- Email: [email protected]
Properties of Anionic–Cationic Adsorption Films in the Presence of Inorganic Electrolytes
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
184, 139–146 (1996)
0604
Properties of Anionic–Cationic Adsorption Films in the Presence of Inorganic Electrolytes DANUTA GO´RALCZYK1 Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry, Jagiellonian University, Krako´w, Poland Received February 16, 1996; accepted July 10, 1996
The composition and mutual interaction of the components in anionic–cationic adsorption films have been determined by using to the mixed monolayers a regular solution model. The effect of the inorganic electrolyte concentration and kind on the properties of monolayers adsorbed from solutions containing decylpyridinium chloride and sodium decylsulfonate in concentration ratios varying over a wide range has been investigated. The calculations have been performed both neglecting and taking into consideration the effect of surface interactions on the partial molar areas of surfactants in the mixed monolayer. It has been found that although calculation results are very different, all conclusions concerning the effect of the solution composition are almost the same in both cases. q 1996 Academic Press, Inc. Key Words: decylpyridinium chloride; sodium decyl sulfonate; anionic–cationic adsorption films; surface interactions; composition of mixed films; regular solution model.
INTRODUCTION
Knowledge of the interaction of components in mixed adsorption films not only is important from the theoretical point of view but also has great practical importance, because these interactions affect the adsorption and micelization behavior of surfactants remarkably. They may therefore improve considerably the useful properties of surfactants, such as washing, foaming, emulsifying, wetting, dispersing, and flotation properties (1). Estimation of the mutual interactions of adsorbed components is possible by using a regular solution model for mixed monolayers, according to which, in a binary system (2, 3), bÅ
ln( a1 c12 /x1 c 71 ) ln( a2 c12 /x2 c 72 ) Å , x 22 x 21
[1]
where a1 and a2 Å 1 0 a1 are the mole fractions of the components in the solution, x1 and x2 are the mole fractions of the components in the mixed monolayer, c 71 , c 72 , and c12 are the concentrations of the individual surfactants and 1
To whom correspondence should be addressed.
their mixture, respectively, required to produce a definite value of the surface tension, and b is a parameter characterizing mutual interactions of the adsorbed components. This parameter assumes positive values in the case of repulsive interactions and negative in the case of attractive interactions. Equation [1] makes it possible to calculate also the mixed monolayer composition, which is usually different from the solution composition. This equation is based upon the assumption that molar areas of the components are the same in the mixed monolayer and in the single-component monolayers, i.e., Ai Å A 7i . This assumption is valid only in the case of weak interactions of adsorbed molecules. However, in the case of stronger interactions it may be assumed that the ratio of partial molar areas of two surfactants in a mixed monolayer equals the ratio of molar areas of two individual surfactants at the same surface tension s, i.e., A1 / A2 Å A 71 / A 72 . Then Eq. [1] assumes the more complicated form ( 4 ) bÅ
Å
F F
ln
ln
a1 c12
x1 c 71 a2 c12
x2 c 72
sA 71 AU 12 10 RT x1 A 71 / x2 A 72
0
sA 72 AU 12 10 RT x1 A 71 / x2 A 72
JCIS 4476
/
6g18$$$$21
10-25-96 15:55:32
DGY DGY
x 22
x 21 , [2]
where AV 12 Å x1 A1 / x2 A2 is the average molar area in the mixed monolayer. The surface attractive interactions are particularly strong in the case of mixtures of anionic and cationic surfactants which form mixed monolayers showing large deviations from ideality (5–8). The surface properties of these mixtures have been investigated in many papers (9–17). It has been stated that the compositions of anionic–cationic adsorption films and the mutual interactions of their components are dependent not only on the structures and concentration ratios of surfactants present in the solution, but also on the concentration and type of added inorganic electrolyte, which indicates that electrostatic factors have great significance. The effect of sodium chloride concentration on the surface properties of mixtures containing surfactants of the
139
AID
S S
0
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
coida
AP: Colloid
´ RALCZYK DANUTA GO
140
same hydrocarbon chain length (decylpyridinium chloride and sodium decyl sulfonate), the composition of which was varied over a wide range, has been investigated in the last paper (17). Similar investigations are now performed for the same mixtures but in the presence of inorganic electrolytes with different anions (sodium bromide and iodide). The obtained results make it possible to state whether the effect of inorganic electrolyte concentration is much the same irrespective of the kind of inorganic anion and what effect a change of the inorganic anion has at the same concentration of added electrolyte. Up to now such investigations have been only performed for mixtures of equimolar composition (16). EXPERIMENTAL
The systems investigated contained decylpyridinium chloride (R10PyCl—component 1) as a cationic surfactant and sodium decyl sulfonate (R10SO3Na—component 2) as an anionic surfactant. Methods of purification of these substances have been described previously (18, 19). Sodium bromide and iodide were used as inorganic electrolytes. They contain a cation common to the examined anionic surfactant (Na / ), whereas anions (Br 0 and I 0 ) are different from those coming from the dissociation of the investigated cationic surfactant (Cl 0 ). However, it should not have a significant effect on the measurement results because the Br 0 and I 0 anions, the specific action of which is much stronger, are in large excess as compared with Cl 0 anions. In most concentrated solutions this excess is tenfold at least and therefore the ionic strength of the mixed solutions may be assumed as a constant value, equal to the concentration of added inorganic electrolyte. In the case of R10PyCl solutions in the presence of NaI is similarly. However, in the case of solutions of single cationic surfactant with addition of NaBr, the situation is quite different; surfactant concentration in most concentrated solutions is relatively great and ionic strength at constant NaBr addition obviously is not constant. For that reason, measurements in the presence of this inorganic electrolyte have not been carried out for decylpyridinium chloride but for decylpyridinium bromide (R10PyBr), the method of purification of which was described previously (20), and sodium bromide was added to the solutions in quantities ensuring the ionic strength constancy. To prepare the solutions, water distilled four times was used, which was boiled down to two thirds of its volume immediately before use in order to remove CO2 and other volatile contaminations that might be present. The surface tension of the solutions was measured using the drop weight method at constant temperature (298 K). The measurement accuracy was {0.2 mN/m. Mixtures containing a predominance of the cationic surfactant (c1 /c2 Å 100/1, 30/1, 10/1, and 3/1) and the equimolar mixture (c1 /c2 Å 1/1) were investigated at three inor-
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
FIG. 1. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.01 M NaBr: (1) R10PyBr, (2) R10SO3Na, (3) R10PyCl / R10SO3Na: c1 /c2 Å 100/1 (3a), 1/100 (3a * ), 30/1 (3b), 10/1 (3c), 1/10 (3c * ), 3/1 (3d), 1/1 (3e).
ganic electrolyte concentrations (0.01, 0.03, and 0.1 M NaBr or NaI). In contrast, for mixtures with a predominance of the anionic surfactant (c1 /c2 Å 1/10 and 1/100), the measurements were only performed at one inorganic electrolyte concentration (0.01 M NaBr or NaI) because it has been stated previously (17) that the effect of the ionic strength of the solution is similar irrespective of whether anionic or cationic surfactant is predominant in the mixture, provided the molar ratio of the components is the same (100/1 and 1/100 or 10/1 and 1/10). For that reason the mixtures of compositions 1/3 and 1/30 were not investigated. Their behavior will be close to that of mixtures 3/1 and 30/1, respectively. RESULTS
Figures 1–6 show the results of the surface tension measurements obtained at different NaBr (Figs. 1–3) and NaI (Figs. 4–6) concentrations for solutions containing single cationic (curves 1) and anionic (curves 2) surfactants and their mixtures (curves 3) for different mole ratios of the components. On the basis of curves concerning individual components it may be stated that both surfactants investigated have similar ability to reduce surface tension in the presence of NaBr. In contrast, the cationic surfactant is much more surface-active than the anionic surfactant in the presence of NaI. It should be mentioned that the anionic surfactant (R10SO3Na) has a greater ability to reduce the surface tension in the presence of NaCl, as was found previously
coida
AP: Colloid
ANIONIC–CATIONIC ADSORPTION FILMS
141
FIG. 2. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.03 M NaBr: Denotations of curves as in Fig. 1.
