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Surface Tension and Adsorption Properties of a Series of Bolaamphiphilic Poly(fluorooxetane)s Charles M. Kausch, Yongsin Kim, Vernon M. Russell, Robert E. Medsker, and Richard R. Thomas* OMNOVA Solutions, Incorporated, 2990 Gilchrist Road, Akron, Ohio 44305-4489 Received February 11, 2003. In Final Form: May 19, 2003 Studies of surface tension, adsorption parameters, and dynamic surface tension were performed on a series of bolaamphiphilic R,ω-(diammonium disulfato)poly(fluorooxetane)s of several perfluoroalkyl chain lengths. Similar measurements were performed on a small-molecule, anionic fluorosurfactant with a -C8F17 perfluoroalkyl group that is known to be an effective flow and leveling aid in aqueous coatings. Molecular area demands for the poly(fluorooxetane)s were found to be relatively small for a polymeric species and may indicate a change in conformation between bulk solution and interface. Other adsorption parameters were found to be similar to those of the small-molecule fluorosurfactant. Diffusion coefficients were found to be slightly smaller for the poly(fluorooxetane)s compared to the small-molecule fluorosurfactant.
Introduction Fluorosurfactants have been used commercially for nearly 50 years. Fluorosurfactants typically provide very effective and efficient surface tension reduction and have found utility as wetting, flow, and leveling aids in a variety of coatings and formulations. Some of the most popular fluorosurfactants in use can be described generically as small molecules having long (≈ -C8F17) perfluoroalkyl (Rf) chains and containing a hydrophilic group of the cationic, anionic, or nonionic type to impart some water compatibility. With these types of fluorosurfactants, it has been shown numerous times that a minimum surface tension in an analogous series of materials is reached when Rf ≈ -C8F17. It is no wonder, therefore, that most fluorosurfactants possess this type of perfluoroalkyl group. In recent years and with the introduction of more sensitive analytical instrumentation, it has been found that fluorochemicals with -C8F17 are becoming widespread and pervasive in the environment.1-6 These materials have been found in a variety of fauna that are rather isolated geographically from industrial centers. This has prompted concern over the persistence and, more importantly, the bioaccumulation potential of these small molecule, long Rf chain fluorosurfactants. In fact, the propensity of this class of materials for bioaccumulation has inspired the largest commercial producer to remove its line of fluorosurfactants based on this type of chemistry from the market. In a number of industries, fluorosurfactants are considered to be necessary items of commerce. Many commercial materials contain fluorosurfactants that provide various important attributes that cannot be demonstrated with hydrocarbon surfactants. Therefore, a need exists to find effective fluorosurfactants with less or no potential impact to the environment. * To whom correspondence should be addressed. (1) Giesy, J. P.; Kannan, K. Environ. Sci. Technol. 2001, 35, 1339. (2) Giesy, J. P.; Kannan, K.; Jones, P. D. Sci. World 2001, 1, 627. (3) Olsen, G. W.; Burris, J. M.; Mandel, J. H.; Zobel, L. R. J. Occup. Environ. Med. 1999, 41, 799. (4) Moody, C. A.; Kwan, W. C.; Martin, J. W.; Muir, D. C. G.; Mabury, S. A. Anal. Chem. 2001, 73, 2200. (5) Moody, C. A.; Martin, J. W.; Kwan, W. C.; Muir, D. C. G.; Mabury, S. A. Environ. Sci. Technol. 2002, 36, 545. (6) Ellis, D. A.; Mabury, S. A.; Martin, J.; Muir, D. C. G. Nature 2001, 412, 321.
