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Structure of Sodium Dodecyl Sulfate and Polyacrylic Acid Adsorption Layer Using Nitroxide Spin Labeled Alumina Xiang Yu† and P. Somasundaran* Langmuir Center for Colloids and Interfaces, Columbia University, New York, New York 10027 Received October 7, 1998. In Final Form: November 1, 1999 Two nitroxide spin probes (7- and 12-doxyl stearic acid) were covalently bonded onto alumina surfaces, and the structures of sodium dodecyl sulfate and poly(acrylic acid) layers adsorbed on them were investigated by monitoring changes in the electron spin resonance spectral characteristics of the probes in comparison to those of electrostatically adsorbed doxyl stearic acid probes. Upon the adsorption of the surfactant and the polymer, the rotational correlation time of the bound probes was found to increase, but to different extent in different adsorption regions. In contrast, while sodium dodecyl sulfate adsorption caused restriction of motion on the adsorbed probes similar to that on bound ones, the poly(acrylic acid) adsorption produced different effects on spin-spin exchange of the bound and adsorbed probes. The spin-spin exchange peak is proposed to be caused by surface migration and aggregation of the probes resulting from the reduction in available surface sites because of competition by the strongly adsorbing polymer species.
Surfactants and polymers have been used to modify particle surface properties for many applications.1 Information on properties of adsorbed surfactant and polymer layers has been obtained in the past by determining adsorption isotherm, zeta potential, and particle hydrophobicity.2 More recently, fluorescence, nuclear magnetic resonance, and electron spin resonance probing techniques have been used to obtain useful information on such properties as viscosity and polarity of the adsorbed layers.3-5 The validity of information from electron spin resonance (ESR) spectroscopic analysis using probes is dependent to a great extent upon the absence of interference by the spin probe itself. A proper probe to investigate adsorption behavior of a substance is one that locates in desired locations but with minimum change in the adsorption characteristics of the adsorbent and adsorbate. One way to locate a probe at the interface is by chemically binding it to the solid. Auteri et al. reported6 a method for covalently attaching a number of spin probes including doxyl stearic acids on different oxides by esterification between the surface sites and spin labels. For this work, chemically bound and adsorbed 7- and 12-doxyl stearic acids were used for studying the structure * To whom correspondence may be addressed. † Current address: Corporate Research Center, International Paper, 1422 Long Meadow Road, Tuxedo, NY 10987. (1) (a) Williams, R. A. In Colloid and Surface Engineering: Applications in the Process Industries; Butterworth-Heinemann, Ltd.: Oxford, 1992. (b) Somasundaran, P., Yu, X. Adv. Colloid Interface Sci. 1994, 53, 31-48. (2) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Phys. Chem. 1964, 68, 3562. (3) (a) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31-36. (b) Chander, P.; Somasundaran, P.; Waterman, K. C.; Turro, N. J. J. Phys. Chem. 1987, 91, 150-154. (4) (a) Tajima, Kazuo; et al. Langmuir 1996, 12, 6651-6658. (b) Ebner, S.; Keul, H.; Ho¨cker, H. Macromolecules 1996, 29, 553-559. (c) P. F. Devaux In Biological Magnetic Resonance; Berliner, L. J., Reuben, J. H., Eds.; Plenum Press: New York, 1983. (d) Berliner, L. J. In Spin Labeling I: Theory and Application; Academic Press: New York, 1979. (5) (a) Alaimo, M. H.; Kumosinski, T. F. Lanmuir 1997, 13, 20072018. (b) Wang, Li-Qiong; et al. Langmuir 1996, 12, 2663-2669. (c) Mears, S. J. Langmuir 1998, 14, 997-1001. (6) Auteri, F. P.; Belford, R. L.; Robinson, B. H.; Clarkson, R. B. Colloids Surf. 1993, 81, 25-42.
