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Langmuir 1999, 15, 1052-1058
Novel Results on the Adsorption Properties of n-Alkyldimethylphosphine Oxides at the Air/Water Interface K. Lunkenheimer,*,† K. Haage,† and R. Hirte‡ Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Rudower Chaussee 5, D-12489 BERLIN, Germany, and Technische Fachhochschule Wildau, Friedrich-Engels-Strasse 63 D-15742 Wildau, Germany Received May 27, 1998. In Final Form: October 29, 1998 The adsorption parameters of alkyldimethylphosphine oxides (from n-hexyl- to n-tetradecyl) at the air/water interface were calculated from equilibrium surface tension (σe) versus concentration (c) isotherms at 295 K. To guarantee absence of measurable surface-active impurities in the adsorption layers, stock solutions of the surfactants were purified until the state of surface-chemical purity was achieved. The evaluation of the σe vs c isotherms was performed by applying a two-state approach to the surface equation of state. All of them are described in the best way by ideal surface behavior. The critical micelle concentrations (cmc) and the calculated adsorption parameters, i.e., standard free energy of adsorption (∆G°ad) and limiting surface area demand per molecule adsorbed (Amin), reveal distinct effects of alternation (even/odd phenomena). The Amin values decrease with rising n-alkyl chain length. The results are discussed with respect to donor-acceptor interactions and surface conformational changes in the adsorption layer. As these surfactants’ adsorption reveals ideal surface behavior, the data may serve as a reliable basis for modeling adsorption.
1. Introduction n-Alkyldimethylphosphine oxides are amphiphiles that represent a lot of advantages with respect to modeling adsorption properties because they are nonionic and chemically very stable compounds. Furthermore, they are hydrated in aqueous solution, revealing “ideal surface behavior”.1-3 Thus, they have been used repeatedly for studying various problems of interfacial science such as thermodynamic,2,4-9 kinetic,10-18 surface-rheological,19-21 * To whom correspondence may be addressed. † Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. ‡ Technische Fachhochschule Wildau. (1) Lucassen-Reynders, E. H. Prog. Surf. Membr. Sci. 1976, 10, 253. (2) Lunkenheimer, K.; Haage, K.; Miller, R. Colloids Surf. 1987, 22, 215. (3) Lunkenheimer, K.; Malysa, K.; Wantke, K. Colloids Surf., in press. (4) Lunkenheimer, K.; Hirte, R. J. Phys. Chem. 1992, 96, 8683. (5) Hirte, R.; Lunkenheimer, K. J. Phys. Chem. 1996, 100, 13786. (6) Todoroki, N.; Tanaky, F.; Ikeda, N.; Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1993, 66, 351. (7) Ravera, F.; Ferrari, M.; Liggieri, L.; Miller, R.; Passerone, A. Langmuir 1997, 13, 4817. (8) Fainerman, V. B.; Miller, R. Langmuir 1997, 13, 409. (9) Kresheck, G. C. J. Colloid Interface Sci. 1997, 187, 542. (10) Miller, R.; Lunkenheimer, K. Colloid Polym. Sci. 1986, 264, 257. (11) Miller, R.; Schano, K.-H. Colloid Polym. Sci. 1986, 264, 277. (12) Miller, Lunkenheimer, K. R.; Schano, K.-H. Mater. Sci. Forum (Chem. Interfaces) 1988, 351. (13) Fang, J. P.; Wantke, K.-D.; Lunkenheimer, K. J. Phys. Chem. 1995, 99, 4632. (14) Loglio, G.; Miller, R.; Stortini, A.; Tesei, U.; Degliinnocente, N.; Cini, R. Colloids Surf. A 1995, 22, 63. (15) Fainerman, V. B.; Zholob, S. A.; Miller, R.; Loglio, G.; Cini, R. Tenside 1996, 33, 452. (16) Ferrari, M.; Liggieri, L.; Ravera, F.; Amodio, C.; Miller, R. J. Colloid Interface Sci. 1997, 186, 40. (17) Miller, R.; Zholob, S. A.; Makievski, A. V.; Joos, P.; Fainerman, V. Langmuir 1997, 13, 5663. (18) Liggieri, L.; Ravera, F.; Ferrari, M.; Passerone, A. C.; Miller, R. J. Colloid Interface Sci. 1997, 186, 46. (19) Loglio, G.; Tesei, U.; Cini, R. J. Colloid Interface Sci. 1979, 71, 316. (20) Loglio, G.; Tesei, U.; Cini, R. J. Colloid Interface Sci. 1984, 100, 393.
