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Investigations of First Adsorption Step of Cationic Dimeric (Gemini) Surfactants onto Silica Surfaces by Analytical and Calorimetric Methods
Journal of Colloid and Interface Science 243, 525–527 (2001) doi:10.1006/jcis.2001.7943, available online at http://www.idealibrary.com on
LETTER TO THE EDITOR Investigations of First Adsorption Step of Cationic Dimeric (Gemini) Surfactants onto Silica Surfaces by Analytical and Calorimetric Methods This paper reports new results on the adsorption of cationic dimeric surfactants (12–s–12 surfactants, where s is the carbon number of the polymethylene spacer) on adsorbents with different surface functions, namely raw and HCl-treated silica. These results were obtained by traditional methods (adsorption isotherms, electrophoretic mobility, and chemical analysis of the equilibrated supernatant) and microcalorimetry. The results showed that the stoichiometry of the first step of the adsorption (ion-exchange step) varies strongly with the spacer carbon number. The binding of one surfactant to the surface brings about the release of between 1 and 2 sodium ions as the spacer carbon number is increased from 2 to 10. Thus the surfactant binding to the surface involves one head group for the 12–2–12 surfactant (short spacer) and two head groups for the 12–10–12 surfactant (long spacer). These results suggest the use of dimeric surfactants as molecular rulers to study the distribution of charged sites on surfaces. The microcalorimetric experiments clearly showed the two adsorption steps. The ion-exchange step gives rise to an endothermal effect having an amplitude that depends strongly on the spacer carbon number. The second adsorption step associated with the formation of surfactant aggregates gives rise to an exothermal effect that also depends on s. °C 2001 Academic Press
In previous studies we have investigated the adsorption of cationic dimeric surfactants 12-s-12 (alkanediyl-α,ω-bis(dodecyldimethylammonium bromide); s = carbon number of the alkanediyl spacer) on two adsorbents with different surface chemical functions, namely raw and HCl-treated silica (1–3). The adsorption was studied using traditional techniques (adsorption isotherms, electrophoretic mobility). The experiments were performed using free systems; that is, the potential of the solid surface and the ionic strength of the surfactant solution were not maintained constant by fixing the pH of the supernatant or adding a swamping electrolyte. These studies did not permit a complete analysis of the adsorption process and of the evolution of the system. Microcalorimetry was used to investigate the thermodynamics of the adsorption (1). The results obtained in these studies showed that the adsorption of dimeric cationic surfactants on silica occurs in two steps, just like the adsorption of conventional surfactants on the same surface (3–7). This letter reports new results on the adsorption of 12-s-12 dimeric surfactants onto raw and HCl-treated silica (SiNa and SiH, respectively). The study was performed using a new experimental approach that combines monitoring of the adsorption by chemical analysis of the equilibrated supernatant, traditional physicochemical measurements, and microcalorimetric measurements. This approach allows a more complete description of the adsorption. The materials (surfactants, silica) and experimental methods have been previously described (1–3).
