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Adsorption Behavior of Surface Chemically Pure N-Cycloalkylaldonamides at the Air/Water Interface Dorota Piłakowska-Pietras,† Klaus Lunkenheimer,‡ Andrzej Piasecki,*,† and Maciej Pietras† Institute of Organic and Polymer Technology, Wrocław University of Technology, 50-370 Wrocław, Poland, and Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, D-14424 Potsdam, Germany Received November 24, 2004. In Final Form: February 18, 2005 Equilibrium surface tension (σe) and electric surface potential (∆Ve) versus concentration isotherms of the homologous series of N-cycloalkylaldonamides synthesized from cycloalkylamines (from cyclopentylto cyclododecylamine) and D-glucono-1,5-lactone (c-CnGA) or D-glucoheptono-1,4-lactone (c-CnGHA) (c-nC ) 5-12) were investigated at the air/water interface. The measurements were performed with aqueous, surface chemically pure surfactant solutions. Equilibrium surface tension vs concentration isotherms were evaluated to get the adsorption parameters, i.e., standard free energy of adsorption, ∆G°ads, saturation surface concentration, Γ∞, minimum surface area demand per molecule adsorbed, Amin, and interaction parameter, Hs. Increasing the size of the cycloalkyl moiety leads to a significant increase of the minimum surface area demand per molecule adsorbed. This fact, together with a decrease of the intermolecular interaction parameter suggests that the introduction of a more bulky cycloalkyl ring (c-nC ) 7 and 8) causes an attenuation of the hydrogen-bond network. This goes in line with the exceptional finding that the higher homologues revealed improved solubility in water. In addition, surface tension investigations suggest occurrence of a phase transition for the N-cyclooctylaldonamides at relatively small surface coverage. This observation is well supported by the surface potential measurements, for which the effect of possible changes in the molecules’ surface orientation is even more pronounced. Moreover, the concentration intervals of N-cyclooctylaldonamide in which the change in orientation is observed for either the surface tension or the surface potential isotherms are in very good agreement.
Introduction Sugar-based saccharide surfactants have evoked great interest from scientists and manufacturers because of a variety of possibilities and low-cost synthesis, due to cheap, natural, and renewable sources.1-5 They open new directions for detergent,5 cosmetic,6 pharmaceutical,7,8 food,9,10 and textile industries. In their molecules, the carbohydrate moiety is linked to a hydrophobic fragment by either an oxygen or a nitrogen atom. Alkylaldo(bio)namides belonging to the N-linked sugar-based surfactants are prepared in a selective reaction of aldonic or aldobionic acid or their derivatives (appropriate lactones) and long-chain alkylamines.5 Their properties depend characteristically on their structure: the general molecular architecture (single* To whom correspondence should be addressed. Fax: (+48-71) 320-36-78. E-mail:
[email protected]. † Wrocław University of Technology. ‡ Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. (1) Rybinski, W.; Hill, K. Alkyl Polyglycosides. In Novel Surfactants: Preparation, Applications, and Biodegradability; Holmberg, K., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1998; Vol. 74, pp 31-85. (2) So¨derman, O.; Johansson, I. Curr. Opin. Colloid Interface Sci. 2000, 4, 391. (3) Stubenrauch, C. Curr. Opin. Colloid Interface Sci. 2001, 6, 160. (4) Burczyk, B. Biodegradable and Chemodegradable Nonionic Surfactants. In Encyclopedia of Surface and Colloid Science; Hubbard, A., Ed.; Marcel Dekker: New York, 2002; pp 724-752. (5) Burczyk, B. Novel Saccharide-Based Surfactants. In Novel Surfactants: Preparation, Applications, and Biodegradability, 2nd ed., Revised and Expanded; Holmberg, K., Ed.; Surfactant Science Series; Marcel Dekker: New York, 2003; Vol. 114, pp 129-192. (6) Busch, P.; Hensen, H.; Tesmann, H. Tenside, Surfactants, Deterg. 1993, 30, 116. (7) Rico-Lattes, I.; Lattes, A. Colloids Surf., A 1997, 123-124, 37. (8) Uchegbu, I. F.; Vyas, S. P. Int. J. Pharm. 1998, 172, 33. (9) Franco, J. M.; Berjano, M.; Munoz, J.; Gallegos, C. Food Hydrocolloids 1995, 9, 111. (10) Akoh, C. C. J. Am. Oil Chem. Soc. 1992, 69, 9.
chain, single-head entities, dichained and dicephalic compounds, bolaamphiphiles, and gemini structures), the structure of the sugar residue (derivatives of mono-, di-, or higher saccharides), and the structure and number of substituents on the nitrogen atom. The derivatives of the aldonic acids and the primary alkylamines are sparingly soluble in water.5,11-13 Hence their applications as surfactants is limited. Derivatization of N-alkylgluconamides by sulfation with chlorosulfonic acid to monosulfate (and neutralization with NaOH) improves their solubility in water and performance properties.14 Insertion of an additional substituent in addition to the long alkyl chain at the nitrogen atom of the N-alkylaldonamides also improves their solubility in water. Thus, for example, N-alkyl-N-methylaldonamides, the derivatives of gluconic and glucoheptonic acids, form micellar solutions at room temperature,11,15,16 whereas N,N-di-n-alkyl-17 and N-alkylN-(2-hydroxyethyl)aldonamides18 do not. In this paper we report on the surface properties of the new group of saccharide surfactants with a cycloalkyl hydrophobic group. The investigation is the continuation of our studies that aim at an explanation of the effect of different substituents, located on the nitrogen atom of the aldonamide-type amphiphiles, on their behavior at (11) Pfannemu¨ller, B.; Welte, W. Chem. Phys. Lipids 1985, 37, 227. (12) Pfannemu¨ller, B.; Ku¨hn, I. Makromol. Chem. 1988, 189, 2433. (13) Andre, Ch.; Luger, P.; Gutberlet, T.; Vollhardt, D.; Fuhrhop, J.-H. Carbohydr. Res. 1995, 272, 129. (14) Mehltretter, C. L.; Furry, M. S.; Mellies, R. L.; Rankin, J. C. J. Am. Oil Chem. Soc. 1952, 29, 202. (15) Syper, L.; Wilk, K. A.; Sokołowski, A.; Burczyk, B. Prog. Colloid Polym. Sci. 1832, 110, 199. (16) Burczyk, B.; Wilk, K. A.; Sokołowski, A.; Syper, L. J. Colloid Interface Sci. 2001, 240, 552. (17) Piłakowska-Pietras, D.; Lunkenheimer, K.; Piasecki, A. J. Colloid Interface Sci. 2004, 271, 192. (18) Piłakowska-Pietras, D.; Lunkenheimer, K.; Piasecki, A. Langmuir 2004, 20, 1572.
