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Adsorption of zwitterionic gemini surfactants at the air–water and solid–water interfaces
Colloids and Surfaces A: Physicochemical and Engineering Aspects 203 (2002) 245– 258 www.elsevier.com/locate/colsurfa
Adsorption of zwitterionic gemini surfactants at the air–water and solid–water interfaces V. Seredyuk a, E. Alami a, M. Nyde´n a, K. Holmberg *, A.V. Peresypkin b, F.M. Menger b a
Department of Applied Surface Chemistry, Chalmers Uni6ersity of Technology, SE-412 96 Go¨teborg, Sweden b Department of Chemistry, Emory Uni6ersity, Atlanta, GA 30322, USA Received 19 March 2001; accepted 24 October 2001
Abstract In the present paper we report the adsorption behavior of a series of zwitterionic gemini surfactants, Cx –PO− 4 – (CH2)2 –N+(CH3)2 –Cy where x+ y=22 and x"y, at the air– water and solid– water interfaces. The critical micelle concentration (CMC), was determined by du Nouy ring tensiometry and by steady state fluorescence. The surface excess concentration of zwitterionic gemini surfactants was calculated from the surface tension versus log concentration curves by applying the Gibbs’ adsorption isotherm. The values of surface area per molecule calculated using Gibbs’ equation were very low, 20–30 A, 2, considering the relatively large size of the gemini surfactants. This probably reflects some kind of surfactant aggregation at the surface. Adsorption at hydrophilic and hydrophobic solid surfaces (silica and silica treated with dichlorodimethylsilane, respectively) was investigated using reflectometry. The adsorbed amount at the hydrophilic surface was much higher than that at the hydrophobic surface. This can be interpreted as formation of either continuous bilayer structures or micelle-like aggregates on the hydrophilic substrate. Assuming monolayer packing, the area per molecule obtained at the hydrophobic surface at a surfactant concentration around the CMC was 42 and 44 A, 2 for the more symmetrical geminis and 159 and 198 A, 2 for the more unsymmetrical surfactants. Higher surfactant concentration resulted in formation of aggregates at the surface. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Zwitterionic gemini surfactants; Gibbs’ adsorption isotherm; CMC
1. Introduction Gemini surfactants, sometimes referred to as twin surfactants or dimeric surfactants, are cur-
* Corresponding author. Tel.: + 46-31-772-2969; fax: + 4631-16-0062. E-mail address: [email protected] (K. Holmberg).
rently attracting a lot of attention. Gemini surfactants contain two polar head groups and two hydrocarbon tails and the ‘surfactant monomers’ are linked by a spacer unit at or in close proximity to the head groups. The spacer, which may be rigid or flexible, hydrophilic or hydrophobic, typically contains 2–8 bridging atoms [1–8]. Cationic geminis are easy to prepare and for this reason they are of particular interest from an industrial
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point of view. Most published information about solution properties of gemini surfactants, therefore, relate to this surfactant class. The synthesis of anionic and nonionic geminis is somewhat more complicated and the literature contains relatively few systematic studies on the physico– chemical behavior of such surfactants. Until recently very little work has been reported on zwitterionic geminis [9,10]. In this presentation, we report in some detail the adsorption behaviour of a series of zwitterionic gemini surfactants at the air– water and solid–water interfaces. All surfactants in the series have the same head group composed of a monovalent phosphate group and a quaternary ammonium group separated by a two carbon spacer. The length of the two hydrocarbon tails, one of which is attached to the phosphate group and one to the quaternary ammonium group, vary between 8 and 14 carbon atoms with the total number of tail carbons being kept constant at 22. We have previously demonstrated that these surfactants are efficient in lowering the surface tension and that they give a very tight packing at interfaces, probably due to the strong electrostatic interactions between the polar groups [10]. They self-assemble at concentrations almost a 100-fold lower than the corresponding conventional surfactants. The zwitterionic geminis contain no counterions; like amino acids they are in the form of ‘inner salts’. In practical applications the surfactants will obviously be exposed to electrolytes. It is, therefore, relevant to investigate the effect of added salt on the physico-chemical properties of the surfactants and such a study is also included. The zwitterionic geminis can be seen as a combination of an anionic surfactant, an alkyl phosphate, and a cationic surfactant, an alkyltrimethylammonium salt. In this work we have compared the behaviour of one of the geminis with that of a 1:1 molar mixture of an alkyl phosphate and an alkyltrimethylammonium salt, keeping the total lengths of the tails of the anionic and the cationic surfactants the same as the corresponding tail lengths of the gemini surfactant.
2. Materials and methods
2.1. Surfactants The gemini surfactants were synthesized as de+ scribed before [9]. Cx –PO− 4 –(CH2)2 –N (CH3)2 – Cy is used as an abbreviation, where x and y are the number of carbon atoms in the hydrophobic chains. Disodium decylphosphate was a gift from Akzo Nobel Surface Chemistry, Sweden and dodecyltrimethylammonium bromide was purchased from Aldrich, USA.
2.2. Preparation of surfaces Measurements of surfactant adsorption were made on hydrophilic, unmodified silica and on hydrophobized silica. The basis for both surfaces was polished silicone wafers, thermally oxidized to produce a SiO2 layer thickness of 100 A, and then cut in about 1×5 cm big plates. The slides were cleaned in a mixture of H2O:H2O2:27% NH4OH (ratio 5:1:1 v/v) at 80 °C for 10 min., after which the plates were rinsed with Milli-Q water. The plates were then treated by a mixture of H2O:30% H2O2:37% HCl (ratio 6:1:1 v/v) at 80 °C for 10 min., and rinsed with Milli-Q water. The wafers were stored in absolute ethanol until used as hydrophilic surfaces. For preparation of hydrophobic plates the same cleaning/activating procedure as above was used. After rinsing the cleaned plates with trichloroethylene they were methylated by exposure to a solution of 10% (v/v) dichlorodimethylsilane in trichloroethylene for 5 min [11]. Finally, the plates were rinsed again with trichloroethylene and absolute ethanol. They were stored in absolute ethanol until use.
2.3. Tensiometry The critical micelle concentration (CMC), was determined by measuring the reduction in air– water interfacial tension as a function of surfactant concentration on a Sigma 70 instrument (KSV, Finland) using du Nouy technique with a 9.54 mm Pt/Ir ring. Milli-Q water was used for all preparations.
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The plot of the surface tension versus log surfactant concentration shows a break at a concentration corresponding to the CMC. All the experiments were carried out at 20 °C.
2.4. Fluorescence measurements All fluorescence measurements were performed using a Spex Fluorolog-3, t-3 (JY Horiba). The spectra were recorded between 350 and 500 nm with exitation wavelength at 335 nm. The data obtained are presented in terms of pyrene fluorescence intensity I/III. All the experiments were carried out at 20 °C.
2.5. Reflectometry Adsorption measurements were performed using optical reflectometry combined with a stagnation-point flow-cell. The relative change in the reflectometer signal DS/S0 is proportional to the adsorbed amount Y (in mg m − 2), provided Y is not too high [12]: Y=Q
DS S0
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3. Results
3.1. Surfactants Fig. 1 shows the chemical structure of the four zwitterionic gemini surfactants investigated. The figure also shows the combination of decyl phosphate and dodecyltrimethylammonium bromide used as reference system.
