MOLECULAR WEIGHT DISTRIBUTION OF NONIONIC

MOLECULAR WEIGHT DISTRIBUTION OF NONIONIC SURFACTANTS. I. SURFACE AND INTERFACIAL TENSION OF NORMAL DISTRIBUTION AND ...
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Oct., 1963

MOLECULAR WEIGHTDISTRIBUTION OF NONIONIC SURFACTANTS

1987

MOLIECULAR WEIGHT DISTRIBUTION OF NONIONIC SURFACTANTS. I. SURFACE AND INTERFACIAL TENSIOS OF SORMAL DISTRIBUTION AND HOMOGENEOUS p,t-OCTYLPHENOXYETHOXYETHAXOLS (OPE’S) BY E. H. CROOK, D. B. FORDYCE, AND G. F. TREBBI Research Laboratories, Rohm & Haas Co., Bristol, Pennsylvania Received March 16, 1963 Surface and interfacial tension (us. isooctane) as a function of concentration have been determined for normal distribution (Poisson) and single species p,t-octylphenoxyethoxyethanols (OPE1-lo) and for mixtures of single species and normal distribution OPE, and OPEN. Areas per molecule ( A ) ,surface tensions a t the critical micelle concentration ( -yc.m.o. ), and critical micelle concentrations (c.m.c.) are presented as a function of ethylene oxide chain length. Such parameters, derived from surface tension data, indicate a preferential adsorption of shorter, ethylene oxide chain length molecules (OPE,-,) at the air-water interface. Similar data, derived from interfacial tension results, indicate a preferential adsorption of longer, ethylene oxide chain length molecules a t the isooctane-water interface.

Introduction have been Although a number of reported in the literature concerning the surface tension of normal distribution nonionic surfactants, relatively few@-8 have dealt with the interfacial properties of relatively homogeneous surface active materials. This investigation presents a detailed study of the interfacial properties of both homogeneous and normal distribution p,t-octylphenoxyethoxyethanols (OPE’s) of ethylene oxide (EO) chain lengths of 1 to 10. From these studies conclusions have been drawn as to which molecular species preferentially orient a t the air-water and isooctane-water interfaces. Experimental Materials.-Single species OPE1, OPE2, and OPE3 were prepared by repeated distillation of the reaction products of recrystallized p,t-octylphenol and the appropriate number of moles of ethylene oxide. Single species OPE4, OPEs, and OPE6 were prepared by the condensation of recrystallized p,t-octylphenol with the appropriate purified polyethylene glycol monochloride. Further purification was accomplished by distillation or column chromatography. Samples of single species OPEI, OPEs, OPEe, and OPElo were prepared by the reaction of hexaoxyethylene glycol with the appropriate OPE, chloride@followed by chromatographic purification. The high purity (95y0 or greater) of these single species materials was confirmed by carbon-hydrogen, ultraviolet, gas chromatography,I0 thin layer chromatography,ll and hydroxyl numberl2 analysis. Further details on the synthesis and characterization of these compounds will be published sh0rtly.1~ Normal distributionl* OPE2-10, OPE16, and OPE40 were prepared by the reaction of recrystallized p,t-octylphenol with ethylene oxide (a recrystallized material was necessary to eliminate interpretative complica,tions arising from the presence of o,t(1) L. Hsiao, €1. N. Dunning, and P. B. Lorens, J . Phys. Chem., 60, 657 (1956). (2) F. V. Nevolin, T. G. Tipisova, N. A. Polyakova, and A . M. Semenova, J . prakt. Chem., 145, 206 (1962). (3) Y. Ishii, T. Kusano, and R. Saito, Kogyo Kagalcu Zasshi, 61, 180 (1958). (4) Y. Ishii, T. ‘Dgawa,and I. Mizutani, ibid., 64, 1794 (1961). (5) W. P. Evans, quoted b y K. Durham, “8urface Activity and Det.ergency,” Macmillan Co., London, 1961, p. 25. (6) M. J. Schick, S. M. Atlas, and F. R. Eirich, J . Phys. Chem., 66, 1326 (1962). (7) J. M. Corkiil, J. F. Goodman, and R. H. Ottewill, Trans. Faraday SOC., 57,1627 (1961). (8) P. Becher, J . Phys. Chem., 64, 1221 (1960). (9) B. A. Gingras and C. H. Bayley, Can. J . Chem., 56, 599 (1957). (10) W. Smith and R. C. Mansfield, unpublished results, 1961. (11) W. Myers and R. C . Mansfield, unpublished results, 1962. (12) C. L. Hilton, Anal. Chem., Si, 1610 (1959). (13) R. C. Mansfield and J. E. Locke, paper in preparation. (14) Normal distribution refers to a n approximately Poisson distribution of molecular weights: J. Kelly and H. L. Greenwald, J . Phys. Chem., 62, 1096 (1958).

