Aggregation of polyoxyethylene glycol monodecyl ether in nonpolar solvents and the effect of water

Aggregation of Polyoxyethylene Glycol Monodecyl Ether in Nonpolar Solvents and the Effect of Water II. Apparent Molar Volumes and Heat Capacities in Cyclohexane and Decane at 298.15 K R E U I T A N A K A ~ AND A K E M I S A I T O

Department of Chemistry, Faculty of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka, Japan 558 Received May 1, 1989; accepted June 23, 1989 The apparent molar volumes V0 and the apparent molar heat capacities Cp,, of the amphiphiles polyoxyethyleneglycolmonodecyl ether Cl0Enfor n = 1 to 8 and decanol were determined in cyclohexane and decane up to 0.8 mole kg-1 at 298.15 K. Those properties of CloE8 in heptane were also measured to complete the series of systems, CloE, + heptane, investigated previously. The process of forming aggregatesdepends on the length of polar chain of the amphiphiles and solvents. Althoughtheir aggregations proceed, in general, gradually with increase in the molality the amphiphile C10E8aggregates as sharply as alkanol in alkane. An abnormal increase in Cp,, at the dilute region due to intramolecular association was observed for CloEn of n N 4 in all of the investigated solvents. By adding a small amount of water the aggregation is enhanced and swollen micelles are formed passing through a sharp transition. The molality where this transition occurs shifts to a smaller value when the polar-chain of the amphiphiles is longer and/or the solvent molecule is a hydrocarbon of a longer straight-chain. © 1990Academic Press, Inc.

INTRODUCTION In the first paper ( 1 ) (which will hereafter be designated as Part I) the apparent m o l a r volumes V+ and the apparent m o l a r heat capacities Cp,o o f p o l y o x y e t h y l e n e glycol m o n o decyl ether C m H 2 1 0 ( C 2 H 4 0 ) n H , C10En for n = 1 to 7, measured in heptane at 298.15 K, have been reported. Those properties for C1oE3 to which a small a m o u n t o f water was added were investigated. It was f o u n d that the aggregations o f these amphiphiles proceed rather gradually with increasing molality o f solutes in c o m p a r i s o n with those o f alkanols which aggregate sharply in alkane (2). It was also f o u n d that the aggregation is dramatically enhanced by adding a relatively small a m o u n t o f water and swollen micelles are formed bey o n d the critical concentration at which a sharp peak in Cp,, appears. Those results are essentially consistent with the conclusion o f Ravey et al. (3) w h o used a small angle neutron scattering technique. F r o m the previous i To whom all correspondence should be addressed.

work it was f o u n d that the heat capacity measurement is very useful in studying the aggregation process o f n o n i o n i c amphiphiles in n o n p o l a r solvents. In this paper, we report the results o f measurements extended to the binary systems o f {C~oEn ( o f n = 1 to 8) or decanol (C10E0) + cyclohexane or decane} and the ternary systems f o r m e d f r o m C~0En o f n = 3 or 6, water, a n d decane at 298.15 K. In order to complete the previous series o f systems the measurements for the binary system o f CloE8 + heptane were also carried out.

EXPERIMENTAL

Materials. The polyoxyethylene glycol monodecyl ether (Nikko Chemicals Co., Ltd.), decanol ( G o l d Label reagent, Aldlich C h e m ical Co.), and decane ( T o k y o Kasei K o g y o Co., Ltd.) were used without further purification. Cyclohexane (Special Grade material, W a k o Pure Chemical Industries Ltd.) was fractionally distilled. H e p t a n e and water were the same samples as used in Part I.

82 0021-9797/90 $3.00 Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990

AGGREGATION OF NONIONIC AMPHIPHILES

Measurements. A vibrating tube densimeter and a flow calorimeter were used for measuring densities p and heat capacities Cv, respectively. The operational procedures for those measurements are described in Part I. In the measurements of Cp each pair of power changes 2u°, due to the difference in the Cp's of the liquids flowed in the sample and reference tubes, was taken by exchanging the sequence of the flowing liquids for (decanol + alkane) systems. This was to cancel out the "boundary effect" (4, 5 ) existing between the two liquids. Nevertheless, only the single values of APwere measured between the mixtures of neighboring molality for (C~0En + decane) systems, since only trivial differences occurred between the results calculated by using a pair of LxP and a single Ap value examined for (CloEn + cyclohexane) systems. In the ternary systems the liquids were flowed in an order such that the concentration of the following system was larger and to avoid the separation of water phase in the mixing region of the two liquids while they were flowed in the cell tubes. The apparent molar volumes Vo and the apparent molar heat capacities Cp,, of solute molecules were calculated from V~ = M / o + ( 1 / p -

llp*)/m,

[1]

and

Cp,, = Mcp + (Cp - c*)/m,

[2]

