Langmuir 1996,11, 2515-2524
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Interaction of Alkanes with Monolayers of Nonionic Surfactants R. Aveyard, B. P. Binks, P. D. I. Fletcher,* and J. R. MacNab Surfactant Science Group, School of Chemistry, University of Hull, Hull HU6 7RX, U.K. Received October 24, 1994. In Final Form: February 21, 1995@ We have investigated the effects of adding small quantities of liquid alkanes to the surfaces of aqueous solutions of nonionic surfactants of general structure H(CH2),(OCH&H2),0H (abbreviated to C,E,). A range of surfactant head and tail lengths was studied (n= 10,12,and 14 and m = 5,7, and 9); surfactant concentrationswere in excess ofthe critical aggregation concentration in water. Short chain length alkanes spread on the aqueous solutions whereas long chain alkanes form lenses in equilibriumwith the surfactant monolayer containing adsorbed oil. The equilibrium spreading coefficients (derived from tension measurements) were found to be zero within the experimental uncertainty of about 0.3 mN/m for all the alkane surfactant combinations investigated. The near-zero equilibrium spreading coefficients arise because adsorption of the alkanes into the chain region of surfactant monolayers at the solution-air surface causes the surface tension to decrease to a value close to the sum of the oil-air plus oil-water tensions. Small droplets of alkanes of various concentrations in a nonadsorbing diluent (squalane) were placed on surfactant solutions, and the tension lowering was recorded. Analysis of the tensions using the Gibbs adsorption equation yielded the extent of adsorption as a function of alkane activity. The adsorption isotherms so obtained approximate to those for ideal 2-D gaslike monolayers for weakly adsorbing long chain alkanes; i.e., the adsorption increases linearly with activity. Shorter alkanes show larger degrees of adsorption for a given oil activity and the isotherms correspond to the formation of multilayer films. For spreading oils, the apparent maximum values of adsorption correspond to oil film thicknesses of a few nanometers even though the spread films show interference colors (indicating the actual film thicknesses are '100 nm). It is argued that this apparent maximum extent of adsorption may correspond to the surface concentration of oil which is significantly energetically different to bulk oil through association with the oil-water interface.
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Introduction The interaction of oils with surfactant monolayers has important consequences for a wide range of phenomena, including microemulsion formation, and is of direct relevance to technological applications such as detergency and solubilization. Of prime interest is the extent to which oils associate with and penetrate surfactant monolayers a t the oil-water interface and how this is related to the molecular structures of both oil and surfactant. In this paper we discuss the information that can be obtained using surface tensiometry of aqueous surfactant solutions onto the surfaces of which small quantities of oils have been placed. Data are presented for a range of linearchain, liquid alkanes with nonionic alkylpoly(oxyethy1ene) surfactants incorporating a range of hydrophilic head groups and hydrophobic tail groups. An oil added to the surface of an aqueous surfactant solution can spread to either form a continuous oil film over the surface or form discrete lenses. The thermodynamic tendency to spread is related to the magnitudes of the three tensions in the system. The equilibrium spreading coefficient of oil on a n aqueous phase (Sow(eq)) is defined as
However, for the systems studied in this work, yaowas not altered by addition of a small quantity of aqueous surfactant solution to the air-oil surface. The equilibrium spreading coefficientSow(eq) is negative for lens formation (nonspreading) and zero for spreading. The value of So,(eq) cannot be p0sitive.l In a system for which Sow(eq)is zero, the addition of the spreading oil to the surface will cause the measured tension to fall from that of the aqueous solution to the tension ofthe composite film including both the surfactant monolayer and the oil film. Small quantities of added oil will yield thin films (say less than a few nanometers) for which the strength of interaction between the oil-water and air-oil interfaces may be sufficient to cause the measured tension to be significantly different to the sum of the bulk phase tensions (yao yo,). The addition of more oil will cause the film to thicken and the measured composite film tension to approach the sum (Yao yo,). Film excess tensions arising from colloidal interactions across thin aqueous films have recently been found to be of the order of 0.01 mN/m for films with thicknesses of a few nanometem2 Hence, the composite film tensions of spread oil films showing interference colors (for which the film thickness is > 100 nm) are expected to be equal to the sum (yao yaw), For cases where the oil does not spread macroscopically, adsorption of the oil from the bulk oil in the lens onto the surfactant monolayer surface can nevertheless occur. At equilibium, lenses coexist with a mixed film of oil and surfactant which can either be a mixed surfactantloil monolayer or a surfactant monolayer with a multilayer of oil. This latter case has been observed for alkanes on
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+
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where yaw, yao,and yow are the equilibrium air-aqueous solution, air-oil, and oil-solution tensions, respectively. As will be seen later, the equilibrium value of yaw (measured in the presence of oil) is generally appreciably different from the air-solution tension of a surfactant solution measured in the absence of oil. In principle, a similar difference may exist between the equilibrium value of yao(measured in the presence of added aqueous phase) and the oil-air tension in the absence of aqueous phase.
* Abstract published in Advance A C S Abstracts, July 1, 1995.
(1) Rowlinson, J. S.; Widom, B. Molecular Theory of Capillarity, InternationalSeries o f Monographs on Chemistry8, Oxford University Press: Oxford, 1989; p 216. (2) See for example: Bergeron, V.; Fagan, M. E.; Radke, C. J . Langmuir 1993, 9,1704.
0743-746319512411-2515$09.00/0 0 1995 American Chemical Society
2516 Langmuir, Vol. 11, No. 7, 1995
aqueous solutions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)using ellipsometry to obtain the thickness of the adsorbed oil film.3r4 The former case, in which oil lenses coexist with a mixed monolayer ofoil and surfactant, has been investigated for a range of apolar oils with various ionic surfactants. For these systems, the amount of oil adsorbed was estimated from measurements of the decrease in the surface tension ( A y )following the addition of oil to the solution ~ u r f a c e . ~Neutron ,~ reflection measurements of a selection of the same systems subsequently confirmed the measurements of the amount of oil adsorbed and provided the first determinations of the microstructure of mixed oillsurfactant films.'t8 Clearly, surface tension measurements of surfactant solutions in the presence and absence of oil can provide important fundamental information about the interaction of oils with surfactant monolayers. However, studies to date have been limited to ionic surfactants with alkane^^-^ since, for these systems, the surfactant does not partition into the oil phase to any significant extent. Surfactant partitioning from aqueous solution to the added oil drop is expected to cause severe artifacts when measuring the tension decrease A y following addition of oil drops to the surface. Nonionic surfactants of general structure H(CHdn(0CH2CH2),0H (abbreviated to C,E,) are known to partition from water to oil to a significant e ~ t e n t . ~ JInO this paper we discuss how artifacts due to surfactant partitioning to the oil can be overcome in the case of nonionic surfactants of the C,E, type and describe oil spreading and adsorption for a range of liquid n-alkanes with aqueous solutions of various C,E, surfactants.