electrochemical,22-24 analytical,25 catalytic26 and ecological27 ones. After having discovered that reliable investigations on the adsorption of surfactants require a particular grade of the surfactant solutions’ purity28-35 some basic new adsorption properties were detected. One of these concerns the finding that, opposite to common sense, the crosssectional area Amin (limiting surface area demand per molecule adsorbed) is not constant within a series of soluble homologues but decreases with increasing chain length until the Amin value of the corresponding insoluble homologues is reached. This novel result was at first detected by us with various soluble members of the homologous series of n-alkyldimethyl- and n-alkyldiethylphosphine oxides.2 After we had put forward a new approach to the surface equation of state4,5 we discovered additional new features in the surfactant adsorption properties. The most striking (21) Noskov, B. A.; Grigoriev, D. O.; Miller, R. J. Colloid Interface Sci. 1997, 188, 9. (22) Do¨rfler, H.-D.; Mu¨ller, E.; Heinze, M. Z. Phys. Chem. (Leipzig) 1980, 216, 969. (23) Mu¨ller, E.; Emons, H.; Do¨rfler, H.-D. J. Colloid Interface Sci. 1981, 79, 567. (24) Mu¨ller, E.; Emons, H.; Do¨rfler, H.-D. Z. Phys. Chem. 1986, 267, 921. (25) Kresheck, G. C.; Gordon, C.; Jones, C. J. Colloid Interface Sci. 1980, 77, 278. (26) Albrizzio, J.; Archila, J.; Rodulfo, T.; Cordes, E. H. J. Org. Chem. 1972, 37, 871. (27) Cini, R.; Ficalbi, A.; Loglio, G. Microchim. Acta 1974, 203. (28) Mysels, K. J.; Florence, A. T. In Clean Surfaces: Their Preparation and Characterization for Interfacial Studies; Goldfinger, G., Ed.; Marcell Dekker: New York 1970; p 227. (29) Mysels, K. J.; Florence, A. T. J. Colloid Interface Sci. 1973, 43, 577. (30) Vijayendran, B. R. J. Colloid Interface Sci. 1977, 60, 418. (31) Lunkenheimer, K.; Miller, R. Tenside 1979, 16, 312. (32) Carroll, B. J. J. Colloid Interface Sci. 1982, 86, 586. (33) Miller, R.; Lunkenheimer, K. Colloid Polym. Sci. 1982, 261, 1148. (34) Mysels, K. J. Langmuir 1986, 2, 423. (35) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1987, 120, 176.
