Langmuir 2008, 24, 3171-3180
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Adsorption of Multiple Ammonium Salts at the Air/Solution Interface Joanna Wegrzyn´ska,† Graz˘ yna Para,‡ Jan Chlebicki,† Piotr Warszyn´ski,*,‡ and Kazimiera A. Wilk† Faculty of Chemistry, Wrocław UniVersity of Technology, Wybrzez˘ e Wyspian´ skiego 27, 50-370 Wrocław, Poland, and Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Krako´ w, Poland ReceiVed August 24, 2007. In Final Form: NoVember 9, 2007 The interfacial behavior of aqueous solutions of newly synthesized bis- and tris-ammonium salts (i.e., bis[2hydroxy-3-(dodecyldimethylammonio)propyl]alkylamine dichlorides and bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]dialkylammmonium trichlorides, respectively) was analyzed, both experimentally and theoretically. The dynamic and equilibrium surface tension of multiple ammonium salt solutions was measured by using a pendant drop shape analysis method. The determined surface tension isotherms indicated the lack of significant differences in surface activity between bis- and tris-ammonium salts, contrary to the expectations for divalent and trivalent surfactant ions. That effect was explained by assuming the formation of multiple surfactant ion-counterion associates. Taking into account the association process, a good correlation between experimental data and theoretical predictions was obtained by means of the “surface quasi two-dimensional electrolyte” (STDE) model of ionic surfactant adsorption. The degree of association necessary to explain the lack of difference in surface activity between bis- and tris-ammonium salts was in quantitative agreement with the results of measurements of the concentration of free chloride anions in the surfactant solution.
Introduction Multiple quaternary ammonium salts are potential compounds for the next generation of surfactants and are attracting a lot of interest.1-5 The research on interfacial properties of dimeric and oligomeric ionic surfactants is scientifically challenging and important from a practical point of view. For instance, bisammonium type representatives have found a wide range of applications in petroleum, chemical, and pharmaceutical industries,6 mainly because of their adsorption and aggregation properties, high viscosity, detergency, solubilization ability, improvement of wetting, and profound antimicrobial activity against bacteria, yeasts, and molds.3,4,7,8 The interfacial behavior and aggregation properties of dimeric and oligomeric surfactants have been recently reviewed by Zana.5 He pointed out that their surface activity is 1-2 orders of magnitude higher than the surface activity of the corresponding monomeric surfactants. Therefore, they exhibit high efficiency in reducing the water surface (interfacial) tension and a much lower critical micelle concentration (cmc). The differences in the interfacial behavior between dimeric or trimeric surfactants and conventional monomeric ones are often attributed to constraining hydrophobic and hydrophilic groups by a spacer.4,5 The length and flexibility of the spacer group determines the distribution of both hydrophobic and hydrophilic entities at the interface, thus influencing the structure of the adsorption layer (e.g., minimum area per surfactant † ‡
Wrocław University of Technology. Polish Academy of Sciences.
(1) Yoshimura, T.; Yoshida, H.; Ohno, A.; Esumi, K. J. Colloid Interface Sci. 2002, 267, 167. (2) Zana, R.; Xia, J. Gemini surfactants; Surfactant Science Series; Marcel Dekker: New York, 2004. (3) Chlebicki, J.; Wegrzyn´ska, J.; Maliszewska, I.; Os´wiecimska, M. J. Surfactants Deterg. 2005, 3, 277. (4) Zana, R. J. Colloid Interface Sci. 2002, 248, 203. (5) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205. (6) Rosen, M. J. Surfactants and interfacial phenomena, 2nd ed.; John Wiley: New York, 1988. (7) Tatsumi, T.; Zhang, W.; Nakatsuji, Y.; Miyake, K.; Matsushima, K.; Tanaka, M.; Furuta, T.; Ikeda, I. J. Surfactants Deterg. 2001, 4, 271. (8) Esumi, K.; Taguma, K.; Koide, Y. Langmuir 1996, 12, 4039.
molecule, which is attainable at the cmc). The same applies to the structure of the micellar aggregates. The presence of a multiple charge on the surfactant molecule is another factor influencing the structure of the adsorption layer at the solution interface. Adsorption of an ionic surfactant is always accompanied by the formation of the electric double layer (EDL) at the interface. The electrical potential of the EDL hinders further adsorption of the ionic surfactant and facilitates the adsorption of counterions; therefore, the electric charge of surfactant hydrophilic groups is the major factor determining the structure and composition of the solution interface.9 Other important parameters are the thickness of the adsorbed (Stern) layer at the interface, the degree of penetration of the Stern layer by counterions, the distribution of electric dipole moments in the adsorbed layer, and its dielectric characteristics. These parameters are in the majority of cases difficult to measure. The most elementary description of surfactant adsorption is based on the Gibbs adsorption equation:
Γ)-
1 dγ nRT d ln c
(1)
where Γ is the surfactant surface excess concentration, γ is the surface tension, c is the surfactant concentration, R is the gas constant, and T is the absolute temperature. The numerical factor n (so-called Gibbs factor) assumes a value of 1 for nonionic surfactants, while for ionic surfactants, in the absence of added electrolyte, its value is determined by the condition of electroneutrality of the adsorbed layer. For a monovalent surfactant and monovalent counterion, n ) 2, while n ) 3 and 4 for fully dissociated divalent and trivalent surfactants with univalent counterions, respectively. As it was demonstrated by Zana in his review,5 there is ambiguity concerning the value of the factor n when the surface tension isotherms of multiple ionic surfactants are described in terms of eq 1. In some studies n ) 2 was used (9) Dukhin, S. S., Kretszchmar, G., Miller, R., Eds. Studies in Interface Science; Elsevier: Amsterdam, 1995.