octylphenol and dioctylphenol adducts). The ethylene oxide content of the compounds was determined by weight increase and chemical hydroxyl number methods. The resultant compounds had the following average ethylene oxide chain lengths: 2.0, 3.0, 4.1, 5.0, 6.0, 7.1, 8.0, 9.1, 9.9, 16, and40, respectively. The determination of the ethylene oxide chain length was uncertain t o f 0 . 1 units. Water distilled from alkaline permanganate solution was used to prepare the surfactant solutions. The ohm-* cm.-I and specific conductance of the water was 1 x thus was substantially free of inorganic comtaminants. Isooctane used for interfacial tension measurements was a “Spectro Grade” Eastman Organic Chemicals product. Procedure .-A DuPIIoiiy tensiometer was used to measure the surface and interfacial tensions of the aqueous surfactant solutions. The instrument was calibrated with water, benzene, and isooctane and an average deviation of -1% was found for the three liquids. The corresponding experimental values for the surfactant solutions are assumed to be of comparable accuracy. Stock solutions or dispersions (ca. 1 x mole/l.) of surfactant were prepared by dissolution in water and equilibrated for 24 hr. Lower concentrations of surfactant then were prepared by dilution of portions of the stock solution. The obtained solutions were equilibrated a t 25 =!= 1’ -fwr ~ m w ~ f ~ ~ - b -surface e f o w tensions were determined (25 ml. of solution in a Tannin dish). A time dependence of the surface tensionl5J6 was uoted with both single species and normal distribution OPE’s. This phenomenon was more accentuated with the normal distribution compounds. Experiments indicated thQt an equilibration time of three minutes was usually sufficient to minimize this effect. The DuNoUy ring was equilibrated in the surface of the liquid for ca. 3 min. a t a stress several tenths of a dyne below the point of maximum stress. The break point was then approached very slowly until rupture occurred. An average of four to six readings was taken as the correct value. Only values with a deviation of < A 0 2 dynes em.-’ were accepted. Surface tension curves as a function of concentration were determined from the data resulting from 12 to 18 concentrations. For the series OPE? through OPElo interfacial tension measurements were carried out coincidentally with the surface tension measurements. After the surface tension measurement had been completed and the ring re-immersed in the aqueous phase, 25 ml. of isooctane was layered over the aqueous solution. Equilibration was allowed to proceed for 10 min.17 and the interfacial tension then was determined using the appropriate correction parameters.’* For the series OPE2 through OPE8 a problem of equilibration arose since the surfactant was primarily soluble in the isooctane phase and the OPE’s were present as aqueous dispersions or solutions. Two procedures were adopted. (1) After a surface tension measurement was completed, 25 (15) J. Leja and J. C. Nixon, Proc. Intern. Congr. Surface Activity, I n d London, 3,297 (1957). (16) W. P. Evans, quoted by K. Durham, “Surface Activity and Detergency,” Maomillan Co., London, 1961, p. 27. (17) I n order to determine whether equilibrium had been established between the isooctane and aqueous phases for the series, OPE7 to OPEio,, a comparison waa made between values recorded 10 min. after incdrporation of isooctane and those obtained 2 weeks later. The results showed t h a t the former values were essentially equilibrium values (deviation i i1%). (18) W. D. Harkinsand H. F. Jordan, J. A m . Chem. SOC.,52,1751 (1930).

E. H. CROOK, D. B. FORDYCE, AND G. F. TREBBI

1988 GO

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Vol. 67

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FOR NORMAL DISTRIBUTION OPE's,

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Fig. 2.-Surface tension us. concentration for normal distribution OPEz-la.16.40,

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Oct., 1963

MOLECULAR WEIGHTDISTRIBUTION OF NONIONIC SURFACTANTS

ml. of the solution was pipetted into an 8-oz. wide-mouth bottle and overlaid with 25 ml. of isooctane. The capped bottle then was shaken vigorously and permitted to equilibrate for 2 weeks, after which the interfacial tension was determined. ( 2 ) A stock solution of surfactant in isooctane was prepared and dilutions made from it accordingly. Twenty-five-milliliter portions of water were pipetted into 8-oz. wide-mouth bottles and overlaid with 25 ml. of the surfactant-rich isooctane. After two weeks’ equilibration the interfacial tensions were determined. Agreement between meaaurernents on the same surfactant concentration prepared by the alternate methods was satisfactory (&I%).

