Langmuir 1990,6, 1499-1504
1499
Adhesion of Trimyristin Fat to Radio Frequency Plasma Treated PVC and Chromium E. Blomberg* and C.-G. Golander Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden Received August 30,1989. I n Final Form: March 14, 1990 There is an increasing belief in the use of surface modification techniques to reduce the adhesion of soil to surfaces so that only weak detergents or mechanical means is required for the soil removal. In this work, we have studied how the soil adhesion is affected by controlled and well-defined modification of the surface. Various surfaces were prepared by radio frequency plasma treatment combined with surface derivatization techniques. Adsorption and displacement of trimyristin, a model soil, were investigated by ellipsometry. Two fundamentally different and successful approaches to realize a good soil-repellant surface were found: (i) strongly polar surfaces of poly(ethy1ene oxide) that interact strongly with water or (ii) surfaces which contain cross-linked fluorocarbon moieties.
Introduction Lately, there is an increasing awareness of the large economic values involved in the cleaning of surfaces. According to a recent estimation,' the cleaning expenses for public houses in Sweden amount to 1%of the gross and the corresponding figure for national product (GNP), industries and private houses is more than twice as much. There are mainly two characteristics intimately connected with the concept of antisoil surfaces, namely, soil resistance and soil release. These characteristics are not necessarily correlated. Thus, it is known from the textile industry that low energy surfaces often give higher soil resistance and high-energy surfaces show better soil release.2 Despite this, the literature is surprisingly scarce in reports on how the cleaning efficiency is affected by systematic and welldefined variation of the chemistry of the surface. One may distinguish between two fundamentally different mechanisms for soil removal from surfaces: gradual solubilization of the soil in the surfactant phase or direct displacement of soil from the surfacea3 The efficiency of solubilization depends on flow conditions as well as the nature of the soil and the detergent but is independent of the nature of the surface onto which the soil is attached. Characteristic requirements for good detergents are efficient adsorption a t the soil/water interface combined with comparatively low adsorption in the water/air interface capacity and efficient kinetics of dissolution at the soil/liquid interface. Since changes in the hydrophilic-lipophilic balance (HLB) of the detergent influence these different requirements in slightly different ways, there exist considerable technological possibilities for optimization of solubilizing detergent systems for each situation and application. On the other hand, the direct soil/surface interaction which determines the probability for direct displacement is a function of soil and surface chemistry and the surrounding media (solvent). Surface morphology and the shape of the interfacial contact zone also influence the magnitude of the interaction force and the accessibility of displacing molecules (surfactants) at the interface? Recent work also suggests that surfaces with highly mobile functional groups suppress the adsorption
* To whom correspondence should be addressed.
(1) Leaf, R. Golu. till Tak. 1981, 7, 19-23. (2) Mankowich, A. J . Am. Oil. Chem. SOC.1961,38,589. (3) See, for instance: Shaw, D. J. Introduction t o Colloid and Surface Chemistry, 3rd ed.; Butterworth London, 1980. (4) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: New York, 1985.
0743-7463/90/2406-1499$02.50/0
Table I. Conditions for Preparation of the Various Plasma Films sample gas effect, W gas flow, SCCMa 0 2 100 20 PVC-ox CH4 50 20 Cr-CHI C2F4 50 20 Cr-CzFd 4 Standard cubic centimeters per minute.
