Langmuir 1996,11, 977-983
977
Adsorption of Semifluorinated Alkanes at Hydrocarbon-Air Surfaces B. P. Binks, P. D. I. Fletcher," W. F. C. Sager,?and R. L. Thompson Surfactant Science Group, School of Chemistry, University of Hull, Hull HU6 7RX, U.K. Received September 13, 1994. I n Final Form: November 29, 1994@ Hydrocarbonand fluorocarbonchains tend to demix. Because of this antipathy, semifluorinatedalkanes (SFAS) of the general diblock structure F(CF2)n(CH&H (abbreviated as FnHm) are expected to show surfactant properties (adsorption and aggregation) in hydrocarbon and fluorocarbon solvents. We have used surface tensiometry to investigate the adsorption of a range of FnHm materials at hydrocarbon-air surfaces. Maximum surface pressures are found to be 0-10 mN/m. The surface pressures generally increase with increasing SFA concentrations up to concentrations very close to the solubility limit in the hydrocarbon solvents investigated. This suggests that micelle aggregate formation is either absent or occurs only over a very narrow concentration range. The extent of adsorption increases with increasing F chain length of the SFA, increasing chain length of the alkane solvent, and decreasing temperature. Weakly adsorbing systems form expanded monolayers with a minimum area per SFA molecule of a few square nanometers. Strongly adsorbing systems form condensed monolayers of minimum area per SFA of 0.26 nm2,corresponding to a fluorocarbon chain density equal to that found in condensed bulk phases of perfluoroalkanes. The transition from weak to strong adsorption behavior occurs in an abrupt manner and can be induced either by changingthe temperature or by changingthe hydrocarbon solventcomposition.
Introduction Hydrocarbons show a n antipathy for fluorocarbon solvents. This point can be illustrated by the immiscibility of hydrogenated linear alkanes (H(CH&H, abbreviated to H,) with the corresponding linear fluorinated alkanes (F(CFz),F, abbreviated to F,). F, and H, solvent pairs are miscible in all proportions above their upper critical solution temperatures (UCST),which provides a measure of the antipathy between the solvents. For a particular F, solvent, the UCST increases strongly with H solvent chain length, e.g. for F, the UCST is 9.2 "C for H5 and 194.1 "C for H15.l For He with F, solvents of different chain lengths, the UCST for F3 is 2.4 "C2 and increases to 30.1 "C for F7.l For toluene, the UCST with perfluoromethylcyclohexane is 88.9 0C,3compared with a value of 26.4 "C for H, with the same fluor~carbon.~ Hence it can be seen that aromatic hydrocarbons are more immiscible with fluorocarbons than linear alkanes of the same number ofcarbon atoms. Overall, the UCST values show that the antipathy between H and F chains can be maximized by increasing both fluorocarbon and hydrocarbon chain lengths and also by using aromatic hydrocarbons. Additionally, we note that the UCST values for fluorocarbonhydrocarbon solvent pairs are much lower than for alkanes with water. Hence, WF antipathy is considerably weaker than that between hydrocarbons and water. This paper is concerned with the adsorption properties of semifluorinated alkane (SFA) molecules of the general structure F(CF&(CHZ)~H, abbreviated as F,H,. These molecules possess the basic diblock architecture common to conventional surfactants except that their affinities for
* Author to whom correspondence should be addressed.
Current address: Department of Physical and Macromolecular Chemistry, Gorlaeus Laboratories, Leiden University, Postbus 9502,Einstein Weg 55,2300 RA Leiden, The Netherlands. @Abstractpublished in Advance ACS Abstracts, February 15, 1995. (1)Young, C.L. Trans. Faraday SOC.1989,65,2639. (2)Hicks, C.P.;Hurle, R. L.; Toczylkin, L. S.; Young, C. L. Aust. J. Chem. 1978,31,19. (3)Hildebrand, J. H.; Cochrane, D. R. F. J . Am. Chem. SOC.1949, t
71. . - 22. I
(4)Hurle, R.L.;Toczylkin, L. S.; Young, C. L. J.Chem. S O ~Faraday . Trans. 2 1977,73,618.
