872
Langmuir 1988,4, 872-878
Removal of Triglycerides from Polymer Surfaces in Relation to Surfactant Packing. Ellipsometry Studies Kjell Backstrom, Bjorn Lindman," and Sven Engstrom Physical Chemistry 1 and Food Technology, Chemical Center, P.O. Box 124, 5'-221 00 Lund, Sweden Received December 30, 1987. I n Final Form: March 15, 1988 The removal of triglycerides from polymer surfaces by surfactant mixtures and by surfactants combined with inorganic salts or organic solvents has been studied by ellipsometry. Important results are that the efficiency of ionic surfactants can be enhanced by addition of inorganic salts and that the efficiency of nonionic surfactants can be enhanced by addition of hydrocarbons. The removal process starts with the adsorption and aggregation of surfactant molecules at the soil/water interface. This aggregation is affected by the geometric properties of the surfactant molecules in the aggregates, such as head group area, hydrocarbon chain length, and volume. It is postulated that maximal removal is related to the packing of the surfactant molecules. The optimal packing is obtained in planar layers, i.e., zero curvature toward water and soil. The addition of salts, solvents, and other surfactants to a surfactant solution affects the packing properties of the surfactant aggregates and changes their curvature. The results show very clearly that a decrease in curvature corresponds to an increase in the soil removal. The assumption that low curvature of the surfactant aggregates favors the removal thus seems to be a useful tool for predicting the detergency properties of surfactant solutions. nent systems. A more important aspect is to try to develop a model which can rationalize the experimental observations in terms of surfactant molecule packing. In this way it becomes possible to utilize our deep understanding of surfactant aggregate geometry in homogeneous phases to predict cleaning efficiency.
Introduction In our society, cleaning of hard surfaces is a very extensive activity with significant economic consequences. In spite of this, our understanding of the physical mechanisms involved is very incomplete, and therefore attempts for systematic improvements have no solid basis. Many studies dealing with textile detergency have been presented, but the significance of the results obtained for hard surface cleaning is often unclear. To facilitate experimental investigations in the field of hard surface cleaning, we recently developed a technique based on ellipsometry. This approach has been used to study the removal of triglycerides by surfactants from chromium and poly(viny1 chloride) (PVC) surfaces. Some of the results have previously been p~blished.l-~Reference 1is an introduction to the field of studying detergency by ellipsometry, and some examples are given of the possibilities of the method. In ref 2 the method is discussed in more depth, and a complete description of the measuring technique is included. Special interest is devoted in ref 2 to the theoretical background of ellipsometry, and some models for evaluation of data are compared. In ref 3 the cleaning behavior of some surfactants is examined in relation to such factors as concentration, temperature, and agitation. In all these cases, well-defined single surfactants were used in order to make the systems simple enough to allow conclusions about the mechanisms involved. In practice, however, the cleaning products contain several components. Besides surfactants, solvents and alkaline builders are often included. It is also customary to mix different surfactants in an attempt to optimize the performance of the cleaner. In this study the fat removal properties of several of such mixtures are investigated. The surfactants are mixed or combined with other components such as hydrocarbons, alcohols, and inorganic salts. The purpose of this work is to throw more light on the surfactant behavior in cleaning processes and as a result to get a better understanding of the cleaning of hard surfaces. Another purpose is to check how the conclusions drawn in previous papers are applicable to multicompo-
Experimental Section Ellipsometry is an optical technique which allows the measuring of properties such as the thickness and refractive index of thin films at interfaces. The method is based on the fact that the film will change the state of polarization of an incident light beam upon reflection at the interface, and the magnitude of this change is dependent on the film properties mentioned above. If some kind of process (adsorption, desorption, etc.) occurs at the interface, ellipsometry makes it possible to follow that process continuously. Equipment. The measuring instrument was a modified automated Rudolph thii-film ellipsometer, type 43603-2003, steered by a personal computer, previously described in ref 2. Surface. The surface used, described previously,*consisted of gray poly(viny1chloride) (PVC without plasticizer). The critical surface tension of PVC is 39 mN/m. Method. Since test slide preparation, film deposition, and measurements were performed according to ref 2 , only a short description of the method will be given. After being cleaned, the substrate was immediately placed in a cuvette containing 4.5 mL of water and the refractive index of the substrate was determined. The substrate was then removed from the ellipsometer,and one drop of triglyceride solution (1-2%) was placed on the surface. The slide was centrifuged in a spinner at 4000 rpm for 5 min. The substrate was again placed in the cuvette, and the amount of triglyceride was determined from the ellipsometer angles as discussed below. A detergent solution (0.5 mL) was added to the cuvette, and the change of the ellipsometer angles was monitored continuously. From these angles it is possible to calculate the thickness and refractive index of a thin layer on a ~urface.~ However, as shown by Cuypers et al.: higher accuracy is obtained if the amount on the surface is calculated from these values. For these calculations, the partial specific volume (V) and the ratio of the molar weight (AI) and the molar refractivity ( A )of the film are needed and are given below. The temperature used in all experiments was 25 "C and the agitation, performed by a magnetic stirrer, was 325 rpm unless otherwise stated.