10.1021/la047112e CCC: $30.25 © 2005 American Chemical Society Published on Web 03/30/2005
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Figure 1. General synthetic routes for N-cycloalkylaldonamides.
the solution/air interface. Our previous studies dealt with surface-active aldonamides with two symmetrical alkyl chains17 and/or with single-chained surfactants with an additional 2-hydroxyethyl moiety.18 In this study the hydrophobic group was altered in order to investigate the effect of its chemical constitution on the adsorption behavior. This seems interesting on the background of some new and unexpected features recently discovered with respect to changes of the aldonamides’ chemical structure. For example, we observed a distinct maximum in the dependence of the area demand per adsorbed molecule, Amin, on the number of carbon atoms in the N,Ndi-n-alkylaldonamides’ n-alkyl chain, nC.17 On the other hand, we only detected slight effects in the dependence of Amin on nC as well as on the structure of the saccharides’ headgroup for the N-alkyl-N-(2-hydroxyethyl)aldonamides.18 Thus, these findings contribute to our knowledge about the surface phenomena of novel aldonamide-type surfactants. It should be explicitly mentioned that these subtle observations could only be obtained by using the required grade of surface chemical purity. Only the purification procedure concerned resulted in accurate and reliable results that allowed precise demonstration of the influence of the surfactants’ structure on the adsorption behavior. Measurements of the electric surface potential were performed in addition to the surface tension studies. As the surface potential also strongly depends on adsorption, we can obtain complementary information about the surface properties of the surfactants. The results of such investigations on soluble surfactants’ electric adsorptions properties can be found elsewhere.19-23 Experimental Section Substances. General synthetic routes for N-cycloalkylgluconamides, c-CnGA, and N-cycloalkylglucoheptonamides, c-CnGHA, are presented in Figure 1. The surfactants under study were obtained in a one-way reaction of D-glucono-1,5-lactone (Sigma-Aldrich, Germany) or D-glucoheptono-1,4-lactone (SigmaAldrich, Germany), with the corresponding cycloalkylamine: cyclopentyl- (n ) 5), cyclohexyl- (n ) 6), cycloheptyl- (n ) 7), (19) Kamien´ski, B.; Waligo´ra, B.; Paluch, M. Bull. Pol. Acad. Sci. 1964, XII, 117. (20) Geeraerts, G.; Joos, P.; Ville, F. Colloids Surf., A 1993, 75, 243. (21) Lunkenheimer, K.; Czichocki, G.; Hirte, R.; Barzyk, W. Colloids Surf., A 1995, 101, 187. (22) Neys, B.; Joos, P. Colloids Surf. 1998, 143, 467. (23) Lunkenheimer, K.; Earnshaw, J. C.; Barzyk, W.; Dudnik, V. Prog. Colloid Polym. Sci. 2000, 116, 95.
cyclooctyl- (n ) 8), and cyclododecylamine (n ) 12) (all amines were purchased from Sigma-Aldrich). Synthesis of N-Cycloalkylaldonamides. General Procedure. A solution of cycloalkylamine (0.1 mol) in 20 mL of methanol was dropped carefully (0.5 h) into the stirred suspension of d-glucono1,5-lactone or D-glucoheptono-1,4-lactone (0.1 mol) in 150 mL of methanol at 50-55 °C. The reaction was continued for another 4 h. In the case of the cyclooctylamine derivatives the reaction mixture was then cooled to room temperature and filtered to remove traces of unreacted lactone and the solvent was removed. The solid residue was crystallized three times from an ethyl acetate/methanol mixture. In the case of cyclopentyl-, cyclohexyl-, cycloheptyl-, and cyclododecylamine derivatives the cooled reaction mixtures were filtered to separate the precipitated crude reaction products. All of them were then purified by repeated crystallization from methanol/water (cyclopentylamine derivatives), methanol (cyclohexylamine derivatives), ethyl acetate/ methanol (cycloheptylamine derivatives), or acetone/methanol (cyclododecylamine derivatives) mixtures. The yields of the crude reaction products were almost quantitative (97-99 mol %); however, the yields of the crystallized products of high purity were dependent on the efficiency of the crystallization procedures (Table 1). Representative 13C NMR analysis results, δ (ppm): for solutions of N-cyclooctylgluconamide, c-C8GA, in DMSO-d6 171.03 (CdO), 73.54, 72.38, 71.44, and 70.11 (C(dO)(CH2OH)4CH2OH), 63.37 (CH2OH), 48.21 (CHNH); carbon atoms of the cyclooctyl ring (their position in relation to the CHNH group is given in parentheses) 31.67 (2), 31.54 and 26.82 (3 and 8), 26.73 and 