10.1021/la702619a CCC: $40.75 © 2008 American Chemical Society Published on Web 02/14/2008
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for dimeric ionic surfactants,10,11 while other authors used the value n ) 3.12-14 The same applies to trivalent surfactant ions for which, according to the thermodynamics, factor n ) 4 should be used. An explanation of this ambiguity is that the surfactant multivalent ion is not fully dissociated, with partial binding of the counterion to the dimeric or trimeric surfactant ion.15 As it is suggested in ref 5, the counterion binding should follow the mass action law; therefore, it should increase with the surfactant concentration. Consequently, the effect of binding should increase with decreasing surface activity of a multiple surfactant, that is, should be more important for a surfactant with shorter hydrocarbon chains. Also, the length of the spacer and its flexibility may influence the degree of counterion binding. To the authors’ best knowledge, the concept of complexation has never been used in any theoretical description of the surface activity of multiple ionic surfactants; however, the cationic gemini surfactant-counterion pairing in the interfacial region of gemini micelles was already discussed by Geng et al.16 The formation of multiple surfactant-counterion complexes is a similar phenomenon to the Manning condensation of counterions at a polyelectrolyte chain.17 The simple criterion for the condensation can be formulated as
l < lB )
e2 4π0kT
(2)
where l is the distance between neighboring charged groups at the polyelectrolyte molecule, e is the elementary charge, k is the Boltzmann constant, 0 is the vacuum dielectric permittivity, is the dielectric constant of the solution, and lB is the Bjerrum length equal to 0.71 nm for aqueous solutions at T ) 25 °C. If that criterion is fulfilled, the counterion condensation can be observed. More detailed theoretical studies of the condensation indicated18 that, for short polyelectrolyte chains, even when eq 2 is not strictly fulfilled, the counterion condensation occurs and its degree increases with the electrolyte concentration, chain flexibility, and decrease in distance between charges. Neutron reflectivity permits a direct determination of the surface excess of some surfactants and its comparison with the value obtained from the Gibbs equation (eq 1). Li et al.15 determined the values of that factor for selected surfactants and showed that for some of them the surfactant-counterion complexation had to occur. In principle, the counterion binding can be determined by measurements of the electric conductivity of surfactant solutions. The binding can be evidenced by a deviation of the dependence of the electrical conductance on surfactant concentration from linearity.19 However, the electric conductivity measurements gave no evidence of surfactant-counterion complexation in systems in which it was evidenced by neutron reflectivity.20 Yet, one has to keep in mind that there is a large difference in surface activity between a divalent surfactant and its monovalent equivalent, complexed with counterions (see below). Therefore, a minute amount of complexes in a solution, (10) Devinsky, F.; Lacko, I.; Bitererova, F.; Tomeckova, L. J. Colloid Interface Sci. 1986, 114, 314. (11) Estoe, J.; Nave, S.; Downer, A.; Paul, A.; Rankin, A.; Tribe, K. Langmuir 2000, 16, 4511. (12) Esumi, K.; Taguma, K.; Koide, Y. Langmuir 1996, 12, 4039. (13) Espert, A.; v. Klitzing, R.; Poulin, P.; Colin, A.; Zana, R.; Langevin, D. Langmuir 1998, 14, 4251. (14) Menger, F. M.; Keiper, J. S.; Azov, V. Langmuir 2000, 16, 2052. (15) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmuir 1999, 15, 4392. (16) Geng, Y.; Romsted, L. S.; Menger, F. J. Am. Chem. Soc. 2006, 128, 492. (17) Manning, G. S. J. Chem. Phys. 1969, 51, 924. (18) Muthukumar, M. J. Chem. Phys. 2004, 120, 9343. (19) Frindl, M.; Michels, B.; Levy, H.; Zana, R. Langmuir 1994, 10, 1140. (20) Zana, R. J. Colloid Interface Sci. 2002, 246, 182.
below the detection limit by electric conductivity measurements, can have a large contribution to the surfactant adsorption. That makes a direct comparison of the results of surface tension, neutron reflectivity, and conductivity measurements concerning the counterion binding to the multiple surfactant impossible, without detailed knowledge of the surface layer composition. That can be done with the use of a theoretical model of ionic surfactant adsorption, taking into account the multiple charges of surfactant molecules. Nevertheless, the time-dependent dynamic and equilibrium surface tension of multiple ionic surfactants solutions has been treated both theoretically and experimentally by a very limited number of investigators.21 Explaining the mechanism of ionic surfactant adsorption at air/liquid interfaces is of importance for a variety of practical phenomena, such as thin film stability, micellization, foamability, and so forth. A theoretical description of this mechanism is complicated because solutions of ionic surfactants by nature are multicomponent systems containing surfactant ions, counterions, and ions of salt added to control the ionic strength or pH of the solution. Recently, we proposed a model of ionic surfactant adsorption22-27 which is based on the assumption of penetration of counterions into the interfacial Stern layer and the formation of a “surface quasi two-dimensional electrolyte” (STDE). The details of this model and the review of its application to the adsorption of monovalent cationic surfactants, whose solutions’ surface tension was investigated with and without inorganic electrolytes, was published in ref 27. In the present paper, we extend our model to describe the adsorption of multivalent ionic surfactants. We applied the extended model for the quantitative description of surface tension isotherms for multiple cationic surfactants, that is, bis- and trisammonium salts, for which the synthesis, surface-activity, antielectrostatic properties, and antimicrobial activity we have previously reported.28,29 Our aim was to describe the behavior of multiple ammonium salts at the solution/air interface and to explain the effect of the number of charged groups in a surfactant molecule on its adsorption by comparing surface properties of solutions of bis- and tris-ammonium salts. In particular, we concentrated on the possibility of the formation of surfactantcounterion complexes and their influence on the interfacial properties of multivalent ionic surfactant solutions.