(1) Backstrom, K.; Engstrom, S.; Lindman, B.; Arnebrant, T.; Nylander, T.; Larsson, K. J . Colloid Interface Sci. 1984, 99, 479. (2) Engstrom, S.; Backstrom, K. Langrnuir 1987, 3, 568. (3) Backstrom, K.; Engstrom, S.J . Am. Oil Chern. SOC.,in press.
(4) Cuypers, P. A. "Dynamic Ellipsometry"; Ph.D. Thesis, Rijkuniversiteit Limburg, The Netherlands, 1976. ( 5 ) Cuypers, P. A.; Corsel, J. W.; Janssen, M. P.; Kop, J. M. M.; Hermens, W. T.; Hemker, H. C. J . Biol. Chern. 1983, 258, 2426.
0743-7463/88/2404-0872$01.50/0
0 1988 American Chemical Society
Langmuir, Vol. 4 , No. 4 , 1988 873
Triglyceride Removal from Polymer Surfaces 100
Table I. Physical Properties of Model Dirta substance mp, "C
TP TO
-5
PA
63
66
n ("C)
7 , mN/m
V, mL/g
MIA, a/mL
1.4381 (80) 1.4621 (40) 1.4255 (60)
25.2
0.98 1.09 1.00
3.31 3.33 3.29
32.3
TP,tripalmitin; TO,triolein; PA, palmitic acid. The removal is defined as the fraction removed after a certain time, usually 4 or 10 min, and is expressed in percent. Dirt. As the model for dirt in this work were used triolein (TO), tripalmitin (TP),and palmitic acid (PA). Before deposition, they were dissolved in octane, toluene, and 1-propanol, respectively. Triolein and tripalmitin (99% purity) were purchased from Sigma Chemical Co. and the palmitic acid from BDH Chemicals Ltd., England. The substances have the physical properties given in Table I, and the values used for the partial specific volume ( V , in mL/g) as well as the ratio of the molar weight and the molar refractivity (MIA;see ref 5 ) are also given. y is the surface tension (at 20 "C, from ref 6), n is the refractive index a t a given temperature, and mp is the melting point. S u r f a c t a n t s . Nonionic surfactants were (a) monodisperse poly(oxyethy1ene) alkyl ethers (CI2En),manufactured by Nikko Chemicals Co, Japan, and (b) poly(oxyethy1ene) nonylphenol ethers (NFE,), with polydisperse EO groups, manufactured by Berol AB, Sweden. Ionic surfactants were (a) dodecylbenzenesulfonic acid (DBS), 9 8 % , manufactured by Ventron GMBH, West Germany, neutralized by sodium hydroxide, (b) sodium dodecanesulfonate (C12S03Na),manufactured by Merck, West Germany, (c) sodium oleate (NaOle), manufactured by Carl Roth, West Germany, and (d) hexadecyltrimethylammonium chloride (CTAC), manufactured by Eastman Kodak Co. The surfactants were not purified further. Although all surfactants are amphiphilic, they will sometimes in this paper be called hydrophilic or hydrophobic, depending on which property dominates, to facilitate the discussion. All concentrations are given in weight percent (wt %). Additives. The chemicals used were sodium chloride, calcium chloride, sodium salt of nitrilotriacetic acid (NTA), decane, ethanol, butanol, hexanol, and octanol. They were all of analytical grade.
Results and Discussion General Aspects. The removal of hydrophobic molecules from a solid surface by surfactants is a complex process involving a large number of individual steps. The mechanism may presumably be different for different cases (different types of surface, dirt, mechanical agitation, etc.), but it seems that the following steps should always be relevant: (1)transport of surfactant (in monomeric or aggregated form) to the soil on the solid surface, (2) adsorption of surfactant a t the soil/solution interface, (3) formation of some sort of mixed surfactant-dirt complex (in a broad sense) a t the surface, (4) desorption of the mixed surfactant-dirt complex, (5) transport of the surfactant-dirt complex away from the surface, and ( 6 ) stabilization of dispersed dirt to prevent redeposition. While the rate-determining step in the cleaning of hard surfaces has not yet been identified, and may presumably be different for different situations (for example, hydrophobic and hydrophilic surfaces), it is to be noted that plots of removal vs surfactant concentration3 resemble many surfactant adsorption isotherms'+ (which were also obtained in parallel studies by the ellipsometry methodlo). (6) Saito, M.; Akihiko, Y. Textile Res. J . 1983, 54, 18. (7) Adsorption from Solution; Ottewill, R. H., Rochester, C. H., Smith, A. L., Eds.; Academic: London, 1983.
(8) Adsorption from Solution at the SolidlLiquid Interface;Parfitt, C. D., Rochester, C. H., Eds.; Academic: London, 1983. (9) Aveyard, R. In Surfactants; Tadros, Th. F., Ed.; Academic: London, 1984.
O
I
,
0.0
I
0.5
,
I
1 .o
,
,
1.5
,
, 2.0
,
2.5
NaCl concentration, %
F i g u r e 1. Removal of palmitic acid after 10 min by 0.05% NaDBS versus concentration of NaCl in surfactant solution (EI, 0.05% NaDBS; 0.2% NaDBS).