10.1021/la980611t CCC: $18.00 © 1999 American Chemical Society Published on Web 01/20/1999
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Table 1: Melting Points (°C) of the Phosphine Oxide Compounds (Measured and Data from Literature) measured literature
C7H15
C8H17
C9H19
C10H21
C11H23
C12H25
C13H27
C14H29
54
63-66 60-6254
67-70
73-75 6555
76-79
80-82 83-8456
83-85
84-85 84-8657
one is the phenomenon of alternation (the so-called evenodd effect) within various homologous series of soluble amphiphiles.35-39 As shown only recently, it is observed not only at air/water but also at liquid/liquid interfaces.40,41 So far there has been no general explanation for this phenomenon. One feasible explanation goes back to Gutmann’s donor acceptor approach.42 This seems to work with surfactants with strong interaction within the monolayer containing hydrogen bridging functions such as hydroxy, carboxy, sulfato, or sulfonato groups. In this work we were interested in the question whether the nonionic homologous series of n-alkyldimethylphosphine oxides reveals also the effect of alternation, although these surfactants do not show measurable effects of surface interaction. That’s why we synthesized also the oddnumbered homologues. To confirm our previous results,2 we evaluated our measurements by applying an improved approach to the surface equation of state.4,5 We are about to systematically investigate the influence of slight variations in certain amphiphiles’ hydrophilic groups on their adsorption properties by successively altering from phosphine oxide to phosphate. The former is to serve as the chemical function’s reference structure then. 2. Experimental Section 2.1. Substances. The alkyldimethylphosphine oxides with one long-chain alkyl group and two methyl groups were synthesized by procedures communicated by us previously.2 We chose the Arbuzov reaction via freshly fractionated alkyl bromides with triethylphosphite (eq 1), yielding the alkylphosphonic acid diethyl
ester. The n-alkyldimethylphosphine oxides with long alkyl chains were synthesized like many other alkylphosphine oxides by the usual procedure with Grignard compounds by treatment of alkylphosphonic acid dichlorides (eqs 2 and 2a), or in the particular treatment of alkylphosphonic acid ester with an excess of methylmagnesium bromide under stronger conditions, result(36) Lunkenheimer, K.; Miller, R. Abh. Akad. Wiss. DDR Abt. Naturwiss. Technol. No. 1N 1986; Akademie-Verlag: Berlin, 1987; p 113. (37) Lunkenheimer, K.; Burczyk, B.; Piasecki, A.; Hirte, R. Langmuir 1991, 7, 1765. (38) Lunkenheimer, K.; Laschewsky, A. Prog. Colloid Polym. Sci. 1992, 9, 239. (39) Lunkenheimer, K.; Czichocki, G.; Hirte, R.; Barzyk, W. Colloids Surf. A 1995, 101, 187. (40) Goebel, A. Ph.D. thesis, Technical University of Berlin, Berlin, 1997. (41) Goebel, A.; Lunkenheimer, K. Langmuir 1997, 13, 369. (42) Gutmann, V. The Donor-Acceptor-Approach to Molecular Interactions; Plenum Press: New York and London, 1978.
ing in magnesium complexes of the alkyldimethylphosphine oxides (eq 3),43 which have to be extracted from aqueous solutions after hydrolysis several times with ether. The collected ether solutions were treated with solid, anhydrous disodium carbonate, and the solvent was then distilled off. However, the procedure depends on the carbon chain length because of the phosphine oxides’ different solubility. Finally, purification was done by fractional distillation excluding moisture or by recrystallization. The remaining products were additionally purified by distillation or recrystallization in hexane or heptane. The chemical purity was proved by elementary analysis, mass spectra, 1H and 13C NMR, and GLC on different methyl silicon rubber phases. All compounds are solids. Their melting points (mp) were also determined (Table 1). 2.2. High-Performance Purification. Stock solutions of submicellar concentrations were purified by an automatically operating high-performance purification apparatus described in ref 44 to get “surface-chemically pure” surfactant solutions guaranteeing reproducible experiments and reliable results. By this technique, surface-active trace impurities are removed by aspirating the surface periodically in a definite manner until the solution has reached the state of “surface-chemical” purity. The grade of purity was monitored by applying the criterion proposed in ref 35. Dilutions of the surface-chemically pure stock solution were used for the measurements of surface tension to obtain the equilibrium surface tension (σe) versus concentration (c) isotherms. 2.3. Surface Tension Measurements. Surface tension was determined by an automatic Lauda tensiometer TE-1M taking into consideration modifications necessary in applying it to surfactant solutions.45-47
