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Langmuir 2007, 23, 6638-6644
On the Adsorption Properties of Surface Chemically Pure CHAPS at the Air/Water Interface Klaus Lunkenheimer,*,† Gohsuke Sugihara,‡ and Maciej Pietras† Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung, D-14424 Potsdam, Germany, and Department of Chemistry, Faculty of Science, Fukuoka UniVersity, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan ReceiVed October 7, 2006. In Final Form: February 22, 2007 CHAPS, a surface-active derivative of the steroids’ basic structure of the cholic acid {3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate} has become a very important material in biological and pharmaceutical application. Investigations of the adsorption properties of aqueous, surface-chemically pure CHAPS solutions at the air/water interface were performed using surface tension and surface potential measurements. Unlike ordinary extendedchain surfactants, the amphiphilic structure of CHAPS is prone to adopt different concentration-dependent surface states of the adsorption layer. These are well reflected in the adsorption isotherm and in the electric surface properties. They are explained by changes in the adsorbate molecule’s orientation and/or conformation as a result of the latter’s different surface area demand. The versatile favorable application properties of the CHAPS molecule are obviously due to its complicated molecular structure, which enables it to comply with rather different interfacial and colloidal challenges.
Introduction A membrane separates the inner and outer sides of a cell or organelle, which are the smallest units of life. It is well known that the bilayer structure of the membranes consists mainly of phospholipids. To keep organic cells alive, their matter, energy, and information are conveyed across the membranes. The proteins, which are also contained in the membrane in substance, perform most of these functions. To analyze the structure and function of the membrane proteins, it is necessary to extract or solubilize and purify them. The most important problem in the solubilization procedure of proteins is to choose a surfactant that is suitable for extracting the objective proteins while remaining active. Surfactants that are convenient for solubilization should generally have the following properties: (i) high solubilization capacity (at least being capable of solubilizing the proteins sufficiently); (ii) not able to inactivate the proteins; (iii) not able to disturb the activity assay in the measuring system; (iv) sufficient solubility even at low temperatures (proteins usually are treated between 0 and +4 °C); (v) appropriate critical micelle concentration (cmc) and suitable micellar size (which are crucial factors when the surfactant is to be separated from the proteins or when gel filtration is performed); and (vi) no UV absorbance (with the quantitative analysis of proteins performed at a wavelength of 280 nm).1 The following surfactants, besides well-known sodium cholate, have mainly been used as protein solubilizers: Triton X-100 and n-alkanoyl-N-methylglucamides. (The latter’s n-octanoyl-, nnonanoyl-, and n-decanoyl-derivatives are denoted MEGA-8, -9, and -10.) It should be mentioned here that no complete * Corresponding author. E-mail:
[email protected]. Tel/Fax: (03338) 66 885 (office). † Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. ‡ Fukuoka University. (1) Tsuchiya, T. Solubilization of Membrane Proteins and Surfactants (in Japanese); Hirokawa Publishing Co.: Tokyo, 1990.
surfactant has been found that is capable of solubilizing all species of proteins. The most suitable surfactants are still selected by trial and error using a screening set for the first choice, such as n-dodecyl-β-D-maltoside (DDM), n-octyl-β-D-glucoside (OG), sodium cholate (CHO), sucrose monolaurate (SM-12), and CHAPS. For crystallization, surfactants such as n-decyl-β-Dmaltoside (DM) and n-octyl-β-D-maltoside (OM) are used in addition to MEGA-10 and DDM.2 The design and the synthesis of the investigated surfactant CHAPS, chemically characterized as 3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate, as a nondenaturing zwitterionic surfactant for membrane biochemistry was reported by Hjelmeland.3 The first report on the solubilization of receptors that reversibly bind opiates and opioide peptides from brain and neuroblastomaglioma hybrid cell NG 108-15 membranes was published by Simonds et al.4 (Figure 1). Following these achievements, CHAPS has become more widely applied. Several examples of studies using CHAPS are compiled in what follows: for the extraction and purification of thyroid peroxidase from the human thyroid gland,5 for the extraction of L-[3H] glutamate binding sites from guinea pig brain membrane,6 for the separation of intra- and extracellular interleucin-1 of human monocyte cultures,7 for investigating detergent effects on the activity of phospholipase C (Bacillus cerus) towards micellar, short-chain phosphatidylcholine,8 for the isolation and purification of three monoclonal antibodies to avian lipoprotein lipase,9 and for the examination of the effect of different surfactants on the (2) Information from the List of Dojindo Detergents, Dojindo Laboratories, Kumamoto 861-2202, Japan, URL: http://www.dojindo.co.jp and E-mail:
[email protected]. (3) Hjelmeland, L. H. Proc. Natl. Acad. Sci. U.S.A. 1980, 77, 6368-6370. (4) Simonds, W. F.; Koski, G.; Streaty, R. A.; Hjelmeland, L. H.; Klee, W. A. Proc. Natl. Acad. Sci. U.S.A. 1980, 77, 4623-4627. (5) Sugawara, M.; Thayer, C. L.; Kita, T.; Kuma, K. Clin. Chim. Acta 1985, 151, 17-22. (6) Koshiya, K. Life Sci. 1985, 37, 1373-1379. (7) Lepe-Zuniga, J. L.; Zigler, J. S., jr.; Zimmerman, M. L.; Gery, I. Mol. Immunol. 1985, 22, 1387-1392. (8) El-Sayed, M. Y.; Roberts, M. F. Biochim. Biophys. Acta 1985, 831, 133141. (9) Gershenwald, J. E.; Bensodoun, A. Biochim. Biophys. Acta 1985, 836, 286-295.