time, min 3 15 5
of foreign molecules due to the increased conformational constraints that are imposed on the mobile groups.5 There is an increasing belief in the use of surface modification techniques to reduce the adhesion of soil to surfaces in applications where extensive wear and abrasion are absent. The intention is to reduce the soil adhesion so that only weak detergents or mechanical means is required for direct displacement of the soil. Two examples of such applications are wallpapers in homes and paints for use in hospitals and in industry. In this paper, we have focused attention on t h e importance of surface chemistry and surface mobility for adhesion of a homogeneous soil film. We have left out surface roughness effects and exclusively studied smooth surfaces prepared by radio frequency plasma treatment combined with surface derivatization techniques. The surfaces were characterized by means of ESCA. Trimyristin was used as a model soil. Soil resistance was characterized by ellipsometry studies of trimyristin deposition from ethanol. Soil release in surfactant solution was also monitored by means of ellipsometry. The instrumental arrangement and measurement procedure were similar to that recently described by Backstrdm et a1.6
Materials and Methods One-millimeter cast PVC films free from plasticizer and other additives were obtained from MIPAB AB, Sweden. Glass slides (25 X 75 mm) with vacuum-deposited 2000-&thick chromium films were a gift from Applied Physics, University of Linkoping. The samples were sonicated in ethanol/water (1:l) for 15 min. Plasma treatment was accomplished in a commercially available plasma reactor, Plasmaprep 100, from Nanotech. Process parameters used for the preparation of the various plasma films are shown in Table I. Oxygen and methane were obtained from AGA Gas, Sweden. PVC treated in oxygen plasma is referred (5) Andrade, J. D.; Nagaoka, S.; Cooper, S.; Okano, T.; Kim, S. W. ASAIO Trans. 1987,332,7584. (6) Engstrom, S.; Blckstrom, K. Langmuir 1987, 3, 568.
0 1990 American Chemical Society
i
Blomberg and Golander
1500 Langmuir, Vol. 6, No. 9, 1990
water plasma
1
PEI
scribed in detail in a previous paper8 (e.g., 0.5% w/w PEOCHO in phosphate buffer, pH 6, containing 10% K z S 0 4 and NaCNBH3 as catalyst, 40 h, 60 "C). All surfaces were finally cleaned thoroughly in Millipore treated water. The molecular composition of various surfaces is schematically shown in Figure 4. Contact Angle. Contact angle measurements on the films were made with water on a goniometer (Model A100, Ram6 Hart). ESCA. ESCA studies were performed by means of a Leybold Heraeus instrument (Model LH 2000) equipped with a A1 Ka X-ray source operating at 12 kV, 17 mA. The pressure in the analyzing chamber was 2 X lo* Torr. Carbon 1s and oxygen 1s spectra was recorded (in single scans) during 10 min on each samples. Spin Cast. A 5% w/w solution of trimyristin in ethanol, Fluka, was spin cast on PVC or chromium substrates with a commercial spin-cast device from Harrick, NY, operating at 4000 rpm. Ellipsometry. In ellipsometry, one measures the change in the state of polarization of light reflecting on a smooth surface. This change is usually expressed by the ratio of the reflection coefficient for electromagneticfield components polarized in the plane of incidence,R,, and in a plane perpendicular to the plane of incidence, R,, respectively
R J R , = tan \k exp A
1
v 0
where tan \k is the amplitude ratio and A is the phase change, between the two components. The refractive index of the film NFand the thickness dF can be calculated by using Drude's equation^.^ From NFand dp, the mass of the film was calculated by using the Cuypers equation
PEO-CHO
v 0
v 0