FIG. 4. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.01 M NaI: (1) R10PyCl, remaining curves as in Fig. 1.
(17). The observed effect of inorganic anion type on the surface activity of long-chain cations is in accordance with the results of other papers (20–23). All mixtures investigated have a greater ability to reduce surface tension than their single components, except for the mixture of composition 100/1 in the presence of 0.1 M NaI, (curve 3a in Fig. 6) which has almost the same surface activity as decylpyridinium chloride alone. This ability in-
creases when the mixture composition tends to the equimolar (curves 3e in Figs. 1–6). It is worthy of note that the difference between the surface activity of the mixtures and of the single cationic surfactant decreases more and more when the inorganic electrolyte concentration is increased, which is distinct particularly in the case of 0.1 M NaI (Fig. 6). Figures 7 and 8 show the effect of inorganic electrolyte
FIG. 3. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.1 M NaBr: Denotations of curves as in Fig. 1.
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
FIG. 5. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.03 M NaI: (1) R10PyCl, remaining curves as in Fig. 1.
coida
AP: Colloid
´ RALCZYK DANUTA GO
142
FIG. 6. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.1 M NaI: (1) R10PyCl, remaining curves as in Fig. 1.
FIG. 8. Dependence of the surface tension on the logarithm of the concentration for solutions of R10PyCl (curves 1) and mixtures of R10PyCl and R10SO3Na: denotations curves as in Fig. 7. Inorganic electolyte concentration: (a) 0.01 M NaI, (b) 0.03 M NaI, (c) 0.1 M NaI.
concentration (NaBr—Fig. 7 and NaI—Fig. 8) on the ability to reduce the surface tension for single cationic surfactant (curves 1) (for single anionic surfactant this effect is quite similar) and some mixtures investigated (c1 /c2 Å 100/1, 10/1, and 1/1). For mixtures this effect is much smaller
than it is in the case of single decylpyridinium chloride and it decreases when the asymmetry of mixture composition diminishes. The effect of NaI concentration is somewhat greater than the effect of NaBr concentration. The great effect of the inorganic electrolyte concentration which is observed in the case of single ionic surfactants is due to the weakness of electrostatic repulsion between long-chain cations or anions. This results in an increased ability to reduce the surface tension. On the other hand, the ionic character of mixed adsorption films formed in the case of mixtures of both types of surfactants is weakened due to mutual compensation of positive and negative charges of adsorbed long-chain ions. For that reason, the effect of inorganic electrolyte concentration is much smaller and it does not appear at all in the case of equimolar mixture (curve 4 in Figs. 7 and 8). Similar behavior has been described by Zhao (9) for mixtures of other anionic and cationic surfactants with hydrocarbon chains of the same lengths. Figure 9 demonstrates the effect of inorganic electrolyte type on ability to reduce the surface tension for single cationic (curves 1) and anionic (curve 2) surfactants and some mixtures investigated (c1 /c2 Å 1/100, 30/1, 1/10, 3/1, and 1/1). A distinct influence of the inorganic electrolyte type on the surface activity of the cationic surfactant is visible in this figure. However, this effect is absent in the case of anionic surfactant, which contains the same cations (counterions) as applied inorganic electrolytes (Na / ). Also, in the case of equimolar mixtures (c1 /c2 Å 1/1), this effect practically does not appear. For the remaining mixtures investigated, the ability to reduce surface tension increases in the
FIG. 7. Dependence of the surface tension on the logarithm of the concentration for solutions of R10PyBr (curves 1) and mixtures of R10PyCl and R10SO3Na: c1 /c2 Å 100/1 (curves 2), 10/1 (curves 3), 1/1 (curve 4). Inorganic electrolyte concentration: (a) 0.01 M NaBr, (b) 0.03 M NaBr, (c) 0.1 M NaBr.
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
coida
AP: Colloid
143
ANIONIC–CATIONIC ADSORPTION FILMS
tained at different NaBr and NaI concentrations are collected in Tables 1 and 2, respectively. The composition of the mixed monolayer expressed both by the mole fraction x1 of the component of greater surface activity (R10PyCl) and the mole ratio x1 /x2 of the adsorbed components are given in these tables for each composition of the mixture. The values of the surface interaction parameter b are also included in Tables 1 and 2. In regard to the accuracy of calculation of the A 71 , A 72 , and AV 12 values, the calculation results obtained on the basis of Eq. [2] are much less precise than those obtained by using Eq. [1]. On the basis of Tables 1 and 2, it may be stated that the values of the surface interaction parameter b calculated on the basis of Eq. [2] are much more negative than those obtained based on Eq. [1]. This is due to the negative contribution of the term
FIG. 9. Dependence of the surface tension on the logarithm of the concentration for solutions of R10PyCl or R10PyBr (curves 1), R10SO3Na (curve 2), and mixtures of R10PyCl and R10SO3Na: c1 /c2 Å 1/100 (curves 3), 30/1 (curves 4), 1/10 (curves 5), 3/1 (curves 6), 1/1 (curve 7). Inorganic electrolyte: (a) 0.01 M NaCl, (b) 0.01 M NaBr, (c) 0.01 M NaI.
order NaCl õ NaBr õ NaI, the same order as in the case of single cationic surfactant, but this effect is much smaller. This may be explained by the weakness of the ionic nature of the mixed monolayer. Moreover, the extent of the effect of inorganic electrolyte type is dependent neither on the concentration ratio of the components nor on whether anionic or cationic surfactant is predominant in the mixture. The compositions of mixed adsorption films which are formed in the systems investigated and the values of surface interaction parameter b have been calculated on the basis of Eqs. [1] and [2] by substituting the appropriate values of a1 Å c1 /(c1 / c2 ) and concentrations c 71 , c 72 , and c12 required to produce a definite value of the surface tension ( s Å 65 mN/m). These concentrations have been read from the plots presented in Figs. 1–6. The calculations should be performed at surface tension value so low that the mixed adsorption layer is completely saturated (the presence of water is neglected). The surface tension used in the present calculations is relatively high (65 mN/m) but it is in the range of the linear course of p vs log c plots for mixed solutions, what means a constant surface excess value. At lower surface tension values, the surfactant concentration in the solution would be considerably greater, which might change the ionic strength. Areas A 71 and A 72 occupied by surfactants in the single-component monolayers and the average molar area AV 12 of the surfactant mixture have been determined on the basis of the Gibbs adsorption equation from slopes of s vs log c plots (Figs. 1–6) for solutions of individual surfactants and their mixtures, respectively. The calculation results ob-
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
0
sA 71 RT
S
0
sA 72 RT
S
10
AU 12
D
AU 12
D
x1 A 71 / x2 A 72
or 10
x1 A 71 / x2 A 72
.
