Langmuir 2000, 16, 5987-5992
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Effect of Surfactant-Polymer Association on the Stabilities of Foams and Thin Films: Sodium Dodecyl Sulfate and Poly(vinyl pyrrolidone) Britta M. Folmer† and Bengt Kronberg* Institute for Surface Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden Received December 17, 1999. In Final Form: April 12, 2000 The foaming behavior of the anionic surfactant sodium dodecyl sulfate (SDS) has been studied in the presence and in the absence of the nonionic polymer poly(vinylpyrrolidone) (PVP). A current model of surfactant-polymer aggregations in bulk solution and at the air/water interface is related to the foam and thin-film stabilities. Tensiometry, foaming tests, and a thin-film balance are used to obtain this relationship. It is found that, at very low surfactant concentrations, where the surfactants are present as unimers in the bulk solution, there is association between surfactants and polymer at the liquid/air surface, giving increased foam and thin-film stabilities as compared to cases for the same surfactant concentrations but without polymer. As the surfactants and polymers associate in the bulk solution, there is desorption of surfactants and polymers from the surface, rendering decreases in foam and thin-film stabilities. At higher surfactant concentrations, the bulk viscosity is significantly increased owing to the presence of both micelles and saturated micelle-polymer complexes. Also, the surfactant surface coverage at the liquid/air surface has reached its maximum value and is similar to that of SDS solution above the cmc when no polymer is present. Both the increased surface viscosity and the increased bulk viscosity contribute to the observed foam and film stabilities. In the thin-film studies, several stratification steps are observed, probably owing to micelles that are being pushed out of the film.
Introduction The stability of foams is an important factor in many industrial applications. Much effort has been devoted to finding factors or components that influence the stability of foams. Several factors are known to affect the stability of a foam film, e.g., surface tension gradients, surface elasticity, surface viscosity, critical packing parameters, molecular interactions, and van der Waals and doublelayer forces. The addition of polymers to surfactant solutions can also significantly enhance the foam stability owing to complex formation of the components.1-3 If the polymer is surface active, the surface viscosity due to complex formation between the surfactants and polymer at the liquid/air surface increases the film stability.4 The addition of polymer typically also increases the bulk viscosity, which may increase the drainage time of the film. The gellike structures formed can be very important for the enhanced long-term stability. Consequently, the rate of interbubble gas diffusion decreases, which leads to a slower rate of disappearance for smaller bubbles.1 For systems of an anionic surfactant and a nonionic polymer, several mechanisms for the association between micelles and polymer have been proposed. These include a reduction of the surfactant hydrocarbon/water contact area in the micelles, an ion-dipole interaction between the surfactant headgroup and the polymer, and a hydrophobic interaction between the surfactant tail and the polymer. For the present study, a well-understood system was chosen, viz., sodium dodecyl sulfate (SDS) and a high molecular weight poly(vinylpyrrolidone) (PVP). The bulk * Corresponding author. E-mail:
[email protected]. † E-mail:
[email protected]. (1) Sarma, S. R.; Pandit, J.; Khilar, K. C. J. Colloid Interface Sci. 1988, 124, 339. (2) Bergeron, V.; Langevin, D.; Asnacios, A. Langmuir 1996, 12, 1550. (3) Regismond, S. T. A.; Winnink, F. M.; Goddard, E. D. Colloids Surf., A 1998, 141, 165. (4) Lionti-Addad, S.; di Meglio, J. M. Langmuir 1992, 8, 324.
properties and surface properties of SDS in the presence of PVP have been studied extensively.5-8 In this paper, we attempt to establish a relationship between (i) the understood interactions of surfactant and polymer in bulk solution and at the interface and (ii) the foaming and thin-film behaviors. This is achieved using an established model for the associations between surfactant micelles and polymer in the bulk solution and at the liquid/air surface. This study concerns the SDS-PVP system, but we expect that our observations can be generalized to other complexes formed by anionic surfactants and nonionic polymers. The reason for choosing the SDS-PVP system is that this system is known to form strong associations, both in bulk solution and at the air/water interface. Also, SDS is a well-known foaming agent. Experimental Section Materials. SDS was purchased from BDH (99% purity), and the PVP was purchased from Sigma. The average molecular weight of the PVP was 360 000. All solutions were prepared with water that was purified by a Milli-RO 4 pretreatment unit and subsequently led through a Milli-Q PLUS unit before being led into a Q-PAK unit consisting of an active carbon unit, a mixed-bed ion exchanger, an Oranex cartridge, and a final 0.22 µm Millipak 40 filter. The foaming experiments were performed directly after the solutions were prepared to minimize the hydrolysis of SDS to dodecyl alcohol. Also, the surface tension versus concentration curve for SDS does not show a minimum (see Figure 2), from which it (5) Lange, V. H. Kolloid Z. Z. Polym. 1971, 243, 101. (6) Li, F.; Li, G. Z.; Wang, H. Q.; Wang, M. Colloid Polym. Sci. 1998, 276, 1. (7) Chari, K.; Hossain, T. Z. J. Phys. Chem. 1991, 95, 3302. (8) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637.
