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Water Interfaces
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
184, 216–226 (1996)
0614
Nonideality of Mixtures of Pure Nonionic Surfactants Both in Solution and at Silica/Water Interfaces F. PORTET,* P. L. DESBENE,*
AND
C. TREINER† ,1
*Laboratoire d’Analyse des Syste`mes Organiques Complexes., Institut Universitaire de Technologie, Universite´ de Rouen, 43 Rue Saint Germain, Evreux 27000, France; and †Laboratoire d’Electrochimie, UA CNRS 430, Universite´ Pierre et Marie Curie, 4 Place Jussieu, Bat. 74, Paris 85005, France Received March 15, 1996; accepted July 22, 1996
The behavior of two pure alkylethoxylated nonionic surfactants, both as single components and as binary mixtures of various composition, has been studied in solution and at silica/water interfaces. In solution, the variation of cmc with surfactant composition showed large negative and positive deviations from thermodynamic ideality. These results, as obtained from surface tension measurements, may be interpreted by a partial demixing of the micelles into segregated aggregates in the C10E6-rich composition range and by an increased stability of the aggregates in the C14E6rich domain. Careful determination of individual isotherms of the surfactants either as single components or as components of mixtures was performed at a nonporous silica/water interface using a reversed phase HPLC technique. The pure nonionic surfactants displayed non superimposable isotherms when sufficiently dilute solution can be attained: a single adsorption plateau is observed with the more hydrophobic component, and a two-plateau behavior is obtained with the more hydrophilic compound. Some experiments with the intermediate surfactant C12E6 were also performed showing a single-plateau profile. Experimental confirmation of the demixing process at the silica/water interface was found in the C10E6-rich end of the diagram. However, the larger than usual favorable interaction between the surfactants in the C14E6-end of the diagram could not be deduced directly from the shape of the individual isotherms. The various isotherms were discussed making use of the known properties of these surfactants in solution as micelle-type aggregates and as lamellar phases. q 1996 Academic Press, Inc.
Key Words: adsorption of nonionic surfactants; mixed surfactant aggregates; HPLC analysis of nonionic surfactants; nonideal behavior of mixed surfactants; alkylethoxylated surfactants; adsorption of silica/water interface.
INTRODUCTION
Nonionic commercial surfactants such as those used in detergency are mixtures of polydispersed compounds with respect to both hydrocarbon chain length and polar groups 1
To whom correspondence should be addressed.
216
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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in a given chemical series. In the course of an investigation on the adsorption of one of these surfactants at solid/water interfaces, it was find useful to study the behavior of mixtures of two pure nonionic surfactants with a large difference between their hydrocarbon chain length. It is generally assumed that mixtures of nonionic compounds in a given series such as polyoxyethylene derivatives in aqueous solutions present small departures from thermodynamic ideality as defined at the critical micelle concentration (cmc) (1–4). This situation is very different from that encountered when the surfactants are mixtures of ionic and nonionic or of ionic compounds of oppositely charged types for which, understandably, strong interactions induce large deviation from thermodynamic ideality. Therefore the free energy of mixing of surfactants in a nonionic series should, in principle, be deduced from the sole knowledge of the individual cmc’s using one of the thermodynamic models for surfactant mixing available. Good agreement is usually observed between experiment and theory (1–4). However, most surfactant binaries which were tested had cmc values which were relatively close to each other. In the case of detergency, surfactants of widely different cmc values may be present in the solution. It is well-known that in the polyoxyethylene series, the variation of polar chain at constant hydrocarbon chain is not very large in comparison to the effect of changing the hydrocarbon chain length at constant number of the polar groups. Furthermore, the larger the difference between the cmc of two surfactants used in a binary system, the larger the difference between the stoechiometric and the actual micellar composition (3). Finally, the thermodynamic approach although indispensable when dealing with such systems does not incorporate the effect of changing structures upon surfactant mixing. The situation is even more complex when surfactant adsorption on solids from aqueous solutions is considered because of the preferential adsorption of one of the component of the mixture onto the solid surface. Attempted have been made to apply modified versions of solution thermodynamic approaches to the solid/liquid interface adsorption with lim-
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ited success (5, 6). The binary systems used were usually mixtures of anionic and nonionic surfactants for reasons of specific interests or because a quantitative evaluation of each surfactant of a nonionic surfactant mixture which are adsorbed onto the solid surface is difficult to determine experimentally by conventional HPLC methods. Thus, most of the finest adsorption studies involving nonionic surfactants bear an aromatic group because of the ease of using UV–visible spectra for supernatant analysis (5–7). The price to pay is that most of these surfactants are polydispersed materials which complicates the analysis of the adsorption isotherms in terms of structures. The question of structures has haunted the literature on surfactant adsorption at solid/liquid interfaces for many years. If a quasi-consensus has been reached for ionic surfactants, the situation is not as clear with nonionic compounds. Hence, micellar-like adsorbed aggregates have been suggested as deduced, for example, from dynamic fluorescence measurement analysis (8), ellipsometry (9), and microcalorimetry (10, 11). Bilayers have been suggested, with caution, from neutron reflection experiments (12). Results from other techniques indicate that the formation of monolayers (13) may be used for the interpretation of some of the physicochemical properties of these dispersions. These conflicting ideas have to be considered in any interpretation of surfactant adsorption experiment. The aim of the present work is to present an analysis of the surfactant adsorption from aqueous solution of binary mixtures of very pure nonionic compounds, whose individual cmc’s differ by a factor of about 100, on a model nonporous silica dispersion. An interesting investigation has been performed recently with the same ethoxylated surfactants using ellipsometry for the direct evaluation of the adsorption isotherms on silicon wafers (9). However, only the global isotherms had been obtained. In the present study, the adsorption isotherms were determined for the individual components in the mixtures after analysis using a refined HPLC technique: reversed phase partition liquid chromatography, with refractometric detection (14). This technique enabled to investigate the shape of the adsorption isotherms in a concentration domain nearly two orders of magnitude lower than in a previous study with nonionic surfactants (9). Furthermore, surface tension measurements were performed in order to obtain the cmc of the surfactant mixtures as a function of composition, a study which had not been performed previously with widely different cmc values. The solid substrate chosen was Aerosil 200, a well-defined nonporous silica. The nonionic surfactants were compounds of high purity: hexaethyleneglycol monodecylether (C10E6 ), hexaethyleneglycol monododecylether (C12E6 ), and hexaethyleneglycol monotetradecyl ether (C14E6 ). MATERIALS AND METHODS
The compounds C10E6 , C12E6 , and C14E6 were from Nikko Chemicals (Japan). Their purity, as assessed by gas chroma-
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TABLE 1 Calibration Curve for Pure C10E6 , C12E6 , and C14E6 : Cs (ppm) Å a(area) / b Surfactant
Slope a
Intercept b
Standard deviation
C10E6 C12E6 C14E6
20.514 31.080 40.422
9.968 10.764 9.233
0.9991 0.9972 0.9990
tography, was of the order of 98%. Purity analysis was supplemented by the HPLC technique (see below). They were used as received. Silica (Aerosil 200) was a nonporous pure compound (99.8%) provided kindly by Degussa-France. Its specific surface (BET), as determined by the manufacturor, was 200 { 25 m2rg 01 . The surfactants were equilibrated with silica for 24 h at 287C (0.1 g of silica for 10 ml of solution) with purified and filtrated water (Millipore, France, a-Q system). The pH of the solution was adjusted to 4.2. The samples were centrifuged at 10,000 rpm for an hour (Sorvall centrifuge from Prolabo). The supernatant was passed through a cartridge with a C18 bonded phase which retains nonionic surfactants: C18Sep Pak Plus (Waters, France). The surfactants were desorbed with pure acetonitrile (HPLC grade from Carlo Erba, France) and quantitatively reconcentrated. They were analyzed by HPLC System Gold (Beckman, USA) equiped with a refractometric detector (RID6A from Shimadzu, Japan). The mobile phase was made of an acetonitrile/water mixture at a volume ratio of 60/40. The present technique enabled to determine traces of nonionic surfactant concentrations in the ppb domain, which is below any previous determinations (14). All experimental details are provided in that publication. Table 1 presents the calibration curves as regression lines obtained for the three pure surfactants considered: C10E6 , C12E6 , and C14E6 . The unknown concentrations were deduced from these curves (in ppm) calculated from the total areas of the chromatographic peaks. Surface tension measurements were performed at 25.00 { 0.057C with a model K10T tensiometer (Kruss, Germany). The usual precautions were made concerning the cleanliness of all glassware which were thoroughly washed with sulfochromic acid. RESULTS
I. Liquid/Air Adsorption Isotherms
Figure 1 presents the Gibbs isotherms for the three pure surfactant solutions in the presence of a constant concentration of 0.01 molrL 01 of sodium chloride. Figure 2 shows the results obtained for C14E6 , C10E6 , and for only three (for the sake of clarity) of the surfactant mixtures studied. Table 2 presents the cmc values obtained, tentatively, from the
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FIG. 1. Gibbs isotherm for the variation of air/water surface tension with concentration for three pure nonionic surfactants at 257C.
