Langmuir 1997, 13, 2953-2959
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Foaming Properties of Modified Ethoxylated Nonionic Surfactants A. Colin,* J. Giermanska-Kahn, and D. Langevin Centre de Recherche Paul Pascal, avenue Docteur Schweitzer, Pessac 33600, France
B. Desbat Laboratoire de Spectroscopie Mole´ culaire et Cristalline, Universite´ de Bordeaux I, 33405 Talence, France Received July 8, 1996. In Final Form: March 25, 1997X The possible origins of the reduced foamability of dilute solutions of chemically modified ethoxylated nonionic surfactants are discussed. First, the organization of the modified molecules at the interface is different from that of the standard ethoxylated compounds. After diffusion from the bulk to the interface, the molecules undergo a rearrangement to take a folded conformation at the surface. This second process, observed in the dynamic surface tension measurements is much slower than the first one. Therefore, during the foam formation, the interfaces of freshly formed films are not sufficiently covered with the surfactant and, hence, are badly protected against film rupture. Moreover, the modified surfactants have a smaller surface viscoelasticity, thus enhancing the thinning and the breaking of foam films.
Introduction Excessive foaming may cause inconveniences when detergent solutions are used. In order to avoid this phenomenon, development of antifoams has therefore been a necessity. Usually, antifoams are mixtures of oils and solid particules which form hydrophobic droplets in foaming solutions. These droplets bridge the foam films and rupture them by a dewetting action. Another way to solve the problem is to design new surfactants that are good detergents and poor foamers. To achieve this goal, it is necessary to understand how the surfactant chemical structure influences the foaming properties. For this purpose we have studied nonionic poly(oxyethylene) surfactants which have been chemically modified. We have started from the series CnH2n+1(OCH2CH2)mOH, usually abreviated to CnEm. A new series of surfactants have been obtained by substituting the terminal OH group by a more hydrophobic part (oxypropylene chain, or chlorine). The modified surfactants C10E10Cl or C10E5P7 are also good detergents but poor foamers. Furthermore the foaming is poor for both concentrated and dilute solutions. In concentrated solutions, the foamability of ethoxylated nonionic surfactants is related to the presence of the socalled cloud point. Above a critical temperature, that depends weakly on the surfactants concentration, the polar heads begin to dehydrate and an attractive force appears between the micelles. Upon heating, the solution becomes cloudy and separates into two phases: a micelle-rich phase and a very dilute one. This phase separation is accompanied by a reduction in foamability. As demonstrated,1-3 the coexistence of the two phases is a key point in the reduction of foam stability above the cloud point. Drops of the micelle-rich phase separate out of the dilute solution and play the role of antifoam agent by emerging into both air/water interfaces of the foam films and rupturing the film by dewetting. If the micelle-rich phase is removed, after complete separation of two phases, the foamability is restored despite of the reduction in total X
Abstract published in Advance ACS Abstracts, May 1, 1997.
(1) Koretskaya Kolloı¨d Zh. 1977, 39, 571. (2) Ross, S.; Nishioka, G. Colloı¨d Polym. Sci. 1977, 255, 560. (3) Bonfillon-Colin, A.; Langevin, D. Langmuir 1997, 13, 599.