It is accepted commonly that three major factors contribute to the persistence and bioaccumulation potential of small-molecule, long Rf chain fluorosurfactants. First, the robustness of fluorinated materials and polymers to chemical, biological, and physical assault is legendary and has been known for many years.7 This is due to the inherent kinetic and thermodynamic properties of perfluoroalkyl groups. Second, the ability to translate across a biological barrier is related directly to size: small molecules will move across barriers faster than will large molecules. Last, the lipophilicity of an Rf group is proportional directly to length. Longer Rf groups, such as -C8F17, impart a great deal of lipophilicity to molecules, causing them to accumulate in a variety of biological tissues and fluids where compatibility is highest. With the aforementioned issues at hand, an attempt was made to prepare functionally active fluorosurfactants that are polymeric in nature and have much shorter Rf chains. In this regard, a series of novel architecture, bolaamphiphilic R,ω-(diammonium disulfato)poly(fluorooxetane)s with Rf groups spanning -CF3 to -(CF2)4F were prepared and characterized by surface tension and adsorption properties. For comparison, similar properties were measured for a typical small-molecule, long Rf anionic fluorosurfactant. Both classes of molecules were found to be effective flow and leveling agents in aqueous coatings. Experimental Section Materials. Water was distilled and deionized prior to use. Buffer solutions were prepared using pHydrion buffers from Aldrich Chemical Co. The preparation of R,ω-(diammonium disulfato)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane, 1, R,ω-(diammonium disulfato)poly(3-methyl-3-[(2,2,3,3,3pentafluoropropoxy)methyl]oxetane, 2, and R,ω-(diammonium disulfato)poly(3-methyl-3-[(2,2,3,3,4,4,5,5,5-nonafluorohexoxy)methyl]oxetane, 3, has been described previously.8 It was found that gel permeation chromatography (GPC) was not a particularly suitable method of analysis for this class of fluorosurfactants. NMR end group analysis provided much more accurate values of degree of polymerization. The target degree of polymerization (7) Thomas, R. R. In Fluoropolymers. 2. Properties; Hougham, G., Cassidy, P., Johns, K., Davidson, T., Eds.; Kluwer Academic/Plenum: New York, 1997; Vol. 2, Chapter 4. (8) Kausch, C. M.; Leising, J. E.; Medsker, R. E.; Russell, V. M.; Thomas, R. R.; Malik, A. A. Langmuir 2002, 18, 5933.
10.1021/la034233q CCC: $25.00 © 2003 American Chemical Society Published on Web 08/05/2003
Properties of Bolaamphiphilic Poly(fluorooxetane)s
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Figure 1. Structures of poly(fluorooxetane)s and a typical small-molecule, long perfluoroalkyl chain surfactant. was 7 and was found to be accurate to (0.5. However, NMR analysis cannot supply polydispersities. Polydispersity values quoted are those obtained from GPC analysis. In an effort to obtain accurate values of polydispersity, samples have been examined by matrix-assisted laser desorption ionization (MALDI) mass spectroscopy. Preliminary results suggest that polydispersities are in the 1-2 range quoted here. This is a subject for further study and will be reported in a separate communication. The fluorinated polyoxetane surfactants 1-3 sequester substantial amounts (20-30 wt %) of water, making the measurements of glass transition temperatures meaningless. The precursor R,ω-poly(fluorooxetane) diols have glass transition temperatures of critical micelle concentration) in pH 8 buffered water, yielding values of 60.4 ( 4, 53.7 ( 4, and 23.7 ( 0.3 mN/m, respectively.8 In light of the relatively high surface tensions obtained for these small molecules with a variety of Rf chain lengths, the surface tension adsorptions show that surfactants 1-3 afford much lower surface tensions based solely on Rf chain length. Note that the surface tension for F(CF2)∼8CH2CH2OSO3- NH4+ is comparable to that obtained for the similar molecule 4. Unlike small-molecule fluorosurfactants, such as 4, the poly(fluorooxetane)s 1-3 are bolaamphiphilic in structure. Adsorption studies on hydrocarbon bolaamphiphiles21-23 and geminis23-31 indi(15) Muller, N.; Simsohn, H. J. Phys. Chem. 1971, 75, 942. (16) Sadtler, V. M.; Giulieri, F.; Krafft, M. P.; Riess, J. G. Chem.s Eur. J. 1998, 4, 1952. (17) Tadros, T. F. J. Colloid Interface Sci. 1980, 74, 196. (18) Taylor, C. K. Paint Coat. Ind. 1999, May, 58. (19) Ulmius, J.; Lindman, B. J. Phys. Chem. 1981, 85, 4131. (20) Johnson, R. E., Jr.; Dettre, R. H. In Wettability; Berg, J. C., Ed.; Marcel Dekker: New York, 1993; Chapter 1. (21) Jong, L. I.; Abbott, N. L. Langmuir 1998, 14, 2235. (22) Muzzalupo, R.; Ranieri, G. A.; La Mesa, C. Langmuir 1996, 12, 3157. (23) Pestman, J. M.; Terpstra, K. R.; Stuart, M. C. A.; van Doren, H. A.; Brisson, A.; Kellogg, R. M.; Engberts, J. B. F. N. Langmuir 1997, 13, 6857.