3. Results 3.1. Purity and Adsorption Behavior. Figure 1 represents the alteration of the equilibrium surface tension value σe in dependence on the number j of purification cycles using the above-mentioned purification apparatus for four different alkyldimethylphosphine oxide solutions. These dependences are to illustrate the irregularities in the “purification characteristics” σe(j) of the different solutions. All of them reveal positive slopes at low numbers j of purification cycles, indicating distinct effects of surfaceactive trace impurities. At the highest cycle numbers, the slopes become zero, proving that the state of “surfacechemical purity” was reached according to the criterion35 dσ/dj ) 0. There is no unique feature in the purification process detectable. Thus, the solution of 3 × 10-2 M dimethylhexylphosphine oxide (C6Me2PO) shows a very steep increase in σe(j) but reaches its plateau value already at low numbers of j. This indicates that the trace impurity component(s) is (are) of comparatively strong surface activity and its (their) bulk concentration(s) is (are) rather small. The opposite behavior was observed with the solution of 8 × 10-2 M dimethylheptylphosphine oxide (C7Me2PO). There is a relatively small progress in the purification procedure characterized by a weak slope in σe(j). However, (43) Haage, K.; Greiner, A. Proc. 7th Internat. Congress on SurfaceActive Substances; Moscow, 1977; Section A, Vol. 1, p 97. (44) Lunkenheimer, K.; Pergande, H.-J.; Kru¨ger, H. Rev. Sci. Instrum. 1987, 58, 2313. (45) Lunkenheimer, K.; Wantke, K.-D. J. Colloid Interface Sci. 1978, 66, 579. (46) Lunkenheimer, K.; Wantke, K.-D. Colloid Polym. Sci. 1981, 259, 354. (47) Lunkenheimer, K. Tenside 1982, 19, 272.
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Figure 1. Equilibrium surface tension σe in dependence on the number j of purification cycles of the automatic purification operations for four different solutions (“purification characteristics”): (i) 3 × 10-2 M dimethylhexylphosphine oxide (C6Me2PO); (ii) 8 × 10-2 M f 6 × 10-2 M dimethylheptylphosphine oxide (C7Me2PO) (see text); (iii) 5 × 10-4 M dimethylundecylphosphine oxide (C11Me2PO); (iv) 7 × 10-5 M dimethyltridecylphosphine oxide (C13Me2PO).
Figure 2. Dynamic surface tension of three n-alkyldimethylphosphine oxide solutions when used “as received” and after surfacechemical purification. Filled symbols: “as received”. Open symbols: “surface-chemically pure” solutions. Key: (2, 4) 3 × 10-2 M dimethylhexylphosphine oxide (C6Me2PO); (b, O) 4 × 10-2 M dimethylheptylphosphine oxide (C7Me2PO); (0, 9) 1 × 10-4 M dimethyldodecylphosphine oxide (C12Me2PO).
the constant value of σe could only be reached at extremely high numbers of purification cycles. As the aspiration of the solution under purification leads to a loss of about 0.1 mL per cycle, this solution had slightly to be diluted twice (8 × 10-2 M f 7 × 10-2 M f 6 × 10-2 M; indicated by broken lines) to ensure continuation of the purification procedure. Thus, one can conclude that the impurity components’ surface activities were not so much higher
than that of the main component but their concentrations should be relatively high. The importance of the necessary grade of purity for investigating amphiphiles’ interfacial properties becomes even more striking in following their adsorption kinetics. In Figure 2 the dynamic surface tension behavior of three different alkyldimethylphosphine oxide solutions is compared in the corresponding states “as received” and
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Figure 3. Equilibrium surface tension (σe) versus concentration (c) isotherms of the homologous series of n-alkyldimethylphosphine oxides at 295 K from dimethyltetradecyl- (left) to dimethylhexylphosphine oxide (right).