S0743-7463(96)00667-1 CCC: $14.00
surfactant concentration. Here, the replacement of the terminal OH group by a more hydrophobic group shifts the cloud point well below room temperature thus reducing foamability. In dilute solutions, this explanation does not hold anymore. In order to find the origin of the low foamability of the dilute solutions, first, we have characterized the structure of the surfactant monolayers at the surface of the solution, and second, we have investigated the surfactant adsorption kinetics. Experimental Part Materials. The aqueous solutions were made with ultrapure water (Millipore MilliQ system). The nonionic surfactants (C10E10, C10E10Cl, C10E5P7) given by Rhoˆne Poulenc were used as received. Experimental Techniques and Data Analysis. Tensiometry. The equilibrium surface tension is measured by using an open frame version of the Wilhelmy plate (to avoid the wetting problems of the classical plate). The reproducibility, including possible long equilibration times, is 0.25 mN/m. In order to determine the surface coverage, we use the Gibbs equation
dπ ) RTΓ d ln c
(1)
where π is the surface pressure, c the surfactant bulk concentration, Γ its surface concentration, R the gas constant, and T the temperature. Ellipsometry. In order to measure the thickness of the monolayer, we use a Plasmos ellipsometer type SD 2300 with a rotating analyser system. This technique does not give a direct thickness measurement, instead two “ellipsometric angles” ψ and ∆ (related to intensity change and phase shift of reflected light) are measured.4 A model is hence required to obtain the adsorbed layer thickness. We analyze our data using a homogeneous and isotropic adsorbed layer model. Under these conditions, the refractive index nm and thickness d of the monolayer can be calculated, in principle, from the changes δψ and δ∆ in the measured values of ψ and ∆, with and without an adsorbed layer. Hence, two sets of measurements are performed, one on pure water and one on surfactant solutions. We find that δψ is zero within experimental error as expected in the case of thin adsorbed layers. Thus, we can only rely on the measurement (4) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and polarized light; North-Holland: Amsterdam, 1992.
© 1997 American Chemical Society
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Table 1. Ellipsometric Data for Three Ethoxylated Surfactants: Molar Refractivities Rs and Refractive Indices ns of Pure Liquids, Refractive Indices nm, and Thicknesses d of the Monolayer (x is a number of molecules of water per surfactant molecule used in calculations) Rs (Å3)
nm
d (Å)
x
ns
C10E10
228.9 234.3
C10E5P7
339.6
31 39 13 17 20 25
10 20 10 20 12 24
1.457
C10E10Cl
1.382 1.373 1.459 1.453 1.422 1.4110
surfactant
1.455 1.453
of δ∆. Since the length of the extended surfactant chain is of the order of 50 Å, which is small compared with the wavelength λ of incident light, the Drude approximation can be used.5 The following expression relates δ∆, d, and nm
(
cos φ tg2φ nw2 na2 + nw2 - nm2 δ∆ d ) 4Π λ
2
2
2
2
)
nw2na2
2
(nw - na )(na tg φ - nw )
nm2
(2)
where Φ is the angle of incidence and nw and na are the refractive indices of water and air, respectively. In order to estimate nm, we assumed the additivity of molar refractivities Ri of film constituents, i.e., water and surfactant molecules. These are defined by the Lorentz-Lorentz relationship
Ri )
Mi ni2 - 1 Fi ni2 + 2
(3)
The molar refractivities of three surfactant molecules evaluated from the addition of the bond and atomic contributions6 are given in Table 1. Then, one obtains
Rm )
nm2 - 1 nm2 + 2
Sd ) Rs + x
nw2 - 1 nw2 + 2
(4)
where Rs and Rm are the molar refractivities of the surfactant molecule and the mean value of the monolayer, respectively, S is the area per surfactant molecule, and x is the average number of water molecules per one surfactant molecule in the monolayer. The thickness of the monolayer can be estimated provided assumptions for x can be made. Fourier Transform Infrared (FTIR) Spectroscopy. In order to analyze the organization of the molecules at the interface, we used the recently developed technique of polarization modulated reflection absorption spectroscopy7,8 (PM IRRAS). This technique combines Fourier transform mid-IR reflection spectroscopy with the fast polarization modulation of incident light between polarization (p) in the plane of incidence and polarization (s) perpendicular to this plane. The polarization modulation is introduced to eliminate isotropic absorptions occurring in the sample environment (especially water vapor or CO2 bands) as well as the fluctuations of the apparatus parameters. The experimental setup and signal treatment procedure were described elsewhere.8 Here, we recall only the main elements of the setup: a Nicolet 740 spectrometer with HgCdTe detector and ZnSe grid IR light polarizer modulated by ZnSe photoelastic modulator. The frequency of polarization modulation is 3000 cm-1. Each spectrum results from a coaddition of 300 scans at a resolution of 4 cm-1. This PM IRRAS signal can be expressed7,8 (5) Drude, P. Ann Phys. 1889, 36, 532, 865. (6) Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.; CRC Press Inc.: Boca Raton, FL, 1983. (7) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pezolet, M.; Turlet, J. M. Appl. Spectrosc. 1993, 47, 869. (8) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pezolet, M.; Turlet, J. M. Thin Solid Films 1994, 242, 146.
