J. phys. Chem. 1982, 86, 541-545
541
Relationship of Structure to Properties in Surfactants. 10. Surface and Thermodynamic OH, in Aqueous Properties of 2-Dodecyloxypoly(ethenoxyethanol)s, C,2H25(OC2H4)x Solution Milton J. Rosen,' Anna W. Cohen, Manila1 Dahanayake, and XI-yuan Huat Department of Chemistry, Brooklyn College, C/ty Unlvers/fy of New Y&, I n Final Form: August 17, 1981)
Brooklyn, New York 11210 (Received: Apdl 13, 1981;
The surface and thermodynamic properties of homogeneous surfactants of structure C12Hzs(OC2H4),0H, where x = 2-8, purified by passage through a column of octadecylsilanized silica gel, have been investigated. Surface tension as a function of concentration of the surfactant in aqueous solution was measured at 10, 25, and 40 "C,using the Wilhelmy plate technique. From these measurements, the maximum surface excess concentration and the minimum area per molecule at the aqueous solution/air interface, the critical micelle concentration, the surface pressure at the cmc, and the standard thermodynamic parameters of adsorption and micellization were calculated. Structural effects on adsorption,micellization, and the effectiveness of surface tension reduction are discussed in terms of these parameters.
Introduction silanized silica ge1,12 to remove any traces of impurities more surface active than the parent compound. Although polyoxyethylenated nonionic surfactants are The concentrations of the nonionic surfactant solutions, extensively used, both in industrial and in fundamental after passage through the columns, were determined by research applications, there is little reliable information a tensammetric method13 developed especially for this on the surface and thermodynamic properties of wellpurpose. characterized, highly purified compounds of this type. Surface Tension Measurements. Measurements were Investigation of these materials has been hampered by the made by two different investigators, using the Wilhelmy difficulty of separating individual compounds from the plate technique,14working independently. In one instruproduct mixture obtained in the usual method of synment, the Wilhelmy plate (sand-blasted platinum, ca. 5-cm thesizing these surfactants, the reaction of ethylene oxide perimeter) was hung from the arm of a Bethlehem dialwith the hydrophobe, and by the tedium of the alternative type torsion balance, in the other from the arm of a Cahn method for their synthesis, the addition of the oxyethylene electrobalance. Solutions measured with the torsion groups one or two at a time. balance were immersed in a constant temperature bath at Our knowledge of the surface and micellar properties the desired temperature h0.02 "C; those measured with of well-characterized compounds of this type is consethe electrobalance were placed in jacketed dishes with quently limited to an investigation1v2of the member of the circulated through the jacket a t the desired temseries, P - ~ ~ ~ ~ - C ~ H ~ , C where ~ H x~=(1-10, ~ Cand ~ H ~ )water ~~H , perature f0.05 "C. Both instruments were calibrated some studies*l0 on about a dozen individual compounds against quartz-condensed water (specific conductivity 1.1 of structure R(OC2H4),0H, where R is an alkyl chain, X lo4 mho cm-' at 25 "C) each day that measurements usually straight chain. were made. Sets of measurements were taken at 15-min The availability from a commercial sourcell of wellcharacterized surfactants of structure C12H25(OC2H4)rOH, intervals until no significant change occurred. Solutions that had been purified by repeated passage through the where x = 1-8, and the introduction of a convenient reverse octadecylsilanized silica gel columns reached equilibrium phase chromatographic method12 for removing small values in about 1h or less; those that had not been purified amounts of highly surface-active impurities from surfacin this manner showed a continued decrease in surface tants has prompted us to investigate the surface and tension even after several hours. thermodynamic properties of these compounds. Experimental Section Purification and Analysis of the Surfactants. Six nonionic surfactants with structures C12H2s(OC2H4)20H ( E W , C12H26(OC&)30H (E039 Ci2H26(OCzH4)4OH ( E W , C I ~ H ~ S ( O C ~ H ~ ) S O H C12H2dOC2H4)70H (E07), and C12H25(OC2H4)80H (EO@were purchased from Nikko Chemical Co., Tokyo, Japan as compounds of >98% purity as indicated by gas chromatography. Before being used for surface tension measurements, aqueous solutions of the materials (in water that had first been deionized, and then distilled twice, the last time from alkaline permanganate solution through a 3-ft high Vigreaux column with quartz condenser and receiver) were further purified by repeated passage through minicolumns (SEP-PAK cl8 Cartridge, Waters ASSOC.,Milford, MA) of octadecylt Visiting Scholar, People's Republic of China.