Model of Ionic Surfactant Adsorption in a Multivalent Electrolyte In our previous papers, we described the adsorption of cationic surfactants at the water/air interface in the presence of various monovalent anions by means of the “surface quasi twodimensional electrolyte” (STDE) model.25-27 In that model, it is assumed that a strong electric field of surfactant ions adsorbed at the interface causes the “adsorption” of counterions within the interfacial Stern layer. The adsorbed ions preserve their freedom of motion in the Stern layer, and therefore, that layer can be considered as a quasi two-dimensional electrolyte, which does (21) Rosen, M. J.; Song, L. D. J. Colloid Interface Sci. 1996, 179, 261. (22) Warszyn´ski, P.; Barzyk, W.; Lunkenheimer, K.; Fruhner, H. J. Phys. Chem. B 1998, 102, 10948. (23) Warszynski, P.; Lunkenheimer, K.; Czichocki, G. Langmuir 2002, 18, 2506. (24) Adamczyk, Z.; Para, G.; Warszyn´ski, P. Langmuir 1999, 15, 8383. (25) Para, G.; Jarek, E.; Warszyn´ski, P.; Adamczyk, Z. Colloids Surf., A 2003, 222, 213. (26) Para, G.; Jarek, E.; Warszyn´ski, P. Colloid Surf., A 2005, 261, 65. (27) Para, G.; Jarek, E.; Warszyn´ski, P. AdV. Colloid Interface Sci. 2006, 122, 39. (28) Wegrzyn´ska, J.; Chlebicki, J. J. Surfactants Deterg. 2006, 9, 221. (29) Wegrzyn´ska, J.; Chlebicki, J.; Maliszewska, I. Pol. J. Chem. Technol. 2006, 8, 59.
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not fulfill the electroneutrality condition. The total charge in the Stern layer is the sum of the positive charge of the adsorbed surfactant headgroups and the negative charge of the counterions. That overall charge determines the potential of the diffuse layer. The model takes explicitly into account the finite size of the headgroups and the counterions and considers lateral electric interactions between them. The accumulation of the surfactant co-ions in the Stern layer is negligible because of the strong electrostatic repulsion of surfactant ions by the charge. The same basic assumptions are applied in the extension of the model on the description of the adsorption of surface active multiple ammonium salts. Adsorption equations for the surfactant cations and their counterions and anions of added electrolyte in the Stern layer can be formulated by exploiting the postulate of equilibrium, that is, by assuming equal electrochemical potentials of each ion for the bulk phase and in the Stern layer. The respective adsorption equations for the system of multivalent cationic surfactant ions with their counterions and anions of added electrolyte can be written as
(
)
aS zseψs (1 - θS - θC1 - θC2) ) RS kT θS exp[-2HSθS] exp
φS (3) kT
for surfactant cations (S) and
(
)
( )
(
)
( )
φC2 aC2 zC2eψs (1 - θS - θC1 - θC2)gC2S ) θC2 exp RC2 kT kT
(4)
σδ 0s
and ψd is the diffuse layer potential at the boundary between the Stern layer and the diffuse part of the EDL, so it has to be calculated numerically using the implicit formula
σ)
x
0kTNA
( ( [ ] )) ∑i ci exp -
zieψd kT
-1
(6)
If the multivalent ions are present in the solution, in contrast to a purely monovalent system, there is no analytical formula connecting the surface charge density: (30) Koryta, J.; Dvorˇa´k, J.; Boha´cˇakova´, V. Lehrbuch der Elektrochemie, Polish ed.; PWN: Warsaw, 1980.
(8)
where the contributions of all ionic species, that is, surfactant ions, their counterions, and ions of added electrolyte with respective valencies zi, are included in the summation. Here, F and NA are the Faraday constant and the Avogadro number, respectively, δ is the thickness of the Stern layer, and s is the dielectric constant in the Stern layer. The activity corrections for the lateral interactions in a two-dimensional electrolyte φi for surfactant ions and counterions can be found from
(9)
where κs ) e2(ΓS + ΓC1 + zC22ΓC2)/0skT is the twodimensional, surface equivalent of the Debye-Hu¨ckel screening length31,32 and asi has the meaning of an effective ionic radius in the interfacial layer. To calculate the total surface excess concentrations of all ionic species, the adsorption in the diffuse part of the EDL has to be considered. It can be found as
ci ) ΓEDL i κ
[
d
]
zieψ -1 kT dψ s(ψ)
exp -
∫0ψ
(5)
for surfactant counterions (C1) and multivalent anions of added electrolyte (C2), respectively. Here, ai ) γici are the activities of surfactant ions and their respective counterions in the solution, γi are activity coefficients, and ci are concentrations. The activity coefficients can be calculated using the extended Debye-Hu¨ckel theory.30 zi are the respective valencies, RS is the “surface activity” of the surfactant cation being a measure of the free energy of the adsorption after separating the contribution of the electric components, and RC1 is the “surface activity” of the counterions, which is a measure of their penetration into the Stern layer due to van der Waals interactions, image forces, and hydration. For the multivalent ions, it also takes into account the correlation effects. θS ) ΓS/ΓS∞ is the relative surfactant surface concentration, and ΓS∞ is the limiting surfactant surface concentration of a closely packed monolayer; θC1 ) ΓC1/ΓC1∞, θC2 ) ΓC2/ΓC2∞, and ΓC1∞ and ΓC2∞ are the same quantities for counterions, gC1S ) ΓS∞/ΓC1∞, gC2S ) ΓS∞/ΓC2∞, and Hs is the interaction parameter accounting for the attractive lateral interactions among the adsorbed surfactant hydrophobic tails. The electric potential of the Stern layer ψs can be found from
ψs ) ψd +
(7)
φi κs 2e )kT 8π0skT 1 + κsasi
()
aC1 zC1eψs φC1 (1 - θS - θC1 - θC2)gC1S ) θC1 exp RC1 kT kT
σ ) F(zsΓS + zC1ΓC1 + zC2ΓC2)
(10)
where
s(ψ) )
x
2e2NA
0kTκ
( [ ] )
∑i ci exp 2
zieψd kT
-1
and κ is the Debye-Hu¨ckel reciprocal length. After the determination of the total surface excess concentrations of the surfactant ions and all other ions present in the solution, the surface tension can be calculated by integrating the Gibbs adsorption equation: T T T -dγ ) ΓST+ dµS+ + ΓC1 - dµC1- + ΓK+ dµK+ + ΓC2- dµC2(11)
where ΓT’s are the total surface excess concentrations (including that in the diffuse part of the EDL, eq 10) for surfactant cations (S+), surfactant counterions (C1-, C2-), and co-ions (K+) and µ’s, µi ) µi0 + RT ln(ai), are the respective chemical potentials, where µi0 is the standard chemical potential. For co-ions, only adsorption (negative) in the diffuse part of the EDL is considered. The quantitative approach, described above, gives a possibility to calculate surface tension isotherms as a function of surfactant concentration and to determine the surface (excess) concentrations (31) Levine, S.; Robinson, K.; Bell, G. M.; Mingins, J. J. Electroanal. Chem. 1972, 38, 253. (32) Derjaguin, B. V.; Churaev, N. V.; Muller, V. M. Surface Forces, Russian ed.; Nauka: Moskva, 1985.