3.2. Determination of critical micelle concentration The CMCs of the surfactants were determined both by tensiometry and by steady state fluorescence. Very good agreement between the two methods was generally obtained (Table 1). The influence on CMC of added salt, NaBr, to the salt-free geminis was also investigated. Fig. 2 shows a typical surface tension vs. surfactant concentration plot for one of the geminis with varying amount of NaBr added. Table 1 gives a full account of all the CMC values and of values of surface tension at the CMC. Fig. 3 shows the surface tension measured by tensiometry for solutions of the anionic decyl phosphate surfactant and the cationic dode-
Here, Q is the sensitivity factor. It is proportional to the refractive index increment dn/dc of the surfactant and can be calculated from optical reflection theory [13]. For adsorption of a single component, it is straightforward to convert DS/S0 to adsorbed amounts. A minor complication occurs when Y becomes too high ( \ 10 mg m − 2). Then the change of signal is no longer strictly linear with the adsorbed amount.
2.6. Contact angle measurements The contact angle of drops of aqueous solutions of the gemini surfactants on paper surfaces was determined by means of a dynamic adsorption tester (DAT 1100 instrument from Fibro Systems, Sweden), which determines contact angle from geometrical drop parameters.
Fig. 1. Structure of the four gemini surfactants and of the combination of decylphosphate and dodecyltrimethylammonium bromide used as reference compound.
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Table 1 CMC and surface tension at the CMC for the gemini surfactants and for the reference compounds in the absence and presence of NaBr Surfactant
CMC from tensiometry (M)
CMC from fluorescence (M)
Surface tension at CMC (mN m−1)
+ C8H17–PO− 4 –(CH2)2–N (CH3)2–C14H29 − C8H17–PO4 –(CH2)2–N+(CH3)2–C14H29+NaBr + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17 − C14H29–PO4 –(CH2)2–N+(CH3)2–C8H17+NaBr + C12H25–PO− 4 –(CH2)2–N (CH3)2–C10H21 − C12H25–PO4 –(CH2)2–N+(CH3)2–C10H21+NaBr + C10H21–PO− 4 –(CH2)2–N (CH3)2–C12H25 − C10H21–PO4 –(CH2)2–N+(CH3)2–C12H25+NaBr C10H21PO4Na2 C12H25N+(CH3)3Br− C12H25N+(CH3)3Br−/ C10H21PO4Na2 (1:1 mol)
6×10−6 3×10−5 1.5×10−5 3.2×10−5 2.4×10−5 4×10−5 1.5×10−5 3×10−5 3×10−4 5×10−3 3.1×10−4
1×10−5 – 1.5×10−5 – 1×10−5 – 1×10−5 – – – 4×10−4
27 26 29 27 28 28 26 29 39 42 33
cyltrimethylammonium bromide surfactants and of the 1:1 molar mixture of these. Fig. 4 shows the fluorescence measurement of the mixture using pyrene as probe molecule and plotting the intensity ratio of the first and the third vibronic peak, I to III, as a function of surfactant concentration. The inflection point of the curve is taken as the CMC in this case. As can be seen from Table 1, addition of NaBr to the different gemini surfactants (1 mol of salt per mole surfactant) results in a slight increase in the CMC. Table 1 also shows that the gemini + surfactant C10H21 – PO− 4 – (CH2)2 – N (CH3)2 – C12H25 has a much lower CMC, both in presence and absence of salt, than the mixture of alkylphosphate and alkyltrimethylammonium bromide with similar alkyl chain lengths. This is in accordance with what one would expect. Normal gemini surfactants, carrying the same charge on the two head groups, are known to have CMC values one to two orders of magnitude lower than the values of the ‘surfactant monomers’ [4]. From Table 1, it can also be concluded that all the gemini surfactants reduce the surface tension to very low values indicating good packing at the air – water interface.
3.3. Adsorption at the air– water interface The surface excess concentration of surfactant at the air–solution interface, Y, and the surface
area per molecule was determined using the Gibbs adsorption isotherm: Y= −
dk 1 nRT d ln C
area= (NaY) − 1
where k is the surface tension in mN m − 1; Y is the adsorbed amount in mol m − 2; T is absolute temperature, R= 8.314 J mol − 1 K − 1; and Na is Avogadro’s number. The value of n ( the number of species at the interface whose concentration at the interface changes with a change in surfactant concentration) is taken as 1 for the gemini surfactants because the surfactants have a net zero charge and carries no counterions. As is shown in Table 2, very low values of the surface area per molecule are obtained for all the geminis. There is a slight trend towards lower values with increasing salt concentration. The more unsymmetrical surfactants, i.e. the geminis with C8 and C14 alkyl chains, seem to occupy the smallest areas per molecule.
3.4. Adsorption at the solid–water interface Reflectometry has been used to monitor the adsorption at silica surfaces. The amount adsorbed at the hydrophilic surface was much higher than that at the hydrophobic surface. This is the typical behavior for surfactant adsorption and is usually interpreted as formation of aggregates in the form of either continuous bilayer structures or
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micelle-like aggregates on the hydrophilic substrate [14]. On hydrophobic surfaces most surfactants give monolayer adsorption [15]. The values of adsorbed amount obtained for the zwitterionic geminis are interesting. Fig. 5(a and b) show adsorption versus time at the hydrophilic silica surface and the hydrophobic, methylated surface, respectively. The concentrations used are 1×10 − 5 M which is around the CMC for all the surfactants. As can be seen, for both surfaces there is a large difference between the more symmetrical and the more unsymmetrical gemini surfactants. The values of area per molecule at the hydrophobic surface for the un+ symmetrical surfactants, C14 – PO− 4 – (CH2)2 – N (CH3)2 –C8 and + C8 – PO− –(CH ) –N (CH ) – C , are high, 198 4 2 2 3 2 14 2 , and 159 A (Table 3). The more symmetrical + geminis, C10 –PO− 4 – (CH2)2 – N (CH3)2 – C12 and + − C12 – PO4 –(CH2)2 –N (CH3)2 – C10, give much smaller values, 42 and 44 A, 2. These values must
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be regarded as extremely low for a gemini surfactant in a monolayer even if the layer is very densely packed.
3.5. Effect of surfactant concentration on adsorbed amount The effect of surfactant concentration on the adsorption profile has also been investigated by reflectometry. The lowest concentration corresponds roughly to the CMC of the surfactant. The adsorbed amount increases with increasing surfactant concentration as shown in Fig. 6(a and + b) for the C14 –PO− 4 –(CH2)2 –N (CH3)2 –C8 and + − the C8 –PO4 –(CH2)2 –N (CH3)2 –C14 surfactant, respectively. The adsorbed amount increases steeply with the concentration at both the hydrophilic and the hydrophobic surface. At the hydrophobic surface the calculated values of area + per molecule for C14 –PO− 4 –(CH2)2 –N (CH3)2 – 2 C8 are 198, 61 and 38 A, for the concentrations
+ Fig. 2. Surface tension vs. concentration, C, for the gemini surfactant C14 – PO− 4 – (CH2)2 – N (CH3)2 – C8 without and with added NaBr.
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Fig. 3. Surface tension vs. concentration, C, for the reference surfactants.