10.1021/la991655k CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000
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Langmuir, Vol. 16, No. 14, 2000
Folmer and Kronberg
Figure 1. Schematic diagram of the thin-film balance.
is concluded that, within the experimental time frame, there is no significant amount of dodecyl alcohol being formed. Surface Tension Measurements. A KSV Sigma 70 instrument using a du Nou¨y ring was used to determine the liquid/air surface tension as a function of surfactant concentration. The Zuidema-Waters method was employed for the ring correction. Surfactant was added using a Metrohm dosimat titration unit. The surface tension measurements of single-surfactant solutions were carried out by starting with pure water to which the surfactant solution was added. Measurements in the presence of polymer were made with a solution of 2 wt % polymer in water to which a surfactant solution containing the same concentration of polymer was added. Foaming. A foam column was used to determine foam stability. Inert nitrogen gas was allowed to flow through 1 mL of solution for 1 min. The decrease in foam volume was recorded 5 min after the gas flow was switched off. The gas flow was determined to be 72 mL/min. An excess bulk liquid was always present during the experiment to ensure equilibrium between the bulk liquid and the foam. Drainage and Film Thickness Measurements. Microscopic films were studied by the microinterferometric method of Scheludko.9 A schematic diagram of the instrument is presented in Figure 1. A horizontal microscopic foam film (diameter 10-2-10-1 cm) is formed between the tips of the menisci of a biconcave drop, which is held in a vertical cylindrical glass tube of 2.2 mm radius by sucking liquid out of the drop. The amount of liquid in the biconcave drop and the film radius are controlled by (9) Scheludko, A. D. Adv. Colloid Interface Sci. 1967, 1, 391.
a Teflon microsyringe which contains the reservoir of solution and leads to contact with the drop through a capillary as indicated. A film liquid of a desired radius is formed in the holder by manipulating the microscrew. The tube and the connected capillary are enclosed in a glass chamber. The measuring cell is attached to the stage of a metallographic microscope (Zeiss IM 35). Light from a quartz-halogen source (100 W) passing through a monochromatic interference filter (λ ) 546 nm) is incident on the film surface. The light signal reflected from a small portion of the film (radius ca. 0.02 mm) is directed onto a photodiode, converted into an electric signal, amplified, and finally recorded on a strip chart recorder as photocurrent versus time. The film is observed in reflected light through a calibrated eyepiece, and the radius of the film is determined visually with an accuracy of ca. 5 nm. Precautions are taken to eliminate the effects of external disturbances (such as vibrations) on film thinning and rupture. The film thickness, h, is determined from
h)
λ arcsin 2πnw
where
x
1+
∆ 4R(1 - ∆)
∆)
I - Imin Imax - Imin
R)
(
and
)
nw - 1 nw + 1
2
(1 - R)2
(1)
Surfactant-Polymer Association
Langmuir, Vol. 16, No. 14, 2000 5989
Figure 2. Surface tension versus concentration of SDS in the presence and absence of PVP.
λ is the wavelength of the light (546 nm), nw is the refractive index of the surfactant solution, which was determined to be 1.33 both with and without polymer present, I is the light intensity, and Imin and Imax are the light intensities of the last interference minimum and maximum, respectively. The determining factor for the stability of foams is the stability of the films separating the gas bubbles in polyhedral foams. Capillary suction transports liquid from the flat part of the film to the Plateau border, causing thinning of the film and eventually leading to rupture. The Reynolds equation, expressing the rate of thinning for a circular foam film, is
VRe ≡ -
∂h 2h3∆P ) ∂t 3µr2
(2)
where h is the film thickness, t is the time, µ is the dynamic viscosity of the liquid, r is the film radius, and ∆P is the driving force of the drainage. At the initial stage of the drainage process, the driving force is simply given by the capillary suction in the plateau border, i.e., ∆P ) Pc ) 2γ/r, where r is the radius of the curvature in the plateau border and γ is the surface tension. For thinner films (