This contribution is more negative for greater surface tension and for larger difference between the value of the average molar area AV 12 Å x1 A1 / x2 A2 and the value calculated on the basis of the molar areas of individual components. In the systems now investigated the value of AV 12 amounts to 40–80% of the value calculated using the values of A 71 and A 72 and so the term
S
10
AU 12
x1 A 71 / x2 A 72
D
assumes relatively large values (0.6–0.2). On the basis of Tables 1 and 2, it may be seen that the values of x1 calculated based on Eq. [2] are less different from the value of 0.5 than those obtained using Eq. [1]. Thus Eq. [2] gives a smaller asymmetry of mixed monolayer composition than Eq. [1]. However, it is difficult to decide which results are closer to the real surface composition. Comparison with the composition calculated on the basis of Gibbs surface excesses, as in (4) and (8), is impossible because of the lack of appropriate experimental results. In spite of distinct differences in the values of x1 and b, all conclusions concerning the effect of the solution composition are similar irrespective of whether simplified Eq. [1] or Eq. [2], which takes into account the changes of molar areas caused by mutual surfactant interactions in the mixed monolayer, is used in calculations. On the basis of results contained in Tables 1 and 2 it may be stated that the content of a given component in the mixed adsorption film rises
coida
AP: Colloid
´ RALCZYK DANUTA GO
144
TABLE 1 Influence of Inorganic Electrolyte Concentration on Properties of Anionic–Cationic Films Adsorbed from Mixtures: R10PyCl (1) / R10SO3Na (2) / NaBr (s Å 65 mN/m) Calculations on the basis of Eq. [1] Inorganic electrolyte
Calculations on the basis of Eq. [2]
c1/c2
x1
x1/x2
b
x1
x1/x2
b
0.01 M NaBr
100/1 30/1 10/1 3/1 1/1 1/10 1/100
0.672 0.615 0.571 0.534 0.504 0.438 0.343
2.1/1 1.6/1 1.3/1 1.2/1 1/1 1/1.3 1/1.9
011.8 013.5 015.4 016.6 017.7 015.4 012.0
0.538 0.527 0.517 0.506 0.498 0.478 0.457
1.2/1 1.1/1 1.1/1 1/1 1/1 1/1.1 1/1.2
054.7 055.7 057.2 058.3 059.3 057.0 054.6
0.03 M NaBr
100/1 30/1 10/1 3/1 1/1
0.710 0.635 0.585 0.543 0.506
2.5/1 1.7/1 1.4/1 1.2/1 1/1
09.4 011.4 012.9 013.6 015.9
0.549 0.536 0.524 0.511 0.500
1.2/1 1.2/1 1.1/1 1/1 1/1
045.0 046.2 047.3 047.8 049.9
0.1 M NaBr
100/1 30/1 10/1 3/1 1/1
0.788 0.677 0.610 0.553 0.508
3.7/1 2.1/1 1.6/1 1.2/1 1/1
06.2 08.3 09.5 010.7 011.7
0.602 0.552 0.526 0.510 0.497
1.5/1 1.2/1 1.1/1 1/1 1/1
021.1 029.5 038.6 041.8 043.9
Accuracy of x1 calculation Accuracy of b calculation
{0.002
{0.005
{0.2
{2.5
TABLE 2 Influence of Inorganic Electrolyte Concentration on Properties of Anionic–Cationic Films Adsorbed from Mixtures: R10PyCl (1) / R10SO3Na (2) / NaI (s Å 65 mN/m) Calculations on the basis of Eq. [1] Inorganic electrolyte
Calculations on the basis of Eq. [2]
c1/c2
x1
x1/x2
b
x1
x1/x2
b
0.01 M NaI
100/1 30/1 10/1 3/1 1/1 1/10 1/100
0.765 0.668 0.617 0.657 0.533 0.466 0.377
3.3/1 1.0/1 1.6/1 1.3/1 1.1/1 1/1.2 1/1.7
08.7 011.6 013.0 015.1 015.9 014.1 011.8
0.585 0.569 0.556 0.543 0.531 0.510 0.487
1.4/1 1.3/1 1.2/1 1.2/1 1.1/1 1/1 1/1
044.3 046.2 047.3 049.1 049.9 048.3 047.1
0.03 M NaI
100/1 30/1 10/1 3/1 1/1
0.855 0.744 0.661 0.595 0.552
5.9/1 2.9/1 2.0/1 1.5/1 1.2/1
07.3 07.9 09.8 011.9 012.7
0.620 0.602 0.583 0.563 0.548
1.6/1 1.5/1 1.4/1 1.3/1 1.2/1
032.3 032.7 034.0 036.0 036.9
0.1 M NaI
100/1 30/1 10/1 3/1 1/1
0.926 0.815 0.719 0.645 0.573
33.5/1 4.4/1 2.6/1 1.8/1 1.3/1
04.6 05.7 06.9 07.4 08.9
0.673 0.645 0.615 0.586 0.560
2.1/1 1.8/1 1.6/1 1.4/1 1.3/1
019.1 019.4 020.0 021.2 022.1
Accuracy of x1 calculation Accuracy of b calculation
AID
JCIS 4476
/
6g18$$4476
{0.002
{0.005
{0.2
{2.5
10-25-96 15:55:32
coida
AP: Colloid
145
ANIONIC–CATIONIC ADSORPTION FILMS
TABLE 3 Influence of Inorganic Electrolyte Type on Properties of Anionic–Cationic Films Adsorbed from Mixtures: R10PyCl (1) / R10SO3NA (2) / Nax (s Å 65 mN/m) Calculations on the basis of Eq. [1] Inorganic electrolyte
Calculations on the basis of Eq. [2]
c1/c2
x1
x1/x2
b
x1
0.01 M NaCla
100/1 10/1 1/1 1/10 1/100
0.649 0.555 0.491 0.424 0.325
1.9/1 1.3/1 1/1 1/1.4 1/2.1
012.1 015.5 019.0 015.5 012.2
0.503 0.487 0.472 0.453 0.434
1/1 1/1 1/1.1 1/1.2 1/1.2
065.2 067.3 070.3 066.8 064.1
0.01 M NaBr
100/1 10/1 1/1 1/10 1/100
0.672 0.571 0.504 0.438 0.343
2.1/1 1.3/1 1/1 1/1.3 1/1.9
011.8 015.4 017.7 015.4 012.0
0.538 0.517 0.498 0.478 0.457
1.2/1 1.1/1 1/1 1/1.1 1/1.2
054.7 057.2 059.3 057.0 054.6
0.01 M NaI
100/1 10/1 1/1 1/10 1/100
0.765 0.617 0.533 0.466 0.377
3.3/1 1.6/1 1.1/1 1/1.2 1/1.7
08.7 013.0 015.9 014.1 011.8
0.585 0.556 0.531 0.510 0.487
1.4/1 1.2/1 1.1/1 1/1 1/1
044.3 047.3 049.9 048.3 047.1
Accuracy of x1 calculation Accuracy of b calculation a
x1/x2
{0.002
{0.005
{0.2
{2.5
b
Results from (17, 24).