0.01, 0.1 and 1 mM (corresponding to around CMC, ten times the CMC and 100 times the CMC), respectively. Similar behaviour was obtained with the other gemini surfactants, as can be seen from Table 3. This indicates that some type of surface aggregation takes place at higher surfactant concentration.
3.6. Effect of salt on adsorbed amount Fig. 7(a and b) show adsorption versus time for the four geminis in the presence of an equimolar amount of NaBr. A comparison between Figs. 5 and 7 show that the presence of salt results in higher amount of surfactant at the surface. It is interesting to note the difference in effect exerted by the salt on surfactant aggregation in micelles and at surfaces. As was shown in Table 1, salt addition leads to a slight increase in the CMC, i.e. decrease in driving force to form aggregates, for
all four gemini surfactants. The same amount of salt leads to increased adsorption both at the hydrophilic and the hydrophobic surfaces. It can also be noted that the adsorption experiments shown in Fig. 7 all give rather stable plateau values which for all the surfactants are higher for the hydrophilic than for the hydrophobic surface. The calculated values of area per surfactant at the hydrophobic surface lie between 44 and 61 with the values for the symmetrical geminis being slightly higher than for the unsymmetrical. On the hydrophilic surface the symmetrical geminis occupy a much smaller area per molecule but the very high values obtained clearly indicate multilayer adsorption. Fig. 8 shows adsorption of the C14 –PO− 4 – (CH2)2 –N+(CH3)2 –C8 gemini surfactant at varying concentrations of added NaBr. As can be seen, increasing the amount beyond the 1:1 molar ratio of salt to surfactant has only a small effect on the adsorption behaviour.
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3.7. Adsorption of the mixture of anionic and cationic surfactant Adsorption of the 1:1 mixture of disodium decylphosphate and dodecyltrimethylammonium bromide used as a reference to the gemini surfac+ tant C10 –PO− 4 –(CH2)2 – N (CH3)2 – C12 (or rather + − to C10 –PO4 –(CH2)2 – N (CH3)2 – C12 +NaBr), as well as of the individual components, i.e. the decylphosphate and the dodecyltrimethyl bromide were also investigated. The results in terms of adsorbed amount and calculated surface area per molecule for the mixture are collected in Table 3. As can be seen, the values obtained are much higher than for the corresponding gemini surfactant at the same concentration. Clearly, the gemini surfactants have different behaviour compared with the mixture at the solid– water interface.
3.8. Contact angle measurements The influence of surfactant concentration on the wetting dynamics of the gemini surfactants
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was studied using paper as a porous substrate. The concentrations used were 0.01 (close to the CMC), 0.1 and 1 mM. The results obtained for + the C10 –PO− 4 –(CH2)2 –N (CH3)2 –C12 surfactant is shown in Fig. 9. It is noteworthy that only the highest concentration which is around 100 times the CMC gives a pronounced reduction in contact angle under the dynamic conditions that prevail during wetting on a penetrating substrate such as paper.
4. Discussion The surface area per molecule values obtained at both the air–water and the hydrophobic solid– water interfaces are extreme and must reflect some kind of aggregation at the surface. Non-charged surfactants usually form monolayers at hydrophobic surfaces and aggregates either in the form of surface micelles or bilayers at hydrophilic surfaces [15,16]. Hydrophobic interactions are believed to be the main driving force on hydrophobic surfaces
Fig. 4. Variation of the intensity ration I/III of the pyrene fluorescence spectrum vs. concentration, C, for the combination of decylphosphate and dodecyltrimethylammonium bromide used as reference.
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Fig. 5. Adsorption kinetics of the four gemini surfactants at (a) hydrophilic silica, and (b) at hydrophobized silica. The surfactant + concentration was 1 ×10 − 5 M. C10C12 stands for C10 –PO− 4 – (CH2)2 −N (CH3)2 – C12, etc.
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+ Fig. 6. Adsorption kinetics for the gemini surfactant C14 –PO− 4 – (CH2)2 – N (CH3)2 – C8 at different concentrations at (a) hydrophilic silica, and (b) hydrophobized silica.
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Table 2 Adsorbed amount and area per molecule at the air–water interface for the gemini surfactants and for the reference compounds in the absence and presence of NaBr Surfactant
Adsorbed amount at the air–water interface (mol m−2)
Area per molecule at the air–water interface (A, 2)
+ C8H17–PO− 4 –(CH2)2–N (CH3)2–C14H29 − C8H17–PO4 –(CH2)2–N+(CH3)2–C14H29+NaBr (1:1 mol) + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17 − C14H29–PO4 –(CH2)2–N+(CH3)2–C8H17+NaBr (1:1 mol) + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17+NaBr (1:2 mol) − C14H29–PO4 –(CH2)2–N+(CH3)2–C8H17+NaBr (1:4 mol) + C12H25–PO− 4 –(CH2)2–N (CH3)2–C10H21 − C12H25–PO4 –(CH2)2–N+(CH3)2–C10H21+NaBr (1:1 mol) + C10H21–PO− 4 –(CH2)2–N (CH3)2–C12H25 − C10H21–PO4 –(CH2)2–N+(CH3)2–C12H25+NaBr (1:1 mol) C10H21PO4Na2 C12H25N+(CH3)3Br− C12H25N+(CH3)3Br−/ C10H21PO4Na2 (1:1 mol)
8.36×10−6 7.97×10−6 5.70×10−6 7.23×10−6 7.37×10−6 8.69×10−6 6.07×10−6 6.33×10−6 5.36×10−6 5.48×10−6 1.67×10−6 2.93×10−6 3.19×10−6
20 20 29 23 22 19 27 26 31 30 99 57 52
Table 3 Adsorbed amount and area per molecule at the hydrophobic solid–water interface for the gemini surfactants and for the reference compounds in the absence and presence of NaBr Surfactant
+ C8H17–PO− 4 –(CH2)2–N (CH3)2–C14H29 − C8H17–PO4 –(CH2)2–N+(CH3)2–C14H29 + C8H17–PO− 4 –(CH2)2–N (CH3)2–C14H29 − C8H17–PO4 –(CH2)2–N+(CH3)2–C14H29 + C8H17–PO− 4 –(CH2)2–N (CH3)2– C14H29+NaBr (1:1) + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17 − C14H29–PO4 –(CH2)2–N+(CH3)2–C8H17 + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17 − C14H29–PO4 –(CH2)2–N+(CH3)2– C8H17+NaBr (1:1) + C12H25–PO− 4 –(CH2)2–N (CH3)2–C10H21 + C12H25–PO− 4 –(CH2)2–N (CH3)2–C10H21 − C12H25–PO4 –(CH2)2–N+(CH3)2–C10H21 + C12H25–PO− 4 –(CH2)2–N (CH3)2– C10H21+NaBr (1:1) + C10H21–PO− 4 –(CH2)2–N (CH3)2–C12H25 + C10H21–PO− –(CH ) –N (CH3)2–C12H25 4 2 2 + C10H21–PO− 4 –(CH2)2–N (CH3)2–C12H25 + C10H21–PO− 4 –(CH2)2–N (CH3)2– C12H25+NaBr (1:1) C12H25N+(CH3)3Br−/C10H21PO4Na2 C12H25N+(CH3)3Br−/C10H21PO4Na2+ NaBr (1:1)
Concentration (mol l−1)
Adsorbed amount at the Area per molecule at the hydrosolid–water interface (mg m−2) phobic solid–water interface (A, 2) Hydrophilic
Hydrophobic
6×10−6 1×10−5 1×10−4 1×10−3 1×10−5
– 1.2 2.6 3.2 2.2
– 0.5 1.3 1.8 1.6
– 159 61 44 50
1×10−5 1×10−4 1×10−3 1×10−5
1.6 2.8 3.5 2.5
0.4 1.3 2.1 1.8
198 61 38 44
1×10−5 1×10−4 1×10−3 1×10−5
3.2 4.0 7.2 4.0
1.8 2.1 6.0 1.5
44 38 13 53
1×10−5 1×10−4 1×10−3 1×10−5
3.2 3.9 5.9 4.3
1.9 5.0 5.8 1.3
42 16 14 61
3.1x10−4 1.5×10−3
– 3.8
– 0.8
– 99
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Fig. 7. Adsorption kinetics for the four gemini surfactants with added equimolar amount of NaBr at (a) hydrophilic silica, and (b) + hydrophobized silica. C10C12 stands for C10 –PO− 4 –(CH2)2 –N (CH3)2 – C12, etc.