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Table 1. Adsorption Parameters for Fluorosurfactants sample
pH
Γ∞ (×106 mol/m2)
β/R (×10-6 m3/mol)
molecular area (Å2/molecule)a
σ2 (mN/m)b
n
molecular area (Å2/molecule)c
cmc (×105 mol/L)
1 1 1 1d 2 3 4
2 6 8 8 8 8 8
2.39 ( 0.06 2.47 ( 0.02 3.50 ( 0.1 6.07 ( 0.5 3.40 ( 0.09 2.46 ( 0.09 6.82 ( 0.3
60.9 ( 2 4.24 ( 0.2 1.13 ( 0.05 0.234 ( 0.04 1.95 ( 0.06 19.1 ( 0.9 0.598 ( 0.04
69.6 ( 2 67.2 ( 2 47.4 ( 2 27.3 ( 2 48.8 ( 1 68.7 ( 1 24.3 ( 1
5.6 14 4.0 1.6 14 9.9 11
1 1 1 1 1 1 1
68.6 ( 5 58.6 ( 5 46.0 ( 4 37.9 ( 1 35.1 ( 0.7 64.4 ( 2 31.5 ( 2
1.24 10.7 9.68 6.70 5.34 5.00 8.64
a From the Davies isotherm fit. b σ2 ) [1/(N - 2)]ΣN (γ 2 i)1 exptl - γtheory) where N is the number of data points and γexptl and γtheory are the experimental surface tension values and those calculated using eq 1, respectively. c From the Gibbs adsorption equation. d 200 mM NaCl.
cate unusual surface tension reduction effectiveness and efficiency compared to surfactants without this type of architecture. In that regard, this represents a rare attempt at characterizing the adsorption properties of fluorinated bolaamphiphiles. To characterize further the adsorption properties of 1-4, surface tension data were fitted parametrically to the Davies adsorption isotherm due to the ionic nature of the materials examined. The Davies adsorption isotherm is a variation of the Langmuir isotherm and accounts for the effect of double-layer charging on adsorption properties and is given in eq 1:32,33
γ ) γ0 + RT Γ∞ ln(1 - θ) -
4RT (2RT(C1∞ + C3∞))1/2 zF zFΨs 1 - cosh (1) 2RT
[
( )]
where γ0 is the surface tension of the solvent, R is the gas constant, T is absolute temperature, Γ∞ is surface excess at saturation, z is the surfactant valence, is the dielectric constant of water, C1∞ is the bulk surfactant concentration, C3∞ is the bulk concentration of any added electrolyte coion, F is Faraday’s constant, and Ψs is the surface potential. The fluorinated poly(oxetane)s 1-3 exhibit small polydispersities (∼1-2).8 Assuming that each mer behaves similarly in regard to adsorption, a pseudo-single-surfactant approach34 was adopted for use in adsorption isotherm data analysis. The term θ is the quotient of surface excess at a given concentration, Γ, and surface excess at saturation and is given by
θ)
βC1∞/R Γ ) Γ∞ exp(zFΨs/RT) + βC1∞/R
(2)
where β/R reflects the adsorption strength. To make eqs 1 and 2 soluble, the surface potential, ψs, must be related to the surface charge density, σ, through the GouyChapman relationship 4
σ ) zFΓeq ) (2RT
∑ i)1
( )
Ci∞)1/2 sinh
zFΨs 2RT
(3)
where C2∞ is the bulk surfactant co-ion concentration and (24) Diamant, H.; Andelman, D. Langmuir 1994, 10, 2910. (25) Diamant, H.; Andelman, D. Langmuir 1995, 11, 3605. (26) Eastoe, J.; Rogueda, P.; Howe, A. M.; Pitt, A. R.; Heenan, R. K. Langmuir 1996, 12, 2701. (27) Eastoe, J.; Dominguez, M. S.; Wyatt, P.; Beeby, A.; Heenan, R. K. Langmuir 2002, 18, 7837. (28) Menger, F. M.; Keiper, J. S.; Azov, V. Langmuir 2000, 16, 2062. (29) Song, L. D.; Rosen, M. Langmuir 1996, 12, 1149. (30) Wettig, S. D.; Nowak, P.; Verrall, R. E. Langmuir 2002, 18, 5354. (31) Boschkova, K.; Feiler, A.; Kronberg, B.; Stålgren, J. J. R. Langmuir 2002, 18, 7930.