as follows
S)C
(Rp - Rs) (Rp + Rs) + J0(φ0)(Rp - Rs)
J2(φ0)
where Rp and Rs are the polarized reflectivities, J0 and J2 are the zero- and second-order Bessel functions, φ0 is the maximum dephasing given by the photoelastic modulator, and C is a constant that depends on the electronic device. The spectra of the monolayer adsorbed at the air/water interface contain the dispersions which comes from the liquid water absorption. In order to eliminate this contribution, one records the differential normalized signal
S)
S(d) - S(0) S(0)
where S(d) and S(0) correspond to the monolayer on the aqueous solution and pure water PM IRRAS spectra, respectively. In that way, the dependence on Bessel function is also removed and the weak absorption bands of the monolayer (orders of magnitude weaker than the substrate bands) are extracted. In the PM IRRAS spectra, the up (down) sense of monolayer bands relative to the base line and their intensity depend on the orientation of the corresponding transition moment, which in turn indicates the orientation of the molecular groups at the interface. This surface selection rule works if only the angle of incidence is chosen correctly (75° for the pure water substrate).7,8 For example, a transition moment lying in the interface plane gives rise to an absorption band oriented positively with the respect to the surface, whereas a transition moment perpendicular to the interface results in negatively oriented band. The general orientation of the transition moment leads to competitive negative and positive contributions. In order to determine the intensity of bands as a function of the orientation of associated transition moments, we used the computer program developed to simulate the PM IRRAS spectra from the anisotropic complex indexes n˜ j(ν) ) nj(ν) + iκj(ν) (j ) x, y, z). First, we determine the isotropic optical indices of three pure liquid surfactants by the micro-ATR (attenuated total reflectance) technique.9 Then, we modify the indices to take into account the differences between oscillator densities in the bulk and in the monolayer. Namely, we apply the reduction factor in the extinction coefficient of each compound, which is deduced from comparison of the area per polar head between bulk and the monolayer (we assume that we are in the limits of Lambert-Beer law). Finally, we assume that the modulus of the transition moment is preserved when passing from the isotropic medium to an anisotropic uniaxial monolayer. The extinction coefficient k along x, y, and z directions can be expressed in terms of the corresponding transition moment coordinates
kx ∝ Mx,2
ky ∝ My2,
and
kz ∝ Mz2
and
1 k ) (kx + ky + kz) 3 Hence, if the transition moment is tilted at an angle R with respect to the interface plane (x, y), simple relations can be applied to obtain the anisotropic extinction coefficient10
kx ) ky )
3 k cos2(R) 2
kz ) 3k sin2(R) The real part of an anisotropic refractive index n(ν) is then calculated from Kramers Kro¨nig analysis using the McLaurin method. We take for n(∞) the values of monolayer refractive (9) Buffeteau, T.; Desbat, B.; Eyquem, D. Vib. Spectrosc. 1995, 406, 29. (10) Blaudez, D.; Turlet, J.-M.; Dufourc, J.; Bard, D.; Buffeteau, T.; Desbat, B. J. Chem. Soc., Faraday Trans. 1996, 92, 525.