Experimental Section Materials. The nonionic surfactants were obtained from the following sources: C12E5 (Nikko), C12E7 (Nikko), Cl2E9 ('98% purity, Fluka), C1& (>97% purity, Fluka), and C1& (Nikko). All Nikko samples showed only a single detectable GC peak in the manufacturer's analyses. Sample purity was further tested by comparisonof the measured cloud point of an aqueous solution of the surfactant with literature values.ll The values agreed within 0.3 "C or better in each case. The alkanes were obtained from Fisons (heptane), Fluka (octane, nonane, and decane), Aldrich (undecane, dodecane, tridecane, tetradecane and pentadecane), and BDH (hexadecane)and were of greater than 99% purity. Squalane (2,6,10,15,19,23-hexamethyltetraetracosane, C&2, 99% purity) was obtained from BDH. Dinonyl phthalate (bis(3,5,5-trimethylhexyl) phthalate, abbreviated as DNP) was obtained from Fluka and was a technical grade which contained a mixture of isomers. All the oils were passed over an alumina column prior to use to remove all polar impurities. Measured oil-air tensions of all the alkanes agreed with literature values12 within 0.1 mN/m. Water was purified by reverse osmosis and further treated with a Milli-Q reagent water system. (3)Kellay, H.; Meunier, J.; Binks, B. P. Phys. Reu. Lett. 1992,69, 1220. (4) Kellay, H.; Binks, B. P.; Hendrikx, Y.; Lee, L. T.; Meunier, J.Adu. Colloid Interface Sci. 1994,49,85. ( 5 ) Aveyard, R.; Cooper, P.; Fletcher, P. D. I. J.Chem. SOC.,Faraday Commun. 1990,86,211. (6) Aveyard, R.;Cooper, P.; Fletcher, P. D. I. J.Chem. Soc.,Faraday Trans. 1990,86,3623. (7) Lu, J . R.; Thomas, R. K.; Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Sokolowski, A.; Penfold, J . J.Phys. Chem. 1992,96,10971. (8)Lu, J . R.; Thomas, R. K.; Binks, B. P.; Fletcher, P. D. I.; Penfold, J . J. Phys. Chem. 1995,99,4113. (9)Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Langmuir 1989,5, 1210. (10)Aveyard, R.;Binks, B. P.; Clark, S.; Fletcher, P. D. I. J . Chem. SOC.,Faraday Trans. 1990,86, 3111. (11)van Os,N.M.; Haak, J. R.; Rupert, L. A. M. Physico-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: Amsterdam, 1993. (12)Selected Values of Properties of Hydrocarbons and Related Compounds; A.P.I. Project 44, Chemical Thermodynamics Properties Centre: College Station, TX,1966; Vol. 11.
Aveyard et al. Methods. Surface tensions of aqueous solutions were measured using a Kriiss K10 du Nouy ring instrument. This apparatus measures the static, maximum pull exerted on a du Nouy ring and thus ensures that the contact angle formed by the aqueous solution with the ring is zero. The standard method of determining the tension decrease following the addition of a few drops of oil to the surface of the aqueous surfactant solution was as follows. The tension of the aqueous surfactant solution in the absence of oil was determined in the usual way. The liquid surface was then raised slightly with the meniscus still attached to the ring and a smallvolume ofoil (typically lOpL, although variation of the exact amount added did not affect the results) was added to the liquid surface using a microliter syringe. A double-walled glass lid (thermostated and fitted with a small hole in the top to allow the shaft ofthe ring through) was then placed in position to ensure good temperature control and to minimize evaporation of the added oil. For either the short chain length, volatile alkanes, or the mixed oil systems, a filter paper soaked in the oil was fitted inside the glass lid to ensure saturation of the vapor space above the aqueous solution. The new (lower)tension of the aqueous solution was then measured. Oil-water interfacial tensions were measured using a Kriiss Site 04 spinning drop tensiometer. An oil drop containing the equilibrium concentration of surfactant (estimated as described later in the text) was injected into the rotating capillary of the instrument containing an aqueous solution of the surfactant a t the required concentration. Densities and refractive index values required for the calculation of the tension were taken from the literature.13 Spreading behavior was determined by visual examination of oil drops (typically 40 pL) added to the surface of aqueous surfactant solutions in a stoppered dish in which both the dish and lid were thermostated. Careful thennostating and sealing of the vessel are necessary to ensure the absence of either evaporation or temperature gradients which can lead to artifacts. As for the tension measurements using the du Nouy ring, for the addition of either pure volatile oils or oil mixtures it was necessary to add filter papers soaked in the oil to ensure saturation of the vapor space. Oils added to the surface generally spread initially. For certain systems the oil film retracted after a minute or so to form one or more circular lenses which were clearly visible on the surface and which were stable for extended times. Oil/ surfactant systems for which such lenses were observed were classified as nonspreading. Other systems, classified as spreading, showed no retraction to lenses even when left for hours. The spreading systems showed regions of oil film on the surface for which interference colors were visible. These colored film regions were observed to move continuously over the aqueous surface despite all precautionstaken to ensure the absence of temperature gradients or draughts in the observation cell. All spreading and tension measurements were made a t 25 "C.
Results and Discussion Effects of Surfactant Partitioning to the Oil Phase. Firstly, we briefly review the relevant phase
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behavior of C,E, type surfactants in alkane water mixtures (see, for example, refs 9, 10, and 14). At low surfactant concentrations, below a critical concentration required for aggregation, only surfactant monomers are present in both the oil and water phases. At concentrations above this, additional surfactant leads to the formation of microemulsion aggregates in either the aqueous phase, the oil phase, or a third phase. At low temperatures, sufficiently below the so-called phase inversion temperature (PIT), a Winsor I system is formed consisting of an oil-in-water microemulsion in equilibrium with a coexisting oil phase. Sufficiently above the PIT, a Winsor I1 system is formed consisting of a water-in-oil microemulsion phase plus a n excess water phase. Over a temperature range centered on the PIT, a third-phase (13) Handbook OfChemistryandPhysics, 62nded.; CRC Press: Boca Raton, FL, 1981. (14)Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Schomacker, R. Langmuir 1988,4,499.