(1) E. H. Crook, D. B. Fordyce, and G. F. Trebbi, J. Phys. Chem., 67, 1987 (1963). (2) E. H. Crook, G. F. Trebbi, and D. B. Fordyce, J. Phys. Chem., 68, 3592 (1964). (3) H. Akasu, M. Ueno, and K. Meguro, J. Am. Oil Chem.SOC.,51,519 (1974). (4) J. M. Corkill, J. F. Goodman, and S. P. Harrold, Trans. Faraday SOC.,60,202 (1964). (5) J. M. Corkill, J. F. Goodman, and R. H. Ottewill, Trans. Faraday Soc., 57, 1627 (1961). (6) P. H. Elworthy and A. T. Florence, Kolloid 2.2.Polym., 195, 23 (1964). (7) P. H. Elworthy and C. B. MacFarlane, J. Pharm. Pharmacol., 14, 100 (1962). (8) R. A. Hudson and B. A. Pethica, Chem. Phys. Appl. Surf. Act. Subst., ROC. Int. Congr., 4th, 1964,2, 631-9 (1967). (9) H. Lange, Kolloid Z.,201, 131 (1965). (10) K. Meguro, H. Akasu, and T. Satake in "Colloid and Interface Science",Vol. 2, M. Kerker, Ed.,Academic Press, New York, 1976, p 421. (11) Nikko Chemicals Co., Ltd., Tokyo, Japan. (12) M. J. Rosen, J. Colloid Interface Sci., 79, 587 (1981). (13) M. J. Rosen, X. Y. Hua, P. Bratin, and A. W. Cohen, Anal. Chem., 53, 232 (1981). (14) L. Wilhelmy, Ann. Phys., 119, 177 (1863).
0 1982 American Chemical Society
The Journal of Physical Chemistty, Vol. 86, No. 4, 1982
542
Rosen et ai.
x
10-C
0 25'C
A 40'C
36
-
32
-
36 -
2Bk
2
4
-
-60
u
L----I__L-
---1__L
-5B
-56
-114
- 5 A
-50
-40 log
c
42
-44
-46
-43
,_-A -38
6C
58
56
54
52
48
50
09
-36
46
44
42-&5
c
Flgure 4. Surface tension vs. log of the concentration of CI2Hz5(OC,H,),OH In aqueous solution.
Flgure 1. Surface tension vs. log of the concentration of C,H,(OIn aqueous solution. C,H,)OH
~___
7-7
601
-
56
x
10-C
C 25'C A 40°C
-
52
52 -
-,
48;
--
441
$1.
E
E
431 k 4c-
___ -FS --3
-56
-'4
-51
-50
48
-50 IO,
-44
46
-42
-40
-38
c
Figure 2. Surface tension vs. log of the concentration of CI2Hz5(0C,H,)30H in aqueous solution. 7 -
C
-54
-52
-48
-50 09
-46
-44
-42
-40
-38
-36
c
Figure 5. Surface tension vs. log of the concentration of C,2H25(0C,H4),0H in aqueous solution.
10C1Hnl.
"2'
on
\
48/-
32
56
_________~___~-
60 t
40
-59
-36
-
\\\
i C -
36
1
32
-
~
28;I
I 1 -60
1
58
1
-56
,
-54
-52
-5C
48
-46
. A -42 -4P 36 35
- o P
.in
L C
*L
5 2
log c
Figure 3. Surface tension vs. log of the concentration of C1,H,5(OC,H,)40H in aqueous solution.
