Langmuir 1998, 14, 5719-5724
5719
Dynamic Surface Tensions and Micelle Structures of Dichained Phosphatidylcholine Surfactant Solutions Julian Eastoe* and James S. Dalton School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
Richard K. Heenan ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K. Received January 12, 1998. In Final Form: May 11, 1998 Surface tensions of aqueous solutions containing dialkyl phosphatidylcholines ((di-Cn)-PCs with n ) 5, 6, 7, and 8) have been studied at the air-water interface, under equilibrium and dynamic conditions using du Nouy and maximum bubble pressure tensiometery, respectively. In the limit of short time (t f 0) the dynamic surface tension (DST or γ(t)) decays were consistent with a diffusion-controlled adsorption. However, at long times, closer to the equilibrium γ values, there was good evidence for a mixed diffusionactivation mechanism. Analysis of the limiting γ(t f ∞) data was consistent with an adsorption barrier of around 4 kJ mol-1. To check for the possible effects of micelles on the DST, approximate first-order micellar dissociation rate constants kmic were determined by stopped-flow spectrophotometry and a probe dye, Eosin Y. For example, with (di-C8)-PC, kmic was about 2 s-1. By use of measured values for kmic for these surfactants, it was shown that, just above the critical micelle concentration, the micelle breakdown kinetics have no discernible effect on the γ(t) decays. Small-angle neutron scattering (SANS) was also used to characterize the micelle structures, and analysis of the SANS data, using appropriate form factor expressions, indicated a micellar shape change spheres f ellipsoids f cylinders on increasing hydrophobic chain length from n ) 5 to n ) 8. These results show that the PCs have similar dynamic and equilibrium surface properties to structurally related dichained glucamides (Langmuir 1996, 12, 2701 and 1998, 14, 979; J. Colloid Interface Sci. 1997, 188, 423).
Introduction Phosphatidylcholines (PCs) are biosynthetically produced surfactants, and man-made PC analogues are useful model compounds for biochemical and physical studies.1 Figure 1 shows the generic structure of the saturated chain, synthetic dialkyl phosphatidylcholines ((di-Cn)PCs) which are studied here; the compounds have alkyl chains (n - 1) carbon atoms long. The purpose of this research was to characterize aqueous solutions of these PCs in terms of the equilibrium and dynamic adsorption properties as a function of concentration, especially dynamic surface tension and the micelle structures. When compared with properties of structurally related dichained glucamides (di-(Cn-Glu’s), with n ) 5, 6, 7, and 8 alkyl carbons2), which have been studied previously,3-5 these results point to a general activated diffusion mechanism for surfactants of this type. Many industrial and technological processes use surfactant solutions under nonequilibrium conditions, and * To whom correspondence should be addressed: e-mail,
[email protected]; tel, U.K. +117-9289180; fax, U.K. +1179250612. (1) Stryer, L. Biochemistry 3rd ed.; W. H. Freeman and Co.: New York. Small, D. M. In The Physical Chemistry of Lipids-The Handbook of Lipid Research; Plenum Press: New York and London, 1986; Vol. 4. (2) These glucamides are (CnH2n+1)2C[CH2NHCO(CHOH)4CH2OH]2 and termed di(Cn-Glu)’s. It should be noted that the hydrophobic groups on (di-C6)-PC are the same length as those on di-(C5-Glu) etc. This accounts for the nomenclature, which includes the carbonyl carbon in the alkyl chain length of the phosphatidylcholines. (3) Eastoe, J.; Rogeuda, Ph. G. A.; Howe, A. M.; Pitt, A. R.; Heenan, R. K. Langmuir 1996, 12, 2701. (4) Eastoe, J.; Dalton, J. S.; Rogueda, Ph. G. A.; Crooks, E. R.; Pitt, A. R.; Simister, E. A. J. Colloid Interface Sci. 1997, 188, 423. (5) Eastoe, J.; Dalton, J. S.; Rogueda, Ph. G. A.; Griffiths P. C. Langmuir 1998, 14, 979.
Figure 1. Molecular structure of symmetric diacyl phosphatidylcholines referred to as (di-Cn)-PC in text.
therefore dynamic surface tension (DST or γ(t)) is an important property.6 This paper develops an insight into whether surfactant type and critical micelle concentration (cmc), as well as the presence, shape, and kinetic stability of micelles, have any discernible effect on surface tension decays. For this (di-Cn)-PC surfactant series, these properties all depend strongly on the alkyl chain length. The cmc and adsorption isotherms have been derived from equilibrium du Nouy tensiometry, while the dynamic surface tensions were determined by a maximum bubble pressure (MBP) method.6 Small-angle neutron scattering (SANS) was used to study the micelle structures, and the micelle dynamics were investigated using stopped-flow spectrophotometry, with a probe dye Eosin Y that partitions between micelles and water, to measure the firstorder dissociation rate constants kmic as described before.7 The dynamic adsorption, Γ(t), has been considered by Ward and Tordai as a diffusion-only process,8 but owing (6) Dukhin, S. S.; Kretzschmar, G.; Miller, R. Dynamics of Adsorption at Liquid Interfaces; Elsevier: Amsterdam, 1995. (7) Zana, R.; Tondre, C. J. Colloid Interface Sci. 1978, 66, 544. (8) Ward, A. F.; Tordai, L. J. Chem. Phys. 1946, 14, 453.