Foaming Properties of Nonionic Surfactants
Figure 1. Equilibrium surface tension of C10E10 and C10E10Cl solutions as a function of the bulk surfactant concentration. indices determined earlier from ellipsometry. Finally, we simulated the PM IRRAS signal of the monolayer on a pure water substrate, with average thicknesses as obtained from ellipsometry. We can neglect the difference between the subphase and pure water, because the critical micelle concentrations are of the order of mM. In order to find which orientation of the transition moment corresponds to the experimental intensity of the PM IRRAS band, we simulate first the band intensity for different transition moment orientations: in the plane of the interface and perpendicular. Then, it is easy to adjust the angle corresponding to the actual direction of the transition moment. From this direction we deduce the mean orientation of the vibrating group in the molecule and finally the shape of the molecule at the interface. Dynamic Surface Tension Measurements. The dynamic surface tension is measured by using a pendent drop apparatus. Details on the experimental setup can be found elsewhere.11 The surfactant solution under investigation is contained in a thermostated cell. An air bubble is formed at the tip of a stainless steel needle immersed in the surfactant solution. The tip of the needle is curved upward. In this way, surfactant depletion in the solution during the adsorption process is negligible because of the large surfactant reservoir. The needle is attached to a syringe which piston is attached to a micrometric screw. Once formed, the bubble is illuminated with a plan parallel beam, its image is captured with a CCD camera (Sony MACC-77) using an objective (Macro Zoom 18-108) that produces little distorsion. The image is digitized with a video image digitizer (Cyclope of Digital Vision, 512 × 512 pixel and 256 gray levels) and stored on a PC 386 25 MHz computer with a fast rate bus (ESDI 1,5 Mo/s). The value of the surface tension is deduced from the shape and the size of the drop by fitting the drop profile to the Laplace equation with the axisymmetric drop shape computer program of Skinner et al.12 Foamability Test. The Ross-Miles method is used to evaluate the foamability of the three compounds. In our device, 100 mL of solution was poured from a height of 70 cm into a cylinder, and the volume of foam produced is used as a measure of the foamability. The decrease of foam height was also recorded to evaluate the foam stability.
Results and Discussion Static Measurements. The surface tension versus bulk concentration curves are given in Figure 1 for C10E10 and C10E10Cl. There is no minimum in surface tension near the critical micelle concentration (cmc), which is indicative of the purity of the sample. The values of the surface coverage at the cmc calculated with the Gibbs equation are given in Table 2 together with the value of the cmc. It can be seen that at room temperature, the modified nonionic surfactants have an area per molecule twice as (11) Bonfillon, A.; Sicoli, F.; Langevin, D. J. Colloid Interface Sci. 1994, 168, 497. (12) Skinner, F. K.; Rotenberg, Y.; Neumann, A. W. J. Colloid Interface Sci. 1989, 130, 25.
Langmuir, Vol. 13, No. 11, 1997 2955
Figure 2. Area per molecule for C10E7P5OH molecule as a function of temperature. Table 2. Critical Micellar Concentration and Area per Molecule of C10E10, C10E7P5, and C10E10Cl As Obtained from Surface Tension versus Bulk Concentration Curves surfactant
temp (°C)
cmc (g/L)
area per molecule (Å2)
C10E10 C10E10Cl C10E5P7 C10E5P7 C10E5P7 C10E5P7
24 24 5 17 27 42
0.2 0.08 0.17 0.12 0.11 0.09
38 80 45 75 85 180
large as the nonmodified one. Moreover, in the case of the oxypropylene derivative we observe a continuous increase of the area per molecule with the temperature (Figure 2). These differences in area per molecule observed for the three surfactants at cmc should be accompanied by differences in thicknesses of the corresponding monolayers. This is because the molecular volumes are similar, at least in the case of C10E10OH and C10E10Cl compounds. To check this hypothesis, we determine the thicknesses by ellipsometry. To solve eq 4, we take either one or two water molecules per oxyethylene (or oxypropylene) segment. The results are presented in Table 1. They depend only slightly on the number of water molecules taken in the calculation. Therefore, although imprecise, they allow comparison of the values of the thicknesses. As expected, the two surfactants which occupy an area of 80 Å2 per molecule form thinner monolayers with average thicknesses 22 and 15 Å, whereas the C10E10 molecule which occupies an area of 40 Å2 is 35 Å thick. Then, it can be conjectured that the modified molecules take preferentially a horizontal position whereas the C10E10 molecule seems to stand more vertical. In order to better understand the differences in conformation of these molecules at the interface, we used the PM IRRAS technique in the range 800-4000 cm-1. For poly(oxyethylene) surfactants, there are two regions of interest: around 1100 cm-1 where the complex band for C-O-C skeletal stretching vibrations is found and between 2800 and 3000 cm-1, for the bands of symmetric and antisymmetric CH2 stretching vibrations of groups belonging to either aliphatic or poly(oxyethylene) chains. In the bulk, the oxyethylene part of the surfactant molecule can exist in three basic forms: a planar extended zigzag conformation, a meander conformation, and a helical conformation. It seems that in dilute aqueous solutions all three forms coexist with the predominance of the helical
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Figure 4. PM IRRAS spectra for the three nonionic surfactants: the CH2 stretching vibrations region.