+,
Surfactants can self-associate into a wide range of different structures in aqueous systems, these aggregates changing in a complex way with surfactant concentration, temperature, solubilizate, electrolyte, etc.l'-" A good understanding of factors determining aggregate shape has been obtained in recent years, and theories are available that make possible the calculation of the most stable aggregate geometry for a given situation.'8-22 Interactions related to the effective shape of surfactant molecules and electrostatic interactions are well understood; others, like hydration forces, are controversial and the subject of much current research.23 Adsorption and aggregation of surfactants on a solid surface is apparently favored if surfactant packing into planar or low curvature aggregates has a lower free energy than other geometries. In our analysis of the cleaning experiments we will examine how far a model based on surfactant packing into planar aggregates can rationalize the results. (Besides adsorption transport to and from the surface may also be favored by planar aggregates, as shown in work on bicontinous micr~emulsions~~). There are different models in which surfactant packing in aggregates is discussed. In one, the hydrophilic and lipophilic properties of the surfactant(s) are considered; for planar packing these should be b a l a n ~ e d . ~ ~Is-, ~ ~ , ~ ~ raelachvili, Mitchell, and Ninham18 based their reasoning on the critical packing parameter (CPP), which characterizes the geometric packing properties of an amphiphilic compound. It is defined as u/aolc,where u is the hydrocarbon volume, a, is the optimal surface (head group) area, (10) Arnebrant, T.; Backstrom, K., Jonsson, B.; Nylander, T. J. Colloid Interface Sci., in press. (11) Ekwall, P. Adv. Liquid Cryst. 1975, 1 , 1. (12) Tiddy, G. J. T. Phys. Rep. 1980, 57, 1. (13) Shinoda, K. Progr. Colloid Polym. Sci. 1983, 68, 1. (14) Laughlin, R. G. Adv. Liquid Cryst. 1978, 3, 41. (15) Laughlin, R. G. In Surfactants;Tadros, T., Ed.; Academic: New
York, 1984; p 53. (16) Fontell, K. Mol. Cryst. Liquid Cryst. 1981, 63, 59. (17) Lindman. B.: Wennerstrom.H. TOD.Current Chem. 1980.87.1. (18) 1sraelachvili;J. N.; Mitchell; D. J.; Ninham, B. W. J . Cheh. Soc.,
Faradav Trans. 2 1976. 72. 1525. (19) israelachvili,J. N. Ihtermolecular and Surface Forces; Academic:
London, 1985. (20) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2
1981, 77, 601. (21) Jonsson, B.; Wennerstrom,H. J . Colloid Interface Sci. 1981,80, 4821. (22) Jonsson, B.; Wennerstrom, H. J . Phys. Chem. 1987, 91, 338. (23) Hydration Forces and Molecular Aspects of Solvation; Jonsson, B., Ed.; University: Cambridge, 1985. (24) Shinoda, K.; Lindman, B. Langmuir 1987,3, 135. (25) Shinoda, K. J. Phys. Chem. 1986,89, 2429. (26) Shinoda, K.; Shibata, Y. Colloids Surf. 1986, 19, 185.
874 Langmuir, Vol. 4, No. 4, 1988 and 1, is the critical chain length. CPP, when taking into account both geometric features and electrostatic head group repulsions, provides a good rationalization of surfactant self-association. Surfactants with an effective CPP < 1/3 preferably form spherical micelles, surfactants with CPP '/3-l/2 cylindrical micelles (rods), surfactants with CPP 1/2-1 vesicles, surfactants with CPP i= 1 planar bilayers or bicontinuous microemulsion, and surfactants with CPP > 1reversed micelles. For adsorption at a planar surface, a CPP of =1 is expected to be favorable, since the closest packing is attained in planar bilayers. Anionic Surfactants. Salt Effects. In comparison with some nonionic surfactants, single-chain anionic surfactants like sodium dodecyl sulfate and sodium dodecyl benzenesulfonate were found to be rather inefficient in removing triglyceride or fatty acid from a solid ~ u r f a c e ; ~ changes in pH or temperature do not change the situation extensively. However, as exemplified in Figure 1 for the removal of palmitic acid by 0.05% NaDBS, salt addition may lead to a dramatic improvement. The removal is low in distilled water, rises sharply to a high value already a t 0.1% NaCl, and then increases slightly for higher NaCl concentrations. NaDBS precipitates in solutions with NaCl content higher than about 2%. This behavior can be explained in terms of electrostatic interactions in different ways. The first step in the cleaning procedure, as discussed in ref 3, involves the adsorption and aggregation of the surfactant at the soil/water and the substrate/soil interfaces. The adhesion energy for a solid particle on a surface is (1) = Ydl + Ysl - Yds where Ydl is the surface energy for dirt/liquid, yalis the surface energy for substrate/liquid, and Yds is the surface energy for dirt/substrate. Adsorption of the surfactant to the dirt lowers in this case ydl and ysl;thus the surfactant adsorption decreases the adhesion energy, and the removal is facilitated. Addition of NaCl or another salt increases the adsorption of ionic surfactant^.'^^^'*^^ The salt effects are best discussed in terms of the effect of electrostatic interactions on the surfactant packing. Single-chain ionic surfactants in general form spherical micelles in a broad concentration range due to a strong repulsion between head groups; this corresponds to a large effective a, and a CPP < 1/3. This is in line with the experimental cleaning results. Salt addition allows for a closer packing and produces an increase in the CPP. This is indeed experimentally observed in a large number of ways: micelle growth from spheres to rods, change in phase behavior, change in microemulsion structure from oil droplets in water to a bicontinuous phase and to water droplets in oil, e t ~ . The ~ ~ ellipsometry p ~ ~ ~ dirt ~ removal results are qualitatively in line with the expected change in the CPP. Besides the action a t the surface, the added salt also influences the solution behavior of the surfactant. Because of the screening properties, the micelle formation is energetically less unfavorable in the presence of salt, and as a result the critical micelle concentration (cmc) is lowered.30 Since the surfactant concentration in Figure 1 is near the cmc and we have shown3that removal is very dependent
--
(27) Koopal, L. K.; Keltjens, L. Colloid Surf. 1986, 17, 371. (28) Jurkiewicz, K. J. Colloid Interface Sci. 1986, 112, 229. (29) Mazer, N. A,; Benedek, G. B.; Carey, M. C. J . Phys. Chem. 1978,
80,1075. (30) Gunnarsson, G.; Jonsson, B.; Wennerstrom, H. J . Phys. Chem. 1980,84, 3114. (31) Gugring, P.; Lindman, B. Langmuir 1985, 1, 464.