intersection between the straight lines below and above the breaks (see below). The cmc’s for C10E6 , C12E6 , and C14E6 agree relatively well with literature values (Table 2). The ratio of the cmc of the two pure compounds, which were used for the mixed systems, namely, C10E6 and C14E6 , is close to 100. Note that thereafter, when the composition of
the surfactant mixtures will be discussed, a Å 1 for pure C14E6 and a Å 0 for pure C10E6 . Inspection of the isotherms reveals several anomalies. From the slope of the Gibbs isotherm, the surface area per molecule at the air/water interface can be calculated. The values obtained are 0.59 nm2 for C10E6 and 0.63 nm2 for C12E6 in agreement with literature values (9, 15). However, the slope is much too steep for C14E6 , leading to an unrealistically small value of 0.165 nm2 for the surface area. Furthermore, one does not find a neat break at the cmc for any of these mixtures with the C14E6 compound as is usually found for pure single or mixed surfactants systems. Therefore, solutions of C14E6 in water was thoroughly investigated. The surface tension equilibration times was lengthened up to 4 h for each concentration with no significant change of the data. Repeated HPLC analysis of the C14E6 sample used showed this surfactant to be a pure compound as was demonstrated by the manufacturer from gas chromatographic experiments. The cloud point was carefully measured and was find to be equal to 42 { 17C, in excellent agreement with the value of 437C published by Mitchell et al. (16). The validity of extrapolating the linear portions of Gibbs isotherm to a common point to define the cmc could be questioned. However, the cmc found here is in very good agreement with the value published by Balmbra et al. (15). Furthermore, when plotting the log(cmc) for C8E6 , C10E6 , and C12E6 as a function of the number of carbon atoms in the hydrocarbon chain using the literature values and extrapolating, the C14E6 compound falls exactly on the experimental point, thus supporting the cmc value determined for this surfactant. As all the mixtures containing C14E6 behaved in the same manner, the values of Table 2 may be taken as true cmc’s values and the abnormal profile of the Gibbs isotherm for the most TABLE 2 Values of Critical Micelle Concentrations (in mol.liter01) for Mixtures of C14E6 / C10E6
C14E6
C10E6 C12E6c
a
dg r103 d log C
cmc (1105)
1.0 0.9 0.7 0.6 0.3 0.2 0.1 0 0
58.2 58 60 56 28 29 24 16.0 15.3
0.80 0.57 0.38 0.74 4.7 5.0 8.2 63 8.5
a
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1.0a
90a,b 8.7a
Reference 15. Corkill, J. M., Goodman, J. F., and Harrold, S. P., Trans. Far. Soc. 60, 202 (1964). c Pure C12E6 . b
FIG. 2. Comparison of the variation of air/water surface tension with concentration for two pure surfactants and three mixed binary systems.
cmc (1105) (literature)
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where C12 and C1 are, respectively, the cmc of the surfactant mixture and of pure surfactant 1; x is the micellar composition. An average value of b Å 08.0 { 1.6 was obtained for a õ 0.3. This value is much larger than expected, indicating a very strong departure from ideality. For mixtures of nonionic surfactants, values of the order of 02 at most (1) are usually found. The dotted line represents the ideal behavior, as calculated by the classical relationship (3): 1 a (1 0 a ) Å / C12 C1 C2
FIG. 3. Variation of the critical micelle concentration with stochiometric mole fraction for the mixed C10E6 / C14E6 ( a Å 1) system. The dotted line represents the ideal behaviour as calculated by Eq. [2].
hydrophobic compound and the surfactant mixtures is not the consequence of some impurity but seems imposed by the structure of the surfactant at the water/air interface. These anomalies have consequences on the adsorption isotherms as will be shown below. Note that this behavior had been observed before for C16E8 and for C18E9 (17) but they were interpreted as the consequence of a nonequilibrium state of the solution/vapor interface. Figure 3 presents the variation of the cmc with the stoechiometric composition a for the mixtures studied. Here again, anomalies appear. Starting from the C14E6-rich corner, the departure from ideality is negative, as expected, indicating an overall favorable interaction between the two surfactants when forming mixed micelles. However, as noted above, available data reveal, generally, close to ideal behavior when homologous nonionic surfactants with only slightly different numbers of polar groups are mixed in the whole composition range domain. The present negative deviation from ideality seems far too large. In order to quantify this deviation, the regular solution theory as applied to surfactant mixtures was used (1) with the introduction of a single interaction parameter b. This empirical coefficient may be calculated from the Eq. 1:
bÅ
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[2]
Figure 3 shows that the experimental curve crosses the ideal one for a Å 0.5. Thus, the deviation from the ideal line becomes positive, a result which, within the framework of the regular solution approximation, means that repulsion interactions have led to partial demixing of the micelles. For a õ 0.1, the experimental curve approaches the ideal one from above. Such a behavior is most unusual. There are a few mixed systems for which a single b value cannot represent the departure from ideality in the whole composition range; such is the case for example with mixtures of sodium dodecyl sulfate and nonionic surfactants (18); there is the well-known case of mixed hydrogenated and perfluorinated surfactants for which demixing of the micelles has been demonstrated by several authors (19, 20). Some mixed nonionic surfactants also present nonideal behavior. Such are the cases of mixtures of siloxanes and alkylethoxylated compounds whose hydrocarbon chain lengths differ by more than two methylene groups (21). The variation of the cmc’s with composition displays a large positive deviation from ideality. Xia et al. (22) report what they call an antimixing behavior for mixtures of C12E8 with dimethyldodecylaminoxide in a pH range where the latter surfactant is partially nonionic and cationic. As the partial demixing of the surfactants, as revealed by the cmc versus composition profile has consequences at the silica/water interfaces, the discussion will be postponed until the presentation of the corresponding adsorption isotherms. II. Solid/Liquid Adsorption Isotherms
The profile of the adsorption isotherm of nonionic surfactants on a hydrophilic silica in aqueous solutions has been described in detail in the literature (23, 24). In broad terms, starting at infinite dilution, the isotherm may be ascribed by the following events. After an initial surfactant adsorption with the polar headgroups interacting with the silanol moieties acting as surface sites through hydrogen bonding (primary adsorption sites) (25) the adsorption increases through lateral interactions between the hydrocarbon chains forming, at least in the case of relatively small polar chains, micellarlike aggregates (12, 26, 27). The type of structure which prevails just below the cmc depends upon the substrate used.
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FIG. 4. Variation of surface coverage with equilibrium concentration for three pure nonionic surfactants adsorbed at the silica/water interface.
For example, with ethoxylated compounds on a hydrophilic silica, strong interactions of the polar moieties with the surface silanol groups are claimed to be compatible with the formation of bilayers (12). With alumina, where surface group interactions with the polar moieties of alkylpolyethoxylated compounds are weak (28), patch-like aggregates are more likely. Close to the adsorption plateau, free micelles are formed. A detailed analysis of the different adsorption isotherms will be attempted first with the pure compounds and then for the mixtures of C10E6 and C14E6 . 1. Pure Compounds Among the various type of isotherms which may be used to present adsorption data, we have chosen to plot the variation of the fraction coverage, u Å G / Gmax defined as the ratio of the quantity of surfactant adsorbed at an equilibrium (free) surfactant concentration Ceq over the maximum adsorption at the adsorption plateau as a function of Ceq . The log u scale was adopted here merely for reasons of convenience because of the magnitude of the change of fraction coverage. Figure 4 presents the adsorption isotherms isotherms for the three pure nonionic compounds: C10E6 , C12E6 and C14E6 . Note the very small u values which could be determined with the present experimental technique. The more hydrophobic compound is more adsorbed at any concentration than the more hydrophilic ones. However, more importantly, the adsorption profiles are different. A sigmoidal curve is observed with and C12E6 and C14E6 whether with C10E6 , a two-plateau isotherm
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is obtained. Both types of isotherms had been obtained before. However, what is usually observed is that in a given series of compounds, the curves may be superimposed using for example a reduced concentration scale. Thus, as they stand the present data disagree in some respects with those found with nonionics of the same type of those used here (9) but also with anionic surfactants adsorbed on alumina (6) or cationic surfactants adsorbed on negatively charged minerals (29). As far as the nonionic series which may be directly compared to the present data (9), the different behavior observed for C10E6 may be due to the limited concentration domain covered in the lower range by the ellipsometric technique in comparison to that of HPLC. This surfactant isotherm, as investigated by ellipsometry does not present a two-stage profile. Note, that the As/l values which are very close to ours for C12E6 and for C14E6 disagree with the present findings for C10E6 . Also, the cmc’s were not determined by the authors who used literature values (9). Concerning the alkylarylethoxylated (Triton X series) compounds adsorbed on a porous silica (8), the effect of chain length presents the same features that those found in the present study although the former compounds are polydispersed. This observation, if general, could be of great interest. Figure 4 also shows that at infinitely dilute solutions, the isotherms corresponding to the three surfactants tend to merge. From the isotherms obtained in the present investigation, three surface coverage domains may be defined: (i) the very low region ( u õ 0.01); (ii) the intermediate region (0.01 õ u õ 0.5); (iii) the concentrated region (0.5 õ u õ 1).
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FIG. 5. Variation of the fraction of nonadsorbed surfactant with surface coverage for the three pure surfactants.