when its content in the solution is increased, although to a considerably smaller extent. Similar behavior has been found previously for mixtures of anionic and cationic surfactants with hydrocarbon chains of different lengths (12, 15) as well as for mixtures containing hydrocarbon cationic and fluorocarbon anionic surfactant (8). Mixed monolayers of almost equimolar composition are formed not only by mixtures of composition 1/1 but also by mixtures of composition 3/1, 10/1, and 1/10. In the case of the remaining mixtures investigated, this component predominates in the adsorption film which prevails in the solution; however, the asymmetry of the monolayer composition is always smaller than the asymmetry of the solution composition. These results are similar to those obtained previously (17) for the same surfactant mixtures in the presence of NaCl. They explain completely why the effect of the inorganic electrolyte concentration on the surface tension decreases when the predominance of one of the surfactants in the mixture is reduced. The increase of the inorganic electrolyte concentration in the solution enhances the asymmetry of the mixed monolayer composition. This effect is greater in the presence of NaI than it is in the presence of NaBr and it is most visible in the case of mixtures containing a great excess of one of the components (c1 /c2 Å 100/1). In all systems investigated the values of parameter b are negative, which indicates attractive interactions between adsorbed components. These interactions are strongest in the
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
case of adsorption from equimolar solutions (the absolute value of b is then largest). They decrease when the solution ionic strength is increased, which indicates their mainly electrostatic character. Similar behavior has been found previously (15, 16) for mixtures of anionic and cationic surfactants with hydrocarbon chains of different lengths. The composition of mixed adsorption films and values of interaction parameter b in the presence of NaCl, NaBr, and NaI of the same concentration equal to 0.01 M are compared in Table 3. The results for 0.01 M NaCl are taken from previous papers (17, 24). On the basis of results contained in Table 3 it may be stated that for a given mixture composition, the content of decylpyridinium ions in the monolayer is greatest in the presence of NaI and is smallest in the presence of NaCl in the solution. The ability of inorganic anions to enhance the adsorption of long-chain cations increases in the order Cl 0 õ Br 0 õ I 0 (20, 25, 26), whereas the adsorption of decyl sulfonate ions remains unchanged (25). Therefore, in the presence of NaI in the solution, the surface composition asymmetry is largest in the case of mixtures with a predominance of the cationic surfactant (e.g., c1 /c2 Å 100/1), whereas it is smallest in the case of mixtures with a predominance of the anionic surfactant (e.g., c1 /c2 Å 1/100). For the given mixture composition the absolute value of b decreases in the order NaCl ú NaBr ú NaI because the ability of inorganic anions to weaken the electrostatic interactions increases in the order Cl 0 õ Br 0 õ I 0 . A similar
coida
AP: Colloid
´ RALCZYK DANUTA GO
146
effect of inorganic electrolyte type on the composition of the mixed adsorption films and the mutual interaction of their components has been stated previously (11, 13, 16) for other mixtures of anionic and cationic surfactants. DISCUSSION AND CONCLUSIONS
In the approach presented the thermodynamic nonideality of the mixed adsorption films is taken into account, whereas the ideal behavior of the solutions is assumed. The complexes formed by long-chain anions and cations are not taken into consideration because the results of measurements of the electrical conductivity of the mixed solutions indicate that the formation of such complexes may be neglected even at higher concentrations than those now considerated (27). The regular solution model applied to anionic–cationic adsorption films is only a rough approximation of the real state of the interface. It takes into consideration the presence of only long-chain anions and cations, while the presence of water and counterions, which are actually constituents of the electrical double layer, is quite neglected. Nevertheless, this model provides some information about the properties of the mixed adsorption films. In the systems investigated, containing decylpyridinium chloride and sodium decyl sulfonate, the composition of mixed adsorption films is dependent on both the mixture composition and the concentration and kind of the inorganic electrolyte present in the solution. Equimolar mixtures and mixtures containing a small predominance of one of the components (3/1, 10/1, or 1/10) form monolayers of almost equimolar composition. In contrast, an asymmetry of the monolayer composition appears in the case of a great predominance of one of the components in the mixture. This asymmetry is larger for greater concentrations of added inorganic electrolyte. In the presence of NaI in the solution the effect is most pronounced. At the same inorganic electrolyte concentration, the content of decylpyridinium ions in the monolayer is greatest in the presence of NaI, whereas it is smallest in the presence of NaCl. This is due to the different effect of inorganic ions on the adsorption of long-chain cations, while the adsorption of long-chain anions remains unchanged. The negative values of the surface interaction parameter b indicate the mutual attraction of adsorbed components, which is greatest in the case of adsorption from equimolar solutions ( the absolute value of b is then largest ) . Similar changes of parameter b with the solution composition have been found by Zhao and Zhu ( 8 ) for solutions containing hydrocarbon cationic and fluorocarbon anionic surfactants, but these changes were considerably smaller than those now obtained. These interactions decrease
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
when the concentration of added inorganic electrolyte is increased. At constant ionic strength they are strongest in the presence of NaCl and weakest in the presence of NaI in the solution. These facts indicate that discussed interactions have mainly electrostatic character. The application of Eq. [2], which takes into consideration changes of the molar areas of the components in the mixed monolayer, reduces the dependence of parameter b on the molar ratio of surfactants in the solution. However, in contrast to results obtained by Gu and Rosen (4), this equation gives nonconstant values of b for systems now investigated. It is worthy of note that recently Wu¨stneck (28) investigated the surface behavior of mixtures of anionic and zwitterionic surfactants and also found that the parameter describing the mutual attraction of adsorbed molecules is greatest when the concentration ratio of the surfactants in the solution is equal to 1/1. Wu¨stneck applied an entirely different approach to mixed monolayers taking into consideration self-interactions of the same kind molecules too (29). REFERENCES 1. Joost, F., Leiter, H., and Schwuger, M. J., J. Colloid Polym. Sci. 266, 554 (1988). 2. Rosen, M. J., and Hua, X. Y., J. Colloid Interface Sci. 86, 164 (1982). 3. Rosen, M. J., ACS Symp. Ser. 311, 144 (1986). 4. Gu, B., and Rosen, M. J., J. Colloid Interface Sci. 129, 537 (1989). 5. Scamehorn, J. F., ACS Symp. Ser. 311, 1 (1986). 6. Holland, P. M., ACS Symp. Ser. 311, 102 (1986). 7. Holland, P. M., Colloids Surf. 19, 171 (1986). 8. Zhao, G.-X., and Zhu, B.-Y., ACS Symp. Ser. 311, 184 (1986). 9. Zhao, G.-X., Chen, Y.-Z., Ou, J.-G., Tien, B.-S., and Huang, Z.-M., Acta Chim. Sin. 38, 409 (1980). 10. Ding, H. J., Wu, X.-L., and Zhao, G.-X., Acta Chim. Sin. 43, 603 (1985). 11. Go´ralczyk, D., Colloids Surf. 59, 361 (1991). 12. Go´ralczyk, D., Colloids Surf. 66, 241 (1992). 13. Go´ralczyk, D., Univ. Iagel. Acta Chim. 36, 153 (1993). 14. Go´ralczyk, D., Colloid Polym. Sci. 272, 204 (1994). 15. Go´ralczyk, D., J. Colloid Interface Sci. 167, 172 (1994). 16. Go´ralczyk, D., Univ. Iagel. Acta Chim. 38, 151 (1995). 17. Go´ralczyk, D., J. Colloid Interface Sci. 179, 211 (1996). 18. Waligo´ra, B., and Go´ralczyk, D., Bull. Acad. Polon. Sci. Se´r. Sci. Chim. 22, 901 (1974). 19. Go´ralczyk, D., Polish J. Chem. 52, 417 (1978). 20. Go´ralczyk, D., Tenside Deterg. 20, 228 (1983). 21. Parreira, H. C., J. Colloid Interface Sci. 29, 235 (1969). 22. Tamaki, K., Colloid Polym. Sci. 252, 547 (1974). 23. Tamaki, K., Bull. Chem. Soc. Jpn. 47, 2764 (1974). 24. Go´ralczyk, D., Bull. Acad. Polon. Sci. Se´r. Sci. Chim., in press. 25. Go´ralczyk, D., Colloids Surf. 11, 287 (1984). 26. Go´ralczyk, D., Tenside Deterg. 30, 356 (1993). 27. Go´ralczyk, D., in ‘‘XI Seminarium, Procesy technologiczne surowco´w mineralnych, Barano´w Sand., 1984,’’ p. 206. 28. Wu¨stneck, R., Miller, R., Krivanek, J., and Holtzbauer, H. R., Langmuir 10, 3738 (1994). 29. Wu¨stneck, R., Miller, R., and Krivanek, J., Colloids Surf. A 81, 1 (1993).
coida
AP: Colloid
184, 139–146 (1996)
0604
Properties of Anionic–Cationic Adsorption Films in the Presence of Inorganic Electrolytes DANUTA GO´RALCZYK1 Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry, Jagiellonian University, Krako´w, Poland Received February 16, 1996; accepted July 10, 1996
The composition and mutual interaction of the components in anionic–cationic adsorption films have been determined by using to the mixed monolayers a regular solution model. The effect of the inorganic electrolyte concentration and kind on the properties of monolayers adsorbed from solutions containing decylpyridinium chloride and sodium decylsulfonate in concentration ratios varying over a wide range has been investigated. The calculations have been performed both neglecting and taking into consideration the effect of surface interactions on the partial molar areas of surfactants in the mixed monolayer. It has been found that although calculation results are very different, all conclusions concerning the effect of the solution composition are almost the same in both cases. q 1996 Academic Press, Inc. Key Words: decylpyridinium chloride; sodium decyl sulfonate; anionic–cationic adsorption films; surface interactions; composition of mixed films; regular solution model.