10.1021/la0629476 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/04/2007
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Figure 1. Chemical formula of the CHAPS molecule.
secondary and oligomeric structures of band 3 of bovine erythrocyte membranes.10 According to a recent literature search for CHAPS papers using SciFinder, more than 3000 entries were found. As mentioned above, CHAPS has been used in various fields of biological, medical, and pharmaceutical sciences. Hence more precise and detailed information on its properties as a surfactant is essential to obtain a better scientific understanding and to develop further utilization. To investigate the specific surface-chemical effects of a surfactant with respect to its chemical structure, it is mandatory to apply a peculiar grade of purity to avoid artifacts.11 The peculiar requirements for the soluble surfactants’ purity are due to the conditions of their syntheses. Generally, an amphiphile is synthesized from two different parent components, with one being rather hydrophobic and the other one being rather hydrophilic in nature. The crucial point is that the former component’s surface activity is often much greater than that of the resultant surfactant product. Because a chemical synthesis is never performed completely, some residual content of the parent components will always remain in the synthesized surfactants, say 1 mol % in the best case, corresponding to an indexed purity “pro analysis” (p.a.). As long as one is dealing with the surfactant’s bulk properties, this grade of purity is usually sufficient. However, concerning adsorption properties this grade will never suffice because the hydrophobic parent component will be preferentially enriched in the adsorption layer according to its greater surface activity. The difference between the surface activities of the two parent components usually amounts to 1 or 2 orders of magnitude. Thus, the surfactant’s adsorption layer does not represent that grade of purity given by the indexed bulk data but rather that of a surface mixture of the surfactant itself and its enriched hydrophobic parent component. Furthermore, there is usually more than one adsorbed trace impurity component.12 Hence, the true structure-function relationship of a surfactant cannot be retrieved reliably from its adsorption properties if it is applied in as-received grade. With respect to any of the surfactant’s surface properties, this chemical grade is insufficient. The true surface properties can be achieved only by applying the particular grade of surface-chemical purity for the corresponding investigations.11,18 (10) Moriyama, R.; Makino, S. Biochim. Biophys. Acta 1985, 832, 135-141. (11) Lunkenheimer, K. Purity of Surfactants and Interfacial Research. In Encyclopedia of Surface and Colloid Science, 2nd Ed.; Taylor & Francis: New York, 2006; pp 5879-5906. (12) Priester, T.; Bartoszek, M.; Lunkenheimer, K. J. Colloid Interface Sci. 1998, 208, 6. (13) Lunkenheimer, K.; Pergande, H.-J.; Kru¨ger, H. ReV. Sci. Instrum. 1987, 58, 2313. (14) Lunkenheimer, K.; Wienskol, G.; Prosser, A. J. Langmuir 2004, 20, 5738. (15) Lunkenheimer, K.; Wedler, C. Tenside, Surfactants, Deterg. 1993, 30, 342. (16) Giacomelli, C. E.; Vermeer, A. W. P.; Norde, W. Langmuir 2000, 16, 4853. (17) Ko, J.-S.; Oh, S.-W.; Kim. K.-W.; Nakashima, N.; Nagadome, S.; Sugihara, G. Colloids Surf., B 2005, 45, 90. (18) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1987, 120, 176.
Figure 2. Equilibrium surface tension values of the as-received CHAPS solutions as a function of concentration in the region of critical micelle formation, cmc.