Results and Dicussion Critical Micelle Concentrations, Maximum Surface Excess Concentrations, Minimum Surface AreaslMolecule, and Surface Pressures at the Critical Micelle Concentration. Plots of the surface tension (y) at 10,25, and 40 "C of aqueous solutions of E 0 2 , E 0 3 , E 0 4 , E05, E 0 7 , and E 0 8 vs. the log of their bulk phase concentration in mol dm-3 (log C ) in water are shown in Figures 1-6. Critical micelle concentrations (cmc) were taken as the concentrations at the point of intersection of the two linear
"E
3-
03
-44
4 6
YL
42
4-
3E
3-
c
Figure 6. Surface tension vs. log of the concentration of CI2Hz5(0C,H4),0H in aqueous solution.
portions of the y-log C plots. The slope of the linear portion of each curve below the cmc was determined by the method of least mean squares. Maximum surface excess concentrations (r"), in mol cm-2,and minimum areas/molecule (Am), in nm2,at the aqueowlair interface were calculated from the relationships 1
=
f
-r?Y
2.303RT(+cC),
1
The Journal of Physical Chemistty, Vol. 86, No. 4, 1982 543
Structure vs. Properties of Surfactants
TABLE I: Surface Properties of Surfactants of Structure C,,H,,( OC,H,),OH cmc/ rmaxl temp/"(= (mol dm-, x l o 5 ) (mol cm-, x
compd
5.34 3.8 3.3 4.73 3.2 4.72 lo 6.3 4.01 25 5.2 3.98 40 5.6 3.90 lo 8.2 3.96 3.63 25 6.4 40 5.9 3.41 lo 9.0 3.42 25 6.4 (6.0;" 5.6b) 3.31 (3.5b) 40 5.9 3.28 10 12.1 2.85 25 8.2 (8.2;" 7.2c) 2.90 (3.2c) 40 7.3 2.77 10 15.6 2.56 25 10.9 2.52 40 9.3 2.46 Reference 3. Reference 10. lo
C12H25(0C2H4)20H
25 40
C12H25(0C2H4)30H
C12H25(0C2H4)40H
C12H25(0C2H4)50H
C,,H,5(OC,H4),0H CI2H2,(OC2H,),OH
Reference 9, at 23 "C.
Aminl
"t \
TcmJ
(nmz x 100)
(mN m-l)
31.1 35.1 35.2 39.3 41.8 42.5 42.0 45.7 48.1 48.6 50.1 (56;" 4 S b ) 50.6 58.3 57.3 (64.8;" 52c) 59.9 64.9 66.0 67.4
47.4 45.1 43.6 45.4 44.1 43.1 43.9, 43.3, 42.0 41.9 41.5 (41.8;" 42.5b) 41.2 38.3 38.3 (38.1;" 38.7c) 38.5 37.4 37.2 31.3
lolo)
TABLE 11: Thermodynamic Parameters of Micellization for Nonionics of Structure C,,Hz5(OC,H4),OH AS"mic/
46
compd
typ/ C
40 C12H25(0C2H4)30H
lo
25 40 C12H25(0C2H4)40H
lo
25 40 C12H25(0C2H4)50H
lo
25 40 C12H25(0C2H4)70H
37 -
36 C12H23(0C2H4)80H
351
lo
25 40
~
i
i
4
i
Q
Number of oxyethylene units in molecule
'
7
'
0
1
Flgure 7. Effectiveness of surface tension reduction, A, vs. number of oxyethylene units in the molecule for C,2H25(OC2H,)xOHat various temperatures.