83

in Part I), 1.6039 for decane, and 1.4337 for cyclohexane. The uncertainties for those compounds were about +0.0002. RESULTS AND DISCUSSION

L Binary Systems The experimental results for V~ and Cp,, of the binary systems {C10En( o f n = 1 to 8) or decanol (C10Eo) + cyclohexane or decane} and (C1oE8 + heptane) are listed in Table I. They are plotted in Figs. 1, 2, and 4 where the smoothed curves were determined by the leastsquares fits to the polynominal equations: V~ or Cp., = ~ ai mi-1 or = Z ai m(i-1)/2. In Table II the extrapolated values at m = 0, Cp~,,, and V~, determined by the least-squares method, are shown. Since erroneous calculations were found in the table for (C10E1 + heptane) reported in Part I the corrected sets of values are also listed in Tables I and II. According to the correction, the values of C~., and V~ are revised only slightly. Although Cp,, values are not seriously changed, the decrease in V, of C10E1 with increasing m is as small as that for Cl0E2. In spite of this revision the essential discussion presented in Part I remains as it is except for the description about the volumetric behavior of CIoE~. The changes in apparent molar volumes with m are plotted as the deviations A V, ( = V, V; °) in Figs. 1 and 2 where the points correspond to each V~ are placed with the same interval on the Y-axis. The values for decanol in decane, calculated from excess volumes reported by Treszczanowicz et al. (7), are shown in Fig. 2, and the calculated values of V~ from their results is 196.0 cm 3 mole -~. Although a systematical difference is seen between the two sets of measurements that may be attributed to a discrepancy in determining the limiting value at m = 0, the relative changes with m are in good agreement. Apparent molar volumes. In each solute a decrease in V, with increasing m is observed and its magnitude is larger in cyclohexane than in decane. The constriction with increasing m -

where M is the molar mass of the solute; p and Cp are the density and the specific heat capacity of the solution, respectively; p* and c* are those quantities of solvents; and m is the molality of the solute. The estimated imprecision in V, and Cp,, have been described in Part I. The values of density p/g cm -3 at 298.15 K were 0.72615 for decane, 0.77385 for cyclohexane, and 0.82672 for decanol with uncertainty of +0.00002. They are compared with the literature values 0.72625 (6), 0.77389 (6), and 0.82670 (7), respectively. The values of heat capacities divided by the volume, (Cp/ V)/J K -~ cm -3 were 1.5238 for heptane (used

Journal of Colloid and Interface Science, Vol. 134, No. I, January 1990

84

T A N A K A A N D SAITO TABLE I Experimental Values for Apparent Molar Heat Capacities Cv,, and Volumes Vo of C~oEn versus Molality m for the Systems (C~oE, + Alkane) at 298.15 K (m ° = 1 mole kg -~, C O = 1 J K ~ mole -~, V° = 1 cm 3 mole -1)