Concerning the extraordinary effects of the special structure of the amphiphilic CHAPS molecule mentioned above, we determined the adsorption parameters of CHAPS from equilibrium surface tension (σe) and surface potential (∆V) versus concentration (c) isotherms using surface-chemically pure aqueous solutions of it. Experimental Section Purification. Because surfactants used as received generally contain traces of impurities that have greater surface activity than that of the indexed surfactant, these impurities have to be removed beforehand to escape artifacts. This was performed by applying the high-performance surfactant purification apparatus HPPS.13,14 By this technique, the adsorption layer of a stock surfactant solution is subsequently built up, compressed, and aspirated automatically during cyclic runs until the state of surface-chemical purity (scp) is achieved. In the case of micelle-forming surfactants, this adsorptive purification must be applied to submicellar concentrations because the impurity components may be solubilized by micelles. Figure 2 shows the values of the equilibrium surface tension of the as-received CHAPS solutions as a function of concentration. From the break in this dependence, the value of the corresponding concentration of critical micelle formation, cmc, of 4.6 × 10-3 M was determined. The linear dependence for concentrations c e cmc indicates that some surface-active contaminants are contained in the CHAPS solutions. These surface-active impurities cause the relationship between σe and log c not to reveal the required thermodynamically reasonable concave shape towards the abscissa.15 Astonishingly, two more recent papers on adsorption and micellization characteristics of CHAPS and its mixtures still show this paradoxical, nonreasonable linear dependence of the σe versus log c isotherm.16,17 Figure 3 shows the equilibrium surface tension of a 3 × 10-3 M stock CHAPS solution as a function of the number of purification operations j. The solution’s state of surface-chemical purity (scp) was reached when (dσe/dj) ) 0, according to the criterion developed in ref 18. This was achieved after 150 runs. From the surface-chemically pure 3 mM stock solution, various dilutions were prepared. The difference in the dynamic surface tension behavior of the 3 mM solution prepared from the as-received material and the same solution after surface-chemical purification is illustrated in Figure 4. It is evident that the long time dependence of the dynamic surface tension is caused by surface-active trace impurities contained in the as-received product, as explained in detail in refs 11 and 18.
6640 Langmuir, Vol. 23, No. 12, 2007
Lunkenheimer et al. The standard free energy of adsorption is calculated as ∆Gad 0 ) RT ln aL
(2)
The ideally diluted concentration interval is described by the TraubeHenry equation: ∆σe ) Kc ) RT Γ
Figure 3. Purification characteristic of the 3 mM CHAPS solution: equilibrium surface tension, σe, as a function of the number of purification cycles j.
(3)
K is called the Henry constant. Attempts to match the entire σe versus c isotherm without discontinuity were made by applying the two-state approach to the adsorption equation as described in ref 20. To do so, it is assumed that the adsorption may occur in two alternative surface states of different orientations. According to this approach, only the flat oriented adsorbate conformation occurs in the Henry region of very low surface concentration, whereas the orientation is perpendicular to the interface at high concentrations. Because there is no phase transition, a gradually continuous transition is assumed to occur that is described by a transition function R(c). Thus, the whole σe versus c isotherm is described by the relation ∆σe ) R∆σ1 + (1 - R)∆σ2
(4)
∆σe ) σw - σe stands for surface pressure with σw being the surface tension of pure water and σe being the equilibrium surface tension of the surfactant solution. The function ∆σ1 denotes the Traube-Henry equation, and ∆σ2 stands for the Langmuir-Szyszkowski equation. As explained below, eq 4 was also applied to higher concentrations where the Henry equation no longer holds but where two different surface states might nevertheless exist. The latter are then described by different parameters of saturation adsorption, Γ∞, and surface activity, aL, for the two states.22 To compare the surface concentrations obtained from the adsorption equations, they were also calculated by the Gibbs adsorption equation Γ)Figure 4. Difference in the dynamic surface tension behavior of the 3 mM CHAPS solutions prepared from the as-received material and from the solution after surface-chemical purification. Surface Tension. The surface tension of the solutions was measured by using the du Nou¨y ring method, taking into consideration the required modifications for surfactant solutions19 Surface Potential Measurements. Surface potential measurements were performed by the vibrating capacitor method, which became known as the Kelvin probe method. The device (Kelvin Probe SP1) was purchased from Nanofilm Technologies GmbH, Germany. The experiments were carried out using a glass vessel of 9 cm diameter and a Shott glass electrode as a reference. The original experimental setup was improved by inserting a special sensor in order to maintain a constant distance between the probe and the sample. The measurements were carried out at room temperature, 22 ( 2 °C. To evaluate the electric properties of the adsorption layers, the difference between the surface potential of pure water and of the corresponding CHAPS solutions was determined. The measuring accuracy was (10 mV or better. Evaluation of Adsorption Isotherms. The experimentally determined σe versus c isotherm was evaluated by the LangmuirSzyszkowski equation
(
∆σe ) RT Γ∞ ln 1 +
)
c aL
(1)
Γ, Γ∞, and aL denote the surface concentration, saturation surface concentration, and bulk concentration for which the adsorption layer reaches the state of half saturation (surface activity, aL), respectively.