where (-dy/d log C), is the slope of the y-log C plot at constant (absolute) temperature, T, R = 8.314 J mol-l K-l, and N is Avogadro's number. Values of cmc, Amin, and Acme (= yo - ycmc,where yo is the surface tension of water and ycmcthe surface tension of the solution at the cmc), are listed in Table I. For comparison, some values from the literature are listed in parentheses. The cmc values show an increase with increase in the number of oxyethylene groups in the molecule from two to eight, as would be expected from the increase in the hydrophilic character of the molecule resulting from this change. A similar increase was reported by Crook and co-workers2for p-tert-odylphenoxypoly(ethenoxyethano1)s as the number of oxyethylene units increased from one to ten. The minimum areas per molecule at the aqueous solution/air interface with increase in the number of oxyethylene units in the molecule in agreement with the relationship observed for other polyoxyethylenated nonion-
lo
25 40
AG'micI
AWmiJ
(kJ mol-')
(kJ mol-')
-23.9 -25.6 -26.9 -22.8 -24.4 -25.8 -22.1 -23.9 -25.4 -21.9 -23.9 -25.3 -21.2 -23.3 -24.8 - 20.6 -22.6 -24.2
(kJ mol-' K-l)
t 4.,
t0.10, t5.,
+0.10,
t8.,
tO.11,
+9.,
tO.11,
t12.,
+0.12,
+13.,
tO.12,
ics9J5that A-n-'I2 (= 21.0-22.9 at 10 "C; 21.7-24.8 at 25 "C; 22.6-24.9 at 40 "C), where n is the number of oxyethylene units in the chain, is almost a constant. The minimum area per molecule also increases with increase in temperature, as would be expected from the increased thermal agitation of the molecules in the surface film. The effectiveness of surface tension reduction, on the other hand, measured by the surface pressure at the cmc,16 rcmC, in these compounds shows a steady decrease with increase in the number of oxyethylene units (Figure 7), in contrast to that reported for the p-tert-octylphenoxypoly(ethenoxyethanol)s,2 where a maximum was observed at three to six oxyethylene units, depending upon the temperature. It is noteworthy here that long-chain alcohols containing six to ten carbon atoms at 20 "C in aqueous solution can produce surface pressures as high as 45 mN of C8H1,0C2H40Hat 25 "C is m-l at 20 OC and that acmc reported by Shinoda and c o - ~ o r k e r s 'as ~ 45 mN m-l, (15)L.Hsiao, H. N. Dunning, and P.B.Lorenz, J.Phys. Chem., 60, 657 (1956). (16)M.J. Rosen, J. Colloid Interface Sci., 56, 320 (1976).
544
Rosen et at.
The Journal of Physlcal Chemistry, Vol. 86, No. 4, 1982
consistent with o w rcmc value of 45.7 mN m-l for ClzH25(OC2H4)20H at the same temperature, whereas Crook and co-workers report a value for p - t e r t C8H1,C6H40C2H40Hof only 37.4 mN m-l. Thermodynamic Parameters of Micellization. Table I1 lists standard free energies, AGOdc, enthalpies, AHomic, and entropies, ASodc, of micellization for the six nonionic surfactants investigated. AGOmicvalues were calculated by use of the relationship AGOmic= RT In cmc (3) The AGOdc values calculated in this manner at each of the three temperatures used (10,25, and 40 "C) were plotted against the number of oxyethylene units in the molecule and smooth curves were drawn through the data points. From these three curves, AHomic and ASodc values at 25 "C were calculated from the relationship A( AGomic)/ AT = -ASomic (4) by using the values for AGOmicat 10 and 40 "C from the smooth curves, and AHomic was calculated from the relationship AHomic = AGomic+ TASomic (5) Table 11lists the values obtained for AHo- and for ASodc at 25 "C. The ASomicvalues in Table I1 are all positive, indicating increased randomness in the system upon transformation of the nonionic surfactant molecules into micelles. The slight increase in the (positive) Modc value with increase in the number of oxyethylene units in the surfactant molecule has been observed by other investigatiorm2q8 The desolvation of the oxyethylene units has been stated2p8J8to be the major contributing factor to the positive entropy of micellization in polyoxyethylenated nonionics. An alternative explanation is that there is less restriction on the motion of the surfactant molecule when it is in the essentially water-free environment of the micelle than in the aqueous phase. This extends to both the hydrophobic alkyl chain, which is in a hydrocarbon-like environment in the interior of the micelle, and the adjacent part of the hydrophilic polyoxyethylene chain, which is freed, on the only partially solvated micellar surface, from some of the restrictions placed upon it by hydrogen bonding to water molecules. This explanation, which assigns the change in entropy to the solute rather than to the solvent, is consistent with a recentlg reevaluation of the concept of entropy of solution. The AHodcvalues in Table 11are all positive, in contrast to those of Crook and co-workers,2who report negative AHodc values at 25 "C for p-tert-octylphenoxypoly(ethenoxyethano1)s continuing less than four oxyethylene units. The values increase with increase in the number of oxyethylene units in the molecule, in accord with the observations of others,2T8indicating that a greater number of hydrogen bonds between polyoxyethylene chain oxygens and water molecules are broken in the micellization process as the number of oxyethylene units in the molecule increases. Thermodynamic Parameters of Adsorption. Table I11 lists of standard free energies, AGOad, enthalpies, A H o a d , and entropies, ASo,,+ of adsorption. AGoad values were calculated by use of the relationship20 AGoad = RT In cmc - ~,,JcmC (6) (17)K.Shinoda,T.Yamanaka, and K. Kinoshita, J.Phys. Chem., 63, 648 (1959). (18)M . J. Schick, J.Phys. Chem., 67, 1796 (1963). (19)D.H.Wertz, J. Am. Chem. Soc., 102,5316 (1980).