m mo

cp,_._e, CO

v, ~

m m°

Cp,~ Co

v~ "~

m mo

Cp,__~ CO

v~ ~

337.90 522.8 552.5

-198.52 195.85

420.3 441.9 500.3 510.7

~ mo

cp,.__~, Co

v,

0.0293 0.0990 0.3814

370.6 590.5 531.1

199.38 197.85 195.55

239.39 238.56 236.69 235.08

0.0287 0.0933 0.3855 0.7644

426.2 454.6 507.2 509.7

239.05 238.19 236.19 234.86

531.1 557.7 589.8 588.2

282.52 281.37 279.70 277.79

0.0305 0.0798 0.3037 0.7118

538.2 567.1 592.6 586.0

282.06 281.07 279.06 277.51

645.0 663.0 681.8 687.8 678.6

321.86 321.06 320.02 318.28 316.71

0.0186 0.0470 0.1094 0.3554 0.7341

643.3 669.5 685.9 686.0 675.1

321.42 320.60 319.61 317.74 316.36

743.9 744.6 755.8 759.3 755.8

362.12 361.26 359.86 358.82 356.47

0.00975 0.0588 0.1414 0.3033 0.7045

760.7 749.2 762.2 760.9 753.3

-360.68 359.29 357.93 356.14

913.5 834.9 827.9 842.8 853.8 847.6

401.98 401.12 400.43 397.75 395.77

0.00697 0.0195 0.0554 0.1144 0.4093 0.7402

899.5 832.6 828.6 843.5 853.1 847.3

402.05 401.82 400.89 399.63 396.99 395.72

1015.7 935.5 934.8 951.5 942.0

-441.15 439.78 437.25 434.93

0.00607 0.0617 0.1124 0.3905

1004.1 929.9 937.5 951.1

443.01 440.44 439.39 436.54

CIoE, + cydohexane CloEo(deeanol) 0.00494 0.0394 0.1468 0.4741

324.6 406.2 604.7 509.9

-199.25 197.07 195.25

0.0095 0.0441 0.2119 0.6117

324.9 434.4 585.9 486.7

-198.97 196.45 194.95

0.00405 0.0383 0.1308 0.4807

409.3 429.4 467.9 510.4

-238.81 237.83 235.75

0.00821 0.0393 0.1944 0.5661

418.6 430.8 484.9 511.1

-238.84 237.33 235.44

0.00464 0.0407 0.0959 0.4168 0.8360

517.8 547.1 572,8 592,0 583,3

-281.79 280.86 278.48 277.22

0.00962 0.0510 0.1412 0.5181

519.0 552.6 582.9 590.1

-281.54 280.27 278.09

0.00875 0.0275 0.0482 0.1419 0.4522

633.9 655.4 670.8 686.4 683.3

-321.27 320.88 319.37 317.30

0.00944 0.0288 0.0714 0.1872 0.5444

636.0 653.9 679.8 688.3 680.4

321.85 321.01 320.31 318.92 316.93

0.00499 0.0202 0.0614 0.1489 0.3991

772.2 741.6 746.9 758.1 759.8

-361.48 360.66 359.31 357.34

0.00675 0.0276 0.0969 0.1781 0.4988

775.2 743.8 754.5 763.3 757.7

-361.45 360.03 358.92 356.88

O.00375 0.00924 0.0304 0.0658 0.1325 0.5089 0.7946

915.1 847.5 826.2 832.0 845.8 851.6 846.0

-402.45 401.60 400.75 399.36 396.55 395.56

0.00460 0.00964 0.0401 0.0974 0.2014 0.6197

910.8 865.3 825.9 841.4 851.3 851.1

401.98 401.48 401.19 399.98 398.57 396.10

0.00296 0.0100 0.0758 0.1454 0.5306

1027.3 980.5 930.1 942.6 948.2

442.72 442.31 440.08 438.90 435.81

0.00368 0.0151 0.0956 0.1933 0.6560

1020.0 959.6 934.2 947.5 945.0

442.47 442,25 439,78 438,22 435,33

0.01903 0.0652 0.3069

CloE1 0.0188 0.0656 0.2902 0.6775

CIOE2 0.0201 0.0586 0.2075 0.6093

CloE3 0.0177 0.0365 0.0916 0.2684 0.6086

CloE4 0.00967 0.0367 0.1009 0.1982 0.6004

C1oE5 0.00517 0.0149 0.0506 0.0992 0.3026 0.7209

CIoE6

Journal of Colloid and Inte(ace Science, Vol. 134,No. 1, January 1990

0.00486 0.0352 0.0961 0.2933 0.7815

85

AGGREGATION OF NONIONIC AMPHIPHILES TABLE

I--Continued

m

Co,._~,

v~

m

c,.~

V,

m

C,.o

v,

m

C,.,

v,

mo

Co

Vo

mo

Co

~

m°

Co

Vo

mo

Co

-'~

0.00305 0.0199 0.0846 0.3052 0.7068

1088.6 1039.9 1037.6 1057.4 1045.0

484.03 482.39 480.10 476.90 474.81

0.00394 0.0301 0.1218 0.4086 0.8223

1116.1 1035.0 1043.8 1055.4 1041.7

483.89 481.55 479.17 476.11 474.35

0.00700 0.0404 0.1499 0.5111