Chemical analysis of the equilibrated supernatant yielded the concentrations of free sodium ion and bromide ion during the course of the adsorption. The results of these measurements are represented in Fig. 1. They show that during the ion-exchange step, the short spacer dimeric surfactant 12–2–12 adsorbs by only one charged head group onto one charged site on the surface and that this adsorption is accompanied by the release of one sodium ion from the surface. Indeed the measurements indicate that for each adsorbed 12–2–12 dimeric surfactant ion the equilibrated supernatant contains one free bromide ion and one sodium ion released from the surface. One bromide ion thus accompanies the dimeric surfactant ion to the surface. Nevertheless, this surface-bound bromide ion should not be viewed as ion-pairing the free surfactant head group. It is there to locally neutralize the particle charge. Figure 2 gives a schematic representation of the adsorbed 12–2–12 dimeric ion. In contrast, during the ion-exchange step, the adsorption of a dimeric surfactant with a long and flexible spacer (s ≥ 10) involves its two head groups. Indeed, the equilibrated supernatant was found to contain two bromide ions and two released sodium ions for each adsorbed surfactant ion. Figures 3 and 4 illustrate this situation for the 12–10–12 surfactant. The results in Figs. 1–4 suggest the use of dimeric surfactants as molecular rulers to study the distribution of distances between charged sites on surfaces. After exhausting the adsorption sites present on the surface a second adsorption step occurs that involves hydrophobic interactions between alkyl chains of adsorbed surfactants. The adsorption process goes on till saturation of the surface. Dimeric surfactants with spacer carbon number in the range 4 ≤ s ≤ 8 showed intermediate behavior because of the heterogeneity in the site distribution. The thermodynamics of adsorption was investigated for the same systems using microcalorimetry. The results obtained at 35◦ C are represented in Figs. 5 and 6. The variation of the differential molar enthalpy of adsorption (1ads h) with the amount of adsorbed surfactant is complex, revealing that the adsorption process is composite (1). The first adsorption step (ion exchange), which only involves electrostatic interactions between surfactant cations and negative sites of the silica surface, is strongly influenced by the spacer length. Indeed, the value of the extremum of the enthalpic curve at the end of this process depends on s. For s = 2, the value of the extremum is positive and decreases when s increases, to reach more and more negative values. Therefore, resulting effects are endothermic for s = 2 and 4, and becomes increasingly exothermic when s increases. Indeed, the enthalpic effect in this first step results from two phenomena: 1. the destructuration of the interfacial water, which gives rise to an endothermic effect (11–13); 2. the exchange sodium ion/dimeric surfactant ion, which involves the release of sodium ions from the surface and their hydration (exothermic effect) (11–12) and a partial dehydration of bromide counterions binding at the interface (endothermic effect) (11–12). This last effect depends on the spacer length and on the stoichiometry of the exchange sodium ion/dimeric surfactant ion. For dimeric surfactants with a long spacer (s ≥ 10), there is no counterion binding and the endothermic effect disappears.
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0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
526
LETTER TO THE EDITOR
FIG. 4. Schematic representation of the adsorption of a 12–10–12 dimeric surfactant onto silica during the ion-exchange adsorption step. FIG. 1. Variation of the concentrations of free bromide ions (r) and free sodium ions (j) in the equilibrated supernatant of the 12–2–12/SiNa system with the amount of adsorbed surfactant.
FIG. 2. Schematic representation of the adsorption of a 12–2–12 dimeric surfactant onto silica during the ion-exchange adsorption step.
FIG. 5. Variation of the differential molar enthalpies of adsorption of 12–2– 12 (r), 12–6–12 (j), and 12–10–12 (d) onto raw silica (SiNa) with the amount of adsorbed surfactant at 35◦ C.
FIG. 3. Variation of the concentrations of free bromide ions (r) and free sodium ions (j) in the equilibrated supernatant of a 12–10–12/SiNa system with the amount of adsorbed surfactant.
FIG. 6. Variation of the differential molar enthalpies of adsorption of 12– 2–12 (r) and 12–8–12 (j) onto HCl-treated silica (SiH) with the amount of adsorbed surfactant at 35◦ C.
LETTER TO THE EDITOR The comparative study of the adsorption onto SiNa and SiH surfaces shows that the global energetic balance of the first step is more exothermic for SiH systems (Figs. 5 and 6). This effect is not due to the adsorption mechanism since the cations exchanged are identical (1). It is due to chemical treatment of the SiNa surface, which brings about a strong decrease in the number of sodium ions bound to the surface (1, 2). The protonation of the silica surface results in a more labile organization of the interfacial water and its destructuration becomes less endothermic (11, 12). Analytic measurements in the bulk phase permitted the determination of the stoichiometry during the cation-exchange step and the quantification of the number of exchanged sites. The results suggest the use of dimeric surfactants with different spacer lengths as “molecular rulers” in order to obtain information on the distribution of distances between sites at the surface. Considering each site of desorption as a nucleation center strengthens the hypothesis about the different types of solloids that may be formed at the surface (1). The results obtained show that the chemical state of the surface and the density of adsorption sites determine the shape of the formed solloids. A more detailed and complete account of the results will be given in a forthcoming paper.