Backstrom et al. on the concentration/cmc ratio near the cmc, the surfactant was also tested a t a higher concentration, 0.2%, well above the cmc. The result is also inserted in Figure 1,and it is clear that addition of salt increases the removal more than addition of surfactant. This indicates that a direct screening effect of the added salt on the surfactant molecule packing is more important than the cmc lowering effect. However, there is a limit for the beneficial salt influence for many surfactants, since the addition of salt raises the surfactant Krafft point and causes precipitation a t a certain concentration. Some other g r o ~ p s have ~ ~ pinvestigated ~~ the influence of salt on the dirt removal by ionic surfactants. Although these investigations were performed with liquid dirt, the results are similar. Ogino et al.32showed that the rolling up of olive oil and oleic acid by NaDBS was accelerated in the presence of sodium sulfate. Muller et showed that the removal of a liquid fat mixture by anionic surfactants was facilitated when the solution contained NaCl or CaC1,. It is thus clear that salt improves the removal by ionic surfactants of both solid and liquid dirt. Nonionic Surfactants. Salt Effects. The effect of inorganic salts is quite different for nonionic than for anionic surfactants. The behavior also seems to be strongly dependent on the nonionic surfactant considered. In our measurements we observed that the removal of triolein by C12E8from PVC is not influenced by addition of NaCl up to 5%, in contrast to the case described above of NaDBS, which improved significantly by such small amounts as 0.1% NaC1. Shinoda and Friberg34have pointed out that addition of salts like NaCl to a nonionic surfactant solution causes a change in the hydrophile-lipophile balance (HLB) of the surfactant toward the hydrophobic side. This also means a lowering of the cloud point. The cloud point of 0.05% CI2E8is depressed to 63 "C by 5% NaC1, to be compared to 78 "C for the pure surfactant solution. Thus the cloud point is still much above the experimental temperature. Another case is illustrated in Figure 7 in connection with the discussion of surfactant mixtures. C12E5 and C12E8are used to remove triolein from PVC in distilled water and in water containing CaC12 in an amount equivalent to a water hardness of 8 (1.8 mm; 1 degree of hardness = 0.71 mg of Ca/100 mL of H20). The calcium ions improve the removal for C12E5 whereas the effect on C12E8is small. The cloud points are slightly depressed, to 34 "C from 35 "C for C12E5 and to 77 "C from 78 "C for ClzE8. Apparently the surfactant that has the cloud point nearest to the solution temperature is most affected by the calcium ions. However, more relevant is a discussion in terms of packing effects, which has been considered in some detail for such nonionic surfactant^.^^-^' The size of C12E5micelles increases strongly with both temperature and surfactant concentration, whereas the micelle radius for C12E, is hardly affected.36 This corresponds to an increase of CPP for C12E5, and since the CPP is less than unity, a better packing is obtained and, consequently, a better removal; the corresponding effect for C12E8should be insignificant, as observed. It is striking to note that for a surfactant (32) Ogino, K.; Aqui, W. Bull. Chem. SOC.Jpn. 1976, 49, 1703. (33) Muller, M.; Quack, J. M.; Vitores, L. Seifen-Ole-Fette- Wachse 1980, 18, 533. (34) Shinoda, K.; Friberg, S. Emulsions and Solubilization; Wiley: New York, 1986. (35) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J . Chem. SOC., Faraday Trans. 1 1983, 79, 975. (36) Nilsson, P.-G.; Wennerstrom, H.; Lindman, B. J . Phys. Chem. 1983,87, 1377. (37) Nilsson, P.-G.; Wennerstrom, H.; Lindman, B. Chem. Scr. 1985, 67.
Langmuir, Vol. 4, No. 4, 1988 875
Triglyceride Removal from Polymer Surfaces 80
70 60
-8
-
60 50 40 30 20 70
-
5040
20
10
-
I
04 0.00
IO-
I
0
0.01
0
Figure 2. Removal of tripalmitin after 4 min by 0.05% NFEe
and decane at various decane concentrations.