1069.9 1031.6 1049.6 1052.0

483.11 481.35 478.83 475.57

0.00892 0.0599 0.2029 0.6131

1090.2 1032.3 1054.6 1048.2

482.54 480.64 477.95 475.15

0.00503 0.00892 0.0375 0.1628 0.3970

1175.7 1171.1 1140.6 1173.2 1168.0

523.54 522.34 521.58 518.05 515.45

0.00508 0.0099 0.0585 0.1977 0.4919

1188.3 1156.6 1147.1 1175.4 1161.6

523.84 523.05 520.64 517.51 514.99

1181.3 1144.1 1153.7 1175.9 1155.8

523.47 522.13 520.02 516.77 514.45

0.00741 0.0268 0.0991 0.2974 0.6914

1167.7 1138.5 1160.7 1173.9 1150.0

522.53 521.86 519.41 516.25 514.09

351.1 592.0 571.8

196.17 194.81 193.08

0.0198 0.1162 0.3922

363.6 618.9 543.8

195.67 194.39 192.77

469.7 511.6 520.2

234.63 233.39 232.14

0.0964 0.2981 0.7181

475.2 516.1 518.1

234.45 233.16 231.88

577.2 607.8 604.2

277.33 275.93 274.67

0.0702 0.2469 0.5973

585.3 608.5 601.7

277.02 275.58 274.45

673.5 703.9 698.0

316.70 315.18 313.65

0.0422 0.1993 0.5445

692.0 704.8 695.1

316.71 314.86 313.37

791.6 775.9 790.8 781.1

-356.45 353.83 352.44

0.0184 0.0660 0.3838

781.9 778.1 790.0

-355.98 353.40

902.3 864.2 886.4 887.9

396.69 395.75 394.02 392.71

0.0188 0.0787 0.2447 0.6072

889.8 866.7 889.7 883.2

396.49 395.38 393.60 392.07

CloE7

C,oE8 0.00700 0.0189 0.0790 0.2474 0.5854

C~oEn + decane CloEo(decanol) 0.00504 0.0399 0.1542 0.4946

348.7 436.0 620.3 521.5

-195.58 193.94 192.57

0.00519 0.0572 0.1932 0.5918

346.9 509.8 609.0 505.2

196.39 0.00975 195.31 0.0874 193.63 0.2956 192.42

0.0209 0.1424 0.4042

441.8 493.8 520.6

235.51 234.08 232.72

0.0462 0.1953 0.5004

455.6 521.1

234,97 233,72 232,41

0.0155 0.0959 0.2995 0.7012

544.9 594.5 608.3 598.8

277.71 276.87 275.38 274.24

0.0373 0.1480 0.3992

566.2 604.7 606.8

277.41 276.23 274.98

0.00488 0.0780 0.2486 0.6517

660.4 697.9 704.2 691.9

-316.08 314.57 313.13

0.0103 0.1224 0.3492

661.9 701.7 701.6

317.42 315.54 314.03

0.00727 0.0241 0.0886 0.4990

804.0 780.9 780.6 787.3

--355.66 353.00

0.00950 0.0264 0.1920 0.5980

797.2 775.2 789.4 784.5

-356.8 354.52 352.71

0.0075 0.0229 0.0955 0.3005 0.7004

923.8 879.0 868.6 891.7 879.6

396.26 396.03 395.19 393.23 391.86

0.00964 0.0341 0.1483 0.3997

921.1 871.8 879.2 890.7

-395.91 394.48 392.73

CIoEI 502.8

0.0769 0.2524 0.6048

CloE2 0.0559 0.1958 0.5022

CloE3 0.0199 0.1594 0.4560

CIoE4 0.0136 0.0426 0.2959 0.7256

CloE5 0.0138 0.0580 0.2014 0.5020

Journal o f Colloid and Interface Science,

Vol. !34, No. 1, January1990

TANAKA AND SAITO

86

TABLE I--Continued m

c.,÷

v,

m

cp_.__~ .,

v+

,n

cp_~

v+

__.m

c,._.__~

mo

-~"

Vo

mo

Co

~

m°

CO

~

m°

Co

v+

CtoE6

0.00744 0.0173 0.0505 0.1616 0.4026

1023.5 975.3 963.7 1008.5 1010.5

436.40 -435.20 433.42 431.56

0.00832 1002.3 -0.0126 0.0183 984.1 436.05 0.0287 0.0614 966.8 435.09 0.0813 0.2118 1017.0 432.80 0.2560 0.4864 1003.7 431.24 0.6051

0.00722 0.0545 0.2189 0.5049

1130.6 1079.9 1160.1 1110.7

47 5 . 9 475.11 472.08 470.50

0.0103 0.1016 0.2584 0.5955

1118.3 1136.0 1152.1 1100.8

477.01 474.02 471.72 470.41

0.00309 0.0188 0.1490 0.4086

1211.7 1145.2 1319.5 1226.4

516.34 515.62 512.55 509.9

0.00482 0.0250 0.2012 0.4932

1206.3 1142.4 1292.3 1211.7

CtoE8 516.55 0.00764 515.64 0.0397 511.07 0.2500 509.66 0.6100

978.2 -0.0129 966.7 -0.0376 983.1 434.33 0.1181 1019.0 432.38 0.3084 994.6 430.95 0.7006