25.02 (5 and 7), 23.44 (4), and 23.32 (6). Analogous chemical shifts of carbon atoms of the D-gluconic acid residue (172.25, 73.6, 72.4, 71.5, and 63.3 ppm, respectively) were observed in the 13C NMR spectrum of N-dodecyl-N,N-bis[3-(gluconylamido)propyl]amine.24 The corresponding 1H NMR analysis for the solution of Ncyclohexylglucoheptonamide, c-C6GHA, in DMSO-d6, δ (ppm): 7.54 (d, 1 H) -NHCO-; 5.43 d, 1 H -C(dO)CH(OH)(CHOH)4CH2OH; 4.76 (d, 1 H) -C(dO)CH(OH)CH(OH)(CHOH)3CH2OH; 4.30-4.57 (m, 5 H) -C(dO)(CHOH)-(CHOH)4CH2OH; 3.303.85 (m, 8 H) -C(dO)(CHOH)5CH2OH and -C(dO)NHCH- (in cyclohexane rings). Residual protons of the cyclohexane ring absorb at δ ) 1.10-1.75 ppm. Methods. Structure Analysis. The chemical structure and the purity of the synthesized aldonamides were evaluated by elemental analysis and by 13C and 1H NMR spectra. The elemental analyses were carried out using a Perkin-Elmer 2400CHN apparatus. The accuracy of the C, H, and N determination was (0.02%. The 13C and 1H NMR spectra of the surfactant solutions in DMSO-d6 or D2O were measured using a Bruker Avance DRX300 spectrometer (Bruker, Karlsruhe, Germany). Melting points were not corrected. High-Performance Purification. Aqueous stock solutions of the N-cycloalkylaldonamides were purified by an automatically (24) Wilk, K. A.; Syper, L.; Burczyk, B.; Sokołowski, A.; Domagalska, B. W. J. Surfactants Deterg. 2000, 3, 185.
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Table 1. Data of Physicochemical Properties of the Homologous Series of N-Cycloalkylgluconamides, c-CnGA, and N-Cycloalkylglucoheptonamides, c-CnGHA compound
yield (mol %)
mp (°C)
formula
mol wt
C
calculated (%) H
N
C
found (%) H
N
c-C5GA c-C5GHA c-C6GA c-C6GHA c-C7GA c-C7GHA c-C8GA c-C8GHA c-C12GA c-C12GHA
94.0 90.0 91.0 86.2 83.0 81.0 79.9 75.3 78.0 79.1
172-174 161-163 176-178 148-150 132-133 109-111 107-109 110-112 170-172 190-192
C11H21O6N C12H23O7N C12H23O6N C13H25O7N C13H25O6N C14H27O7N C14H27O6N C15H29O7N C18H35O6N C19H37O7N
263.29 293.32 277.32 307.35 291.35 321.38 305.37 335.40 361.48 391.51
50.17 49.14 51.97 50.80 53.59 52.32 55.06 53.72 59.81 58.29
8.04 7.90 8.36 8.20 8.65 8.47 8.91 8.72 9.76 9.53
5.32 4.78 5.05 4.56 4.81 4.36 4.58 4.18 3.87 3.58
50.30 49.21 51.71 50.90 53.50 52.30 55.28 53.60 59.70 58.20
7.92 8.01 8.20 8.32 8.80 8.51 8.76 8.64 9.70 9.41
5.14 4.79 5.05 4.31 4.71 4.31 4.36 4.21 3.97 3.48
Figure 2. Equilibrium surface tensions, σe, vs logarithm of concentration, c, isotherms for the homologous series of N-cycloalkylaldonamides: N-cycloalkylgluconamides, c-CnGA (b), and N-cycloalkylglucoheptonamides, c-CnGHA (O), measured at 295 K. operating high-performance purification apparatus.25 In this technique the surface is aspirated periodically in order to remove the comparatively stronger surface-active impurities until the state of “surface chemical” purity is achieved. The grade of purity was judged by applying the criterion proposed in refs 26 and 27. Surface Tension Measurements. Surface tension of aqueous solutions of surfactants was determined by using the du Nou¨y ring technique and an automatic ring tensiometer, Lauda TE1M, at 295 K (22 °C). Modifications necessary for surfactant solutions were applied.28,29 Surface Potential Measurements. Surface potential was measured using the vibrating capacitor method also known as the Kelvin probe.30,31 The device (Kelvin Probe SP1) was purchased from Nanofilm Technologie GmbH, Germany. The measurements were carried out at room temperature, 22 ( 2 °C, and the readings after 1 h were used for surface potential evaluation. The accuracy of measurement was (10 mV and better.
Results and Discussion Surface Tension Measurements. The equilibrium surface tension vs concentration isotherms of surface chemically pure aqueous solutions of N-cycloalkylaldonamides are presented in Figure 2. The surfactants under study do not form micelles in aqueous solutions at room temperature, comparable to other aldonamides such as N,N-di-n-alkylaldonamides17 or N-alkyl-N-(2-hydroxy(25) Lunkenheimer, K.; Pergande, H.-J.; Kru¨ger, H. Rev. Sci. Instrum. 1987, 58, 2313. (26) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1987, 120, 176. (27) Lunkenheimer, K.; Miller, R. J. Tenside Deterg. 1979, 16, 312. (28) Lunkenheimer, K. Tenside Deterg. 1982, 19, 272. (29) Lunkenheimer, K.; Wantke, K.-D. Colloid Polym. Sci. 1981, 259, 354. (30) Potter, E. F. J. Am. Chem. Soc. 1937, 59, 1883. (31) Lunkenheimer, K.; Barzyk, W.; Hirte, R.; Rudert, R. Langmuir 2003, 19, 6140.