These various domains will be discussed in sequence for the two surfactants used as single or mixed binary systems. A. C10 E6 . The first portion of the isotherm on Fig. 4, below u Å 0.01, begins with a profile of the Langmuir type. It indicates that as the first molecules adsorb, most certainly with the polar and the nonpolar groups lying flat onto the silica surface, they block any further adsorption as the total concentration increases (23). This is best shown on Fig. 5 where the variation of the mole fraction of free surfactant is plotted as a function of the surface coverage. At a u value close to 0.01, there is a minimum adsorption, which corresponds to the pseudo adsorption plateau for that surfactant. The primary adsorption sites (silanol groups) should be considered here as saturated. For u values from 0.01 to about 0.5, the adsorption isotherm is part of a sigmoidal curve. It implies a cooperative process concerning mostly the polar moieties parallel to the surface and the alkyl chains perpendicular to that surface (23). This configuration favors van der Waals forces between surfactants and therefore the increase of adsorption as the monomer concentration further increases. Aggregates may be formed as patches of molecules (27). When the surface coverage is high, the aggregates increase in numbers but not in size (8). Figure 5 shows that close to the cmc, a minimum is observed which corresponds, in the present case, to 15% of the monomers which are not adsorbed onto the silica surface. This means that the aggregates have reached their maximum size before the cmc. Note finally that the adsorption plateau starts at the cmc. B. C14 E6 . The first portion of the isotherm, up to u Å 0.01, presents a sigmoidal profile, a situation which is differ-
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ent from that of C10E6 . This may mean that a cooperative process is already at work with only the polar portions interacting with the surface sites with the hydrocarbon chains perpendicular to the interface. For u around 0.01, a very small surface coverage value indeed, the surface occupied by a molecule of C10E6 at the silica/water interface (As / l Å 59 nm2 ) is almost twice that occupied by C14E6 (see below). Thus, it seems that some type of aggregation occurs for the former surfactant even at very low u values. For u values between 0.01 and 0.5, the curve may be superimposed to that of C10E6 and must correspond therefore to the same cooperative adsorption mechanism. For u values between 0.5 and 0.95, the adsorption relatively decreases and a clear plateau is attained slightly above the cmc contrary to the case of C10E6 . The same type of profile has been observed in a series of polydispersed surfactants of the Triton X series adsorbed on silica (8), and polyoxyethylene oxides adsorbed on silver iodide crystals (30). However in the former case, the hydrocarbon chain was constant and the average number of polar groups was varied. The slope before the plateau was steeper the smaller the number of ethylene oxide groups, i.e., the more hydrophobic the surfactant, as in the present case. Finally, a small (on the logarithm scale) step-wise increase of adsorption is noted on Fig. 4 just before a final plateau. The accuracy of the data justifies this observation. It may correspond to a rearrangement of the aggregates leading to some coalescence close to the cmc, much as in the case of lamellar phases in solution. It has been suggested (31) that, in aqueous solutions, nonionic surfactants with long hydrocarbon chains tend to
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TABLE 3 Surface Area Occupied per Molecule at the Silica/Water Interface According to Eq. [3] at the Adsorption Plateau for Each Individual Surfactant Surfactants (As / l (nm2)
Mole fraction of surfactant a
C14E6
C10E6
C12E6
1.0 0.9 0.7 0.3 0.1
0.32 (0.32)a 0.38 0.51 — 3.80
0.43 (0.66)a 0.43 — 1.10 4.10
0.36 (0.40)a
a
2. Surfactant Mixtures
Reference 9.
form large oblate micelles with a transition to a lamellar phase, whether in the case of smaller hydrocarbon chains, the globular micelles which are supposed to form do not lead to such a transition upon increasing concentration, at least not at room temperature. The average surface occupied by a monomer at the solid/liquid interface is instructive in that respect. We used the classical relation (in nm2 ): As / l Å
S(BET) 1 10 18 NAGmax
[3]
where S is equal to 200 m2 /g, Gmax is the adsorption at the plateau, and NA is Avogadro’s number. Table 3 presents the values obtained from the present investigation at the adsorption plateau of the solid/liquid interface together with those deduced from the Gibbs isotherms at the vapor/liquid interface. Two observations may be put forward: (i) As / l is much smaller than Av / l . (ii). As / l decreases as the hydrocarbon chain length increases. Hence, As / l decreases from 0.43 to 0.32 nm2 as the hydrocarbon chain length increases from 10 to 14 carbon atoms. Such crude calculations imply that the whole particle surface (the BET surface) is covered by the surfactant, which may not be the case here (12). Thus, the As / l values of Table 3 must be considered as minimum values. In the case of mesophases formed by nonionic surfactants in aqueous solutions, it was proposed that for a surface area per molecule less than 0.47 nm2 , only bilayers and discs should be present (32). Of course, the geometrical packing constraints on which this figure is based cannot apply to a two-dimensional structure such as that which exists at a solid/liquid interface. Nevertheless, the two observations noted above indicate that a bilayer most certainly forms at the cmc at least in the case of C14E6 where the As / l value is almost exactly half that obtained at the vapor/liquid monolayer interface. Certainly, the situation is not as clear in the case of C10E6 . As expected, the value of As / l for C12E6 is situated between the two other surfactant values. It has been indicated that in this ethoxylated surfactant series, the size
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of the micelles increases by a factor of 10 as the hydrocarbon chain is increased by two methylene groups (33). Such variations are not matched by similar changes at the adsorption plateau. Note that bilayers have been deduced from various neutron reflection experiments at or close to the cmc of C12E6 with a bilayer thickness of 0.49 { 0.04 nm. The area per molecule was found equal to As/e Å 0.44 { 0.04 nm2 (12). However, the definition of the area/molecule is different from that deduced from Eq. 3 as it takes into account only that portion of the silica surface covered with the surfactant.
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A. Some general observations. The most important results are shown on Fig. 6 where the individual adsorption isotherms for the two single surfactants and for four surfactant mixtures investigated are presented. Note the very low equilibrium surfactant concentrations which could be analyzed. Before the shape of the individual isotherms is discussed, some general observations may be put forward. C10E6 and C14E6 display in each mixture two different isotherms. The isotherms corresponding to the latter surfactant appear at lower concentrations than that of the former. It has been recalled above that the onset of the adsorption plateau often coincides with the surfactant cmc because adsorbed aggregates are formed onto silica surfaces from the surfactant monomers only. This general rule is found valid for pure C10E6 and C14E6 as well as for two of the binaries investigated: i.e., within experimental uncertainty. It is the case of C14E6 , for a Å 0.90 and 0.70, but not for a Å 0.10 (see Table 2 and Fig. 7). (Note that on Fig. 7, the onset of the plateau for pure C14E6 is considered to correspond to the first plateau). The plateau onset coincides with the cmc because, as could be expected, the first micelle is richer with the more hydrophobic surfactant (3). All C14E6 monomers are then incorporated into the mixed micelles. Therefore, the isotherms of C10E6 will appear at a higher total surfactant concentration as shown on Fig. 7. This result may be interpreted as the consequence of a partial demixing into two types of micelles at each a value: C14E6-rich micelles at lower surfactant concentrations and the C10E6-rich at higher total concentrations. The cmc difference between the two surfactants is so large that the surface either of the silica or of the liquid/vapor interface is already saturated by C14E6 as C10E6 monomers appear in the solution. The latter monomers cannot be seen on Gibbs’ water/air isotherms, and they only appear on the individual silica/water isotherms. It may be pointed out that the composition of the mixed aggregates does not change above the cmc. In most cases, the whole process stops around the adsorption plateau. Moreover, the actual aggregate composition is then equal to the stoechiometric composition. In this respect, surface aggregates are different from bulk micelles. In this latter case, the stoechiometric and actual micellar composition converge only at larger surfactant concentrations.
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FIG. 6. Individual adsorption isotherms for the two individual surfactants in four binary mixtures and as single compounds.
The adsorption isotherms corresponding to the global concentrations, as opposed to the individual isotherms of Fig. 6 are displayed on Fig. 8. The global isotherms nearly coincide with the individual C10E6 isotherms because the latter
FIG. 7. Variation of the plateau onset of the adsorption isotherms of the global and for the individual isotherms with surfactant composition. The variation of cmc is recalled for the purpose of comparison.