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Chart 1. Structural Formulas of Multiple Ammonium Salts, (bisAmC12)Cn and (trisAmbisC12)bisCn
of all species in the system as well as the electric potential of the electric double layer. Experimental Section Materials. The bis-ammonium salts, bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]-alkylamine dichloride, and tris-ammonium salts, bis[2-hydroxy-3-dodecyldimethylammonio)propyl]dialkylammmonium trichloride, of the general formulas shown (together with their abbreviations) in Chart 1 were synthesized according to the method described in ref 28. After synthesis, all surfactants were purified by repeated recrystallization and dried thoroughly. Methods. Surface Tension Measurements. The equilibrium surface tension measurements were performed using the Wilhelmy Plate method and the pendant drop shape analysis method. A platinum plate was roasted in a flame before each experiment to remove contaminants from its surface. During the surfactant purification procedure (see below), the surface tension was controlled with the use of the automatic du Nouy ring tensiometer (Lauda TE-1M). All necessary modifications of the measuring procedure for surfactant solutions were applied.33,34 The experimental setup for the dynamic and equilibrium surface tension determination by the pendant drop shape analysis method is described in detail elsewhere.25 The method is based on fitting the solution of the Young-Laplace equation of capillarity to the shape of the pendant drop,35 which is recorded by a digital camera. The measured surface tension value, being the only unknown parameter in that equation, corresponds to the best-fit value. In the experimental setup used, the dynamic surface tension measurements can be carried out every 5 s. The agreement between the results of surface tension measurements performed with purified surfactant solutions with the Wilhelmy plate, du Nouy ring, and pendant drop shape analysis techniques was better than 1mN/m. All surface tension measurements were performed at 295 K. High-Performance Purification. Aqueous solutions of multiple ammonium salts were additionally purified by automatically operating the high-performance purification apparatus developed by Lunkenheimer et al.36 In this technique, the surface is aspirated periodically to remove the comparatively more surface active impurities until the state of “surface chemical” purity is achieved. All necessary precautions were taken to avoid an uncontrolled decrease in surfactant concentration during the purification process. The grade of purity was judged by applying the criterion proposed by Lunkenheimer et al.37,38 The results of dynamic surface tension measurements performed by the pendant drop shape analysis technique served as an additional check of surfactant solution purity. A lack of surface (33) Lunkenheimer, K. Tenside Deterg. 1982, 19, 272. (34) Lunkenheimer, K.; Wantke, K.-D. Colloid Polymer Sci. 1981, 259, 354. (35) Rotenberg, Y.; Boruvka, L.; Neuman, A. W. J. Colloid Interface Sci. 1983, 93, 169. (36) Lunkenheimer, K.; Pergande, H. J.; Kruger, H. ReV. Sci. Instrum. 1987, 58, 2313. (37) Lunkenheimer, K.; Miller, K. J. J. Colloid Interface Sci. 1987, 120, 176. (38) Lunkenheimer, K.; Miller, K. J. Tenside Deterg. 1979, 16, 312.
tension drift over time scales longer than the period needed to attain the adsorption equilibrium by the diffusion of the surfactant to the surface of the pendant drop39 was selected as a criterion for the surfactant purity. ConductiVity and Chloride ActiVity Measurements. The surfactant solution conductivity was measured with an electrical conductivity meter (Elmetron CP-501) with a conductivity cell type E-60 and with platinum electrodes embedded in glass (K ) 1.0 ( 0.2 cm-2). The chloride ion activity was measured with a pH meter (Elmetron CP-501) with a chloride ion specific electrode (Hydromet type ECl01) in conjunction with a reference electrode (Ag, AgCl). First, the calibration curve was obtained by measuring the electromotive force (emf) of the KCl solution in the concentration range 10-5-10-1 M. Next, the solutions of ammonium salts were measured. The solutions of 50 mL of surfactant were freshly prepared and thermostated for 20 min before measurement. The equilibrium values of the emf for ammonium salts were reached after 1-2 min, more slowly than those for KCl solutions. To all solutions (KCl and ammonium salts), 1 mL of stabilizing solution was added before measurement. The stabilizing solution was prepared from 425 g of NaNO3 and 29 mL of CH3COOH dissolved in 1 L of water. All measurements were performed in a thermostated double-walled glass container at 25 °C. Water used in all experiments was double distilled and purified by means of a Millipore (Bedford, MA) Milli-Q purification system.