REFERENCES 1. Grosmaire, L., Chorro, M., Chorro, C., and Partyka, S., J. Colloid Interface Sci. 242, 395 (2001). 2. Chorro, C., Chorro, M., Dolladille, O., Partyka, S., and Zana, R., J. Colloid Interface Sci. 199, 169 (1998). 3. Chorro, M., Chorro, C., Dolladille, O., Partyka, S., and Zana, R., J. Colloid Interface Sci. 210, 134 (1999). 4. Fan, A., Somasundaran, P., and Turro, N. J., Langmuir 13, 506 (1997). 5. Zajac, J., and Partyka, S., in “Adsorption on New and Modified Inorganic Sorbents” (A. Dabrowski and V. A. Tertykh, Eds.), Chap. 3.6. Elsevier, Amsterdam, 1996.
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6. Somasundaran, P., and Fuerstenau, D. W., J. Phys. Chem. 70, 90 (1966). 7. Birjsterbosch, B. H., J. Colloid Interface Sci. 47, 186 (1974). 8. Wangnerud, P., and J¨onsson, B., Langmuir 10, 3542 (1994). 9. B¨ohmer, M. R., and Koopal, L. K., Langmuir 8, 2649 (1992); 8, 2660 (1992). 10. Yeskie, M. A., and Harwell, J. H., J. Phys. Chem. 93, 3372 (1989). 11. Samoilov, S., J. Struct. Chem. 3, 314 (1962). 12. Halliwel, R., and Nyburg, J., Trans. Faraday Soc. 59, 1126 (1963). 13. Tanford, C., “The Hydrophobic Effect.” Wiley, New York, 1973. L. Grosmaire C. Chorro M. Chorro S. Partyka1 R. Zana∗ LAMMI University of Montpellier II Place E. Bataillon 34095 Montpellier Cedex, France ∗ ICS (CNRS, ULP) 6 Rue Boussingault 67083 Strasbourg Cedex, France Received July 18, 2001; accepted August 30, 2001
1 To whom correspondence should be addressed. E-mail: [email protected].
LETTER TO THE EDITOR Investigations of First Adsorption Step of Cationic Dimeric (Gemini) Surfactants onto Silica Surfaces by Analytical and Calorimetric Methods This paper reports new results on the adsorption of cationic dimeric surfactants (12–s–12 surfactants, where s is the carbon number of the polymethylene spacer) on adsorbents with different surface functions, namely raw and HCl-treated silica. These results were obtained by traditional methods (adsorption isotherms, electrophoretic mobility, and chemical analysis of the equilibrated supernatant) and microcalorimetry. The results showed that the stoichiometry of the first step of the adsorption (ion-exchange step) varies strongly with the spacer carbon number. The binding of one surfactant to the surface brings about the release of between 1 and 2 sodium ions as the spacer carbon number is increased from 2 to 10. Thus the surfactant binding to the surface involves one head group for the 12–2–12 surfactant (short spacer) and two head groups for the 12–10–12 surfactant (long spacer). These results suggest the use of dimeric surfactants as molecular rulers to study the distribution of charged sites on surfaces. The microcalorimetric experiments clearly showed the two adsorption steps. The ion-exchange step gives rise to an endothermal effect having an amplitude that depends strongly on the spacer carbon number. The second adsorption step associated with the formation of surfactant aggregates gives rise to an exothermal effect that also depends on s. °C 2001 Academic Press
In previous studies we have investigated the adsorption of cationic dimeric surfactants 12-s-12 (alkanediyl-α,ω-bis(dodecyldimethylammonium bromide); s = carbon number of the alkanediyl spacer) on two adsorbents with different surface chemical functions, namely raw and HCl-treated silica (1–3). The adsorption was studied using traditional techniques (adsorption isotherms, electrophoretic mobility). The experiments were performed using free systems; that is, the potential of the solid surface and the ionic strength of the surfactant solution were not maintained constant by fixing the pH of the supernatant or adding a swamping electrolyte. These studies did not permit a complete analysis of the adsorption process and of the evolution of the system. Microcalorimetry was used to investigate the thermodynamics of the adsorption (1). The results obtained in these studies showed that the adsorption of dimeric cationic surfactants on silica occurs in two steps, just like the adsorption of conventional surfactants on the same surface (3–7). This letter reports new results on the adsorption of 12-s-12 dimeric surfactants onto raw and HCl-treated silica (SiNa and SiH, respectively). The study was performed using a new experimental approach that combines monitoring of the adsorption by chemical analysis of the equilibrated supernatant, traditional physicochemical measurements, and microcalorimetric measurements. This approach allows a more complete description of the adsorption. The materials (surfactants, silica) and experimental methods have been previously described (1–3).