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concentrations are so much larger for this surfactant that it completely outweight the presence of the more hydrophobic component. Table 3 presents the results of the As / l data for each surfactant in each of the four surfactant mixtures studied. Wherever a comparison can be made between the surface area per molecules for each individual surfactant, one note that the values are close to each other. B. Analysis of the various isotherms for surfactant mixtures. With the first addition of C10E6 to C14E6 ( a Å 0.90), the global plateau value decreases slightly with respect to pure C14E6 , as expected. The corresponding C14E6 isotherm is very similar to that of the pure compound. The main consequence of the mixing is that the plateau value is attained more rapidly at a concentration close to the cmc. This was not the case with pure C14E6 which displayed, as noted before, a two-stage plateau formation. It seems therefore that the mixing stabilizes the larger surfactant aggregates (oblates) induced by that surfactant. The situation is reminiscent of that produced by the mixture of phospholipids with small or large double hydrocarbon chains. Here also, the surfactants with the smaller hydrophobic chains might stabilize the mixed aggregates by reducing the edge energy (32). In strong contrast with the preceeding case, the addition of C14E6 to C10E6 ( a Å 0.10) shows a completely different profile from that of the pure compound. One may assume that at very low surfactant coverage ( u õ 0.01), the C10E6 molecules adsorb flat on the Aerosil 200 surface, but the situation changes rapidly with increasing surfactant concentration. The increase of non adsorbed monomers observed for the pure compound has disappeared. The interpretation of this behavior can only be of a speculative nature. It is
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FIG. 8. Global adsorption isotherms for the binary mixtures. The corresponding isotherms for the single compounds are also presented for the sake of completeness.
possible that in the presence of an excess of the more hydrophobic surfactant, self aggregation of C10E6 monomers is favored at lower concentration; however at higher concentration the presence of an excess of C14E6 seems to hamper the surface aggregation process. This is clearly seen by the much lower slope of the isotherm. Nevertheless, the percentage of adsorption at the cmc is the same as for the pure C10E6 (about 85% of this surfactant total concentration is adsorbed before the plateau). Therefore the present results suggest that the pure C10E6 aggregates present the same size and shape as that displayed at the stoechiometric composition of 0.10. This type of aggregate segregation has been suggested before for binary cationic surfactant mixtures (34). Figure 9 is a schematic drawing of the possible configuration of the mixed aggregates. The C10E6 aggregates is viewed as inducing a discontinuity in the C14E6 adsorbed bilayers. The opposite behavior of the 0.10 and 0.90 mixtures is in line with that displayed by the mixed cmc variation with composition (Fig. 3) although a more direct comparison could only be made by studying the actual composition of
the mixed micelles at the cmc as can be performed by crossflow ultrafiltration experiments (35). The increase of the percentage of C10E6 in the mixture to a mole fraction of 0.30 decreases only slightly the value of the global Gmax value. Furthermore the C14E6 isotherm profile is changed. The aggregation mechanism is favored by the increased proportion of C10E6 . At small coverage, the adsorption of C14E6 is hardly changed, but as u increases, the proportion of C14E6 aggregates increases much more rapidly than in the case of the pure compound. Comparison of the isotherms on Fig. 6 seems to indicate that at the cmc, the size of the aggregates increase as the C10E6 content increases compared to pure C14E6 . The shape of the C10E6 isotherm is also modified at a Å 0.70 of C14E6 . As for the previous system, the lateral interactions start earlier in the presence of C14E6 , but the slope of that portion of the isotherm is smaller, indicating a decreased adsorption efficiency. The first addition ( a Å 0.10) of the more hydrophobic component to the hydrophilic one increases the maximum
FIG. 9. Schematic representation of mixed aggregates in each rich-end of the binary surfactant compositions.
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adsorption with respect to pure C10E6 at all surfactant coverage values. Comparatively, the addition of a small ( a õ 0.10) quantity of C14E6 has more consequences on the adsorption of C10E6 than the addition of a larger quantity. The isotherm profile is striking. After a rapid adsorption increase, the isotherm displays a dramatic change of slope: it goes backwards. All experiments have been performed in duplicate to ensure the validity of this particular section of the isotherm. Figure 6 shows that at a Å 0.30, the same phenomenon is still observed although the magnitude of the effect is smaller. This most unusual type of behavior has been noted before by Somasundaran et al. (7) for C12E8 in a a Å 0.26 mixture with sodium octylbenzenesulfonate at an alumina/water interface. The suggested interpretation for the rapid increase of the nonionic surfactant adsorption leading to this curious isotherm was based upon a reverse charge effect of the anionic surfactant on the positively charged alumina (36). In the present situation, the same isotherm profile suggests, evidently, a different mechanism. Small C14E6 aggregates are formed at u õ 0.01, independently from the presence of the C10E6 molecules which still adsorb flat at the silica surface. One recalls that this is the consequence of the very large cmc difference between the two surfactants. These molecules first inhibit the growth of the C14E6 aggregates. For a Å 0.10, the C10E6 aggregates begin to form and cosolubilize C14E6 molecules, hence the very rapid adsorption and, conversely, the decrease of the equilibrium concentration of the more hydrophobic compound. This situation is favored by the presence of the excess of C10E6 up to the plateau adsorption. The same interpretation should apply to the mixture with a Å 0.3. It must be emphasized that the points corresponding to a Å 0.1 and a Å 0.3 on the cmc versus composition curve on Fig. 3 are effectively above the ideal line as described by Eq. [3]. This observation provides a strong support to the suggestion of a partial demixing both in the bulk of the solution and at the solid/liquid interface in the C10E6-rich portion of the diagram. CONCLUSIONS
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displayed in the C10E6-rich composition range. Thus, as far as the mixtures are concerned, some degree of non mixing must be admitted as deduced from the bulk as well as the surface behavior in the C10E6-rich composition domain. The addition of up to 30% of the lower surfactant homologue to the higher one does not modify the adsorption of the latter surfactant except at concentrations close to the cmc. At that concentration, C10E6 stabilizes the larger C14E6-rich aggregates. The cosolubilization of C10E6 , although slight, seems to increase regularly with that surfactant proportion in the mixed C14E6-rich aggregates up to a maximum in the region of 30–40%, at which composition the growth of C14E6 aggregates is hampered by a maximum compacity effect at the silica surface. The addition of the higher homologue to the lower one increases dramatically the adsorption of the latter for a surface coverage u between 0.001 and 0.5 because of the cosolubilization of a small quantity of C14E6 . This effect attains a threshold at a 10% composition. The increasing adsorption of C10E6 at small surfactant coverage is hampered by the cosolubilization of C14E6 . Surfactant mixing does not modify greatly the maximum adsorption amount of either surfactant except when a is equal to or less than 0.10. This is most certainly due to some segregation of two types of mixed aggregates at least in the C10E6-rich domain. ACKNOWLEDGMENTS We are grateful to M. H. Mannebach for performing the surface tension measurements. This work was financially supported by the French Ministry of Environment and the French Association of Soaps and Detergent Industries.
REFERENCES 1. 2. 3. 4. 5. 6.
The adsorption of two nonionic surfactants which cmc’s differ by 2 orders of magnitude due to a four methylene groups difference of their hydrocarbon chain have been determined on a hydrophilic surface. The lower homologue, C10E6 , displays a two-stage isotherm profile at the solid/ liquid interface, whereas the more hydrophobic derivative, C14E6 , presents a single sigmoidal curve. Thus, the classical observation according to which, in a given surfactant series, the isotherms may be superimposed merely by a translational operation may not be of general validity. The cmc variation with stochiometric composition indicates a favorable synergistic behavior in the C14E6-rich end of the diagram whereas an unfavorable effect of mixing is
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7. 8. 9. 10. 11. 12. 13. 14.
Holland, P. M., and Rubingh, D. N., J. Phys. Chem. 87, 1984 (1983). Nagarajan, R., Langmuir 1, 331 (1985). Clint, J., J. Chem. Soc. 71, 1327 (1975). Puwada, S., and Blanckstein, D., ACS Symp. Ser. 501, 96 (1992). Harwell, J. H., Roberts, B. L., and Scamehorn, J. F., Colloids Surf. A., 32, 1 (1988). Scamehorn, J. F., Schechter, R. S., and Wade, W. H., J. Colloid Interf. Sci. 85, 494 (1982). Somasundaran, P., Fu, E., and Xu, Q., Langmuir 8, 1065 (1992). Levitz, P., Van Damme, H., and Keravis, D., J. Phys. Chem. 88, 2228 (1984). Tiberg, F., Jonsson, B., Tang, J., and Lindman, B. Langmuir 10, 2294 (1994). Denoyel, R., Giordano, F., and Rouquerol, J., Colloids Surf. A. 76, 141 (1993). Lindheimer, M., Keh, E., Zaini, S., and Partyka, S., J. Colloid Interf. Sci. 138, 83 (1989). MacDermott, D. C., Lu, J. R., Lee, E. M., and Thomas, R. K., Langmuir 8, 1204 (1992). Kunjappu, J. T., and Somasundaran, P., J. Colloid Interf. Sci. 175, 520 (1989). Desbene, P. L., Portet, F. I., and Goussot, G. J., J. Chromatogr. A, 209, 730 (1996).
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15. Balmbra, R. R., Clunie, J. S., Corkill, J. M., and Goodman, J. F., Trans. Faraday Soc. 60, 979 (1964). 16. Mitchell, D. J., Tiddy, G. J. T., Waring, L., Bostock, T., and MacDonald, M. P., J. Chem. Soc., Faraday Trans 1, 79, 975 (1983). 17. Cox, M. F., J. Am. Oil. Chem. Soc. 63, 367 (1989). 18. Carrion Fite´, F. J., Tenside Det. 22, 225 (1985). 19. Mukerjee, P., and Yang, A. Y. S., J. Phys. Chem. 80, 1388 (1976). 20. Asakawa, T., Miyagishi, S., and Nishida, M., Langmuir 3, 821 (1987). 21. Hill, R. M., ACS Symp. Ser. 501, 278 (1992). 22. Xia, J., Dubin, P. L., and Zhang, H., ACS Symp. Ser. 501, 234 (1992). 23. Giles, C. M., MacEwan, T. H., Nakhwa, S. N., and Smith, D., J. Chem. Soc. 3973 (1960). 24. Zhu, B. Y., and Gu, T., J. Chem. Soc., Faraday Trans. 85, 3813 (1989). 25. Trens, P., and Denoyel, R., Langmuir 9, 519 (1993). 26. Lee, E. M., Thomas, R. K., Cummins, P. G., Staples, E. J., Penfold, J., and Rennie, A. R., Chem. Phys. Lett. 162, 196 (1989).