60
I
decane e withdecane - with + withoutdecane without decane
%$
-
601 40
-
20
-
O2
'
4
'
6
'
I
8
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2
,
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4
.
,
6
. 8,
,
1
Carbon atoms in alcohol
Decane concentration,%
Ow ow
,
I
10
'
1
EOgroups in surfactant
Figure 3. Removal of tripalmitin after 4 min by 0.05% NFE,
with different lengths of the poly(oxyethy1ene) chain with and without 0.008% decane (+, without decane; m, with decane). which is close to the balanced state even quite small additions cause significant effects. Nonionic Surfactants. Hydrocarbon Addition. In Figure 2 is illustrated the removal of tripalmitin by 0.05% NFE,, in the presence of up to 0.013% decane, and the beneficial influence of small additions of decane up to 0.008% is evident. (At 0.008% the molar ratio between decane and NFE,, is 0.81.) However, as shown in Figure 3, the effect of hydrocarbon addition on triglyceride removal is complex and depends on the surfactant. For the case of the nonionic surfactants alone the removal has a maximum for a certain length of the EO chain. Such a nonmonotonic behavior can again be qualitatively understood from the surfactant molecular packing: a surfactant with a short EO chain will be too hydrophobic and have CPP > 1, while with a long EO chain it will be too hydrophilic and have CPP < 1;a t some intermediate EO chain length, CPP i= 1 and optimal packing a t a planar surface is achieved. The model of rationalization of the cleaning efficiency data is much supported by the observed hydrocarbon effects. Penetration of hydrocarbon into the hydrophobic part of the aggregates increases the volume v , which in turn raises the CPP. Let us assume that the optimal surfactants have zero curvature toward triglyceride and water and consequently a CPP = 1. Then the more hydrophobic surfactants have CPP > 1and the more hydrophilic surfactants CPP < 1. Since a CPP of 1is desired, it is evident that the addition of decane, which enhances the CPP, is beneficial to the surfactants with CPP < 1 and unfavorable for surfactants with CPP > 1, just as the results indicate. This change in curvature is also evident from many ternary phase diagrams with the components water-surfactant-hydro~arbon.~~If one starts from the L1 area
Figure 4. Removal of tripalmitin after 10 min by 0.05% NFE,, and 0.03% of different 1-alcohols.
(micellar solution) a t the water-surfactant edge and increases the hydrocarbon content, i.e., moves against the hydrocarbon corner, one normally passes a D (lamellar) phase and finally enters a L2 phase (reversed micellar solution). The curvature of the surfactant aggregates is thus convex toward water in hydrocarbon-free solutions and then passes through zero and is concave near the hydrocarbon corner. The curvature changes in these systems due to other factors, such as temperature and salt content, are discussed in ref 24. Nonionic Surfactants, Alcohol Addition. Normal alcohols (Cz,4,s,a)from a homologous series were added to a surfactant solution containing NFE,,. In Figure 4 the results pertaining to the removal of tripalmitin are given as a function of the number of carbons in the alcohol. The removal clearly increases markedly with increasing hydrocarbon chain length of the alcohol. Since "Elo is more hydrophilic than the optimal surfactant, this result is in line with the results obtained with decane and is according to expectation for the model based on the packing in the surfactant aggregates. The smallest alcohol, ethanol, is water miscible and is therefore not expected to interfere significantly with the surfactant aggregates a t these concentrations. With increasing number of carbon atoms in the alcohol, the water solubility decreases and the tendency for incorporation in surfactant aggregates increases. Since there is a hydrophile-lipophile gradient in nonionic surfactant aggregates, the less polar alcohols are expected to accumulate in the interior of the aggregates and the more polar alcohols nearer to the hydrophilic moiety of the aggregates. The resulting effect is that the higher alcohols to a greater extent increase the volume of the hydrophobic part of the aggregates and hence enhance the value of CPP. Since NFE,, is more hydrophilic than the optimal surfactant, its CPP is less than 1and the long-chain alcohols decrease the curvature of the surfactant aggregates and allow a more efficient packing a t the soil/water interface. It is indeed observed that rather small amounts of long-chain alcohol induce a transition from spherical micelles to lamellar liquid crystalline phase.39 No studies regarding hard surface cleaning have been found in the literature when combinations of nonionic surfactants and hydrocarbons or alcohols were used. Cox et al.@investigated the effect of partially replacing nonionic surfactants by butyl glycol on the removal of grease soil. The general result was a deterioration of the cleaning, (38)Kuneida, H.; Shinoda, K. J.Dispersion Sci. Technol. 1982,3,233. (39)Ekwall,P.;Mandell, L.; Fontell, K. Mol. Cryst. Liquid Cryst. 1969,8,157. (40) Cox,M. F.; Mataon, T. P. J. Am. Oil Chem. SOC.1984,61,1273.
Backstrorn et al.
