- Email: [email protected]
Adsorption of Polyoxyethylenic Surfactants on Quartz, Kaolin, and Dolomite: A Correlation between Surfactant Structure and Solid Surface Nature
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
181, 571–580 (1996)
0414
Adsorption of Polyoxyethylenic Surfactants on Quartz, Kaolin, and Dolomite: A Correlation between Surfactant Structure and Solid Surface Nature D. M. NEVSK AIA, 1 A. GUERRERO-RUI´Z,
AND
J.
DE
D. LO´PEZ-GONZA´LEZ
Departamento de QuıB mica Inorga´nica y QuıB mica Te´cnica, Facultad de Ciencias, Universidad Nacional de Educacio´n a Distancia (UNED), Madrid 28040, Spain Received October 27, 1995; accepted March 1, 1996
The influence of variables such as adsorption temperature, polar chain length, and nature of functional groups on the adsorption, from aqueous solutions, of various surfactants (TX-114, TX-100, TX-165, TX-305, NP1P4E, NP4P1E, NP4S, NP10S, and NP25S) has been investigated. Several nonporous solids, including various samples of quartz, QA (5.1 m2 g 01 ), QB (5.5 m2 g 01 ), and QC (4.0 m2 g 01 ), kaolin (19.3 m2 g 01 ), and dolomite (0.3 m2 g 01 ), were studied. Conformational changes of adsorbed surfactant molecules on QC quartz, when the oxyethylenic length of Tritons increases, have been detected. For all the other solid samples the surface is not completely covered by Tritons. On quartz, the surfactants are adsorbed by hydrogen bonds between the surfactant’s ether groups and the silanol groups of the solid surface. These hydroxyl groups must be free and sufficiently separated from other hydroxyls of the solid surface. When the number of propoxy groups increases (from NP1P4E to NP4P1E) the adsorbed amount of surfactant on the solid studied decreases. Anionic surfactants are adsorbed on quartz in lower amounts than the corresponding nonionic surfactants. However, the adsorbed amounts of Tritons and sulfated Tritons on kaolin are similar, probably due to the positive charges on the edges of this material. q 1996 Academic Press, Inc. Key Words: polyoxyethylenic surfactants; aggregate conformation; nature of surface.
INTRODUCTION
Adsorption of a surfactant at a liquid–solid interface makes up the basis of many technological processes such as detergency (1, 2), flotation, water treatment (3), and enhanced oil recovery (4). The phenomena of adsorption processes at a liquid–solid interface are complicated in comparison with adsorption at a solid–gas interface due to the presence of a third component, the solvent. For instance, the solvent can also be adsorbed onto the solid surface, compet1 To whom correspondence should be addressed. Fax: 34-1-3986697. Email: [email protected].
ing with the surfactant (adsorbate). Moreover, interactions between surfactant and solvent should also be allowed for. There are some internal factors such as the length and nature of the polar chain of the surfactant and nature of the solid surface that must be taken into account. Somasundaran et al. (5) related the length of the ethoxyl chain with the conformational behavior of the adsorbed molecules. Sobisch (6) observed that the adsorbed amount increases with a decrease in the of degree of ethoxylation. Bo¨hmer et al. (7), using an extension of the self-consistent-field theory of Scheutjens and Fleer for chain molecules (8), calculated the adsorption isotherms of polyoxyethylenic surfactants. They found, in agreement with experimental findings, that the critical micelle concentration (cmc) increases and the aggregation number of micelles decreases as the hydrophilic block becomes longer. Some external factors which affect the adsorption behavior (9–15) are adsorption temperature, salinity, pH, etc. Thus, Gaufres et al. (16) found conformational changes as a function of temperature in an En Em (oxyethylenic surfactants)/water system. In many adsorption studies authors pay much more attention to surfactants than to solid surfaces (5, 7, 15, 17). For instance, adsorption temperature, salinity, or polar chain length is studied. In addition to the above-mentioned factors, which can affect the interactions between surface and surfactants in various ways, the chemical nature of the adsorbent should be considered. In many cases a silica is chosen as the solid adsorbent for this kind of study. The reason for this is that the interaction of surfactant and solid surface is a process that is difficult to evaluate and where, usually, interactions between functional groups of the solid surface and the surfactant are involved. For example, if there are different functional groups on the solid surface, an additional problem concerning understanding the surfactant molecule–solid surface interactions arises. One way to simplify this problem is to choose a very well known and not very complicated surface on which to carry out the adsorption processes.
571
AID
JCIS 4294
/
6g12$$$621
07-04-96 03:05:01
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
572
On the other hand, a widely studied group of surfactants is the polyoxyethylenic [( –OCH2 –CH2 – )n )] (5–7, 15–21) group, which has the advantage that its hydrophobic nature can be controlled not only by modifying the number of carbon atoms in its hydrophobic chain, but also by varying the oxyethylenic chain. Consequently, this type of surfactant exhibits a wide range of physical properties and it is possible to select the more appropriate surfactant for a given application. The aim of this work is to study adsorption on a liquid– solid interface. For this purpose, the effect of some variables has been investigated. Adsorption temperature, as an external variable, has been considered. With respect to surfactants, the effects of the length of the polar chain on possible conformational changes of the adsorbed molecules and of the nature of the functional groups on the shape of adsorption isotherms have been studied. Also, a number of solids of different natures (quartz, kaolin, and dolomite) have been chosen in order to investigate how these solid surfaces affect the adsorption processes because these kinds of materials are frequently found in oil reservoirs. EXPERIMENTAL
The solid sample used in this study was a natural quartz (Sifraco C-600), 98.8% purity, which is denoted as QA. This sample (QA) was treated with a HCl solution (pH 1.5) and subsequently washed with distilled water until there were no chloride ions in the washing water as detected with a solution of AgNO3 . Afterward, the sample was dried in a vacuum oven for a few hours and denoted as QB. Sample QC was obtained by calcination of sample QB in an oven at 1273 K for 9 h. A kaolin sample from Sigma and a natural dolomite sample from Granada (Spain) was also subjected to study. The nonionic surfactants used were a series of Tritons (from Rohm and Haas Co.) with purity ú98% and the general formula C8H17 –Ph– (OCH2 –CH2 )n –OH, with n Å 7 for TX-114, n Å 9.5 for TX-100, n Å 16 for TX-165, and n Å 30 for TX-305. Two nonylphenols (from ICI, Petrochemicals and Plastics Division) were used, with purity ú96% and the general formula C9H19 –Ph– (OCH2 –CH2 –CH2 )x – (OCH2 –CH2 )n –OH, with x Å 1 and n Å 4 for NP1P4E and x Å 4 and n Å 1 for NP4P1E. A series of anionic surfactants were used as well, with purity ú94% and the general formula
AID
JCIS 4294
/
6g12$$$622
07-04-96 03:05:01
TABLE 1 Some Characteristics of Surfactants Surfactant
n
x
TX-114 TX-100 TX-165 TX-305 NP1P4E NP4P1E NP4S NP10S NP25S
7 9.5 16 30 4 1 4 10 25
— — — — 1 4 — — —
cmc (mol/kg)a 2.1 2.7 7.0 1.0 5.0 3.0 4.0 3.0 1.5
1 1 1 1 1 1 1 1 1
1004 1004 1004 1003 1005 1005 1004 1004 1004
a
Determined in the Thermodynamic and Microcalorimetric Centre (CTM) or (CNRS) of Marseille by the Wilhelmy slide method at 293 K.
