Adsorption Behavior of Surface-Chemically Pure N-Alkyl-N-(2

N-alkyl-N-(2-hydroxyethyl)aldonamides (alkyl: n-C6H13, n-C8H17, n-C10H21, n-C12H25, and n-C14H29) were obtained in the reaction of long-chain ...
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1572

Langmuir 2004, 20, 1572-1578

Adsorption Behavior of Surface-Chemically Pure N-Alkyl-N-(2-Hydroxyethyl)aldonamides at the Air/Water Interface Dorota Piłakowska-Pietras,†,§ Klaus Lunkenheimer,# and Andrzej Piasecki*,† Institute of Organic and Polymer Technology, Wroclaw University of Technology, 50-370 Wroclaw, Poland and Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, D-14424 Potsdam, Germany Received July 21, 2003. In Final Form: October 28, 2003 N-alkyl-N-(2-hydroxyethyl)aldonamides (alkyl: n-C6H13, n-C8H17, n-C10H21, n-C12H25, and n-C14H29) were obtained in the reaction of long-chain N-alkyl-N-(2-hydroxyethyl)amines with D-glucono-1,5-lactone and D-glucoheptono-1,4-lactone. The adsorption isotherms were obtained from surface tension measurements of aqueous solutions of surface-chemically pure surfactants. The experimental equilibrium surface tension versus concentration isotherms were evaluated by the Frumkin adsorption equation to get the adsorption parameters, namely, standard free energy of adsorption, ∆G°ad, saturation adsorption, Γ∞, minimum surface area demand per molecule adsorbed, Amin, and interaction parameter, Hs. The investigated functionalized alkylaldonamides show improved solubility in comparison with the corresponding sugar derivatives of the primary amines. The introduction of the -CHOH moiety into the saccharide headgroup causes a noticeable increase of the hydrophobic character of surfactant. The minimum surface area demand, Amin, is slightly greater for glucoheptonamides than for the corresponding gluconamides. The practically constant Amin value within the homologue series of the aldonamides indicates that the obtuse hydroxyethyl residue is the determining factor for the arrangement of the adsorbed surfactants in the interfacial layer.

Introduction The nonionic saccharide surfactants of the aldonamide type are usually obtained in the reaction of an amine or its derivative with aldonic acid or the related aldonlactone. These compounds are characterized by the formula A-C(O)-NR1R2, where A-C(O)- denotes the saccharide residue and R1 and R2 denote the saturated or unsaturated hydrocarbon chains of chain lengths C1-C18. Aldonamides derived from primary alkylamines and derivatives of monosaccharides (free lactonic acids or their lactones) have very low solubility in water. They form supramolecular assemblies when their aqueous solutions are heated and subsequently cooled. Extensive studies on saccharide surfactant’s organogels and fibers are reported in the literature.1-6 The formation of suprastructures in water is explained by very dense packing of the molecules in the crystal’s unit cell because of the strong hydrogen-bond network between the hydrophilic headgroups. The solubility of the aldonamides in water can be improved by introducing an additional substituent at the nitrogen atom, such as a methyl group7 or a second alkyl chain.8 It is suggested that in this case the geometric conditions cause * To whom correspondence should be addressed. Fax: (+48-71) 320-36-78; e-mail: [email protected]. † Wroclaw University of Technology. § Present address: Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung, D-14424 Potsdam, Germany. # Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. (1) Fuhrhop, J. H.; Schnieder, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387. (2) Fuhrhop, J. H.; Svenson, S.; Boettcher, Ch.; Ro¨ssler, E.; Vieth H. M. J. Am. Chem. Soc. 1990, 113, 4307. (3) Frindi, M.; Michels, B.; Zana, R. J. Phys. Chem. 1992, 96, 8137. (4) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. M. J. Org. Chem. 1999, 64, 413. (5) Svenson, S.; Ko¨ning, J.; Fuhrhop, J. H. J. Phys. Chem. 1994, 98, 1022. (6) Hafkamp, R. J. H.; Kokke, B. P. A.; Danke, I. M.; Geurts, H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Chem. Commun. 1997, 545.