1 dσe RT d ln c
(5)
as explained in ref 20. The surface area demand per molecule adsorbed, A, is given by A ) (ΓNL)-1, with NL denoting Avogadro’s number.
Results and Discussion The equilibrium surface tension versus concentration isotherm of the surface-chemically pure CHAPS solutions at 295 K is given in Figure 5. It covers the concentration interval from 4 × 10-6 to 3 × 10-3 M. Thus, the entire isotherm from the onset of the Henry region down to the critical concentration of micelle formation (cmc) extends to more than three decades. This σe versus log c isotherm was evaluated by applying the Langmuir adsorption equation (eq 1). It turned out that the entire experimental isotherm could not satisfactorily be matched within the overall concentration interval up to the highest concentration of 3 mM. Therefore, we tried to apply the two-state approach to the surface equation of state as proposed in ref 20 (eq 4). In doing so, it was found that the transition function of this approach was degenerated into a negligibly small concentration interval between 9 × 10-4 and 1 × 10-3 M. This means that there is no continuous transition from that part of the isotherm extending to the lower concentrations below 9 × 10-4 M and the one covering the concentration region of 1 × 10-3 M e c e 3 × 10-3 (19) Lunkenheimer, K.; Wantke, K.-D. Colloid Polym. Sci. 1981, 259, 354. (20) Lunkenheimer, K.; Hirte, R. J. Phys. Chem. 1992, 92, 8683. (21) Pallas, N. R.; Pethica, B. A. Langmuir 1985, 1, 509. (22) Hirte, R.; Lunkenheimer, K. J. Phys. Chem. 1996, 100, 13786.
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Figure 5. Equilibrium surface tension vs concentration isotherm of a surface-chemically pure CHAPS solutions at 295 K.
M, but a discontinuous phase transition occurs. It can be deduced from Figure 5 that the corresponding equilibrium surface tension values within the small interval between 9 × 10-4 and 1 × 10-3M remain constant within experimental error. As one can take from the corresponding surface pressure (π) versus surface area (A) isotherm calculated from the adsorption isotherm (see below), this phase transition is first order and is characterized by a plateau in (∂π/∂Α)p,T ) 0.21 Applying the Frumkin adsorption equation, which also takes surface interaction into consideration, to the interval 4 × 10-6 M e c e 9 × 10-4 M resulted in reasonable adsorption parameters and a good best-fit quality of (0.15 mN/m, which is within experimental error. However, we found experimentally that there are obviously another two small concentration intervals of the σe versus log c isotherm for which some unusual scatter in the surface tension measuring values is observed. A more careful investigation of these regions, in particular, the first one, did not improve this uncertainty very much. Thus, for example, the three surface tension values of the first interval are 9 × 10-5 M (62.3 mN/m), 1 × 10-4 M (61.8 mN/m), and 1.1 × 10-4 M (62.3 mN/m). Thus, this narrow region could also be well characterized by a plateau surface tension value of 62.1 mN/m. These two unusual adsorption regions cover the concentration intervals of 9 × 10-5 M e c e 1.2 × 10-4 M and 1.2 × 10-5 M e c e 1.5 × 10-5 M. To decide whether these two intervals do indeed belong to two additional distinguished regions in which some molecular reorientation and/or restructuring within the interfacial layer takes place, we again applied the two-state approach to the adsorption equation to different parts of the σe versus log c isotherm. In addition, we investigated the surface potential (∆V) versus log c isotherms of these scp CHAPS solutions. Matching the σe versus log c isotherm by using the HenryLangmuir (H-L) option in the concentration interval of 4 × 10-6 M e c e 8 × 10-4 M resulted in a sharp transition between the Henry and Langmuir regions at a concentration of 1.2 × 10-5 M. This is in agreement with the visual discontinuity of the isotherm. Now excluding the pure Henry part of 4 × 10-6 M e c e 1 × 10-5 M and applying the two-state approach to the interval of 2 × 10-5 M e c e 8 × 10-4 M using the Langmuir I-Langmuir II (LI-LII) option clearly revealed another sharp transition between the Langmuir I adsorption region and that of Langmuir II at a concentration of 1.1 × 10-4 M. Thus, a distinct transition within the second concentration interval between 9 × 10-5 and 1.1 × 10-4 M was also observed, for which a small plateau in
Figure 6. Equilibrium surface potential (∆V) vs log c isotherm of surface-chemically pure CHAPS solutions at 295 K.