INTRODUCTION
Knowledge of the interaction of components in mixed adsorption films not only is important from the theoretical point of view but also has great practical importance, because these interactions affect the adsorption and micelization behavior of surfactants remarkably. They may therefore improve considerably the useful properties of surfactants, such as washing, foaming, emulsifying, wetting, dispersing, and flotation properties (1). Estimation of the mutual interactions of adsorbed components is possible by using a regular solution model for mixed monolayers, according to which, in a binary system (2, 3), bÅ
ln( a1 c12 /x1 c 71 ) ln( a2 c12 /x2 c 72 ) Å , x 22 x 21
[1]
where a1 and a2 Å 1 0 a1 are the mole fractions of the components in the solution, x1 and x2 are the mole fractions of the components in the mixed monolayer, c 71 , c 72 , and c12 are the concentrations of the individual surfactants and 1
To whom correspondence should be addressed.
their mixture, respectively, required to produce a definite value of the surface tension, and b is a parameter characterizing mutual interactions of the adsorbed components. This parameter assumes positive values in the case of repulsive interactions and negative in the case of attractive interactions. Equation [1] makes it possible to calculate also the mixed monolayer composition, which is usually different from the solution composition. This equation is based upon the assumption that molar areas of the components are the same in the mixed monolayer and in the single-component monolayers, i.e., Ai Å A 7i . This assumption is valid only in the case of weak interactions of adsorbed molecules. However, in the case of stronger interactions it may be assumed that the ratio of partial molar areas of two surfactants in a mixed monolayer equals the ratio of molar areas of two individual surfactants at the same surface tension s, i.e., A1 / A2 Å A 71 / A 72 . Then Eq. [1] assumes the more complicated form ( 4 ) bÅ
Å
F F
ln
ln
a1 c12
x1 c 71 a2 c12
x2 c 72
sA 71 AU 12 10 RT x1 A 71 / x2 A 72
0
sA 72 AU 12 10 RT x1 A 71 / x2 A 72
JCIS 4476
/
6g18$$$$21
10-25-96 15:55:32
DGY DGY
x 22
x 21 , [2]
where AV 12 Å x1 A1 / x2 A2 is the average molar area in the mixed monolayer. The surface attractive interactions are particularly strong in the case of mixtures of anionic and cationic surfactants which form mixed monolayers showing large deviations from ideality (5–8). The surface properties of these mixtures have been investigated in many papers (9–17). It has been stated that the compositions of anionic–cationic adsorption films and the mutual interactions of their components are dependent not only on the structures and concentration ratios of surfactants present in the solution, but also on the concentration and type of added inorganic electrolyte, which indicates that electrostatic factors have great significance. The effect of sodium chloride concentration on the surface properties of mixtures containing surfactants of the
139
AID
S S
0
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
coida
AP: Colloid
´ RALCZYK DANUTA GO
140
same hydrocarbon chain length (decylpyridinium chloride and sodium decyl sulfonate), the composition of which was varied over a wide range, has been investigated in the last paper (17). Similar investigations are now performed for the same mixtures but in the presence of inorganic electrolytes with different anions (sodium bromide and iodide). The obtained results make it possible to state whether the effect of inorganic electrolyte concentration is much the same irrespective of the kind of inorganic anion and what effect a change of the inorganic anion has at the same concentration of added electrolyte. Up to now such investigations have been only performed for mixtures of equimolar composition (16). EXPERIMENTAL
The systems investigated contained decylpyridinium chloride (R10PyCl—component 1) as a cationic surfactant and sodium decyl sulfonate (R10SO3Na—component 2) as an anionic surfactant. Methods of purification of these substances have been described previously (18, 19). Sodium bromide and iodide were used as inorganic electrolytes. They contain a cation common to the examined anionic surfactant (Na / ), whereas anions (Br 0 and I 0 ) are different from those coming from the dissociation of the investigated cationic surfactant (Cl 0 ). However, it should not have a significant effect on the measurement results because the Br 0 and I 0 anions, the specific action of which is much stronger, are in large excess as compared with Cl 0 anions. In most concentrated solutions this excess is tenfold at least and therefore the ionic strength of the mixed solutions may be assumed as a constant value, equal to the concentration of added inorganic electrolyte. In the case of R10PyCl solutions in the presence of NaI is similarly. However, in the case of solutions of single cationic surfactant with addition of NaBr, the situation is quite different; surfactant concentration in most concentrated solutions is relatively great and ionic strength at constant NaBr addition obviously is not constant. For that reason, measurements in the presence of this inorganic electrolyte have not been carried out for decylpyridinium chloride but for decylpyridinium bromide (R10PyBr), the method of purification of which was described previously (20), and sodium bromide was added to the solutions in quantities ensuring the ionic strength constancy. To prepare the solutions, water distilled four times was used, which was boiled down to two thirds of its volume immediately before use in order to remove CO2 and other volatile contaminations that might be present. The surface tension of the solutions was measured using the drop weight method at constant temperature (298 K). The measurement accuracy was {0.2 mN/m. Mixtures containing a predominance of the cationic surfactant (c1 /c2 Å 100/1, 30/1, 10/1, and 3/1) and the equimolar mixture (c1 /c2 Å 1/1) were investigated at three inor-
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
FIG. 1. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.01 M NaBr: (1) R10PyBr, (2) R10SO3Na, (3) R10PyCl / R10SO3Na: c1 /c2 Å 100/1 (3a), 1/100 (3a * ), 30/1 (3b), 10/1 (3c), 1/10 (3c * ), 3/1 (3d), 1/1 (3e).
ganic electrolyte concentrations (0.01, 0.03, and 0.1 M NaBr or NaI). In contrast, for mixtures with a predominance of the anionic surfactant (c1 /c2 Å 1/10 and 1/100), the measurements were only performed at one inorganic electrolyte concentration (0.01 M NaBr or NaI) because it has been stated previously (17) that the effect of the ionic strength of the solution is similar irrespective of whether anionic or cationic surfactant is predominant in the mixture, provided the molar ratio of the components is the same (100/1 and 1/100 or 10/1 and 1/10). For that reason the mixtures of compositions 1/3 and 1/30 were not investigated. Their behavior will be close to that of mixtures 3/1 and 30/1, respectively. RESULTS
Figures 1–6 show the results of the surface tension measurements obtained at different NaBr (Figs. 1–3) and NaI (Figs. 4–6) concentrations for solutions containing single cationic (curves 1) and anionic (curves 2) surfactants and their mixtures (curves 3) for different mole ratios of the components. On the basis of curves concerning individual components it may be stated that both surfactants investigated have similar ability to reduce surface tension in the presence of NaBr. In contrast, the cationic surfactant is much more surface-active than the anionic surfactant in the presence of NaI. It should be mentioned that the anionic surfactant (R10SO3Na) has a greater ability to reduce the surface tension in the presence of NaCl, as was found previously
coida
AP: Colloid
ANIONIC–CATIONIC ADSORPTION FILMS
141
FIG. 2. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.03 M NaBr: Denotations of curves as in Fig. 1.