of sodium dodecyl sulfate and poly(acrylic acid) adsorbed layers on alumina. The Auteri method was adopted for the spin labeling of alumina particles. The study was done under natural pH conditions (∼6) so that alumina is positively charged and can adsorb the anionic surfactant (sodium dodecyl sulfate) and the polymer (poly(acrylic acid)). The probe mobility is expressed in terms of rotational correlation time calculated from the spectrum using the following equation7
τb ) (6.25 × 10-10)∆H[(h-1/h0)1/2 - (h+1/h0)1/2] where ∆H is the peak-to-peak line width of the center field line and η-1, η0, and η+1 are peak-to-peak heights of the low-, center-, and high-field lines, respectively. First, the ESR spectra of chemically bound probes were compared with those of physically adsorbed spectra in the absence of any additives. The spectra of the bound and the adsorbed probes at the water-alumina interface are shown in Figure 1 along with the spectra of the probe in solution.8 It is clear that the manner of probe introduction has a significant effect on the spectral shape. For each given probe, the general trend can be depicted as τbbound > τbadsorbed > τbsolution. This was expected since the mobility of spin probes was most restricted for the chemical binding on the solid surface followed by the physical adsorption. The spin probes in solution can freely rotate with no restriction. The effect of mobility restriction of spin probes was further observed by comparing those obtained with the two different probes. In solution, the position of the doxyl group had little effect on the spectrum shape, while at the interface the closer position of the doxyl group to the surface caused the mobility of space of 7-doxyl stearic acid to be more significantly reduced. Even though Auteri et al. reported the ester bond to slowly break in an aqueous environment, we found the mixing of the labeled alumina at pH 6 with water for 3 (7) Kivelson, D. J. Chem. Phys. 1960, 33, 1094. (8) In all experiments conducted, 1 × 10-5 M spin probes was used. For suspensions, the probe concentrations were the initial addition, and the supernatant was free of probes. The solid-liquid ratio was 2.5%.
10.1021/la981400r CCC: $19.00 © 2000 American Chemical Society Published on Web 02/08/2000
Structure of Adsorbed Layers on Alumina
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Figure 1. ESR spectra of 12- (I) and 7-doxyl stearic acids (II) (a) in solution, (b) adsorbed on alumina, and (c) covalently bound on alumina.
Figure 3. Effect of poly(acrylic acid) addition on the rotational correlation time of 12-doxyl stearic acid bound to alumina.
Figure 2. Effect of sodium dodecyl sulfate adsorption on the rotational correlation time of 12-doxyl stearic acid adsorbed on alumina in water (O, bound; 0, adsorbed).
days on a wrist-action shaker to cause neither any change in the spectral characteristics nor any detectable probe in the supernatant. Evidently, cleavage of the ester bond is rather slow and there was no significant detachment of the probe during the current tests. The effect of surfactant adsorption on the rotational correlation times of bound and adsorbed 12-doxyl stearic acid is illustrated in Figure 2. The adsorption of surfactant caused an increase in rotation correlation time of both the adsorbed and bound probes. It is to be noted that changes in the rotational correlation time at the interface corresponded to changes in slopes of the adsorption isotherm.2,3 A similar effect was also found with 7-doxyl stearic acid as the probe. The increase in the rotational correlation time of the spin probe starting at ∼8 × 10-4 mol of SDS is caused by the adsorption of SDS molecules and the formation of hemimicelles hindering the mobility of the probes. The trend continued until the critical micelle concentration (∼8 × 10-3 mol for SDS), above which with micelle formation there was no further SDS adsorption. While this observation is in agreement with those of Esumi et al.9 for the LiDS-alumina system, it is quite different from those obtained by Waterman et al.10 Results of the latter showed that in the absence of the surfactant and at low surfactant surface coverage the doxyl stearic acid probes aggregate heavily resulting to a spin-spin exchange spectrum consisting of one broad peak, while the spin exchange peak gradually disappears and eventually an anisotropic spectrum is obtained at higher adsorption (9) Esumi, K.; Otsuka, H.; Meguro, K. J. Colloid Interface Sci. 1990, 136, 1, 224-230. (10) Waterman, K. C.; Turro, N. J.; Chandar, P.; Somasundaran, P. J. Phys. Chem. 1986, 90, 26, 6828-6830.
