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Surface Activity of C17-oligo(propylene oxide-b-ethylene oxide)s in the Absence and Presence of Sodium Oleate Davide Beneventi,† Bruno Carre´,† and Alessandro Gandini*,‡ Centre Technique du Papier, Ressources Fibreuses, Domaine Universitaire, B.P. 251, 38044 Grenoble Cedex 9, France, and Ecole Franc¸ aise de Papeterie et des Industries Graphiques (INPG), B.P. 65, 38402 St-Martin d’He` res, France Received July 9, 2001. In Final Form: October 29, 2001 To improve our understanding of the complex interface features related to deinking in paper recycling, model surfactant systems were investigated. Novel nonionic structures incorporating a C17 hydrophobic chain and block PO-EO sequences of equal composition but different total length were tested in terms of both their surface activity and their competitive adsorption at the air/water interface with respect to the oleate anion. Static and dynamic measurements provided useful information about the diffusion to and molecular orientation at the interface of these species. In particular, the oligoether portion of the nonionic surfactants played a significant role in these phenomena, thanks to the changes of its chain conformation as a function of the crowding and composition of the interface.
Introduction This paper is the third part of a comprehensive investigation aimed at a deeper insight into the complex physicochemical phenomena associated with ink flotation in paper recycling.1,2 This major industrial operation calls upon the separation of the cellulosic fibers from ink particles suspended in an alkaline aqueous medium by the convection of air bubbles, to which the ink must adhere. The presence of various surfactants is essential, not only for the optimization of the actual separation but also to generate the froth at the top of the reservoir, which collects the ink particles. Since the flotation process is based on the adhesion of the hydrophobic ink particles to the airbubble/water interface and their segregation into a stable froth, the understanding of the behavior of surfactants at that particular interface has a fundamental role in the comprehension of the flotation mechanisms. Given the often empirical nature of the publications dealing with ink flotation, it was deemed necessary to tackle these complex problems through a progressive understanding of simpler related systems, such as those previously reported, which dealt with the surface activity and solubility of unsaturated fatty acid salts1 and the adsorption dynamics and foaming properties of various surfactants.2 The heterocoagulation of precipitated calcium soap (resulting from water hardness and the addition of fatty acid sodium salts) with inks,3 fillers, and fibers4 and the ensuing increase in the suspended-solid floatability5-7 are considered the main causes of the intrinsically low ink/ fiber flotation selectivity provided by fatty acid collectors. * Corresponding author:
[email protected]. † Centre Technique du Papier. ‡ Ecole Franc ¸ aise de Papeterie et des Industries Graphiques. (1) Beneventi, D.; Carre, B.; Gandini, A. J. Colloid Interface Sci. 2001, 237, 142. (2) Beneventi, D.; Carre, B.; Gandini, A. Colloids Surf., A 2001, 189, 65. (3) Rutland, M.; Pugh, R. J. Colloids Surf., A 1997, 125, 33. (4) Fernandez, C.; Garnier, G. J. Pulp Paper Sci. 1997, 23 (4), J144. (5) Johansson, B.; Wickman, M.; Stro¨m, G. J. Pulp Paper Sci. 1996, 22 (10), J381. (6) Julien Saint Amand, F. Int. J. Miner. Process. 1999, 56, 277. (7) Drabek, O.; Sterne, J.; Van De Ven, T. G. M. J. Pulp Paper Sci. 1998, 24 (4), J116.
It has recently been claimed8 that deinking flotation selectivity is improved when nonionic surfactants, mostly based on poly(propylene oxide)-poly(ethylene oxide) (PPO-PEO) block copolymers, are used in conjunction with conventional fatty acid collectors. The main reason for this improvement was attributed to the affinity between the ink and the surfactant hydrophobic moiety. Recent investigations9,10 gave particular emphasis to the negative influence of excessive foam generation on the efficiency of the flotation process, which is in apparent contradiction with the fact that fatty-alcohol-ethoxylated surfactants, well-known for their frothing ability, perform adequately as deinking agents. The understanding of the behavior of nonionic surfactants at the air/water interface should be thus considered as one of the main requirements for the comprehension of the flotation process in terms both of particle adhesion to air bubbles and of frothing behavior.2 When nonionic surfactant-fatty acid formulations are added to a pulp suspension, the fatty acids are dissolved through their ionization in the alkaline medium and subsequently precipitate (at least in part) as calcium soaps. The relative solubility of unsaturated calcium soaps1 suggests that, even in the presence of a large excess of calcium ions, a significant amount of fatty acid is still present in solution, in its anionic form, thus modifying, together with the nonionic surfactants, the properties of the air bubble/water interface. The purpose of the present study was to investigate, on one hand, the effect of the hydrophilic chain length on the surface activity of a series nonionic surfactancts and, on the other hand, the competitive role between these molecules and a conventional soap in occupying the air/ water interface. Materials and Methods Three high-purity nonionic surfactants based on the general structure C17-oligo(propylene oxide-b-ethylene oxide), with a constant PPO/PEO molar ratio and different lengths of total PPO + PEO sequence, were kindly supplied by EKA chemicals. They were characterized by narrow molecular weight distribu(8) Nagarajan, R.; Moon, T. 4th Research Forum on Recycling, Chaˆteau Frontenac, Quebec, Canada, 7-9 Oct. 1997, 33. (9) Ajersch, M; Pelton, R. J. Pulp Paper Sci. 1996, 22 (9), J338. (10) Deng, Y. Tappi J. 2000, 83 (6), 61.