whereas adsorption on hydrophilic surfaces is governed by a combination of hydrophobic and polar interactions. The best studied polar interac-
tion is the hydrogen bonding between polyoxyethylene chains of alcohol ethoxylates and silanol groups at silica surfaces.
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Zwitterionic surfactants have been shown to adsorb in a manner that resembles adsorption of nonionic surfactants [17,18]. Adsorption on hydrophobic solid substrates normally results in a monolayer at the surface with the exception of graphite at which surface aggregates of hemimicellar type are being formed, as demonstrated by atomic force microscopy [18]. Such surface aggregates on graphite are not unique to zwitterionic surfactants, however, but are found for a wide variety of both charged and noncharged surfactants [15,19]. Manne et al. have studied adsorption of a series of surfactants, including geminis, at the solid– water interface [20]. Both symmetrical geminis (with two C12 chains and either a C2 or a C4 spacer) and a highly unsymmetrical gemini (with one C18 and one C1 chain and with a C3 spacer) were included and the adsorption was compared with that of conventional single chain and double
chain surfactants. All surfactants were cationics. At the polar but uncharged mica surface surfactant aggregates were formed that resembled the aggregates formed in solution but with lower curvature. At the hydrophobic graphite the surfactants formed aggregates which were surface -controlled and relatively independent on surface geometry. As mentioned above, surfactant adsorption at graphite is a special case because of the marked influence of the graphite lattice on the geometry of the aggregates formed, as is further discussed in [15]. In this work, the surface area per molecule values obtained at the lower surfactant concentration (around the CMC) differed very much between the two symmetrical and the two unsymmetrical geminis. At the hydrophobic surface the former gave area per molecule values of 42 and 44 A, 2, indicating an extremely tight packing. The unsymmetrical geminis gave values be-
+ Fig. 8. Adsorption kinetics for the gemini surfactant C14 –PO− 4 – (CH2)2 – N (CH3)2 – C8 at different concentrations of added NaBr.
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+ Fig. 9. Contact angle vs. time for a drop of an aqueous solution of the gemini surfactant C10 – PO− 4 – (CH2)2 – N (CH3)2 – C12 on paper.
tween 150 and 200 A, 2, which represent loose packing. Formation of ordered monolayers is evidently favored when the two hydrocarbon tails are of approximately equal length. The difference between the symmetrical and the unsymmetrical gemini surfactants is retained at higher concentrations. The area per molecule values obtained at the hydrophobic solid–water interface at the higher surfactant concentrations are extreme and must reflect some kind of surface aggregation. Most likely, a surface aggregation occurs also at the air–water interface. At both interfaces the calculated values are much too low to be realistic for a tightly packed monolayer. The slow dynamics obtained in the wetting of a porous substrate, see Fig. 9, is in accordance with formation of surface aggregates which block the transport of surfactant unimers to the moving front of the wetting aqueous solution. An addi-
tional factor that may contribute to the poor performance in the wetting experiment is the very low CMC of the gemini surfactants which leads to low unimer concentrations. We have also seen in diffusion NMR experiments that the rate of exchange between the surfactant aggregates is very low [10]. In addition, we have observed that transport of surfactant unimers to a newly created air–water interface was found to be very sluggish. Attempts to measure dynamic surface tension using the maximum bubble pressure method failed for all the gemini surfactants. No reduction in surface tension was obtained during the short time intervals studied with the technique (data not shown). We believe that the poor performance in dynamic wetting and dynamic surface tension reduction is caused by formation of surfactant aggregates at the solid– water and air–water interfaces, respectively. At this stage, we have no suggestion as to the
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structure of the surface aggregates. The structure of the geminis, with a zwitterionic headgroup and with two rather long hydrocarbon tails, seem to be ideal for close packing in mono- and bilayers with all the tails aligned in parallel and with the phosphate group of one gemini surfactant in close contact with the quaternary ammonium group of a neighboring surfactant. However, other packing patterns are also possible and we plan to investigate this issue further by using ellipsometry to measure the thickness of the surfactant layer at the solid– water and possibly also at the air– water interfaces.
5. Conclusion The important result from the present work is that the zwitterionic gemini surfactants give extremely tightly packed monolayers. At higher surfactant concentrations the area per molecule values obtained at the hydrophobic solid– water interface indicate aggregate formation at the surface. Also at the air–water interface aggregates are likely to form since the values of surface area per molecule obtained from Gibbs’ adsorption equation are lower than what is possible to obtain by monolayer packing. The surface aggregates seem to be responsible for a poor performance of these gemini surfactants in terms of dynamic wetting and dynamic surface tension lowering.
Acknowledgements We thank the Swedish Institute for a grant to V.S. The Competence Center for Surfactants from Natural Products is acknowledged for support for E. Alami, A.V. Peresypkin and F.M. Menger
were supported by the National Institutes of Health.