FIG. 4. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.01 M NaI: (1) R10PyCl, remaining curves as in Fig. 1.
(17). The observed effect of inorganic anion type on the surface activity of long-chain cations is in accordance with the results of other papers (20–23). All mixtures investigated have a greater ability to reduce surface tension than their single components, except for the mixture of composition 100/1 in the presence of 0.1 M NaI, (curve 3a in Fig. 6) which has almost the same surface activity as decylpyridinium chloride alone. This ability in-
creases when the mixture composition tends to the equimolar (curves 3e in Figs. 1–6). It is worthy of note that the difference between the surface activity of the mixtures and of the single cationic surfactant decreases more and more when the inorganic electrolyte concentration is increased, which is distinct particularly in the case of 0.1 M NaI (Fig. 6). Figures 7 and 8 show the effect of inorganic electrolyte
FIG. 3. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.1 M NaBr: Denotations of curves as in Fig. 1.
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
FIG. 5. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.03 M NaI: (1) R10PyCl, remaining curves as in Fig. 1.
coida
AP: Colloid
´ RALCZYK DANUTA GO
142
FIG. 6. Dependence of the surface tension on the logarithm of the concentration for solutions in 0.1 M NaI: (1) R10PyCl, remaining curves as in Fig. 1.
FIG. 8. Dependence of the surface tension on the logarithm of the concentration for solutions of R10PyCl (curves 1) and mixtures of R10PyCl and R10SO3Na: denotations curves as in Fig. 7. Inorganic electolyte concentration: (a) 0.01 M NaI, (b) 0.03 M NaI, (c) 0.1 M NaI.
concentration (NaBr—Fig. 7 and NaI—Fig. 8) on the ability to reduce the surface tension for single cationic surfactant (curves 1) (for single anionic surfactant this effect is quite similar) and some mixtures investigated (c1 /c2 Å 100/1, 10/1, and 1/1). For mixtures this effect is much smaller
than it is in the case of single decylpyridinium chloride and it decreases when the asymmetry of mixture composition diminishes. The effect of NaI concentration is somewhat greater than the effect of NaBr concentration. The great effect of the inorganic electrolyte concentration which is observed in the case of single ionic surfactants is due to the weakness of electrostatic repulsion between long-chain cations or anions. This results in an increased ability to reduce the surface tension. On the other hand, the ionic character of mixed adsorption films formed in the case of mixtures of both types of surfactants is weakened due to mutual compensation of positive and negative charges of adsorbed long-chain ions. For that reason, the effect of inorganic electrolyte concentration is much smaller and it does not appear at all in the case of equimolar mixture (curve 4 in Figs. 7 and 8). Similar behavior has been described by Zhao (9) for mixtures of other anionic and cationic surfactants with hydrocarbon chains of the same lengths. Figure 9 demonstrates the effect of inorganic electrolyte type on ability to reduce the surface tension for single cationic (curves 1) and anionic (curve 2) surfactants and some mixtures investigated (c1 /c2 Å 1/100, 30/1, 1/10, 3/1, and 1/1). A distinct influence of the inorganic electrolyte type on the surface activity of the cationic surfactant is visible in this figure. However, this effect is absent in the case of anionic surfactant, which contains the same cations (counterions) as applied inorganic electrolytes (Na / ). Also, in the case of equimolar mixtures (c1 /c2 Å 1/1), this effect practically does not appear. For the remaining mixtures investigated, the ability to reduce surface tension increases in the
FIG. 7. Dependence of the surface tension on the logarithm of the concentration for solutions of R10PyBr (curves 1) and mixtures of R10PyCl and R10SO3Na: c1 /c2 Å 100/1 (curves 2), 10/1 (curves 3), 1/1 (curve 4). Inorganic electrolyte concentration: (a) 0.01 M NaBr, (b) 0.03 M NaBr, (c) 0.1 M NaBr.
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
coida
AP: Colloid
143
ANIONIC–CATIONIC ADSORPTION FILMS
tained at different NaBr and NaI concentrations are collected in Tables 1 and 2, respectively. The composition of the mixed monolayer expressed both by the mole fraction x1 of the component of greater surface activity (R10PyCl) and the mole ratio x1 /x2 of the adsorbed components are given in these tables for each composition of the mixture. The values of the surface interaction parameter b are also included in Tables 1 and 2. In regard to the accuracy of calculation of the A 71 , A 72 , and AV 12 values, the calculation results obtained on the basis of Eq. [2] are much less precise than those obtained by using Eq. [1]. On the basis of Tables 1 and 2, it may be stated that the values of the surface interaction parameter b calculated on the basis of Eq. [2] are much more negative than those obtained based on Eq. [1]. This is due to the negative contribution of the term
FIG. 9. Dependence of the surface tension on the logarithm of the concentration for solutions of R10PyCl or R10PyBr (curves 1), R10SO3Na (curve 2), and mixtures of R10PyCl and R10SO3Na: c1 /c2 Å 1/100 (curves 3), 30/1 (curves 4), 1/10 (curves 5), 3/1 (curves 6), 1/1 (curve 7). Inorganic electrolyte: (a) 0.01 M NaCl, (b) 0.01 M NaBr, (c) 0.01 M NaI.
order NaCl õ NaBr õ NaI, the same order as in the case of single cationic surfactant, but this effect is much smaller. This may be explained by the weakness of the ionic nature of the mixed monolayer. Moreover, the extent of the effect of inorganic electrolyte type is dependent neither on the concentration ratio of the components nor on whether anionic or cationic surfactant is predominant in the mixture. The compositions of mixed adsorption films which are formed in the systems investigated and the values of surface interaction parameter b have been calculated on the basis of Eqs. [1] and [2] by substituting the appropriate values of a1 Å c1 /(c1 / c2 ) and concentrations c 71 , c 72 , and c12 required to produce a definite value of the surface tension ( s Å 65 mN/m). These concentrations have been read from the plots presented in Figs. 1–6. The calculations should be performed at surface tension value so low that the mixed adsorption layer is completely saturated (the presence of water is neglected). The surface tension used in the present calculations is relatively high (65 mN/m) but it is in the range of the linear course of p vs log c plots for mixed solutions, what means a constant surface excess value. At lower surface tension values, the surfactant concentration in the solution would be considerably greater, which might change the ionic strength. Areas A 71 and A 72 occupied by surfactants in the single-component monolayers and the average molar area AV 12 of the surfactant mixture have been determined on the basis of the Gibbs adsorption equation from slopes of s vs log c plots (Figs. 1–6) for solutions of individual surfactants and their mixtures, respectively. The calculation results ob-
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
0
sA 71 RT
S
0
sA 72 RT
S
10
AU 12
D
AU 12
D
x1 A 71 / x2 A 72
or 10
x1 A 71 / x2 A 72
.