Results and Discussion Figure 1 presents optimized molecular structures of multiple ammonium salts, (bisAmC12)Cn and (trisAmbisC12)bisCn. Optimization was performed using first AM1 semiempirical quantum mechanical calculations followed by ab initio calculations using a 6-31G(d) basis set. Optimization was performed with the use of the Gaussian 0340 quantum chemical modeling package. In Table 1, some characteristic molecular parameters obtained for the optimized molecular geometries of multiple ammonium salts are presented. The calculation of the degree of hydrophobicity, defined as the logarithm of the compound octanol-water partition coefficient, log P, was carried out using atomic parameters derived by Ghose and co-workers41 using the algorithm contained in the HYPERCHEM 7.5 chemical modeling package.42 It can be seen that the theoretically calculated hydrophobicity for dimeric and trimeric pairs (bisAmC12)C2, (trisAmbisC12)bisC2 and (bisAmC12)C6, (trisAmbisC12)bisC6 is similar, without taking into account electric charges. On the other hand, the cross sections of the hydrophilic parts of all molecules are similar. The distances (39) Adamczyk, Z. Particles at interfaces: Interactions, Deposition, Structure; Academic Press-Elsevier: London, 2006. (40) Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian Inc.: Wallingford, CT, 2004. (41) Viswanadhan, V. N.; Ghose, A. K.; Revankar, G. N.; Robins, R. K. J. Chem. Inf. Comput. Sci. 1989, 29, 163. (42) HYPERCHEM, version 7.5; available from HyperCube Inc., 1115 NW 4th Street, Gainesville, FL 32601.
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Figure 1. Molecular structures of multiple ammonium salts (bisAmC12)Cn and (trisAmbisC12)bisCn. The chloride couterions are not shown. Table 1. Characteristics of the Molecular Structures of the Investigated Surfactants molecular mass volume [nm3] log P cross section of the hydrophilic part [nm2] distance between charged amine groups [nm] radius of gyration [nm]
(bisAmC12)C2
(bisAmC12)C6
586 0.397 7.52 0.64
642 0.441 9.18 0.66
615 0.431 7.39 0.70
727 0.510 10.70 0.85
0.96
0.91
0.55 (1.10)
0.52 (1.04)
1.24
1.26
1.16
1.15
between the charge groups either are fulfilling the Manning criterion for counterion condensation (for trimeric surfactants) or are about to do so (for dimeric surfactants). It has to be additionally considered that the spacer between the charged amine groups has some flexibility, so in a medium of high dielectric constant (water) the distance between them will be smaller as a result of diminished repulsion between charged groups. The dynamic surface tension measurements were performed in a wide range of concentrations for all investigated surfactants. Examples (for (trisAmbisC12)bisC6) of typical results of dynamic surface tension measurements for investigated bis- and tris-
Figure 2. Dependence of the dynamic surface tension γ of (trisAmbisC12)bisC6 solutions on the time t elapsing from the drop formation (determined by the drop shape analysis method) at various bulk surfactant concentrations, i.e., (1) 10-5 M, (2) 3 × 10-5 M, (3) 8 × 10-5 M, (4) 2 × 10-4 M, and (5) 3 × 10 -4 M.
(trisAmbisC12)bisC2
(trisAmbisC12)bisC6
ammonium salts are shown in Figure 2. As it can be seen, after a certain period needed for the diffusion of ammonium salts to the interface,39 the adsorption equilibrium is established. The equilibrium surface tension was obtained from all performed measurements by taking the surface tension value in the limit of long times. As it was pointed out above, the lack of a long time drift of the surface tension was considered as a proof of surfactant purity. Figure 3 presents a comparison of the dependence of the equilibrium surface tension on the surfactant concentration in a solution for the simple cationic surfactant having 12 carbon atoms in the hydrocarbon chain, dodecyldimethylammonium chloride (DTACl)27, and the dimeric (bisAmC12)Cn and trimeric (trisAmbisC12)bisCn surfactants.
Figure 3. Dependence of the surface tension on the concentration of surfactant for (1) (bisAmC12)C2 2Cl-, (1′) (bisAmC12)C6 2Cl-, (2) (trisAmbisC12)bisC2 3Cl-, (2′) (trisAmbisC12)bisC6 3Cl-, and (3) DTACl.
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Figure 4. Experimental adsorption isotherm of (bisAmC12)C2 (points) together with theoretical isotherms calculated for hypothetical monovalent, divalent, and trivalent surfactant salts with the same molecular structure as (bisAmC12)C2, i.e., containing 24 carbon atoms in the hydrophobic chain.
Approximately 2 orders of magnitude difference in the surface activity between simple and multiple cationic surfactants corresponds to the observation of Zana.5 Multiple salts having hexyl chains in the spacer are more surface active (more hydrophobic) in accordance with the data presented in Table 1. On the other hand, the similar surface activities of dimeric (divalent) and trimeric (trivalent) surface active ammonium salts seems puzzling. Since the adsorption of ionic surfactants is strongly influenced by their charge,25-27,43,44 it can be argued that, contrary to the experimental results, there should be a significant difference between the surface activities of divalent and trivalent surfactants. It is illustrated in Figure 4 where we present the experimental isotherm of (bisAmC12)C2 together with the surface tension isotherms calculated for hypothetical gemini surfactants with the same molecular structure as (bisAmC12)C2 (with salt but with various electric charges of the hydrophilic part of the molecule: +e, +2e, and +3e). The calculations were performed by means of the STDE model of ionic surfactant adsorption using parameters extrapolated from those obtained previously for surfactants with a smaller number of carbon atoms in the hydrophobic part.27 It is demonstrated that due to electric charge the surface activity of divalent salts should be ∼2 orders of magnitude lower than that of the monovalent ones. The surface activity of trivalent gemini salts is the lowest, at least 1 order of magnitude lower than that for divalent ones. As it can be seen in Figure 4., the experimental surface tension isotherm of (bisAmC12)C2 falls in between the theoretical isotherms for mono- and divalent surfactant salts. Therefore, it may suggest that, in the case of multiple ionic surfactants, both bare surfactant ions and associates with chloride counterions are present in the solution. The presence of higher surface active associates shifts the region of surface activity toward lower surfactant concentrations. Also, the results presented in Table 1 concerning the distance between charged amine groups indicate the possibility of surfactant-counterion associates due to the phenomenon of counterion condensation predicted by Manning.17 The concept of counterion binding to dimeric surfactants was used by Li et al.15 to reconcile the results obtained from surface tension measurements with those obtained by means of neutron reflectivity. Nevertheless, the nominal ionic strength of the tris-ammonium salt should be over 2 times higher than the one of the (43) Okuda, H.; Imae, T.; Ikeda, S. Colloids Surf. 1987, 27, 187. (44) Fainerman, V. B. Colloids Surf. 1991, 57, 249.