997.2 962.2 1001.0 1017.6 986.8

436.16 435.72 433.83 432.01 430.81

1085.0 476.13 0.0286 1156.8 473.40 0.1818 1144.1 471.46 0.4038 1093.0 470.26

1074.8 475.88 1165.9 472.51 1125.7 470.93

1173.9 517.28 0.0102 1154.7 515.07 0.0986 1271.2 510.63 0.3039 1196.6 509.44 0.7131

1163.3 1317.0 1252.7 1186.4

CIoE7

0.0192 0.1279 0.2986 0.6898

515.12 513.04 510.30 509.36

CloEn + heptane CIoEI

0.00872 0.0876 0.5231 2.017

433.7 -0.0163 4 6 7 . 3 232.16 0.1366 524.8 230.38 0.6546 502.7 229.13

0.00706 0.0199 0.0925 0.2971 0.7109

1182.4 -0.00805 1117.1 -0.0298 1174.2 506.88 0.1458 1217.8 505.71 0.3960 1169.1 505.47

436.0 -0.02128 4 3 9 . 2 485.2 231.87 0.2594 511.4 524.2 230.04 0.8221 521.9 CloE8 1173.0 -0.00972 1108.6 -0.0399 1225.3 506.35 0.1946 1202.4 505.56 0.4948

is quite large in C w E v a n d C~0E8 in b o t h solvents. These negative contributions to Vo arise no t only f r o m the attractive forces b e t w e e n the p o l ar chains o f a m p h i p h i l e s b u t also f r o m interstitial a c c o m m o d a t i o n o f solvent m o l e cules w i t h i n the aggregates o f a m p h i p h i l e s as has b e e n suggested in ( a l k a n o l + a l k a n e ) m i x tures (7, 8 ). T h e values o f V ~ are plotted as a f u n c t i o n o f n in Fig. 3 w h e r e those in h e p t a n e r e p o r t e d in Part I are also shown. T h e y are strictly linear against n. T h e m a g n i t u d e o f V ; ° for each a m phiphile decreases slightly d e p e n d i n g on the Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990

1149.3 1124.0 1232.8 1189.5

232.71 0.0590 231.21 0.3822 229.83 1.276

--506.04 505.43

0.0164 0.0458 0.2346 0.6204

4 5 7 . 6 232.65 521.9 230.75 513.5 229.42

1112.9 -1112.2 506.95 1228.3 505.77 1176.9 505.48

solvents in o r d er o f c y c l o h e x a n e > d e c a n e > heptane. T h o s e differences b e c o m e large as the length o f o x y e t h y l e n e chain increases. T h e i n c r e m e n t s in V ; ° values o f C10E, for each additional oxyethylene group are 40.42, 40.01, an d 39.02 c m 3 m o l e - t in cyclohexane, decane, a n d heptane, respectively. Apparent molar heat capacities. T h e experi m e n t a l results o f a p p a r e n t m o l a r heat capacities Cp,~ are s h o w n in Fig. 4 al o n g with those for (Ct0 En + h e p t a n e ) reported in Part I. Th er e a p p e a r to be no significant differences in Cp,~ curves for each solute f r o m d e c a n o l (C1oEo)

A G G R E G A T I O N OF NONIONIC AMPHIPHILES 0

i

i

r

i

r

J

I

87

TABLE II

I

Apparent Molar Heat Capacities C~,~ and Volumes V~ of Cl0E, at the Infinite Dilution for the Systems (C~oE, + Alkane) at 298.15 K (C O= 1 J K -1 mole-l, Vo = 1 em 3 mole -1 ) -5

QoE.

E

n

9"

-~o

-15I

~ 0

J

I

I

0.3

m/(mo[

I

i

L

O'

A "7 0

__c~,,

v~

__c~*

v~

V°

Co

V°

CO

V°

327 410 511 622 814 945 1057 ll20 1213

Decane

200.3 239.7 283.0 322.2 362.2 402.3 442.9 483.8 523.6

353 429 526 651 825 947 1054 1169 1253

Heptane

196.4 235.7 277.9 317.4 356.9 396.6 436.6 476.8 516.6

(314) a 428 c 523 c 616 ~ 843 c 928 c 1050 ~ 1192 c 1236

(194.8) b 232.9 c 275.7 c 314.8 c 352.8 ~ 392.3 ~ 429.9 c 467.8 c 507.5

0.9

• k g-t )0.6

FIG. 1. Deviations of apparent molar volumes A Vo ( = Vo - V~) of CloE~ in cyclohexane at 298.15 K. Labels indicate n. Marks represent experimental points.