C9H19 –Ph– (OCH2 –CH2 )n –SO4 –Na / , with n Å 4 for NP4S (from Stepan Europe), n Å 10 for NP10S (from CFPI), and n Å 25 for NP25S (from Witco Chemicals). Some characteristics of the surfactants mentioned are included in Table 1. The specific surface areas of the solids were determined by nitrogen adsorption at 77 K, taking 16.2 nm2 as the crosssectional area for the adsorbed nitrogen molecule. An automatic Micromeritics ASAP 2000 volumetric system was used to obtain the corresponding gas adsorption isotherms. The values obtained for specific surface areas are 5.1 for QA, 5.5 for QB, 4.0 for QC, 19.3 for kaolin, and É0.3 m2 g 01 for dolomite with accuracy §0.1 m2 g 01 . The surfactant adsorption isotherms from water solutions were obtained by the immersion method (22). A few grams of solid was put in contact with surfactant water solutions (20 cm3 ) of known concentrations. Equilibrium was reached after 24 h with ellipsoidal stirring at 273, 298, and 308 K; the stirring rate was 100 rpm. The solids were then separated by centrifugation, at their corresponding temperatures, at 3000g. The amount adsorbed at 273, 298, and 308 K was calculated on the basis of the concentration change determined by UV spectroscopy at 225 and 275 nm. The experimental error was less than 5%. More details about this experimental method are given elsewhere (23). The pH of the solutions was measured before and after 24 h of contact between the solids and the solutions, using a PHB-62 pH meter. The obtained pH values for each solid do not present significant variations with different surfactants. They are as follows: 8.6 { 0.2 for QA; 5.7 { 0.3 for QB; 7.2 { 0.1 for QC; 5.0 { 0.5 for kaolin; and 9.6 { 0.2 for dolomite. The XRD patterns were obtained with a Seifert 3000P diffractometer using nickel-filtered copper Ka radiation ( l Å 0.1538 nm). The infrared spectra were recorded in a Nicolet ZDX Fourier-transformed spectrophotometer with a resolution of 4 cm01 . Self-supporting wafers of the samples with
coidas
AP: Colloid
LIQUID-PHASE OXYETHYLENIC SURFACTANT ADSORPTION
FIG. 1. FTIR spectra of kaolin at (A) 773 K, (B) 373 K, and (C) 296 K.
573
kaolin sample. Evolution of IR spectra with the pretreatment temperature under vacuum is also presented. At 296 K a wide band is observed between 3730 and 3570 cm01 , which corresponds to hydroxyl groups which are hydrogen-bound (25), (Fig. 1C). When treatment temperature increases (up 373 K), adsorbed water is almost eliminated and a series of bands appear (3660, 3635, and 3580 cm01 ) (Fig. 1B). These bands can be attributed to hydrogen-bound hydroxyl groups. A characteristic peak at 3420–3430 cm01 (Figs. 1B and 1C) is assigned to molecular adsorbed water (26). Finally, on the kaolin pretreated at 773 K (Fig. 1A), narrow bands can be observed at 3745, 3700, 3660, 3625, and 3570 cm01 . The new two peaks at higher wavelength can be assigned to OH linked to two octahedral Al 3/ (3745 cm01 ), and to one OH coordinated to three octahedral Al 3/ (3700 cm01 ) (27). It should also be noted that an OH coordinated to one Al 3/ in an octahedral site, which should give a band between 3785 and 3800 cm01 , does not appear in this sample. This assignment, for which only Al–OH bonds are considered, is consistent with the structure of kaolin. For this solid only OH groups are bonded to the octahedral layer, which contains Al 3/ ions (27). Thus, when the kaolin sample is heated, the hydroxyl-bound groups begin to suffer a condensation process (28). Brindley and colleagues (29) showed that kaolin is deshydroxylated at 723–823 K to produce metakaolin. Thus, at room temperature, it seems that almost all of the hydroxyl groups on the surface of kaolin are hydrogenbound. Effect of the Adsorption Temperature
weight-to-surface ratios of about 10 mgrcm02 were placed in a vacuum greaseless stopcock cell, which enabled us to carry out ‘‘in situ’’ pretreatment. RESULTS AND DISCUSSION
Solids Characterization The three quartz samples have been characterized in a previous work (23). The state of the silica surface strongly affected the process of adsorption of Triton X-100 from aqueous solution. It was shown that the parameter which determines the amount of adsorbed surfactant was not the amount of OH nm02 but the nature of the surface hydroxyl groups. The three quartz samples exhibit b-quartz structure (XRD). Furthermore, on the QA sample surface, impurity traces of Na, Ca, and Al were detected by X-ray photoelectron spectroscopy (23). Most of the surface hydroxyl groups of samples QA and QB are hydrogen-bound while in sample QC they are free and isolated (23). X-ray diffraction patterns of kaolin and dolomite allow us to identify our samples as a typical kaolinite and dolomite (24). Figure 1 shows FTIR spectra in the vibration range of the hydroxyl groups of the
AID
JCIS 4294
/
6g12$$$622
07-04-96 03:05:01
Figure 2 shows the adsorbed amount ( G in mmol/m 2 ) of TX-114 on kaolin as a function of the equilibrium concentration (Ceq in mol/liter) at 273, 298, and 308 K. The evolution of the adsorption isotherms with temperature is similar for all the surfactants and all samples studied. This behavior during Triton adsorption has been explained by many authors (30–34). At low Ceq or a low degree of coverage u ( u Å G / Gmax ) the monomers are adsorbed on the solid surface, displacing water molecules. This kind of process has an exothermic nature. When the adsorption temperature is increased, the amount of adsorbed surfactant decreases. When the majority of the adsorption sites are covered, interactions take place between surfactant molecules to form micelles. This process is endothermic. Thus, at high Ceq or a high degree of coverage, when the adsorption temperature is increased, the adsorbed amount also rises. According to modern polymer adsorption theory (8), this kind of behavior is expected since the Flory–Huggins parameter x for the oxyethylenic (EO) part increases with T. The parameter xxy has been defined as the energy change (in units of kT) associated with the transfer of a segment of type x from a solution of pure x to a solution of pure y (35). Kjellander and
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
574
FIG. 2. Adsorption isotherms of TX-114 on kaolin at ( j ) 273 K, ( l ) 298 K, and ( m ) 308 K.
FIG. 3. Adsorption isotherms of ( j ) TX-114, ( l ) TX-100, ( m ) TX165, and ( . ) TX-305 on QC at 298 K.
Florin (36) studied the physical meaning of the interaction polymer–solvent parameter, x, of Flory. They found that evolution between 298 and 348 K is visibly linear, and they attain the numerical expression
of the larger size of the molecules (37). A displacement toward higher Ceq of the value at which the ‘‘plateau’’ is reached is also observed. The reason for this is that when oxyethylenic length rises, the monomer solubility in water is higher and, thus, the cmc is also higher (38). The adsorption capacity of the samples for the four Tritons, in general, follows the trend QC ú QB ú QA ú kaolin ú dolomite (Table 2), except for TX-114, for which QA and kaolin positions are inverted, and for TX-305, for which the values of the last three samples are very close. Usually it is accepted that adsorption of this kind of surfactant is
x(T ) Å (5.879 1 10 03 )T 0 1.6583.