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27. Cummins, P. G., Staples, E., and Penfold, J., J. Phys. Chem. 94, 3740 (1990). 28. Jansen, J., Treiner, C., Vaution, C., and Puisieux, F., Int. J. Pharm. 103, 19 (1994). 29. Cases, J. M., and Mutafschiev, B., Surface Sci. 9, 5772 (1968). 30. Mathai, K. G., and Ottewill, R. H., Trans. Faraday Soc. 62, 750, 759 (1971). 31. Hofland, H. E. J., Bouwstra, J. A., Gouris, G. S., Spies, F., Talsma, H., and Junginger, H. E., J. Colloid Interface Sci. 161, 366 (1993). 32. Lin, T. L., J. Colloid Interface Sci. 154, 444 (1992). 33. Ruppert, L. A. M., J. Colloid Interface Sci. 153, 92 (1992). 34. Meguro, K., J. Colloid Interface Sci. 146, 313 (1991). 35. Makayssi, A., Lemordant, D., and Treiner, C., Langmuir 9, 2808 (1993). 36. Giles, C. M., Smith, D., and Huitson, A., J. Colloid Interface Sci. 47, 755 (1974).
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0614
Nonideality of Mixtures of Pure Nonionic Surfactants Both in Solution and at Silica/Water Interfaces F. PORTET,* P. L. DESBENE,*
AND
C. TREINER† ,1
*Laboratoire d’Analyse des Syste`mes Organiques Complexes., Institut Universitaire de Technologie, Universite´ de Rouen, 43 Rue Saint Germain, Evreux 27000, France; and †Laboratoire d’Electrochimie, UA CNRS 430, Universite´ Pierre et Marie Curie, 4 Place Jussieu, Bat. 74, Paris 85005, France Received March 15, 1996; accepted July 22, 1996
The behavior of two pure alkylethoxylated nonionic surfactants, both as single components and as binary mixtures of various composition, has been studied in solution and at silica/water interfaces. In solution, the variation of cmc with surfactant composition showed large negative and positive deviations from thermodynamic ideality. These results, as obtained from surface tension measurements, may be interpreted by a partial demixing of the micelles into segregated aggregates in the C10E6-rich composition range and by an increased stability of the aggregates in the C14E6rich domain. Careful determination of individual isotherms of the surfactants either as single components or as components of mixtures was performed at a nonporous silica/water interface using a reversed phase HPLC technique. The pure nonionic surfactants displayed non superimposable isotherms when sufficiently dilute solution can be attained: a single adsorption plateau is observed with the more hydrophobic component, and a two-plateau behavior is obtained with the more hydrophilic compound. Some experiments with the intermediate surfactant C12E6 were also performed showing a single-plateau profile. Experimental confirmation of the demixing process at the silica/water interface was found in the C10E6-rich end of the diagram. However, the larger than usual favorable interaction between the surfactants in the C14E6-end of the diagram could not be deduced directly from the shape of the individual isotherms. The various isotherms were discussed making use of the known properties of these surfactants in solution as micelle-type aggregates and as lamellar phases. q 1996 Academic Press, Inc.
Key Words: adsorption of nonionic surfactants; mixed surfactant aggregates; HPLC analysis of nonionic surfactants; nonideal behavior of mixed surfactants; alkylethoxylated surfactants; adsorption of silica/water interface.
INTRODUCTION
Nonionic commercial surfactants such as those used in detergency are mixtures of polydispersed compounds with respect to both hydrocarbon chain length and polar groups 1
To whom correspondence should be addressed.
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0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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in a given chemical series. In the course of an investigation on the adsorption of one of these surfactants at solid/water interfaces, it was find useful to study the behavior of mixtures of two pure nonionic surfactants with a large difference between their hydrocarbon chain length. It is generally assumed that mixtures of nonionic compounds in a given series such as polyoxyethylene derivatives in aqueous solutions present small departures from thermodynamic ideality as defined at the critical micelle concentration (cmc) (1–4). This situation is very different from that encountered when the surfactants are mixtures of ionic and nonionic or of ionic compounds of oppositely charged types for which, understandably, strong interactions induce large deviation from thermodynamic ideality. Therefore the free energy of mixing of surfactants in a nonionic series should, in principle, be deduced from the sole knowledge of the individual cmc’s using one of the thermodynamic models for surfactant mixing available. Good agreement is usually observed between experiment and theory (1–4). However, most surfactant binaries which were tested had cmc values which were relatively close to each other. In the case of detergency, surfactants of widely different cmc values may be present in the solution. It is well-known that in the polyoxyethylene series, the variation of polar chain at constant hydrocarbon chain is not very large in comparison to the effect of changing the hydrocarbon chain length at constant number of the polar groups. Furthermore, the larger the difference between the cmc of two surfactants used in a binary system, the larger the difference between the stoechiometric and the actual micellar composition (3). Finally, the thermodynamic approach although indispensable when dealing with such systems does not incorporate the effect of changing structures upon surfactant mixing. The situation is even more complex when surfactant adsorption on solids from aqueous solutions is considered because of the preferential adsorption of one of the component of the mixture onto the solid surface. Attempted have been made to apply modified versions of solution thermodynamic approaches to the solid/liquid interface adsorption with lim-
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ited success (5, 6). The binary systems used were usually mixtures of anionic and nonionic surfactants for reasons of specific interests or because a quantitative evaluation of each surfactant of a nonionic surfactant mixture which are adsorbed onto the solid surface is difficult to determine experimentally by conventional HPLC methods. Thus, most of the finest adsorption studies involving nonionic surfactants bear an aromatic group because of the ease of using UV–visible spectra for supernatant analysis (5–7). The price to pay is that most of these surfactants are polydispersed materials which complicates the analysis of the adsorption isotherms in terms of structures. The question of structures has haunted the literature on surfactant adsorption at solid/liquid interfaces for many years. If a quasi-consensus has been reached for ionic surfactants, the situation is not as clear with nonionic compounds. Hence, micellar-like adsorbed aggregates have been suggested as deduced, for example, from dynamic fluorescence measurement analysis (8), ellipsometry (9), and microcalorimetry (10, 11). Bilayers have been suggested, with caution, from neutron reflection experiments (12). Results from other techniques indicate that the formation of monolayers (13) may be used for the interpretation of some of the physicochemical properties of these dispersions. These conflicting ideas have to be considered in any interpretation of surfactant adsorption experiment. The aim of the present work is to present an analysis of the surfactant adsorption from aqueous solution of binary mixtures of very pure nonionic compounds, whose individual cmc’s differ by a factor of about 100, on a model nonporous silica dispersion. An interesting investigation has been performed recently with the same ethoxylated surfactants using ellipsometry for the direct evaluation of the adsorption isotherms on silicon wafers (9). However, only the global isotherms had been obtained. In the present study, the adsorption isotherms were determined for the individual components in the mixtures after analysis using a refined HPLC technique: reversed phase partition liquid chromatography, with refractometric detection (14). This technique enabled to investigate the shape of the adsorption isotherms in a concentration domain nearly two orders of magnitude lower than in a previous study with nonionic surfactants (9). Furthermore, surface tension measurements were performed in order to obtain the cmc of the surfactant mixtures as a function of composition, a study which had not been performed previously with widely different cmc values. The solid substrate chosen was Aerosil 200, a well-defined nonporous silica. The nonionic surfactants were compounds of high purity: hexaethyleneglycol monodecylether (C10E6 ), hexaethyleneglycol monododecylether (C12E6 ), and hexaethyleneglycol monotetradecyl ether (C14E6 ). MATERIALS AND METHODS
The compounds C10E6 , C12E6 , and C14E6 were from Nikko Chemicals (Japan). Their purity, as assessed by gas chroma-
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TABLE 1 Calibration Curve for Pure C10E6 , C12E6 , and C14E6 : Cs (ppm) Å a(area) / b Surfactant
Slope a
Intercept b
Standard deviation
C10E6 C12E6 C14E6
20.514 31.080 40.422
9.968 10.764 9.233
0.9991 0.9972 0.9990
tography, was of the order of 98%. Purity analysis was supplemented by the HPLC technique (see below). They were used as received. Silica (Aerosil 200) was a nonporous pure compound (99.8%) provided kindly by Degussa-France. Its specific surface (BET), as determined by the manufacturor, was 200 { 25 m2rg 01 . The surfactants were equilibrated with silica for 24 h at 287C (0.1 g of silica for 10 ml of solution) with purified and filtrated water (Millipore, France, a-Q system). The pH of the solution was adjusted to 4.2. The samples were centrifuged at 10,000 rpm for an hour (Sorvall centrifuge from Prolabo). The supernatant was passed through a cartridge with a C18 bonded phase which retains nonionic surfactants: C18Sep Pak Plus (Waters, France). The surfactants were desorbed with pure acetonitrile (HPLC grade from Carlo Erba, France) and quantitatively reconcentrated. They were analyzed by HPLC System Gold (Beckman, USA) equiped with a refractometric detector (RID6A from Shimadzu, Japan). The mobile phase was made of an acetonitrile/water mixture at a volume ratio of 60/40. The present technique enabled to determine traces of nonionic surfactant concentrations in the ppb domain, which is below any previous determinations (14). All experimental details are provided in that publication. Table 1 presents the calibration curves as regression lines obtained for the three pure surfactants considered: C10E6 , C12E6 , and C14E6 . The unknown concentrations were deduced from these curves (in ppm) calculated from the total areas of the chromatographic peaks. Surface tension measurements were performed at 25.00 { 0.057C with a model K10T tensiometer (Kruss, Germany). The usual precautions were made concerning the cleanliness of all glassware which were thoroughly washed with sulfochromic acid. RESULTS
I. Liquid/Air Adsorption Isotherms
Figure 1 presents the Gibbs isotherms for the three pure surfactant solutions in the presence of a constant concentration of 0.01 molrL 01 of sodium chloride. Figure 2 shows the results obtained for C14E6 , C10E6 , and for only three (for the sake of clarity) of the surfactant mixtures studied. Table 2 presents the cmc values obtained, tentatively, from the
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FIG. 1. Gibbs isotherm for the variation of air/water surface tension with concentration for three pure nonionic surfactants at 257C.