hydrocarbon or fluorocarbon solvents replace the conventional surfactant amphiphilicity based on affinities for oil and water solvents. Thus SFAs can be regarded as a novel class of structurally simple surfactants which possess no charged or polar groups. A complete understanding of conventional surfactant water oil systems requires detailed consideration of all the interaction forces (electrostatic, dipolar, hydrophobic, solvation, etc.) present between surfactant head and tail groups and the oil and water solvents. This situation is greatly more complex than can be dealt with using current computer modeling method^.^,^ Systems containing SFA + alkane perfluoroalkane potentially offer a much greater simplicity in the interactions present (i.e. no electrostatic or dipolar interactions) and additionally contain only F and H chains. We are interested in studying the surfactant behavior of SFAs in order to gain a deeper insight into the relationship between molecular structure and surfactant properties. We envisage that SFA systems may provide examples of surfactantbehavior more amenable to computer modeling using more realistic intergroup interaction potentials than is currently possible for conventional surfactants. Since the antipathy between hydrocarbons and fluorocarbons is relatively weak compared to that between hydrocarbons and water, SFA molecules might be expected to be relatively poor surfactants. SFA species have the additional feature of possessing a relatively rigid perfluorinated chain attached to a more flexible hydrogenated chain and it is of interest to explore the consequences of this on the surfactant properties. There is a limited amount of relevant literature concerning the physicochemical behavior of solutions of SFAs in hydrocarbon or fluorocarbon solvents. At high concentrations and low temperatures, SFAs in alkane solvents precipitate from solution to yield two-phase mixtures of a solid phase plus a saturated solution. The two-phase mixtures commonly exhibit gel behavior which is thought to arise as a result of the formation of a network of anisotropic crystals containing entrapped solvent.
+
+
+
( 5 ) Smit, B.; Hilbers, P. A. J.;Esselink, K.; Rupert, L. A. M.; van Os, N. M.; Schlijper,A. G. Nature, 1990,348,624. (6)Smit, B.;Hilbers, P. A. J.;Esselink, IC In Structure andDynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Chen, S-H., et al., Eds.; Kluwer: Amsterdam, 1992,p 519.
0743-746319512411-0977$09.00/0 0 1995 American Chemical Society
Binks et al.
978 Langmuir, Vol. 11, No. 3, 1995 Phase behavior and microstructure for these systems are discussed in r e f ~ . ~ - lAl small number of SFAIsolvent systems have been shown to exhibit micelle-like aggregation. Hopken et al.9report a light scattering studyofFlzH10 in HE a t 35 "C. A Zimm plot of the data over the concentration range 3-25 mM showed the presence of aggregates containing ca. 130 monomers. Similar data at 60 "C indicated that the aggregation number decreases with increasing temperature. Results for FlzHlo in toluene a t 60 "C indicated that these solutions contained only monomers. Turberg and Brady12 investigated the aggregation of F8H16 in F8 a t 40 "C. They detected a break point in the scattering intensity and in the extent of solubilization of a dye a t a n SFA concentration of ca. 5 wt %. They estimated the aggregation number of the aggregates formed to be ca. 4-6. They also report that F8H12 does not aggregate in Fg a t concentrations up to 10 wt %. Lo Nostro and Chen13 reported a more detailed study of F8H16 solutions (0-12 wt %) in F8 a t 41 "C using viscosity and light and neutron scattering measurements. They concluded that this system forms small, spherical aggregates containing ca. 95 monomers a t concentrations above the cmc of 4 wt %. The aggregates are strongly penetrated by the solvent. It is ofinterest to know whether aggregation of SFA species in hydrocarbon or fluorocarbon solvents is common or restricted to relatively few systems. Since the liquid-vapor surface tensions of fluorocarbons are generally lower than the corresponding alkanes, SFA species are expected to adsorb a t the hydrocarbon-air surface but not a t the fluorocarbon-air surface. Gaines has measured the surface tensions of dodecane solutions of FlzH, ( m = 4,8,14, and 18) and F,H12 ( n = 8 and 10) a t room temperature.14 The surface activity was found to increase with increasing H chain length of the SFA. Maximum surface pressures were only 0-4 mN/m, and no behavior indicative of micelle formation a t concentrations less than the gel formation limit was observed. The surface activity of a range of SFAs in solution in "vaseline oil" was investigated by Napoli et al.15 They observed maximum surface pressures of up to 10 mN/m a t 20 "C and showed that the surface activity (judged by the initial slope of the tension versus concentration curve) increases with increasing F chain length and passes through a minimum with increasing H chain length. Neither Gaines nor Napoli et al. report any analysis of the concentration dependence of the tensions to yield SFA surface concentrations. In this study we have used surface tension measurements to investigate the extent of adsorption and adsorbed film structures for SFAs a t hydrocarbon-air surfaces. The effects of solvent and temperature variation have been studied.