[1]
This means that an increase in adsorption temperature favors contacts between polyoxyethylenic chains over water–polyoxyethylenic chain interactions. Or, in other words, micelle formation is favored when the temperature increases. This effect is in agreement with the experimental observation that when temperature increases, the cmc diminishes. Effect of the Polar Chain Length In Fig. 3, adsorption isotherms of four Tritons (TX-114, TX-100, TX-165, and TX-305) on quartz QC at 298 K are represented. Similar patterns were obtained for QA, QB, kaolin, and dolomite samples, which are not reported as figures for the sake of brevity. As can be observed, when oxyethylenic chain length increases, there is a decrease in the adsorbed amounts (on the five solid samples) because
AID
JCIS 4294
/
6g12$$$622
07-04-96 03:05:01
TABLE 2 Adsorbed Amounts of Triton Surfactants, in mmol/m2, at the ‘‘Plateau’’ at 298 K Sample
TX-114
TX-100
TX-165
TX-305
QA QB QC Kaolin Dolomite
1.47 2.69 4.79 1.74 1.20
0.72 1.59 2.71 0.66 0.42
0.44 0.96 1.88 0.44 0.30
0.23 0.49 0.87 0.24 0.23
coidas
AP: Colloid
LIQUID-PHASE OXYETHYLENIC SURFACTANT ADSORPTION
produced by interactions between ether groups of oxyethylenic chains and surface hydroxyl groups. Moreover, these hydroxyls must be free and sufficiently separated from other hydroxyls on the solid surface (23). Previously it has been stated that the QC surface has free and isolated hydroxyl groups while in the rest of the samples hydrogen-bound hydroxyl groups are predominant. This may be why QC quartz adsorbs more Tritons (Table 2) than the rest of the samples. Also, impurities present on the surface of QA quartz can diminish the amount adsorbed (23). As can be observed from Fig. 3, the shape of the isotherms changes when the oxyethylenic chain length increases, from S to L shape. According to the classification of Giles et al. (39), S-shaped isotherms are obtained when the solvent is strongly adsorbed on the surface or there are strong intermolecular interactions, or when the longitudinal axes of the adsorbed molecules are perpendicular to the adsorbent surface. Lshaped isotherms are also called Langmuir isotherms. They are obtained when there is not strong competition between the solvent and the adsorbent to occupy adsorbent surface sites. In this case, longitudinal axes of adsorbed molecules are parallel to the adsorbent surface. Furthermore, Rudzinski et al. (17) consider that S-shaped isotherms indicate that there are interactions between hydrophobic moieties of surfactant molecules, while in L-shaped isotherms this kind of interaction is not present. This is due to the fact that the large ethylene oxide moiety (as for TX-305, for example) unfolds hydrophobic moieties at long distances on the surface, which weakens the interactions between the hydrophobic moieties. In the case of our solids, the shape transition does not occur with the same surfactant. For example, samples QA and QB are S-shaped with all Tritons, and QC and kaolin are S-shaped with TX-114, TX-100, and TX-165 and L-shaped with TX-305. Dolomite is S-shaped only with TX114. Partyka et al. (40) explained this effect by the progressive disappearance of surfactant behavior and appearance of polymeric character for surfactant molecules when the polar chain length increases. Following this explanation the S–L shape transition should be produced, on all samples, with the same surfactant. However, it seems clear that this does not occur in our case. Therefore, the influence of the solid surface of the nature can be detected in this way. It can be concluded that surface nature has great importance in adsorption behavior. Moreover, if a series of surfactants with differences in only polar chain length are studied, the trend of adsorption for different solids can be predicted. Figure 4 shows adsorption isotherms of Tritons on quartz QA at 298 K represented as the degree of coverage vs Ceq / cmc. As can be seen, at low coverage, where interactions between chains are not predominant, when oxyethylenic length increases the adsorbed amount also rises. This is attributed to the fact that more hydrogen bonds are produced
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
575
FIG. 4. Adsorption isotherms of ( j ) TX-114, ( l ) TX-100, ( m ) TX165, and ( . ) TX-305 on QA at 298 K.
between surfactant segments ( –OCH2 –CH2 ) and surface hydroxyls (5, 41). Degree of Coverage Whether a given surface is completely covered or whether its ‘‘real’’ degree of coverage by a surfactant is known is not a well-resolved problem. Levitz and co-workers (42) compared the similarity of surface aggregates on a silica surface with regular micelles. They calculated the apparent surface area per molecule ( sp )ag at the plateau of the isotherm, assuming that the surface aggregates are oblate ellipsoids, and compared this value to that from experiment (on Spherosil). They concluded that on the plateau surface aggregates are close to aqueous micelles. To obtain the apparent surface area per molecule from experimental results, the following expression has been used (42), sp,exp Å SA / Gr NA ,
[2]
where SA is the specific surface area of the solid, G is the adsorbed amount at the plateau, and NA is the Avogadro constant.
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
576
TABLE 3 Apparent Surface Area per Triton Molecules (sp,exp) Sample QA QB QC Kaolin Dolomite
TX-114 ˚ 2) (A
TX-100 ˚ 2) (A
TX-165 ˚ 2) (A
TX-305 ˚ 2) (A
113 61 34 95 138
231 104 61 251 390
374 172 88 379 544
734 334 191 691 725
When a surfactant is adsorbed on a given surface, if the entire surface is covered, the value of sp,exp for adsorbed surfactant molecules should be very close to the value of sp,ag for surface aggregates. However, if not all of the surface is covered with surfactant, the value of sp,exp obtained experimentally will be higher than that corresponding to the surface aggregate, sp,ag . The comparison of the sp values presented in Tables 3 and 4 indicates that for sample QC, a sufficiently well-fitted value between theoretical (42) and experimental results is achieved. It can be observed that surfactants with a shorter polar chain (TX-114 and TX-100) fitted better, on the QC sample, with a ‘‘meander’’ (43) conformation. In addition, for shorter polar chains, a meander conformation would be more realistic (44). However, TX-165 and TX305, which have longer polar chains, are better fitted, on the QC sample, with a ‘‘coiled’’ (45) conformation. Tanford et al. (46) suggested that for long polar chains (TX-102 or TX-305) the coil conformation is preferred. However, for ˚ 2 ) and experimental sp,exp TX-305, theoretical sp,ag (177 A 2 ˚ ) present enough difference, so it appears that, in (191 A this case, not all QC surface is covered (92.1%). Therefore, when the polar chain increases, a conformational change of adsorbed molecules on QC quartz can be postulated. The rest of the samples have experimental sp,exp values higher than the corresponding theoretical values. Thus, it can be assumed that not all solid surfaces have been covered TABLE 4 Apparent Surface Area (sp) of Surfactants Obtained by Levitz and Van Damme (42) ˚ 2) sp (A Surfactant
NOE
‘‘Meander’’
‘‘Coil’’
Experiment
TBE6 TX-100 TX-102 TX-165a TX-305
6 9.5 12.5 16 30
30 59 95 127 440
30 54 76 90 177
33 60 78 — 250
a Theoretical values of ‘‘meander’’ and ‘‘coil’’ conformations for TX165 were calculated by us by interpolation of aggregation numbers.