Figure 1. Outline of the reaction pathway used for grafting poly(eth lene oxide) to chromium slides. Trichlorotriazine (TCT) = polymin SN, branched poly(ethy1enimine);PEO=$PEI C 0 = monomethoxy monoaldehyde poly(ethy1ene oxide) (M, = 1900). to as PVC-ox, and methane and tetrafluoroethylene (Matheson) plasma films on chromium are referred to as Cr-CHI and Cr-C*F4, respectively. The deposition rates for polymer films from CHI or CzH, plasma were measured by means of an oscillating quartz thickness monitor (Inficon, XTC). Poly(ethy1ene oxide) (PEO) films, covalently attached to chromium slides and referred to as Cr-PEO, were prepared in the following way (see also Figure 1). Chromium slides were activated in water plasma at 30 W, 40 standard cubic centimeters per minute (SCCM), for 3 min. The activated surface, presumably containing reactive chromium oxide OH groups, was then exposed to a 2% w/w solution of trichlorotriazine (TCT) in dioxane containing KHCOs as catalyst. T h e reaction between TCT and surface OH groups was carried out at 0 "C for 60 min. After the surface was rinsed in ultrafiltrated and deionized water (Millipore System), polymin SN (BASF), a branched poly(ethylenimine), was allowed to react with the TCT surface in a 0.5% w/w water solution at pH 9 for 60 min at 60 "C. Subsequently, after another rinsing in water, monomethoxy monoaldehyde PEO (Aldrich) with molecular weight 1900 (referred to as PEO-CHO and synthesized according to Harris et al.') was allowed to react with the primary amino groups on the poly(ethylenimine) surface. The reaction conditions have been de-
where d is the film thickness, nF the refractive index of the film, nB the refractive index of the buffer, M the molecular weight, A the molar refractivity, and Vw the partial molar volume of the adsorbing layer at 20 "C. The desorption of the trimyristin layers in detergent solution containing 1% w/w ethoxylated nonylphenol, NF(E0)lo (IC1Ltd., UK), was studied with a null-ellipsometer, type Auto ELI11 from Rudolph Research. The instrumental arrangement has been described in detail elsewhere.e*8The desorption experiments were carried out in a 0.5-cm3 flow cell with a recirculation of 4 mL/min. For the mass calculations, a one-layer model in the following form was assumed: surfactant solution/fat (+surfactant) film/polymer film + substrate. Firstly the composite refractive index was calculated for the polymer film covered substrate. This refractive index was subsequently used in studying the deposition and displacement of the fat layer. The ambient refractive index (surfactant solution) was N A = 1.3362 (white light).
Results and Discussions S u r f a c e Characteristics. C o n t a c t angles, film refractive indices and film thicknesses obtained from ellipsometry, and surface elementary ratios of oxygen to carbon, chlorine to carbon, and fluorine to carbon from ESCA analysis of the various samples are shown in Table 11. There is an uncertainty in the separate determination of N a n d d from ellipsometry. However, t h e film mass calculated from the product of N a n d d will be more accurately determined since t h e fluctuations in t h e two terms are correlated. T h e low refractive index measured ~~~
(7) Harris, J. M.; Struck, E. C.; Case, M. G.;.Paley, M. S.; Yalpani, M.; van Alstine,J. M.; Brooks, D. E. J . Polym. Sa.,Polym. Chem. Ed. 1984, 22, 341-352.
(8)Kiss, E.; GBlander, C.-G.Progr. Colloid Polym. Sci. 1987, 74,113-
119. (9) Drude, P. Ann. Physik 1889, 272, 532, 865.
Adhesion of Trimyristin Fat to PVC
Langmuir, Vol. 6, No. 9, 1990 1501
Table 11. Characterization of the Various Films by Means of ESCA and Ellipsometry. sample 8, deg N d, A O/C Cl/C F/C PVC 75 1.538 0.13 0.41 PVC-ox 15 1.353 1500 0.38 0.13 CrCH4 90 1.53 100 0.12 C P C ~ F ~ 130 1.375 lo00 0.00 1.41 Cr-PEO 20 1.52 180 0.67 a ESCA intensities from oxygen Is, chlorine 2p, and fluorine 1s signals in relation to the carbon 1s signal. 0 =contact angle (advancing),N = refractive index, d = thickness. ~
~~~