10.1021/la0110343 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/10/2002
Surface Activity of Nonionic Surfactants
Figure 1.
1H
Langmuir, Vol. 18, No. 3, 2002 619
NMR spectrum of the C17(PO-EO)22 nonionic surfactant (solvent D2O, frequency 250 MHz).
Figure 2. FTIR spectrum of the C17(PO-EO)22 nonionic surfactant (liquid phase, NaCl disks). Table 1. Molecular Properties of the Three Nonionic Surfactants Used in This Work, As Determined by 1H NMR Spectrometry and Model Calculation surfactant alkyl chain PO + EO units PO/EO 22POEO 37POEO 94POEO
C17 C17 C17
22 37 94
0.3/0.7 0.3/0.7 0.3/0.7
Mw
AT (Å2)
1318 2042 4793
61 80 153
tions, viz., Ip < 1.3. After dissolution of each surfactant in D2O, the number of EO and PO units and the corresponding ratio were determined for each surfactant by 1H NMR spectroscopy. Figure 1 shows the spectrum obtained for the molecule bearing the shortest PPO + PEO chain. The expected typical peaks related to the alkyl moieties and the oligoether blocks were present in the three spectra. Calculations of each molecular structure were performed taking the aliphatic -(CH2)- peak as reference. Table 1 gives the composition and molecular weight of each surfactant. The only anomaly in these spectra was the excessive intensity of the OH peak at 2.39 ppm, which was shown to be the result of the presence of moisture in the samples (ca. 0.1% w/w, as determined by the Karl Fischer method). It was however confirmed that the three molecules bore one OH function at the end of the PEO block. The FTIR spectra confirmed these structures as shown in the representative example of Figure 2. The conformation of C17-oligo(propylene oxide-b-ethylene oxide) at the air/water interface was assumed to be similar to that
proposed for PPO-PEO block copolymers,11 namely, with the neighboring alkyl-PPO blocks oriented toward the air and bearing a random coil conformation. At low surface pressures, the PEO chains adsorbed at the air/water interface with an extended configuration and, as the surface pressure was increased, they switched to a random coil conformation oriented toward the water phase,12 thus affecting the packing density11,13 of the adsorbed molecules. According to this model, depicted in Figure 3, the theoretical area, AT, of each unperturbed PEO chain was calculated from its radius of gyration under the assumption of free rotation about each bond
na2 s02 )
1 + cos(180 - θ) 1 - cos(180 - θ) 6
(1)
where n is the number of bonds in the chain backbone, a the bond length, and θ the bond angle. (11) Alexandridis, P.; Athanassiou, V.; Fukuda, S.; Hatton, T. A. Langmuir 1994, 10, 2604. (12) Phipps, J. S.; Richardson, R. M.; Cosgrove, T.; Eaglesham, A. Langmuir 1993, 9, 3530. (13) Rennie, A. R.; Crawford, R. J.; Lee, E. M.; Thomas, R. K.; Crowley, T. L.; Robert, S.; Qureshi, M. S.; Rechards, R. W. Macromolecules 1989, 22, 3466.