a significant attenuation of the hydrogen-bond network. An improved solubility of aldonamides can also be achieved by increasing the hydrophilic character of the surfactant molecule, for example, by using di- or trisaccharides,9 by introducing two sugar headgroups (bolaform, gemini, or dicephalic surfactants),10,11,12 or by strong polar groups (carbonate or sulfate).13,14 Because of the ecological and dermatological safety of the sugar-based amphiphiles, the field of their potential use in cosmetics, pharmaceutics, and food is very wide.15-17 Saccharide surfactants containing an amide moiety seem also to be promising in biological and biochemical applications, such as protein extraction and solubilization.18,19 In the present contribution, we report on the synthesis of a new series of amide-type sugar surfactants with an additional hydroxyethyl substituent at the nitrogen atom. The size of the hydrophilic saccharide group has been modified by the -CHOH moiety. Although many authors have so far extensively described surface properties of (7) Burczyk, B.; Wilk, K. A.; Sokolowski, A.; Syper, L. J. Colloid Interface Sci. 2001, 240, 552. (8) Pilakowska-Pietras, D.; Lunkenheimer, K.; Piasecki, A. J. Colloid Interface Sci., in press. (9) Zhang, T.; Marchant, R. E. J. Colloid Interface Sci. 1996, 177, 419. (10) Eastoe, J.; Rogueda, P. Langmuir 1994, 10, 4429. (11) Rico-Lattes, I.; Lattes, A. Colloids Surf. 1997, 123-124, 37. (12) Wilk, K. A.; Burczyk, B.; Sokolowski, A.; Domagalska, B. W. J. Surf. Detergents 2000, 3, 185. (13) Mehltretter, C. L.; Furry, M. S.; Mellies, R. L.; Rankin, J. C. J. Am. Chem. Soc. 1952, 202. (14) Van Au; Vermeer, R.; Harichian, B. (Lever Brothers Company, Division of Conopco, Inc.). U.S. Patent 5,686,603, 1997. (15) Franco, J. M.; Berjano, M.; Munoz, J.; Gallegos, C. Food Hydrocolloids 1995, 9, 111. (16) Busch, P.; Hensen, H.; Tesmann, H. Tenside Surf. Det. 1993, 30, 116. (17) Uchegbu, I. F.; Vyas, S. P. Int. J. Pharm. 1998, 172 (1-2), 33. (18) De Foresta, B.; Le Maire, M.; Orlowski, S.; Chameil, P.; Lund, S.; Moller, J. V.; Michelangeli, F.; Lee, A. G. Biochemistry 1989, 179, 145. (19) Garelli-Calvet, R.; Latge, P.; Rico, I.; Lattes, A.; Puget, A. Biochim. Biophys. Acta 1992, 1109, 55.

10.1021/la0302955 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/04/2004

Adsorption Behavior

Figure 1. The general synthetic routes for N-alkyl-N-(2hydroxyethyl)aldonamides.

Langmuir, Vol. 20, No. 5, 2004 1573 High-Performance Purification. To guarantee reliable experiments and proper analysis of the adsorption behavior, stock solutions of N-alkyl-N-(2-hydroxyethyl)aldonamides were purified by an automatically operating high-performance purification apparatus.20 In this technique, the surface is aspirated periodically to remove contaminations of comparatively stronger surface activity to achieve the state of “surface-chemical” purity. The grade of purity was judged by applying the criterion proposed in refs 21 and 22. 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, as described in refs 23 and 24, were applied. The evaluation of the adsorption parameters was performed using Frumkin’s equation of state for regular surface behavior:

{

(

∆σe ) σw - σe ) - RTΓ∞ ln 1 -

different aldonamides, some of these data are doubtful. We suggest this to be due to an insufficient purity of the investigated compounds. Therefore, a careful purification process of the aqueous surfactants’ solutions has been undertaken. The purification procedure allowed us to get reliable adsorption parameters. Thus, a reliable correlation between the chemical structure and the corresponding adsorption properties can be performed. Experimental Section Substances. The schematic routes of the synthesis of gluconamides, CnHEGA, and glucoheptonamides, CnHEGHA, are presented in Figure 1. The surfactants under discussion were obtained by the amidation reaction of the corresponding aldonlactones with N-alkyl-N-(2-hydroxyethyl)amines. Synthesis of N-alkyl-N-(2-hydroxyethyl)amines. General Procedure. A mixture of 0.5 mole of an appropriate 1-bromoalkane and 1.5 mole of 2-hydroxyethylamine (ethanolamine) was stirred for 10 h at 130 °C. A 0.5 molar solution of potassium hydroxide (KOH) in ethanol was added after cooling. The precipitated potassium bromide was filtered off and the ethanol evaporated. The residue was dissolved in hexane and extracted several times with water to remove the remaining KBr and the excess of unreacted ethanolamine. The crude product was obtained after drying of the hexane solution and evaporation of the solvent. The solid white product was purified by crystallization from cold hexane. Only in the case of the N-hexyl-N-(2-hydroxyethyl)amine, the crude product was purified by fractional distillation. Synthesis of N-alkyl-N-(2-hydroxyethyl)aldonamides. General Procedure. The N-alkyl-N-(2-hydroxyethyl)amine solution in methanol was dropped carefully to the stirred suspension of D-glucono-1,5-lactone or D-glucoheptono-1,4-lactone in methanol at 60 °C (reactants were used in equimolar ratio). The reaction was continued for an additional 10 h. After cooling to room temperature, the reaction mixture was left for 13 h to let the product crystallize. The precipitated white N-alkyl-N-(2-hydroxyethyl)gluconamides, CnHEGA, or N-alkyl-N-(2-hydroxyethyl)glucoheptonamides, CnHEGHA, were purified by crystallization from methanol/acetone or ethanol/acetone mixture. Representative 1H NMR analysis results for solution of C8HEGA: δ [ppm] 0.79 (t, J ) 6.5 [Hz], 3H, CH3(CH2)5CH2CH2N), 1.05-1.25 (m, 10H, CH3(CH2)5CH2CH2N), 1.281.55 (m, 2H, CH3(CH2)5CH2CH2N), 3.05-4.85 (mm, 18H, CH2N(CH2CH2OH)[C(O)(CHOH)4CH2OH], and for solution of C8HEGHA: δ [ppm] 0.79 (t, J ) 6.5 [Hz], 3H, CH3(CH2)5CH2CH2N), 1.10-1.25 (m, 10H, CH3(CH2)5CH2CH2N), 1.25-1.55 (m, 2H, CH3(CH2)5CH2CH2N), 3.00-4.75 (mm, 20H, CH2N(CH2CH2OH)[C(O)(CHOH)5CH2OH]. Methods. Structure Analysis. The chemical structure and the purity of the synthesized aldonamides were proven by elemental analysis and by 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 1H NMR spectra of the surfactant solutions in DMSO-d6 were measured using a Bruker Avance DRX300 spectrometer (Bruker, Karlsruhe, Germany).

) ( )}

Γ Γ + a′ Γ∞ Γ∞

2

(1a)

together with

(

c ) aL

) (

)

2HsΓ Γ exp Γ∞ - Γ RTΓ∞

(1b)

The symbols denote Γ, surface excess; Γ∞, saturation surface concentration; c, bulk concentration; aL, concentration of half surface coverage (“surface activity”); σw and σe, surface tension of water and equilibrium surface tension of the aqueous surfactant solution, respectively; a′ ) Γ∞Hs, Frumkin’s interaction parameter; and Hs, the interaction parameter according to LucassenReynders.25 To verify the data of adsorption, that is, the surface excess, Γ, and the surface area demand, Amin, calculated from the adsorption isotherms (eq 1), they were also calculated by using the Gibbs equation for diluted solutions:

Γ)-

( )

1 dσe RT d ln c

(2)

Results and Discussion The reaction of long-chain secondary amines with D-glucono-1,5-lactone and D-glucoheptono-1,4-lactone leads to the novel type of nonionic aldonamide surfactants. With respect to the aldonamide sugar residue in the surfactant molecule, this new group should exhibit a good biodegradability in natural environment. The yields of the amidation reaction decrease with increasing alkyl chain length and with increasing molecular weight of the lactone used. This has already been observed previously for N,Ndi-n-alkylaldonamides.8 According to the results of elemental analysis (see Table 1), the surfactants obtained are of comparatively high purity. However, there is one aspect that seems to have been neglected by many scientists. The stronger surface-active impurities, mainly consisting of trace parent compounds, that are not detectable in the bulk phase, are preferentially enriched at the interface. Hence, even trace amounts of these contaminations generally effectively influence the adsorption behavior, leading to nonreliable results and misleading conclusions.26 This fact plays a very important role in the characterization of surface properties of most (20) Lunkenheimer, K.; Pergande, H. J.; Kru¨ger, L. Rev. Sci. Instrum. 1987, 58, 2313. (21) Miller, R.; Lunkenheimer, K. Colloid Polym. Sci. 1986, 264, 273. (22) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1987, 130, 176. (23) Lunkenheimer, K. J. Colloid Interface Sci. 1989, 131, 580. (24) Lunkenheimer, K. Tenside Detergents 1982, 19, 272. (25) Lucassen-Reynders, E.-H. Prog. Surf. Membr. Sci. 1973, 10, 253. (26) 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.