“surface-chemically pure”. Except the solution of 1 × 10-4 M dimethyldodecylphosphine oxide (C12Me2PO) with a short relaxation period for establishing the adsorption equilibrium, the equilibrium is established so fast, i.e., within an interval smaller than about 30 s, that its time dependence cannot be followed by the applied ring method. Opposite to it, a pronounced dynamic surface tension behavior is observed with the original solutions. Their slowly decaying surface tension values do not yet adjust within 1 h or so. Thus, the kinetics of the solutions is retarded by at least 2 orders of magnitude. The findings given in Figures 1 and 2 give evidence for the importance of the solutions’ purity if you intend to escape misconclusions. This especially concerns the amphiphiles’ sorption kinetics. Unfortunately, although there are quite a lot of investigations dealing with adsorption properties of alkyldimethylphosphine oxides (cf. Introduction) only a few of them2,10-13,27 took into consideration this important aspect. Therefore, despite big efforts in the chemical purification of the alkylphosphine oxides, an additional endeavor was necessary to arrive at the required grade of “surfacechemical purity”. Figure 3 represents the σe vs c isotherms of the whole homologous series between hydrocarbon chain lengths nC of 6 (n-hexyl) and 14 (n-tetradecyl) measured at 295 K. Each isotherm is characterized by a continuous course without any break. The isotherms’ slope increases monotonically with rising surfactant concentration. 3.2. Evaluation of Equilibrium Surface Tension (σe) versus Concentration (c) Isotherms. The σe vs c isotherms (cf. Figure 3) were evaluated by a two-state approach to the surface equation of state put forward only recently in refs 4 and 5. This approach takes into account two different molecular configurations of the amphiphile in the adsorption layer, which are assumed to occur at either low or high surface coverage. The two alternative regions are connected by a transition region, allowing for a mixture of both surface configurations. With respect to the evaluation of the experimentally determined σe vs c isotherm we have the relation
∆σ ) R∆σ1 + (1 - R)∆σ2
(4)
∆σ ) σw - σe stands for surface pressure, with σw surface tension of pure water and σe equilibrium surface tension of the surfactant solution. The function ∆σ1 denotes the Traube/Henry equation,
∆σ1 ) Kc ) RTΓ(c)
(5)
and ∆σ2 stands for the Langmuir/Szyszkowski equation,
∆σ2 ) RTΓ∞ ln(1 + c/aL)
(6)
and/or the Frumkin equation
∆σ2 ) -{RTΓ∞ ln(1 - Γ/Γ∞) + Γ∞Hs(Γ/Γ∞)2} (7a) with
c ) aLΓ/(Γ∞ - Γ) exp(-2HsΓ/RTΓ∞)
(7b)
R(c) represents the transition function, varying between 1 and 0. Γ, Γ∞, aL, and Hs denote surface concentration (adsorption) and saturation surface concentration (saturation adsorption), surface activity parameter (coefficient describing distribution between bulk and surface phase), and surface interaction parameter, respectively. The interesting adsorption data standard free energy of adsorption ∆G°ad and limiting surface area demand per molecule adsorbed Amin (minimal cross-sectional area) are obtained as
∆G°ad ) RT ln aL
(8a)
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Figure 4. Cross-sectional area Amin of the n-alkyldimethylphosphine oxides with dependence on the number nC of carbon atoms in the n-alkyl chain. Circles: even members; crosses: odd members. (The standard deviations of the calculated Amin values amount to (2 Å2 at maximum.)
