J. Phys. Chem. 1996, 100, 3179-3189
3179
A Scanning Angle Reflectometry Investigation of Block Copolymer Adsorption to Insoluble Lipid Monolayers at the Air-Water Interface James R. Charron and Robert D. Tilton* Department of Chemical Engineering and Colloids, Polymers, and Surfaces Program, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213 ReceiVed: July 28, 1995X
We modified the technique of scanning angle reflectometry to measure the surface excess concentration as water-soluble polystyrene-poly(ethylene oxide) (PS-PEO) diblock copolymers adsorb from solution to dipalmitoylphosphatidylcholine (DPPC) monolayers spread at the air-water interface. Polymers adsorb by penetrating the lipid monolayer. Surface pressure data and fluorescence microscopy indicate that PS-PEO adsorption drives the liquid expanded-to-liquid condensed phase transition in the DPPC monolayer by decreasing the available area per lipid. PS-PEO adsorption is therefore functionally equivalent to mechanical compression of the monolayer. Accordingly, the extent of PS-PEO adsorption accommodated by DPPC monolayers in different regimes of the surface pressure-area isotherm correlates with the compressibility of the penetrated monolayers. Polymer adsorption to liquid expanded monolayers increases the interface compressibility to equal that of the phase transition regime. Polymer segments that adsorb to monolayers in the phase transition and the liquid expanded regimes force lipids from the expanded to the condensed state, thereby creating accessible interfacial area for the polymer. Polymer surface concentrations in those monolayer regimes are indistinguishable from those attained at the air-water interface in the absence of a spread monolayer. Liquid condensed monolayers cannot accommodate polymer segments in this way, and as a result, the extent of polymer adsorption is diminished.
Introduction An amphiphilic block copolymer in a selective solvent has much in common with a surfactant. Both have the capacity to self-assemble and adsorb readily to interfaces. Amphiphilic block copolymer adsorption may be driven by the incompatibility of the insoluble block with the selective solvent. It is generally thought that the insoluble block primarily forms trains, while the soluble block extends into solution as a single, highly extended tail.1 The resultant thick layer is referred to as a polymer brush, and it has the important ability to impart steric stabilization to colloidal structures.2 Our understanding of amphiphilic block copolymer adsorption has progressed considerably in recent years, due to significant developments in both experimental and theoretical methods.3 Previous studies have focused primarily on idealized systems wherein the block copolymer is the only surface active molecule present. Nevertheless, many complex fluid formulations contain surface active species of widely varying molecular weight, solubility, and surface affinity. Examples include paints, printing inks, detergent products, hygienic products, pharmaceuticals, and foods. The structure and composition of the adsorbed layers in such systems depend on interspecies interactions. These may be competitive or cooperative, and important interactions occur at both fluid interfaces, e.g, in foams and emulsions, and solid interfaces, e.g., in colloidal dispersions and macroscopic wetting phenomena. Insoluble lipid monolayers at the air-water interface provide excellent experimental models to begin to address the competition between macromolecules and small surface active molecules for access to available interfacial area. Polymer penetration into lipid monolayers raises the important issue of interface perturbation, wherein the act of polymer adsorption dramatically alters * To whom correspondence should be addressed. E-mail: tilton@ andrew.cmu.edu. X Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-3179$12.00/0
the structure of the interface itself, as the lipids must rearrange to accommodate penetrating polymer segments. By decreasing the area available to each lipid, insertion of polymer segments represents a form of monolayer compression, since displacement of the sparingly soluble lipid into the subphase is unfavorable. Here, polymer-“surfactant” interactions are confined to the interface, and we focus on the importance of monolayer perturbation in the polymer adsorption mechanism. Polymersurfactant binding in solution and surfactant displacement from the interface are not at issue in the insoluble monolayer system, although these effects are very important in soluble surfactant systems. Our hypothesis motivating the present investigation is that the resistance of a monolayer to compression may play an important role in polymer adsorption mechanisms in fluid systems containing polymers plus sparingly soluble surface active molecules. Interfacial perturbations of the sort considered here are of course not addressed in single-component adsorption studies. The well-defined surface pressure-area isotherms of many insoluble lipid monolayers allow the experimenter to easily and systematically vary the physical state of the interface prior to polymer adsorption and in this way attempt to decouple the roles of the initial lipid density in the monolayer (or the corresponding surface pressure π) and the monolayer compressibility