C4∞ is the bulk added counterion concentration. Numerically, ψs was found by using a root finder with eq 3. Next, eq 1 was solved by substitution using the numerical value for ψs. The adsorption parameters (β/R and Γ∞) were estimated by minimizing the difference between γ obtained from eq 1 and experimental surface tension values while maintaining the relationship between θ and β/R given in eq 2. The adsorption isotherm parameters are shown in Table 1 along with other relevant parameters such as critical micelle concentration (cmc). Surface excess at saturation values, Γ∞, for the poly(fluorooxetane) amphiphiles 1-3 were all smaller than for the small-molecule, long Rf chain surfactant 4, leading to surface areas at saturation in the range of 45-70 Å2/molecule compared to ∼24 Å2/molecule for 4. The surface area at saturation for 4 is not surprising. Cross-sectional perfluoroalkyl chain areas have been estimated to be ∼30 Å2 for many smallmolecule, long Rf chain amphiphiles.35-37 The surface areas at saturation for 1-3 require comment. The areas suggest that 1-2 Rf groups are occupying the surface at saturation. This implies that molecules 1-3 are not lying flat on the surface. In solution, the structures of 1-3 would be expected to adopt an extended chain conformation due to Coulombic repulsion between charged terminal groups. Molecular modeling estimates the end-to-end distance of an extended chain conformer of 1 to be ≈42 Å with a diameter of ≈12 Å leading to a cross-sectional area of ≈500 Å2. Clearly, amphiphiles 1-3 are not lying flat on the surface nor is 4. It is more likely that molecules 1-3 are adopting a “bent” conformation placing the ionic terminal groups in the water phase while orienting some of the Rf groups, perhaps as a “loop,” into the air phase. The conformation of a molecule at a surface will be the result of a balance between enthalpic and entropic losses versus those gained by surface tension reduction.38 This is the domain of molecular dynamics simulations and is beyond the context of the current study. It is important to mention that the architecture of polymers, such as the accessibility of many conformations and placement of ionic groups (terminal or bolaamphiphilic, in the present case), allows for a great deal of flexibility in designing a new generation of fluorosurfactants. Adsorption strength or propensity judged by values of β/R is in the range of (120) × 106 m3/mol for 1-3 and increases with increasing Rf chain length compared to a value of 6 × 105 m3/mol for (32) Davies, J. T. Proc. R. Soc. London, Ser. A 1958, 245, 417. (33) Datwani, S. S.; Stebe, K. J. Langmuir 2001, 17, 4287. (34) Daniel, R. C.; Berg, J. C. Langmuir 2002, 18, 5074. (35) Bernett, M. K.; Zisman, W. A. J. Phys. Chem. 1963, 67, 1534. (36) Hann, R. A. In Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum: New York, 1990; Chapter 2. (37) Acero, A. A.; Li, M.; Lin, B.; Rice, S. A.; Goldmann, M.; Azouz, I. B.; Goudot, A.; Rondelez, F. J. Phys. Chem. 1993, 99, 7214. (38) van der Gucht, J.; Besseling, N. A. M.; Fleer, G. J. Macromolecules 2002, 35, 2810.