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SURFACE VI.

TENSION EO CHAIN

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1989 I OPE’S

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0 SINGLE SPECIES A NORMAL DISTRIBUTlON 0 T H E O R E T I C A L NORMAL O I S T R I B U T I O N

80 Results and Discussion AREA PER MOLECULE lo - vs. EO CHAIN LENGTH Surface Tension.-In Fig. 1 and 2 are presented the data of surfacle tension as a function of concentration u 50 for single species OPEl-lo and normal distribution 2 40 OPEz-lo,la,e~.I n Fig, 3 is presented the surface ten30 sion a t the critical micelle concentration (Y ~ . ~ . ~ area .), d per molecule (A),19 and critical micelle concentration r 20 4 lo-. as a function of ethylene oxide chain length. From the C.M.C. vs EO CHAIN LENGTH -. ~ EO . chain length for both single species plot of Y ~ . ~ us. and normal distribution OPE’s the following can be concluded: (1) OPE4 is the most surface active member of the OPE series; (2) as the EO chain length increases . ~ . and (3) as the EO chain from 4-10, Y ~ . ~ increases; length increases from 1 to 3, Y ~ . ~decreases. . ~ . I n comparing normal distribution and single species compounds, with the exception of OPE4, the values of -yc.m.c. for the single species compounds are greater than EO CHAIN L E N G T H . normal distribution compounds a t corresponding EO Fig. 3.-Surface tension at the c.m.c., area per molecule, and chain lengths with the divergence increasing with inc.m.c. us. ethylene oxide chain length for single species and normal creasing EO chain length. Area per molecule increases distribution OPE’S. continuously as a function of EO chain length. As . ~ . the values of ( A ) are in the case of the Y ~ , . ~ data I I I I greater for single species compounds than for the corresponding normal distribution compounds. C.m.c. is an increasing function of EO chain length for both single species and normal distribution OPE’s (the discontinuities a t low EO chain lengths are probably due to experimental error) with higher values being obtained for single species than for the corresponding normal distribution compounds. That adsorption a t the airwater interface of nonionics is matchstick in nature is indicated by the sourface area per molecule of OPE,, which value of 23 A.2 molgcule-1 approximates closely to reported values of 22 A.2 2O mglecule-1 for an aliphatic hydrocarbon chain and 25 A.2 21 molecule-’ for a benzene group. The one ethylene oxide unit contributes very little to ithe total area per molecule; however, further addition of ethylene oxide to the chain increases the area per molecule markedly. At longer EO chain lengths the packing of the OPE molecules is determined primarily by the bulkiness of the hydrated Molecularly D i s l i l l e d NPE’S ethylene oxide chain and not by the hydrophobic alkyl LSchick.eto1, J. Phy5 Chem,Q,I3W,!@ phenyl group. The lower values of the area per molecule and Y ~ . ~ . ~ . for the normal distribution compounds in comparison to the corresponding single species compounds can be attributed to a preferential adsorption of the more surI face active species extant in the normal distribution compounds a t the air-water interface; e.g., normal dis0-5 I I I I I I tribution OPES exhibits a surface area per molecule approximately equal to that of a single species OPE6 0

I

(19) Areas per molecule were computed from the slope of the surface tension VS. Concentration curve previous t o the 0.m.c. (20) W. D. Harkins and R. 1’.Florence, J . Chem. Phgs., 6,847 (1938). (21) M. J. Schick and E. A. Beyer, J . Am. Oil Chemists’ Soc., 40, 66 (1963).

Fig. 4.--Comparison of log c.m.c. vs. ethylene oxide chain length relationship for OPE’s and NPE’s.

(53.8 compared to 53.0 respectively). One method of demonstrating this difference between bulk

E. H. CROOK,D. B. FORDYCE, -4ND G. F. TREBBI

1990 I I , /

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SURFACE

FOR

TENSION

surface tension a t the critical micelle concentration for a given normal distribution compound can be calculated . ~ . assuming a linear combination of the Y ~ . ~ contributions of the individual OPE’s that constitute the compound, i.e.