References [1] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1991) 1451. [2] M.J. Rosen, Chemtech 23 (1993) 30. [3] R. Zana, Current Opin. Coll. Interf. Sci. 1 (1996) 566. [4] R. Zana, in: K. Holmberg (Ed.), Novel Surfactants, Surfactants Science Series 74, Marcel Dekker, New York, 1998, p. 241. [5] L. Fang, M.J. Rosen, J. Coll. Interf. Sci. 224 (2000) 265. [6] F.M. Menger, J.S. Keiper, Angew. Chem. Int. Ed. 39 (2000) 1906. [7] K. Holmberg, In: K. Holmberg (Ed.), Handbook of Applied Surface, and Colloid Chemistry, Wiley, Chichester, 2001, 385. [8] R. Zana, M. Benroau, R. Rueff, Langmuir 7 (1991) 1072. [9] A.V. Peresypkin, F.M. Menger, J. Org. Lett. 9 (1999) 1347. [10] V. Seredyuk, E. Alami, M. Nyde´ n, K. Holmberg, A.V. Peresypkin, F.M. Menger, Langmuir, 17 (2001) 5160. [11] H. Elwing, S. Welin, A. Askendal, U. Nilsson, I. Lundstro¨ m, J. Coll. Interf. Sci. 119 (1987) 35. [12] R. Miller, V.B. Fainerman, K.-H. Schano, A. Hofmann, W. Heyer, Tenside. Surf. Det. 5 (1997) 34. [13] J.C. Dijt, M.A. Cohen Stuart, G.J. Fleer, Adv. Coll. Interf. Sci. 50 (1994) 79. [14] B. Jo¨ nsson, B. Lindman, K. Holmberg, B. Kronberg, Surfactants, Polymers in Aqueous Solution, Wiley, Chichester, 1998 Chapter 13. [15] F. Tiberg, J. Brinck, L. Grant, Current Opin. Coll. Interf. Sci. 4 (2000) 411. [16] L.M. Grant, T. Ederth, F. Tiberg, Langmuir 16 (2000) 2285. [17] W.A. Dukker, D.R. Clarke, Coll. Surf. A 94 (1994) 275. [18] W.A. Ducker, L.M. Grant, J. Phys. Chem. 100 (1996) 11507. [19] S. Manne, J.P. Cleveland, H.E. Gaub, G.D. Stucky, P.K. Hansma, Langmuir 10 (1994) 4409. [20] S. Manne, T.E. Scha¨ ffer, Q. Huo, P.K. Hansma, D.E. Morse, G.D. Stucky, I.A. Aksay, Langmuir 13 (1997) 6382.
Adsorption of zwitterionic gemini surfactants at the air–water and solid–water interfaces V. Seredyuk a, E. Alami a, M. Nyde´n a, K. Holmberg *, A.V. Peresypkin b, F.M. Menger b a
Department of Applied Surface Chemistry, Chalmers Uni6ersity of Technology, SE-412 96 Go¨teborg, Sweden b Department of Chemistry, Emory Uni6ersity, Atlanta, GA 30322, USA Received 19 March 2001; accepted 24 October 2001
Abstract In the present paper we report the adsorption behavior of a series of zwitterionic gemini surfactants, Cx –PO− 4 – (CH2)2 –N+(CH3)2 –Cy where x+ y=22 and x"y, at the air– water and solid– water interfaces. The critical micelle concentration (CMC), was determined by du Nouy ring tensiometry and by steady state fluorescence. The surface excess concentration of zwitterionic gemini surfactants was calculated from the surface tension versus log concentration curves by applying the Gibbs’ adsorption isotherm. The values of surface area per molecule calculated using Gibbs’ equation were very low, 20–30 A, 2, considering the relatively large size of the gemini surfactants. This probably reflects some kind of surfactant aggregation at the surface. Adsorption at hydrophilic and hydrophobic solid surfaces (silica and silica treated with dichlorodimethylsilane, respectively) was investigated using reflectometry. The adsorbed amount at the hydrophilic surface was much higher than that at the hydrophobic surface. This can be interpreted as formation of either continuous bilayer structures or micelle-like aggregates on the hydrophilic substrate. Assuming monolayer packing, the area per molecule obtained at the hydrophobic surface at a surfactant concentration around the CMC was 42 and 44 A, 2 for the more symmetrical geminis and 159 and 198 A, 2 for the more unsymmetrical surfactants. Higher surfactant concentration resulted in formation of aggregates at the surface. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Zwitterionic gemini surfactants; Gibbs’ adsorption isotherm; CMC
1. Introduction Gemini surfactants, sometimes referred to as twin surfactants or dimeric surfactants, are cur-
* Corresponding author. Tel.: + 46-31-772-2969; fax: + 4631-16-0062. E-mail address: [email protected] (K. Holmberg).
rently attracting a lot of attention. Gemini surfactants contain two polar head groups and two hydrocarbon tails and the ‘surfactant monomers’ are linked by a spacer unit at or in close proximity to the head groups. The spacer, which may be rigid or flexible, hydrophilic or hydrophobic, typically contains 2–8 bridging atoms [1–8]. Cationic geminis are easy to prepare and for this reason they are of particular interest from an industrial
0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 1 1 0 6 - 2
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point of view. Most published information about solution properties of gemini surfactants, therefore, relate to this surfactant class. The synthesis of anionic and nonionic geminis is somewhat more complicated and the literature contains relatively few systematic studies on the physico– chemical behavior of such surfactants. Until recently very little work has been reported on zwitterionic geminis [9,10]. In this presentation, we report in some detail the adsorption behaviour of a series of zwitterionic gemini surfactants at the air– water and solid–water interfaces. All surfactants in the series have the same head group composed of a monovalent phosphate group and a quaternary ammonium group separated by a two carbon spacer. The length of the two hydrocarbon tails, one of which is attached to the phosphate group and one to the quaternary ammonium group, vary between 8 and 14 carbon atoms with the total number of tail carbons being kept constant at 22. We have previously demonstrated that these surfactants are efficient in lowering the surface tension and that they give a very tight packing at interfaces, probably due to the strong electrostatic interactions between the polar groups [10]. They self-assemble at concentrations almost a 100-fold lower than the corresponding conventional surfactants. The zwitterionic geminis contain no counterions; like amino acids they are in the form of ‘inner salts’. In practical applications the surfactants will obviously be exposed to electrolytes. It is, therefore, relevant to investigate the effect of added salt on the physico-chemical properties of the surfactants and such a study is also included. The zwitterionic geminis can be seen as a combination of an anionic surfactant, an alkyl phosphate, and a cationic surfactant, an alkyltrimethylammonium salt. In this work we have compared the behaviour of one of the geminis with that of a 1:1 molar mixture of an alkyl phosphate and an alkyltrimethylammonium salt, keeping the total lengths of the tails of the anionic and the cationic surfactants the same as the corresponding tail lengths of the gemini surfactant.
2. Materials and methods
2.1. Surfactants The gemini surfactants were synthesized as de+ scribed before [9]. Cx –PO− 4 –(CH2)2 –N (CH3)2 – Cy is used as an abbreviation, where x and y are the number of carbon atoms in the hydrophobic chains. Disodium decylphosphate was a gift from Akzo Nobel Surface Chemistry, Sweden and dodecyltrimethylammonium bromide was purchased from Aldrich, USA.
2.2. Preparation of surfaces Measurements of surfactant adsorption were made on hydrophilic, unmodified silica and on hydrophobized silica. The basis for both surfaces was polished silicone wafers, thermally oxidized to produce a SiO2 layer thickness of 100 A, and then cut in about 1×5 cm big plates. The slides were cleaned in a mixture of H2O:H2O2:27% NH4OH (ratio 5:1:1 v/v) at 80 °C for 10 min., after which the plates were rinsed with Milli-Q water. The plates were then treated by a mixture of H2O:30% H2O2:37% HCl (ratio 6:1:1 v/v) at 80 °C for 10 min., and rinsed with Milli-Q water. The wafers were stored in absolute ethanol until used as hydrophilic surfaces. For preparation of hydrophobic plates the same cleaning/activating procedure as above was used. After rinsing the cleaned plates with trichloroethylene they were methylated by exposure to a solution of 10% (v/v) dichlorodimethylsilane in trichloroethylene for 5 min [11]. Finally, the plates were rinsed again with trichloroethylene and absolute ethanol. They were stored in absolute ethanol until use.