Langmuir, Vol. 11, No. 7, 1995 2517
Alkane-Surfactant Interaction microemulsion coexists with excess oil and excess water phases (Winsor I11system). In order to prevent surfactant aggregate transfer to the oil phase, it is necessary that the oil water surfactant mixture forms a Winsor I two-phase system at equilibrium. For this reason, the present study (made at 25 "C)was restricted to nonionic surfactant oil combinations for which the PIT (values given in Table 1)is sufficiently in excess of 25 "C. In addition to possible transfer to the oil ofthe surfactant aggregates, it is also necessary to consider the equilibrium concentration of surfactant monomers in the oil phase. For Winsor I systems, the critical aggregation concentration required for the formation of microemulsion droplets in the aqueous phase is generally very similar to the critical micelle concentration (cmcwater) found in binary water surfactant mixtures in the absence of oil.l0 In oil water two-phase systems, monomers of nonionic surfactants of the C,E, type of the chain lengths used in this work distribute strongly in favor of the oil. Hence, a Winsor I system consists of an aqueous phase containing aggregates and a monomer concentration equal to cmcwater coexisting with an oil phase containing surfactant monomer at a concentration given approximately by the product of the partition coefficient and cmcwater.This equilibrium oil phase concentration is designated c*,il. Both CmCwakr and c*,il are expected to be virtually independent of the concentration of aggregated surfactant in the aqueous phase of a Winsor I system. Values of CmCwakr and C*oil for the systems investigated here are summarized in Table 1. Following the addition of a drop of a pure alkane to the surface of an aqueous C,E, solution (for which a Winsor I system is formed at equilibrium), it is thus expected that monomeric surfactant will transfer to the oil until the oil phase concentration reaches c*,il. Oil adsorption should cause the surfactant solution surface tension to decrease by an amount Ay equal to the surface pressure of the adsorbed oil film. Preliminary experiments were made in which pure alkane drops were added to aqueous solutions of nonionic surfactants and the tension changes measured. Particularly for low aqueous phase surfactant concentrations (only just in excess of cmcwater), it was commonly observed that the apparent values of Ay can be negative; i.e., the addition of oil causes the tension to increase. A likely explanation is that surfactant transfers from the water to the oil leading to a decrease in the aqueous phase surfactant concentration to below cmcwater and a concomitant rise in tension. We depict in Figure 1 the surface tensions of C12E7 solutions at various aqueous phase concentrations in excess of cmcwater in the absence of dodecane, with lenses of pure dodecane and with dodecane containing 0.15 wt % of the surfactant (equal to the value of c*,il). In the presence of pure dodecane, the apparent value of Ay (given by the difference between curves i and ii increases with increasing aqueous phase surfactant concentration. Since effects due to aqueous phase surfactant depletion leading to apparently low values of Ay are likely to be lessened at higher aqueous phase surfactant concentration, the data appear to be consistent with the proposed explanation. In the presence of dodecane containing a surfactant concentration equal to c*"il, Ay (given by the difference between curves i and iii) is independent of surfactant
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+
+
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(15) Aveyard, R.; Lawless, T. A. J. Chem. Soc., Faraday Trans. 1
1986,82,2951. (16) Kunieda, H.; Shinoda, K. J. Dispersion Sci. TechnoZ. 1982, 3, 233. (17) Kunieda,H.; Shinoda,K. J.CoZZoidInterfaceSci. 1985,107,107 (18) Kahlweit, M.; Strey, R.; Firman, P. J.Phys. Chem. 1986, 90, fi7l. - . -.
(19) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989.
Table 1. Summary of PIT Values and Critical Aggregation Concentrations (at 25 "C) for the Surfactant Oil Systemsa alkane CmcwatelJ C*oil/ PIT/ surfactant chain length mM wt% "C 7 1.29 29 0.064 C12E5
+
(no oil) 29.8 mN/m at 6.4 mM yaw
Ci2Ei
Yaw
(no oil)
32.5 mN/m at 5.0 mM
8 9 10 11 12 13 14 16 7 8 9 10 11 12 13 14 16
0.050
7
0.10
Cl2E9 yaw
8 9 10 11 12 13 14 16 7-16
(no oil)
34.8 mN/m a t 1.0 mM
CioEi Yaw (no oil)
0.95
33.8 mN/m at 9.5 mM 7-16
C14E7
0.0095
1.13 1.01 0.91 0.81 0.76 0.70 0.65 0.57 0.26 0.22 0.20 0.18 0.16 0.15 0.14 0.13 0.11 0.043 0.038 0.034 0.030 0.028 0.025 0.023 0.022 0.019 0.3
32 35 38 40 43 44 46 50 58 61 64 66 68 71 74 76 81 72 75 77 79 82 84 86 89 94 >60
used for all alkanes
for all alkanes
0.3
>40 for all
used for all alkanes
yaw (no oil) 33.3 mN/m at 0.95 mM
alkanes
a PITvalues were takenfrom refs 11and 15-18, cmcwate,values from refs 11and 19, and c*,il values from refs 20 and 21.
35
.
25 -10
-8
-6
-4
In ([C,,E,I,JIM
Figure 1. Variation of yawwith aqueous phase concentration of C12E7. The curves refer to (i) the absence of added oil, (ii) the addition of pure dodecane, and (iii)the addition of dodecane containing 0.15 w t % C12E7, equal to C*oil.
concentration (pOst-cmhater).A similar independencehas been noted for the addition of alkanes to aqueous ionic surfactants for which surfactant partitioning to the oil does not occur.6
Aveyard et al.
2518 Langmuir, Vol. 11, No. 7, 1995
increasing surfactant concentration in the oil until the
I c*,d is reached when the tension should remain constant.
36
26 0
0.2
0.4
0.6 0.8 1 wt% C,,E, in dodecane
Figure 2. Variation of yawin the presence of added dodecane with oil phase surfactant concentration. The curves refer to ClzEs (triangles), C12E7 (circles), and (squares). The I %) and yaw(mN/m)in the absence of oil are values of C * ~ ~(wt 0.76 and 30.1, 0.15 and 33.7, and 0.025 and 35.8 for C1&5, C1&, and ClzEg, respectively. The aqueous phase concentrations of the surfactants were fured at 1.5 times cmcwakrin each
case.