876 Langmuir, Vol. 4,No. 4, 1988 100
90
ao 70
-ae m
E
a
60 50 40
30 20
10
0.2
0.0
Weight fraction of C12ES
Figure 5. Removal of triolein after 10 min by different combinations of CEE8and NaDBS. The totalsurfactant concentration is 0.05%.
claimed to be due to dilution effects of the surfactant. Butyl glycol is highly water soluble at room temperature. The conclusion from the experiments with decane and alcohol addition is that the removal by a hydrophilic surfactant can be greatly enhanced by small additions of hydrophobic substances. This behavior is argued to be due to changes in the spontaneous curvature of the surfactant aggregates, giving better ability to form dense aggregates a t the soillwater interface. Mixtures of Nonionic and Anionic Surfactants, Studies have been performed in hard surface cleaning concerning the effect of mixing nonionic and anionic surfactants, but no general picture has emerged. Some authors41 find no synergistic effects between the surfactants, while others do.424 It was therefore interesting to see if any synergistic effects between different surfactants could be discovered by our technique. In Figure 5 the effect of mixtures of C12E8and NaDBS on the removal of triolein from PVC is shown. The total surfactant concentration was 0.05%. As the nonionic surfactant is partially substituted by the anionic surfactant, the removal is decreased down to approximately zero for low nonionic surfactant content. In fact, the removal is lower than expected if the nonionic surfactant is diluted, so the anionic surfactant alone has an antagonistic effect on the nonionic surfactant. The cmc values of the two surfactants are 0.0038% (C12E8)and 0.042% (NaDBS).45 It should be noted that the anionic surfactant alone has no effect, although its concentration is above (yet rather near) the cmc. Aronson et al.41obtained similar results when they removed mineral oil drops from plastic films by nonionic surfactants and mixtures of nonionic and anionic surfactants. The anionic surfactant deteriorated the removal. They also measured the interfacial tension between the oil and the surfactant solution and found that the interfacial tension increased with increasing concentration of the anionic surfactant. As a lowering of the interfacial tension is a result of surfactant adsorption at the interface, the presence of anionic surfactant seems to disturb the adsorption of nonionic surfactant. Schwuger and Smolka& studied the adsorption of mixtures of SDS and octa(oxy(41) Aronson, M. P.; Gum, M. L.; Goddard, E. D. J.A m . Oil Chem. SOC.1983,60, 1333. (42) Mankowich, A. M. J.Am. Oil Chem. SOC.1963,40, 674. (43) Mankowich, A. M. J. Am. Oil Chem. SOC.1964,41, 47. (44) Mankowich, A. M. J. Am. Oil Chem. SOc. 1964,41, 449. (45) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems, NSRDS-NBS 36; US. Government Printing Office: Washington, D.C.,1971. (46) Schwuger, M. J.; Smolka, H. G. Colloid Polym. Sci. 1977,255, 589.
0.4
1 .o
0.8
0.6
Weight fraction of Cl2E8
Figure 6. Removal of palmitic acid after 5 min by different combinations of ClzEsand NaDBS in 0.1% NaCl. The total surfactant concentration is 0.05%, with pH =7.
1
2
3
4
5
6
7
8
9
Figure 7. Ftemoval of triolein after 4 rnin by 0.05% C12E5 or Cl2ES and 0.05% of different ionic surfactants. Water hardness (within parentheses) is 0 or 8: 1, C12E5 (0); 2, C12E5 + C12S03Na(0); 3, C12E5+ CTAC (0); 4, C12E5 (8); 5, C12E5 + C12S0W(8);6,C12E5 + CTAC (8); 7 , C12E8(0); 8, Cl2ES(8);9, C12Es+ CI2SO3Na(8).
ethylene) octylphenol ether (OFEJ on charcoal. They showed that a t equilibrium the adsorption of the nonionic surfactant is superior and also that SDS adsorbs more initially but will then be interchanged with the nonionic surfactant. These results do not agree with the results obtained by M a n k o w i ~ h .He ~ ~studied ~ the removal of asphalt from steel panels by a surfactant builder mixture, containing 0.39% OFE,,, 0.44% anionic surfactant (SDS, sodium oleate, or sodium alkylbenzenesulfonate), and 6.8% inorganic builders (sodium metasilicate and sodium phosphate). All three anionic-nonionic combinations removed asphalt better than the nonionic or anionic surfactant alone. One complication is, however, that the single surfactant solutions contained less surfactant than the mixtures, so the results could partially be explained by dilution effects. T r a ~ t m a n nalso ~ ~found synergistic effects between nonionic and anionic surfactants in removal of a fat mixture from PVC and ceramic surfaces. The total surfactant concentration was constant (0.15%), and the best results were obtained for mixtures in which the anionic content was 60-70%. Evidently there are some factors in these studies with synergistic effects that favor removal by anionic surfactants. In these cases the removal by a single anionic surfactant was comparable to the removal by a single nonionic surfactant. Important factors may be the higher surfactant concentration, the content of some polar compounds in the dirt, and also the test method used. Different dirt removal procedures can change the relative (47) Trautmann, M. Tenside Detergents 1985,22, 311
Langmuir, Vol. 4, No. 4, 1988 877
Triglyceride Removal from Polymer Surfaces Table 11. Cloud Point (in "C) in Relation to Water Hardness water hardness substance 0 a C12E6+ C12S03Na C12E5 + CTAC
I
A
60
Q
C12En
+ C12E4Ci2E8
~~
~
C12E6
80
35 >lo0
>loo
34 cloudy at 25