2.3. Tensiometry The critical micelle concentration (CMC), was determined by measuring the reduction in air– water interfacial tension as a function of surfactant concentration on a Sigma 70 instrument (KSV, Finland) using du Nouy technique with a 9.54 mm Pt/Ir ring. Milli-Q water was used for all preparations.
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The plot of the surface tension versus log surfactant concentration shows a break at a concentration corresponding to the CMC. All the experiments were carried out at 20 °C.
2.4. Fluorescence measurements All fluorescence measurements were performed using a Spex Fluorolog-3, t-3 (JY Horiba). The spectra were recorded between 350 and 500 nm with exitation wavelength at 335 nm. The data obtained are presented in terms of pyrene fluorescence intensity I/III. All the experiments were carried out at 20 °C.
2.5. Reflectometry Adsorption measurements were performed using optical reflectometry combined with a stagnation-point flow-cell. The relative change in the reflectometer signal DS/S0 is proportional to the adsorbed amount Y (in mg m − 2), provided Y is not too high [12]: Y=Q
DS S0
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3. Results
3.1. Surfactants Fig. 1 shows the chemical structure of the four zwitterionic gemini surfactants investigated. The figure also shows the combination of decyl phosphate and dodecyltrimethylammonium bromide used as reference system.
3.2. Determination of critical micelle concentration The CMCs of the surfactants were determined both by tensiometry and by steady state fluorescence. Very good agreement between the two methods was generally obtained (Table 1). The influence on CMC of added salt, NaBr, to the salt-free geminis was also investigated. Fig. 2 shows a typical surface tension vs. surfactant concentration plot for one of the geminis with varying amount of NaBr added. Table 1 gives a full account of all the CMC values and of values of surface tension at the CMC. Fig. 3 shows the surface tension measured by tensiometry for solutions of the anionic decyl phosphate surfactant and the cationic dode-
Here, Q is the sensitivity factor. It is proportional to the refractive index increment dn/dc of the surfactant and can be calculated from optical reflection theory [13]. For adsorption of a single component, it is straightforward to convert DS/S0 to adsorbed amounts. A minor complication occurs when Y becomes too high ( \ 10 mg m − 2). Then the change of signal is no longer strictly linear with the adsorbed amount.
2.6. Contact angle measurements The contact angle of drops of aqueous solutions of the gemini surfactants on paper surfaces was determined by means of a dynamic adsorption tester (DAT 1100 instrument from Fibro Systems, Sweden), which determines contact angle from geometrical drop parameters.
Fig. 1. Structure of the four gemini surfactants and of the combination of decylphosphate and dodecyltrimethylammonium bromide used as reference compound.
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Table 1 CMC and surface tension at the CMC for the gemini surfactants and for the reference compounds in the absence and presence of NaBr Surfactant
CMC from tensiometry (M)
CMC from fluorescence (M)
Surface tension at CMC (mN m−1)
+ C8H17–PO− 4 –(CH2)2–N (CH3)2–C14H29 − C8H17–PO4 –(CH2)2–N+(CH3)2–C14H29+NaBr + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17 − C14H29–PO4 –(CH2)2–N+(CH3)2–C8H17+NaBr + C12H25–PO− 4 –(CH2)2–N (CH3)2–C10H21 − C12H25–PO4 –(CH2)2–N+(CH3)2–C10H21+NaBr + C10H21–PO− 4 –(CH2)2–N (CH3)2–C12H25 − C10H21–PO4 –(CH2)2–N+(CH3)2–C12H25+NaBr C10H21PO4Na2 C12H25N+(CH3)3Br− C12H25N+(CH3)3Br−/ C10H21PO4Na2 (1:1 mol)
6×10−6 3×10−5 1.5×10−5 3.2×10−5 2.4×10−5 4×10−5 1.5×10−5 3×10−5 3×10−4 5×10−3 3.1×10−4
1×10−5 – 1.5×10−5 – 1×10−5 – 1×10−5 – – – 4×10−4
27 26 29 27 28 28 26 29 39 42 33
cyltrimethylammonium bromide surfactants and of the 1:1 molar mixture of these. Fig. 4 shows the fluorescence measurement of the mixture using pyrene as probe molecule and plotting the intensity ratio of the first and the third vibronic peak, I to III, as a function of surfactant concentration. The inflection point of the curve is taken as the CMC in this case. As can be seen from Table 1, addition of NaBr to the different gemini surfactants (1 mol of salt per mole surfactant) results in a slight increase in the CMC. Table 1 also shows that the gemini + surfactant C10H21 – PO− 4 – (CH2)2 – N (CH3)2 – C12H25 has a much lower CMC, both in presence and absence of salt, than the mixture of alkylphosphate and alkyltrimethylammonium bromide with similar alkyl chain lengths. This is in accordance with what one would expect. Normal gemini surfactants, carrying the same charge on the two head groups, are known to have CMC values one to two orders of magnitude lower than the values of the ‘surfactant monomers’ [4]. From Table 1, it can also be concluded that all the gemini surfactants reduce the surface tension to very low values indicating good packing at the air – water interface.
3.3. Adsorption at the air– water interface The surface excess concentration of surfactant at the air–solution interface, Y, and the surface
area per molecule was determined using the Gibbs adsorption isotherm: Y= −
dk 1 nRT d ln C
area= (NaY) − 1
where k is the surface tension in mN m − 1; Y is the adsorbed amount in mol m − 2; T is absolute temperature, R= 8.314 J mol − 1 K − 1; and Na is Avogadro’s number. The value of n ( the number of species at the interface whose concentration at the interface changes with a change in surfactant concentration) is taken as 1 for the gemini surfactants because the surfactants have a net zero charge and carries no counterions. As is shown in Table 2, very low values of the surface area per molecule are obtained for all the geminis. There is a slight trend towards lower values with increasing salt concentration. The more unsymmetrical surfactants, i.e. the geminis with C8 and C14 alkyl chains, seem to occupy the smallest areas per molecule.
3.4. Adsorption at the solid–water interface Reflectometry has been used to monitor the adsorption at silica surfaces. The amount adsorbed at the hydrophilic surface was much higher than that at the hydrophobic surface. This is the typical behavior for surfactant adsorption and is usually interpreted as formation of aggregates in the form of either continuous bilayer structures or
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micelle-like aggregates on the hydrophilic substrate [14]. On hydrophobic surfaces most surfactants give monolayer adsorption [15]. The values of adsorbed amount obtained for the zwitterionic geminis are interesting. Fig. 5(a and b) show adsorption versus time at the hydrophilic silica surface and the hydrophobic, methylated surface, respectively. The concentrations used are 1×10 − 5 M which is around the CMC for all the surfactants. As can be seen, for both surfaces there is a large difference between the more symmetrical and the more unsymmetrical gemini surfactants. The values of area per molecule at the hydrophobic surface for the un+ symmetrical surfactants, C14 – PO− 4 – (CH2)2 – N (CH3)2 –C8 and + C8 – PO− –(CH ) –N (CH ) – C , are high, 198 4 2 2 3 2 14 2 , and 159 A (Table 3). The more symmetrical + geminis, C10 –PO− 4 – (CH2)2 – N (CH3)2 – C12 and + − C12 – PO4 –(CH2)2 –N (CH3)2 – C10, give much smaller values, 42 and 44 A, 2. These values must
249
be regarded as extremely low for a gemini surfactant in a monolayer even if the layer is very densely packed.