-
E
z
-2 E
0 36
0.2
0.4
1 0.6 0.8 1 wt% C , q in dodecane
I
I
3
' E 30
z
E
This behavior is indeed observed for the different surfactants. As seen in Table 1 and Figure 2, for C12 surfactants with different numbers of ethylenoxy units in the head group, the oil-phase surfactant concentrations a t which the tensions reach constant values are close to the c*,il values taken from the literature.20,21Where necessary, some of the c*,il values were estimated by extrapolation using plots of measured c*,il values versus surfactant chain length. In the absence of measured values for different oil chain lengths, values were estimated assuming that c*,il in mole fraction units for a particular C,E, surfactant is independent of alkane chain length. The value of c*,il (expressed as a mole fraction) has been found experimentally to be the same for C12E5 in heptane, decane, and tetradecane.20 The correlation observed between the oil phase concentration a t which the tension reaches a constant value and the independently estimated c*,il values provides convincing evidence that the interpretation based on surfactant monomer partitioning is correct. Figure 3 shows the variation of tension with oil phase surfactant concentration for systems in which the surfactant tail group chain length and the alkane chain length are varied. The curves for dodecane with C14E7 and C12E7 (Figure 3a)are qualitatively similar to those of Figure 2. Octane and hexadecane on C10E7 solutions show relatively little variation of tension with oil phase surfactant concentration (Figure 3b). This lack of tension change is probably a consequence of the relatively high aqueous phase concentration used for C I O E ~ (approximately 1.5 mM for CloE7 compared with, for example, 0.075 mM for C12E7). As seen in Figure 1,such a high aqueous phase surfactant concentration reduces tension effects due to surfactant partitioning to the oil. Overall, the experimental data demonstrate that Ay for surfactant/oil systems where the monomeric surfactant partitions to the oil phase to a significant degree can be determined reliably by ensuring (i) that the oil phase surfactant concentration is equal to or greater than c*,il and (ii) that the aqueous phase concentration is equal to cmcwateror higher. One note of caution is added here. The maximum value of b y (Ay(max)) is obtained for pure oils for which the equilibrium oil phase mole fraction activity is unity. Incorporation of high concentrations of surfactant (and possibly associated water) may, in principle, reduce the oil activity significantly and this should be remembered when attempting to compare data for different systems. For the systems considered here, the maximum surfactant concentration in the oil phase is 1.3 wt % (corresponding to less than 1mol %) and, hence, these surfactants do not cause a significant reduction of oil activity. SpreadingBehavior of Oils on Aqueous Solutions of C,E, Surfactants. We first discuss the visual observations of the spreading behavior of the various alkane C,E, combinations. When an oil drop (containing a surfactant concentration equal to c*,il) is placed on a n aqueous surfactant solution of C,E, (at a concentration above cmcwater), the oil generally spreads initially. Nonspreading oils retract within about a minute to produce visible stable circular lenses. Oils classified a s spreading did not give lenses but formed irregular patches of spread film showing characteristic interference colors. Table 2 summarizes the visual observations for the various systems. As will be seen later, all the alkane/surfactant combinations tested are rather close to spreading (Le.,
26H 28
24
22 I 0
I
0.1
0.2
0.3
0.4 0.5 wt% ClOE, in alkane
Figure 3. Graph a shows the variation of yawin the presence of added dodecane with oil phase surfactant concentrationfor C1& (circles) and C12E7 (squares). The values Of C*oil (wt %) and yaw(mN/m) in the absence of oil are 0.15 and 33.7 for C1&. For C1&, yaw(no oil) is 34.1 mNlm but no estimate of was available from the literature. Graph b shows similar plots for C10E7 with octane (squares) and hexadecane (circles). The value of yaw(mN/m) in the absence of oil was 34.8 mN/m. The aqueous phase concentrationof the surfactant was fixed at 1.5 times cmcwaterin all cases.
Figures 2 and 3 show the results of a series of experiments in which drops of alkanes containing different concentrations of surfactant were placed on aqueous phases containing surfactant concentrations in excess of cmcwater.From the phase behavior discussed above, it is expected that the measured tension should decrease with
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(20) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Ye, X. J. Chem. Technol. Biotechnol. 1992,54, 231. (21)Shinoda, K.; Fukuda, M.; Carlsson, A. Langmuir 1990,6,334.
Alkane-Surfactant Interaction
Langmuir, Vol. 11, No. 7, 1995 2519
Table 2. Summary of the Spreading Observations at 25 "C for the Surfactant + Oil Svstems
surfactant
oil chain length
oil spreads ?
7
Yes no no
8 9 10 7 8 9 10 8 9 10 11 11 12 13 14 8 9 10 11 12 13
I
12,
no
no no no
no Yes Yes Yes no Yes Yes no no Yes Yes no no no no
7
9
11
13
15
17
Alkane chain length
-
12 I
-3
-2
I
7
9
11
13
15
17
Alkane chain length
Sow(eq)is close to zero) and hence the oil lenses formed are generally rather flat. The flatness of the equilibrium lenses and the initial spreading of all the oils combine to make it difficult to be certain of the equilibrium spreading behavior by visual observation for some of the systems on the border between spreading and nonspreading. We estimate there is a n uncertainty of 2 or 3 in the chain length of the alkane for which the spreading transition occurs. Despite this uncertainty, it can be seen that the alkane chain length a t which the transition from equilibrium spreading behavior to nonspreading occurs is affected by the nature of the surfactant. The spreading behavior of linear alkanes on aqueous solutions of C,E, surfactants can be compared with that for spreading on pure water and on solutions of some ionic surfactants a t concentrations above cmcWater.For pure water, the transition from spreading to nonspreading behavior has been reported to occur a t alkane chain lengths between 6 and 7 a t 24.5 0C,22between 7 and 8 a t 15 0C,23 between 5 and 6 at 24 "C (ref 24, where it was reported that pentane did not apparently spread macroscopically but showed a more complex behavior), and between 6 and 7 for a range of temperature^.^^ At room temperature, the spreading transition was found to be between alkane chain lengths of 9 and 10 for aqueous solutions of alkyltrimethylammonium bromides with chain lengths of 10,12,and 16 carbon atoms.26 At 25 "C, the transition occurred between undecane and dodecane for sodium bis(2-ethylhexyl) sulfosuccinate (AOT) a t a concentration above the cmc in the presence of 30 mM NaC1.27 Surfactant monolayers generally increase the tendency of alkanes to spread (i.e., the transition moves to higher alkane chain lengths). We now discuss the magnitudes of the spreading coefficients in the light of the oil spreading behavior observed visually. Equation 1 defines the equilibrium spreading coefficientin terms of the equilibrium tensions. (22)Johnson, R.E., Jr.; Dettre, R. H. J. Colloid Interface Sci. 1966, 21, 610.