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Table 1. Data of Physicochemical Properties of the Homologous Series of N-Alkyl-N-(2-Hydroxyethyl)aldonamides calculated (%)

found (%)

compound

melting point (°C)

formula

molecular weight

C

H

N

C

H

N

C6HEGA C6HEGHA C8HEGA C8HEGHA C10HEGA C10HEGHA C12HEGA C12HEGHA C14HEGA C14HEGHA

95-97 145-146 100-102 129-131 104-106 127-129 107-108 129-131 109-111 131-133

C14H29O7N C15H31O8N C16H33O7N C17H35O8N C18H37O7N C19H39O8N C20H41O7N C21H43O8N C22H45O7N C23H47O8N

323.39 353.41 351.44 381.47 379.49 409.52 407.55 437.57 435.60 465.63

52.00 50.98 54.68 53.53 56.97 55.73 58.94 57.64 60.66 59.33

9.04 8.84 9.46 9.25 9.83 9.60 10.14 9.90 10.41 10.17

4.33 3.96 3.99 3.67 3.69 3.43 3.44 3.20 3.22 3.01

51.93 50.91 54.61 53.50 56.81 55.60 59.18 57.59 60.72 59.64

9.03 8.80 9.41 9.21 9.76 9.52 10.08 9.86 10.36 9.98

4.37 3.98 4.02 3.69 3.72 3.36 3.55 3.31 3.16 3.05

Figure 2. Dynamic surface tension of 3 × 10-4 M aqueous solution of N-decyl-N-(2-hydroxyethyl)glucoheptonamide (C10HEGHA): “as received” (a); “as received” solution-adsorption layer compressed to half of its original surface area in a Langmuir trough after 130 min of adsorption (b); after surface-chemical purification (c).

surfactants, since they are usually obtained from substrates that possess much higher surface activity. Moreover, because of our experience, the presence of an additional surface-active component can also lead to other falsifying effects, such as an apparent increased solubility. Thus, with respect to the subject of this investigation, the already good purity of the “as received” products is nevertheless insufficient for their correct surface characterization. To allow reliable and accurate surface studies, all surface-active impurities must effectively be removed. The influence of the trace impurities upon the dynamic surface tension behavior of the main surfactant at the air/water interface is illustrated in Figure 2. In the first stage (Figure 2a), the adsorption behavior was investigated until surface tension had reached an almost constant value. After it, the surface area was compressed to half of its initial size and the dependence of the surface tension on time of desorption was measured (Figure 2b). As shown in Figure 2a, the time of adsorption is very long; the equilibrium value of surface tension is not yet reached even after 2 h of adsorption. On the other hand, a strong decrease of surface tension because of compression of the surface was observed. This behavior clearly indicates the presence of additional surface-active components that influence the adsorption characteristic of the main surfactant. Having removed the surface-active contaminations, no dynamic behavior is observed at all (Figure 2c). For the solution of the grade “surface-chemically pure”, the change of the surface tension within adsorption time is negligible and the equilibrium is reached instantaneously.

Figure 3. Change of the equilibrium surface tension value, σe, in dependence on the number of purification cycles, j, for 9 × 10-2 M N-hexyl-N-(2-hydroxyethyl)gluconamide, C6HEGA (-O-), and 5 × 10-4 M N-decyl-N-(2-hydroxyethyl)glucoheptonamide, C10HEGHA (-b-).