densities. The differences between the two studies are possibly due to the differences in nature of the alumina powder used. The alumina powder used by Waterman et al. is porous, and hence the actual surface accessible for surfactant adsorption might be smaller than that provided by BET test. The limited surface available results in surface aggregation occurring at lower initial concentrations.11 The spin probes were also used to study the fluidity of polymer layers near the surface. With the chemically bound probes, as in the case of sodium dodecyl sulfate, poly(acrylic acid) adsorption caused an increase in their rotational correlation time (Figure 3). However, effect of the polymer on the probe mobility is much less than that of the adsorbed surfactant layer. This is attributed to the adsorption of the polymer as a network at the interface with a less compact structure than that of the surfactant in the form of hemimicelle aggregates. The extent of reduction in the probe mobility depends on the position of the doxyl group and the molecular weight of the adsorbed polymer. By examining the effect of polymer adsorption on the rotational correlation time of stearic acids with the doxyl group in different positions, it should be possible to estimate the polymer segment distribution as a function of distance from the solid surface.12 In contrast to the above, the effect of polymer adsorption on the characteristics of spectra of the adsorbed probes was much more complex. As can be seen from Figure 4, in the case of the adsorbed 12-doxyl-alumina system, the poly(acrylic acid) adsorption causes a gradual appearance of a broad peak, characteristic of spin-spin exchange. A component of spin exchange peak appeared when the polymer concentration reached 100 ppm corresponding to a surface coverage of ∼0.3. Individual peaks totally diminished when the alumina surface became fully covered at a poly(acrylic acid) concentration of 500 ppm. A similar effect of polymer addition was observed also for 7-doxylalumina system. The appearance of spin-spin exchange suggests aggregation of spin probes. The observation was significantly different from any other combination of probes and surfactants/polymers. In the case of subsequent surfactant adsorption, the probe molecules formed mixed (11) Even if the surface areas of the two kinds of alumina powders happen to be the same, possible difference in surface heterogeneity of functional groups distribution may cause the aggregation to happen at different concentrations. (12) For example, the probe position where the spin rotation is most restricted should correspond to where there is highest segment concentration.
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Figure 4. ESR spectra of adsorbed 12-doxyl stearic acid at the alumina-water interface with added poly(acrylic acid): (a) 0 ppm; (b) 100 ppm; (c) 250 ppm; (d) 1000 ppm.
hemimicells with surfactant molecules. In the case of poly(acrylic acid) adsorbed onto alumina with probes chemically bound, the adsorbed polymer layers subsequently restricted the movement of probes, causing a decrease in
Yu and Somasundaran
the rotational correlation time. Here, with the adsorption of poly(acrylic acid) molecules, the available surface was decreased. Because of the limited compatibility of the probe with the polymer, the probe molecules would likely migrate to form aggregates in order to minimize the total free energy. The spin-spin exchange is the result of such probe aggregation. The observation that individual adsorbed probes can migrate on the solid surface and form aggregates is in agreement with the results of Waterman et al.10 Their results can be interpreted as due to breakup of probe aggregates and migration of probe molecules on the solid surface. It is seen that spin probes can be used as a powerful tool to investigate interfacial behavior of surfactants and polymers. When behavior of chemical bound probes, which cannot migrate on the surface, is studied along with that of adsorbed probes, information on stability and migration of adsorbed species and particularly aggregates can be obtained. From the current study, it can be concluded that polymer layers at the interface form a much looser structure than the surfactant hemimicelles. It was also clear that adsorbed surfactant aggregates can break and migrate on the surface under conditions used here. Acknowledgment. Financial support from National Science Foundation (NSF/EEC-9804618 and NSF/CTS9632479) is acknowledged. Stimulating and encouraging discussions with Dr. Maltesh are deeply appreciated. LA981400R