for oxidized PVC is most reasonably due to a slightly porous and rough surface structure induced by plasma sputtering. The low refractive index here obtained for the Cr-C2F4 film is expected for fluoropolymers.1° The comparatively high oxygen to carbon ratio of unmodified PVC (O/C = 0.13) shows that atmospheric surface oxidation has occurred on the plasticizer and additive-free PVC. The carbon 1s peak from unmodified PVC in Figure 2a indicates that oxidized carbon is present predominantly in the C-0 form. Simultaneously, the measured chlorine to carbon ratio, Cl/C = 0.41, is lower than the theoretical value for pure PVC (Cl/C = 0.5), suggestingthat decomposition and spontaneous HCl release to some extent have occurred on the unmodified PVC. However, the advancing contact angle in water was 7 5 O , which is only slightly lower than the value for pure PVC (80°).11 Oxygen plasma treatment of PVC results in a drastic increase of the polarity, O/C = 0.38, and the wetting characteristics. The carbon 1s peak (Figure 2b) indicates that extensive chain scission has occurred since a large fraction of the carbon now appears as carboxylate, COO. Also, the decrease in the Cl/C ratio indicates t h a t fragmentation reactions or further HC1 release has occurred as a result of the plasma treatment in oxygen. The contact angle of the Cr-CH4 was slightly higher than for bare PVC. The deposition rate of CH4 on chromium was low, less than 5 A/min. The low rate is due to competition between deposition and ablation processes. The high oxygen content (O/C = 0.12) of the CICCH4 film (see Table 11) is due to spontaneous oxidation reactions occurring when exposing the film, containing residual trapped polymer radicals, to oxygen atmosphere.12 Judging from ellipsometry measurements at various position, the film was smooth and homogeneous and the substrate chromium signal was barely observable above the background noise signal. The CI'C2F4 film does not contain any oxygen (see Table 11). Hence, the gas-phase deactivation reactions commonly occurring in the dark immediately after the glow discharge has been turned off here probably lead to radical recombinations and elimination of HF or F2. The film deposition rate in C2F4 plasma was around 20 A/min, and the film was strongly hydrophobic with a contact angle of 130". The carbon 1s peak in Figure 2d also shows that the Cr-C2F4 film contains carbon substituted in various degrees with fluorine. Although it is difficult to definitely assign every peak or shoulder in the carbon 1s signal, the relative positions of some distinct peaks indicate contributions from CF2, CF, and C-CF2 groups. We (10) Brandrup, J., Immergut, E. H., Eds. Polymer Handbook; Wiley:
New York, 1975.
(11) Ziaman, W. A. In Contact Angle, Wettability and Adhesion; Gould, E. F., Ed.; Advance8 in Chemistry 43; American Chemical Society: Washington, DC, 1964;p 1. (12) Yasuda, H. K. P h m a Polymerization; Academic Press:New York,
1985; Chapter 6.
293
291
I
I
295
293
289 287 285 283 BINDING ENERPY (eV) 1
291
I
289
281
279
283
281
I
287
285
279
BINDING ENERQY (aV)
Figure 2. (a) Carbon ESCA (1s) peak of pure PVC. The differentchemical shifta of the carbon is indicated by labels. The dashed line represents the CH2 carbon. The 286.5-eV label is assigned to the C-Cl and the C-0 carbons. The carboxy group, COO-, is positioned at 288.5-289.0 eV. (b) Oxidized PVC (PVCox). (c) Methane plasma treated chromium slides (Cr-C&). (d) Tetrafluoroethylene plasma treated chromium slides (CrC2F4). The shift at 287 eV corresponds to CF, at 289.5 eV to C-CF2, and at 291.5 eV to CF2 carbon. (e) PEO on chromium. The C-0 shift at 286.5 eV dominates the spectra.
therefore conclude that the Cr-CzFr film is not a linear poly(tetrafluoroethy1ene)polymer but rather an extensively cross-linked fluorocarbon film. The Cr-PEO film shows a contact angle in water of approximately 20" at room temperature. The oxygen to carbon ratio, O/C = 0.67, is slightly larger than what would be expected from pure PEO (O/C = 0.5). Since the carbon 1s peak, Figure 2e, reveals C-0 groups originating solely from the PEO polymer, a portion of the oxygen signal must
Blomberg and Golander
1502 Langmuir, Vol. 6, No. 9, 1990 Table 111. Deposition of Trimyristin to Different Surfaces. sample deposited mass, pg/cm2 PVC 10.00 f 0.3 PVC-ox 11.25 f 0.5 Cr-CHI 16.25 f 0.5 Cr-CZFd 2.20 f 0.2 Cr-PEO 2.20 f 0.2
a
1207
:
a The masses are calculated from the Cuypers equation and a layer model of surfactant solution/trimyristin + surfactant/polymer film + substrate. Scattering is expressed in S D .
