, (4)0.140, (5) 0.200,(6)0.280, (7) 0.400, ( 8 ) 0.580, (9)0.799,(10)1.
Y
4
4
0.0
0.2
0.6
0.4
0.8
1.0
x2
Figure 4. Surface density vs composition curves: (0) rH at (A) rHat 40 mN m-l, (0) rH at 50 mN m-l; (1)rzH cmc (rHgC), at 40 mN m-l, ( 2 )TzHat 50 mN m-l, (3) TIH at 40 mN m-l, (4) rlHat 50 mN m-l. and show saturated adsorption a t a concentration near the cmc. The THvs m curve moves gradually to the right with decreasingxz. It can be said that a mixture a t lower XZrequires higher total molality to attain the same surface density as that of a mixture at higher XZ. The TH value a t the cmc, denoted by TH,C,is plotted againstXz in Figure 4. It is observed that rH,C increases with decreasing XZ in the XZ range higher than about 0.3, while in the XZ range below about 0.3 it decreases. Next, let us examine the dependence of TH on XZ a t constant y . From the TH vs m curves a t constant XZin Figure 3 and the m vs XzHcurves at constant y in Figure 2, the THVSXZ curves a t constant surface tensions (40 and 50 mN m-l) are obtained and shown in Figure 4. They vs XZcurve in shape. The decrease of resemble the TH and THZCa t low XZ is probably attributable to the nonideal behavior of the solutions because of the relatively high concentration of NaC1. The surface densities of NaCl and DAC, TIHand TzH, respectively, are evaluated from Figures 3 and 4. The values of TIH and TzH a t 40 and 50 mN m-l are depicted as functions ofXz in Figure 4. TzHis observed to increase more rapidly than TH with decreasing XZ. On the other hand, TIHhas a negative value which decreases from 0 a t XZ = 1 with decreasing XZ;the absolute value of TIH is fairly large compared with that of the surface density of pure NaCl (XZ= 0) shown in Figure 3. Further, although THIC
-
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 X2'
e.Xy'
Figure 5. Critical micelle concentration vs composition C vs X z ; (- - -) C vs XzHP; (- - -) C vs XzM. curves: (0)
TzHis larger at 40 mN m-l than a t 50 mN m-17TIH seems to be almost independent of y except for the lowXz region. These facts are the reason why XzHis larger than 1and has a smaller asymptotic value a t a high total molality a t lower surface tension (see Figure 2). Moreover,since there are the relations
and
'4
-1
10
rH= rlH+ rzH= r N a + H +
=
rc,-H
where r N a + H , rDA+H, and TC1-H are the surface densities of sodium, dodecylammonium, and chloride ions, respectively, the curves in Figure 4 can be seen as the dependence of the surface densities of those ions on the solution composition a t constant y . Therefore, it is understood that the large negative adsorption of NaCl is due to the large desorption of Na+ ions caused by the electrostatic repulsive interaction with the DA+ ions in the adsorbed film. Furthermore, it is found that the smaller adsorption of C1- ions than that of DA+ ions corresponds to the desorption of Na+ ions. Our next stage is to discuss the miscibility of NaCl and DAC in the micelles. The DAC micelles formed in the aqueous NaCl solutions can be regarded as the mixed micelles of NaCl and DAC. The critical micelle concentration (cmc) and the surface tension a t the cmc, yc7 give information about the mixed micelle and adsorbed film which are in equilibrium with each other.15 In our thermodynamic treatment of mixed micelles,14the cmc is defined by using the total molality and denoted by C. The values of C and yc of the mixtures and DAC were determined from the break of the y vs m curves in Figure 1. Although the break seems to become obtuse gradually with decreasingxz, it is sharp enough to determine the values of C and yc even for the mixture ofXz = 0.100. The C vs XZcurve is drawn in Figure 5. While yc decreases slightly with decreasing Xz, C increases rapidly. Since the micelles and adsorbed film are in equilibrium with each other, the micellar and surface compositions, XzM and XzHsC,a t the cmc, can be calculated from the composition dependence of C and y c by using
Adsorption and Micelle Formation of Mixtures
X2M= x, - (2xlx2/c)(ac/ax2),,
Langmuir, Vol. 10, No. 9, 1994 2953
(9)
and
X2H,C= X2M- (X~X/RlrH~C)(ayC/ax2X,)Tp (10) The micellar composition is defined by use of the micellar surface excess number of molecules or ions;14 i.e.
XZM= NZM/NM
(11)
where M
N I M= NNa+
(12)
NzM= NDA+M
(13)
and
+
N M= NlM NzM=
+ NDA+M= NCI-M
(14)
In Figure 5, the C value is plotted against XZ,XzM,and XzHsC.Since the combination of the C vs XZand C vs XZM curves shows the relationship between the compositions of the mixture in the monomeric and micellar states, we refer to it as the composition diagram of micelle formation (CDM). It is found from the CDM that XzMhas a value larger than 1and increases with increasing the cmc. This behavior is similar to that ofXzHpcand the shape of CDM resembles that of CDA. Therefore, we may say that the micellar surface excess number of Naf ion is negative and become more negative a t lowerXz. Further, although the dependence of C onXzMis similar to that onXZH,C,XZM has a larger value than X Z ~ ,This ~ . may be due to difference in geometry between the micelles and the adsorbed film. Thus, we can say that, in the similar manner to the twosurfactant systems,15 our thermodynamic approach to elucidate the adsorption and micelle formation by means of the CDA and CDM is applicable to the system of an inorganic salt and an ionic surfactant and provides useful information about the miscibility of the solutes in the adsorbed film and micelle.