S0743-7463(98)00053-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/12/1998
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Eastoe et al.
to an incalculable Volterra integral, accounting for back diffusion from the subsurface, the equation cannot be solved analytically. Instead Miller has shown that asymptotic solutions are useful in applying of the Ward-Tordai equation to surfactant DSTs.6,9 These solutions give γ(t) at short times, where the tension is close to that for the pure solvent γ0, and at long times, where the tension is close to the equilibrium value for the solution γeq. For zwitterionic surfactants the equations are
γ(t)tf0 ) γ0 - 2RTc
(Dtπ )
1/2
(1a)
and
γ(t)tf∞ ) γeq +
RTΓ2 π 2c Dt
1/2
( )
(1b)
where c, Γ, and D represent the bulk concentration, equilibrium surface excess, and monomer diffusion coefficient of the surfactant in solution. The validity of these equations is tested here, as applied to solutions of (diCn)-PC surfactants as a function of chain length and concentration. The results support recent findings that suggest a mixed diffusion-activation adsorption mechanism,4,5,10-14 and this is clearest at the end of the DST decays where the surface pressures are highest. Liggieri et al. introduced this adsorption barrier Ea in terms of an effective diffusion coefficient Deff10
Deff ) D exp(-Ea/RT)
(2)
For these zwitterionic surfactants, at the temperatures and concentrations studied, it was found that Ea is approximately 4 kJ mol-1. This compares with an average value of 10 kJ mol-1 found for a range of pure CiEj n-alkylpoly(ethylene oxide)s, and also 70 kJ mol-1 for the di-(C6-Glu).5 When taken together, all of these results indicate that DST decays of pure, neutral surfactants generally obey a mixed adsorption mechanism. Experimental Section The phosphatidylcholine surfactants (>99%) were purchased from Avanti Polar Lipids (USA), stored at -20 °C, but otherwise used as received. As shown in Figure 2 surface tension measurements indicated a high purity. H2O was from a Milli Q reverse-osmosis purification system, and D2O (99.9% D-atom) was obtained from Fluorochem Ltd. (UK). Tensiometry. Surface tensions were measured using a Kru¨ss K10 du Nouy apparatus and a Lauda MPT1 MBP tensiometer. These instruments were calibrated over the range 70 to 22 mN m-1 against water, benzyl alcohol (Aldrich) and absolute ethanol/ water mixtures,4,12 and this defined the absolute uncertainties in γ as (0.10 mN m-1 (du Nouy) or (0.20 mN m-1 (MBP). In the range 5 ms f 10 s, DST curves from Triton X-100 (Aldrich) solutions agreed to (1.0 mN m-1 with literature data obtained (9) Fainerman, V. B.; Makievski, A. V.; Miller R. Colloids Surf., A 1994, 87, 61. (10) Liggieri, L.; Ravera, F.; Passerone, A. Colloids Surf. A 1996, 114, 351. (11) Ravera, F.; Liggieri, L.; Steinchen, A. J. Colloid Interface Sci. 1993, 156, 109. (12) Eastoe, J.; Dalton, J. S.; Rogueda, Ph. G. A.; Sharpe, D.; Dong, J.; Webster J. R. P. Langmuir 1996, 12, 2706. (13) Lin, S.-Y.; Tsay, R.-Y.; Lin, L.-Y.; Chen, S.-I. Langmuir 1996, 12, 6530. (14) Cho, D.; Narsimhan, G.; Franses, E. I. J. Colloid Interface Sci. 1997, 191, 312.