and
Amin ) (NLΓ∞)-1
(8b)
NL denotes the Lochschmidt number. The standard deviation of the best-fits amounted to ( 0.1 to (0.3 mN/ m. (For details cf. refs 4 and 5.) The macroscopic transition is described by a convenient symmetrically smeared step function of variable width such as, for example, the tanh function or alternatively a polynomial.4,5 It is also possible to describe the transition function by the ratio of the two cross-sectional areas of the alternative surface configurations.5 This approach enables us to describe the complete σe vs c isotherm of any soluble amphiphile (provided its solutions are surface-chemically pure) with utmost accuracy and reliable, reasonable adsorption parameters.4,5,37-40 The σe vs c isotherms of the n-alkyldimethylphosphine oxides are matched in the best manner possible by a twostate fit of Henry-Szyskowski/Langmuir according to ref 4. This means that the whole homologous series obeys “ideal surface behavior”1 with negligible interactions between the molecules adsorbed. 3.3. Adsorption Properties. The relationship between the number nC of carbon atoms in the n-alkyl chain and the resulting adsorption parameters are illustrated in Figures 4-6. Figure 4 shows the homologues’ cross-sectional areas Amin with dependence on nC. The Amin values decrease with increasing chain length. This was already reported by us for the even-numbered members.2 However, it is interesting to note that also the complete n-alkyldimethylphosphine oxides’ Amin(nC) dependence clearly reveals an even/ odd characteristic. A weak but distinct effect of alternation
is also found in the dependence of the standard free energy of adsorption ∆G°ad on nC (Figure 5). The latter relationships are described as
{∆G°ad}evenβ ) -2.645nC + 2.081
(9a)
{∆G°ad}oddβ ) -2.428nC + 0.408
(9b)
For reason of completeness we also give the homologues’ critical micellar concentrations (cmc) with dependence on nC in Figure 6. However, these values were determined with the original solutions, which were not purified further. (High-performance purification to obtain the grade of surface-chemical purity was only performed at submicellar concentrations because surface-active contaminants are usually solubilized in the surfactant micelles.) Hence, the cmc values are not as exact as the other adsorption parameters. Nevertheless, the effect of alternation seems to be reflected in the cmc vs nC dependence, too. The relationships between the cmc values and nC are described as
log(cmc)even ) -0.460nC + 2.099
(10a)
log(cmc)odd ) -0.514nC + 2.704
(10b)
The adsorption data are compiled in Table 2. 4. Discussion According to their chemical and physical properties, the n-alkylphosphine oxides are very convenient model amphiphiles suitable to determine adsorption properties exactly.
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Figure 5. Standard free energy of adsorption ∆G°ad of the dimethylalkylphosphine oxides with dependence on the number nC of carbon atoms in the n-alkyl chain. Circles: even members; crosses: odd members. (The standard deviation in ∆G°ad was (0.1 kJ/mol on average.) Table 2: Adsorption Parameters of n-Alkyldimethylphosphine Homologues nC C6Me2PO C7Me2PO C8Me2PO C9Me2PO C10Me2PO C11Me2PO C12Me2PO C13Me2PO C14Me2PO
Γ∞, 10-10 Amin, mol‚cm-2 Å2/molecule 3.45 3.10 3.84 3.81 3.68 4.56 4.15 4.98 4.49
48.1 53.6 43.2 43.6 45.1 36.4 40.0 33.3 37.0
a L, mol‚dm-3 3.66 × 10-3 1.06 × 10-3 4.03 × 10-4 1.71 × 10-4 5.17 × 10-5 2.39 × 10-5 4.92 × 10-6 2.76 × 10-6 6.79 × 10-7
∆G°ad, cmca kJ‚mol-1 mol‚dm-3 -13.75 -16.78 -19.15 -21.26 -24.18 -26.07 -29.94 -31.36 -34.80
5 × 10-2 1.2 × 10-2 4 × 10-3 1.2 × 10-3 4 × 10-4 1.0 × 10-4 3.9 × 10-5
a These data refer to original solutions (from “as received” material) only, but not to surface-chemically pure solutions.
Figure 6. Critical concentrations of micelle formation (cmc) of chemically pure, but not surface-chemically pure, dimethylalkylphosphine oxides, with dependence on the number nC of carbon atoms in the n-alkyl chain. Circles: even members; crosses: odd members.