Experimental Section Materials. The SFA materials studied here, synthesized according to the method of Rabolt et a1.,16were all linear chain species except for FgHlo, which contained a branched perfluor(7)Twieg, R. J.;Russell, T. P.; Siemens, R.; Rabolt, J. F. Mucromolecules 1985,18,1361. (8) Rabolt, J. F.; Russell, T. P.; Siemens, R.; Twieg, R. J.; Farmer, B.Polym. Preprints(Am. Chem. Sac.,Diu. Polym. Chem.) 1986,27,223. (9)Hopken,J.;Pugh, C.; Richtering,W.; Moller, M. Makromol. Chem. 1988,189,911. (10)Hopken, J. Ph.D. Thesis, University of Twente, 1991. (11)Hopken, J.; Moller, M. Macromolecules 1992,25,2482. (12)Turberg, M. P.; Brady, J. E. J . A m . Chem. SOC.1988,110,7797. (13) Lo Nostro, P.; Chen, S-H. J.Phys. Chem. 1993,97, 6535. (14)Gaines, G. L., Jr. Langmuir 1991,7,3054. (15)Napoli, M.; Fraccaro,C . ;Scipioni, A.;Alessi, P. J . Fluorine Chem. 1991,51, 103. (16)Rabolt, J. F.; Russell, T. P.; Twieg, R. J. Macromolecules 1984, 17, 2786.
oisononyl chain attached to a linear decyl hydrogenated chain. F12H14 was synthesized by Fluorochem and the remaining SFAs used in this work were made by Synprotec (Manchester, UK). Analysis of the samples using GC with mass spectroscopic detection yielded a single chromatographic peak in each case which was estimated to correspond to a minimum purity of ca. 98%. F8H16 was found to contain a minor dodecane-insoluble impurity which did not show up on the GC-MS. This impurity was removed by passing a pentane solution of the SFA over a n alumina column and evaporating the solvent (yield = 93%).The hydrocarbons toluene (May and Baker, Pronalys AR),octane (Fluka, > 99.5%), dodecane (Aldrich, 99+%), and pentadecane (Fluka, '99.5%) were passed over an alumina column prior to use to remove polar impurities. The absence of surface active impurities in the hydrocarbons was confirmed by measuring the surface tensions ofthe pure solvents. These were found to agree with literature values17 within 0.1 mN/m. Methods. Solution-vapor surface tensions were measured by the drop volume technique using stainless steel tips of approximate diameter 5 mm held on an Agla syringe. Tension values were calculated from the measured drop volumes and the solution densities using the correction factors of Harkins and Brown.18 The apparatus was thermostated within 0.1 "C of the required temperature by immersion in a water bath. The average of 10 or so drop volumes was recorded for each tension measurement, giving a typical accuracy of better than 0.1 mN/ m. Densities were measured using a Paar DMA55 densimeter. Vapor pressure osmometry ( W O ) measurements were made using a Gonotec Osmomat 070 instrument. The instrument was calibrated for ideal solution behavior using benzil (Aldrich,98%) solutions in toluene, and eicosane (Sigma, 99%) solutions in dodecane. The Gonotec W O instrument can only be used for temperatures greater than 35 "C and with solvents having a vapor pressure greater than a few Torr.