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importance of the contributing factors, such as wetting, solubilization, and so on, due to differences, for example, in agitation, time, and soil deposition. In order to investigate a system where the anionic surfactant shows good performance, some conditions were changed. As dirt the polar solid substance palmitic acid was chosen, and 0.1% NaCl was incorporated in the cleaning solution. The results are shown in Figure 6. Also, it is clear that the pure nonionic surfactant is more efficient than all combinations tested. To avoid dilution effects of the nonionic surfactants, their concentrations were held constant, and various ionic surfactants were added in equal amounts. The results are shown in Figure 7. The dirt is triolein, and experiments were performed both in distilled water and in water of moderate hardness (degree of hardness 8). Two observations can be made directly. The first is that the ionic surfactants impede the removal by the nonionic surfactants in all cases studied. The second is that when the well-balanced C12E5 is included in the surfactant solution the behavior is improved in the presence of Ca2+ions (see above). The incorporation of ionic surfactants naturally changes the properties of the nonionic surfactant solutions. The cmc values of C12E5, C12S03Na,NaOle, and CTAC are 0.0028%, 0.2390, 0.076%, and 0.045%, r e ~ p e c t i v e l y . ~ ~ Schick and Manning48 showed that the cmc of a mixture of a nonionic and anionic (SDS) surfactant increases only slowly when the weight fraction of the anionic surfactant is increased from 0 to 0.9 and then increases sharply up to the cmc of the ionic substance. Here a t least the cmc values of NaOle and CTAC are well below the concentration used (0.1%). The results cannot be explained by an increase in cmc above the concentration used. The cloud points for the surfactant solutions are shown in Table 11. CTAC and C12S03Naraise the cloud point considerably in distilled water and CTAC also in the water containing Ca2+. The solution containing calcium and NaDBS is cloudy, due to the Krafft point raising effect of Ca2+(low solubility of Ca(DBS)2). Again, the removal results seem to be explained from the packing and curvature point of view. C12E5 is very efficient for removing triolein from PVC, as described earlier, probably due to good packing properties a t the surface; the CPP is slightly below 1. The ionic surfactants have large surface areas due to electrostatic repulsion and increase the surface area of the aggregates more than the volume of the hydrocarbon moiety. Introduction of ionic surfactants in the aggregates therefore increases the curvature of the aggregates (decreases CPP), and consequently the removal efficiency decreases. Mixtures of Nonionic Surfactants. In emulsion technology it is knownMthat the stability of an oil-in-water emulsion is affected by the distribution of the poly(oxyethylene) chain lengths of a nonionic surfactant mixture. A polydisperse surfactant yields more stable emulsions than a monodisperse one, and a mixture of a mainly lipophilic and a mainly hydrophilic surfactant gives better emulsion stability than a single surfactant. It is common (48) Schick, M. J.; Manning, D. J. J.Am. Oil Chem. SOC.1966,43,133.
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EOgroups in surfactant
Figure 8. Removal of triolein after 4 min by 0.05% Cl2E, with different average lengths of the poly(oxyethy1ene) chain (a, monodisperse C12E,; +, combinations of C12E4and C12E8).
practice in many hard surface cleaning applications to use blends of several nonionic surfactants in order to obtain better p e r f ~ r m a n c e . ~The ~ underlying idea is that the detergency process consists of many different steps, and that the optimal surfactant is not the same in every step. For example, it may be argued that the more hydrophobic surfactant may have better adsorption and oil penetration properties, whereas the more hydrophilic surfactant may have better dispersion stabilization properties. In two studies the removal of oil/pigments," grease, and wax oilrnby mixtures of nonionic surfactants with the same hydrophobic parts and different hydrophilic parts was examined, but no general conclusions could be drawn from the results. According to Kubitschek and S ~ h a r e the r~~ maximal removal was obtained with a certain mixture, but these authors did not compare the results with those of the pure surfactants, which also give maximal performance at a certain EO content. Cox and Matson40 found that mixtures behave better than single surfactants in the removal of various soils, but since the surfactant concentration was as high as 590,the result is not relevant for normal detergency. To increase the understanding of these matters, removal studies were made in which monodisperse CI2E, was compared to different mixtures of C12E4 and C12E8.The mixing was done in such a way that the average EO content of the surfactant molecules corresponded to the various monodisperse surfactants investigated. The removal of triolein by 0.05% surfactant solutions is shown in Figure 8. It is obvious that the two curves almost coincide and thus that no advantage is obtained by mixing nonionic surfactants with different EO chain lengths, a t least when the dirt is homogeneous. This finding agrees with the model based on surfactant packing since the CPP would be the same for a single surfactant and a mixture of surfactants having on average the same EO groups as the single surfactant (average value of a the same in the two cases).
Conclusions The results show that the performance of a surfactant is very sensitive to other components in the cleaning solution and that this sensitivity is dependent on the type of surfactant. Knowledge about how various additives affect the removal behavior tells something about the mode of action of the surfactants. One example is the influence of sodium chloride. The anionic surfactants increase their removal capacity dramatically in the presence of salts. This shows that electrostatic forces have an influence on (49) Kubitschek, H. E.; Scharer, D. H. Soap Cosm. Chem. Spec. 1979, 8, 30.