Chemical analysis of the equilibrated supernatant yielded the concentrations of free sodium ion and bromide ion during the course of the adsorption. The results of these measurements are represented in Fig. 1. They show that during the ion-exchange step, the short spacer dimeric surfactant 12–2–12 adsorbs by only one charged head group onto one charged site on the surface and that this adsorption is accompanied by the release of one sodium ion from the surface. Indeed the measurements indicate that for each adsorbed 12–2–12 dimeric surfactant ion the equilibrated supernatant contains one free bromide ion and one sodium ion released from the surface. One bromide ion thus accompanies the dimeric surfactant ion to the surface. Nevertheless, this surface-bound bromide ion should not be viewed as ion-pairing the free surfactant head group. It is there to locally neutralize the particle charge. Figure 2 gives a schematic representation of the adsorbed 12–2–12 dimeric ion. In contrast, during the ion-exchange step, the adsorption of a dimeric surfactant with a long and flexible spacer (s ≥ 10) involves its two head groups. Indeed, the equilibrated supernatant was found to contain two bromide ions and two released sodium ions for each adsorbed surfactant ion. Figures 3 and 4 illustrate this situation for the 12–10–12 surfactant. The results in Figs. 1–4 suggest the use of dimeric surfactants as molecular rulers to study the distribution of distances between charged sites on surfaces. After exhausting the adsorption sites present on the surface a second adsorption step occurs that involves hydrophobic interactions between alkyl chains of adsorbed surfactants. The adsorption process goes on till saturation of the surface. Dimeric surfactants with spacer carbon number in the range 4 ≤ s ≤ 8 showed intermediate behavior because of the heterogeneity in the site distribution. The thermodynamics of adsorption was investigated for the same systems using microcalorimetry. The results obtained at 35◦ C are represented in Figs. 5 and 6. The variation of the differential molar enthalpy of adsorption (1ads h) with the amount of adsorbed surfactant is complex, revealing that the adsorption process is composite (1). The first adsorption step (ion exchange), which only involves electrostatic interactions between surfactant cations and negative sites of the silica surface, is strongly influenced by the spacer length. Indeed, the value of the extremum of the enthalpic curve at the end of this process depends on s. For s = 2, the value of the extremum is positive and decreases when s increases, to reach more and more negative values. Therefore, resulting effects are endothermic for s = 2 and 4, and becomes increasingly exothermic when s increases. Indeed, the enthalpic effect in this first step results from two phenomena: 1. the destructuration of the interfacial water, which gives rise to an endothermic effect (11–13); 2. the exchange sodium ion/dimeric surfactant ion, which involves the release of sodium ions from the surface and their hydration (exothermic effect) (11–12) and a partial dehydration of bromide counterions binding at the interface (endothermic effect) (11–12). This last effect depends on the spacer length and on the stoichiometry of the exchange sodium ion/dimeric surfactant ion. For dimeric surfactants with a long spacer (s ≥ 10), there is no counterion binding and the endothermic effect disappears.
525
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
526
LETTER TO THE EDITOR
FIG. 4. Schematic representation of the adsorption of a 12–10–12 dimeric surfactant onto silica during the ion-exchange adsorption step. FIG. 1. Variation of the concentrations of free bromide ions (r) and free sodium ions (j) in the equilibrated supernatant of the 12–2–12/SiNa system with the amount of adsorbed surfactant.