3.5. Effect of surfactant concentration on adsorbed amount The effect of surfactant concentration on the adsorption profile has also been investigated by reflectometry. The lowest concentration corresponds roughly to the CMC of the surfactant. The adsorbed amount increases with increasing surfactant concentration as shown in Fig. 6(a and + b) for the C14 –PO− 4 –(CH2)2 –N (CH3)2 –C8 and + − the C8 –PO4 –(CH2)2 –N (CH3)2 –C14 surfactant, respectively. The adsorbed amount increases steeply with the concentration at both the hydrophilic and the hydrophobic surface. At the hydrophobic surface the calculated values of area + per molecule for C14 –PO− 4 –(CH2)2 –N (CH3)2 – 2 C8 are 198, 61 and 38 A, for the concentrations
+ Fig. 2. Surface tension vs. concentration, C, for the gemini surfactant C14 – PO− 4 – (CH2)2 – N (CH3)2 – C8 without and with added NaBr.
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Fig. 3. Surface tension vs. concentration, C, for the reference surfactants.
0.01, 0.1 and 1 mM (corresponding to around CMC, ten times the CMC and 100 times the CMC), respectively. Similar behaviour was obtained with the other gemini surfactants, as can be seen from Table 3. This indicates that some type of surface aggregation takes place at higher surfactant concentration.
3.6. Effect of salt on adsorbed amount Fig. 7(a and b) show adsorption versus time for the four geminis in the presence of an equimolar amount of NaBr. A comparison between Figs. 5 and 7 show that the presence of salt results in higher amount of surfactant at the surface. It is interesting to note the difference in effect exerted by the salt on surfactant aggregation in micelles and at surfaces. As was shown in Table 1, salt addition leads to a slight increase in the CMC, i.e. decrease in driving force to form aggregates, for
all four gemini surfactants. The same amount of salt leads to increased adsorption both at the hydrophilic and the hydrophobic surfaces. It can also be noted that the adsorption experiments shown in Fig. 7 all give rather stable plateau values which for all the surfactants are higher for the hydrophilic than for the hydrophobic surface. The calculated values of area per surfactant at the hydrophobic surface lie between 44 and 61 with the values for the symmetrical geminis being slightly higher than for the unsymmetrical. On the hydrophilic surface the symmetrical geminis occupy a much smaller area per molecule but the very high values obtained clearly indicate multilayer adsorption. Fig. 8 shows adsorption of the C14 –PO− 4 – (CH2)2 –N+(CH3)2 –C8 gemini surfactant at varying concentrations of added NaBr. As can be seen, increasing the amount beyond the 1:1 molar ratio of salt to surfactant has only a small effect on the adsorption behaviour.
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3.7. Adsorption of the mixture of anionic and cationic surfactant Adsorption of the 1:1 mixture of disodium decylphosphate and dodecyltrimethylammonium bromide used as a reference to the gemini surfac+ tant C10 –PO− 4 –(CH2)2 – N (CH3)2 – C12 (or rather + − to C10 –PO4 –(CH2)2 – N (CH3)2 – C12 +NaBr), as well as of the individual components, i.e. the decylphosphate and the dodecyltrimethyl bromide were also investigated. The results in terms of adsorbed amount and calculated surface area per molecule for the mixture are collected in Table 3. As can be seen, the values obtained are much higher than for the corresponding gemini surfactant at the same concentration. Clearly, the gemini surfactants have different behaviour compared with the mixture at the solid– water interface.
3.8. Contact angle measurements The influence of surfactant concentration on the wetting dynamics of the gemini surfactants
251
was studied using paper as a porous substrate. The concentrations used were 0.01 (close to the CMC), 0.1 and 1 mM. The results obtained for + the C10 –PO− 4 –(CH2)2 –N (CH3)2 –C12 surfactant is shown in Fig. 9. It is noteworthy that only the highest concentration which is around 100 times the CMC gives a pronounced reduction in contact angle under the dynamic conditions that prevail during wetting on a penetrating substrate such as paper.
4. Discussion The surface area per molecule values obtained at both the air–water and the hydrophobic solid– water interfaces are extreme and must reflect some kind of aggregation at the surface. Non-charged surfactants usually form monolayers at hydrophobic surfaces and aggregates either in the form of surface micelles or bilayers at hydrophilic surfaces [15,16]. Hydrophobic interactions are believed to be the main driving force on hydrophobic surfaces
Fig. 4. Variation of the intensity ration I/III of the pyrene fluorescence spectrum vs. concentration, C, for the combination of decylphosphate and dodecyltrimethylammonium bromide used as reference.
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Fig. 5. Adsorption kinetics of the four gemini surfactants at (a) hydrophilic silica, and (b) at hydrophobized silica. The surfactant + concentration was 1 ×10 − 5 M. C10C12 stands for C10 –PO− 4 – (CH2)2 −N (CH3)2 – C12, etc.
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253
+ Fig. 6. Adsorption kinetics for the gemini surfactant C14 –PO− 4 – (CH2)2 – N (CH3)2 – C8 at different concentrations at (a) hydrophilic silica, and (b) hydrophobized silica.