Figure 2. Equilibrium surface tension γeq vs concentration for (di-Cn)-PC surfactants: (9) (di-C6)-PC at 25 °C; (O) (di-C7)-PC at 25 °C; ([) (di-C8)-PC at 40 °C. For pre-cmc data, the fit is to a quadratic line. by various different DST methods.15 For (di-C5, C6, C7)-PC the samples were thermostated to 25 ( 0.1 °C, while for (di-C8)-PC it was 40 ( 0.1 °C. Both the equilibrium and dynamic surface tensions were reproducible. Small-Angle Neutron Scattering. SANS measurements were performed using the LOQ time-of-flight instrument on ISIS at the Rutherford Appleton Laboratory, U.K. The measurements determine the absolute scattering probability I(Q) (cm-1) as a function of momentum transfer Q (Å-1) ) (4π/λ sin(θ/2) with λ the incident neutron wavelength (2.2 f 10 Å) and θ the scattering angle (1000 Å. The di-(C7-Glu) surfactant also exhibited thin cylinder micelles, but of apparently shorter length, ∼ 200 Å.3 Estimates for the aggregation numbers, obtained using the fitted scale factors, were 12 ( 2, 58 ( 5, and 120 ( 10 for the (di-C5), (di-C6), and (di-C7)-PC, respectively. Because it was not possible to determine an accurate length for the di-C8 micelles, no reliable estimate for the aggregation number could be obtained. The results given in the two sections above, and Tables 1 and 2, indicate close similarities in the evolution of surfactant properties and micelle structure as a function of hydrophobe chain length for the two classes of dichained phosphatidylcholine and glucamide surfactants. Below, dynamic surface tensions of (di-Cn)-PC solutions will be discussed, with the aim of comparing this behavior with the di-(Cn-Glu) series3-5 and of investigating if any generalities can be made about the adsorption kinetics of such neutral dichain surfactants. In both of these two series the alkyl chain length has an important effect on the micelle structure, as well as the more obvious effects on the cmc. Therefore, it is possible to see what effects, if any, the surfactant type, cmc, and structure of micelles have on the dynamic surface tension decays. (23) Lin, T.-L.; Gabriel, N. E.; Roberts, M. F.; Chen, S. H. J. Am. Chem. Soc. 1986, 108, 3499 (24) Lin, T.-L.; Gabriel, N. E.; Roberts, M. F.; Chen, S. H. J. Phys. Chem. 1987, 91, 406. (25) (a) Kotlarchyk, M.; Chen, S.-H.; Huang, J. S.; Kim, M. W. Phys Rev. A 1984, 29, 2054. (b) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022.
spherical micelle (di-C5)-PC
concn × cmc
φmic
R/Å
1.5 f 5
0.028 f 0.221
12 ( 2 ellipsoidal micelle
(di-C6)-PC
concn × cmc
φmic
a/Å
b/Å
2 f 20
0.006 f 0.11
15 ( 2
30 ( 2
cyclindrical micelle (di-C7)-PC (di-C8)-PC
concn × cmc
φmic
r/Å
L/Å
5 f 100 20 f 500
0.003 f 0.076 0.0015 f 0.040
16 ( 2 18 ( 2
120 ( 20 >1000
a R is the sphere radius and a and b are the minor and major axes of an ellipsoid, with r and L the cross-section radius and length of a cylinder.
Figure 5. Dynamic surface tensions for (di-C7)-PC solutions concentrations × 103/mol dm-3: 5.0 ([), 2.0 (0), 1.0 (2), 0.5 (]), 0.3 (f), 0.2 (b), 0.1 (+), 0.05 (O), T ) 25 °C.
(3) Dynamic Surface Tension. DSTs for the (di-C6)-, (di-C7)-, and (di-C8)-PCs were measured over the concentration range 5 × 10-3 to 5 × 10-5 mol dm-3, and Figure 5 shows representative runs for (di-C7)-PC. For the 5 × 10-3, 2 × 10-3, and 1 × 10-3 mol dm-3 solutions the tension has already decreased from the solvent value (∼ 72 mN m-1) before the first data point at about 5 ms. As previously with nonionics,4 these curves were analyzed in terms of a diffusion only mechanism, eq 1, at long and short times. Recently, Miller et al.26 have warned against using the long time approximation (eq 1b) for finding the equilibrium value γeq and questioned its applicability to compounds of low surface activity very close to equilibrium. However, they concluded that this approximation was valid for estimating the adsorption mechanism, by measuring an effective diffusion coefficient. Monomer diffusion coefficients were calculated using the relationship D1/D2 ) (MW2/MW1)1/2 and a reference value of D ) 2.7 × 10-10 m2 s-1, for di-(C6-Glu) at 25 °C obtained from pulsed-field-gradient spin-echo NMR.11 For the PCs the estimates for D are 3.1 × 10-10 m2 s-1 for the (di-C6)-PC and 3.0 × 10-10 m2 s-1 for both the (di-C7)- and (di-C8)-PC compounds. The effect of temperature on D (26) Makievski, A. V.; Fainerman, V. B.; Miller, R.; Bree, M.; Liggieri, L.; Ravera, F. Colloid Surf., A 1997, 122, 269.