(23)Hauxwell, F.; Ottewill, R. H. J. Colloid Interface Sci. 1970,34, 473. (24)del Cerro, C.; Jameson, G. J. J. Colloid Interface Sci. 1980,78, 362. (25)Takii, T.; Mori, Y. H. J. Colloid Interface Sci. 1993,161, 31. (26)Cooper, P. A. Ph.D. Thesis, University of Hull, 1991; p 122. (27)Aveyard, R.;Binks, B. P.; Fletcher, P. D. I.;Peck, T.-G.; Garrett, P. R. J. Chem. SOC.,Faraday Trans. 1993,89,4313.
Figure 4. Variation of Sow(init) (squares)and S,,(eq) (circles) with alkane chain length for (a) C12E7 and (b) C12Es. All the oils on these two surfactant solutions were judged t o be nonspreading at equilibrium by visual observation. Table 3. Equilibrium Air-Water, Air-Oil, and Oil- Water Tensions and Equilibrium Spreading CoefKcients (units mN/m) at 25 "C for Octane and Hexadecane with the Various Nonionic Surfactants surfactant oil chain length yaw yao yOw S,,(eq) C12E5 CizEi CizEs C10E7 C14E7
8 16 8 16 8 16 8 16 8 16
21.7 28.2 22.1 28.2 22.8 28.2 23.3 29.8 22.0 27.9
21.3 27.0 21.3 27.0 21.3 27.0 21.3 27.0 21.3 27.0
0.051 0.90 0.77 1.28 1.34 1.52 1.75 2.8 0.52 0.80
0.3 0.3 0.0 -0.1 0.2 -0.3 0.25 0.0 0.2 0.1
We can also define a n initial spreading coefficient (SOw(init)) as
where yaoand yow have the same meaning as previously but yaw(nooil) is the tension of the aqueous surfactant solution in the absence of oil. Unlike Sow(eq),S,,(init) can be positive. Figure 4 illustrates the variation of both Sow(eq)and S,,(init) with alkane chain length for CnE, and C12E5. It can be seen that the initial spreading coefficients are positive but that the equilibrium values are all zero within the estimated uncertainty in S (f0.3 mN/m). The visual observation that oil drops generally spread initially on the surfactant monolayers is consistent with the positive initial spreading coefficients. The low values of the equilibrium spreading coefficientsmean that they cannot be used t o predict reliably whether o r not spreading will occur a t equilibrium. Values of S,,(eq) for octane and hexadecane on solutions of C12E9, C10E7, and &E7 (summarized in Table 3) are also all zero within the accuracy. Since intermediate alkane chain lengths are unlikely to show a different behavior, it is concluded that all the surfactant/oil combinations investigated here are rather close to spreading at equilibrium. Inspection of eqs 1and 2 shows that the tension decrease following oil addition to the surface A y is the difference
Aveyard et al.
2520 Langmuir, Vol. 11, No. 7, 1995 12
t
12
10 10
. H
-
'E 2 8
'E
P -P 6
E
v
66
6
4
4
2
2
0
7
9
II
0
13 15 17 Alkane chain length
7
9
11
13 15 17 Alkane chdin length
Figure 5. Variation of Ay(max)for the addition of pure alkane with alkane chain length for CIZtail group surfactants with different head groups at aqueousphase concentrationsin excess of cmcwater(100 times cmcwaerin the case of the nonionic surfactants). In descending order the curves refer to ClzEs, C12E7,dodecyltrimethylammonium bromide, sodium dodecyl sulfate, and C12E5. For each curve, the filled symbols refer to spreading oils and unfilled symbols refer to nonspreadingoils. The data for the ionic surfactants were taken from ref 6.
Figure 6. Variation of Ay(max)for the addition of pure alkane with alkane chain length for C,E7 surfactants and two cationic surfactants of Werent chain lengths at concentrationsin excess of cmcwater.The three upper curves refer to n = 14 (circles),12 (triangles),and 10 (squares). The two lower curves show data for dodecyl-(diamonds)and decyltrimethylammoniumbromide (invertedtriangles)for comparison (data from ref 6). The filled symbols refer to spreading oils and unfilled symbols refer to nonspreading oils.
between the initial and equilibrium spreading coefficients. For spreading oils, Sow(eq)is zero and hence Ay is equal to Sow(init)in this case. For nonspreading oils, b y is greater than Sow(init)by an amount equal to Sow(eq). However, a s seen in Figure 4 and Table 3, the equilibrium spreading coefficients for nonspreading oils are generally close to zero and, hence, A y is not significantly different to S,(init). This signifies that the addition of oil to a surfactant solution generally reduces the air-water tension to a value very close to the sum of yow plus yao. Surface Tension and the Extent of Oil Adsorption onto SurfactantMonolayers. The values of Ay for the addition of a range of spreading and nonspreading alkanes (containing a concentration c*,il of surfactant) are shown in Figures 5 and 6. Data for sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (taken from ref 6) are shown in Figure 5 to allow comparison of results from different surfactant head groups. Different surfactant tail lengths are compared in Figure 6. For the nonionic surfactants Ay for a particular alkane increases with increasing head and tail chain lengths of the surfactant. We now consider how tension measurements can be used to estimate the extent of adsorption of oil onto the surfactant monolayers. At constant temperature,tension change is related to surface concentrations (rifor species i) and changes in chemical potential ( Q i for species i) according to the Gibbs adsorption isotherm.
done previously for the surface of pure water23,28and for water coated with insoluble monolayer^.^^ An alternative method of varying the chemical potential of the adsorbing oil is to add to the surface drops of the oil mixed with nonadsorbing diluent over a range of activity. We then consider the Gibbs equation for a system containing water, surfactant, adsorbing oil, and diluent oil (denoted respectively by the subscripts w, s, ao, and do) in which the mixed monolayer of surfactant plus oil is in equilibrium with lenses of bulk (mixed) oil.