ethyl)aldonamides.18 The isotherms extend close to the solubility limits of the amphiphiles in water (Table 2). It is remarkable, that the homologues containing from five up to eight carbon atoms in the cycloalkyl ring have almost identical solubility in water. Generally within a homologous series of standard surfactants the increase of the carbon atom number in the hydrophobic n-alkyl chain causes a noticeable decrease of their solubility in water (as, for example, in straight-chain aliphatic alcohols32,33 or carboxylic acids34,35). The low solubility of the Ncycloalkylaldonamides with small cycloalkyl rings is attributed to strong intermolecular hydrogen bond interactions similar to those observed for the homologous N-alkylaldonamides CH3(CH2)nNHCOCH2(CHOH)xCH2OH.11 With increase in the ring size up to cyclooctyl, the hydrogen bonds in the crystal phase are probably attenuated (this is also reflected in a considerable lowering of the melting point, see Figure 3). This in turn results in an increase of the solubility of the compounds in water, although their surface activity increases. However, a further increase of the cycloalkyl ring size (up to derivatives of cyclododecylamine) again leads to an increase of their melting points and to a dramatic decrease of their solubility in water. This is why the investigation of the adsorption isotherms of the homologous cyclododecylamine derivatives c-C12GA and c-C12GHA was impossible (their solubility in water was only 3 × 10-4 and lower than 1 × 10-4 mol/dm3, respectively). Interesting observations were also made for the adsorption behavior of the N-cycloalkylaldonamides. The equilibrium surface tension vs concentration isotherms of the N-cyclooctylaldonamides differs from the corresponding ones of the lower homologues. The isotherms of N-cyclopentylgluconamide, the N-cyclohexylaldonamides, and the N-cycloheptylaldonamides were evaluated to get the adsorption parameters by using Frumkin’s adsorption equation for regular surface behavior
{
(
(
) (
∆σe ) σw - σe ) - RTΓ∞ ln 1 -
) ( )}
Γ Γ + a′ Γ∞ Γ∞
2
(1a)
with
c ) aL
)
Γ 2HsΓ exp Γ∞ - Γ RTΓ∞
(1b)
where Γ is the surface concentration, Γ∞ is the saturation surface concentration, c is the bulk concentration, aL is (32) Kinoshita, K.; Ishikawa, H.; Shinoda, K. Bull. Chem. Soc. Jpn. 1958, 31, 1081. (33) Yaws, C. L.; Hopper, J. R.; Sheth, S. D.; Han, M.; Pike, R. W. Waste Manage. 1997, 17, 541. (34) Brimblecombe, P.; Clegg, S. L.; Khan, I. J. Aerosol Sci. 1992, 23, 901. (35) Cyberlipid Center www home page http://www.cyberlipid.org/ fa/acid0001.htm
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Table 2. Solubility in Water and Adsorption Parameters of N-Cycloalkylgluconamides, c-CnGA, and N-Cycloalkylglucoheptonamides, c-CnGHA
surfactant
solubility limit 103 (mol/dm3)
equation of state
Γ∞ (10-6) (mol/m2)
aL (mol/dm3)
Amin (Å2/molecule)
-∆G°ads (kJ/mol)
Hs (kJ/mol)
s (mN/m)
µtot (D)
Vmax (V)
c-C5GA c-C6GA c-C6GHA c-C7GA c-C7GHA c-C8GA c-C8GHA
∼300 ∼100 ∼100 ∼80 ∼80 ∼100 ∼100
Frumkin Frumkin Frumkin Frumkin Frumkin Henry-Frumkin Henry-Frumkin
6.22 6.23 5.71 3.82 4.00 3.79 4.09
5.01 × 10-1 1.99 × 10-1 2.07 × 10-1 5.59 × 10-2 5.34 × 10-2 1.37 × 10-2 1.04 × 10-2
26.7 26.6 29.1 43.5 41.5 43.8 40.5
1.7 3.9 3.9 7.1 7.2 10.5 11.2
1.70 2.59 2.73 3.25 2.61 2.30 0.947
(0.17 (0.09 (0.17 (0.13 (0.20 (0.23 (0.21
0.280 0.260 0.386 0.435 0.453 0.489
0.400 0.340 0.335 0.395 0.390 0.455
Figure 3. Melting points in dependence on the number of carbon atoms in the cycloalkyl ring, c-nC, of the N-cycloalkylaldonamides: N-cycloalkylgluconamides, c-CnGA (b), and Ncycloalkylglucoheptonamides, c-CnGHA (O).
the surface activity parameter, σw and σe are the surface tension of pure water and equilibrium surface tension of surfactant solution, respectively, a′ ) Γ∞Hs (Frumkin’s interaction parameter) where Hs is the interaction parameter according to Lucassen-Reynders,36 R is the universal gas constant, and T is the absolute temperature. This equation was used for the calculation of the adsorption parameters of N-cyclopentyl-, N-cyclohexyl-, and Ncycloheptylaldonamides (Table 2). This equation was already used to calculate the adsorption parameters of the N,N-di-n-alkylaldonamides17 and N-alkyl-N-(2-hydroxyethyl)aldonamides.18 Evaluating the σe vs log(c) isotherms of N-cyclooctylgluconamide and N-cyclooctylglucoheptonamide, it turned out that the original Frumkin equation was unsatisfactory for describing the experimental isotherm within the entire concentration interval. To get a satisfactory description of the experimental data, the two-state approach proposed by Lunkenheimer and Hirte37,38 was applied. In this approach it is assumed that the adsorbate’s surface orientation can occur in two alternative ways: a flat one at comparatively low surface coverage and an (more or less) upright one at higher surface densities. The equation for describing the adsorption consists of two contributions, one each for the adsorbate’s particular surface orientation
∆σe ) R∆σe(1) + (1 - R)∆σe(2)
(2)
where ∆σe(1) covers the Traube-Henry region (for low surfactant concentrations)
∆σe(1) ) Kc ) RTΓ(c)
(3)
whereas ∆σe(2) covers the concentration interval of higher surfactant concentrations that is described by the Frumkin equation (eqs 1a and 1b). K is called Henry’s constant, (36) Lucassen-Reynders, E.-H. Prog. Surf. Membr. Sci. 1973, 10, 253. (37) Lunkenheimer, K.; Hirte, R. J. Phys. Chem. 1992, 96, 8683. (38) Hirte, R.; Lunkenheimer, K. J. Phys. Chem. 1996, 100, 13786.