(di-Cn)-PC Surfactants
Figure 6. Dynamic surface tension of a (di-C8)-PC solution at 2 × 10-4 mol dm-3. The solid lines are diffusion-only fits calculated by eq 1 with D ) 3.0 × 10-10 m2 s-1. The dotted line is for eq 1b with Deff ) 0.24 × 10-10 m2 s-1 corresponding to an activation barrier of 6.3 kJ mol-1 via eq 2.
with (di-C8)-PC at 40 °C was accounted for using the Stokes-Einstein equation. The method for obtaining an effective diffusion coefficient (Deff) from DST data using eq 1 has been given previously.4 If the adsorption mechanism were simply diffusion controlled, then these values for the monomer diffusion coefficient D and the measured surface excesses Γ (e.g., Figure 3) given above should account well for the measured DST decays. The most obvious explanations for any observed discrepancies between measured DST data and predictions using the limiting equations would be that the mechanism is not purely diffusion controlled and/or micelles play a role in determining the flux of monomer to the interface. In particular below the cmc, if Deff < D, this would be consistent with an adsorption barrier. In the case of (di-C7)-PC, the decays for the lower four concentrations were amenable to analysis in terms of eq 1 at short time. Treatment of similar short time decay gave Deff values essentially identical to D, for all three PCs, and this implies that the start of the process may be thought of as essentially diffusion controlled. This short time-diffusion-only mechanism was also consistent with DST decays for nine separate nonionic CiEjs and glucamides.4 However, at long times the effective diffusion coefficients are smaller than the expected D values, and the ratio Deff/D was between 0.20 and 0.05. Furthermore, there were no obvious effects of the cmc’s on the DST decays or on values of Deff/D resulting from the analyses. Figure 6 shows experimental data and tensions calculated from eqs 1a and 1b for (di-C8)-PC at 2 × 10-4 mol dm-3. The agreement at short times is obviously good, but closer to equilibrium the measured value of tension is consistently higher than that predicted by the diffusion law. The dashed line was fitted to the data with an effective diffusion coefficient Deff ) 0.24 × 10-10 m2 s-1. This effect has been seen recently, with DSTs as a function of temperature, for di-(C6-Glu) below its cmc,5 and can be explained the presence of a weak adsorption barrier via eq 2.10,11 The values of activation energies derived in this way for the phosphatidylcholine series were all in the range 3-7 kJ mol-1, and there was no obvious concentration or chainlength dependence. This compares favorably with the average value of ∼10 kJ mol-1 found recently for the nine different nonionic surfactants.4 The likely origin of this
Langmuir, Vol. 14, No. 20, 1998 5723
Figure 7. Dynamic surface tension of a (di-C8)-PC solution at 2 × 10-4 mol dm-3. The lines are calculations using eq 6 with kmic ) 2 s-1 and (s) D ) 3.0 × 10-10 m2 s-1 or (- - -) Deff ) 0.24 × 10-10 m2 s-1.
barrier is that to adsorb into the layer a molecule needs energy to overcome the increasing surface pressure. (4) Effect of Micelle Breakdown on DST. For the most hydrophobic surfactant in this study, (di-C8)-PC, the DST curves were measured for concentrations up to 32 times the cmc. The adsorption barrier derived from eq 2 was very similar to that found for all the other surfactants, even though a majority of the samples were above the respective cmc. This has also been noted before for the nonionics,4 and hence it can be concluded that at low micelle concentrations the presence of aggregates has no discernible effect on the DST. In other words, close to the cmc there is nothing special about the presence of micelles in terms of dynamic surface tensions. Furthermore, it would appear that up to about 100 × cmc the total surfactant concentration can be used in eqs 1a and 1b. This conclusion is further supported by analyzing the DST decay for post-cmc (diC8)-PC solutions in terms of Fainerman’s equation27 to account for the release of monomer from micelle breakdown events:
γ(t)tf∞ ) γeq +
( )
RTΓ2 1 2ct Dkmic
1/2
(6)
The first-order rate constant for micelle breakdown is kmic, and for (di-C8)-PC micelles values have been measured by the stopped-flow dye probe method.4,7 For various combinations of concentration jump, from above to below the cmc, the relaxation traces were all characteristic of a first-order decay and a rate constant of 2 ( 0.5 s -1. This is comparable to the value of kmic ) 1 s -1 found in similar experiments for the di-(C7-Glu).4 This experimental value for kmic was used in eq 6 to calculate the DST decay, and as shown in Figure 7 the agreement with the measured data was poor. On Figure 7, the solid line was obtained using D ) 3.0 × 10-10 m2 s-1, while the dashed line with the effective value Deff ) 0.24 × 10-10 m2 s-1 was obtained as described above. Similar discrepancies between eq 6 and the measured DST curves were also seen for other concentrations and surfactant chain lengths. It can be deduced that the effects of micellar breakdown may only be important when there is very little monomer in the (27) Fainerman, V. B.; Makievski A. V. Kolloid Zh. 1992, 54, 890, 897.
5724 Langmuir, Vol. 14, No. 20, 1998
subsurface and when values of kmic are very small, i.e., for hydrophobic surfactants that have very low cmc’s (