We assume that the addition of the oil mixture to the solution surface causes negligible change to the chemical potentials of either water or surfactant, i.e., dpw and Qs both equal zero. This is certainly justified for water in which the alkanes are virtually insoluble. For the surfactant, we recall that the aqueous phase cmc values for &E5 measured in the absence of added alkane and in alkane saturated solutions have been found to be almost identicallo and thus dps is also likely to be zero. Noting that -dy = dAy, we obtain dAy = rao
@ao
-k rdo
*do
(5)
For a mixed oil drop containing a mole fraction xao of adsorbing oil andxdoof diluent oil, from the Gibbs-Duhem equation we obtain %do = -(XadXdo) dpaoand hence
(3) According to eq 3, in order to obtain the surface concentration of a n adsorbing oil, it is necessary to measure the variation of tension as a function of chemical potential of the adsorbing species. For oil adsorption this can be done by recording the tension for different set partial pressures of oil vapor in equilibrium with the surface as has been
Substituting dpi = kT In ai, where k is the Boltzmann constant, T is the absolute temperature, and ai is the activity of species i, yields (28) Jones, D. C.; Ottewill, R. H. J. Chem. SOC.1955, 4076. (29) Dean, R. B.; Hayes, K. H. J . Am. Chem. SOC.1951,73, 5583.
Alkane-Surfactant Interaction (dAy/d In aao)= kT{rao- (X,JXdo)rdo}
Langmuir, Vol. 11, No. 7, 1995 2521 4,
(7)
The activity of the adsorbing oil in the oil mixture (aao) is equal to the product of the activity coefficient and the mole fraction xao,i.e., faSao. In what follows, the activity scale used is that for which the activity of the pure liquid is unity; Le., the standard state is the pure liquid. For this choice, the activity coefficient f a 0 tends to unity as xao tends to unity. It can be seen from eq 7 that the surface concentration ofthe adsorbing oil Taocan be obtained from measurements of Ay as a function of adsorbing oil activity aao(varied by dilution of the added oil with diluent oil) only if rdo is zero. If r d o = 0, eq 7 becomes (dAy/d In uao)= k T rao
rao(max)= Ay(max)/kT
(9)
In this case, the maximum surface concentration of adsorbed oil can be obtained from a single measurement of the tension lowering following the addition of a drop of the pure oil to the surface. However, verification of the validity of eq 9 requires the measurement of Ay over a range of adsorbing oil activity. Equation 9 was found to be valid for various alkanelcationic surfactant system^.^,^ Adsorption Isotherms for Alkanes on C,E, Solutions. Figure 8 shows the variation of Ay with mole (30)Ashworth, A. J.;Everett, D.H.Faraday SOC.Trans. 1960,56, 1609.
3 -
*
E
%.2 k
1 -
-0
0.2
0.4
0.2
0.4
0.8 Mole fraction
0.6
I
(b)
3 -
"
(8)
The lack of adsorption of the diluent oil onto the pure surfactant monolayer can be checked by measuring Ay following the addition of a drop of the pure diluent oil to the surface. However, a zero Ay for the pure diluent oil does not guarantee that no adsorption of the diluent oil occurs for the mixed adsorbing oillsurfactant films, i.e., when a drop of the mixed oil is added to the surface. For most of the results obtained here, squalane was used as a diluent oil as this high molar volume oil has been shown to give zero Ay when added to the surface of a number ofionic surfactant solutions.6 However, in order to test the validity of eq 8, the adsorption of dodecane onto aqueous solutions of C12E5 was examined using both squalane and dinonyl phthalate (DNP) as diluent oil. The activity coefficients of alkanes in squalane and in DNP were obtained from ref 30. Some extrapolation, using the appropriate equations given in ref 30, was required to estimate activity coefficients for mixtures of the longer chain length alkanes with either squalane or DNP. For dodecane, the activity coefficient varies from 0.83 (at Xdodeeane = 0) to 1in squalane mixtures, and from 1.62 (at Xdodecane = 0) to 1 in DNP mixtures. Figure 7 shows the variation of Ay with mole fraction of dodecane (upper graph) and with mole fraction activity, i.e. corrected for nonideality (lower graph). It can be seen that the activity coefficient correction reduces the curves for squalane and DNP to a common line. This result strongly suggests that neither squalane or DNP adsorbs to any significant extent across the entire mole fraction range with dodecane. In principle, it is possible that squalane and DNP do adsorb significantly but to the same extent; this appears unlikely in view of their different polarities. When Ay increases linearly with aao and r d o is zero, the quantity (dAyld In aao)in the limit that aaotends to 1is equal to the maximum tension lowering Ay(max)obtained following the addition of a drop of pure adsorbing oil to the surfactant solution. The maximum surface concentration of adsorbing oil rao(max)is then
I
E
. 0
0.6 0.8 1 Mole fraction activity
Figure 7. Variation of A y with (a)mole fraction and (b) mole fraction activity of dodecane for the addition of mixtures with squalane (circles) or DNP (squares) on aqueous solutions of C12E5(6.4 mM). The squalaneoil mixtures all contain a constant of 0.32 mol % and the DNP mixtures all mole fraction of contain a constant mole fraction of 0.64 mol %.
10
8
8 6
5k
4
0
0.2
0.4 0.6 0.8 I mole fraction activity of linear alkane
Figure 8. Variation of A y with mole fraction activity of octane (squares)and hexadecane (circles)for the addition of squalanel alkane mixtures on aqueous solutionsof C12E7 (5 mM). The oil mixtures all contain a constant mole fraction of of 0.052 mol %, equal to C * ~ ~ I .
fraction activity for octane and hexadecane (in squalane) on C12E7 monolayers. It can be seen that pure squalane does adsorb appreciably in the case of C12E7 monolayers. As seen in Figure 6, C12E7generally gives larger A y values for pure alkanes than seen previously for C12 tail group ionic surfactants. The nonzero Ay for pure squalane on C12E7 means that eq 8 cannot be used directly to obtain the surface concentration of the alkanes using the data of Figure 8. However, it is noteworthy that hexadecane shows a near linear variation of Ay with activity whereas the data for octane curves strongly upward consistent with a stronger adsorption in the case of octane. The addition of drops of pure squalane gave values of Ay (in units of
2522 Langmuir, Vol. 11, No. 7, 1995 9 ,
8
Aveyard et al. I
I
1
A
4
I. 6 .
0
0.2
0.4 0.6 0.8 1 mole fraction activity of linear alkane
Figure 9. Variation of Ay with mole fraction activity for the addition of squalane/alkanemixtures on aqueous solutions of ClzE5(6.4mM). In descendingorder the curves refer to octane, nonane, decane, dodecane, and hexadecane. The oil mixtures all contain a constant mole fraction of C12E5 of 0.32 mol %, equal to ~ * ~ i l .