Results and Discussion 1. F,H, Adsorption at the Toluene-Air Surface. Figure 1shows plots of surface pressure Il (equal to the tension of the solvent minus that of the solution, i.e. Il = ysolvent- ysolution) versus SFA concentration (in mol %) for a range of SFA materials in toluene a t 20 "C. In each case the maximum concentration shown is just less than the solubility limit except for F8H16 and FgHlo for which the solubilities were relatively high and were not determined. It can be seen that the Flz chain length SFA adsorbs strongly whereas the shorter F chain length materials show relatively weak adsorption. Conventional surfactants in water reduce the tension from the value for pure water (72 mN/m) to a value of a similar magnitude to that of a hydrocarbon-air surface (35 mN/m or so). Hence, conventional surfactants in water typically achieve a maximum surface pressure of around 35 mN/m. Above the critical micelle concentration (cmc) the surface pressure remains virtually constant with increasing surfactant concentration. For SFA films a t hydrocarbon-air surfaces, the maximum surface pressure is expected to be of the order of the difference in surface tension between hydrocarbon-air and fluorocarbon- air surfaces (typically 10 mN/m or so). The low surface pressures seen here are consistent with this picture. None of the surface pressure curves show a clear break point followed by a concentration range over which the pressure is virtually constant, indicative of a critical micelle concentration (cmc). The two highest concentrations for F12H14 show a levelling-off of the pressure, implying that SFA aggregation may occur over a very narrow concentration range preceding the solubility limit. However, since the solubility limits were judged visually and the formation ofthe gel or a precipitate was commonly (17)Selected Values of Properties of Hydrocarbons and Related Compounds, A.P.I. Project 44,Chemical Thermodynamics Properties Centre, Texas, 1966,Vol. 2. (18)Harkins, W.D.;Brown, F. E. J . Am. Chem. SOC.1919,41,499.
Adsorption of F,H, at Hydrocarbon-Air Surfaces
Langmuir, Vol. 11, No. 3, 1995 979 12
10
i
8
9
VPO signal JV
6
F12H14
6 surface pressure /mN n i l 4
3
Benzil
0
F9H10
0
*f
4
2
6
conc. / mol% 400
2 VPO
signal 200 I mV 0 -4
-2
eicosane
0 In (clmol%)
Figure 1. Variation of surfacepressure with SFAconcentration in toluene at 20 "C. The solid lines show the polynomial-fitted
functions. Unshaded data points were not included in the fit. slow (a few days for solutions of low SFA concentration), it is uncertain whether these two concentrations truly fall below the solubility limit. Generally, it appears that micelle formation is suppressed in SFA hydrocarbon systems relative to conventional surfactant water systems. In this context it is relevant to note that micelle formation by SFAs in hydrocarbon would require the formation of a micelle containing a core of perfluorinated chains. The high rigidity and cross sectional area of the F chain portion of SFA species relative to the H chain group are both factors which are likely to contribute to the lack of micelle formation in these systems. The surface pressure data can be used to obtain the average area occupied per SFA molecule (A)using the appropriate form of the Gibbs adsorption equation:
+
+
0 0
2
4 6 conc. I mol% Figure 2. Variation of W O signal with concentrationfor (a) F12H14 and benzil (ideal)solutions in toluene at 35 "C and (b) F12H14 and eicosane (ideal) solutions in dodecane at 50 "C. Table 1. Summary of Fitting Equations for WmN m-l versus ln(c/mol %) Data for SFAs in Various Solvents SFA solvent temp/"C fitting equation (ln(c/mol %) = x ) 20 TI = 8 . 1 0 0+ ~ ~50.12~ + 78.56 ll = O.577lx2 3.408~ 5.313 20 n= 1.140~ 3.328 20 n= 0.88312+ 2.846 20
+ + ll = 2 . 6 4 9 + ~ ~12.712+ 13.19 +
35 35 20 20 20
n = 0 . 0 5 5 +~ ~1.088~+ 3.989 flat line n = 12.97~~ + 62.77~+ 75.98 for x = -3.50 to -2.60 n = 0.721422+ 4.9852 + 8.658 for x = -2.60 to -2.46
ll = 2 . 4 6 8 + ~ ~27.88~+ 56.44