Figure 3 represents the change of the equilibrium surface tension value, σe, in dependence on the number of purification cycles, j, for a 9 × 10-2 M solution of C6HEGA

Adsorption Behavior

Langmuir, Vol. 20, No. 5, 2004 1575

N-methylaldonamides,7 and N,N-di-n-alkylaldonamides,8 that the functionality of the resulting aldonamide amphiphiles is decisively determined by the chemical structure of the additional substituent, that is, by the 2-hydroxyethyl- and the methyl- and the alkyl groups, respectively. The difference between the adsorption isotherms of the N-alkyl-N-(2-hydroxyethyl)gluconamides and the related N-alkyl-N-(2-hydroxyethyl)glucoheptonamides becomes evident by regarding the parameter of the standard free energy of adsorption, ∆G°ad. The values of ∆G°ad, calculated as ∆G°ad ) RT ln(aL), are listed in Table 2. The CnHEGHA reveal a slightly stronger tendency for adsorption (∆G°ad values are more negative) than the related CnHEGA. Thus, the introduction of an additional -CHOH residue into the sugar headgroup results in a weak increase of the surfactant’s hydrophobicity. The plots of the dependence of standard free energy of adsorption on the number of carbon atoms in the alkyl chain, as shown in Figure 5, reveals a linear relationship. The corresponding relationships are given by

∆G°ad ) -2.80 nc + 6.17 for CnHEGA, Figure 4. Surface tensions σe vs logarithm of concentration isotherms for the homologous series of N-alkyl-N-(2-hydroxyethyl)aldonamides: N-alkyl-N-(2- hydroxyethyl)gluconamides, CnHEGA (-O-), and N-alkyl-N-(2-hydroxyethyl)glucoheptonamides, CnHEGHA (-b-), measured at 295 K.

and a 5 × 10-4 M solution of C10HEGHA. As shown in this figure, the equilibrium surface tension values of the purified surfactant solutions are higher than the corresponding values of the solutions prepared from the “as received” compounds. This difference amounts to 4.5 and 5.5 mN/m for C6HEGA and C10HEGHA, respectively. However, the concentrations of the impurities are relatively small, which are indicated by the negligible changes of the equilibrium surface tensions at higher purification cycles when the impurities had been removed. Nevertheless, this arbitrary difference between the original and the surface-chemically pure solutions proves how important the state of surface-chemical purity is, especially if one intends to investigate the change of the adsorption parameters within a homologous series of surfactants. Figure 4 shows dependence of the equilibrium surface tension (σe) on concentration (c) for the homologues series of CnHEGA and CnHEGHA. The measurements were performed at 295 K. However, at room temperature the C12 and C14 homologues are practically insoluble in water and their surface tension versus concentration isotherms could not be studied. Although one might expect, with regard to the chemical structure, that is, the presence of the bulky hydrophilic group, that the investigated surfactants would be favored to form micelles, no hint of aggregates was observed in aqueous solution. The N,Ndi-n-alkylaldonamides8 show a similar behavior. This behavior is quite opposite to that observed for the N-alkylN-methylaldonamides7 which do not reveal solubility limit at room temperature (295 K) and do form micelles within the entire homologue series. We assume that the introduction of the 2-hydroxyethyl grouping improves the solubility of the investigated aldonamides when compared to the derivatives of primary amines. On the other hand, however, the presence of the hydroxyl group of 2-hydroxyethyl substituent seems to effectively maintain the hydrogen-bond network that results in the limited solubility of the aldonamides studied. Hence, we can assume, comparing the different N-substituted aldonamides, namely, N-alkyl-N-(2-hydroxyethyl)aldonamides, N-alkyl-

correlation coefficient R2 ) 0.9999 (3a) ∆G°ad ) -2.88 nc + 6.60 for CnHEGHA, correlation coefficient R2 ) 0.998 (3b) The linear characteristic of the dependence of ∆G°ad versus nc was previously observed for the nonionic monosubstituted n-alkyl surfactants27,28 and the aldonamide derivatives with two symmetrical alkyl chains,8 too. However, some clear distinctions are noticeable that cannot be explained only according to the differences in the hydrophilic group. The mean contribution of the methylene group to the standard free energy of adsorption, - ∆G°ad/CH2, is about 2.8 and 2.9 kJ/mol for N-alkyl-N(2-hydroxyethyl)gluconamides and N-alkyl-N-(2-hydroxyethyl)glucoheptonamides, respectively. The corresponding value for the N,N-di-n-alkylaldonamides8 with two symmetric alkyl chains is about 3.1 kJ/mol, and in the range from 2.4 to 2.6 kJ/mol for single-chained surfactants.27,28 Hence, we can conclude that the protrusion of the alkyl groups is noticeably affected not only by the hydrophobic part of the aldonamide surfactants but also by the second substituent at the hydrophilic headgroup. According to Traube’s rule, the amphiphiles’ surface activity within a homologous series changes by a constant factor for subsequent homologues.29,30 Traube originally suggested a factor of 3.0 for adjacent homologues for the homologous series of n-alkanoic acids, that is, surface activity of a homologue with two additional methylene groups in the hydrocarbon chain is stronger by a factor of almost 10. This constant factor usually varies a little for different homologous series of surfactants. Thus, for example, the average of it for the soluble even dimethylalkylphosphine oxides is 8.65.27 For the N-alkyl-N-(2-hydroxyethyl)aldonamides, the average of this factor is 9.9, that is, it corresponds well to Traube’s rule. It is easily calculated by the division of the corresponding adsorption parameters aL as aL,n/aL,n+2, where n denotes the number of carbon atoms in CnHEGA or CnHEGHA molecules. (27) Lunkenheimer, K.; Haage, K.; Hirte, R. Langmuir 1999, 15, 1052. (28) Lunkenheimer, K.; Barzyk, W.; Hirte, R.; Rudert, R. Langmuir 2003, 19, 6140. (29) Traube, I. Liebigs Ann. Chem. 1891, 265, 27. (30) Stauff, J. Kolloidchemie; Springer-Verlag: Berlin, 1960.