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
FIG. 5. Adsorption isotherms of ( l ) NP1P4E and ( m ) NP4P1E on dolomite at 298 K.
by these surfactants. This means that when the plateau is reached, it should be considered that the saturation of adsorption capacity of a given surface is reached but not necessarily that all the solid surface is covered by surfactant. Effect of the Polar Chain Nature The NP1P4E surfactant, on all samples, is adsorbed in a greater amount than NP4P1E, as can be observed from Fig. 5, where adsorption isotherms of NP1P4E and NP4P1E at 298 K on dolomite are represented. The adsorption isotherms corresponding to other solids are somewhat similar, and are not reported as figures for the sake of brevity. It is now generally accepted that the adsorption of nonionic surfactants on silica involves hydrogen bonds between the oxygen atoms of their oxyethylenic chains and the silanol groups (47–49), and when the polar chain of nonylphenols increases, a systematic decrease in the amount of adsorbed surfactant is observed (50). However, it seems that the decrease of the adsorbed amount of NP4P1E in relation to NP1P4E is due not only to the increase of the polar chain but also to the presence of additional –CH2 – in the propoxy group. This ‘‘extra’’ –CH2 – probably diminishes the polar-
coidas
AP: Colloid
577
LIQUID-PHASE OXYETHYLENIC SURFACTANT ADSORPTION
TABLE 5 Adsorbed Amount (G) at the ‘‘Plateau’’ and Apparent Surface Area (sp,exp) per Molecule of NP1P4E 273 K
298 K
308 K
Sample
G (mmol/m2)
˚ 2) sp (A
G (mmol/m2)
˚ 2) sp (A
G (mmol/m2)
˚ 2) sp (A
QA QB QC Kaolin Dolomite
0.74 1.64 1.60 0.44 2.46
224 101 103 3.77 67
1.60 2.77 3.30 3.52 8.10
103 60 50 47 20
9.24 3.64 3.61 9.73 34.83
18 46 46 17 5
ity of the propoxy groups with respect to the ethoxy group. Consequently, when the propoxy groups are predominant as in the case of NP4P1E, the adsorbed amount decreases. In the case of nonylphenols, adsorption capacity follows the order dolomite ú kaolin ú QC ú QB ú QA, which is opposite to that found for Triton adsorption. As is obvious, this behavior indicates that dolomite and kaolin are less polar than quartz. This is consistent with the above-mentioned transition of the isotherm shapes from S to L. The trend of the three quartz samples is also expected; the surface of QC is less polar than those of the other quartz samples because the thermal treatment reduces the number of surface OH groups. However, the fact that the trend between the quartz samples is the same with nonylphenols as with Tritons indicates that the predominant factor in adsorption processes is the nature of the hydroxyl groups. Although in the case of nonylphenols theoretical values of sp,ag cannot be calculated because the aggregation number has not been measured, experimental values of sp,exp were obtained. The adsorbed amounts of NP1P4E are comparatively very high (Table 5), so the possibility of bilayer formation could be considered. Assuming that the steric constraints in a bilayer are due to the alkyl chains (including the phenol group), the volume of a bilayer portion of N molecules can be estimated as (42) Va Å Nr£a Å pR 2r(2La ),
˚ 2 ). values close to the theoretical sp,ag at 298 K ( sp Å 20 A Thus, it could be considered that a bilayer is formed at 298 K on dolomite and at 308 K on QA and kaolin. For dolomite, ˚ 2 ) is too low; at 308 K, the experimental value of sp (5 A it may be that a multilayer is formed on the dolomite surface. Adsorption of Anionic Surfactants Figure 6 shows the amount of the sulfated nonylphenols adsorbed on kaolin at 298 K. As in the case of the Tritons,
[3]
where R is the radius of the disk, La is the minor semiaxis of the aliphatic core, £a is the average volume of the nonyl chain, and N is the aggregation number. In this case sp,ag would be (42) sp,ag Å pR 2 /Nr Å £a /2Larr,
[4]
where r Å 1 for an infinite bilayer and r Å 0.907 for a close˚ , and £a Å 387 A ˚2 packed arrangement of disks, La Å 12 A 2 ˚ (51). Values for sp,exp of 17–18 A have been obtained. If theoretical and experimental values are compared (Table 5), it can be observed that, at 308 K, QA quartz and kaolin have ˚ 2 , while dolomite presents some sp,exp values of 17–18 A
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
FIG. 6. Adsorption isotherms of ( j ) NP4S, ( l ) NP10S, and ( m ) NP25S on kaolin at 298 K.
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
578
TABLE 6 Adsorbed Amounts of Sulfated Nonylphenols, in mmol/m2, at the ‘‘Plateau’’ at 298 K Sample
NP4S
NP10S
NP25S
QB Kaolin
0.074 0.650
0.055 0.421
0.032 0.271
when the oxyethylenic chain length increases the adsorbed amount diminishes. The anionic sulfated nonylphenols are negatively charged as well as the surfaces of oxides (52– 55) and so our samples also have a negative sign at our given pH values (56). This means that very small amounts of these surfactants should be adsorbed (Table 6). This is true in the case of quartz QB; however, kaolin adsorbs eight times more (of the same order as Tritons; see Table 2). This behavior can be attributed to positive charges on the kaolin edges (57). For enhanced oil recovery (EOR) applications it is convenient to have very low adsorption of the surfactant on the solid surface. The reason for this is that, in this way, the main part of the surfactant could form a microemulsion with oil. It is obvious that a surfactant which has the same type of charge that an adsorbent has on its surface will be less adsorbed. However, ionic surfactants are more expensive that nonionic ones and they can precipitate in a saline medium, such as an oil reservoir. This is the reason for searching for a nonionic surfactant. As can be observed in Fig. 7,
FIG. 8. Adsorbed amount at the ‘‘plateau’’ on kaolin at 298 K.
where the adsorbed amounts of all the surfactants at the plateau for the QB sample at 298 K are presented, the sulfated nonylphenols are less adsorbed in comparison with the rest of the nonionic surfactants. However, for kaolin (Fig. 8) TX-305 is the surfactant less adsorbed on the solid surface. Therefore, due to differences in the nature of the samples studied, finding a surfactant that is optimum for all types of surfaces involved is a difficult task. Comparison with an Apolar Solvent In Fig. 9 adsorption isotherms of TX-100 on kaolin at 298 K, using water and decane as solvents, are compared. In decane, on the kaolin surface, TX-100 is adsorbed four times more than in water. This behavior is easily explained by the Traube rule (58), which states that a polar (apolar) adsorbent preferentially adsorbs the more polar (apolar) component from the apolar (polar) solution. However, for EOR applications, where the solvent is normally a mixture of oil and water, it is difficult to apply this kind of rule. CONCLUSIONS
FIG. 7. Adsorbed amount at the ‘‘plateau’’ on QB at 298 K.
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
At the ‘‘plateau’’ of the adsorption isotherms, only the QC quartz surface is completely covered (or almost covered in the case of TX-305) by the four Tritons studied. QA quartz is covered by É31%, QB quartz by É50%, kaolin by É25%, and dolomite by É16%. So, when maximum adsorption is reached, it should be considered simply that
coidas
AP: Colloid
LIQUID-PHASE OXYETHYLENIC SURFACTANT ADSORPTION
579
ACKNOWLEDGMENTS We thank the group of Professor J. Rouquerol from CTM of CNRS of Marseille, and especially Dr. L. Luciani for measurements of cmc of surfactants. We also thank the group of Professor P. Sermon from Brunel University. The authors acknowledge financial support from Project JOULE by the E.U. (Contract CT-91-0062).