TABLE I11 : Thermodynamic Parameters of Adsorption for Nonionics of Structure C,,H,,(OC,H,),OH ASoad/
compd
C,,H,,(OC,H,),OH C,,H,,(OC,H,),OH C,,H,,(OC,H,),OH C,,H,,(OC,H,),OH C,,H,,(OC,H,),OH C,,H,,(OC,H,),OH
AG",/
APad/
teyp/ C
(kJ mol-')
(kJ mol-')
10 25 40 10 25 40 10 25 40 10 25 40 10 25 40 10 25 40
-32.8 -35.2 -36.2 -33.5 -35.5 -36.8 -33.3 -35.9 -37.7 -34.2 -36.2 -37.9 -34.7 -36.9 -38.7 -35.2 -37.4 -39.3
(kJ mol-'
K-')
-I..,
+0.11,
-0.3
+0.11,
+O.,
+0.12,
+O.,
t0.12,
+2.,
+0.13,
+3.,
+0.13,
The standard state for the adsorbed surfactant here is a hypothetical monolayer at its minimum surface area/ molecule, but a t zero surface pressure. The last term in eq 6 expresses the surface work involved in going from zero surface pressure to surface pressure, rcmoat constant minimum surface area/molecule, A- (= A d . When rm, is in mN m-l, A,, in nm2, and R is in J mol-l K-l, with AGOad in kJ mol-l, eq 6 becomes AGoad
= RT In cmc - 6.023 X 10-l~cmJcmc (7)
AHo,*and ASoad values at 25 "C listed in Table I11 are obtained as before from relationships corresponding to eq 4 and 5. The ASoad values are all positive, and slightly greater than the ASo& values for the same compounds. This may reflect the greater freedom of motion of the hydrocarbon chains at the planar air/aqueous solution interface compared to that in the relatively cramped interior beneath the convex surface of the micelle. The change in ASoad with increase in the number of oxyethylene units in the molecule, on the other hand, approximates that observed in ASomic. This is reasonable, since groups at the micellar surface would not experience the space restriction imposed upon groups extending into the interior. The Mead values are all less positive than the AHomic values and become slightly more positive with increase in the number of oxyethylene units in the molecule. This indicates that less bonds between polyoxyethylene chain oxygen and water molecules are broken in the process of adsorption at the air/aqueous solution interface than in micellization, but that, as in micellization, the number of bonds broken increases with increase in the number of oxyethylene units in the molecule. Structural Effects on Adsorption, Micellization, and Effectivenessof Surface Tension Reduction. From eq 3 and 6, it follows that rcmdcmc E TcmJmin
= AG'mic - AGoad
(8)
i.e., the rmJmb product expresses the work involved in transferring the surfactant molecule from a monolayer at zero surface pressure to the micelle. AGOmic- AGOad (= rCmJmin) values are listed in Table IV. (20)M.J. Rosen and S. Aronson, Colloids Surf., 3, 201 (1981).