where Fz is Boltzmann's constant, T is the absolute temperature, and a is the activity of the SFA solute. For ideal solutions the activity can be equated with the concentration. Vapor pressure osmometry (VPO)was used to assess the ideality of selected SFAholvent pairs. Figure 2 shows a comparison of the VPO signal obtained for SFA solutions with that of ideal solutions. For the two systems shown, the SFA solutions were found to behave ideally up to the solubility limit, and eq 1can be applied without correction for nonideality. In the absence of any evidence to the contrary (except for one particular system; see later text), ideal behavior was assumed for all SFNsolvent mixtures investigated here. The surface pressure -ln(concentration) curves were fitted to polynomials of order 1or 2, and the fitted curves are shown as solid lines in the relevant figures. The fitting equations are summarized in Table 1. Differentiation of the fitted equations yielded the corresponding values of the area occupied per SFA molecule (A). The surface pressure-area isotherm for F12H14 adsorbed a t the toluene-air surface is shown in Figure 3, which also
F&14 dodecane F12H14 dodecane F12H14 dodecane
20 35 50
+
Il = 1 2 . 9 7 ~ ~62.7%
+
75.98 ll = 0.2802~~0.69852 0.5119 n= 0.51212 0.4975
+
+ +
includes theoretical ll-A isotherms calculated according to the ideal 2-D gas (eq 2) and Volmer (eq 3) surface equations of state:
rrA=kT
(2)
II(A - A,) = KT
(3)
Both of the latter isotherms correspond to 2-D "gas-like" monolayers. The Volmer equation includes a correction for a "hard-disk excluded area of A,, per molecule. The value of A,, assumed for the calculated Volmer isotherm was 0.28 nm2, which corresponds to the cross-sectional area of a n all-trans CF2 chain.lg It can be seen in Figure 3 that the isotherm for F12H14 corresponds to a surface (19)Wunderlich, B.Macromolecular Physics; Academic Press: New York, 1973;Vol. 1, p 97.
Binks et al.
980 Langmuir, Vol. 11, No. 3, 1995
Table 2. Summary of Limiting areas per Molecule at the Hydrocarbon-Vapor Surface and Solubilities for SFAs in Various Solvents SFA solvent tempPC lim area/nm2 solubility/mol3'2 0.25 0.15 F12H14 toluene 20 1.87 0.34 F10H16 toluene 20 3.5 23.07 FgHlo toluene 20 4.6 22.2 F8H16 toluene 20 0.34 0.85 F12H14 toluene 35 3.3 23.7 FloHlo toluene 35 infinite 0.45 F12H14 octane 20 0.26 0.16 (cmc?) F12H14 dodecane 20 0.26 0.12 F12H14 pentadecane 20
9
6
surface pressure /mN m-l
F12H14 dodecane F12H14 dodecane F12H14 dodecane
20 35 50
0.26 3.7 7.9
0.16 (cmc?) 2.1 5.0
3
10
8 0
0
50
100
150
200
area / A2 Figure 3. Surfacepressure-area isothermfor F12H14 in toluene at 20 "C (curve iii). Curves i and ii show theoretical curves corresponding to the Volmer and ideal 2-D gas surfaceequations of state, respectively. phase which is much more condensed and incompressible than a 2-D gas-like monolayer. For the SFAs with F chain lengths shorter than 12, the limiting (minimum) areas per SFA achieved before the solubility limit correspond to relatively expanded films (Table 2). The limiting areas fall progressively with increasing F chain length, consistent with the notion that the increased F chain length of the SFA leads to a n increased driving force for adsorption. This change is consistent with the increase in UCST with increasing F chain length for binary solvent mixtures noted earlier. F12H14 at the toluene-air surface shows a limiting area of 0.25 nm2, close to the cross-sectional area of a n alltrans CF2 chain, indicating that the fluorocarbon chain region of this film is highly condensed. Adsorbed monolayers on aqueous solutions of conventional surfactants with a single hydrocarbon tail typically reach a limiting area of 2-3 times the cross-sectional area of a CHZchain (0.18 nm2) and thus are relatively expanded. Crudely, the limiting area reached by adsorbed monolayers of conventional surfactants a t the air-water surface is a consequence of the balance between cohesion arising mainly from the tail groups and repulsion mainly from the head groups. This balance of opposing forces is also important in determining whether surfactant aggregation is limited to the formation of finite aggregation number micellar aggregates or proceeds to complete phase separation ofprecipitated surfactant. Such a phase separation can be envisaged as the formation of aggregates of infinite aggregation number and might be expected for systems which show little repulsion between head groups. The highly condensed state of the F12H14 monolayer a t the hydrocarbon-air surface is consistent with a relatively weak repulsion of the H chains ("head groups)') which, in turn, is consistent with the suppression of micelle formation in favor of precipitation. Figure 4 shows the surface pressure-ln(concentrati0n) curves for F12H14 and FloHlo at 35 "C. The surface pressure curve for F12H14 a t 35 "C is shifted to higher concentrations