K)
h -1 dA A h dπ
(1)
In this definition, A h is the mean molecular area per lipid. Previous investigators4-6 have monitored the surface pressure and surface potential during adsorption of “Pluronic” triblock (PEO-PPO-PEO) copolymers to natural lecithin blends, inferring the polymer surface excess concentration from a modified Gibbs adsorption equation. Miyano et al.7,8 used a reflectance spectroscopy technique to measure adsorption kinetics of © 1996 American Chemical Society
3180 J. Phys. Chem., Vol. 100, No. 8, 1996
Charron and Tilton
polyelectrolyte dyes to charged monolayers, where the adsorption is controlled by electrostatic interactions. In this investigation, we use in-situ scanning angle reflectometry to measure directly the surface excess concentration of nonionic polystyrenepoly(ethylene oxide) (PS-PEO) diblock copolymers as they adsorb from aqueous solution to insoluble monolayers of the zwitterionic lipid dipalmitoylphosphatidylcholine (DPPC) spread at the air-water interface in a Langmuir trough. Unlike the natural lecithin blends, DPPC displays a well-defined π-A h isotherm with distinct regimes having markedly different compressibilities. We adsorb PS-PEO to DPPC monolayers in different regimes of the π-A h isotherm and simultaneously measure the polymer-induced surface pressure change and the block copolymer surface excess concentration during adsorption. Our hypothesis that monolayer compressibility may influence the polymer adsorption mechanism is based on the assumption that monolayer penetration represents a form of compression. Therefore, we also present fluorescence microscopy evidence that polymer adsorption does indeed compress the monolayer. Experimental Section Materials. We purchased the PS-PEO diblock copolymer (Mn ) 375 000, 8 wt % polystyrene, Mw/Mn ) 1.20, NPS/NPEO ) 288/7840) from Polymer Laboratories. We determined that the apparent critical micelle concentration of this copolymer is 23 µg/mL, using the pyrene fluorescence method of Wilhelm et al.9 The lipid DPPC and the fluorescent lipid probe 1-palmitoyl-2-[12-[(7-nitro-2-(1,3-benzoxadiazol)-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (NBD-PPC) were purchased from Avanti Polar Lipids, Inc., and used without further purification. We purchased Certified ACS grade NaCl from Fisher Scientific. Since the surface tensions of NaCl solutions prepared from roasted and nonroasted salt were identical, the NaCl was deemed sufficiently free from surface active contaminants and used as received. The subphase contained 0.01 M NaCl in all experiments. We distilled Fisher HPLC grade chloroform prior to use as the monolayer spreading solvent. All water was purified with the Milli-Q Plus system from Millipore Corp. Scanning Angle Reflectometry. Reflectometry is based on the dependence of the reflectivity of a composite interface on the thickness, d, and refractive index, n, of each constituent interfacial layer and on the angle of incidence of polarized light. At the Brewster angle, the reflectivity of parallel (p) polarized light is zero for an ideal, clean interface. The Brewster angle,
θB ) tan-1(nt/ni)
(2)
depends on the refractive indexes of the media containing the transmitted (nt) and incident (ni) light. When molecules adsorb to the interface, the reflectivity changes in response to the changing refractive index distribution in the interfacial region. Since the sensitivity to adsorption is greatest at angles of incidence near θB, we measure the intensity of reflected, linearly polarized light at a 2° range of incident angles centered on θB (53.12° for the air-water interface). Leermakers and Gast10 have published a modification of the scanning angle reflectometry method, originally developed by Schaaf et al.,11 that allows continuous resolution of adsorption kinetics. These investigators and Dijt et al.12 developed reflectometric techniques to study macromolecule adsorption at solid-liquid interfaces. The current instrument is a further modification of the Leermakers and Gast design and is better suited for studies of the air-water interface. Their instrument recorded the reflectivity profile of p-polarized light by focusing