The Journal of Physlcal Chemistry, Vol. 86, No. 4, 1982 545
Structure vs. Properties of Surfactants
TABLE IV : Structural Effects on Micellization and Adsorption
+8.9 +9.6 +9.3 t 10.7 +11.1 + 11.0 +11.2 +12.0 + 12.3 t 12.3 +12.4 + 12.6 t 13.5 t13.6 + 13.9 + 14.6 t14.8 t 15.1
t 5.,
-3.9
+6.,
-4.8
+8.,
-3.6
+9.,
- 3.,
+lo.,
- 3.,
+9.,
- 5.,
It is apparent that the "work of transfer", which measures the ease of adsorption to form a monolayer a t zero surface pressure relative to the ease of micellization, shows little change with temperature in the 10-40 "C range, but increases steadily with increase in the number of oxyethylene units in the molecule. As mentioned above, the positive values for this work of transfer stem from two sources: (1) the greater positive entropy change upon adsorption than upon micellization, and (2) the smaller positive enthalpy change upon adsorption than upon micellization. Table IV also lists AHomic - mead and T(ASomi, - ASoad) values. The entropy contribution to AGOmic- AGOad remains fairly constant at 3-5 kJ mol-l while the enthalpy contribution increases steadily (from 5., to kJ mol-l) with increase in the number of oxyethylene units in the molecule. The increase in the work of transfer with increase in the number of oxyethylene units in the molecule can, therefore, be seen to be due mainly to the enthalpy factor. These observations are consistent with previous ones that were interpreteda1as indicating that steric factors inhibit micellization more than they do adsorption at the air/aqueous solution interface. In the present case, the structural elements in the surfactant molecule that may cause "steric inhibition" of (21) M. J: h e n in "Solution Chemistry of Surfactants",Vol. I, K. L. Mittal, Ed., Plenum, New York, 1979, pp 45-61.
micellization are (1)the alkyl chain and (2) the polyoxyethylene chain. AB mentioned above, the greater restriction on the motion of the alkyl chain in the relatively cramped interior of the micelle compared to the planar airlaqueous solution interface may be the cause for the entropy contribution to the positive value of the work of transfer. This contribution would be expected to remain essentially unchanged with increase in the number of oxyethylene units in the surfactant molecule. On the other hand, the positive - m 0 , d and their steady increase with values for AHomic increase in the number of oxyethylene units in the molecule indicate that greater dehydration of the polyoxyethylene chain is required for micellization than for adsorption at the airlaqueous solution interface. This implies that the space available to the hydrophilic group at the surface of the micelle is more restricted than at the planar airlaqueous solution interface. Data on other surfactants,2l both ionic and nonionic, also indicate that increase in the size of the hydrophilic group inhibits micellization more than adsorption at the airlaqueous solution interface. From eq 8 it follows also that rcmc
= (AG'mic - AG"a&/Ami,
(9)
This relationship in modified form has been pointed out by us previously.16 Since, as discussed above, bulky hydrophilic groups inhibit micellization more than they do adsorption at the air/aqueous solution interface, rmC, the effectiveness of surface tension reduction, would normally increase as the bulkiness of the hydrophilic group is increased if A- remained constant. In the present case, an increase of one in the number of oxyethylene units in the molecule causes an approximately 15% increase in Ad,.,, while producing an increase of less than 10% in the value of AGOmic- AGOad, resulting in the observed decrease in rcmc (Table I and Figure 7) as the length of the polyoxyethylene chain is increased. The percentage change in (AGOdc - AGOad)with change in temperature from 10 to 40 "C is more or less constant with increase in the number of oxyethylene units in the surfactant molecule, while the percentage change in Ami, which, at lesser oxyethylene content, is greater than the percentage change in (AGOmic- AGOa&, decreases, and approaches that of the latter when the molecule contains more than five oxyethylene units. This accounts for the fusion of three temperature curves in Figure 7 to one, at that oxyethylene content. Acknowledgment. The authors express their thanks to Professor Seymour Aronson of this Department for helpful discussions. This material is based upon work supported by the National Science Foundation under Grant No. ENG-7825930.