6
surface pressure /mN m-l
n-
0 0
-8
-6
, -4
-2
0
2
In (c/mol%) Figure 4. Variation of surfacepressure with SFAconcentration
in toluene at 35 "C. The solid lines show the polynomial-fitted functions. Unshaded data points were not included in the fit. relative to the corresponding curve a t 20 "C (Figure 1). This comparison shows the extent of adsorption decreases with increasing temperature, indicating that the adsorption process is exothermic. FloHlo shows weak adsorption in comparison to F12H14. 2. F12H14Adsorption at the Surfaces of Linear Alkanes at 20 "C. It was noted previously that fluorocarbonhydrocarbon antipathy increases strongly with increasing hydrocarbon chain length. In order to investigate the effect of this increased antipathy on adsorption properties, the surface pressure-ln(concentration) curves for F12H14 solutions in the alkanes Hs, H12, and HI5 were measured and are shown in Figure 5. Octane solutions show virtually no adsorption up to the solubility limit, whereas in the higher alkanes greatly increased adsorption occurs, consistent with a n increased driving force. The concentration required to achieve a sharp increase in surface pressure for F12H14 in toluene (Figure 1)is similar to that seen in Figure 5 for dodecane as solvent. Extrapolation of the UCST data in ref 2 shows that the UCST for perfluoromethylcyclohexane with toluene is approximately equal to that for perfluoromethylcyclohexane with
Adsorption of Fa,,, at Hydrocarbon-Air Surfaces
Langmuir, Vol. 11, No. 3, 1995 981
5
4
surface pressure I mN m-'
0
T
surface 3 pressure
/mNm-' 2
0
loo 0 2 area I A
200
0
400
800
I
-4
-3
-2
surface pressure I mN m-l
-1
In (c/mol%)
Figure 5. Variation of surface pressure with F12H14 concenand pentadecane (Hd tration in octane (Ha), dodecane (Hd, at 20 "C.The solid lines show the polynomial-fittedfunctions. Unshaded data points were not included in the fit. The short
vertical lines show the solubilitylimits. The inset graph shows the variation of tension (in mN/m) with F12H14 concentration (in mol %) in pentadecane. undecane, i.e. toluene and undecane are approximately equivalent in terms of their antipathy for fluorocarbons. Hence it appears that the extent of adsorption of F12H14 in the different hydrocarbon solvents broadly correlates with the strength of antipathy between a n F chain and the hydrocarbon solvent. The surface pressure curve for dodecane solutions shows a maximum before the solubility limit. This behavior is similar to the minima in surface tension curves which have been observed for conventional surfactants in water, where the surfactant contains a surface active impurity. The best known example of this behavior is that of sodium dodecyl sulfate with dodecanol as impurity.20 The interpretation of the surface tension minimum (or surface pressure maximum) is that the impurity adsorbs competitively with the surfactant in the surface monolayer until the cmc is reached when the impurity is then extracted into the micelles formed. Further work is required t o confirm whether the surface pressure maximum observed here for the F12H14 H12 system does correspond to a cmc. For Hi5 a s solvent, the surface pressure curve shows a decreasing slope a t the highest concentrations measured. Since it is unlikely that the surface excess concentration decreases with increasing bulk concentration, it appears that the activity coefficient of F12H14 may decrease progressively in the concentration range preceding the solubility limit. This would be consistent with a small degree of progressive aggregation of the SFA in Hi5 over this concentration range. Unfortunately, it was not possible to verify this nonideal behavior using VPO since the instrument could not be operated a t 20 "C and the vapor pressure of H15 is too low to yield reliable data. The high SFA concentration data for Hi5 as solvent was not included in the polynomial fit used to obtain the values of area per SFA. The II-A isotherms for the Hlz and Hi5 solutions are shown in Figure 6. In HI2 (part a of the figure), the SFA
+
(20) Elworthy, P. H.; Mysels, K. J. J . Colloid Sci. 1966,21, 331.