intersection between the straight lines below and above the breaks (see below). The cmc’s for C10E6 , C12E6 , and C14E6 agree relatively well with literature values (Table 2). The ratio of the cmc of the two pure compounds, which were used for the mixed systems, namely, C10E6 and C14E6 , is close to 100. Note that thereafter, when the composition of
the surfactant mixtures will be discussed, a Å 1 for pure C14E6 and a Å 0 for pure C10E6 . Inspection of the isotherms reveals several anomalies. From the slope of the Gibbs isotherm, the surface area per molecule at the air/water interface can be calculated. The values obtained are 0.59 nm2 for C10E6 and 0.63 nm2 for C12E6 in agreement with literature values (9, 15). However, the slope is much too steep for C14E6 , leading to an unrealistically small value of 0.165 nm2 for the surface area. Furthermore, one does not find a neat break at the cmc for any of these mixtures with the C14E6 compound as is usually found for pure single or mixed surfactants systems. Therefore, solutions of C14E6 in water was thoroughly investigated. The surface tension equilibration times was lengthened up to 4 h for each concentration with no significant change of the data. Repeated HPLC analysis of the C14E6 sample used showed this surfactant to be a pure compound as was demonstrated by the manufacturer from gas chromatographic experiments. The cloud point was carefully measured and was find to be equal to 42 { 17C, in excellent agreement with the value of 437C published by Mitchell et al. (16). The validity of extrapolating the linear portions of Gibbs isotherm to a common point to define the cmc could be questioned. However, the cmc found here is in very good agreement with the value published by Balmbra et al. (15). Furthermore, when plotting the log(cmc) for C8E6 , C10E6 , and C12E6 as a function of the number of carbon atoms in the hydrocarbon chain using the literature values and extrapolating, the C14E6 compound falls exactly on the experimental point, thus supporting the cmc value determined for this surfactant. As all the mixtures containing C14E6 behaved in the same manner, the values of Table 2 may be taken as true cmc’s values and the abnormal profile of the Gibbs isotherm for the most TABLE 2 Values of Critical Micelle Concentrations (in mol.liter01) for Mixtures of C14E6 / C10E6
C14E6
C10E6 C12E6c
a
dg r103 d log C
cmc (1105)
1.0 0.9 0.7 0.6 0.3 0.2 0.1 0 0
58.2 58 60 56 28 29 24 16.0 15.3
0.80 0.57 0.38 0.74 4.7 5.0 8.2 63 8.5
a
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1.0a
90a,b 8.7a
Reference 15. Corkill, J. M., Goodman, J. F., and Harrold, S. P., Trans. Far. Soc. 60, 202 (1964). c Pure C12E6 . b
FIG. 2. Comparison of the variation of air/water surface tension with concentration for two pure surfactants and three mixed binary systems.
cmc (1105) (literature)
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where C12 and C1 are, respectively, the cmc of the surfactant mixture and of pure surfactant 1; x is the micellar composition. An average value of b Å 08.0 { 1.6 was obtained for a õ 0.3. This value is much larger than expected, indicating a very strong departure from ideality. For mixtures of nonionic surfactants, values of the order of 02 at most (1) are usually found. The dotted line represents the ideal behavior, as calculated by the classical relationship (3): 1 a (1 0 a ) Å / C12 C1 C2
FIG. 3. Variation of the critical micelle concentration with stochiometric mole fraction for the mixed C10E6 / C14E6 ( a Å 1) system. The dotted line represents the ideal behaviour as calculated by Eq. [2].
hydrophobic compound and the surfactant mixtures is not the consequence of some impurity but seems imposed by the structure of the surfactant at the water/air interface. These anomalies have consequences on the adsorption isotherms as will be shown below. Note that this behavior had been observed before for C16E8 and for C18E9 (17) but they were interpreted as the consequence of a nonequilibrium state of the solution/vapor interface. Figure 3 presents the variation of the cmc with the stoechiometric composition a for the mixtures studied. Here again, anomalies appear. Starting from the C14E6-rich corner, the departure from ideality is negative, as expected, indicating an overall favorable interaction between the two surfactants when forming mixed micelles. However, as noted above, available data reveal, generally, close to ideal behavior when homologous nonionic surfactants with only slightly different numbers of polar groups are mixed in the whole composition range domain. The present negative deviation from ideality seems far too large. In order to quantify this deviation, the regular solution theory as applied to surfactant mixtures was used (1) with the introduction of a single interaction parameter b. This empirical coefficient may be calculated from the Eq. 1:
bÅ
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Figure 3 shows that the experimental curve crosses the ideal one for a Å 0.5. Thus, the deviation from the ideal line becomes positive, a result which, within the framework of the regular solution approximation, means that repulsion interactions have led to partial demixing of the micelles. For a õ 0.1, the experimental curve approaches the ideal one from above. Such a behavior is most unusual. There are a few mixed systems for which a single b value cannot represent the departure from ideality in the whole composition range; such is the case for example with mixtures of sodium dodecyl sulfate and nonionic surfactants (18); there is the well-known case of mixed hydrogenated and perfluorinated surfactants for which demixing of the micelles has been demonstrated by several authors (19, 20). Some mixed nonionic surfactants also present nonideal behavior. Such are the cases of mixtures of siloxanes and alkylethoxylated compounds whose hydrocarbon chain lengths differ by more than two methylene groups (21). The variation of the cmc’s with composition displays a large positive deviation from ideality. Xia et al. (22) report what they call an antimixing behavior for mixtures of C12E8 with dimethyldodecylaminoxide in a pH range where the latter surfactant is partially nonionic and cationic. As the partial demixing of the surfactants, as revealed by the cmc versus composition profile has consequences at the silica/water interfaces, the discussion will be postponed until the presentation of the corresponding adsorption isotherms. II. Solid/Liquid Adsorption Isotherms
The profile of the adsorption isotherm of nonionic surfactants on a hydrophilic silica in aqueous solutions has been described in detail in the literature (23, 24). In broad terms, starting at infinite dilution, the isotherm may be ascribed by the following events. After an initial surfactant adsorption with the polar headgroups interacting with the silanol moieties acting as surface sites through hydrogen bonding (primary adsorption sites) (25) the adsorption increases through lateral interactions between the hydrocarbon chains forming, at least in the case of relatively small polar chains, micellarlike aggregates (12, 26, 27). The type of structure which prevails just below the cmc depends upon the substrate used.
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FIG. 4. Variation of surface coverage with equilibrium concentration for three pure nonionic surfactants adsorbed at the silica/water interface.
For example, with ethoxylated compounds on a hydrophilic silica, strong interactions of the polar moieties with the surface silanol groups are claimed to be compatible with the formation of bilayers (12). With alumina, where surface group interactions with the polar moieties of alkylpolyethoxylated compounds are weak (28), patch-like aggregates are more likely. Close to the adsorption plateau, free micelles are formed. A detailed analysis of the different adsorption isotherms will be attempted first with the pure compounds and then for the mixtures of C10E6 and C14E6 . 1. Pure Compounds Among the various type of isotherms which may be used to present adsorption data, we have chosen to plot the variation of the fraction coverage, u Å G / Gmax defined as the ratio of the quantity of surfactant adsorbed at an equilibrium (free) surfactant concentration Ceq over the maximum adsorption at the adsorption plateau as a function of Ceq . The log u scale was adopted here merely for reasons of convenience because of the magnitude of the change of fraction coverage. Figure 4 presents the adsorption isotherms isotherms for the three pure nonionic compounds: C10E6 , C12E6 and C14E6 . Note the very small u values which could be determined with the present experimental technique. The more hydrophobic compound is more adsorbed at any concentration than the more hydrophilic ones. However, more importantly, the adsorption profiles are different. A sigmoidal curve is observed with and C12E6 and C14E6 whether with C10E6 , a two-plateau isotherm
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is obtained. Both types of isotherms had been obtained before. However, what is usually observed is that in a given series of compounds, the curves may be superimposed using for example a reduced concentration scale. Thus, as they stand the present data disagree in some respects with those found with nonionics of the same type of those used here (9) but also with anionic surfactants adsorbed on alumina (6) or cationic surfactants adsorbed on negatively charged minerals (29). As far as the nonionic series which may be directly compared to the present data (9), the different behavior observed for C10E6 may be due to the limited concentration domain covered in the lower range by the ellipsometric technique in comparison to that of HPLC. This surfactant isotherm, as investigated by ellipsometry does not present a two-stage profile. Note, that the As/l values which are very close to ours for C12E6 and for C14E6 disagree with the present findings for C10E6 . Also, the cmc’s were not determined by the authors who used literature values (9). Concerning the alkylarylethoxylated (Triton X series) compounds adsorbed on a porous silica (8), the effect of chain length presents the same features that those found in the present study although the former compounds are polydispersed. This observation, if general, could be of great interest. Figure 4 also shows that at infinitely dilute solutions, the isotherms corresponding to the three surfactants tend to merge. From the isotherms obtained in the present investigation, three surface coverage domains may be defined: (i) the very low region ( u õ 0.01); (ii) the intermediate region (0.01 õ u õ 0.5); (iii) the concentrated region (0.5 õ u õ 1).