the σe versus log c isotherm, characterized by constant surface tension values, is confirmed. However, to exclude any doubt about the thermodynamic attempts to ascertain the phase transition, we investigated the electric surface properties (∆V) of the CHAPS adsorption layers. The results of the (∆V) versus log c isotherms are illustrated in Figure 6. As can be seen, the investigations of the surface potential support the conclusions about the different concentration intervals that are separated from each other by characteristic concentrations. The surface potential reaches a maximum value each at the end and a minimum value at the onset of these distinguished concentration intervals. However, it is interesting that the third interval of 9 × 10-4 M e c e 1 × 10-3 M, for which a first-order phase transition was detected, is only weakly reflected in the ∆V versus log c isotherm. As we shall see below, this is connected to the fact that the transition at the highest concentrations occurs at conditions of the almost-saturated adsorption layer. The fact that under conditions of a condensed monolayer there is no indication of a phase transition detectable in the surface potential versus area curve, though it becomes quite obvious in the corresponding surface pressure versus surface area relationship, was already observed by Harkins for spread monolayers of eicosanol and pimeric acid.23 To better understand surface tension together with surface potential investigations, we calculated the corresponding surface pressure (π ) σwater - σsolution) and surface potential (∆V) versus surface area (A) isotherms from the adsorption equations. For the latter, we evaluated the following concentration intervals separately to avoid any intermingling influence of a transition region. Thus we have the following separate intervals: (i) Henry region: 4 × 10-6 M e c e 1 × 10-5 M; (ii) Langmuir I: 1.5 × 10-5 M e c e 9 × 10-5 M; (iii) Langmuir II: 1.3 × 10-4 M e c e 8 × 10-4 M; and (iv) Langmuir III: 1 × 10-3 M e c e 3 × 10-3 M. The corresponding transition concentrations are ctrI ) 1.2 × -5 10 M, ctrII ) 1.1 × 10-4 M, and ctrIII ) 9.8 × 10-4 M. In summary, we can characterize the thermodynamic behavior of the CHAPS adsorption layer between the onset of the Henry region and critical micelle concentration (cmc) formation as H f ctrI r LI f ctrII r LII f ctrIII r LIII f cmc. (23) Harkins, W. D. The Physical Chemistry of Surface Films, 2nd ed.; Reinhold Publishing Corporation: New York, 1954.
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Figure 7. Surface pressure (π) vs surface area (A) isotherm of surface-chemically pure CHAPS solutions as calculated for the separate concentration intervals according to the characterization given in the text.
Figure 8. Surface potential (∆V) vs surface concentration (Γ) for surface-chemically pure CHAPS adsorption layers in the Henry region of low surface pressure (π e 2.8 mN/m).
The resulting π versus A isotherm (i.e., the calculated surface pressure as a function of the calculated area per molecule adsorbed, A, is shown in Figure 7). In evaluating the surface potential (∆V) versus area (A) isotherms, another interesting finding appeared. Figure 8 exhibits the dependence of ∆V on the surface concentration Γ for the separate Henry region. As one can see, there are two straight lines, each of which has a constant but quite different slope of d(∆V)/dΓ. The slope of the first linear curve is about 8 times that of the latter. This means that there is an additional characteristic concentration at which the adsorption layer seems to change discontinuously from the gaseous state to another state. The latter can still be described as a Henry state but with a completely different Henry constant. The critical concentration of 4 × 10-6 M, at which the slope of d∆V/∆Γ in the Henry region does change abruptly, corresponds to a surface area of about 400 Å2/molecule. The molecular geometry of the CHAPS molecule let us suggest that at very low surface concentrations the molecule will have a flat arrangement
Lunkenheimer et al.