originate from the chromium oxide. Accordingly, two peaks, one from PEO and one from chromium, were observed in the oxygen 1s signal (not shown here). From these results, we can conclude that the HzO plasma activated Cr surface reacts with trichlorotriazine, the coupling agent, without further activation of the OH groups.13 The composite PEO/polymin SN film has a thickness of approximately 180 A and a refractive index of approximately 1.52 when measured in air. Independent measurements have shown that the thickness of a branch poly(ethy1enimine) ( olymin SN, BASF) layer adsorbed This means that the thickness of at pH 9 is about 40 the pure PEO layer is about 100-150 A, which is close to the length of the extended PEO molecule (approximately 110 A). In water, the PEO layer swells considerably, and the thickness and the refractive index calculated separately vary considerably between measurements. However, the product of the refractive index and thickness, which is proportional to the adsorption density, stays constant during the swelling and amounts to 1.7 f 0.1 bg/cm2 in both air and water. Deposition of Trimyristin. A first indication of the strength of adhesion of trimyristin to the various surfaces is obtained from the myristin deposition experiments. As seen in Table 111, similar amounts, 10-20 rg/cm2, were deposited on bare PVC, PVC-ox, and Cr-CH4 films during spin casting, while 1order of magnitude lower deposited masses were obtained on the Cr-CZFd and the Cr-PEO surfaces. The differences in the deposited masses directly reflect differences in the interaction between the surface and the trimyristin film in ethanol. The relatively small amount of trimyristin deposited on the Cr-CzF4 and Cr-PEO samples indicates that the adhesion of the triglyceride film onto these surfaces is poor. Displacement of Trimyristin. In pure water, no desorption of trimyristin was measured on the PVC, PVCox, Cr-CzF4, or Cr-C& films after 45 min of equilibration at 20 "C. However, at 40 "C the films on Cr-CZFd were completely displaced, indicating poor attraction between trimyristin and the Cr-CzF4 surface. On the Cr-PEO surface, complete removal of t h e layer occurred immediately when the sample was immersed into water, evidently due to the high contact free energy (interfacial tension) between the fat and hydrated PEO. The NF(E0)lo surfactant used in this study adsorbs with a higher affinity and higher plateau adsorption values to many polymer surfaces compared to longer EO chain NF surfactant^.'^ The surfactant concentration used here is in excess of the cmc (=0.043 mg/mL). Hence, its adsorption to surfaces in general can be regarded as being independent of concentration since the chemical potential of the surfactant is roughly constant.
:
~
,
~
0 10
0
r
100
20
30 40 Time (min)
50
60
70
1,
a
c
8060-
"n Yo
40..
1
I
,
- ;02 0
1
(13)Gombotz, W. R.; Guanghui, W.; Hoffman, A. S. J. Appl. Polym. Sci. 1988, 35, 1-17. (14) Kronberg,B.; KHI1, L.; Stenius, P. J.Dispersion Sci. Technol. 1981, 2 (2, 3), 215-232.
C
..
0.
%
0. 0
i
10
20
30 40 Time (min)
50
60
70
"0
20
30 40 Time (min)
50
60
70
90
,
04 0
Figure 3. (a) Fraction ( % ) of the trimyristin film on PVC remaining after desorption in 1% NF(E0)lo surfactant solution and 40 "C ( 0 ) . (b) Desorpmeasured by ellipsometry a t 20 (0) tion of trimyristin from oxidized PVC (see Figure 3a). (c) Desorption of trimyristin from methane plasma treated chromium (see Figure 3a), (B)measurements at 30 "C. (d) Desorption of trimyristin from tetrafluoroethylene plasma treated chromium (see Figure 3a).
A rapidly cooled melt of trimyristin, as obtained from spin casting, will preferably crystallize in its a-form, which has a melting point around 32 OC.15 Hence, the solubilization power of the surfactant for trimyristin is expected to be low at 20 OC but will increase with temperature, especially around the melting point. The amount of trimyristin films on different surfaces remaining after exposure to a circulating 1% surfactant solution a t 20 and 40 "C for various times is shown in Figure 3a-d. At 20 OC, no displacement was observed on bare PVC. Instead, t h e film mass increases by (15)Hernqvist,L. Polymorphism of
fats, Thesis, Lund, 1984.