and R denotes the transition function, varying with c between 1 and 0. Following this approach, the transition from the adsorbate’s surface orientation at low coverage to that at high coverage occurs successively within a definite concentration interval the width of which depends on the amphiphile’s geometric conditions. The interval is called the “transition region”. The advantage of this approach is that the complete experimental σe vs log c isotherm can be matched satisfactorily with high precision and reliability. The adsorption parameters of N-cyclooctylaldonamides calculated by this approach are given in Table 2. This two-state approach was successfully employed for describing the adsorption behavior of n-alkanoic acids,31 n-alkyldimethylphosphine oxides,39 hemicyanine dyes,40 and 2-n-alkyl-1,3-dioxane derivatives.41,42 However, opposite to the σe vs log c isotherms of these surfactants, those of N-cyclooctylgluconamide and N-cyclooctylglucoheptonamide do not show a monotonic dependence but reveal a kink at a narrow particular concentration range. Moreover, after evaluation of the N-cyclooctylaldonamide σe vs log c isotherms by the two-state approach, it turned out that the resulting width of the transition interval β became extremely small and/or zero. This feature indicates that the usual “transition region” does not exist, but there is a phase transition. This means that the change of the adsorbate’s surface conformation and/or orientation does not occur continuously but discontinuously by an all-ornothing mechanism. Evaluating the two parts of the isotherm for the concentration regions below and/or above the critical concentration of that “transition interval” separately results in two branches of the corresponding surface tension decrease, ∆σe ) σw - σe, vs surface area, A, isotherm (Figure 4). As one can take from this figure the discontinuities in the ∆σe vs A isotherms are more distinct than the σe vs log c isotherms. Thus, these characteristics underline phenomena of phase transition, presumably of first order. True phase transitions in surface chemically pure surfactant adsorption layers at room temperature are very rare. Generally, a specific chemical structure is required that would allow extremely strong interaction between the adsorbed amphiphiles. Such amphiphiles are of limited solubility and do not form micelles. In such cases there is a critical value of the interaction parameter for phase transition, Hcrs g 2RT. The adsorption isotherm of n-dodecanoic acid adsorption is an example for it.31 However, as we can see from Figure 5, the experimentally determined Hs values for the N-cycloalkylaldonamides are relatively small, still well below the critical value of (39) Lunkenheimer, K.; Haage, K.; Hirte, R. Langmuir 1999, 15, 1052. (40) Lunkenheimer, K.; Laschewsky, A.; Warszynski, P.; Hirte, R. J. Colloid Interface Sci. 2002, 248, 260. (41) Lunkenheimer, K.; Burczyk, B.; Piasecki, A.; Hirte, R. Langmuir 1991, 7, 1765. (42) Lunkenheimer, K.; Piasecki, A.; Burczyk, B.; Hirte, R. Langmuir 2000, 16, 6982.
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Figure 6. Model structures of axial (A) and equatorial (E) conformers of N-cyclooctylglucoheptonamide, c-C8GHA. Figure 4. Surface tension decrease, ∆σe ) σw - σe, vs surface area, A, isotherm of N-cyclooctylgluconamide resulting from the separate evaluation of the concentration intervals of the corresponding σe vs log c isotherms below and/or above the critical transition interval.
Figure 5. Surface interaction parameter, Hs, of the homologous series of N-cycloalkylaldonamides in dependence on the number of carbon atoms in the cycloalkyl ring, c-nC: N-cycloalkylgluconamides, c-CnGA (b), and N-cycloalkylglucoheptonamides, c-CnGHA (O).
phase transition, Hcrs ) 2RT. So far we do not yet know the exact mechanism of this phase transition although the ∆σe vs A isotherms suggest a first-order phase transition. However, it was only recently that another type of phase transition in adsorbed monolayers of different surfactants was observed43 that need not necessarily be induced by extreme surface interaction but may occur at negligible surface interaction, too.44,45 Furthermore, the investigations show an attenuation of the intermolecular interactions with the increase of the cycloalkyl ring from cycloheptyl to cyclooctyl (Figure 5). This parameter grows initially with the increase of the ring size in the N-cycloalkylgluconamides, being maximal for the cycloheptyl derivative, but decreasing again for the most bulky cyclooctyl derivative. In the case of the N-cycloalkylglucoheptonamides, the cyclohexyl and cycloheptyl derivatives have almost identical interaction but that for cyclooctyl is again considerably lower. To under(43) Prosser, A.; Retter, U.; Lunkenheimer, K. Langmuir 2004, 20, 2720. (44) Lunkenheimer, K.; Barzyk, W.; Hirte, R. Unusual Phase Transition in Adsorption Layers. Lecture delivered at the XVIth Conference of the European Colloid and Interface Society, Paris, 2002, Book of Abstracts OC-M11. (45) Prosser, A. J.; Retter, U.; Lunkenheimer, K. Investigations of Phase Transitions in the Adsorbed Monolayers of Aqueous Nonionic Surfactant Systems. Poster at the XVIIth Conference of the European Colloid and Interface Society, Florence, 2003, Book of Abstracts P6/ 101; to be submitted.