in the adsorption layer. The three axial hydroxy groups in positions 3, 7, and 12 of the cholic acid skeleton provide the hydrophobic cyclopentano-phenanthrene body with some solubility for the aqueous phase. Because they are in axial positions,24,25 which are directed toward the aqueous phase, the hydrophobic cyclopentano-phenanthrene part will necessarily have a flat arrangement across the interface. The extended chain residue containing the polar carbonamido together with the cationic dimethylammonio and the anionic sulfonato groups, which are connected by three short aliphatic chains, possess alternating amphiphilic character along the cyclopentano-phenanthrene molecule’s side group. Thus, it is reasonable to assume that at very low surface coverage the whole CHAPS molecule will be arranged in a flat orientation across the interface. A rough estimation of the flat surface area arrangement results in an occupied area of 250 to 300 Å2/molecule. Compared with the minimal possible surface area per molecule adsorbed, which amounts to about 100 Å2/ molecule, this indicates that well below this concentration the adsorbate molecules float as individual entities within the surface layer. However, at slightly higher concentration they obviously begin to interact with each other somehow, perhaps in a kind of loose cluster formation,26,27 which may also lead to a partial compensation of the individual molecular dipole’s contribution to the electric surface potential by convenient surface assembly. The above given estimations well suit the findings of the surface potential measurements insofar as such conditions might be attained at the critical concentration of 4 × 10-6 M. The steep slope in the first Henry region amounting to 7. 44 × 1012 mV cm2/mol is twice that observed for the univalent sodium dodecyl sulfate surfactant in the Henry region, which amounts to 3.6 × 1012 mV cm2/mol.28 This hints at the fact that the surface potential in the first Henry region is mainly brought about by the electric dipole of the betainic dimethylammoniopropane sulfonate group of the CHAPS molecule that carries two opposite charges, which are separated by a propyl chain. The dipole moment of the CHAPS molecule is approximately twice that of the completely dissociated univalent sodium dodecyl sulfate surfactant. This is due to the fact that part of the sodium counterions of the electric double layer are embedded within the adsorption layer. Thus, they partially neutralize the potential drop of the condenser consisting of completely separated sheets of ions and counterions directed perpendicularly to the boundary layer.29 The much lower surface potential gradient in the second part of the Henry region at almost the maximum level means that the adsorption of additional molecules does not likewise contribute to the rise in surface potential. This might occur by some weak electric interaction via the adsorbate molecules’ dipolar grouping of -N+(CH3)2-CH2-CH2-CH2-SO3- in the boundary layer. Thus, a certain clustering of adsorbate molecules may occur that also results in a weaker contribution to the surface potential rise as compared to that observed in the first Henry region as a result of a partial neutralization of the potential determining charges. Such clustering in adsorbed layers, called reversible nucleation, was only recently observed for some slightly soluble amphiphiles. (24) Beyer, H. Lehrbuch der Organischen Chemie; S. Hirzel-Verlag: Leipzig, Germany, 1961. (25) Fuhrhop, J.; Penzlin, G. Organic Synthesis, 2nd ed.; VCH: Weinheim, Germany, 1994. (26) Prosser, A. J.; Retter, U.; Lunkenheimer, K. Langmuir 2004, 20, 2720. (27) Prosser, A. J.; Retter, U.; Lunkenheimer, K. XVIIth Conference of the European Colloid and Interface Society, Florence, Italy, Sept 21-26, 2003, Abstract Book P6/101. (28) Lunkenheimer, K.; Czichocki, G.; Hirte, H.; Barzyk, W. Colloids Surf., A 1995, 101, 187. (29) Warszynski, P.; Lunkenheimer, K.; Czichocki, G. Langmuir 2002, 18, 2506.