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MIXTURES

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Vol. 67

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Fig. 5.-Surface tension vs concentration for mixtures of single species and normal distribution OPE4 and OPElo.

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Surface Tension At vs.

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phase and interfacial concentrations is to assume that the same Poisson distribution of molecular weights of OPE members is found a t the air-water interface as is found in the bulk of the solution. A semitheoretical

i=l

(X,YiC.,.O.)

(1)

where xz. is the mole fraction of i and y’,, is the surface tension of pure component i a t the c.m.c. It was assumed that there were no interactions between individual molecules to give nonadditive effects. Where Y ~ . ~values . ~ . of OPE’s having EO chain lengths greater than 10 were required for the calculation, extrapolation of the experimental data yielded the necessary values. The calculated values are presented in Fig. 3. It is clear that the experimental values of Y ~ . ~ are . ~ .substantially lower than the theoretically determined values of Y ~ . ~which . ~ . would be expected if there were the same distribution of molecular species at the airwater interface as exist in the bulk of the solution. This differelice could only result if there is a preferential adsorption of the more surface active species a t the air-water interface. Although the c.m.c. us. EO chain length data are not as consistent as the corresponding area per molecule and Y ~ . ~ data, . ~ . it is clear that the values of c.m.c. of both normal distribution and single species compounds increase with increasing EO chain length with single species OPE’s having greater c.m.c. than the corresponding normal distribution materials. This is probably related to the presence in the normal distribution compounds of shorter ethylene oxide chain length OPE’S which are inherently less water soluble than the longer ethylene oxide chain length members. Such inherent insolubility leads to the formation of micelles at a lower concentration than in the case of the single species materials where only one type of molecule is available for micellization. Hsiao, et al.,I have proposed that critical micelle concentration and ethylene oxide chain length for a homologous series of alkyl phenoxyethoxyethanols should be related by the equation In c.m.c.

~

c

=

A

+ Bn

(2)

where n is the ethylene oxide chain length of a given nonionic surfactant. That this relationship is not applicable a t ethylene oxide chain lengths less than ca. 20 units is clearly shown in Fig. 4, ih which the present c.m.c. results for p,t-octylphenoxyethoxyethanolsand the c.m.c. results of Schick, et u Z . , ~ for nonylphenoxyethoxyethanols are given. Nonlinearity a t shorter ethylene oxide chain lengths is not surprising since the hydrophobicity of the surfactant changes substantially with each decreasing ethylene oxide unit when the total EO chain length is short. From the data of Schick, et aZ.,6 a plot of logarithm of the aggregation number us. ethylene oxide chain length is also not linear at ethylene oxide chain lengths less than 15. This is in accord with a large increase in micelle size as the hydrophobicity of the compound increases. With materials which are above their cloud points (OPE7 and shorter ethylene oxide chain length molecules) at 2 5 O , there is little doubt that the c.m.c. indicates the onset of a highly aggregated state.22

Oct., 1963

MOLECULAR WEIGHTDISTRIBUTION OF NONIONIC SURFACTANTS I

i

INTERFACIAL TENSION

I 1 1 1 1 1

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Fig. 7.-Interfacial

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Liter-’).

tension vs. concentration for single species OPEI-N.

An interesting feature of the surface tension us. concentration plots is the rise in surface tension above the c.m.c. for normal distribution OPE’s (this phenomenon is not observed with single species corn pound^^,^). This behavior in the case of anionic surfactants has been shown2a to be due to solubilization of surface active impurities within the micelles a t concentrations above the c.m.c. With nonionic materials such as the normal distribution OPE’s the so-called “impurities” are inherent in the compound since the shorter chain length molecules are more surface active than the longer chain length molecules. At concentrations at, or just above, the c.m.c., the air-water interface consists primarily of shorter ethylene oxide chain length OPE’s; however, as the concentration of surfactant increases and more micelles are formed the hydrophobic short chain OPE’s can be extracted from the air-water interface and solubilized in the micelles present in the bulk of the solution. The site a t the air-water interface vacated by the hydrophobic molecule is subsequently occupied by a less surface active, hydrophilic molecule. Such a mechanism gives rise to an interface which would be less rich in hydrophobic species with increasing surfactant concentration until a limiting value of the surface tension is attained. It is noteworthy that the postc.m.c. curvature (Fig. 2) becomes greater with increasing ethylene oxide chain length, Le., as the molecular weight distribution broadens. From Fig. 1 and 2 a curvature previous to the c.m.c. is noted in the surface tension us. log C plots of some of (22) K.Kuriyama, Kollozd-Z., 181, 144 (1962). (23) G.D.Miles :and L. Shedlovsky, 1.Phys. Chem., 48,57 (1944).