stand the observed behavior, the molecular structure of surfactants is taken into account. The investigated Ncycloalkylaldonamides can exist as two conformers due to the presence of the nonplanar, monosubstituted cycloalkyl ring, and the different (axial or equatorial) positions of the amide group in the cycloalkyl ring (Figure 6). Spectroscopic investigations, especially low-temperature NMR studies showed, however, that the differences in the energy of formation among monosubstituted cycloalkane stereoisomers are so insignificant that the existence of two stable conformers was confirmed only at low temperatures (e-70 °C).46 At room temperature they exist in dynamic equilibrium (for the monosubstituted derivatives of cyclohexane the rate of ring inversion was estimated as 2 × 105 s-1, practically independent of the kind of the substituent) with negligible preference of equatorial conformers. Modeling optimal structures with minimal total energy of the investigated N-cycloalkylaldonamides (with energetically favorable conformations of cycloalkane rings: envelope, chair, twisted chair, and boat-chair for cyclopentane, cyclohexane, cycloheptane, and cyclooctane, respectively) proved that in every case the total energies of the two possible conformers are so close to each other that it was not possible to show the predominant one. These results need not necessarily mean that the same is true for the stereochemical behavior of the amphiphilic N-cycloalkylaldonamide molecules in the adsorption layer at the air/water interface. An average cross-sectional area of about 27Å2 per molecule adsorbed, which is found for the lower ring homologues of c-nC e 6, corresponds roughly to the crosssectional area of the extended (hydrated) aldonamide group. The cross section of the flat cyclopentyl and/or cyclohexyl ring corresponds approximately with this value. Thus, with respect to the cycloalkyl aldonamides’ arrangement in the adsorption layer, it does not matter whether an axial or an equatorial conformation was realized as long as the lower ring homologues with c-nC e 6 are concerned. However, increasing the ring size of the hydrophobic cycloalkane makes the cross section in the flat oriented position get successively bigger than that of the cross section of the hydrophilic extended aldonamide entity. In this case, i.e., for c-nC g 7, it seems to be probable that at low surface concentrations an axial conformation of the adsorbate is favorable over the equatorial one because of less contact of the hydrophobic hydrocarbon ring with water. At higher surface concentrations, however, an even closer packing in the adsorption layer can be realized by the equatorial conformation, provided that the orientation of the adsorbed amphiphile is altered such that the whole molecule becomes directed upright to the (46) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of organic compounds; John Wiley & Sons: New York, 1994; Chapter 11.
Adsorption Behavior
Figure 7. Cross-sectional area, Amin, of the adsorbed Ncycloalkylaldonamides in dependence on the number of carbon atoms in the cycloalkyl ring, c-nC: N-cycloalkylgluconamides, c-CnGA (b), and N-cycloalkylglucoheptonamides, c-CnGHA (O).
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Figure 8. Standard free energy of adsorption, ∆G°ads, of the homologous series of N-cycloalkylaldonamides in dependence on the number of carbon atoms in the cycloalkyl ring, c-nC: N-cycloalkylgluconamides, c-CnGA (b), and N-cycloalkylglucoheptonamides, c-CnGHA (O).
interface. Thus, a more favorable orientation in the adsorbed layer is realized by a change in the molecule’s conformation although the overall cross-sectional area is bigger then. As we have found previously that the interfacial properties well reflect even most subtle structural variations such as alternation in homologous series of amphiphiles and stereogeometrical and counterion effects47 it is well imaginable that the negligible thermodynamic differences between the stereoisomers observed in the gas phase may become distinguishable at the air/water interface. The importance of the intermolecular hydrogen-bond network on the adsorption behavior of N-cycloalkylaldonamides becomes more evident when we consider the minimum area demand per molecule adsorbed, Amin. The dependence of Amin on the number of carbon atoms of the cycloalkyl ring, c-nC, is shown in Figure 7. It is interesting that for the N-cyclopentyl- and N-cyclohexylaldonamides the area demand per molecule adsorbed is comparatively small. The values of Amin are very similar to those for simple surfactants with a single alkyl chain, like nalkanols or n-alkanoic acids.31 Such close packing of the molecules in the adsorption layer can occur because of strong intermolecular hydrogen-bond interactions. It seems that increasing the adsorbates’ ring size from cyclopentyl to cyclohexyl cannot significantly weaken the prevailing interactions. This behavior will be better understood if we consider the related aldonamide derivatives with a single alkyl chain. The latter are practically insoluble in water, and this is attributed to the very strong hydrogen-bond network.11,12,48,49 Thus, with respect to these facts, the relatively low solubility of the N-cyclopentyl- and N-cyclohexylaldonamides can better be understood. In the case of higher homologues a steep increase of Amin of about 15 Å2/molecule is observed. A regular increase of the cycloalkyl ring size by one methylene group causes an irregular increase of the distance between the adsorbed molecules. This might facilitate penetration of water molecules between the adsorbed surfactants, increase their hydration, and reduce the intermolecular interaction. This effect may well explain the lowering of the interaction between the adsorbate molecules (Figure 5) and the increase of the minimal surface area demand