Figure 5. Dependence of the electric specific conductivity on the concentration of solution of multiple ammonium salts curves. (top) Gemini with alkyl chain C2 in the spacer: (1, circles) bis-ammonium salt and (2, triangles) tris-ammoium salt. (bottom) Gemini with alkyl chain C6 in the spacer: (1′, circles) bis-ammonium salt and (2′, triangles) tris-ammoium salt.
bis-ammonium salt at the same bulk concentration; the results of conductivity measurements shown in Figure 5 are rather ambiguous and cannot be used as proof of the formation of surfactant-counterion complexes. Therefore, to prove the hypothesis of the formation of surfactant-counterion associates, we measured the concentration of free chloride ions in surfactant solutions using a Cl- selective electrode. The results of measurements obtained in the case of dimeric and trimeric surfactant salts are presented in Figure 6. The dotted lines in Figure 6 represent the chloride concentration for the case when all surfactant molecules were fully dissociated, forming divalent or trivalent surfactant cations accompanied by the corresponding number of chloride anions. We can notice that, at higher surfactant concentrations but still lower than the cmc’s for the investigated surfactants, the concentration of Clions in the solution is lower than predicted for the fully charged surfactant molecules and the respective number of chloride counterions. That indicates the formation of surfactant-chloride associates. However, this difference in the case of (bisAmC12)C2 is small, at the level of experimental error, and it can be easily overlooked in conductivity measurements. The lack of significant differences between the bis and tris forms of the surfactants can be also explained by the protonation of amine groups in the spacer of the bis surfactants. However,
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Langmuir, Vol. 24, No. 7, 2008 3177
Figure 6. Concentration of chloride ions in the solution of a dimeric and trimeric surfactant: (A) (bisAmC12)C2, (B) (bisAmC12)C6, (C) (trisAmbisC12)bisC2, and (D) (trisAmbisC12)bisC6, as a function of surfactant contents. Symbols denote experimental data, dotted lines denote chloride ion concentration for fully dissociated surfactants, and solid lines denote chloride ion concentration calculated from the mass action law taking into account the formation of surfactant-chloride associates.
as it was shown in Figure 4. the trivalent (with the effective charge +3e) salts should be much less surface active than the mono- or divalent forms. Moreover, a clear decrease in the chloride concentration with increasing concentration of Gemini surfactants, demonstrated in Figure 5, which is more pronounced for tris surfactants, favors the idea of counterion condensation. Even if protonation occurs, its effect is counterbalanced by surfactant complex formation. To additionally verify the role of protonation of bis surfactants, we performed measurements of surface tension of (bisAmC12)C2 and (trisAmbisC12)bisC2 at pH ) 11, regulated by adjusting the concentration of KOH (results not presented here). However, increasing pH of the solution, we simultaneously increased the electrolyte concentration. Therefore, the charges of the hydrophilic groups were more screened and the surfactants’ surface activities increased.27 The screening of surfactant charges could not be directly separated from the effect of deprotonation (which also decreases the charge), so additional experiments were performed in a KCl solution of the same ionic strength as the KOH solution used to adjust the pH. The results of these two experiments showed that addition of the same concentration of KOH and KCl resulted in a similar shift of surface activity. The differences between isotherms for bis and tris surfactants in the range of 10-5-10-4 M were slightly larger in KOH than in pure surfactant solutions but equal to the differences observed between isotherms obtained for KCl. In our opinion, that proves that protonation of bis surfactants cannot by its own explain the lack of differences between the surface activities of bis and tris gemini surfactants. As it was suggested in ref 5, we applied the mass action law to calculate the concentrations of surfactant ions, chloride
counterions, and surfactant-counterion associates:
K1 )
cS2+Cl, cS2+cCl-
c ) cS2+ + cS2+Cl-,
2c ) cCl- + cS2+Cl(12)
for dimeric surfactants and
K1 )
cS3+Cl, cS3+cCl-
K2 )
cS3+Cl2, cS3+Cl-cCl-
c ) cS3+ + cS3+Cl- + cS3+Cl2-, 3c ) cCl- + cS3+Cl- + 2cS3+Cl2- (13) for trimeric surfactants. K1 and K2 are the respective association constants, c is the total surfactant concentration, cS2+ and cS3+ are the concentrations of fully dissociated dimeric and trimeric surfactant salts, respectively, cS2+Cl-, cS3+Cl-, and cS3+Cl2- are concentrations of surfactant-counterions associates, and cCl- is the concentration of free chloride ions in the solution. The solid lines in Figure 6 represent the dependence of the concentration of free chloride ions in the investigated surfactant solution as a function of total surfactant concentration calculated with the use of eqs 12 and 13. The values of the association constants used for these calculations are gathered in Table 2. To account for the effect of the formation of surfactantcounterion associates in surface active dimeric and trimeric ammonium salt solutions on their interfacial properties, we calculated the dependence of the surface tension on the surfactant concentration using the STDE model of adsorption. The model was adapted to take into account the presence of two (for dimeric)
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Wegrzyn´ ska et al.