Properties of Bolaamphiphilic Poly(fluorooxetane)s
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increasing the Debye reciprocal length, κ,39
κ)
Figure 3. Surface tension isotherms for poly(fluorooxetane) 1 in pH 2 (9), 6 (b), and 8 (2) solutions along with fits to the adsorption isotherm using eq 1 at pH 2 (solid line), 6 (dashed line), and 8 (dotted line).
4. This is a reflection of the relatively large effectiveness of 1-3 at reducing surface tension and also is an indication of their relative solubilities in water. Considering the somewhat unusual adsorption properties of 1-3, poly(fluorooxetane) 1 was studied as a function of solution pH and in the presence of added electrolyte in the form of NaCl. Shown in Figure 3 is the surface tension isotherm of 1 as a function of solution pH. Somewhat surprising is that the molecular areas at saturation decrease from ∼70 Å2/molecule at pH 2 to ∼48 Å2/molecule at pH 8, indicating more efficient packing at the interface. The pH range examined should span the pKa of the sulfate group (SO3H + NH3 T SO3- NH4+), and more efficient packing should be observed at lower pH values due to a lessened effect of Coulombic repulsion. Adsorption strengths were found to decrease from ∼6.1 × 107 m3/mol at pH 2 to ∼1 × 106 m3/mol at pH 8 in accordance with the lessened expected solubility of a free sulfate acid versus the sulfate salt. The effect of added nonadsorbing electrolyte was examined using 1 in pH 8 solution with 200 mM added NaCl, and the surface tension isotherm is shown in Figure 4. The effect of added electrolyte is not dramatic as compared to observations made using other materials with regard to achieving an ultimately lower surface tension at the cmc.33 Molecular areas at saturation decreased from ∼47 Å2/molecule in the absence of added electrolyte to ∼27 Å2/molecule in the presence of 200 mmol of NaCl, indicating an increase in packing efficiency by
(4)
where e is the elementary electrostatic charge and n0 is the molar number concentration of electrolyte. The value of κ is estimated to increase from ∼3.3 × 10-4 Å-1 with no added electrolyte to ∼0.15 Å-1 with 200 mmol added NaCl. In earlier work,8 surface tension isotherms for 1 and 2 were reported in a water/methanol mixture. Adsorption strength (β/R) was less by an order of magnitude approximately and surface excess at saturation (Γ∞) was less by a factor of 2-3 indicating greater molecular area demands in this solvent mixture compared to the pure aqueous buffers used in this study. This is an artifact of the lesser dielectric constant of the water/methanol mixture (∼65) versus water (80) and its effect on ion dissociation and, hence, surface excess in addition to increased solubility in methanol decreasing adsorption free energy. A comparison was made between surface excesses at saturation and resultant molecular area demands obtained from fitting surface tension data to the Davies adsorption isotherm (eq 1) and the Gibbs adsorption equation given by eq 5:40
Γ∞ ) Figure 4. Surface tension isotherms for poly(fluorooxetane) 1 in pH 8 solution (9) and one containing 200 mmol NaCl (b) along with adsorption isotherm fits according to eq 1 for pH 8 solution (solid line) and one containing 200 mmol NaCl (dashed line).