Adhesion of Trimyristin Fat to PVC
Langmuir, Vol. 6, No. 9,1990 1503
TRIMYRISTIN
Hydrocarbon tail
-0
P o l a r headgroup
p"c
PVC-ox dSCMOL/ u\ Cl
COOH
Cr-CH,
Cr-C2 F4
Figure 4. Molecular composition of various surfaces shown
schematically.
approximately 0.1 f 0.05 pg/cm2 within approximately 1 min, indicating adsorption of the NF(E0)lo surfactant on the trimyristin film. This is in agreement with previous adsorption studies for this surfactant on PVC latex, which show that adsorption saturation occurs at concentrations slightly above cmc and results in adsorption values of around 0.18 pg/cm2.14 According to the same study, the adsorption was larger to partially polar surfaces compared to completely hydrophobic surfaces. On the PVC-ox surface, desorption of 905'% of the initial mass occurs in approximately 3 min. This was followed by a 5-1096 (0.1 f 0.05 Kg/cm2) increase of the mass in 15-30 min, probably due to comparatively slow surfactant adsorption to the bare polar PVC-ox substrate. Evidently, the nonionic surfactant penetrates along the PVC-ox/trimyristin interface, adsorbs to both surfaces, and displaces the entire film. Orientation of polar head groups in trimyristin toward the hydrophilic PVC-ox surface may favor this interfacial penetration. On the Cr-CH4 surface, a less than 1 % initial decrease of the film mass was observed after which the mass was constant. This indicates a strong hydrophobic interaction between the layer and the surface. On the hydrophobic Cr-CzF4 surface, a low initial adsorption (0.02 f 0.01 pg/cm2) of surfactant occurred followed by a sudden displacement of approximately 50 % of the film, which as indicated above was very thin on this substrate. Obviously, the "hydrophobic" interaction between the hydrocarbon and the fluorocarbon surface was poor. The residual trimyristin observed after surfactant exposure may possibly be due t o t h e presence of hydrocarbon spots deficient in fluorine, to which trimyristin shows a stronger affinity. At 40 "C,above the melting point of a-trimyristin, the soil removal mechanism was quite different. On pure PVC,
a slow decrease of the film mass was observed during the whole 60-min desorption time studied. These differences in kinetics suggest t h a t solubilization rather than displacement of the trimyristin film occurs in this case. The removal rate was on the average 0.01 pg/cmz/min at the poor stirring conditions used here, slightly larger in the beginning and slightly smaller a t the end of the measurement. The removal rate will of course be strongly dependent on the stirring conditions. In contrast to pure PVC, PVC-ox behaved similarly at 20 and 40 "C; e.g., the displacement reaction is quantitative and occurs within 5 min. Displacement reactions also seemed to dominate on the Cr-CH4 surface. Evidently, surfactant diffusion and penetration along the interface are favored at the higher temperature. Furthermore, with increasing temperature the attractive forces between the trimyristin and the Cr-CH4 film may have decreased compared to the purely hydrophobic attraction between trimyristin and NF(EO)1o.l4 Similar to the situation on the Cr-CH4, the trimyristin film was quantitatively displaced from the Cr-CzFe surface at 40 "C. The weak adhesion between trimyristin and c&2F4 is also confirmed by the spontaneous displacement observed in pure water (no surfactant) at 40 "C. Finally, we note that the adsorption of surfactant on the Cr-CzF4 is approximately 50% smaller than on the other surfaces studied, indicating a lower affinity of the hydrocarbon to this surface. It should be pointed out that hydrolysis of trimyristin may result in an "apparent" displacement at high pH. However, at the pH used in our experiment, i.e., 5.5, no spontaneous decrease in film thickness of trimyristin was observed from the PVC surface in pure water at 20 "C. From a thermodynamical point of view, we can formulate the free energy condition for displacement of the film as
+