Table 2. Values of Association Constants for the Formation of Surfactant-Chloride Associates association constants [m-3] K1 K2
(bisAmC12)C2
(bisAmC12)C6
(trisAmbisC12)bisC2
(trisAmbisC12)bisC6
0.08
0.16
0.3 0.08
0.3 0.07
Table 3. Best-Fit Parameters for the Description of Surface Tension Isotherms of Bis- and Tris-Ammonium Salts in Terms of the STDE Model parameter
(bisAmC12)C2
(bisAmC12)C6
(trisAmbisC12)bisC2
(trisAmbisC12)bisC6
Rs [mol/dm ] ΓS∞ [mol/cm2] Hs [kJ/mol] gClRCl- [mol/dm3] asCl- [nm] δ ) as [nm] s
(2.2 ( 0.1) × 10 (1.6 ( 0.1) × 10-10 0.1 ( 0.1 0.18 ( 0.02 17 000 0.33 0.45 26
(1.3 ( 0.1) × (1.4 ( 0.1) × 10-10 0.3 ( 0.1 0.15 ( 0.02 17 000 0.33 0.45 26
(1.1 ( 0.1) × (1.5 ( 0.2) × 10-10 0.5 ( 0.1 0.16 ( 0.02 17 000 0.33 0.45 26
(3.2 ( 0.3) × 10-13 (1.35 ( 0.1) × 10-10 0.6 ( 0.1 0.15 ( 0.02 17 000 0.33 0.45 26
3
-11
or three (for trimeric) surface active cations. The relative content of fully dissociated surfactant ions and surfactant-chloride associates in the whole range of surfactant concentrations studied was calculated according to the mass action law, and using the association constants given in Table 2. The results of our calculations are shown in Figure 7. The values of the STDE model parameters concerning the surfactant ions, which correspond to the best fit of the theoretical model to the experimental data, are collected in Table 3. The model parameters for chloride ions and the dielectric properties of the surface layer in the presence of a surfactant containing 12 carbon atoms in the
Figure 7. Dependence of surface tension vs logarithm of surfactant concentration for multiple ammonium salts. Curves 1 and 1′ are bis-ammonium salts, curves 2 and 2′ are tris-ammoium salts, with alkyl chains C2 (curves 1 and 2) and C6 (curves 1′ and 2′). Solid lines denote fits of the theoretical STDE model of ionic surfactant adsorption, which accounts for the formation of multivalent surfactant-counterion associates.
10-12
10-11
hydrocarbon chain were assumed to be the same as those in our previous works.27,45 As it can be seen, combining the mass action law for the description of multivalent ionic surfactant-counterion associate formation with the STDE model of ionic surfactant adsorption can provide a good description of the surface behavior of surface active multiple ammonium salts. As it was demonstrated in Figure 4, the multivalent surfactant ions are much less surface active than their monovalent equivalents with the same molecular structure. This results from the electrostatic repulsion between the electric charges accumulated in the surface layer due to surfactant adsorption and that of surfactant ions in the solution. That repulsion is much stronger for multivalent surfactants (see eq 3). Therefore, the presence of even a small number of surfactant-counterion associates, for which the electric charge is reduced in comparison with that of fully dissociated surfactant ions, increases the surface activity of the surfactant solution. The relative concentrations (normalized to total surfactant concentrations) of fully dissociated surfactant ions and surfactant-chloride counterion associates, calculated according to eqs 12 and 13, for all investigated surfactant solutions are shown in Figure 8. It can be seen that the relative amount of monovalent associates Surf2+Cl- in the dimeric surfactant solutions at a concentration close to the cmc does not exceed 0.2 for (bisAmC12)C2 and 0.3 for (bisAmC12)C6. For trimeric ammonium salts, there are two types of associates: divalent Surf3+Cl- and monovalent Surf3+Cl-2. The results of our calculations indicate that, in the case of both trimeric surfactants, (trisAmbisC12)C2 and (trisAmbisC12)C6, at a concentration close to the cmc, about half of the surfactant in solution is present in the form of surfactantchloride complexes. Taking into account the large difference in surface activity between fully dissociated trimeric surfactant molecules and their associates, one can completely neglect the presence of fully dissociated surfactant ions during the calculation of the surface properties of (trisAmbisC12)C2 and (trisAmbisC12)C6 solutions. On the other hand, the ratio of the divalent and monovalent associates in the trimeric ammonium salt solutions is very similar the one in solutions of the respective dimeric surfactants (bisAmC12)C2 and (bisAmC12)C6. It leads to a very similar surface activity of dimeric (divalent) and trimeric (trivalent) surface active ammonium salts, as demonstrated in Figure 3. The STDE model of ionic surfactant adsorption allows the calculation of the surface (excess) concentration of surfactant (45) Para, G.; Warszyn´ski, P. Colloids Surf., A 2007, 300, 346.
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Figure 8. (a) Relative amounts of divalent surfactant ions, Cl- ions, and surfactant-chloride associates as a function of the total surfactant concentration for (left) (bisAmC12)C2 and (right) (bisAmC12)C6 solutions. (b) Relative amounts of divalent surfactant ions, Cl-, ions and surfactant-chloride associates as a function of the total surfactant concentration for (left) (trisAmbisC12)C2 and (right) (trisAmbisC12)C6 solutions.
ions and counterions in both the Stern and diffuse parts of the electric double layer. Therefore, it is possible to calculate the apparent Gibbs factor using the following relation:
where all surface active ions, irrespective of the degree of their association with counterions, are taken into account. The dependence of the Gibbs factor calculated using the STDE model of adsorption on the surfactant concentration for all dimeric and trimeric surfactants studied is shown in Figure 9.