AA

vz

C~

Cyclohexane

0 1 2 3 4 5 6 7 8

I

cz,

•

1

•

-5

Extrapolated value from the results in Ref. (2). b From Ref. (8). c Determined previously (Part I).

to C1oE4 due to the solvents. The large and sharp maximum in Cp,~ for decanol indicates that the solutes aggregate sharply when their concentration is increased near the molality where the peak appears (1, 2). On the other hand the present amphiphiles, especially with 55

r

i

~

2]

i

i

z

i

r

E

z.,5 u

"5 E

~s'_lO

~E35 u 2:< -15

25

,

,

0.3

.

O.6

,

,

0.9

r n / ( m o l . k g -1 )

FIG. 2. Deviations of apparent molar volumes AVo (=Vo - V~) of C~oE, in decane at 298.15 K. Labels indicate n. Marks represent experimental points. A, deeanol from Ref. (7).

150

1

J

3

r

4 n

r

5

,

6

,

7

FIG. 3. Apparent molar volumes at the infinite dilution Vg° of C~oE, in alkanes. Marks represent experimental points: A, cyclohexane; ©, decane; [3, heptane. Journal of Colloid and Interface Science, V o l .

134, N o . 1, J a n u a r y

1990

88

TANAKA AND SAITO

9 I],

'

. . . . . .

1/*

(o)

.

t

,

,

,

,

,

"

o

~

'(b)

1

8

_

°10

~-~

I

o

I

I

I

o13

I

m /(mo[.kg -1)

I

'

i

i

i

m/(mol.kg -t

FIG. 4. Apparent molar heat capacities Cp,, of Cx0E, in alkanes at 298.15 K. Labels indicate n. Marks represent experimental points: L~, and 4, cyclohexane; [3, heptane; © and e, decane.

a short polar-chain, have no such sharp maximum in Cp,,, and accordingly, it is suggested that those amphiphiles aggregate gradually with increasing m. Nakamura et al. (9) have found from IR measurements that the OH group ofpolyoxyethylene glycol monododecyl ether C~2E, in CC14 forms intramolecular hydrogen bondings with the oxygen of neighboring oxyethylene groups and also with others in the molecules with a longer polar chain. Thus, the O H group of C10E~ is partially blocked due to intramolecular hydrogen bondings so that the predominant attractive forces for their intermolecular interactions are not hydrogen bondings but van der Waal's forces. Therefore, the process of forming aggregates in these amphiphiles is gradual with increasing m contrastive to the alkanols in saturated hydrocarbons. A split in Cp,, curves occurs for C10E, of n 5 due to the solvents and it becomes remarkable as n increases. The peak in Cp,, is very low and broad for all the amphiphiles in cyclohexane. This suggests that the aggregates Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990

for C10En are formed very gradually with increasing m in cyclohexane. However, by exchanging the solvent from cyclohexane to a straight chain alkane the peak becomes large and sharp. It becomes sharper and shifts to a smaller m in the sequence of solvent: cyclohexane > heptane > decane. From these systematical changes in Cp,o with n of C10E, and the solvents it appears that the aggregations are enhanced in C10En, that has a longer polarchain, and are favored in the solvents of hydrocarbons of a longer straight-chain. Actually the Cp,o of CloE8 in decane accompanies as large and sharp a maximum as in the case of decanol in alkane, indicating that its aggregation proceeds very sharply with increasing rn. These conclusions agree well with those of Ravey et al. (3) who used amphiphiles C~2E~. According to their report the aggregation numbers of C12E4 are, for instance, at 293 K and at 0.28 mole kg -I, 3 in cyclohexane, 5 in heptane, and 9 in decane. The aggregation number for C12E6 in decane increases to 23 at the same temperature and concentration. The