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Table 2. Adsorption Parameters of the Homologous Series of N-Alkyl-N-(2-Hydroxyethyl)aldonamides surfactant C6HEGA C6HEGHA C8HEGA C8HEGHA C10HEGA C10HEGHA

equation of state Frumkin Frumkin Frumkin Frumkin Frumkin Frumkin

Γ∞ (10-6) [mol/m2] 3.09 3.04 3.12 3.07 3.15 3.11

aL [mol/ dm3] 10-2

1.31 × 1.22 × 10-2 1.31 × 10-3 1.39 × 10-3 1.36 × 10-4 1.24 × 10-4

Amin [Å2/ molecule]

-∆G°ad [kJ/ mol]

HS [kJ/ mol]

s [mN/m]

53.8 54.6 53.2 54.0 52.7 53.3

10.6 10.8 16.3 16.1 21.8 22.3

1.1 1.9 2.2 2.9 3.0 3.4

(0.16 (0.18 (0.14 (0.17 (0.22 (0.26

Figure 5. Standard free energy of adsorption, ∆G°ad, of the homologous series of N-alkyl-N-(2-hydroxyethyl)aldonamides in dependence on the number of carbon atoms in the alkyl chain, nC: CnHEGA (-O-); CnHEGHA (-b-).

Figure 6. Cross-sectional area, Amin, of the adsorbed N-alkylN-(2-hydroxyethyl)aldonamides in dependence on the number of carbon atoms in the alkyl chain, nC: CnHEGA (-O-); CnHEGHA (-b-).

The influence of the chemical structure of the surfactant molecule on the adsorption behavior is also sensitively reflected in the characteristic of the surface area demand per adsorbed molecule, Amin. The values of Amin are calculated by Amin ) (Γ∞NL)-1; where NL denotes Lochschmidt’s number. These values are identical to those calculated by the Gibbs equation, eq 2, determined from the tangent to the experimental equilibrium surface tension versus concentration by using a spline function.28 We observed linear dependencies of Amin on the alkyl chain length, nc, for hydrocarbon chain lengths between 6 and 10. They are presented in Figure 6 for the homologues series of the N-alkyl-N-(2-hydroxyethyl)gluconamides and N-alkyl-N(2-hydroxyethyl)glucoheptonamides:

chain length is nevertheless significant. Generally, a decrease in dAmin/dnC is found for homologous series of surface-chemically pure surfactants.27,28,31 However, unlike the aldonamide type surfactants, this decrease is usually comparatively greater for soluble surfactants amounting to values between -0.6 e -(dAmin/dnC) e -1.3 Å2/molecule for even homologues and almost twice these values for odd homologues. The relatively slow decrease of the Amin value of the adsorbed aldonamide surfactant indicates that the cross-sectional area is mainly determined by the structure of the hydrophilic headgroup. The insertion of an additional -CHOH group in the sugar residue of the CnHEGHA is still detectable in the corresponding Amin values. This becomes evident by the comparison with the values of the related CnHEGA. This indicates that the orientation of the adsorbed gluconamides occurs presumably in such a manner that the extended sugar entities of the adsorbed amphiphiles are not directed perpendicular into the adjacent bulk water phase but possess a certain bend to the plane interface. To figure out the possible conformation of the surfactant molecule at the air/water interface, we performed a molecular modeling carried out with the HyperChem 5.0 software (HyperCube Inc, Ontario, Canada). The optimized structures for the N-octyl-N-(2-hydroxyethyl)aldonamides, obtained for the MM+ force field in vacuo, are presented in Figure 8. If we assume that the alkyl chain is directed perpendicular toward the air phase, then,