the longer chain length OPE’s (OPEs-lo,16,40). This behavior is found for both single species and normal distribution compounds and is accentuated as the ethylene oxide chain length increases. It is probable that this behavior is an effect of the packing of the hydrated ethylene oxide chains. The picture might be as follows: a t lower concentrations the packing of the molecules a t the air-water interface is not too tight and the hydrophilic ethylene oxide chains are free to orient in a random configuration. With increasing concentration the packing a t the interface becomes increasingly tighter and the E O chains are forced into a more extended configuration with a corresponding decrease in free energy and entropy. Superposition of this effect on the normal lowering of surface tension with concentration would produce nonlinearity in the surface tension us. concentration relationship previous to the critical micelle concentration. 24 Such behavior would be expected to be independent of whether the compound were single species or normal distribution. This is confirmed by the experimental results. Surface Tension of Mixtures.-Plots of surface tension us. log C for mixtures of single species and normal distribution OPE4 and OPElo are presented in Fig. 5 . From these plots areas per molecule, c.m.c., and Yc.m.0, were calculated and are presented in Fig. 6 as a function of mole per cent OPElo. From area and Y ~ . ~ data . ~ . it is clear that for mixtures of single species OPE4 and OPElo the molecule (24) The authors wish t o thank one of the referees for suggesting this explanation for the observed behavior of the surface tension VE. concentration plots in the pre-c.m.c. region.

E. H. CROOK, D. B. FORDYCE, AND G. F. TREBBI

1992 YO

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PHASE

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Vol. 67 I

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Fig. 8.-Interfacial

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CONCENTRATION

OPE’s

ISOOCTANE) OPEx

Concentration

1

(Moles

I

Liter-’).

tension us. concentration for normal distribution OPE2-la,16,40,

OPE4 preferentially adsorbs a t the air-water interface. With the mixtures of normal distribution OPE, and OPE10 there is a continuous change of the area per molecule from that equivalent to single species OPE3 to that equivalent to an area intermediate between that of single species OPEs and OPEs. The surface tension is approximately constant from a 0 : l to 2 : l OPEloOPE, mole ratio, then increases to a value between that of single species OPE7 and OPEs. These data indicate that for the normal distribution mixtures the air-water interface is predominantly occupied by OPES, OPE4, OPEC, and OPE6 molecules in ratios such that the average surface tension value is approximately independent of the OPElo-OPE4mole ratio, at least up t o a 2: 1 OPElo-OPE, mole ratio (at this ratio 31.5% of the total mixture still consists of OPEs-6 molecules, with this percentage being even greater a t lower OPEloOPE, ratios). The behavior of the c.m.c. us. mole ratio plots is indicative of mixed micelle formation. The lower values of the c.ni.c. obtained for mixtures of normal distribution OPE, and OPElo compared to the corresponding mixtures of single species compounds suggest a higher micellar molecular weight for the normal distribution mixtures. The considerable negative deviation from linearity of the c.1n.c. us. mole per cent OPE10 plot is a strong indication that the shorter EO chain length molecules substantially increase the molecular weight of the mixed micelles in a nonadditive manner, Le., the composition of the micelles is not stoichiometrically equivalent to the nominal ratio of OPEloto OPB. Interfacial Tension.-From plots (Fig. 7 and 8) of