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As compared with this investigation, it is interesting that the phenomenon of reversible nucleation was also observed at almost identical bulk concentrations and at very low surface pressures for quite different kinds of surfactants.26,27 In the second part of the Henry region at concentrations of c g 4 × 10-6 M, there is still a net contribution to the surface potential from increasing adsorption, although a much lower one due to the partial neutralization of the electric charges. Further discussion of the surface potential versus log c isotherm must necessarily remain vague for several reasons. The components of ∆V of a monolayer are related to three different contributions to the vertical component of the dipole moment of the film µ⊥.30 They are due to the component of the substrate (water) dipoles µ1, of the dipole moment of the head group µ2, and of the component of the terminal dipoles in the hydrophobic chain in the nonpolar phase (the methyl group in n-alkyl surfactants). Thus, the Helmholtz surface potential equation is read as
∆V ) 4πΓµ1 + 4πΓµ2 + 4πΓµ3
(6)
Γ denotes the number of adsorbate molecules per unit area. Because µ1 cannot be measured and the µ3 term is generally constant because n-alkyl chains usually form the hydrophobic part of common surfactants, eq 6 simplifies to
∆V ) 4πΓµD
(7)
where µD ) µ1 + µ2 + µ3 denotes the overall effective dipole moment that is characteristic of the dipole moment of the head group of the adsorption layer’s molecules.30,31 For ionic surfactants, a fourth component, ψ0, contributes to the surface potential, which is attributed to the electric double layer. Thus, eq 7 is then read as
∆V ) 4πΓµD + ψ0
(8)
To make the matter even more complicated, Demchak and Fort have pointed out that it is not enough to discriminate the various components of the surface potential; the corresponding local permittivities of the three-layer capacitor model of Davies and Rideal32 must also be used. According to them, eq 6 ought to be applied as
∆V ) 4πΓ
[
]
µ1 µ2 µ3 + + 1 2 3
(9)
The two kinds of parameters, µi and i, depend on the adsorption density. They cannot be discriminated experimentally and can only formally be separated. It is only the ratio of µi/i that can be determined from experimental ∆V versus Γ isotherms.31 Coming back to the discussion of the adsorption properties of the CHAPS solutions, including the results of the surface potential measurements, we would like to emphasize that these findings can only supplement the thermodynamic findings. In Figure 7, we have marked the various transition concentrations, ctr, determined from the thermodynamic evaluation. As you can see, the characteristic concentrations ctr are always observed at those surface potential regions in which the surface potential has already passed the preceding maximum. This means that the maximum value of ∆V was reached when the adsorption of this surface (30) Davies, J. T.; Rideal, E. K. Interfacial Phenomena, 2nd ed.; Academic Press: New York, 1963. (31) Barzyk, W.; Pomianowski, A.; Lunkenheimer, K. Bull. Pol. Acad. Sci., Chem. 1997, 45, 189. (32) Demchak, R. J.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191.
Figure 9. Surface potential (∆V) vs surface concentration (Γ) isotherm for surface-chemically pure CHAPS adsorption layers in the bulk concentration region of higher surface pressures (1.25 × 10-5 M e c e 1.4 × 10-3M; π g 2.8 mN/m; beyond the Henry region).
state had already been completed. It is interesting that the onset of the following new surface state leads first to a noticeable decrease in the surface potential value (Figure 9). Only upon further increases in the chemical potential does the surface potential again rise until the subsequent maximum value is reached. Furthermore, it is interesting that the four different maximum surface potential values are almost identical (i.e., 390, 400, 410, and 426 mV, respectively). However, the most striking result of the surface potential investigations is that the surface potential has already reached its maximum value at the end of the Henry region at a concentration as low as 1 × 10-5 M with a surface pressure of only 2.8 mN/m. This means that the relative contribution of the electric charge to the increase in the surface potential gradually vanishes with rising adsorption. How can this be understood? When we look at the limiting cross-sectional areas belonging to the corresponding surface states, this results in L I: AminI ) 109.4 Å2/molecule and L II: AminII ) 105.4 Å2/molecule and/or L III: AminIII ) 88.1 Å2/molecule. From these data, we conclude that upon increasing adsorption the CHAPS molecule’s limiting surface area demand is distinctly different for each characteristic adsorption state. This seems reasonable. Consequently, these changes are also to be reflected in the corresponding surface conformations of the CHAPS molecule. This is imaginable only by turning off the CHAPS molecule’s side group from the flat orientation to a more upright orientation. Now considering the complete dipolar dimethylammoniopropanesulfonate (APS) group and assuming a perpendicular orientation, this ought to result in the maximum contribution to the surface potential. This is not in agreement with the experimental findings. However, this side group also contains the adjacent polar carbon amide group. Its dipole component is directed opposite to that of the dipolar APS group. Thus, turning the whole side group toward a more upright position against the boundary layer will not reveal the full contribution of its dipole to the surface potential of the CHAPS adsorption layer but a weaker effective one. In addition, by such conformation the dipolar APS group might become more deeply immersed in the sublayer. This in turn would also weaken its influence on the electric adsorption properties. By a mechanism such as this, the surface potential behavior can be understood reasonably well. Within each characteristic