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FIG. 5. Variation of the fraction of nonadsorbed surfactant with surface coverage for the three pure surfactants.
These various domains will be discussed in sequence for the two surfactants used as single or mixed binary systems. A. C10 E6 . The first portion of the isotherm on Fig. 4, below u Å 0.01, begins with a profile of the Langmuir type. It indicates that as the first molecules adsorb, most certainly with the polar and the nonpolar groups lying flat onto the silica surface, they block any further adsorption as the total concentration increases (23). This is best shown on Fig. 5 where the variation of the mole fraction of free surfactant is plotted as a function of the surface coverage. At a u value close to 0.01, there is a minimum adsorption, which corresponds to the pseudo adsorption plateau for that surfactant. The primary adsorption sites (silanol groups) should be considered here as saturated. For u values from 0.01 to about 0.5, the adsorption isotherm is part of a sigmoidal curve. It implies a cooperative process concerning mostly the polar moieties parallel to the surface and the alkyl chains perpendicular to that surface (23). This configuration favors van der Waals forces between surfactants and therefore the increase of adsorption as the monomer concentration further increases. Aggregates may be formed as patches of molecules (27). When the surface coverage is high, the aggregates increase in numbers but not in size (8). Figure 5 shows that close to the cmc, a minimum is observed which corresponds, in the present case, to 15% of the monomers which are not adsorbed onto the silica surface. This means that the aggregates have reached their maximum size before the cmc. Note finally that the adsorption plateau starts at the cmc. B. C14 E6 . The first portion of the isotherm, up to u Å 0.01, presents a sigmoidal profile, a situation which is differ-
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ent from that of C10E6 . This may mean that a cooperative process is already at work with only the polar portions interacting with the surface sites with the hydrocarbon chains perpendicular to the interface. For u around 0.01, a very small surface coverage value indeed, the surface occupied by a molecule of C10E6 at the silica/water interface (As / l Å 59 nm2 ) is almost twice that occupied by C14E6 (see below). Thus, it seems that some type of aggregation occurs for the former surfactant even at very low u values. For u values between 0.01 and 0.5, the curve may be superimposed to that of C10E6 and must correspond therefore to the same cooperative adsorption mechanism. For u values between 0.5 and 0.95, the adsorption relatively decreases and a clear plateau is attained slightly above the cmc contrary to the case of C10E6 . The same type of profile has been observed in a series of polydispersed surfactants of the Triton X series adsorbed on silica (8), and polyoxyethylene oxides adsorbed on silver iodide crystals (30). However in the former case, the hydrocarbon chain was constant and the average number of polar groups was varied. The slope before the plateau was steeper the smaller the number of ethylene oxide groups, i.e., the more hydrophobic the surfactant, as in the present case. Finally, a small (on the logarithm scale) step-wise increase of adsorption is noted on Fig. 4 just before a final plateau. The accuracy of the data justifies this observation. It may correspond to a rearrangement of the aggregates leading to some coalescence close to the cmc, much as in the case of lamellar phases in solution. It has been suggested (31) that, in aqueous solutions, nonionic surfactants with long hydrocarbon chains tend to
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TABLE 3 Surface Area Occupied per Molecule at the Silica/Water Interface According to Eq. [3] at the Adsorption Plateau for Each Individual Surfactant Surfactants (As / l (nm2)
Mole fraction of surfactant a
C14E6
C10E6
C12E6
1.0 0.9 0.7 0.3 0.1
0.32 (0.32)a 0.38 0.51 — 3.80
0.43 (0.66)a 0.43 — 1.10 4.10
0.36 (0.40)a
a
2. Surfactant Mixtures
Reference 9.
form large oblate micelles with a transition to a lamellar phase, whether in the case of smaller hydrocarbon chains, the globular micelles which are supposed to form do not lead to such a transition upon increasing concentration, at least not at room temperature. The average surface occupied by a monomer at the solid/liquid interface is instructive in that respect. We used the classical relation (in nm2 ): As / l Å
S(BET) 1 10 18 NAGmax
[3]
where S is equal to 200 m2 /g, Gmax is the adsorption at the plateau, and NA is Avogadro’s number. Table 3 presents the values obtained from the present investigation at the adsorption plateau of the solid/liquid interface together with those deduced from the Gibbs isotherms at the vapor/liquid interface. Two observations may be put forward: (i) As / l is much smaller than Av / l . (ii). As / l decreases as the hydrocarbon chain length increases. Hence, As / l decreases from 0.43 to 0.32 nm2 as the hydrocarbon chain length increases from 10 to 14 carbon atoms. Such crude calculations imply that the whole particle surface (the BET surface) is covered by the surfactant, which may not be the case here (12). Thus, the As / l values of Table 3 must be considered as minimum values. In the case of mesophases formed by nonionic surfactants in aqueous solutions, it was proposed that for a surface area per molecule less than 0.47 nm2 , only bilayers and discs should be present (32). Of course, the geometrical packing constraints on which this figure is based cannot apply to a two-dimensional structure such as that which exists at a solid/liquid interface. Nevertheless, the two observations noted above indicate that a bilayer most certainly forms at the cmc at least in the case of C14E6 where the As / l value is almost exactly half that obtained at the vapor/liquid monolayer interface. Certainly, the situation is not as clear in the case of C10E6 . As expected, the value of As / l for C12E6 is situated between the two other surfactant values. It has been indicated that in this ethoxylated surfactant series, the size
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of the micelles increases by a factor of 10 as the hydrocarbon chain is increased by two methylene groups (33). Such variations are not matched by similar changes at the adsorption plateau. Note that bilayers have been deduced from various neutron reflection experiments at or close to the cmc of C12E6 with a bilayer thickness of 0.49 { 0.04 nm. The area per molecule was found equal to As/e Å 0.44 { 0.04 nm2 (12). However, the definition of the area/molecule is different from that deduced from Eq. 3 as it takes into account only that portion of the silica surface covered with the surfactant.
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A. Some general observations. The most important results are shown on Fig. 6 where the individual adsorption isotherms for the two single surfactants and for four surfactant mixtures investigated are presented. Note the very low equilibrium surfactant concentrations which could be analyzed. Before the shape of the individual isotherms is discussed, some general observations may be put forward. C10E6 and C14E6 display in each mixture two different isotherms. The isotherms corresponding to the latter surfactant appear at lower concentrations than that of the former. It has been recalled above that the onset of the adsorption plateau often coincides with the surfactant cmc because adsorbed aggregates are formed onto silica surfaces from the surfactant monomers only. This general rule is found valid for pure C10E6 and C14E6 as well as for two of the binaries investigated: i.e., within experimental uncertainty. It is the case of C14E6 , for a Å 0.90 and 0.70, but not for a Å 0.10 (see Table 2 and Fig. 7). (Note that on Fig. 7, the onset of the plateau for pure C14E6 is considered to correspond to the first plateau). The plateau onset coincides with the cmc because, as could be expected, the first micelle is richer with the more hydrophobic surfactant (3). All C14E6 monomers are then incorporated into the mixed micelles. Therefore, the isotherms of C10E6 will appear at a higher total surfactant concentration as shown on Fig. 7. This result may be interpreted as the consequence of a partial demixing into two types of micelles at each a value: C14E6-rich micelles at lower surfactant concentrations and the C10E6-rich at higher total concentrations. The cmc difference between the two surfactants is so large that the surface either of the silica or of the liquid/vapor interface is already saturated by C14E6 as C10E6 monomers appear in the solution. The latter monomers cannot be seen on Gibbs’ water/air isotherms, and they only appear on the individual silica/water isotherms. It may be pointed out that the composition of the mixed aggregates does not change above the cmc. In most cases, the whole process stops around the adsorption plateau. Moreover, the actual aggregate composition is then equal to the stoechiometric composition. In this respect, surface aggregates are different from bulk micelles. In this latter case, the stoechiometric and actual micellar composition converge only at larger surfactant concentrations.