interfacial tension as a function of concentration for single species and normal distribution OPE’S,areas per molecule ( A ) , critical micelle concentrations (c.m.c.), 25 and interfacial tensions a t the c.m.c. ( ~ i ( ~ . can ~ . be ~ calculated. These data are presented in Fig. 9 as a function of EO chain length. From the ~ i ( ~ . ~ us. . ~ EO . ) ) chain length plot it can be concluded that: (1) as the EO chain length increases, ~ i ( ~ . ~ increases . ~ . ) for both single species and normal distribution OPE’s; (2) from OPE, to OPE10 lower values of the interfacial tension are attainable with single species OPE’Sthan with the corresponding normal distribution compounds; and (3) a t EO chain lengths . ~ . ) are obtained with greater than 10 lower ~ i ( ~ . ~values normal distribution compounds than with single species compounds. From the plot of area per molecule U S . EO chain length it can be seen that: (1) A generally increases with increasing EO chain length; (2) the A values for single species compounds are generally lower than those of the corresponding normal distribution compounds; and (3) a t EO chain lengths greater than ea. 12, A values of single species OPE’S are greater than those of the corresponding normal distribution compounds. The plot of c.m.c. us. EO chain length indicates that: (1) in general, c.m.c. decreases with increasing EO chain length; (2) in the range of EO chain lengths from 2-7 the c.m.c. values are greater for single species compounds than for the corresponding normal distribution compounds; and (3) in the range of EO chain lengths 8-10 the observed c.m.c. values for (23) C.m.c. values were determined under the condition of equal volumes of water and isoiictane for surfaotant distribution.

Oct., 1963

1993

MOLECULAR WEIGHTDISTRIBUTION OF NONIONIC SURFACTANTS

normal distribution compounds are greater than those for the corresponding single species compounds. With single species OPE’s there is no dificulty in interpreting the experimental results because only one entity is present in the bulk of the water and isooctane phases and a,t the oil-water interface, but with normal distribution compounds the individual solubilities of the various EO chain length OPE molecules in the isooctane phase must be considered. Greenwald, et al.,26,27 studied the partition coefficients of some normal distribution and single species OPE’S in the system, isooctane-water. Also, a recent study?*deals extensively with the distribution of single species and normal distribution OPEl-lo in the system, isooctane-water. These investigations indicate that: (1) OPE’s with a n EO chain length less than nine are more soluble in isooctane than in water; (2) for OPE’S with an EO chain length less than nine the single species compounds are more soluble in isoijctane than the corresponding normal distribution compounds; and (3) a t EO chain lengths greater than nine single species OPE’S are more soluble in water than the corresponding normal distribution compounds. Thus, when a normal distribution OPE dissolved in water is equilibrated with isooctane, the compound is fractionated, with shorter chain length OPE’s being concentrated in the oil phase while the longer chain length OPE’s concentrate in the aqueous phase. This partition of molecules according to solubility will lead to a lower average molecular weight of the OPE molecules dissolved in the isooctane phase and a higher average molecular weight of the OPE molecules dissolved in the aqueous phase, compared to the average molecular weight of the original compound. The decrease of c.m.c. with increasing EO chain length is explained by the decrease of oil solubility accompanying such an increase in hydrophilicity, i.e., more surfactant is available to the aqueous phase for micelle formation. With the shorter chain length OPE’s most of the surfactant is not available to the aqueous phase and the total concentration of surfactant that is added to the system must be high to bring about micellization in the aqueous phase. Distribution considerations also account for the higher c.m.c. values for single species than for normal distribution OPE’s a t the shorter EO chain lengths and the reverse situation a t longer EO chain lengths. I n the former case only very water-iinsoluble molecules are present with single species OPE’S while the normal distribution compounds contain some water-soluble material. I n the latter case the normal distribution compounds contain some shorter EO chain length water-insoluble molecules while the corresponding single species OPE’s have greater water than isooctane solubility. Examination of the ~ i ( ~ . ~us. . ~ EO . ) chain length values indicates thak in no case was it possible to obtain as low interfacial tensions with normal distribution compounds as with single species OPE’S. This is due to the presence in the normal distribution compounds of longer EO chain length species which orient a t the oilwater interface. The shorter EO chain length molecules were so soluble in the isooctane that there was (26) H.L.Gre,enwald, E. B. Xice, M. Kenly, and J. Kelly, Anal. Chem., 33,465 (1961). (27) H. L. Greenwrtld, unpublished results, 1956. (28) E. H.Crook, D.33. Fordycel and 0.FATrebbi, paper in preparation.

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Lenglh.