AGGREGATION OF NONIONIC AMPHIPHILES formations of these aggregates, which take an extended (hank-like) form ( 3 ), are favorable in straight-chain alkanes because those hydrocarbons tend to take a parallel molecular arrangement, or in other words, an orientation correlation exists between neighboring chains (10, l l ) . A sharp increase in Cp,, with decreasing m below 0.05 mole kg -I is again observed for C10En o f n ~ 4 in decane and cyclohexane, as was also the case in heptane. In extremely dilute regions the intermolecular interactions become scarce so that those abnormal increases in Cp,, are attributed to the thermal relaxation due to an intramolecular association a m o n g the oxyethylene groups, the conformations of which are in equilibrium between extended form and rounded form. From these results we speculate that the molecules Cl0En with n >~ 4 take more or less a rounded form at extremely dilute regions. They become extended forms when the molality is increased beyond the region where Cp,, reaches minim u m , while m a n y of the solutes still remain as m o n o m e r i c one. We have no explanation, however, how m o n o m e r i c ones are stabilized in extended forms in this region. The extrapolated values Cv~,, at m = 0 are shown in Fig. 5 as a function of n where the previous results obtained in heptane are also included. These values are in linear relation with n within experimental error, but experience a j u m p due to a conformational change in the m o n o m e r s between n = 3 and 4. It is interesting that the values of C~,, are independent of the solvents. All the points determined for the three solvents were treated similarly and the gradient of Cp~°, with n was calculated by the least-squares method: each addition of the oxyethylene group gives an increase of 99.2 J K -~ mole -~ for n ~ 3 and 103 J K -] mole -1 for n ~> 4.

H. Ternary Systems Apparent molar heat capacities. Since the a m o u n t of added water is relatively small the mixed solute (Cl0E~ + r-H20) was regarded

13

i

i

I

11

89

// r

i

~

i

"T

/

s. 7

.oJ

J

I

I

I

4

5

6

7

I'1

FIG. 5. Apparent molar heat capacities at the infinite dilution Cp~,~of C~0E,in alkanes at 298.15 K. Marks represent experimental points: A, cyclohexane;D, heptane; ©, decane.

as a c o m p o n e n t molecule and the molar mass M of the mixed solute was defined, as in Part I, by M(mix) = M(CIoE,) + r- M(H20).

[3]

The ratio r of the a m o u n t of water added to C~0E, was adjusted to an accuracy better than 99.9% by using a microsyringe. Figure 6 shows the experimental results of Cp,~ for (C~0E3 + r . H 2 0 ) in decane along with those for (CIoE3 + 0.3. H 2 0 ) in heptane as reported in Part I. The change in Co,, of the present systems with rn is very similar to that in heptane. In the very dilute region the solution is transparent and Cp,, increases linearly with increasing m because the water molecules are in equilibrium between m o n o m e r i c ones and binding ones on the monomeric amphiphiles. By further increment of m the phase of water separates out as expressed with dotted lines. And then, the system turns emulsion accompanying a sharp increase in Co,,. The mixture of (C10E3 + 0.2. HeO + decane) is a transitional one: it has neither a two-phase region Journal of Colloid and Interface Science, VoL 134, No. 1, J a n u a ry 1990

90

TANAKA AND SAITO 90

,

,

¢

E ~eO

o 70

0

t

0~2

~

m/(mo|.kg

)0 ~

-1 .4

0.6

FIG. 6. Apparent molar heat capacities Cp,o of mixed solutes (CloE3 + r . H20) in alkanes at 298.15 K. Labels indicate r. Marks represent experimental points: e, heptane (from Part I); O, [], and A, decane.

nor a sharp peak in Cp,o but has emulsion in the region from 0.06 to 0.14 mole k g - 1 . The emulsion changes transparent and swollen micelles which are formed at a higher molality where Cp,~ reaches a maximum. This very sharp peak suggests that the transition between the emulsion and the micellar phase is a first-order type. Therefore, we may assign a critical concentration to form swollen micelles at the molality where the peak appears. It is seen that the peak in Cp,~ for (CmE3 + 0.3. H 2 0 ) observed in heptane is shifted to a lower m in the system with decane. This indicates that the formation of swollen micelles are, as in the binary systems, enhanced more in decane than in heptane. Figure 7 shows Cp,~ of(Cl0E6 + r . HzO) in decane. In these mixtures the water phase is not separated out as long as the amount of water is increased up to r = 1,0 while phase separation occurred in (C10E3 + r . H 2 0 q- decane) with r = 0.3. This suggests that the water molecules are stabilized while trapped in the flexible oxyethylene chain of C10E6 in the dilute region where most of the amphiphiles exist as monomers. Emulsion was observed for the mixtures with r 1t>0.6 in a very narrow range of about 0.02 mole kg -z at just below the molality where the peak in Cp,~ appears. From these results it is concluded that the sharp peak Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990