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Figure 3. Schematic representation of the conformation of C17(PO-EO)n molecules at the air/water interface: (a) at low surface pressures; (b) at high surface pressures. The surfactants were dissolved in deionized water at a concentration of 1.2 × 10-4 mol L-1 and a pH of 9, adjusted with NaOH. Solutions with concentrations ranging from 2 × 10-8 to 5 × 10-5 mol L-1 were obtained by the addition of increasing volumes of the mother concentrated solution to 50 mL of deionized water. Each addition was followed by the readjustment of the pH at 9 and 3 min of equilibration under stirring. The surface tension was then measured at 35 °C using a De Nou¨y ring apparatus. Results were rationalized using the Gibbs equation
Γ)-
1 dγ RT d ln c
(2)
where Γ is the surface excess, γ the surface tension, and c the bulk concentration of the surfactant. The dynamic surface tension (DST) of 2.5 × 10-6 mol L-1 solutions of each surfactant was measured at 35 °C using an image analysis tensiometer, based on the oscillating bubble technique.14 The corresponding diffusion coefficients were obtained from the short-time approximation of the Ward and Tordai equation15
γ(t) ) γ0 - 2RTc0(Dt/π)1/2
(3)
where γ0 is the solvent surface tension, c0 the bulk concentration of the surfactant, and D the diffusion coefficient. The diffusion relaxation frequency, ωrel, was estimated using the approximation
ωrel ) D(dc/dΓ)2 ≈ D(c/Γ)2
(4)
The same technique was adopted to evaluate the surface elastic modulus, E, of the air/water interface. After the surface tension had reached a constant value, the bubble surface (which ranged from 21 to 25 mm2) was perturbed with a sinusoidal area variation at a frequency of 0.125 Hz and an amplitude of 5 mm2. The surface modulus was then sampled for 30 s at a frequency of 5 Hz. The foaming properties of 5 × 10-5 mol L-1 nonionic surfactant solutions were compared using the air bubbling test:16 200 mL of the foaming solution was introduced in a 5 cm diameter graduated cylinder kept at 35 °C and a constant air flow was bubbled into the solution through a porosity 4 sintered glass disk. The height of the foam rising in the cylinder was measured as a function of time. High-purity sodium oleate (Fluka) was used to investigate the competitive adsorption of fatty acid/nonionic surfactant mixtures at the air/liquid interface. A 5 × 10-3 mol L-1 mother solution was prepared, and the equilibrium surface tension was then measured following the same procedures adopted for nonionic surfactants. Thereafter, a 4 × 10-7 mol L-1 solution of each nonionic surfactant was prepared, to which increasing volumes of the oleate mother solution were added. The surface tension of the ensuing mixtures was then measured at 35 °C and (14) Mobius, D.; Miller, R. Drops and Bubbles in Interfacial Research; Elsevier: Amsterdam, 1998; pp 1-138. (15) Ward, A. F.; Tordai, L. J. Chem Phys. 1946, 14, 453.
Figure 4. Adsorption isotherms of C17(PO-EO)n (with n ) 22, 37, 94) nonionic surfactants, pH 9, T ) 35 °C (nPOEO represents C17(PO-EO)n). pH ) 9, after each addition and 3 min relaxation. The volumes added were very small compared with that of the starting solution, and therefore the dilution effect on the nonionic surfactant concentration was neglected. The surface excess of sodium oleate was determined using the Gibbs’ adsorption equation for binary surfactant mixtures
dγ ) -RT(Γ1 d ln a1 + Γ2 d ln a2)
(5)
where Γ1, Γ2 and a1, a2 are the surface excesses and the activity coefficients of oleate and nonionic surfactant, respectively. Since surface tension measurements were performed with low surfactant concentrations, activity coefficients ai could be replaced by the corresponding bulk concentrations ci. Moreover, the nonionic surfactant concentration c2 was kept constant to 4 × 10-7 mol L-1. Under these conditions eq 5 becomes
|
Γ1 ) -
dγ RT d ln c1
|
(6)
c2
in the absence of specific interactions between surfactant molecules (see results below). The surface excess of oleate as a function of its bulk concentration could then be calculated from the adsorption isotherm of the surfactant mixture. The adsorption dynamics at the air/water interface, as a function of the sequence of surfactant adsorption, was also investigated using dynamic surface tension measurements. After the surface tension of a sodium oleate solution had reached equilibrium, a concentrated solution of nonionic surfactant was injected and the dynamic surface tension measured until it had reached a constant value. The adsorption sequence was then inversed, by the injection of solutions of sodium oleate into the nonionic surfactant solution. To avoid surface tension variations due to dilution effects, measurements were performed with concentrations always above the corresponding critical micelle concentration (cmc).
Results and Discussion Adsorption from Nonionic Surfactant Solutions. The increase in the PPO-PEO chain length was clearly reflected by the different adsorption behavior at the air/ water interface. The adsorption isotherms, given in Figure 4, show that at low concentrations (