As it can be seen in Figure 9, for low surfactant concentrations, when the degree of association is negligible, the apparent Gibbs factor calculated from the STDE model of ionic surfactant adsorption is in agreement with its value resulting from thermodynamics, that is, electroneutrality condition of the surface layer, and is equal to 3 for the divalent surfactant ions and to 4 for the trivalent surfactant ions. However, for higher surfactant concentrations, when the surfactant-counterion association begins to be significant, the value of the Gibbs factor deviates from its thermodynamic value. Therefore, any attempt to calculate the surface concentration of a multiple ionic surfactant from the slope of the surface tension isotherm, using solely the nominal valency of surfactant ions or any constant value of the Gibbs factor, may lead to a significant error.
Figure 9. Dependence of the apparent Gibbs factor on the concentration of the surfactant for (1) (bisAmC12)C2 2Cl-, (1′) (bisAmC12)C6 2Cl-, (2) (trisAmbisC12)bisC2 3Cl-, and (2′) (trisAmbisC12)bisC6 3Cl-.
Multiple quaternary ammonium salts are potential compounds for the next generation of surfactants; therefore, research on the interfacial properties of dimeric and oligomeric ionic surfactants is scientifically challenging and important from a practical point of view. We performed experimental investigations of the interfacial behavior of “surface chemically pure” aqueous solutions of bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]alkylamine dichlorides and bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]dialkylammmonium trichlorides. Our surface tension measurements indicate a 2 orders of magnitude difference in the surface activities between the studied multiple cationic surfactants and their simple single chain equivalent, DTAC, in accordance with earlier observations. According to the theoretical prediction, the adsorption of ionic surfactants should be strongly influenced by their charge, due to the electrostatic repulsion between the surface charge of the adsorbed surfactant molecules
RT
∑
( ) dγ
1
n)-
Γi
d ln c
(14) fitted
surfactant ions
Conclusions
3180 Langmuir, Vol. 24, No. 7, 2008
and the ones in the solution. That is why there should be a significant difference between the surface activities of divalent and trivalent surfactants. In contrast to the theoretical predictions, similar surface activities of dimeric (divalent) and trimeric (trivalent) surface active ammonium salts was observed in our experiments. Since the surface tension experiments were performed with the use of carefully purified surfactant solutions, the effect of impurities could be eliminated. Our experimental findings can be explained by the formation of surfactantcounterion complexes of an effective charge lower than the nominal one, assuming full dissociation of the surface active multivalent salts. Partial neutralization of the surfactant charge leads to lowering of the electrostatic barrier for adsorption at the solution interface and consequently to a higher apparent surface activity of the surfactant solution. We used the previously developed STDE model of ionic surfactant adsorption based on the assumption that the surfactant ions and counterions undergo nonequivalent adsorption within the Stern layer at the air/solution interface. The model was modified to describe the surface tension isotherms of aqueous solutions of multivalent cationic surfactants. Additionally, the model was adapted to take into account the presence of two (for dimeric) or three (for trimeric) surface active cations. The relative ratio of surfactant ions and their associates with counterions was calculated using the mass action law. We found that combining the mass action law for the description of multivalent ionic surfactant-counterion associate formation with the STDE model of ionic surfactant adsorption can provide a good description of the surface behavior of the investigated surface active multiple ammonium salts. The calculated surface tension isotherms described well our experimental results assuming, at surfactant concentration close to the cmc, the presence of 10-20% Surf2+Classociates in solutions of bis-ammonium salts and ∼50% Surf3+Cl- and 10% Surf3+Cl2- associates in solutions of trisammonium salts. For lower surfactant concentrations, the amount of associates in the solution decreases, and for concentrations below 10-5 M only fully dissociated surfactant ions are present in the solutions. These results correspond to the measurements of the concentration of free chloride in the solutions performed with a chloride specific electrode. It can be concluded that the high efficiency in reducing the water surface (interfacial) tension by multiple ionic surfactants is due to the effect of counterion condensation. Otherwise, the effect of the double hydrophobic chain leading to an increase in the surface activity of multiple
Wegrzyn´ ska et al.
surfactants would be counterbalanced by the presence of an electrostatic barrier for the adsorption of multiple charged surfactant molecules. Since the STDE model of ionic surfactant adsorption allows calculation of the surface (excess) concentration of surfactant ions, it was possible to calculate the numerical factor (Gibbs factor) in the Gibbs adsorption equation. This equation is frequently used to directly determine the amount of adsorbed surfactant from the slope of the dependence of surface tension versus the logarithm of surfactant concentration. We found that, for a low surfactant concentration, when the degree of association is negligible, the apparent Gibbs factor calculated from the STDE model of ionic surfactant adsorption corresponds with its value resulting from the thermodynamics, that is, electroneutrality condition of the surface layer. This value is equal to 3 for divalent surfactant ions and to 4 for trivalent surfactant ions. However, for higher surfactant concentrations, when the surfactantcounterion association begins to be significant, the value of the Gibbs factor deviates from its thermodynamic value. This means that any attempt of direct calculation of the amount of the adsorbed multiple ionic surfactant, on the basis of the Gibbs adsorption equation, with the constant Gibbs numerical factor respective for the nominal charge of the surfactant and without knowing the degree of the surfactant-counterion association, may lead to erroneous results. Therefore, the degree of counterion association in the multiple ionic surfactant solution may be the main reason for the discrepancies, reported in the literature, between the results obtained by surface tension, neutron reflectivity, and electric conductivity measurements. Additionally, we pointed out that the measurement of a free surfactant counterion concentration (chloride ions in our case) provides valuable information about the counterion condensation. This method should be preferred over the conductivity measurements when comparing the surface tension and neutron reflectivity results concerning the numerical factor in the Gibbs adsorption equation applied for the adsorption of multiple ionic surfactants. Acknowledgment. The cooperation between Wrocław University of Technology and Institute of Catalysis and Surface Chemistry was possible in the framework of the Scientific Network “Surfactants and Dispersed System in Theory and Practice-SURUZ” (Contract No. INCO-CT-2003-003355). LA702619A