AW = yst- ytw- ysw RTJ:rt
d In c
+
R T P r , d In c > 0 where yxyis the interfacial tensions between the trimyristin (t),substrate (s), and water phase; rxthe amount of surfactant adsorbed on the trimyristin (t) film and the substrate (s), respectively; a n d c t h e surfactant concentration. The first three terms originate from the work of adhesion in pure water. The last two terms are the free energy gain due to adsorption of the surfactant.2 The ytw,rt,and rsvalues can be measured easily by standard methods and yst and yswfrom adhesion force measurements.I6 Here we consider the displacement results in rough qualitative terms only: Chemical modification of the surface will affect all quantities either directly, as for yaw,yat, and rs, or indirectly, since the surface will have an impact on the orientation of trimyristin molecules at the interface which in turn influences ytwand rt. One may expect, however, that ytw is close t o 50 mN/m (hydrocarbon/water contact).17 On the bare (hydrophobic) PVC, yawis most likely also close to 50 mN/m. Hence, the first three terms will become approximately -100 mN/m (we assume that the interfatial tension Tatbetween the trimyristin and the hydrophobic PVC is close to zero) and have t o be counterbalanced by large free energies associated with the surfactant adsorption to the two contacting surfaces before displacement of the trimyristin will occur spontaneously. The ystwill increase (decreasing contact angle) and ysw (16) Claesson, P. M.; Golander, C.-G. J. Colloid Interface Sci. 1987, 11 7, 366.
(17)Tanford, C. The Hydrophobic Effect; Wiley: New York, 1973.
1504 Langmuir, Vol. 6, No. 9,1990 will decrease with increasing polarity on the surface. Since Fa is comparatively larger on partially polar surfaces,14
displacement will be facilitated. On the PEO surface, yst will be around 50 mN/m and yswis almost zero,I6 and displacement will occur spontaneously in pure water. To explain the displacement on the Cr-CzF4 surface, one may assume that yethas a finite value which increases with temperature and yawis comparatively low for this surface, so that A W is larger than zero even in the absence of surfactant, in particular a t 40 "C. In contrast, on the CrCH4 surface, significant contributions from the last two terms (surfactant adsorption) are required before displacement occurs spontaneously.
Conclusions The deposition of trimyristin fat on a PEO surface or a cross-linked fluorocarbon film deposited in a tetrafluoroethylene plasma is 1 order of magnitude lower than deposition on a bare (or 02 plasma oxidized) PVC surface or a film depceited from a radio frequency plasma discharge in methane. By use of ellipsometry, it was possible to distinguish between two different trimyristin removal mechanisms, e.g., solubilization and instantaneous displacement of the
Blomberg and Golander
film. Trimyristin is most easily removed from strongly polar surfaces such as poly(ethy1ene oxide) (PEO) and oxygen plasma treated PVC by water displacement (on PEO) or surfactant displacement (on 0 2 plasma treated PVC). On hydrophobic surfaces such as PVC or CH4 plasma treated chromium, the spin-cast trimyristin film is not displaced by NF(E0)lo surfactant a t room temperature (e.g., below the temperature, 32 "C, at which a-crystals of trimyristin are formed). At 40 "C, trimyristin will slowly solubilize in the surfactant phase. On a cross-linked C2F4 plasma film, the trimyristin film is easily displaced already a t room temperature. This indicates poor adhesion between the hydrocarbon and the cross-linked fluorocarbon film. Acknowledgment. We thank Agneta Askendahl for providing us with fresh chromium slides and AnnCharlotte Malmvik for synthesis of the PEO-aldehyde. We are grateful to Per Stenius for fruitful discussions and criticism of this paper. Registry No. PVC, 9002-86-2; PEO,25322-68-3;NF(EO)lo, 9016-45-9; Cr, 7440-47-3; CtF4, 9002-84-0; CHI (homopolymer), 27936-85-2;trimpistin, 555-45-3;water, 7732-18-5;oxygen, 778244-7.