Figure 3. PM IRRAS spectra for the three nonionic surfactants: the COC antisymmetric stretching band region.
one.13 This results in a broad band around 1100 cm-1, which includes some contributions from CC stretching and CH2 rocking. Nevertheless, the main C-O-C antisymmetric stretching vibration has a transition moment oriented mainly along the oxyethylene chain. Therefore its intensity is related to the average direction of oxyethylene segments. In Figure 3 the PM IRRAS spectra of the three compounds are shown together with the simulated spectra, calculated for a transition moment lying in the surface plane, perpendicular to the interface and for a random orientation. It can be noted that the sign of the absorption is opposite for the two first situations. (13) Cooney, R. P.; Barraclough, C. G.; Healy, T. W. J. Phys. Chem. 1983, 87, 1868.
In the case of C10E10Cl, the average direction of the oxyethylene segments is close to the interface plane (at about 20°), whereas for C10E10 this orientation is further away from the plane (45°). In fact, with an area of 40 Å2, the polar chains are not closely packed in a vertical direction (area of 20 Å2), but they are closer to the vertical than the modified molecules (area of 80 Å2). Finally, for C10E7P5OH, the orientation of the oxyethylene segments is intermediate, being close to the case of random orientation. For the modified molecules, the alkyl part being strongly hydrophobic is in contact with air; the poly(oxyethylene) hydrophilic part has to lay more along the interface plane because chlorine or poly(oxypropylene) are less soluble in water and have to be closer to air. The differences in monolayer thicknesses and in oxyethylene segment organization observed between the two modified molecules can be also understood. The chlorine atom is more hydrophobic than the oxypropylene group. Moreover the chlorine derivative is above the cloud point, the polar head being dehydrated and, hence, more hydrophobic. Therefore the tendency of oxyethylene segments to lay along the interface is even more pronounced. The oxyethyleneoxypropylene derivative is still below cloud point, so its conformation depends on degree of dehydration of polar heads. As seen by tensiometry, the area per molecule increases as the critical temperature approaches. The CH2 vibrations bands are much more difficult to interpret. The relative intensities of symmetric and antisymmetric vibrations (Figure 4) are different at the water/air interface and in the bulk solution of the pure liquids. In fact, the environment is different, resulting probably from differences in interactions leading to differences in bond strengths. In the IR spectra, both aliphatic and oxyethylene CH2 groups are contributing to the antisymmetric vibration band, whereas mostly aliphatic ones contribute to the symmetric vibration band. The intensity ratio between the nonmodified and the modified surfactant for this latter band is about 2, as expected from the number of corresponding oscillators densities. These studies lead to the conclusion that the organization of the surfactant with the respect to the interface is different for C10E10 and its derivatives. For C10E10, the monolayer is quite compact and the molecules stand vertical. For C10E10Cl the monolayer is less compact and the molecules take a horizontal position in the interface
Foaming Properties of Nonionic Surfactants
Langmuir, Vol. 13, No. 11, 1997 2957
Figure 5. Schematic molecular models of C10E10 and C10E10Cl at the interface.