in Cp,, observed by adding water is not inherent in the transition between the emulsion and the swollen micellar phase but an essential phenomenon in the formation of the swollen miceUes. It is clearly seen, from the shift of the peak to a lower m, that the formation of aggregates is enhanced as the amount o f water is increased. The maximum becomes sharper by increasing the amount of water, but the water is still gentle up to r = 0.3. This shows that their aggregation process is a "pseudotransition" of second-order type. By further addition of water (r ~ 0.4) the maximum turns to a sharp peak and this suggests that the aggregates are formed very sharply while passing through a transition of first-order type. The critical concentration, where the peak in Cp,~ appears, is shifted to a lower m by increasing r and reaches asymptotically a constant m of about 0.03 mole kg -1. The motive forces of enhancing the coalescence of the amphiphiles are the intermolecular hydrogen bondings made by water among the amphiphiles. Those bridges of water can be located at favorite sites

14

~

i

J

~... 13

~: 12

1.0

o ull

/i0 "2

0

02 0,4 m I ( m o t - k g "1)

0.6

FIG. 7. Apparent molar heat capacities Cp,~ of mixed solutes (C1oE6 + r . H20) in decane at 298.15 K. Marks represent experimental points. Labels indicate r.

AGGREGATION OF NONIONIC AMPH1PHILES

~55

445 E.

325'

315

r

O J2

~

m / (mot.kg-)

10.4

0.6

FIG. 8, Apparent molar volumes V~ of mixed solutes (C~0E, + r. H20 ) in decane at 298.15 K. Labels indicate r. Marks represent experimental points: O, D, and /x, CmE6; • and m, C~0E3.

of the flexible polar-chains suffering less sterichindrance, thereby less entropy loss; the intermolecular forces are much more effective in swollen micelles than those in alkanol-alkanol. The shift of critical concentration to a lower m in (C~oE6 q- r . H 2 0 ) compared to that in (C~0E3 + r . H 2 0 ) observed in decane shows that a longer chain of oxyethylene groups helps stabilize the swollen micelles. The behavior in Cp,, for (CIoE6 + r . H20) in the extremely dilute region could not be made clear because those solutions were unstable when they were flowed in the cell tubes and the precision of measurements was quite poor. Apparent molar volumes. The experimental results of V, for the ternary systems are shown in Fig. 8. The two-phase regions are expressed

91

by the dotted line. As in (CIoE3 q- r . H 2 0 + heptane), the curves of V~ for the present systems changes continuously with m passing over the critical concentration of forming swollen micelles. By increasing the amount of water, the decrease in volume with increasing m becomes large, especially at infinitely dilute regions. This result is acceptable consistently with the fact that water molecules dramatically enhance the formation of aggregates as discussed so far. From the plot of limiting values V, vs r, which is the same method used in Part I, the molar volume of water dispersed as monomers in decane was estimated to be 20 __+2 cm 3 mole -~. This is compared with the molar volume obtained in heptane, 18 + 1 cm 3 mole -1 reported in Part 1. The present value in decane is higher than that of pure water as has been mentioned by Caron and Desnoyers (12). REFERENCES 1. Tanaka, R., J. ColloidlnterfaceSci. 122, 220 (1988). 2. Tanaka, R., Toyama, S., and Murakami, S., J. Chem. Thermodyn. 18, 63 ( 1986 ). 3. Ravey, J. C., Buzier, M., and Picot, C., J. Colloid Interface Sci. 97, 9 (1984). 4. Fortier, J.-L, Benson, G. C., and Picker, P., J. Chem. Thermodyn. 8, 289 (1976). 5. Tanaka, R., J. Chem. Thermodyn. 14, 259 (1982). 6. Riddick, J. A., and Bunger, W. B., "Organic Solvents," Vol. II, " Techniques of Chemistry" (A. Weissberger, Ed). Weily-Interscicnce,New York, 1970. 7. Treszczanowicz, A. J., Kiyohara, O., and Benson, G. C., J. Chem. Thermodyn. 13, 253 (1981). 8. Treszczanowicz, A, J., and Benson, G. C., J. Chem. Thermodyn. 10, 967 (1978). 9. Nakamura, M., Miura, T., and Yano, N., lnt. Congr. Surf. Act. Subst. 7th 2, 773 (1978). 10. Clement, C., J. Chim. Phys. Phys.-Chim. Biol. 75, 747 (1978). 11. Bhattacharyya, S. N., and Patterson, D., J. Solution Chem. 9, 247 (1980). 12. Caron, G., and Desnoyers, J. E., J. Colloid Interface Sci. 119, 141 (1987).

Journal of Colloid and Interface Science, Vol. 134, No. 1, January 1990