02
area I A Figure 6. Surface pressure-area isotherms for F12H14 in dodecane (curve iii in a) and in pentadecane (curve iii in b) at 20 "C. For both a and b, curves i and ii refer to the Volmer and ideal 2-D gas surface equations of state. The apparent first order phase transition in the lI-A isotherm in b is shown as the horizontal dashed line. forms a condensed monolayer and reaches a limiting area of 0.26 nm2, close to the cross-sectional area of a n alltrans F chain. In Hi5 (part b of the figure), the monolayer is again highly condensed for surface pressures above 0.6 mN/m. The abrupt change of slope in the surface pressure-ln(concentration) curve (Figure 5) at 0.6 mN/m suggests the presence of a first-order phase transition in the monolayer a t which regions of condensed and expanded surface phases coexist. The inset to Figure 5 shows the data in the region of the apparent phase transition replotted as the variation of surface tension with SFA concentration. This inset plot highlights the abrupt change of slope and shows that the tensions for SFA concentrations above 0.08 mol % do not extrapolate back to the pure solvent tension. This adds weight to the assertion that a phase transition is present. The firstorder phase transition is indicated as the horizontal region in the JI-A isotherm shown in Figure 6b. A similar transition may also be present in the HE system. However the low surface pressure of the transition (relative to the accuracy ofthe I 3values, approximately 0.2 d / m ) makes it difficult to determine this with certainty. The group of Motomura has reported surface phase transitions for a range of conventional soluble surfactants at the air-water and oil-water interfaces. The phase transitions occur a t surface pressures of 1-5 mN/m. Charged surfactants generally give transitions from a gaseous to an "expanded" filmz1whereas transitions from "expanded" to condensed monolayers have been observed ~~~~
~~
(21) Aratono, M.; Uryu, S.;Hayami, Y.; Motomura, K.; Matuura, R. J . Colloid Interface Sci. 1984,98, 33.
Binks et al.