Amin ) -0.275nc + 55.433 for CnHEGA 6 e nc e 10 (4a) Amin ) -0.325nc + 56.567 for CnHEGHA 6 e nc e 10 (4b) The slope of the Amin(nc) dependence of the CnHEGA homologues is almost identical to that of the CnHEGHA homologues. The cross-sectional areas for the CnHEGA and CnHEGHA series very slightly depend on the hydrocarbon chain length. The decrease in the surface area with the increase of alkyl chain length by one methylene group, ∆Amin, for the CnHEGA homologues, as well as for the CnHEGHA homologues, is about 0.3 Å2/molecule. As can be taken from the errors in the calculated Amin values (Figure 6), the slight decrease -(dAmin/dnC) with increasing

(31) Lunkenheimer, K.; Burczyk, B.; Piasecki, A.; Hirte, R. Langmuir 1991, 7, 1765.

Adsorption Behavior

Langmuir, Vol. 20, No. 5, 2004 1577

Figure 7. Interaction parameter, Hs, of the homologous series of N-alkyl-N-(2-hydroxyethyl)aldonamides in dependence on the number of carbon atoms in the n-alkyl chain, nC: CnHEGA (-O-); CnHEGHA (-b-).

because of the specific conformation at the nitrogen atom, any alteration in the bulky hydrophilic headgroup will necessarily be reflected in the corresponding limiting crosssectional area of the adsorbed molecule. Moreover, the obtuse 2-hydroxyethyl group is obviously mainly responsible for the comparatively large cross-sectional areas of the adsorbed aldonamide surfactants. However, as can be taken from Figure 7, a noticeable interaction between the hydrophobic alkyl chains of the aldonamide is maintained within the adsorbed layers. The considerable increase of the interaction parameter, Hs, with increasing chain length, often observed for alkyl surfactants, is accompanied by the slight decrease of the corresponding Amin values. The Hs values seem to approach a limiting value at chain lengths nC > 10. Thus, one can conclude that also the Amin values will approach a certain limiting value at nC > 10, which is mainly determined by the geometric restriction given by the 2-hydroxyethyl group. However, the glucoheptonamides CnHEGHA, although having slightly greater Amin values than the related gluconamides CnHEGA, possess a distinctly stronger surface interaction within the adsorbed layer than the latter by about 1 kJ/mol. This cannot be explained by the interaction between the alkyl chains only because the chain lengths are comparatively short. It indicates that the main contribution to the surface interaction is brought about by hydrogen bonding between the hydroxyl groups of the hydrophilic group, presumably by hydrogen bonding between the extended 2-hydroxyethyl residues. Finishing these considerations, the results of related surfactant structures reported in the literature are compared. The Amin values for alkylaldonamides reported in refs 32 and 33 are about 40 Å2/molec. Thus, the distinct increase of about 15 Å2/molecule in the N-alkyl-N-(2hydroxyethyl)aldonamides can be attributed to the presence of the functional 2-hydroxyethyl residue, leading to (32) Syper, L.; Wilk, K. A.; Sokolowski, A.; Burczyk, B. Colloid Polym. Sci. 1998, 110, 199. (33) Zhu, Y.-P.; Rosen, M. J.; Vinson, P. K.; Morrall, S. W. J. Surf. Detergents. 1999, 2, 357.

Figure 8. Modeled structures of N-octyl-N-(2-hydroxyethyl)gluconamide, C8HEGA, and N-octyl-N-(2-hydroxyethyl)glucoheptonamide, C8HEGHA.

the effective growth of the hydrophilic headgroup. The Amin values of the saccharide surfactants of refs 32 and 33 usually show stronger influence of the hydrophobic group, as well as of the sugar residue in comparison to the N-alkylN-(2-hydroxyethyl)aldonamides. Thus, for example, Syper et al.32 have investigated N-dodecyl-N-methylaldonamides that differ with respect to the chemical structure from the surfactants reported on in this work only by the functional group at the nitrogen atom. They found greater crosssectional areas for the glucoheptonic derivative than for the gluconic ones. This finding is in agreement with our results. The data on the Amin values of Zhu et al.33 for N-alkanoyl-N-methylglucamines show a significant dependence on the alkyl chain length. The Amin decreases from 43.7 to 35.5 Å2/molecule for substituted N-methylglucamides with increasing chain length of the acyl