REFERENCES
FIG. 9. Adsorption isotherms of TX-100 ( m ) in water and ( j ) in decane on kaolin at 298 K.
the adsorption capacity of a given surface is saturated and not necessarily that the entire solid surface is covered by the surfactant. On the QC surface there is a conformational change of adsorbed surfactant molecules, from a ‘‘meander’’ to a ‘‘coil’’ conformation, when the length of the oxyethylenic chain increases. For a given series of surfactants, which are distinguished by their oxyethylenic length, it is possible to predict which sample will be more adsorbed by studying the adsorption behavior of only one of the surfactants of the series. NP1P4E seems to form a bilayer on QA quartz and kaolin at 308 K and on dolomite at 298 K. However, when the amount of propoxy groups in the oxyethylenic chain increases, the adsorbed amount on the solid surface diminishes. Sulfated nonylphenols are adsorbed in smaller amounts on quartz than on the rest of the surfactants; this may be due to the negative charges of these surfactants and of the solid surface. By contrast, kaolin exhibits a positive charge on its edges, and in this case the amounts of sulfated nonylphenols adsorbed are similar to those of TX-100, TX-165, and TX-305. On kaolin, using an organic solvent (decane) TX-100 is adsorbed four times more than in water, probably due to the fact that there is no (or less) competition between solvent and surfactant for the solid surface groups.
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
1. Chiu, Y. C., Chen, L. J., and Pien, W. Y., Colloids Surf. 34, 23 (1988/ 1989). 2. Lim, J. C., and Miller, C. A., Langmuir 7, 2021 (1991). 3. Schambil, F., and Schwuger, M. J., Colloid Polym. Sci. 265, 1009 (1987). 4. Dawe, R. A., J. Chem. Tech. Biotechnol. 51, 361 (1991). 5. Somasundaran, P., Snell, E. D., Fu, E., and Xu, Q., Colloids Surf. 3, 49 (1992). 6. Sobisch, T., Colloids Surf. 66, 11 (1986). 7. Bo¨hmer, M. R., Koopal, L. K., Janssen, R., Lee, E. M., Thomas, R. K., and Rennie, A. R., Langmuir 8, 2228 (1992). 8. Scheutjens, J. M. H. M., and Fleer, G. J., J. Phys. Chem. 83, 1619 (1979). 9. Cases, J. M., and Villieras, K., Langmuir 8, 1251 (1992). 10. Nagarajan, R., and Ruckenstein, E., Langmuir 7, 2934 (1991). 11. Lijour, Y., Calves, J. Y., and Saumagne, P., J. Chem. Soc. Faraday Trans. 1 83, 3283 (1987). 12. Rouquerol, J., Partyka, S., and Rouquerol, F., in ‘‘Adsorption at the Gas–Solid and Liquid–Solid Interface’’ (J. Rouquerol and K. S. W. Sing, Eds.). Elsevier, Amsterdam, 1982. 13. Howard, G. J., and McConnell, P., J. Phys. Chem. 71, 2974 (1967). 14. Boomgaard, Th van den., Tadros, Th. F., and Lyklema, J., J. Colloid Interface Sci. 116, 8 (1987). 15. Levitz, P., Ph. thesis. University of Orleans, France, 1985. 16. Gaufres, R., Sportouch, S., Ammour, J., and Maillols, J., J. Phys. Chem. 94, 4635 (1990). 17. Rudzinski, W., Dominko, A., Partyka, S., and Bruin, B., Adsorpt. Sci. Technol. 2, 207 (1985). 18. Wormuth, K. R., Langmuir 7, 1622 (1991). 19. Cummins, P. G., Staples, E., Penfold, J., and Heenan, R. K., Langmuir 5, 1195 (1989). 20. Corti, M., Minero, C., and Degiorgio, V., J. Phys. Chem. 88, 309 (1984). 21. Karlstro¨m, G., J. Phys. Chem. 89, 4962 (1985). 22. Everett, D., Pure Appl. Chem. 58, 967 (1986). 23. Nevskaia, D. M., Rojas Cervantes, M. L., Guerrero RuıB z, A., and Lo´pez Gonza´lez, J. D., J. Chem. Technol. Biotechnol. 63, 249 (1995). 24. ASTM Powder Diffraction Files ed., Joint Commitee on Powder Diffraction Standards, Pennsylvania, 1979, Files 14-164 and 11-78. 25. Kiselev, A. V., in ‘‘Infrared Spectra of Surface Compounds,’’ Chap. 4. Publishing House, Jerusalem, 1975. 26. Van der Voort, P., Gillis-D’Hamers, Y., Vrancken, K. C., and Vansant, E. F., J. Chem. Soc. Faraday Trans. 1 87, 3899 (1991). 27. Kno¨ringer, H., and Ratnasamy, P., Catal. Rev. Sci. Eng. 17, 31 (1978). 28. Gillis-D’Hamers, I., Cornelissens, I., Vrancken, K. C., Van der Voort, P., and Vansant, E. F., J. Chem. Soc. Faraday Trans. 1 88, 723 (1988). 29. Brindey, G. W., and Lamaitre, J., in ‘‘Chemistry of Clays and Clay Minerals’’ (A. C. D. Newman, Ed.). Wiley, London, 1987. 30. Thomas, F., Bottero, J. Y., Partyka, S., and Cot, D., Thermochim. Acta 122, 197 (1987). 31. Partyka, S., Lindheimer, M., Zaini, S., Keh, E., and Brun, B., Langmuir 2, 101 (1986).
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
580
32. Keh, E., Partyka, S., and Zaini, S., J. Colloid Interface Sci. 129, 363 (1989). 33. Lindheimer, M., Keh, E., Zaini, S., and Partyka, S., J. Colloid Interface Sci. 138, 83 (1990). 34. Denoyel, R., Rouquerol, F., and Rouquerol, J., in ‘‘Adsorption from Solution’’ (C. H. Rochester and A. L. Smith, Eds.), p. 225. Academic Press, London. 1983. 35. Evers, O. A., Scheutjens, J. M. H. M., and Fleer, G. J., Macromolecules 23, 5221 (1990). 36. Kjellander, R., and Florin, E., J. Chem. Soc. Faraday Trans. 1 77, 2053 (1981). 37. Denoyel, R., and Rouquerol, J., J. Colloid Interface Sci. 143, 555 (1991). 38. Saito, Y., and Sato, T., J. Phys. Chem. 89, 2110 (1985). 39. Giles, C. H., D’Silva, A. P., and Easton, I. A., J. Colloid Interface Sci. 43, 766 (1974). 40. Partyka, S., Lindheimer, M., and Faucompre, B., Colloids Surf. A: Phys. Eng. Asp. 76, 267 (1993). 41. Denoyel, R., Ph. thesis, Marseille University, Marseille, 1987. 42. Levitz, P., and Van Damme, H., J. Phys. Chem. 90, 1302 (1986). 43. Rosch, M., in ‘‘Nonionic Surfactants, Surfactant Science Series’’ (J. Schick, Ed.), Vol. 1, p. 753. Dekker, New York, 1967.
AID
JCIS 4294
/
6g12$$$624
07-04-96 03:05:01
44. Robson, R. J., and Dennis, E. A., J. Phys. Chem. 81, 1075 (1977). 45. Flory, P. J., ‘‘Principles of Polymer Chemistry,’’ p. 399. Cornell Univ. Press, Ithaca, NY, 1953. 46. Tanford, C., Nozak, V., and Rohde, N. J., J. Phys. Chem. 81, 16 (1977). 47. Rochester, C. H., and Gellan, A., J. Chem. Soc. Faraday Trans. 1 81, 2235 (1985). 48. Furlong, D. N., and Aston, J. R., Colloids Surf. 12, 255 (1984). 49. Scamehorn, J. F., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 85, 494 (1982). 50. Trochet-Mignard, L., Taylor, P., Bognolo, G., and Tadros, Th. F., Colloids Surf. A: Phys. Eng. Asp. 95, 37 (1995). 51. Levitz, P., Langmuir 7, 1595 (1991). 52. Casey, W. H., J. Colloid Interface Sci. 163, 407 (1994). 53. Yates, D. E., and Healy, T. W., J. Colloid Interface Sci. 55, 9 (1976). 54. Axelos, M. A. V., Tchoubar, D., and Bottero, J. Y., Langmuir 5, 1185 (1989). 55. Kosmulski, M., Colloids Surf. A: Phys. Eng. Asp. 95, 81 (1995). 56. Nevskaia, D. M., Guerrero-Ruiz, A., and Lo´pez-Gonza´lez, J. de D., in preparation. 57. Peng, F. F., and Pingkuan, D., J. Colloid Interface Sci. 164, 229 (1994). 58. Traube, I., Ann. 265, 27 (1891).
coidas
AP: Colloid
181, 571–580 (1996)
0414
Adsorption of Polyoxyethylenic Surfactants on Quartz, Kaolin, and Dolomite: A Correlation between Surfactant Structure and Solid Surface Nature D. M. NEVSK AIA, 1 A. GUERRERO-RUI´Z,
AND
J.