FIG. 2. Schematic representation of the adsorption of a 12–2–12 dimeric surfactant onto silica during the ion-exchange adsorption step.
FIG. 5. Variation of the differential molar enthalpies of adsorption of 12–2– 12 (r), 12–6–12 (j), and 12–10–12 (d) onto raw silica (SiNa) with the amount of adsorbed surfactant at 35◦ C.
FIG. 3. Variation of the concentrations of free bromide ions (r) and free sodium ions (j) in the equilibrated supernatant of a 12–10–12/SiNa system with the amount of adsorbed surfactant.
FIG. 6. Variation of the differential molar enthalpies of adsorption of 12– 2–12 (r) and 12–8–12 (j) onto HCl-treated silica (SiH) with the amount of adsorbed surfactant at 35◦ C.
LETTER TO THE EDITOR The comparative study of the adsorption onto SiNa and SiH surfaces shows that the global energetic balance of the first step is more exothermic for SiH systems (Figs. 5 and 6). This effect is not due to the adsorption mechanism since the cations exchanged are identical (1). It is due to chemical treatment of the SiNa surface, which brings about a strong decrease in the number of sodium ions bound to the surface (1, 2). The protonation of the silica surface results in a more labile organization of the interfacial water and its destructuration becomes less endothermic (11, 12). Analytic measurements in the bulk phase permitted the determination of the stoichiometry during the cation-exchange step and the quantification of the number of exchanged sites. The results suggest the use of dimeric surfactants with different spacer lengths as “molecular rulers” in order to obtain information on the distribution of distances between sites at the surface. Considering each site of desorption as a nucleation center strengthens the hypothesis about the different types of solloids that may be formed at the surface (1). The results obtained show that the chemical state of the surface and the density of adsorption sites determine the shape of the formed solloids. A more detailed and complete account of the results will be given in a forthcoming paper.
REFERENCES 1. Grosmaire, L., Chorro, M., Chorro, C., and Partyka, S., J. Colloid Interface Sci. 242, 395 (2001). 2. Chorro, C., Chorro, M., Dolladille, O., Partyka, S., and Zana, R., J. Colloid Interface Sci. 199, 169 (1998). 3. Chorro, M., Chorro, C., Dolladille, O., Partyka, S., and Zana, R., J. Colloid Interface Sci. 210, 134 (1999). 4. Fan, A., Somasundaran, P., and Turro, N. J., Langmuir 13, 506 (1997). 5. Zajac, J., and Partyka, S., in “Adsorption on New and Modified Inorganic Sorbents” (A. Dabrowski and V. A. Tertykh, Eds.), Chap. 3.6. Elsevier, Amsterdam, 1996.
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6. Somasundaran, P., and Fuerstenau, D. W., J. Phys. Chem. 70, 90 (1966). 7. Birjsterbosch, B. H., J. Colloid Interface Sci. 47, 186 (1974). 8. Wangnerud, P., and J¨onsson, B., Langmuir 10, 3542 (1994). 9. B¨ohmer, M. R., and Koopal, L. K., Langmuir 8, 2649 (1992); 8, 2660 (1992). 10. Yeskie, M. A., and Harwell, J. H., J. Phys. Chem. 93, 3372 (1989). 11. Samoilov, S., J. Struct. Chem. 3, 314 (1962). 12. Halliwel, R., and Nyburg, J., Trans. Faraday Soc. 59, 1126 (1963). 13. Tanford, C., “The Hydrophobic Effect.” Wiley, New York, 1973. L. Grosmaire C. Chorro M. Chorro S. Partyka1 R. Zana∗ LAMMI University of Montpellier II Place E. Bataillon 34095 Montpellier Cedex, France ∗ ICS (CNRS, ULP) 6 Rue Boussingault 67083 Strasbourg Cedex, France Received July 18, 2001; accepted August 30, 2001
1 To whom correspondence should be addressed. E-mail: [email protected].