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grouping from C11 to C14. Moreover, the decrease in the surface area per one methylene group ∆Amin/CH2 is about 3 Å2/molecule. This value is of an order of magnitude higher than that of the N-alkyl-N-(2-hydroxyethyl)gluconamides. Hence, the effect of the kind of the second substituent at the N-atom seems to be much more pronounced. Analogies can also be found when the size of the carbohydrate residue is considered. Thus, the molecular areas of the N-dodecanoyl-N-methylglucamines, C11H23CON(CH3)CH2(CHOH)xCH2OH, decrease slightly in the order of C12-glucamide (x ) 4) > C12-xylamide (x ) 3) > C12glyceramide (x ) 1), reflecting the decreasing size of the hydrophilic group.33 This observation is supported by data of So¨derberg,34 who observed a monotonical increase of Amin with the sequential addition of galactose structural units (sucrose < raffinose < stachyose) for the 6-Ododecanoyl-D-glucosides. However, for cyclic sugar derivatives, for which the hydrophilic headgroup remains unaltered, it seems that the influence of the alkyl chain length on the Aminvalues becomes negligible. Thus, Coppola et al.35 found practically constant values of Amin with increasing alkyl chain length for 6-O-acyl-R,β-glucopyranoses. They assumed that the molecular areas are essentially determined by interaction between the glucoside polar headgroups. The negligible effect of the hydrophobic group can be attributed to the existence of the bulky sugar ring, which, opposite to the extended acyclic saccharide residue, occupies much higher surface area. Conclusions Concluding the discussion, we would like to point out that for the detailed analysis of the adsorption properties of the N-alkyl-N-(2-hydroxyethyl)aldonamides with respect to their structure-performance relationship, as performed in this contribution, one should be cautious in comparing these results with corresponding data presented in the literature, because for the latter the grade of surface-chemical purity was not given. (34) So¨deberg, L.; Drummond, C. J.; Furlong, D. N.; Godkin, S.; Matthews, B. Colloids Surf. 1995, 102, 91. (35) Coppola, L.; Gordano, A.; Procopio, A.; Sindona, G. Colloids Surf. 2002, 196, 175.

Piłakowska-Pietras et al.

The results presented in this work show how small changes in the amphiphile’s structure are sensitively reflected in its adsorption properties. This, however, was only possible by strictly obeying the requirement of the surfactants’ surface-chemical purity. Thus, the introduction of the -CHOH residue into the sugar headgroup increases the hydrophobic character of the N-alkyl-N-(2-hydroxyethyl)aldonamides. An opposite effect was observed for N,N-di-n-alkylaldonamides,8 where the gluconic derivatives had slightly higher surface activity than the glucoheptonic ones. The surface behavior of the reported alkylaldonamide surfactants corresponds almost to that behavior that was originally assumed to generally hold for alkyl surfactants possessing constant cross-sectional area within homologues series, and the Traube factors well suiting the originally suggested ones. Thus, although recent investigations showed that this assumption does generally not hold for adsorbed alkyl surfactants,27,28,31 the abovedescribed alkylaldonamide surfactants could also represent convenient models for basic investigations on surface properties. In addition, it would be interesting to know whether an effect of alternation (even/odd phenomenon) within these homologous series will exist and of what extent it would be. Following recent investigations, this phenomenon seems to be of common occurrence in the adsorption and association properties of n-alkyl amphiphiles. Unfortunately, this question cannot yet be answered because the odd representatives were not available. Concerning applied properties, foam investigations are in progress. The detailed studies will be presented separately. The first results show that the N-alkyl-N-(2hydroxyethyl)aldonamides form foams of medium stability. Morever, the foamability of N-alkyl-N-(2-hydroxyethyl)glucoheptonamides is higher than for the corresponding N-alkyl-N-(2-hydroxyethyl)gluconamides. Acknowledgment. We are grateful to Prof. R. Hirte for providing us with the programs of various adsorption equations to evaluate the adsorption isotherms. One of us, D.P., is grateful to the Max-Planck-Institute of Colloid and Interface Research for a research grant. LA0302955