(Figure 5). In the case of C10E7P5OH, this conformation depends on temperature. In the following, we are going to analyze the consequences of the differences in molecular shape on dynamic surface tension and on foaming properties. Dynamic Measurements. When a fresh surface is created between a surfactant solution and air, the initial surface tension is that of the bare air/water interface. Then the surfactant reaches the interface and the surface tension becomes time dependent. A series of sucessive steps are involved in the equilibration process and one or two steps may happen simultaneously. First, the surfactant molecules migrate from the bulk toward the interface by convection or diffusion. Second, they adsorb at the interface and reorient to find their final position: a polar head in the water and an aliphatic tail away from water. Usually, for low bulk concentrations and in the absence of convection, diffusion is expected to be the slowest step of the adsorption process. We assume here that when the surfactant has reached the interface, it can adsorb at any loci without restrictions and that the time required for reorientation at the interface is very short. In this case, the interfacial tension at the air/water interface can be calculated using the penetration depth theory instead of solving the diffusion equations. Joos and Serrien14 thus obtain:
[ (πτ4t ) ]
σ ) σe + (σ0 - σe) exp -
1/2
(5)
where σ, σe, and σ0 are the dynamic, equilibrium, and pure solvent surface tension, respectively. The diffusion relaxation time τ is defined as τ ) (1/D)(dΓ/dc)2, where D is the diffusion coefficient, Γ is the surface concentration, and c is the bulk concentration. This equation is valid if (σ - σe) is small (linearization allowed). The typical dynamic surface tension curves of aqueous C10E10 solutions are shown in Figure 6. These results are in good agreement with the model of Joos and Serrien (eq 5). As expected, the value of τ decreases when the bulk concentration increases (see Table 3). The adsorption of C10E10 is then limited by surfactant diffusion toward the interface. There is no observable adsorption barrier. The dynamic surface tension curves of modified nonionic surfactant cannot be described by this model. The time needed to reach equilibrium increases with the bulk concentration. This fact indicates the presence of an additional slower process. We suggest that the molecules are still supplied to the surface by simple diffusion, but (14) Serrien, G.; Joos, P. J. Colloid Interface Sci. 1990, 139, 149.
Figure 6. Dynamic surface tension of C10E10 solutions as a function of bulk concentration C (C ) 0.005, 0.01, 0.02, 0.1, 0.2 g/L; T ) 24 °C). The experimental curves have been fitted by using eq 5. Table 3. Diffusion Relaxation Time τ and Reaction Rate k as a Function of Concentration and Temperature for the Three Ethoxylated Surfactants surfactant
bulk concn (g/L)
τ (s)
k-1 (s)
0.005 0.01 0.02 0.005 0.01 0.02 0.04 0.08
11 ( 2 7(1 5 ( 0.5 253 ( 20 129 ( 10 12 ( 2 1.27 ( 0.12 0.33 ( 0.04
0 0 0 0 154 ( 12 279 ( 54 393 ( 21 419 ( 60
C10E10 C10E10Cl
surfactant
temp (°C)
τ (s)
k-1 (s)
C10E5P7
5 17 27 35
1.48 ( 0.17 0.58 ( 0.05 0.24 ( 0.069 0.26 ( 0.03
199 ( 20 260 ( 21 310 ( 42 419 ( 60
once at the surface they undergo a rearrangement. We have seen in the static measurements that the modified molecules take more horizontal conformations in the monolayer than C10E10. These folded conformations, allowing the hydrophobic end to be in contact with air, are probably very different from the bulk conformations. One can imagine that first the molecule arrives vertical at the interface and then reorients in a time scale which increases with monolayer density. In order to analyze our data, we have used the two-stage model of Joos et al.15 This model, developed for proteins, describes the diffusion to a clean surface followed by subsequent reorientations at the interface. Using the diffusion penetration theory, Joos et al. showed that the surface tension varies as
{
[ (πτ4t ) ] + β} exp(-kt)
σ ) σe + R exp -
1/2
(6)
where R and β are parameters such that R + β ) σ0 - σe. The first term exp[-(4t/πτ)1/2e] accounts for the diffusion process and the second one exp(-kt) for the reorientation process. The diffusion relaxation time τ is defined as:
τ)
( )
1 dΓ1 D dc
2
Γ2
where Γ1 is the adsorption of the molecule in the state 1, Γ2 is the adsorption of the molecule in the state 2, and k (15) Serrien, G.; Geeraerts, G.; Ghosh, L.; Joos, P. Colloids Surf. 1992, 68, 219.