x
2e2n0 0RT
dγ 1 nRT d ln[M]
(5)
where [M] is the molar concentration of surfactant and n is the electrolyte type of the surfactant. Theoretically, n ) 1 for a nonionic surfactant and n > 1 for an ionic surfactant depending on its state of dissociation and/or valence. For example, poly(fluorooxetane)s 1-3 should yield n ) 3 and 4 should give n ) 2 if they are dissociated completely. Surface excesses at saturation and molecular area demands calculated using eq 5 are shown in Table 1 along with those calculated using the Davies adsorption isotherm in eq 1. Best fits were obtained with the Gibbs n parameter set equal to 1, while n g 2 gave unacceptably large values of molecular area demands. The adsorption parameters for 4 are a good test of eqs 1 and 5. The crosssectional area of an Rf group is ≈30 Å2. Molecular area demand calculated using eq 1 or eq 5 yields values of 24 and 32 Å2/molecule, respectively, for surfactant 4. These results, therefore, seem quite reasonable. Gibbs n values of ≈1 have been found previously in studies of hydrocarbon anionic surfactants with small counterions suggesting nondissociation.41,42 Dynamic Surface Tensions and Diffusion Coefficients. Dynamic surface tension data were collected for surfactants 1-4. Dynamic surface tension data for poly(fluorooxetane) surfactant 1 and 4 in pH 8 solution are shown in Figures 5A and 6A, respectively. Dynamic surface tension data (not shown) collected for 2 and 3 are similar in nature to those for 1 and exhibit no unusual features. Diffusion coefficients were obtained by solution of the mass (39) Chatelier, R. C.; Drummond, C. J.; Chan, D. Y. C.; Vasic, Z. R.; Gengenbach, T. R.; Griesser, H. J. Langmuir 1995, 11, 4122. (40) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; Chapter III. (41) Lunkenheimer, K.; Burczyk, B.; Piasecki, A.; Hirte, R. Langmuir 1991, 7, 1765. (42) Lunkenheimer, K.; Czichocki, G.; Hirte, R.; Barzyk, W. Colloids Surf., A 1995, 101, 197.
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Figure 5. (A) Dynamic surface tension data for poly(fluorooxetane) 1 in pH 8 solution at a concentration of 1.13 × 10-5 M (0), 1.24 × 10-4 M (O), 3.73 × 10-4 M (4), and 1.03 × 10-3 M (3) and (B) dynamic surface tension data at 1.24 × 10-4 M along with the solid line fitted using eq 6 with D ) 5.1 × 10-8 cm2/s.
Figure 7. Diffusion coefficients for poly(fluorooxetane)s 1 (9), 2 (b), and 3 (2) and the fluorosurfactant 4 (1) along with lines fitted according to eq 8 for 1 (solid line), 2 (dashed line), 3 (dotted line), and 4 (dashed-dotted line). Table 2. Diffusion Coefficient Data for Fluorosurfactants sample
pH
-ma
-ba
rb
1 2 3 4 1 1
8 8 8 8 2 6
1.18 ( 0.2 1.20 ( 0.4 1.39 ( 0.2 0.894 ( 0.3 1.55 ( 0.2 1.43 ( 0.4
12.0 ( 1 11.7 ( 2 13.1( 1 9.89 ( 1 13.7 ( 0.8 12.6 ( 2
0.948 0.768 0.957 0.849 0.977 0.841
a Log D ) m log[M] + b. b Correlation coefficient from linear least-squares fit.
be fitted reasonably well statistically by
log D ) m log[M] + b
Figure 6. Dynamic surface tension data for fluorosurfactant 4 in pH 8 solution at a concentration of 8.28 × 10-6 M (0), 4.55 × 10-5 M (O), 9.15 × 10-5 M (4), and 1.00 × 10-3 M (3) and (B) dynamic surface tension data at 4.55 × 10-5 M along with the solid line fitted using eq 6 with D ) 3.67 × 10-6 cm2/s.
(Dπ )
Γ(t) ) 2
1/2
[C0t1/2 -
∫0T
1/2
Cs d[(T - t)1/2]]
(6)
where Γ(t) is surface excess as a function of t, D is the diffusion coefficient, C0 is the bulk surfactant concentration, T is the time needed to reach an equilibrium surface tension, and Cs is the subsurface concentration as a function of t and is calculated from eq 2 after solving for C1∞ as a function of θ.