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FIG. 6. Individual adsorption isotherms for the two individual surfactants in four binary mixtures and as single compounds.
The adsorption isotherms corresponding to the global concentrations, as opposed to the individual isotherms of Fig. 6 are displayed on Fig. 8. The global isotherms nearly coincide with the individual C10E6 isotherms because the latter
FIG. 7. Variation of the plateau onset of the adsorption isotherms of the global and for the individual isotherms with surfactant composition. The variation of cmc is recalled for the purpose of comparison.
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concentrations are so much larger for this surfactant that it completely outweight the presence of the more hydrophobic component. Table 3 presents the results of the As / l data for each surfactant in each of the four surfactant mixtures studied. Wherever a comparison can be made between the surface area per molecules for each individual surfactant, one note that the values are close to each other. B. Analysis of the various isotherms for surfactant mixtures. With the first addition of C10E6 to C14E6 ( a Å 0.90), the global plateau value decreases slightly with respect to pure C14E6 , as expected. The corresponding C14E6 isotherm is very similar to that of the pure compound. The main consequence of the mixing is that the plateau value is attained more rapidly at a concentration close to the cmc. This was not the case with pure C14E6 which displayed, as noted before, a two-stage plateau formation. It seems therefore that the mixing stabilizes the larger surfactant aggregates (oblates) induced by that surfactant. The situation is reminiscent of that produced by the mixture of phospholipids with small or large double hydrocarbon chains. Here also, the surfactants with the smaller hydrophobic chains might stabilize the mixed aggregates by reducing the edge energy (32). In strong contrast with the preceeding case, the addition of C14E6 to C10E6 ( a Å 0.10) shows a completely different profile from that of the pure compound. One may assume that at very low surfactant coverage ( u õ 0.01), the C10E6 molecules adsorb flat on the Aerosil 200 surface, but the situation changes rapidly with increasing surfactant concentration. The increase of non adsorbed monomers observed for the pure compound has disappeared. The interpretation of this behavior can only be of a speculative nature. It is
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FIG. 8. Global adsorption isotherms for the binary mixtures. The corresponding isotherms for the single compounds are also presented for the sake of completeness.
possible that in the presence of an excess of the more hydrophobic surfactant, self aggregation of C10E6 monomers is favored at lower concentration; however at higher concentration the presence of an excess of C14E6 seems to hamper the surface aggregation process. This is clearly seen by the much lower slope of the isotherm. Nevertheless, the percentage of adsorption at the cmc is the same as for the pure C10E6 (about 85% of this surfactant total concentration is adsorbed before the plateau). Therefore the present results suggest that the pure C10E6 aggregates present the same size and shape as that displayed at the stoechiometric composition of 0.10. This type of aggregate segregation has been suggested before for binary cationic surfactant mixtures (34). Figure 9 is a schematic drawing of the possible configuration of the mixed aggregates. The C10E6 aggregates is viewed as inducing a discontinuity in the C14E6 adsorbed bilayers. The opposite behavior of the 0.10 and 0.90 mixtures is in line with that displayed by the mixed cmc variation with composition (Fig. 3) although a more direct comparison could only be made by studying the actual composition of
the mixed micelles at the cmc as can be performed by crossflow ultrafiltration experiments (35). The increase of the percentage of C10E6 in the mixture to a mole fraction of 0.30 decreases only slightly the value of the global Gmax value. Furthermore the C14E6 isotherm profile is changed. The aggregation mechanism is favored by the increased proportion of C10E6 . At small coverage, the adsorption of C14E6 is hardly changed, but as u increases, the proportion of C14E6 aggregates increases much more rapidly than in the case of the pure compound. Comparison of the isotherms on Fig. 6 seems to indicate that at the cmc, the size of the aggregates increase as the C10E6 content increases compared to pure C14E6 . The shape of the C10E6 isotherm is also modified at a Å 0.70 of C14E6 . As for the previous system, the lateral interactions start earlier in the presence of C14E6 , but the slope of that portion of the isotherm is smaller, indicating a decreased adsorption efficiency. The first addition ( a Å 0.10) of the more hydrophobic component to the hydrophilic one increases the maximum
FIG. 9. Schematic representation of mixed aggregates in each rich-end of the binary surfactant compositions.
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adsorption with respect to pure C10E6 at all surfactant coverage values. Comparatively, the addition of a small ( a õ 0.10) quantity of C14E6 has more consequences on the adsorption of C10E6 than the addition of a larger quantity. The isotherm profile is striking. After a rapid adsorption increase, the isotherm displays a dramatic change of slope: it goes backwards. All experiments have been performed in duplicate to ensure the validity of this particular section of the isotherm. Figure 6 shows that at a Å 0.30, the same phenomenon is still observed although the magnitude of the effect is smaller. This most unusual type of behavior has been noted before by Somasundaran et al. (7) for C12E8 in a a Å 0.26 mixture with sodium octylbenzenesulfonate at an alumina/water interface. The suggested interpretation for the rapid increase of the nonionic surfactant adsorption leading to this curious isotherm was based upon a reverse charge effect of the anionic surfactant on the positively charged alumina (36). In the present situation, the same isotherm profile suggests, evidently, a different mechanism. Small C14E6 aggregates are formed at u õ 0.01, independently from the presence of the C10E6 molecules which still adsorb flat at the silica surface. One recalls that this is the consequence of the very large cmc difference between the two surfactants. These molecules first inhibit the growth of the C14E6 aggregates. For a Å 0.10, the C10E6 aggregates begin to form and cosolubilize C14E6 molecules, hence the very rapid adsorption and, conversely, the decrease of the equilibrium concentration of the more hydrophobic compound. This situation is favored by the presence of the excess of C10E6 up to the plateau adsorption. The same interpretation should apply to the mixture with a Å 0.3. It must be emphasized that the points corresponding to a Å 0.1 and a Å 0.3 on the cmc versus composition curve on Fig. 3 are effectively above the ideal line as described by Eq. [3]. This observation provides a strong support to the suggestion of a partial demixing both in the bulk of the solution and at the solid/liquid interface in the C10E6-rich portion of the diagram. CONCLUSIONS
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displayed in the C10E6-rich composition range. Thus, as far as the mixtures are concerned, some degree of non mixing must be admitted as deduced from the bulk as well as the surface behavior in the C10E6-rich composition domain. The addition of up to 30% of the lower surfactant homologue to the higher one does not modify the adsorption of the latter surfactant except at concentrations close to the cmc. At that concentration, C10E6 stabilizes the larger C14E6-rich aggregates. The cosolubilization of C10E6 , although slight, seems to increase regularly with that surfactant proportion in the mixed C14E6-rich aggregates up to a maximum in the region of 30–40%, at which composition the growth of C14E6 aggregates is hampered by a maximum compacity effect at the silica surface. The addition of the higher homologue to the lower one increases dramatically the adsorption of the latter for a surface coverage u between 0.001 and 0.5 because of the cosolubilization of a small quantity of C14E6 . This effect attains a threshold at a 10% composition. The increasing adsorption of C10E6 at small surfactant coverage is hampered by the cosolubilization of C14E6 . Surfactant mixing does not modify greatly the maximum adsorption amount of either surfactant except when a is equal to or less than 0.10. This is most certainly due to some segregation of two types of mixed aggregates at least in the C10E6-rich domain. ACKNOWLEDGMENTS We are grateful to M. H. Mannebach for performing the surface tension measurements. This work was financially supported by the French Ministry of Environment and the French Association of Soaps and Detergent Industries.
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The adsorption of two nonionic surfactants which cmc’s differ by 2 orders of magnitude due to a four methylene groups difference of their hydrocarbon chain have been determined on a hydrophilic surface. The lower homologue, C10E6 , displays a two-stage isotherm profile at the solid/ liquid interface, whereas the more hydrophobic derivative, C14E6 , presents a single sigmoidal curve. Thus, the classical observation according to which, in a given surfactant series, the isotherms may be superimposed merely by a translational operation may not be of general validity. The cmc variation with stochiometric composition indicates a favorable synergistic behavior in the C14E6-rich end of the diagram whereas an unfavorable effect of mixing is
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27. Cummins, P. G., Staples, E., and Penfold, J., J. Phys. Chem. 94, 3740 (1990). 28. Jansen, J., Treiner, C., Vaution, C., and Puisieux, F., Int. J. Pharm. 103, 19 (1994). 29. Cases, J. M., and Mutafschiev, B., Surface Sci. 9, 5772 (1968). 30. Mathai, K. G., and Ottewill, R. H., Trans. Faraday Soc. 62, 750, 759 (1971). 31. Hofland, H. E. J., Bouwstra, J. A., Gouris, G. S., Spies, F., Talsma, H., and Junginger, H. E., J. Colloid Interface Sci. 161, 366 (1993). 32. Lin, T. L., J. Colloid Interface Sci. 154, 444 (1992). 33. Ruppert, L. A. M., J. Colloid Interface Sci. 153, 92 (1992). 34. Meguro, K., J. Colloid Interface Sci. 146, 313 (1991). 35. Makayssi, A., Lemordant, D., and Treiner, C., Langmuir 9, 2808 (1993). 36. Giles, C. M., Smith, D., and Huitson, A., J. Colloid Interface Sci. 47, 755 (1974).
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