DE
D. LO´PEZ-GONZA´LEZ
Departamento de QuıB mica Inorga´nica y QuıB mica Te´cnica, Facultad de Ciencias, Universidad Nacional de Educacio´n a Distancia (UNED), Madrid 28040, Spain Received October 27, 1995; accepted March 1, 1996
The influence of variables such as adsorption temperature, polar chain length, and nature of functional groups on the adsorption, from aqueous solutions, of various surfactants (TX-114, TX-100, TX-165, TX-305, NP1P4E, NP4P1E, NP4S, NP10S, and NP25S) has been investigated. Several nonporous solids, including various samples of quartz, QA (5.1 m2 g 01 ), QB (5.5 m2 g 01 ), and QC (4.0 m2 g 01 ), kaolin (19.3 m2 g 01 ), and dolomite (0.3 m2 g 01 ), were studied. Conformational changes of adsorbed surfactant molecules on QC quartz, when the oxyethylenic length of Tritons increases, have been detected. For all the other solid samples the surface is not completely covered by Tritons. On quartz, the surfactants are adsorbed by hydrogen bonds between the surfactant’s ether groups and the silanol groups of the solid surface. These hydroxyl groups must be free and sufficiently separated from other hydroxyls of the solid surface. When the number of propoxy groups increases (from NP1P4E to NP4P1E) the adsorbed amount of surfactant on the solid studied decreases. Anionic surfactants are adsorbed on quartz in lower amounts than the corresponding nonionic surfactants. However, the adsorbed amounts of Tritons and sulfated Tritons on kaolin are similar, probably due to the positive charges on the edges of this material. q 1996 Academic Press, Inc. Key Words: polyoxyethylenic surfactants; aggregate conformation; nature of surface.
INTRODUCTION
Adsorption of a surfactant at a liquid–solid interface makes up the basis of many technological processes such as detergency (1, 2), flotation, water treatment (3), and enhanced oil recovery (4). The phenomena of adsorption processes at a liquid–solid interface are complicated in comparison with adsorption at a solid–gas interface due to the presence of a third component, the solvent. For instance, the solvent can also be adsorbed onto the solid surface, compet1 To whom correspondence should be addressed. Fax: 34-1-3986697. Email: [email protected].
ing with the surfactant (adsorbate). Moreover, interactions between surfactant and solvent should also be allowed for. There are some internal factors such as the length and nature of the polar chain of the surfactant and nature of the solid surface that must be taken into account. Somasundaran et al. (5) related the length of the ethoxyl chain with the conformational behavior of the adsorbed molecules. Sobisch (6) observed that the adsorbed amount increases with a decrease in the of degree of ethoxylation. Bo¨hmer et al. (7), using an extension of the self-consistent-field theory of Scheutjens and Fleer for chain molecules (8), calculated the adsorption isotherms of polyoxyethylenic surfactants. They found, in agreement with experimental findings, that the critical micelle concentration (cmc) increases and the aggregation number of micelles decreases as the hydrophilic block becomes longer. Some external factors which affect the adsorption behavior (9–15) are adsorption temperature, salinity, pH, etc. Thus, Gaufres et al. (16) found conformational changes as a function of temperature in an En Em (oxyethylenic surfactants)/water system. In many adsorption studies authors pay much more attention to surfactants than to solid surfaces (5, 7, 15, 17). For instance, adsorption temperature, salinity, or polar chain length is studied. In addition to the above-mentioned factors, which can affect the interactions between surface and surfactants in various ways, the chemical nature of the adsorbent should be considered. In many cases a silica is chosen as the solid adsorbent for this kind of study. The reason for this is that the interaction of surfactant and solid surface is a process that is difficult to evaluate and where, usually, interactions between functional groups of the solid surface and the surfactant are involved. For example, if there are different functional groups on the solid surface, an additional problem concerning understanding the surfactant molecule–solid surface interactions arises. One way to simplify this problem is to choose a very well known and not very complicated surface on which to carry out the adsorption processes.
571
AID
JCIS 4294
/
6g12$$$621
07-04-96 03:05:01
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
572
On the other hand, a widely studied group of surfactants is the polyoxyethylenic [( –OCH2 –CH2 – )n )] (5–7, 15–21) group, which has the advantage that its hydrophobic nature can be controlled not only by modifying the number of carbon atoms in its hydrophobic chain, but also by varying the oxyethylenic chain. Consequently, this type of surfactant exhibits a wide range of physical properties and it is possible to select the more appropriate surfactant for a given application. The aim of this work is to study adsorption on a liquid– solid interface. For this purpose, the effect of some variables has been investigated. Adsorption temperature, as an external variable, has been considered. With respect to surfactants, the effects of the length of the polar chain on possible conformational changes of the adsorbed molecules and of the nature of the functional groups on the shape of adsorption isotherms have been studied. Also, a number of solids of different natures (quartz, kaolin, and dolomite) have been chosen in order to investigate how these solid surfaces affect the adsorption processes because these kinds of materials are frequently found in oil reservoirs. EXPERIMENTAL
The solid sample used in this study was a natural quartz (Sifraco C-600), 98.8% purity, which is denoted as QA. This sample (QA) was treated with a HCl solution (pH 1.5) and subsequently washed with distilled water until there were no chloride ions in the washing water as detected with a solution of AgNO3 . Afterward, the sample was dried in a vacuum oven for a few hours and denoted as QB. Sample QC was obtained by calcination of sample QB in an oven at 1273 K for 9 h. A kaolin sample from Sigma and a natural dolomite sample from Granada (Spain) was also subjected to study. The nonionic surfactants used were a series of Tritons (from Rohm and Haas Co.) with purity ú98% and the general formula C8H17 –Ph– (OCH2 –CH2 )n –OH, with n Å 7 for TX-114, n Å 9.5 for TX-100, n Å 16 for TX-165, and n Å 30 for TX-305. Two nonylphenols (from ICI, Petrochemicals and Plastics Division) were used, with purity ú96% and the general formula C9H19 –Ph– (OCH2 –CH2 –CH2 )x – (OCH2 –CH2 )n –OH, with x Å 1 and n Å 4 for NP1P4E and x Å 4 and n Å 1 for NP4P1E. A series of anionic surfactants were used as well, with purity ú94% and the general formula
AID
JCIS 4294
/
6g12$$$622
07-04-96 03:05:01
TABLE 1 Some Characteristics of Surfactants Surfactant
n
x
TX-114 TX-100 TX-165 TX-305 NP1P4E NP4P1E NP4S NP10S NP25S
7 9.5 16 30 4 1 4 10 25
— — — — 1 4 — — —
cmc (mol/kg)a 2.1 2.7 7.0 1.0 5.0 3.0 4.0 3.0 1.5
1 1 1 1 1 1 1 1 1
1004 1004 1004 1003 1005 1005 1004 1004 1004
a
Determined in the Thermodynamic and Microcalorimetric Centre (CTM) or (CNRS) of Marseille by the Wilhelmy slide method at 293 K.