mN/m) of 2.5 for C12E9, 2.1 for C12E7, 0.1 for C12E5, 0.15 for C10E7, and 2.5 for C14E7. The surfactant solutions showing negligible adsorption of squalane into the pure surfactant monolayers (C12E5 and C10E7) were thus selected for detailed adsorption studies for a range of linear alkanes. Plots of A y versus mole fraction activity in squalane for (in decreasing order of A y ) octane, nonane, decane, dodecane, and hexadecane on C12E5 solutions are presented in Figure 9. All of these alkanes (either pure or mixed with squalane) appear to behave as nonspreading oils asjudged visually. The curves of A y versus oil activity were fitted to polynomial functions of order 2,3, or 4 and the fits are shown in Figure 9 as the solid curves. Differentiation of the polynomial fitting functions (summarized in Table 4) then yield Tao as a function of the activity of the adsorbing oil. The surface concentrations of the oils on solutions of C12E5 are shown in Figure 10 a s a function of activity. For both planar air-water surfactant monolayers6 and for curved monolayers coating microemulsion droplet^,^ alkane adsorption is known to increase with decreasing alkane chain length. Only the longest chain length alkane studied here (hexadecane) shows a n approximately linear with oil activity corresponding to the ideal increase of rao 2-D gas surface equation of state ( A y = ra&tZT). In this
0
0.2
0.4 0.6 0.8 1 mole fraction activity of linear alkane
Figure 10. Adsorption isotherms for alkanes on aqueous solutions of C12E5. Experimental conditions and symbols as for Figure 9.
case, Taofor pure alkane (0.46molecules/nm2) is similar to the value (0.39 molecules/nm2) estimated using the approximate equation (9). For all the shorter chain alkanes, the adsorption isotherms curve strongly upward and the maximum surface concentrations are, as expected, considerably higher than the values estimated using eq 9. The upward curvature signifies that adsorption is progressively more favored with increasing alkane content of the mixed alkane/surfactant film. The effect is more marked for dodecane on C12E5 than that measured previously for dodecane on dodecyltrimethylammonium bromide solutions (Figure 2 of ref 6)even though the values of Ay(max)are similar for the two surfactants. For C12E5, the maximum adsorption seen here (for octane) corresponds to greater than four molecules of alkane per nm2. Since the minimum cross-sectional area of a n alkyl chain is approximately 0.2 nm2 and the surfactant contributes approximately 2 alkyl chains per nm2,l0the mixed oil/ surfactant film must be in excess of a monolayer in some cases, even for oils which appear (as judged visually) not to spread macroscopically. In contrast, dodecane films on monolayers of alkyltrimethylammonium bromide surfactants with chain lengths of 12, 14, and 16 give nearly linear adsorption isotherms and the mixed surfactantloil films have been shown to be monolayer^.^,^ This comparison illustrates how the amounts of adsorption and the shapes of the adsorption isotherms for alkanes on surfactant monolayers are highly dependent on the
Table 4. Polynomial Equations Used To Fit the Data of Ay versus Oil Mole Fraction Activity (a)= surfactant C12E7 C12E5
C10E7
oil chain length 8 16 8 9 10 12 16 8 10 12 14
fitting equation
+
+
Ay = 7.8971~~ - 1.5402~~1.0535~ 2.9931
+ + + + + + +
+ + + +
A y = 1.3134~~0.8407~ 2.1052 Ay = 6 . 2 8 6 0~ ~1.5876~~3.2629~ 0.0816 A y = 2.1229~~2.9059~~1.8833~ 0.0832 A y = 1.0454~~2.8235~~1.9558~ 0.0698 Ay = 2.7069~~ - 2.0271~~3.0948~ 0.0933 A y = 0 . 4 4 9 7 ~ ~0.9956~ 0.1204 Ay = 20.5195~~ - 25.8353~~13.2917~~2.3628~ 0.1060 Ay = 1.5239~~2.6351~~3.6222~ 0.0989 Ay = -1.6674~~4.0856~~3.1590~ 0.0756 Ay = -0.1543~' 4.6446~ 0.0398
+ +
+ + + +
+
+ + +
+
+
+
+
a The fitting equations correspond to the solid lines shown in Figures 8, 9 and 12. For octane with C&7, the fitting equation applies only over the range of a from 0.4 to 1.
Alkane-Surfactant Interaction i.2
1
Langmuir, Vol. 11, No. 7, 1995 2523
A
10
-
I I
8 -
8
0.8
B E
5 a
0.6
.
0.4
0.2
0 0
0.4 0.6 0.8 1 mole fraction activity of linear alkane
0.2
Figure 11. Adsorption data of Figure 9 expressed as a thickness of adsorbed oil (liquid alkane density assumed). molecular structure of the surfactant. No explanation for these effects is currently available. The adsorption data in Figure 10 are represented in Figure 11 in terms of the equivalent thickness of the adsorbed oil film (obtained by multiplying Tao by the molecular volume ofthe oil in bulk liquid). The equivalent thickness corresponds to the volume of alkane adsorbed per unit area and increases with decreasing alkane chain length (as does the number of molecules of adsorbed oil). The equivalent thicknesses are in the range 0-1.2 nm, much lower than the film thickness expected for macroscopic spreading leading to interference colors (> 100 nm). Hence, the adsorption data are consistent with the visual observation of nonspreading behavior for these alkanes on C12E5 monolayers. For alkanes containing a concentration c*oil of the surfactant (in the absence of diluent oil) added to C10E7 solutions, the transition from spreading to nonspreading behavior is judged visually to occur between dodecane and tridecane. Plots of Ay versus oil activity for a range of alkanes spanning the transition from spreading to nonspreading are shown in Figure 12. Tetradecane (nonspreading) shows approximately linear behavior whereas the plot for dodecane (spreading)has appreciable curvature. However, we note there is no clear correlation between the linearity of the plot and the spreading behavior since the plots for a range of nonspreading alkanes on C12E5 are curved (Figure 9). In cases where spreading of the pure adsorbing oil occurs, mixing with squalane leads to a transition from spreading to nonspreading since pure squalane does not spread. Equation 8 can only be used over the mixed oil composition range where the drops are nonspreading since, for an aqueous surface with a thin (but macroscopic) film of oil, the measured tension corresponds to the sum of yaw + yao. For squalane/decane mixtures on C10E7, the transition from nonspreading to spreading behavior was estimated visually to occur at decane mole fractions of between 0.85 and 1. Although the visual observation is rather imprecise, it provides some support for the use of eq 8 to obtain the surface concentrations of oil using the tension data over the mole fractions of the adsorbing alkane from zero to close to 1. The adsorption isotherms for both spreading and nonspreading alkanes on CloE, solutions are compared in
mole fraction activity of linear alkane
Figure 12. Variation of Ay with mole fraction activity for the addition of squalanelalkane mixtures on aqueous solutions of C10E7 (9.5 mM). In descendingorder the curves refer to octane (spreading), decane (spreading), dodecane (spreading), and tetradecane (nonspreading). The oil mixtures all contain a constant weight fractionof C10E7 of 0.3 wt % (this concentration is in excess of c*,il).