Cs )
R/β exp(zFΨs/RT) 1-θ
(7)
The first term in brackets on the right side of eq 6 accounts for the adsorption rate of surfactant to the interface, while the second term reconciles the desorption rate that becomes more important at long times and/or higher concentrations. The convolution integral in eq 6 was evaluated graphically. Fits of eq 6 to one concentration each of 1 and 4 are shown in Figures 5B and 6B, respectively. Diffusion coefficient data for poly(fluorooxetane) surfactants 1-3 and the fluorosurfactant 4 collected in pH 8 solution are shown in Figure 7. The diffusion coefficients are concentration-dependent and can (43) Ward, A. F. H.; Tordai, L. J. Chem. Phys. 1946, 14, 453.
and the fitted parameters are given in Table 2. On average, diffusion coefficients for the poly(fluorooxetane)s 1-3 were ∼1-1.5 orders of magnitude lower than that of the smaller fluorosurfactant 4 at a given concentration. The StokesEinstein equation gives the relationship between diffusion coefficient and radius, r, of a spherical body,44
D)
transfer equation given by Ward and Tordai,43
(8)
kT 6πrµ
(9)
where k is the Boltzmann constant and µ is kinematic viscosity, and predicts that the diffusion coefficient will be inversely proportional to the radius. Diffusion coefficients taken near the cmc yield estimates of r ≈ 416, 86, and 327 Å for 1, 2, and 3, respectively. A similar analysis for 4 gives r ≈ 42 Å. Obviously, the fluorosurfactants studied presently are not spheres but use of eq 9 in examining the reasonableness of the measured diffusion coefficients is valuable. The value of r estimated for 4 is quite realistic based on the small-molecule nature of this species. The values of r obtained for 1-3 seem a little high and may indicate premicellar aggregation. A radius of gyration (Rg) calculation for 1 in the extended chain conformation gives a value of ≈147 Å. The order-ofmagnitude agreement between a calculated Rg and r obtained from a spherical approximation to the shape of 1 lends credibility to measured diffusion coefficient values. Conclusions Equilibrium and dynamic surface tensions have been measured for a series of bolaamphiphilic poly(fluorooxetane)s with perfluoroalkyl chain lengths ranging from -CF3 to -(CF2)4F. Similar properties were measured for (44) Einstein, A. Investigations on the Theory of Brownian Movement; Dover: New York, 1956.
Properties of Bolaamphiphilic Poly(fluorooxetane)s
a typical, small-molecule, long Rf chain anionic fluorosurfactant. While equilibrium surface tensions (∼25-30 mN/m) measured for the poly(fluorooxetane)s were higher than that of the small-molecule fluorosurfactant (∼15 mN/ m), they were lower than expected based simply on Rf chain length. Adsorption strengths, in the measure of the Davies isotherm β/R parameter, were comparable to that of the small-molecule, long Rf fluorosurfactant. Surface molecular area demands were found in the range of ∼4770 Å2/molecule. Despite being larger than the value for a single Rf chain containing surfactant (∼24 Å2/molecule), they are unexpectedly small for a polymeric molecule. It is thought that the bolaamphilicity and polymeric nature of the poly(fluorooxetane)s allow them to adopt a favorable configuration that allows for more efficient packing at the interface. The adsorption properties of one poly(fluorooxetane) variant containing pendent -CF3 groups were examined as a function of pH and added electrolyte. Adsorption strength and molecular area demand decreased with increasing pH from 2 to 8. This is due to the pH range spanning the pKa of the sulfate groups. In the
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acid form, less solubility is expected and observed in the increased value of β/R. In the presence of added electrolyte (200 mM NaCl), the molecular area demand was found to decrease from 47 to 27 Å2/molecule. The decrease would be expected based on the increased Debye reciprocal length with increasing electrolyte concentration. Dynamic surface tension data and adsorption parameters were used to calculate diffusion coefficients. As expected based on size and hydrodynamic radii, the diffusion coefficients measured for the poly(fluorooxetane)s were approximately an order of magnitude less than that measured for the smallmolecule fluorosurfactant at comparable concentrations. In summary, adsorption properties will depend on a number of parameters. These include a balance of entropic and enthalpic changes occurring during configurational changes upon adsorption, adsorption strength or solubility in the form of the β/R parameter, and packing efficiency at the interface dictated by conformations accessible. LA034233Q