in the adsorption layer by the increase of the cycloalkyl ring’s size. It explains also the relatively high solubility in water of compounds with cycloheptyl and cyclooctyl rings. Additional information about the adsorption properties of the N-cycloalkylaldonamides can be taken from their standard free energies of adsorption, ∆G°ads, calculated as ∆G°ads ) RT ln aL. Among the group of the Nfunctionalized aldonamide surfactants,16-18 the N-cycloalkylaldonamides reveal the lowest tendency for adsorption. A not straight-line dependency of, ∆G°ads, on the number of carbon atoms in the cycloalkyl rings, c-nC, is presented in Figure 8. The values of standard free energies of adsorption for the two aldonamide derivatives of cyclohexyl and cycloheptyl rings are practically identical (Table 2). This means that the introduction of another -CHOH entity into the gluconamide group of the cyclohexyl and/or cycloheptyl derivatives does not have any effect on the amphiphiles surface activity. However, for the cyclooctyl derivatives there seems to be a small but distinct difference between the surface activities of the gluconamide and the glucoheptonamide structure. NCyclooctylglucoheptonamide’s surface activity is slightly stronger (more negative ∆G°ads value) than that of the corresponding N-cyclooctylgluconamide. This then means that the insertion of an additional -CHOH group into the sugar residue slightly increases the hydrophobic character of the resulting N-cycloalkylglucoheptonamide if the hydrophobic cycloalkyl ring exceeds a certain size. (This fact again hints to a different adsorption mechanism of the bigger cycloalkyl ring homologues.) An effect like this was already observed for the homologous series of N-alkylN-(2-hydroxyethyl)aldonamides.18 Summarizing, we would like to underline that we are reluctant to draw further conclusions about the influence of the geometrical structure on the adsorption properties because this would require investigation of additional homologous members of these series. That’s why we give here the linear dependence of, ∆G°ads, on the number of carbon atoms in the cycloalkyl rings, c-nC, averaged for the two homologues series of cycloaldonamides. The corresponding relationship is
(47) Lunkenheimer, K. Purity of Surfactants and Interfacial Research. In Encyclopedia of Surface and Colloid Science; Hubbard, A., Ed.; Marcel Dekker: New York, 2002; pp 3739-3772. (48) Sack, I.; Macholl, S.; Fuhrhop, J.-H.; Buntkowsky, G. Phys. Chem. Chem. Phys. 2000, 2, 1781. (49) Messerschmidt, Ch.; Svenson, S.; Stocker, W.; Furhhop, J.-H. Langmuir 2000, 16, 7445.
for c-CnGA and c-CnGHA with c-nC ) 6, 7. The average contribution of one methylene group to the overall free energy of adsorption of the N-cycloalkylaldonamides, ∆∆G°ads/CH2, was estimated as ca. -3.3 kJ/CH2. The absolute value is relatively higher than those of N-alkyl-
∆G°ads ) -3.25 × c-nC + 15.6
(4)
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Piłakowska-Pietras et al.
Figure 9. Equilibrium surface potential, ∆Ve, vs logarithm of concentration, c, isotherms for the homologous series of N-cycloalkylaldonamides: N-cycloalkylgluconamides, c-CnGA (b), and N-cycloalkylglucoheptonamides, c-CnGHA (O), measured at 295 K.
N-(2-hydroxyethyl)aldonamides18 and comparable with the values obtained for N,N-di-n-alkylaldonamides.17 The reported contributions for different single n-alkyl-chained surfactants21,39,42,50 are about 30% lower than those for the N-cycloalkylaldonamides. This result is reasonable because of the branched structure of the cycloalkyl hydrophobic residue. In fact, one could regard this configuration approximately as two parallel short alkyl chains. Surface Potential Measurements. The equilibrium surface potential, ∆Ve, vs concentration isotherms of the surface chemically pure solutions of N-cycloalkylaldonamides are presented in Figure 9. The increase of the surface potential with rising number of carbon atoms in the hydrophobic group is rather unusual. For homologous series of simple surfactants, like n-alkanols and n-alkanoic acids, elongation of the alkyl chain leads to a significant decrease of the surface potential.20 The observed increase can be explained when an increase of hydration of the adsorbed molecules with the increase of the cycloalkyl ring is considered. The contribution of water molecules to the measured surface potential is much higher than that of the surfactant molecules. It means that the real surface potential of water is high and the adsorbing surfactant molecules decrease the initial value of the interfacial potential. This conclusion is in agreement with considerations of Kamien´ski.51 For the description of electric properties of surfactant solutions the simplified form of the Rideal and Davies equation52 can be applied
∆Ve ) (2.27 × 109)Γµov
(5)
where ∆Ve is the experimental value of the equilibrium surface potential, Γ is the surface concentration (determined from the adsorption isotherm), and µov is the overall surface dipole moment. The experimental values of the surface potential, ∆Ve, were used for the evaluation of the surface potential at saturation, ∆Vmax, and the total surface dipole moment, µtot (if ∆Ve ) ∆Vmax then µov ) µtot). The values obtained for the surface potential at saturation of the interface, ∆Vmax, and for the total surface dipole moment, µtot, are listed in Table 2. (50) Lunkenheimer, K.; Laschewsky, A. Prog. Colloid Polym. Sci. 1992, 9, 239. (51) Kamien´ski, B. In Materiały Konferencji Elektrochemicznej PAN, Warszawa, 1957; pp 17-70. (52) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York and London, 1963.
Figure 10. Equilibrium surface potential, ∆Ve, in dependence on surface area per molecule adsorbed, A, of N-cyclooctylgluconamide, c-C8GA (b) and N-cyclooctylglucoheptonamide, c-C8GHA (O).