Fig. 9.-Interfacial tension a t the c.m.c., area per molecule, and c.m.c. us. ethylene oxide chain length for single species and normal distribution OPE’S.

little driving force for them to orient a t the oil-water interface. Thus, a normal distribution OPE5 corresponds to a single species OPE7 in terms of interfacial tension lowering. I n one case (single species) the interface contains only one species (0pE.1) while in the other case the interface consists of a mixture of species which give a net result of lowering the interfacial tension to a value comparable to that brought about by single species OPE,. The intersection of the two curves in the vicinity of OPElo is understandable since the inherent interfacial tension lowering of single species OPE’s decreases with increasing EO chain length as the molecules become more hydrophilic and less surface active. With normal distribution compounds, however, there are still present some shorter EO chain length molecules which can orient a t the oil-water interface and thus depress the interfacial tension below that of the comparable long EO chain single species compound. The area per molecule us. EO chain length data are entirely in accord with the explanation offered for the Yi(c.m.o.) data. Interfacial Tension of Mixtures.-Plots of interfacial tension us. concentration are presented in Fig. 10 for mixtures of single species and normal distribution OPElo and OPE, in the sytem water-isooctane. From these plots areas per molecules, c.m.c., and ~ i ( ~ . ~ . values were calculated and are presented in Fig. 11 as a function of mole per cent OPElo. A comparison of the calculated areas per molecule for the mixtures of single species OPE4 and OPElo indicates that the same entity is concentrating a t the oil-water interface, i.e., single species OPElo. Most of the OPE4 is solubilized in the isooctane phase and the portion in the aqueous phase does not contribute sub-

~ . )

E. H. CROOK, D. B. FORDYCE. AXD G. F. TREBBI

1994

30

I

\

INTERFACIAL MIXTURES (OIL

Vol. 67

TENSION

DATA

OF O P E 4

-OPE10

FOR

PHASE ISOOCTANE)

//’

C.M.C.

lnterfactal Tension A i T h e vs, Mole % OPE10

I N T E R F A C I A L TENS1 ON

FOR

OF

MIXTURES OIL

PHASE

-

vs. CON CE N T R ATION OPE4

-

OPEio

ISOOCTANE

t = 25OC.

Area

Per M o l e c u l e v s . Mole % OPEIO

o o:i

1

p Single Species Normal

Distribution

A

il L

Conceniroiion

(Moles

Liter-[I.

-

Fig. 10.-Interfacial tension us. concentration for niixt.ures of single species and normal distribution OPE4 and OPElo.

stantially t o the observed area per molecule. Further corroboration of this conclusion is indicated by a comparison of y l ( c m.c.) values of the various mixtures with . ~ . )of single species OPElo. the ~ i ( ~ . ~value With mixtures of normal distribution OPE, and OPE10 the calculated areas per molecule are of the same order of magnitude as those found in the case of mixtures of single species OPEI and OPElo. It is probable that the values obtained are an average of contributions from some shorter and longer EO chain length OPE’s which give a result which is approximately equal to that of single species OPElo(note that with normal distribution OPElothe observed area value is equal to that which would be expected for a single species OPE12). The observed values of ~ i ( ~ . ~for. ~ the mixtures are of the same order as that observed for single species OPEloand are consistent with the aforementioned explanation. The plots of c.m.c. os. EO chain length for the mixtures of single species and normal distribution OPElo and OPE4 indicate that lower c.m.c. are obtained for mixtures of single species OPE4 and OPE10 than for normal distribution OPE4-OPEloat corresponding mole ratios of the surfactants. This is caused by the extraction of a majority of the shorter EO chain length OPE’s present in the normal distribution compounds into the isooctane phase, thus lowering the effective

0

Narmol

Distribution

10.2

vs. Mole % OPE10

C.M.C.1

L

i

I

o

I

I

I

IO

PO

30

I 40

I

so

I

I

60

70

I

eo

I

I

90

100

I

Fig. 11.-Interfacial tension a t the c.m.c., area per molecule, and c.m.c. us. mole per cent OPELOfor mixtures of single species and normal distribution OPE4 and OPElo.

.

concentration available to forin micelles in the aqueous phase. Since the percentage of individual OPE molecules which are inore soluble in the isooctane phase than in the aqueous phase is greater with the mixtures of the normal distribution OPE’s than with the mixtures of the single species compouiids the observed c.m.c. values are correspondingly greater. This effect diminishes )as the percentage of OPE4 in the mixtures of single species compounds becomes greater. Finally, there is an intersection of the curves since single species OPE, is more oil soluble than normal distribution OPE4 which contains considerable quantities of longer EO chain length molecules that are quite hydrophilic. Acknowledgment.-The authors wish to thank Mr. R. C. Mansfield and Mr. J. E. Locke for synthesis of the homogeneous and normal distribution OPE’s, Mr. TV. Myers for characterization of the homogeneous compounds via thin layer chromatography, and hlr. IT. Smith for characterization of OPEI--8 via gas chromatography.