2958 Langmuir, Vol. 13, No. 11, 1997
Colin et al. Table 4. Foamability and Foam Lifetime for the Three Ethoxylated Surfactants
Figure 7. Dynamic surface tension of C10E10Cl solutions as a function of bulk concentration C (C ) 0.005, 0.01, 0.02, 0.04, 0.08 g/L; T ) 24 °C). The experimental curves have been fitted by using eq 6.
Figure 8. Dynamic surface tension of C10E5P7 solutions as a function of temperature T (T ) 5, 17, 27 and 35 °C; C ) 0.03 g/L). The experimental curves have been fitted by using eq 6.
is the rate constant for the transformation between these two states. It should be recalled that this equation it valid if (σ - σe) is small (linearization allowed). All the curves for modified nonionic surfactants (Figures 7 and 8) have been fitted by eq 6, with the parameters given in Table 3. We see that the Joos model describes well our data. When the bulk concentration increases the diffusion relaxation time τ decreases, as expected. The reaction rate k diminishes as well; this can be related to the fact that the energy barrier for the reorientation is higher when the surface coverage increases: the molecule has to push away more neighbors to reach its equilibrium configuration at the interface. (For proteins, it is supposed that this energy barrier is proportional to σ0 - σe). Moreover, in the case of C10E5P7, the diffusion time τ decreases as the temperature increases. We have noticed previously that this surfactant becomes more and more dehydrated when heating. Its surface affinity and hence (dc/dΓ1) increases, which leads to a smaller diffusion time. On the contrary, the relaxation time related to the reorientation process increases when heating, because the energy barrier for the reorientation process (related to the surface pressure) increases. These studies show that the chemical modification of the molecules leads to different behaviors during adsorption at the interface. The adsorption of C10E10 is diffusion controlled whereas the adsorption of modified molecules involves two steps. First, the molecules are supplied to the interface by diffusion and adsorb in a first state. Second, they rearrange at the interface to reach their
surfactant
foamability (mL)
lifetime (s)
C10E10 C10E10Cl C10E5P7
90 0 40
2500 100
equilibrium positions. The characteristic time of this later step increases as the surface pressure increases. Foaming Properties. The results of the Ross-Miles test for dilute solutions (concentration near the cmc) are reported in Table 4. C10E10OH solutions produce large amounts of stable foam, C10E10Cl does not foam at all, and C10E5P7OH produces small amounts of a quite unstable foam. In the following, we will recall what are the first steps in the foam life. First, gas is incorporated into the water and forms bubbles. Under buoyancy forces, the bubbles move toward the interface. During the transport, they are submitted to the Archimedes force and to the Stokes friction. This leads to a characteristic time needed to reach the interface being equal to 9lη/2r2Fg, where η is the water viscosity, F the water density, r the radius of the bubble, g the gravity constant, and l a characteristic distance (for example the distance between the bottom of the vessel and the water/air interface). In the Ross-Miles test, this time is of the order of 10-2 to 10-1 s. Since it takes time to the surfactant to move toward the bubbles and to adsorb at their surface, the bubble may arrive at the interface with a surface coverage very far from equilibrium. At this stage, the liquid films that separate two bubbles or a bubble and air at the top of the foam column have to be stable in order to avoid foam rupture. As these films are thick (thicker than 2000 Å), there are no interactions between their surfaces. Foaming is therefore not related to surface forces (disjoining pressure). Foam stability under these dynamic conditions is related to rheological properties of the monolayers. Recently, Joye, Hirasaki, and Miller16,17 have shown that surface properties, and in particulary surface shear viscosity, are the key factors controlling the stability of