Studies of the electric phenomena can give complementary information on the adsorption behavior of surfactants at the air/water interface. It was interesting to know whether the investigations on the surface potential could support our observations and conclusions achieved from the surface tension measurements. The dependencies of the equilibrium surface potential on the surface area demand per adsorbed molecule of the N-cyclooctylaldonamides (∆Ve-A isotherms) presented in Figure 10 reveal a characteristic behavior. In a similar way as for the surface tension vs concentration isotherms there is a certain small transition concentration region in the ∆Ve-A isotherms in which the continuous run of the low concentration branch does not match that one for the higher concentrations. However, as with the former, at this stage we cannot yet be sure whether there is indeed a plateau or whether there is an inflection in the two isotherms within the transition region. In any case, the ∆Ve-A isotherms underline the existence of phase transition of a still unknown type. The presence of a transition region with a distinct plateau in the ∆σe-A isotherms (i.e., no inflection region) indicating an abrupt change in the spread molecules’ orientation via a first-order phase transition was proved convincingly by Pallas and Pethica53 for insoluble monolayers of really pure n-pentadecanoic and n-hexadecanoic acids, and comprehensively discussed by Lunkenheimer et al.31 for the homologous series of soluble and insoluble n-alkanoic acids. The presence of transition regions in the ∆σe-A isotherms of N-cycloalkylaldonamides is almost indiscernible but in the ∆Ve-A isotherms it is clearly visible. It seems that the electric properties are more sensitive to the conformational changes of surfactant molecule within the adsorption layer. It is remarkable, however, that the results of the surface tension and surface potential measurements are in very good agreement. The corresponding transition concentrations fit with high accuracy the range of concentrations where the kinks in the adsorption isotherms are observed. Most of the theoretical approaches describing electrical properties of the amphiphile’s adsorption layer at the air/ water interface are based on the assumption that the surfactant’s surface orientation at the interface remains unaltered in the whole range of concentration. Consequently, the successively adsorbing molecules give a nonchanging, steady contribution to the surface potential value, and therefore the overall surface dipole moment (53) Pallas, N. R.; Pethica, B. A. Langmuir 1985, 1, 509.
Adsorption Behavior
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Figure 11. Equilibrium surface potential, ∆Ve, in dependence on surface coverage, Θ, of aqueous solutions of N-cycloheptylaldonamides: c-C7GA (1) and c-C7GHA (3), and N-cyclooctylaldonamides: c-C8GA (b) and c-C8GHA (O) (solid lines). Dotted and dashed lines denote behaviors calculated by eq 5.
Figure 12. Surface dipole moment in dependence on the surface coverage, Θ, of aqueous solutions of N-cycloheptylaldonamides: c-C7GA (1) and c-C7GHA (3) and N-cyclooctylaldonamides: c-C8GA (b) and c-C8GHA (O) (solid lines). Dotted and dashed lines denote behaviors calculated by eq 5.
does not change during adsorption. The dependencies of the surface potential and the overall surface dipole moment on the surface coverage for several N-cycloalkylaldonamides are shown in Figures 11 and 12. Following the above-discussed conclusions, the N-cyclooctylaldonamides should behave much different from that of their homologues with smaller hydrophobic rings. Indeed, as we can see from Figures 11 and 12, the electric properties are very exceptional. For N-cyclooctylaldonamides the experimental values of the equilibrium surface potential, ∆Ve, and the total surface dipole moment, µtot, agree with the theoretically predicted values (from eq 7, assuming µov to be constant and equal to µtot) only for concentrations, where the surface is (almost) saturated. It is reasonable because in this range of concentration the upright orientation of surfactant molecules in the adsorption layer is thermodynamically favored. The observed discrepancies from the theoretical models can be explained only if we assume that the cyclooctyl surfactants’ surface orientation changes during adsorption. It is especially pronounced in the values of the surface dipole moment (Figure 11) where characteristic maxima are observed. Furthermore, besides very distinct deviations from the theoretical behavior observed with the N-cyclooctylaldonamides they become already noticeable for the solutions of the N-cycloheptylaldonamides (Figures 11 and 12), but to a much smaller extent. This result makes us suggest that the change in the adsorption of the N-cycloalkylaldonamide molecules is not specific for the cyclooctyl derivatives but obviously sets already in with the smaller cycloheptyl ring. However, this behavior was only observed by surface potential, but not by surface tension measurements. It can be understood when taken into account that the most significant changes of the examined electric properties take place within a narrow range of surface coverage only. Summarizing, we can conclude that phase transitions do occur not only for
the N-cyclooctyl- but obviously also for the N-cycloheptylaldonamides. Conclusions The adsorption behavior of the new class of saccharide surfactants, the hydrophobic group of which consists of cycloalkyl instead of n-alkyl (N-cycloalkylgluconamides and N-cycloalkylglucoheptonamides), shows some peculiar features on the influence of the molecular structure on the surface properties. Unlike n-alkyl surfactants, Ncycloalkylaldonamides’ limiting surface area demand per molecule adsorbed increases with increasing cycloalkyl ring size. In addition, the cyclooctyl derivatives reveal a phase transition at low surface pressures that can probably be attributed to the amphiphiles’ conformation changes from axial to equatorial position of the cycloalkyl residue. Surface potential investigations, which seem to be more sensitive to the molecules’ conformation changes in the adsorbed layers at low surface pressures, suggest that such change in conformation sets in already for the smaller cycloheptyl derivatives. It is interesting to note that even bulk properties reflect the unusual trend within the homologous series of the N-cycloalkylaldonamide surfactants. Thus, the solubility in water does not decrease with increasing number of carbon atoms in the cycloalkyl ring. This is true also for the melting points of these compounds which are minimal for either c-nC ) 7 (glucoheptonamide) and/or c-nC ) 8 (gluconamide). These novel results about the influence of the amphiphiles’ subtle structural changes on their adsorption properties could only be achieved by strictly obeying the requirements for surface chemical purity of the surfactant solutions. The role of hydrogen bonds on the adsorption behavior observed with the N-cycloalkylaldonamides is under our intensive investigation. LA047112E