C9H19 –Ph– (OCH2 –CH2 )n –SO4 –Na / , with n Å 4 for NP4S (from Stepan Europe), n Å 10 for NP10S (from CFPI), and n Å 25 for NP25S (from Witco Chemicals). Some characteristics of the surfactants mentioned are included in Table 1. The specific surface areas of the solids were determined by nitrogen adsorption at 77 K, taking 16.2 nm2 as the crosssectional area for the adsorbed nitrogen molecule. An automatic Micromeritics ASAP 2000 volumetric system was used to obtain the corresponding gas adsorption isotherms. The values obtained for specific surface areas are 5.1 for QA, 5.5 for QB, 4.0 for QC, 19.3 for kaolin, and É0.3 m2 g 01 for dolomite with accuracy §0.1 m2 g 01 . The surfactant adsorption isotherms from water solutions were obtained by the immersion method (22). A few grams of solid was put in contact with surfactant water solutions (20 cm3 ) of known concentrations. Equilibrium was reached after 24 h with ellipsoidal stirring at 273, 298, and 308 K; the stirring rate was 100 rpm. The solids were then separated by centrifugation, at their corresponding temperatures, at 3000g. The amount adsorbed at 273, 298, and 308 K was calculated on the basis of the concentration change determined by UV spectroscopy at 225 and 275 nm. The experimental error was less than 5%. More details about this experimental method are given elsewhere (23). The pH of the solutions was measured before and after 24 h of contact between the solids and the solutions, using a PHB-62 pH meter. The obtained pH values for each solid do not present significant variations with different surfactants. They are as follows: 8.6 { 0.2 for QA; 5.7 { 0.3 for QB; 7.2 { 0.1 for QC; 5.0 { 0.5 for kaolin; and 9.6 { 0.2 for dolomite. The XRD patterns were obtained with a Seifert 3000P diffractometer using nickel-filtered copper Ka radiation ( l Å 0.1538 nm). The infrared spectra were recorded in a Nicolet ZDX Fourier-transformed spectrophotometer with a resolution of 4 cm01 . Self-supporting wafers of the samples with
coidas
AP: Colloid
LIQUID-PHASE OXYETHYLENIC SURFACTANT ADSORPTION
FIG. 1. FTIR spectra of kaolin at (A) 773 K, (B) 373 K, and (C) 296 K.
573
kaolin sample. Evolution of IR spectra with the pretreatment temperature under vacuum is also presented. At 296 K a wide band is observed between 3730 and 3570 cm01 , which corresponds to hydroxyl groups which are hydrogen-bound (25), (Fig. 1C). When treatment temperature increases (up 373 K), adsorbed water is almost eliminated and a series of bands appear (3660, 3635, and 3580 cm01 ) (Fig. 1B). These bands can be attributed to hydrogen-bound hydroxyl groups. A characteristic peak at 3420–3430 cm01 (Figs. 1B and 1C) is assigned to molecular adsorbed water (26). Finally, on the kaolin pretreated at 773 K (Fig. 1A), narrow bands can be observed at 3745, 3700, 3660, 3625, and 3570 cm01 . The new two peaks at higher wavelength can be assigned to OH linked to two octahedral Al 3/ (3745 cm01 ), and to one OH coordinated to three octahedral Al 3/ (3700 cm01 ) (27). It should also be noted that an OH coordinated to one Al 3/ in an octahedral site, which should give a band between 3785 and 3800 cm01 , does not appear in this sample. This assignment, for which only Al–OH bonds are considered, is consistent with the structure of kaolin. For this solid only OH groups are bonded to the octahedral layer, which contains Al 3/ ions (27). Thus, when the kaolin sample is heated, the hydroxyl-bound groups begin to suffer a condensation process (28). Brindley and colleagues (29) showed that kaolin is deshydroxylated at 723–823 K to produce metakaolin. Thus, at room temperature, it seems that almost all of the hydroxyl groups on the surface of kaolin are hydrogenbound. Effect of the Adsorption Temperature
weight-to-surface ratios of about 10 mgrcm02 were placed in a vacuum greaseless stopcock cell, which enabled us to carry out ‘‘in situ’’ pretreatment. RESULTS AND DISCUSSION
Solids Characterization The three quartz samples have been characterized in a previous work (23). The state of the silica surface strongly affected the process of adsorption of Triton X-100 from aqueous solution. It was shown that the parameter which determines the amount of adsorbed surfactant was not the amount of OH nm02 but the nature of the surface hydroxyl groups. The three quartz samples exhibit b-quartz structure (XRD). Furthermore, on the QA sample surface, impurity traces of Na, Ca, and Al were detected by X-ray photoelectron spectroscopy (23). Most of the surface hydroxyl groups of samples QA and QB are hydrogen-bound while in sample QC they are free and isolated (23). X-ray diffraction patterns of kaolin and dolomite allow us to identify our samples as a typical kaolinite and dolomite (24). Figure 1 shows FTIR spectra in the vibration range of the hydroxyl groups of the
AID
JCIS 4294
/
6g12$$$622
07-04-96 03:05:01
Figure 2 shows the adsorbed amount ( G in mmol/m 2 ) of TX-114 on kaolin as a function of the equilibrium concentration (Ceq in mol/liter) at 273, 298, and 308 K. The evolution of the adsorption isotherms with temperature is similar for all the surfactants and all samples studied. This behavior during Triton adsorption has been explained by many authors (30–34). At low Ceq or a low degree of coverage u ( u Å G / Gmax ) the monomers are adsorbed on the solid surface, displacing water molecules. This kind of process has an exothermic nature. When the adsorption temperature is increased, the amount of adsorbed surfactant decreases. When the majority of the adsorption sites are covered, interactions take place between surfactant molecules to form micelles. This process is endothermic. Thus, at high Ceq or a high degree of coverage, when the adsorption temperature is increased, the adsorbed amount also rises. According to modern polymer adsorption theory (8), this kind of behavior is expected since the Flory–Huggins parameter x for the oxyethylenic (EO) part increases with T. The parameter xxy has been defined as the energy change (in units of kT) associated with the transfer of a segment of type x from a solution of pure x to a solution of pure y (35). Kjellander and
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
574
FIG. 2. Adsorption isotherms of TX-114 on kaolin at ( j ) 273 K, ( l ) 298 K, and ( m ) 308 K.
FIG. 3. Adsorption isotherms of ( j ) TX-114, ( l ) TX-100, ( m ) TX165, and ( . ) TX-305 on QC at 298 K.
Florin (36) studied the physical meaning of the interaction polymer–solvent parameter, x, of Flory. They found that evolution between 298 and 348 K is visibly linear, and they attain the numerical expression
of the larger size of the molecules (37). A displacement toward higher Ceq of the value at which the ‘‘plateau’’ is reached is also observed. The reason for this is that when oxyethylenic length rises, the monomer solubility in water is higher and, thus, the cmc is also higher (38). The adsorption capacity of the samples for the four Tritons, in general, follows the trend QC ú QB ú QA ú kaolin ú dolomite (Table 2), except for TX-114, for which QA and kaolin positions are inverted, and for TX-305, for which the values of the last three samples are very close. Usually it is accepted that adsorption of this kind of surfactant is
x(T ) Å (5.879 1 10 03 )T 0 1.6583.