This contribution is more negative for greater surface tension and for larger difference between the value of the average molar area AV 12 Å x1 A1 / x2 A2 and the value calculated on the basis of the molar areas of individual components. In the systems now investigated the value of AV 12 amounts to 40–80% of the value calculated using the values of A 71 and A 72 and so the term
S
10
AU 12
x1 A 71 / x2 A 72
D
assumes relatively large values (0.6–0.2). On the basis of Tables 1 and 2, it may be seen that the values of x1 calculated based on Eq. [2] are less different from the value of 0.5 than those obtained using Eq. [1]. Thus Eq. [2] gives a smaller asymmetry of mixed monolayer composition than Eq. [1]. However, it is difficult to decide which results are closer to the real surface composition. Comparison with the composition calculated on the basis of Gibbs surface excesses, as in (4) and (8), is impossible because of the lack of appropriate experimental results. In spite of distinct differences in the values of x1 and b, all conclusions concerning the effect of the solution composition are similar irrespective of whether simplified Eq. [1] or Eq. [2], which takes into account the changes of molar areas caused by mutual surfactant interactions in the mixed monolayer, is used in calculations. On the basis of results contained in Tables 1 and 2 it may be stated that the content of a given component in the mixed adsorption film rises
coida
AP: Colloid
´ RALCZYK DANUTA GO
144
TABLE 1 Influence of Inorganic Electrolyte Concentration on Properties of Anionic–Cationic Films Adsorbed from Mixtures: R10PyCl (1) / R10SO3Na (2) / NaBr (s Å 65 mN/m) Calculations on the basis of Eq. [1] Inorganic electrolyte
Calculations on the basis of Eq. [2]
c1/c2
x1
x1/x2
b
x1
x1/x2
b
0.01 M NaBr
100/1 30/1 10/1 3/1 1/1 1/10 1/100
0.672 0.615 0.571 0.534 0.504 0.438 0.343
2.1/1 1.6/1 1.3/1 1.2/1 1/1 1/1.3 1/1.9
011.8 013.5 015.4 016.6 017.7 015.4 012.0
0.538 0.527 0.517 0.506 0.498 0.478 0.457
1.2/1 1.1/1 1.1/1 1/1 1/1 1/1.1 1/1.2
054.7 055.7 057.2 058.3 059.3 057.0 054.6
0.03 M NaBr
100/1 30/1 10/1 3/1 1/1
0.710 0.635 0.585 0.543 0.506
2.5/1 1.7/1 1.4/1 1.2/1 1/1
09.4 011.4 012.9 013.6 015.9
0.549 0.536 0.524 0.511 0.500
1.2/1 1.2/1 1.1/1 1/1 1/1
045.0 046.2 047.3 047.8 049.9
0.1 M NaBr
100/1 30/1 10/1 3/1 1/1
0.788 0.677 0.610 0.553 0.508
3.7/1 2.1/1 1.6/1 1.2/1 1/1
06.2 08.3 09.5 010.7 011.7
0.602 0.552 0.526 0.510 0.497
1.5/1 1.2/1 1.1/1 1/1 1/1
021.1 029.5 038.6 041.8 043.9
Accuracy of x1 calculation Accuracy of b calculation
{0.002
{0.005
{0.2
{2.5
TABLE 2 Influence of Inorganic Electrolyte Concentration on Properties of Anionic–Cationic Films Adsorbed from Mixtures: R10PyCl (1) / R10SO3Na (2) / NaI (s Å 65 mN/m) Calculations on the basis of Eq. [1] Inorganic electrolyte
Calculations on the basis of Eq. [2]
c1/c2
x1
x1/x2
b
x1
x1/x2
b
0.01 M NaI
100/1 30/1 10/1 3/1 1/1 1/10 1/100
0.765 0.668 0.617 0.657 0.533 0.466 0.377
3.3/1 1.0/1 1.6/1 1.3/1 1.1/1 1/1.2 1/1.7
08.7 011.6 013.0 015.1 015.9 014.1 011.8
0.585 0.569 0.556 0.543 0.531 0.510 0.487
1.4/1 1.3/1 1.2/1 1.2/1 1.1/1 1/1 1/1
044.3 046.2 047.3 049.1 049.9 048.3 047.1
0.03 M NaI
100/1 30/1 10/1 3/1 1/1
0.855 0.744 0.661 0.595 0.552
5.9/1 2.9/1 2.0/1 1.5/1 1.2/1
07.3 07.9 09.8 011.9 012.7
0.620 0.602 0.583 0.563 0.548
1.6/1 1.5/1 1.4/1 1.3/1 1.2/1
032.3 032.7 034.0 036.0 036.9
0.1 M NaI
100/1 30/1 10/1 3/1 1/1
0.926 0.815 0.719 0.645 0.573
33.5/1 4.4/1 2.6/1 1.8/1 1.3/1
04.6 05.7 06.9 07.4 08.9
0.673 0.645 0.615 0.586 0.560
2.1/1 1.8/1 1.6/1 1.4/1 1.3/1
019.1 019.4 020.0 021.2 022.1
Accuracy of x1 calculation Accuracy of b calculation
AID
JCIS 4476
/
6g18$$4476
{0.002
{0.005
{0.2
{2.5
10-25-96 15:55:32
coida
AP: Colloid
145
ANIONIC–CATIONIC ADSORPTION FILMS
TABLE 3 Influence of Inorganic Electrolyte Type on Properties of Anionic–Cationic Films Adsorbed from Mixtures: R10PyCl (1) / R10SO3NA (2) / Nax (s Å 65 mN/m) Calculations on the basis of Eq. [1] Inorganic electrolyte
Calculations on the basis of Eq. [2]
c1/c2
x1
x1/x2
b
x1
0.01 M NaCla
100/1 10/1 1/1 1/10 1/100
0.649 0.555 0.491 0.424 0.325
1.9/1 1.3/1 1/1 1/1.4 1/2.1
012.1 015.5 019.0 015.5 012.2
0.503 0.487 0.472 0.453 0.434
1/1 1/1 1/1.1 1/1.2 1/1.2
065.2 067.3 070.3 066.8 064.1
0.01 M NaBr
100/1 10/1 1/1 1/10 1/100
0.672 0.571 0.504 0.438 0.343
2.1/1 1.3/1 1/1 1/1.3 1/1.9
011.8 015.4 017.7 015.4 012.0
0.538 0.517 0.498 0.478 0.457
1.2/1 1.1/1 1/1 1/1.1 1/1.2
054.7 057.2 059.3 057.0 054.6
0.01 M NaI
100/1 10/1 1/1 1/10 1/100
0.765 0.617 0.533 0.466 0.377
3.3/1 1.6/1 1.1/1 1/1.2 1/1.7
08.7 013.0 015.9 014.1 011.8
0.585 0.556 0.531 0.510 0.487
1.4/1 1.2/1 1.1/1 1/1 1/1
044.3 047.3 049.9 048.3 047.1
Accuracy of x1 calculation Accuracy of b calculation a
x1/x2
{0.002
{0.005
{0.2
{2.5
b
Results from (17, 24).