982 Langmuir, Vol. 11, No. 3, 1995 5
2.5
T = 5OoC 4
2 H
1.5
0 3
H
surface pressure
T=20°C
surface pressure
/mN m-l
/mN m-l
H 1
2
H
H 1
I
0
0.2
1
0.4 0.6 mol fraction H 12
, 0.8
0
1
Figure 7. Variation of surface pressure of F12H14 with mole fraction of Hl2 in H a 1 2 mixtures at 20 "C.The concentration of F12H14 in the mixed solvent is constant at 0.1371 mol %. for uncharged surfactants.22 This suggests that transitions to condensed monolayers require the absence of longrange electrostatic repulsive forces between surfactant molecules in the adsorbed monolayer. Thus, the presence of the transition to the highly condensed film in the SFA monolayers is broadly in line with the behavior of more conventional surfactants. The transition from nonadsorbing behavior in the case of octane to adsorbing in the case of dodecane was investigated further by measuring the variation of surface pressure with mole fraction of Hlz in mixtures with Hs a t a fixed concentration of F12H14. For the solvent mixtures, the solvent surface tension in the absence of SFA was calculated using the method described in ref.23 The solvent mixture surface tension was measured for one representative mixture and agreed closely (better than 0.1 mN/m) with the calculated value. The variation of surface pressure with solvent composition is shown in Figure 7. It can be seen that the surface pressure is very low below a mole fraction of Hlz of ca. 0.4. At higher mole fractions the surface pressure increases approximately linearly with the Hlz mole fraction. The sharp break in the curve suggests that the antipathy between the F12H14 and the solvent mixture must reach a critical value before appreciable adsorption occurs. 3. Effect of Temperature on the Adsorption of F12H14 at the H12-Air Surface. Figure 8 shows surface pressure-ln(concentration) curves for F12H14 solutions in HI^ a t 20, 35, and 50 "C. The two higher temperatures show weak adsorption, giving minimum areas per SFA of 3.7 and 7.9 nm2 at 35 and 50 "C, respectively. At 20 "C, the monolayer forms a condensed film (minimum area 0.26 nm2)at considerably lower concentrations. It is clear that the adsorption of SFAs a t hydrocarbon-air surfaces is promoted at lower temperatures and thus the adsorption is an exothermic process. (22) Matubayasi, N.; Motomura, K.; Aratono, M.; Matuura, R. Bull. Chem. SOC.Jpn. 1978,51, 2800. (23) Aveyard, R. Trans. Faraday SOC.1967, 63, 2778.
In (c/mol%) Figure 8. Variation of surface pressure with concentration of FlzHlr in dodecane at 20,35, and 50 "C. The solid lines show the polynomial-fitted functions. Unshaded data points were not included in the fit.
H
4H H
surface pressure 1mNm-l
-
I H
I
". .. 17.5
20
22.5
5
temperature / C Figure 9. Variation of surfacepressure of 0.1371 mol % F12H14 in dodecane with temperature. The transition from weak to strong adsorption with decreasing temperature was investigated further by measuring the surface pressure of a fixed concentration of F12H14 in H12 as a function of temperature (Figure 9). Above 22.5 "C, the surface pressure is low and virtually constant. Below 22.5 "C the surface pressure increases approximately linearly with decreasing temperature. The abrupt change of slope a t 22.5 "C again suggests that the strong adsorption to give the condensed monolayer occurs in a highly concerted manner.
Langmuir, Vol. 11, No. 3, 1995 983
Adsorption of F,H, at Hydrocarbon-Air Surfaces
Conclusions The main conclusions from this study can be summarized as follows. (1)Adsorption of SFA materials a t hydrocarbon-air surfaces is favored by (i)long F chains in the SFA, (ii)long chain lengths of the alkane solvents, and (iii) low temperatures. (2) The adsorbed monolayers show maximum surface form pressures Of ‘-lo Weaklyadsorbing expanded (minimum area Occupied per SFA Of the Order Of nm2)whereas adsorbing SFAS condensed (minimum area o*26 form nm2)in which the fluorocarbon chain density is similar to that in condensed phases Of perfluoroalkanes’ The transition from weak to strong adsorption behavior occurs very abruptly. (3) The SFAhydrocarbon solvent systems investigated here show no evidence for a wide concentration range in which micelle-like aggregation occurs preceding the
“”’
solubility limit. The formation of condensed monolayers and the kppression ofmicelle formation is consistent kith a lack of repulsive forces between the hydrocarbon chain “head groups” of the SFA materials. (4)Adsorbedmonolayersof F12H14 a t the Hls-air surface a t 20 “C exhibit a phase transition from an expanded state to a highly condensed state.
Acknowledgment. We are grateful to Dr. W. D. Cooper (Shell Research Ltd., Thornton, U.K.) for helpful discussions and to Mr. M. R, Cooper (University of Hull) for obtaining some of the surface tension data. We thank Shell Research Ltd., Thornton, the EPSRC (UK) and the swiss National Science FoundationPThe Royal Society for financial support. We also thank Dr. w. Mahler (DuPont, Central Research and Development, Wilmington, DE) for supplying us with unpublished data. LA940735F