O
L
0
0.2
04
06
08
1
mole fraction activity of linear alkane
0
0.2
{
I
0.4 0.6 0.8 1 mole fraction activity of linear alkane
Figure 13. Adsorption isotherms for alkanes on aqueous solutions of C1&. Experimental conditions and symbols as for Figure 12. The inset shows a comparison of adsorption data for octane on pure water (filledcircles, data replotted from ref 311, ClzEs (filled squares), and C& (unfilled triangles). Figures 12,13, and 14 which show the adsorption data in terms of Ay, rao,and equivalent oil film thickness, respectively. As for C&5, the curvature of the Ay plots increases with decreasing alkane chain length. The inset in Figure 13shows a comparison ofthe adsorption of octane on pure water (filled circles, data replotted from ref 31), on C12E5 solutions (filled squares) and on C10E7 solutions (triangles). Octane spreads on the latter solution but forms lenses on water and ClzE5 solutions. It can be seen that the adsorption isotherm is strongly affected by the presence and the nature of the surfactant monolayer. (31)Hauxwell, F. Ph.D. Thesis, University of Bristol, 1969.
Aveyard et al.
2524 Langmuir, Vol. 11, No. 7, 1995
We now speculate concerning the possible significance of the maximum values of Taofor the case of the alkanes for which the pure oil shows spreading behavior. The 3~ visual observation of spreading oil films showing inter2.5 ference colors implies the oil film thicknesses are of the for order of 100 nm or larger. The maximum values of rao the spreading oils (Figure 13) correspond to oil film 2 thicknesses which are orders of magnitude less than this. E The maximum Taovalues are similar to those reported by -$ Hawwell and Ottewill for pentane on pure water for which 4 1.5 films showing interference colors were also o b s e ~ e d . ~ ~ The measured air-water tension in the presence of oil (i.e., the tension of the composite film including the surfactant monolayer plus the oil film) will decrease with increasing oil film thickness as adsorption increases with increasing activity of the oil. However, at oil film thicknesses greater than a few molecular diameters the film disjoining pressure is likely to be very small and, hence, the tension decrease with further thickening of the oil film is expected to become insignificant relative to the precision of the tension measurements (approximately 0 0.2 0.4 0.6 0.8 1 0.2 mN/m). Thus, the tension measurements only probe mole fraction activity of h e a r alkane oil film thicknesses up to the transition between “thin” Figure 14. Adsorption data of Figure 12 expressed as a and “duplex” films and are expected to be insensitive to thickness of adsorbed oil (liquid alkane density assumed). the macroscopic thickness of bulk oil with properties not significantly affected by the surface. If this argument is 2. The alkane chain length at which the transition from correct, the maximum values of Tao for spreading oils spreading to nonspreading occurs is affected by the (obtained from measurements of the air-water surface) molecular structure of the surfactant. provide a measure of the amount of surface-perturbed oil 3. The equilibrium spreading coefficientsfor all systems associated with the oil-water interface (including the investigated are all close (within 0.3 mN/m) to zero. This surfactant monolayer) which exists in the presence of a is true for systems judged visually to be either spreading spread film of the pure oil. or nonspreading. A crude estimate of the thickness of this “interfacial” oil associated with the oil-water interface was obtained 4. Alkane adsorption (expressed either as number of by converting the adsorption data of Figure 13 into the molecules of alkane or volume of alkane per unit area) equivalent oil film thickness (Figure 14). The spreading increases with decreasing alkane chain length. The oils (octane, decane, and dodecane) show maximum adsorption isotherms also show a greater upward curthicknesses varying from 0.6 to 2.3 nm. The thickness of vature for shorter alkanes signifying that the adsorption the surface oil layer associated with the oil-water becomes progressively more favored with increasing interface increases with decreasing alkane chain length. alkane content of the mixed surfactant/alkane film. Tetradecane (nonspreading) shows a lower thickness of 5. We speculate that the adsorption isotherms for oils approximately 0.5 nm. In the absence of direct measurewhich spread as the pure oil on aqueous surfactant ments of the nonspreading tetradecane film thickness, it solutions yield information about the amount of “surfaceis uncertain whether the total oil film thickness in perturbed” oil associated with the oil-water interfacial equilibrium with the lenses of bulk tetradecane is larger region. For CIOE,monolayers, the thickness of this region than this value. It should be noted that the thicknesses is estimated to be in the range 0.6-2.3 nm and is found shown in Figure 14are the contributions from the adsorbed to increase with decreasing alkane chain length. alkane only; the actual thickness of the surface region will, of course, include a contribution from the surfactant Acknowledgment. We thank Unilever Plc (Port monolayer. Sunlight Laboratory) for provision of a Studentship for J.R.M.We are grateful to Drs. Abid Khan-Lodhi and Tim Conclusions Finch (Unilever Plc, Port Sunlight Laboratory) for fruitful The main conclusions of this study are listed below. discussions and support of this work and to Ms. A. C . 1. Tension changes caused by oil adsorption on the Wicks of the University of Hull for making some of the surfaces of aqueous solutions of nonionic surfactants can surface tension measurements. be measured reliably so long as proper allowance is made LA940834T for the partitioning of surfactant into the added oil.
1
I
R