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the removal action. The removal behavior of the nonionic surfactants on the other hand is not appreciably affected by moderate amounts of sodium chloride. In contrast to inorganic salts, organic substances may have a dramatic effect on the performance of the nonionic surfactants. The hydrophilic-lipophilic balance of the additives and the surfactants is then very important. From the results it is also possible to draw conclusions of direct value in practical cleaning. However, the properties of the soil and the surface markedly affect the reyoval process, and it is therefore important to remember that the results are obtained with hydrophobic dirt on hydrophobic surfaces. The results are most useful in those fields where removal of oils and fats from plastic surfaces is concerned. The following conclusions are important: Nonionic surfactants having cloud points moderately above the experimental temperature are superior to single-chain ionic surfactants. The size of the hydrophilic and hydrophobic parts of the surfactant must be balanced in such a way that the surfactant aggregate curvature against water, soil, and substrate is as close to zero as possible. For a pure nonionic surfactant that means that the cloud point should be near or below the temperature used. The efficiency of a nonionic surfactant may be much enhanced by addition of hydrocarbons or other hydrophobic compounds. For this improvement to take place it is required that the surfactant be more hydrophilic than the optimal surfactant without addition of hydrocarbon. The efficiency of a single-chain ionic surfactant may be enhanced by addition of inorganic salts. Combinations of nonionic and ionic surfactants give no improvement compared to a single nonionic surfactant. Combinations of nonionic surfactants give no improvement compared to a single nonionic surfactant. While the present study thus has provided some information related to the efficiency of different surfactants and surfactant-additive mixtures in hard surface cleaning, a more important goal of future work is to present models which can rationalize experimental results. In this way
good understanding of the packing of aggregated surfactant molecules under various conditions, which has been obtained in experimental and theoretical work during the last decades, can be used efficiently in the formulation of cleaning products. In the present work we have been able only to touch very moderately on this important matter. The cleaning process is certainly a very complex one, and it would be presumptuous to believe that a single simple model would be able to rationalize the large body of observations made in the field. In the present work the cleaning process is studied in a simple and rather welldefined way. An attempt has been made to compare the experimental results with simple geometric and electrostatic features of surfactant packing, which have been so successful in recent years in rationalizing, for example, micelle shape, phase diagrams, and microemulsion structure. The agreement between experimental results and model is throughout good, and although the comparison is simple-minded and mainly qualitative the approach seems encouraging for future studies.
Note Added in Proof. F. Schambil and M. J. Schwuger (Colloid Polym. Sci. 1987,265,1009) have recently studied the removal of oil from fabrics by nonionic surfactants and obtained results which in important respects parallel those obtained in the present work for hard surface cleaning. This would indicate a marked generality of the cleaning mechanism. Schambil and Schwuger correlated their results with the phase behavior of ternary systems. On the basis of the established relation between phase behavior and surfactant molecular packing we would expect the model presented above to be of interest also in types of cleaning other than hard surface cleaning. Acknowledgment. Financial support was obtained from the Swedish Work Environment Fund. Registry No. TO, 122-32-7;TP, 555-44-2; PA, 57-10-3;NFE,, 9016-45-9;DBS-Na, 25155-30-0; NaOle, 143-19-1;CTAC, 112-02-7; NTAeNa, 10042-84-9; CI2SO3N,, 2386-53-0; NaC1, 7647-14-5; calcium chloride, 10043-52-4;decane, 124-18-5;ethanol, 64-17-5; butanol, 71-36-3; hexanol, 111-27-3; octanol, 111-87-5.
Enhancement of Hydrogen Bonding in Vicinal Water: Heat Capacity of Water and Deuterium Oxide in Silica Pores Frank M. Etzler* Picosecond & Quantum Radiation Laboratory, Texas Tech University, Lubbock, Texas 79409 Received December 30, 1987. I n Final Form: March 22, 1988 The heat capacities of water and deuterium oxide confined to silica pores of various radii at 25 O C are reported. Pore radii were varied from 1.2 to 12.1 nm. The data are discussed in terms of their implication for the structure of vicinal (interfacial) water. The results show that the heat capacity of both H20and DzO passes through a maximum near 7 nm when plotted as a function of pore radius. In the case of D20 the maximum appears to be less pronounced than that of HzO. The relative magnitudes of the observed heat capacity maxima can be understood by using the model for vicinal water proposed earlier by this author.
Introduction The properties of liquid water are known to be significantly modified by propinquity to solid surfaces. DrostH a n ~ e n l - in ~ ,particular ~~ has reviewed the properties of *Present address: Institute of Paper Chemistry, Appleton, WI 54912.
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vicinal (interfacial) water and discussed their biophysical significance. LOW^-^ has measured and reviewed the (1)Drost-Hansen, W. Ind. Eng. Chem. 1969, 61, 10.
(2) Drost-Hansen, W. In Chemistry of the Cell Interface, Part B; Brown, H. D., Ed.; Academic: New York, 1971. ( 3 ) Drost-Hansen, W. In Biophysics of Water; Franks, F., Ed.; Wiley: New York, 1982.
0 1988 American Chemical Society