the incident beam in the plane of incidence and spatially resolving the reflected intensities with a linear photodiode array. Since the entire range of angles was sampled simultaneously, the instrument’s time resolution was limited only by the integration time of the photodiode array. Our modifications of this technique are motivated primarily by difficulties associated with the air-water interface. The previous scanning angle reflectometry method required an initial measurement of the incident laser intensity profile, I0p(θ), and the reflectivity of p-polarized light was calculated by normalizing all subsequent intensity measurements during an experiment by the stored I0p(θ) data:
Rp(θ) ≡ Ip(θ)/I0p(θ)
(3)
assuming that I0p(θ) does not change during the course of an experiment. This was not difficult to do in the Leermakers and Gast instrument, where the interface was a solution/prism interface. They measured I0p(θ) by totally internally inflecting the incident focused beam at the prism/air interface before filling the flow cell with solvent. We do not have the opportunity to measure I0p(θ) conveniently via total internal reflection at the air-water interface. Furthermore, this profile may change during the course of any experiment for a variety of reasons, regardless of whether one studies solid or fluid interfaces. Any slight change in the laser intensity profile during an experiment would render the stored I0p(θ) profile invalid. Drifts in the laser pointing direction that are within manufacturer’s specifications, and slight changes in the water level tend to slightly misalign the reflected profile and the previously stored incident profile. Because the Gaussian laser beam has imperfections, this misalignment produces spurious distortions in the calculated reflectivity profile Rp(θ). For these reasons, instead of measuring only the parallel polarized reflectivity profile, we separately measure the reflected intensities of both parallel and perpendicular (s) polarized light and calculate the normalized parallel reflectivity (NPR):
NPR(θ) ≡
Ip(θ) Ip(θ) + Is(θ)
(4)
where Ip and Is are the reflected intensities of parallel and perpendicular polarized light, respectively. NPR can be expressed in terms of the theoretically calculated reflectivities Rp and Rs:
NPR(θ) )
Rp(θ) Rp(θ) + ζ tan2(R) Rs(θ)
(5)
where R is the angle between the plane of incidence and the polarization vector and ζ is an instrumental factor that accounts for losses in the optical components. Note that NPR does not depend on the incident intensity and is therefore insensitive to changes in the laser output or beam profile. NPR profiles may be predicted from eq 5 using simple optical models of the interface, as described in the Data Analysis section. We analyze experimental reflectivity data to determine the average thickness and refractive index of the adsorbed layers by comparison of NPR profiles to the theoretical predictions. Apparatus. Optics. The reflectometer optical train is shown in Figure 1a. We use the 488 nm line of an air-cooled, intensitystabilized 150 mW argon ion laser (Omnichrome). Initially, a small percentage of the beam is redirected via a Fresnel beam splitter to serve as a reference beam. The measuring beam is then p-polarized by a Glan-Laser polarizer. The polarization angle R is set by a half-wave plate oriented at R/2. We then
Block Copolymer Adsorption to Lipid Monolayers
Figure 1. (a) Schematic of the reflectometer optical train. The light source is an air-cooled argon ion laser. The Glan-laser polarizer, halfwave plate, and final polarizer determine the polarization state that reaches the interface. The half-wave plate sets the angle R in eq 5. A planocylindrical lens focuses the light onto the interface. The reflected light intensity profile is measured by the photodiode array. To monitor the level of the water surface, a small percentage of the beam is redirected and reflected at glancing incidence onto a reference position on the photodiode array. (b) Integration of the optical and trough systems. The PTFE trough is fitted with two symmetrically driven Delrin barriers, and the surface pressure is measured using the Wilhelmy plate. The Wilhelmy plate and the point of reflection actually are positioned near the center of the trough; they are shown off-center for clarity. The circulation system is driven by a peristaltic pump, and polymer is injected through the three-way valve.
select either the parallel or the perpendicular component with a polaroid polarizer mounted on a precision rotation stage. In practice, we first measure Ip(θ), then rotate the second polarizer by 90°, and measure Is(θ). Since Rs . Rp for all measured incident angles, R must be kept small (