So far most of the data on adsorption of soluble amphiphiles such as n-alkyldimethylphosphine oxides, n-alkanoic acids, n-alkanols, sodium n-alkylsulfates, etc. have not met the rigorous requirement of surface-chemical purity. Hence, it is interesting to know whether the
surfactants’ basic relationships established in the past lacking sufficient purity will also hold for the case of “surface-chemical purity”. There is, first of all, Traube’s fundamental rule saying that the homologues’ surface activity changes by a constant factor for subsequent members of the homologous series.48,49 The Traube factor is calculated by the quotient of the surface activity parameters aL of subsequent homologues. The parameter aL is calculated from the corresponding surface equation of state, eq 6, as aL ) exp{∆G°ad/RT}. The aL(nC) dependence reveals an alternation characteristic. Hence, it is evident that Traube’s rule also holds. The average values of these quotients for the subsequent even- and odd-numbered members, i.e., for aL,2n/aL,2n+2 and for aL,2n-1/aL,2n+1 (n ) 3-6), amount to 8.65 and 7.34, respectively. Thus, the Traube factor is slightly different for the evenand odd-numbered members. Basically, the surface activities of the odd homologues are relatively weaker by about 15%, as compared to the even ones. This can be seen in Figure 5. The course of the two straight lines ∆G°ad(nC) has a slightly different slope. The reason for it is to be discussed below. Second, we have given evidence by using various homologous series that the previously well-accepted assumption about the constancy of the amphiphiles’ cross(48) Traube, I. Liebigs Ann. Chem. 1891, 265, 27. (49) Stauff, J. Kolloidchemie; Springer-Verlag: Berlin Go¨ttingen Heidelberg, 1960.
1058 Langmuir, Vol. 15, No. 4, 1999
sectional areas within a series does not hold for soluble amphiphiles.2,36,37,39,41 Usually, the Amin values depend on nC, decreasing with rising chain length. If one extrapolates these values to those of the longer-chain, insoluble homologues, one arrives at the more or less constant values of the insoluble ones, which are known from surface pressure (π) versus area (A) isotherms. This was shown by us for the n-alkyldiethylphosphine oxides in ref 36. It is clear from Figure 4 that a relationship like this holds for the odd- and the even-numbered members each. Unfortunately, π(A) isotherms are not known for nalkyldimethylphosphine oxides. It would be interesting to know whether the effect of alternation is maintained in the insoluble monolayer behavior. Although the alternation effect has now been observed repeatedly varying considerably in its amount, the reasons for it are not yet elucidated satisfactorily. There is no parallel shift between the ∆G°ad values of the even- and the odd-numbered homologues; i.e., the Traube factor is different for the two kinds of representatives. This hints to the fact that the n-alkyl chain conformation is influenced by the adsorbate’s entire state of adsorption, i.e., by its hydrophobic as well as its hydrophilic part. It seems that the phenomenon of alternation represents a general feature of n-alkyl amphiphiles. Phenomenologically two kinds of it can be distinguished. The first one is met with amphiphiles of relatively simple molecular structures, such as n-alkyldimethylphosphine oxides, n-alkanoic acids, and n-alkanols, revealing rather small differences between the even- and the odd-membered characteristics.36,37 The alternative kind is characterized by strong differences in the adsorption features of both kinds of homologues. Thus, for example, the cross-sectional areas of certain odd-numbered ionic surfactants may be up to 30 to 40% greater than those of the related even-numbered homologues.38,39 One reasonable explanation for the alternation phenomenon goes back to Gutmann’s concept of donoracceptor interaction.42 According to this approach, a donor-aceptor bond is always shortened, whereas that of the neighboring bond is lengthened to a certain amount. According to Gutmann’s second rule, the alternative shortening and lengthening of the bond lengths is extended throughout the whole molecule. Thus, the n-alkyl chains’ terminal groups should be in a somewhat different molecular state depending on whether they belong to the odd or the even members. However, if we think of extended chains directed perpendicular toward the surface plane, one can hardly understand why the cross-sectional areas of the two kinds of n-alkyl chain members should be different. In addition, if there was some difference in the molecules’ binding conditions, one should expect that the two dependences Amin(nC)even and Amin(nC)odd ought to show a parallel shift. This, however, does not hold. Trying to understand the phenomenon under discussion, we have to realize that an adsorbed layer of an amphiphile is far from assuming a solidlike structure. If possible, this might be met for the insoluble homologues, the only homologues characterized by a real phase transition in their monolayers. It is clear now that the cross-sectional areas in the adsorbed monolayers are distinctly larger than those of the insoluble homologues. Thus, there will be enough freedom in the state of adsorption to allow for
Lunkenheimer et al.