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adsorption state of the CHAPS molecule, its surface orientation remains unaltered. The maximum surface potential is reached at the adsorbate’s highest surface concentration in each state. The onset of the next adsorption state results in another surface orientation with a somewhat smaller surface potential. Only upon further increasing adsorption will the effective dipole moment, characteristic of this particular state, again gradually increase the surface potential until another slightly higher maximum value is achieved (Figure 9). Interestingly, passing into the surface state L III leads to a comparatively small change in the surface potential only but to a comparatively large decrease in surface area (cf. Figures 6 and 7). Furthermore, at the onset of state L III the surface excess has almost reached the state of saturation. This could be understood by taking into consideration the cholic acid molecule’s ability to alter its trans conformation to cis.24,25 This means that ring A of the cyclopentano-phenanthrene body will no longer remain linked to the B ring in the flat trans configuration but will turn into the tilted cis conformation by its reduction of the surface area requirement to a certain extent. It is to be expected that by this change in conformation the electric properties of the adsorption layer will not be altered very much because there remains only the hydrophobic part that will contribute to the change in surface potential. Concluding this discussion, we would like to repeat that a further discussion of the electric surface properties of this system must remain vague. So far, it is hardly possible to correctly describe the ∆V versus log c isotherm of a simple nonionic surfactant such as that of the homologous series of n-alkanols or n-alkanoic acids with respect to their molecular structure, conformation, and/or orientation. In comparison with them, the amphiphilic CHAPS molecule represents a rather complicated structure that carries various polar groups in addition to two oppositely charged ionic groups being distributed across the complicated molecular structure. It is not only the nontypical hydrophobic body that deviates extensively from the general surfactant’s extended n-alkyl chain but also the accumulation of various polar and ionic groups bound to it that results in the quite unusual amphiphilic structure of the CHAPS molecule. In addition, in the discussion of the surface potential behavior, we have neglected any possible change in the water structure and in the permittivities of the components of the adsorption layer, which may also influence its effective dipole moment. Furthermore, we also would like to point to the fact that we always used the Langmuir adsorption isotherm for the piecewise fitting of the σe versus log c isotherm. This method can adequately be used on a first approach basis. However, it is also possible that at least the LII and LIIIadsorption states have to be described by the Frumkin adsorption equation for regular surface behavior, which additionally takes into account surface interaction. It has been demonstrated that the surface excess calculation based on the Langmuir equation agrees well with the one independently obtained by the Gibbs equation (eq 5). Because we are mainly interested in a reliable surface pressure (π) versus surface area (A) isotherm and in a qualitatively correct decision on the reasonability of different adsorption states, we did not
Lunkenheimer et al.
attempt to discuss further details with regard to surface interaction. This would require further assessment, for instance, more measuring values within the different adsorption states. This is due to the interaction parameter of the Frumkin isotherm, which represents an additional free parameter for the matching problem.
Conclusions The fact that we have found various states in the adsorption of the CHAPS molecule at the air/water interface is extremely unusual for an amphiphilic molecule. As far as we know, this has never been observed. Any possible artifact caused by the possible interference of trace impurities, which has often been observed in surface chemistry, can be excluded because we have been dealing only with surface-chemically pure CHAPS solutions. The investigations of electric surface properties using surface potentials are complementary to the thermodynamic findings. The sensitivity of the surface potential measurement is extremely good for highly dilute solutions, where the adsorbate’s surface orientation changes the most. Concerning the decision on the occurrence of different surface states, the findings obtained with both methods are mainly in agreement with each other. By using a sectioned fitting of the adsorption isotherm within the characteristic intervals of the different adsorption states, it is then possible to deduce differences in the standard free energies of adsorption concerned, ∆∆G0ad. The numerical values of these differences are on the order of 1 kJ/mol. This will serve as additional support for the reasonable assumption of different adsorption states of the CHAPS molecule. We are not able to give a more detailed thermodynamic characterization of the kind of surface phases and/or phase transitions other than the first-order phase transition that occurs at a concentration of about 1 × 10-3 M. So far, we can only qualitatively determine that there are various additional surface states. An exact thermodynamic characterization requires further research work, namely, a specific investigation regarding the temperature dependence of the adsorption isotherm. Finally, the question must be raised as to why the CHAPS structure has become such an important application in biology. Thus, for example, in considering properties of n-alkyl surfactants, it seems rather unusual that CHAPS is able to form micelles. Our results indicate that the peculiar application properties of CHAPS mentioned in the Introduction are, of course, caused by the special structure within. The CHAPS molecule is chemically related to other biologically important substances. In turn, this results in rather exceptional adsorption properties that have never been observed with typical extended n-alkyl chain surfactants. They are obviously brought about by the ability of the CHAPS molecule to adopt different concentration-dependent adsorption states, which may comply with different biological and colloidal challenges. Acknowledgment. We thank Mrs. G. Wienskol for the purification of the solutions and the surface tension measurements, Professor R. Hirte for support in fitting the adsorption isotherm, and Dr. A. J. Prosser for preliminary measurements of dynamic surface tension. LA0629476