the thick films. If the surface viscosity is too small, a dimple instability appears and the film breaks. It is then important to know what are the surface rheological properties of the bubble when it arrives in contact with another bubble or with air at the top of the foam. Our dynamic studies do not concern exactly the time scale involved during the foaming process (10 s instead of 10-2 s) and convection is absent. However, we can extract some information about the surface coverage of the bubbles. In the case of C10E10OH, it seems that the surface of the bubble is nearly saturated in 10-1 s. We do not have points in this time scale for the dynamic surface tension curve at the cmc (see concentration 0.2 g/L in Figure 6), but an extrapolation leads to a surface tension of 40 mN/m at 10-1 s. This corresponds to a surface coverage of 86% of the equilibrium coverage (from Gibbs equation). On the contrary, the surface of the bubbles in the solutions of modified surfactants is still far from equilibrium. The same extrapolation for a solution of C10E10Cl at the cmc leads to a surface tension of 59 mN/m at 10-1 s and a surface coverage of 41%. The molecules have enough time to adsorb at the interface but not to undergo the rearrangements. Therefore, the bubbles are less protected against thinning because the surface coverage (16) Joye, J. L.; Hirasaki, G. J.; Miller, C. A. Langmuir 1994, 10, 3174. (17) Joye, J. L.; Hirasaki, G. J.; Miller, C. A. J. Colloid Interface Sci. 1996, 177, 542.
Foaming Properties of Nonionic Surfactants
is not yet complete and the surface shear viscosity is still small (this viscosity increases with surface coverage). Moreover, even if the surface were saturated, the surface shear viscosity of the modified surfactants monolayers would be smaller than for C10E10. A crude estimation of this viscosity can be made by comparison with the threedimensional case. Knowing the composition of interface (surface area), we can assume that ηs ) ηbl, where l is the thickness of the monolayer and ηb the viscosity of the bulk phase which has the same composition as the monolayer. The surface viscosity for the modified surfactant is then smaller than for C10E10OH because the monolayer is thinner and less dense. This may lead to a faster thinning and breaking. In order to check these points, thinning experiments and surface shear measurements are under way. The first measurements of thinning rate confirm the above ideas.18 Conclusion In this paper we have studied the foaming properties of modified nonionic surfactants. We have shown that small chemical modifications of the molecule have huge effects on the foaming properties of the solution. In the concentrated solutions, the reduction of the (18) Espert, A.; Colin, A. In preparation.
Langmuir, Vol. 13, No. 11, 1997 2959
foamability of nonionic surfactants is related to the existence of the cloud point. The surfactant solution separates into two phases: a dilute micellar phase and a concentrated one, the latter acting as an auto-antifoam. For the dilute solutions, a long conformational rearrangement at the surface is responsible for the decrease of the foaming ability. Similar chemical modifications may have similar effects with others surfactants. Adding a hydrophobic part at the end of the polar head leads to a change in the adsorption configuration of the molecules. The dynamic surface tension is also altered. The adsorption process involves two separate steps for the modified surfactant. In addition to diffusion, there is an energy barrier for the reorientation of the molecules at the interface. This leads to large differences in surface coverage of the air bubbles when they arrive to the liquid surface in the Ross-Miles test. These bubbles have a lifetime of only 10-1 s, and in the case of a modified surfactant they arrive at the surface with a surface coverage quite far from equilibrium. This low coverage facilitates the thinning and the breaking of the film. Thinning experiments and measurements of surface shear viscosity are under way in order to investigate further these aspects. LA960667S