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Table 2 Adsorbed amount and area per molecule at the air–water interface for the gemini surfactants and for the reference compounds in the absence and presence of NaBr Surfactant
Adsorbed amount at the air–water interface (mol m−2)
Area per molecule at the air–water interface (A, 2)
+ C8H17–PO− 4 –(CH2)2–N (CH3)2–C14H29 − C8H17–PO4 –(CH2)2–N+(CH3)2–C14H29+NaBr (1:1 mol) + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17 − C14H29–PO4 –(CH2)2–N+(CH3)2–C8H17+NaBr (1:1 mol) + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17+NaBr (1:2 mol) − C14H29–PO4 –(CH2)2–N+(CH3)2–C8H17+NaBr (1:4 mol) + C12H25–PO− 4 –(CH2)2–N (CH3)2–C10H21 − C12H25–PO4 –(CH2)2–N+(CH3)2–C10H21+NaBr (1:1 mol) + C10H21–PO− 4 –(CH2)2–N (CH3)2–C12H25 − C10H21–PO4 –(CH2)2–N+(CH3)2–C12H25+NaBr (1:1 mol) C10H21PO4Na2 C12H25N+(CH3)3Br− C12H25N+(CH3)3Br−/ C10H21PO4Na2 (1:1 mol)
8.36×10−6 7.97×10−6 5.70×10−6 7.23×10−6 7.37×10−6 8.69×10−6 6.07×10−6 6.33×10−6 5.36×10−6 5.48×10−6 1.67×10−6 2.93×10−6 3.19×10−6
20 20 29 23 22 19 27 26 31 30 99 57 52
Table 3 Adsorbed amount and area per molecule at the hydrophobic solid–water interface for the gemini surfactants and for the reference compounds in the absence and presence of NaBr Surfactant
+ C8H17–PO− 4 –(CH2)2–N (CH3)2–C14H29 − C8H17–PO4 –(CH2)2–N+(CH3)2–C14H29 + C8H17–PO− 4 –(CH2)2–N (CH3)2–C14H29 − C8H17–PO4 –(CH2)2–N+(CH3)2–C14H29 + C8H17–PO− 4 –(CH2)2–N (CH3)2– C14H29+NaBr (1:1) + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17 − C14H29–PO4 –(CH2)2–N+(CH3)2–C8H17 + C14H29–PO− 4 –(CH2)2–N (CH3)2–C8H17 − C14H29–PO4 –(CH2)2–N+(CH3)2– C8H17+NaBr (1:1) + C12H25–PO− 4 –(CH2)2–N (CH3)2–C10H21 + C12H25–PO− 4 –(CH2)2–N (CH3)2–C10H21 − C12H25–PO4 –(CH2)2–N+(CH3)2–C10H21 + C12H25–PO− 4 –(CH2)2–N (CH3)2– C10H21+NaBr (1:1) + C10H21–PO− 4 –(CH2)2–N (CH3)2–C12H25 + C10H21–PO− –(CH ) –N (CH3)2–C12H25 4 2 2 + C10H21–PO− 4 –(CH2)2–N (CH3)2–C12H25 + C10H21–PO− 4 –(CH2)2–N (CH3)2– C12H25+NaBr (1:1) C12H25N+(CH3)3Br−/C10H21PO4Na2 C12H25N+(CH3)3Br−/C10H21PO4Na2+ NaBr (1:1)
Concentration (mol l−1)
Adsorbed amount at the Area per molecule at the hydrosolid–water interface (mg m−2) phobic solid–water interface (A, 2) Hydrophilic
Hydrophobic
6×10−6 1×10−5 1×10−4 1×10−3 1×10−5
– 1.2 2.6 3.2 2.2
– 0.5 1.3 1.8 1.6
– 159 61 44 50
1×10−5 1×10−4 1×10−3 1×10−5
1.6 2.8 3.5 2.5
0.4 1.3 2.1 1.8
198 61 38 44
1×10−5 1×10−4 1×10−3 1×10−5
3.2 4.0 7.2 4.0
1.8 2.1 6.0 1.5
44 38 13 53
1×10−5 1×10−4 1×10−3 1×10−5
3.2 3.9 5.9 4.3
1.9 5.0 5.8 1.3
42 16 14 61
3.1x10−4 1.5×10−3
– 3.8
– 0.8
– 99
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255
Fig. 7. Adsorption kinetics for the four gemini surfactants with added equimolar amount of NaBr at (a) hydrophilic silica, and (b) + hydrophobized silica. C10C12 stands for C10 –PO− 4 –(CH2)2 –N (CH3)2 – C12, etc.
whereas adsorption on hydrophilic surfaces is governed by a combination of hydrophobic and polar interactions. The best studied polar interac-
tion is the hydrogen bonding between polyoxyethylene chains of alcohol ethoxylates and silanol groups at silica surfaces.
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Zwitterionic surfactants have been shown to adsorb in a manner that resembles adsorption of nonionic surfactants [17,18]. Adsorption on hydrophobic solid substrates normally results in a monolayer at the surface with the exception of graphite at which surface aggregates of hemimicellar type are being formed, as demonstrated by atomic force microscopy [18]. Such surface aggregates on graphite are not unique to zwitterionic surfactants, however, but are found for a wide variety of both charged and noncharged surfactants [15,19]. Manne et al. have studied adsorption of a series of surfactants, including geminis, at the solid– water interface [20]. Both symmetrical geminis (with two C12 chains and either a C2 or a C4 spacer) and a highly unsymmetrical gemini (with one C18 and one C1 chain and with a C3 spacer) were included and the adsorption was compared with that of conventional single chain and double
chain surfactants. All surfactants were cationics. At the polar but uncharged mica surface surfactant aggregates were formed that resembled the aggregates formed in solution but with lower curvature. At the hydrophobic graphite the surfactants formed aggregates which were surface -controlled and relatively independent on surface geometry. As mentioned above, surfactant adsorption at graphite is a special case because of the marked influence of the graphite lattice on the geometry of the aggregates formed, as is further discussed in [15]. In this work, the surface area per molecule values obtained at the lower surfactant concentration (around the CMC) differed very much between the two symmetrical and the two unsymmetrical geminis. At the hydrophobic surface the former gave area per molecule values of 42 and 44 A, 2, indicating an extremely tight packing. The unsymmetrical geminis gave values be-
+ Fig. 8. Adsorption kinetics for the gemini surfactant C14 –PO− 4 – (CH2)2 – N (CH3)2 – C8 at different concentrations of added NaBr.
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257
+ Fig. 9. Contact angle vs. time for a drop of an aqueous solution of the gemini surfactant C10 – PO− 4 – (CH2)2 – N (CH3)2 – C12 on paper.
tween 150 and 200 A, 2, which represent loose packing. Formation of ordered monolayers is evidently favored when the two hydrocarbon tails are of approximately equal length. The difference between the symmetrical and the unsymmetrical gemini surfactants is retained at higher concentrations. The area per molecule values obtained at the hydrophobic solid–water interface at the higher surfactant concentrations are extreme and must reflect some kind of surface aggregation. Most likely, a surface aggregation occurs also at the air–water interface. At both interfaces the calculated values are much too low to be realistic for a tightly packed monolayer. The slow dynamics obtained in the wetting of a porous substrate, see Fig. 9, is in accordance with formation of surface aggregates which block the transport of surfactant unimers to the moving front of the wetting aqueous solution. An addi-
tional factor that may contribute to the poor performance in the wetting experiment is the very low CMC of the gemini surfactants which leads to low unimer concentrations. We have also seen in diffusion NMR experiments that the rate of exchange between the surfactant aggregates is very low [10]. In addition, we have observed that transport of surfactant unimers to a newly created air–water interface was found to be very sluggish. Attempts to measure dynamic surface tension using the maximum bubble pressure method failed for all the gemini surfactants. No reduction in surface tension was obtained during the short time intervals studied with the technique (data not shown). We believe that the poor performance in dynamic wetting and dynamic surface tension reduction is caused by formation of surfactant aggregates at the solid– water and air–water interfaces, respectively. At this stage, we have no suggestion as to the
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structure of the surface aggregates. The structure of the geminis, with a zwitterionic headgroup and with two rather long hydrocarbon tails, seem to be ideal for close packing in mono- and bilayers with all the tails aligned in parallel and with the phosphate group of one gemini surfactant in close contact with the quaternary ammonium group of a neighboring surfactant. However, other packing patterns are also possible and we plan to investigate this issue further by using ellipsometry to measure the thickness of the surfactant layer at the solid– water and possibly also at the air– water interfaces.
5. Conclusion The important result from the present work is that the zwitterionic gemini surfactants give extremely tightly packed monolayers. At higher surfactant concentrations the area per molecule values obtained at the hydrophobic solid– water interface indicate aggregate formation at the surface. Also at the air–water interface aggregates are likely to form since the values of surface area per molecule obtained from Gibbs’ adsorption equation are lower than what is possible to obtain by monolayer packing. The surface aggregates seem to be responsible for a poor performance of these gemini surfactants in terms of dynamic wetting and dynamic surface tension lowering.
Acknowledgements We thank the Swedish Institute for a grant to V.S. The Competence Center for Surfactants from Natural Products is acknowledged for support for E. Alami, A.V. Peresypkin and F.M. Menger
were supported by the National Institutes of Health.
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