when its content in the solution is increased, although to a considerably smaller extent. Similar behavior has been found previously for mixtures of anionic and cationic surfactants with hydrocarbon chains of different lengths (12, 15) as well as for mixtures containing hydrocarbon cationic and fluorocarbon anionic surfactant (8). Mixed monolayers of almost equimolar composition are formed not only by mixtures of composition 1/1 but also by mixtures of composition 3/1, 10/1, and 1/10. In the case of the remaining mixtures investigated, this component predominates in the adsorption film which prevails in the solution; however, the asymmetry of the monolayer composition is always smaller than the asymmetry of the solution composition. These results are similar to those obtained previously (17) for the same surfactant mixtures in the presence of NaCl. They explain completely why the effect of the inorganic electrolyte concentration on the surface tension decreases when the predominance of one of the surfactants in the mixture is reduced. The increase of the inorganic electrolyte concentration in the solution enhances the asymmetry of the mixed monolayer composition. This effect is greater in the presence of NaI than it is in the presence of NaBr and it is most visible in the case of mixtures containing a great excess of one of the components (c1 /c2 Å 100/1). In all systems investigated the values of parameter b are negative, which indicates attractive interactions between adsorbed components. These interactions are strongest in the
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
case of adsorption from equimolar solutions (the absolute value of b is then largest). They decrease when the solution ionic strength is increased, which indicates their mainly electrostatic character. Similar behavior has been found previously (15, 16) for mixtures of anionic and cationic surfactants with hydrocarbon chains of different lengths. The composition of mixed adsorption films and values of interaction parameter b in the presence of NaCl, NaBr, and NaI of the same concentration equal to 0.01 M are compared in Table 3. The results for 0.01 M NaCl are taken from previous papers (17, 24). On the basis of results contained in Table 3 it may be stated that for a given mixture composition, the content of decylpyridinium ions in the monolayer is greatest in the presence of NaI and is smallest in the presence of NaCl in the solution. The ability of inorganic anions to enhance the adsorption of long-chain cations increases in the order Cl 0 õ Br 0 õ I 0 (20, 25, 26), whereas the adsorption of decyl sulfonate ions remains unchanged (25). Therefore, in the presence of NaI in the solution, the surface composition asymmetry is largest in the case of mixtures with a predominance of the cationic surfactant (e.g., c1 /c2 Å 100/1), whereas it is smallest in the case of mixtures with a predominance of the anionic surfactant (e.g., c1 /c2 Å 1/100). For the given mixture composition the absolute value of b decreases in the order NaCl ú NaBr ú NaI because the ability of inorganic anions to weaken the electrostatic interactions increases in the order Cl 0 õ Br 0 õ I 0 . A similar
coida
AP: Colloid
´ RALCZYK DANUTA GO
146
effect of inorganic electrolyte type on the composition of the mixed adsorption films and the mutual interaction of their components has been stated previously (11, 13, 16) for other mixtures of anionic and cationic surfactants. DISCUSSION AND CONCLUSIONS
In the approach presented the thermodynamic nonideality of the mixed adsorption films is taken into account, whereas the ideal behavior of the solutions is assumed. The complexes formed by long-chain anions and cations are not taken into consideration because the results of measurements of the electrical conductivity of the mixed solutions indicate that the formation of such complexes may be neglected even at higher concentrations than those now considerated (27). The regular solution model applied to anionic–cationic adsorption films is only a rough approximation of the real state of the interface. It takes into consideration the presence of only long-chain anions and cations, while the presence of water and counterions, which are actually constituents of the electrical double layer, is quite neglected. Nevertheless, this model provides some information about the properties of the mixed adsorption films. In the systems investigated, containing decylpyridinium chloride and sodium decyl sulfonate, the composition of mixed adsorption films is dependent on both the mixture composition and the concentration and kind of the inorganic electrolyte present in the solution. Equimolar mixtures and mixtures containing a small predominance of one of the components (3/1, 10/1, or 1/10) form monolayers of almost equimolar composition. In contrast, an asymmetry of the monolayer composition appears in the case of a great predominance of one of the components in the mixture. This asymmetry is larger for greater concentrations of added inorganic electrolyte. In the presence of NaI in the solution the effect is most pronounced. At the same inorganic electrolyte concentration, the content of decylpyridinium ions in the monolayer is greatest in the presence of NaI, whereas it is smallest in the presence of NaCl. This is due to the different effect of inorganic ions on the adsorption of long-chain cations, while the adsorption of long-chain anions remains unchanged. The negative values of the surface interaction parameter b indicate the mutual attraction of adsorbed components, which is greatest in the case of adsorption from equimolar solutions ( the absolute value of b is then largest ) . Similar changes of parameter b with the solution composition have been found by Zhao and Zhu ( 8 ) for solutions containing hydrocarbon cationic and fluorocarbon anionic surfactants, but these changes were considerably smaller than those now obtained. These interactions decrease
AID
JCIS 4476
/
6g18$$$$22
10-25-96 15:55:32
when the concentration of added inorganic electrolyte is increased. At constant ionic strength they are strongest in the presence of NaCl and weakest in the presence of NaI in the solution. These facts indicate that discussed interactions have mainly electrostatic character. The application of Eq. [2], which takes into consideration changes of the molar areas of the components in the mixed monolayer, reduces the dependence of parameter b on the molar ratio of surfactants in the solution. However, in contrast to results obtained by Gu and Rosen (4), this equation gives nonconstant values of b for systems now investigated. It is worthy of note that recently Wu¨stneck (28) investigated the surface behavior of mixtures of anionic and zwitterionic surfactants and also found that the parameter describing the mutual attraction of adsorbed molecules is greatest when the concentration ratio of the surfactants in the solution is equal to 1/1. Wu¨stneck applied an entirely different approach to mixed monolayers taking into consideration self-interactions of the same kind molecules too (29). REFERENCES 1. Joost, F., Leiter, H., and Schwuger, M. J., J. Colloid Polym. Sci. 266, 554 (1988). 2. Rosen, M. J., and Hua, X. Y., J. Colloid Interface Sci. 86, 164 (1982). 3. Rosen, M. J., ACS Symp. Ser. 311, 144 (1986). 4. Gu, B., and Rosen, M. J., J. Colloid Interface Sci. 129, 537 (1989). 5. Scamehorn, J. F., ACS Symp. Ser. 311, 1 (1986). 6. Holland, P. M., ACS Symp. Ser. 311, 102 (1986). 7. Holland, P. M., Colloids Surf. 19, 171 (1986). 8. Zhao, G.-X., and Zhu, B.-Y., ACS Symp. Ser. 311, 184 (1986). 9. Zhao, G.-X., Chen, Y.-Z., Ou, J.-G., Tien, B.-S., and Huang, Z.-M., Acta Chim. Sin. 38, 409 (1980). 10. Ding, H. J., Wu, X.-L., and Zhao, G.-X., Acta Chim. Sin. 43, 603 (1985). 11. Go´ralczyk, D., Colloids Surf. 59, 361 (1991). 12. Go´ralczyk, D., Colloids Surf. 66, 241 (1992). 13. Go´ralczyk, D., Univ. Iagel. Acta Chim. 36, 153 (1993). 14. Go´ralczyk, D., Colloid Polym. Sci. 272, 204 (1994). 15. Go´ralczyk, D., J. Colloid Interface Sci. 167, 172 (1994). 16. Go´ralczyk, D., Univ. Iagel. Acta Chim. 38, 151 (1995). 17. Go´ralczyk, D., J. Colloid Interface Sci. 179, 211 (1996). 18. Waligo´ra, B., and Go´ralczyk, D., Bull. Acad. Polon. Sci. Se´r. Sci. Chim. 22, 901 (1974). 19. Go´ralczyk, D., Polish J. Chem. 52, 417 (1978). 20. Go´ralczyk, D., Tenside Deterg. 20, 228 (1983). 21. Parreira, H. C., J. Colloid Interface Sci. 29, 235 (1969). 22. Tamaki, K., Colloid Polym. Sci. 252, 547 (1974). 23. Tamaki, K., Bull. Chem. Soc. Jpn. 47, 2764 (1974). 24. Go´ralczyk, D., Bull. Acad. Polon. Sci. Se´r. Sci. Chim., in press. 25. Go´ralczyk, D., Colloids Surf. 11, 287 (1984). 26. Go´ralczyk, D., Tenside Deterg. 30, 356 (1993). 27. Go´ralczyk, D., in ‘‘XI Seminarium, Procesy technologiczne surowco´w mineralnych, Barano´w Sand., 1984,’’ p. 206. 28. Wu¨stneck, R., Miller, R., Krivanek, J., and Holtzbauer, H. R., Langmuir 10, 3738 (1994). 29. Wu¨stneck, R., Miller, R., and Krivanek, J., Colloids Surf. A 81, 1 (1993).
coida
AP: Colloid