different surface conformations. This was discussed already by Vold in ref 50. It is interesting to note that an even/odd characteristic is already revealed in Vold’s data calculated for various surface conformational parameters.51 Noticably, there is also a nonparallel shift between the cross-sections of the even and the odd members. Thus, whether the donoracceptor approach is able to explain the even/odd phenomenon seems to depend on the possibility to induce differences in the alkyl chains’ surface conformations. The other main feature in the n-alkyldimethyl(and -diethyl-)phosphine oxides’ adsorption is the dependence of the cross-sectional area Amin on the carbon chain length. Why does Amin decrease with increasing chain length? It is reasonable to assume that a certain limiting Amin value of a homologous series exists, being realized for the homologues with the longer chains, which are insoluble. In refs 2 and 37-39 we tried to understand it by taking into account the increasing interaction between the n-alkyl chains with rising chain length. However, this does not work within the homologous series of the n-alkyldimethylphosphine oxides, as they follow ideal surface behavior; i.e., there is negligible interaction between the adsorbed molecules. Hence, in ref 2 we assumed that the decreasing Amin values with rising chain lengths could be due to differing conditions of hydration of the phosphine oxides’ headgroup. However, it does not seem realistic that alterations in hydration as big as those observed can be caused by varying the n-alkyl chain length of a substituent only. According to ref 50 “isolated n-alkyl chains possess a propensity to adopt irregular conformations”. This is obviously the main reason for the occurrence of the alternation phenomena, although dimethylalkylphosphine oxides were not considered in the conformational analysis of ref 50. However, calculated and experimental data are far from being in coincidence. Thus, for example, Vold presented well-distinguished differences in the calculated adsorptions for several n-alkyl chain surfactants in Gibbs monolayers. However, the trend found in the calculations is opposite to that observed experimentally; i.e., the calculated cross-sectional areas increase with rising chain length.50 Hence, we think that progress in the understanding of adsorption of amphiphiles can be gained by placing reliable data on adsorption at the theoreticians’ disposal. In addition, more subtle experimental techniques other than thermodynamic, like vibrational sum frequency spectroscopy (V-SFS),52,53 might help to get further experimental information about the amphiphiles’ real conformations in adsorption layers. Thus, following the results of VSFS investigations of some simple ionic surfactants,52,53 the decreasing cross-sections with increasing alkyl chain length could be due to a reduction of gauche defects in the hydrocarbon chain. LA980611T (50) Vold, M. J. J. Colloid Interface Sci. 1984, 100, 224. (51) Warszynski, P. Private communication. (52) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. J. Phys. Chem. B 1997, 101, 6724. (53) Gragson, D. E.; McCarthy, B. M.; Richmond, G. L. J. Am. Chem. Soc. 1997, 119, 6144. (54) Hoechst AG, DE 1902444, 1969; Chem. Abstr. 1969, 73, 88023. (55) Unilever Co. GB 976974, 1964; Chem. Abstr. 1965, 62, 8014. (56) Kleiner, H.-J. Liebigs Ann. Chem. 1974, 751. (57) Maier, L. Helv. Chim. Acta 1966, 49, 1249.