[1]
This means that an increase in adsorption temperature favors contacts between polyoxyethylenic chains over water–polyoxyethylenic chain interactions. Or, in other words, micelle formation is favored when the temperature increases. This effect is in agreement with the experimental observation that when temperature increases, the cmc diminishes. Effect of the Polar Chain Length In Fig. 3, adsorption isotherms of four Tritons (TX-114, TX-100, TX-165, and TX-305) on quartz QC at 298 K are represented. Similar patterns were obtained for QA, QB, kaolin, and dolomite samples, which are not reported as figures for the sake of brevity. As can be observed, when oxyethylenic chain length increases, there is a decrease in the adsorbed amounts (on the five solid samples) because
AID
JCIS 4294
/
6g12$$$622
07-04-96 03:05:01
TABLE 2 Adsorbed Amounts of Triton Surfactants, in mmol/m2, at the ‘‘Plateau’’ at 298 K Sample
TX-114
TX-100
TX-165
TX-305
QA QB QC Kaolin Dolomite
1.47 2.69 4.79 1.74 1.20
0.72 1.59 2.71 0.66 0.42
0.44 0.96 1.88 0.44 0.30
0.23 0.49 0.87 0.24 0.23
coidas
AP: Colloid
LIQUID-PHASE OXYETHYLENIC SURFACTANT ADSORPTION
produced by interactions between ether groups of oxyethylenic chains and surface hydroxyl groups. Moreover, these hydroxyls must be free and sufficiently separated from other hydroxyls on the solid surface (23). Previously it has been stated that the QC surface has free and isolated hydroxyl groups while in the rest of the samples hydrogen-bound hydroxyl groups are predominant. This may be why QC quartz adsorbs more Tritons (Table 2) than the rest of the samples. Also, impurities present on the surface of QA quartz can diminish the amount adsorbed (23). As can be observed from Fig. 3, the shape of the isotherms changes when the oxyethylenic chain length increases, from S to L shape. According to the classification of Giles et al. (39), S-shaped isotherms are obtained when the solvent is strongly adsorbed on the surface or there are strong intermolecular interactions, or when the longitudinal axes of the adsorbed molecules are perpendicular to the adsorbent surface. Lshaped isotherms are also called Langmuir isotherms. They are obtained when there is not strong competition between the solvent and the adsorbent to occupy adsorbent surface sites. In this case, longitudinal axes of adsorbed molecules are parallel to the adsorbent surface. Furthermore, Rudzinski et al. (17) consider that S-shaped isotherms indicate that there are interactions between hydrophobic moieties of surfactant molecules, while in L-shaped isotherms this kind of interaction is not present. This is due to the fact that the large ethylene oxide moiety (as for TX-305, for example) unfolds hydrophobic moieties at long distances on the surface, which weakens the interactions between the hydrophobic moieties. In the case of our solids, the shape transition does not occur with the same surfactant. For example, samples QA and QB are S-shaped with all Tritons, and QC and kaolin are S-shaped with TX-114, TX-100, and TX-165 and L-shaped with TX-305. Dolomite is S-shaped only with TX114. Partyka et al. (40) explained this effect by the progressive disappearance of surfactant behavior and appearance of polymeric character for surfactant molecules when the polar chain length increases. Following this explanation the S–L shape transition should be produced, on all samples, with the same surfactant. However, it seems clear that this does not occur in our case. Therefore, the influence of the solid surface of the nature can be detected in this way. It can be concluded that surface nature has great importance in adsorption behavior. Moreover, if a series of surfactants with differences in only polar chain length are studied, the trend of adsorption for different solids can be predicted. Figure 4 shows adsorption isotherms of Tritons on quartz QA at 298 K represented as the degree of coverage vs Ceq / cmc. As can be seen, at low coverage, where interactions between chains are not predominant, when oxyethylenic length increases the adsorbed amount also rises. This is attributed to the fact that more hydrogen bonds are produced
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
575
FIG. 4. Adsorption isotherms of ( j ) TX-114, ( l ) TX-100, ( m ) TX165, and ( . ) TX-305 on QA at 298 K.
between surfactant segments ( –OCH2 –CH2 ) and surface hydroxyls (5, 41). Degree of Coverage Whether a given surface is completely covered or whether its ‘‘real’’ degree of coverage by a surfactant is known is not a well-resolved problem. Levitz and co-workers (42) compared the similarity of surface aggregates on a silica surface with regular micelles. They calculated the apparent surface area per molecule ( sp )ag at the plateau of the isotherm, assuming that the surface aggregates are oblate ellipsoids, and compared this value to that from experiment (on Spherosil). They concluded that on the plateau surface aggregates are close to aqueous micelles. To obtain the apparent surface area per molecule from experimental results, the following expression has been used (42), sp,exp Å SA / Gr NA ,
[2]
where SA is the specific surface area of the solid, G is the adsorbed amount at the plateau, and NA is the Avogadro constant.
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
576
TABLE 3 Apparent Surface Area per Triton Molecules (sp,exp) Sample QA QB QC Kaolin Dolomite
TX-114 ˚ 2) (A
TX-100 ˚ 2) (A
TX-165 ˚ 2) (A
TX-305 ˚ 2) (A
113 61 34 95 138
231 104 61 251 390
374 172 88 379 544
734 334 191 691 725
When a surfactant is adsorbed on a given surface, if the entire surface is covered, the value of sp,exp for adsorbed surfactant molecules should be very close to the value of sp,ag for surface aggregates. However, if not all of the surface is covered with surfactant, the value of sp,exp obtained experimentally will be higher than that corresponding to the surface aggregate, sp,ag . The comparison of the sp values presented in Tables 3 and 4 indicates that for sample QC, a sufficiently well-fitted value between theoretical (42) and experimental results is achieved. It can be observed that surfactants with a shorter polar chain (TX-114 and TX-100) fitted better, on the QC sample, with a ‘‘meander’’ (43) conformation. In addition, for shorter polar chains, a meander conformation would be more realistic (44). However, TX-165 and TX305, which have longer polar chains, are better fitted, on the QC sample, with a ‘‘coiled’’ (45) conformation. Tanford et al. (46) suggested that for long polar chains (TX-102 or TX-305) the coil conformation is preferred. However, for ˚ 2 ) and experimental sp,exp TX-305, theoretical sp,ag (177 A 2 ˚ ) present enough difference, so it appears that, in (191 A this case, not all QC surface is covered (92.1%). Therefore, when the polar chain increases, a conformational change of adsorbed molecules on QC quartz can be postulated. The rest of the samples have experimental sp,exp values higher than the corresponding theoretical values. Thus, it can be assumed that not all solid surfaces have been covered TABLE 4 Apparent Surface Area (sp) of Surfactants Obtained by Levitz and Van Damme (42) ˚ 2) sp (A Surfactant
NOE
‘‘Meander’’
‘‘Coil’’
Experiment
TBE6 TX-100 TX-102 TX-165a TX-305
6 9.5 12.5 16 30
30 59 95 127 440
30 54 76 90 177
33 60 78 — 250
a Theoretical values of ‘‘meander’’ and ‘‘coil’’ conformations for TX165 were calculated by us by interpolation of aggregation numbers.
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
FIG. 5. Adsorption isotherms of ( l ) NP1P4E and ( m ) NP4P1E on dolomite at 298 K.
by these surfactants. This means that when the plateau is reached, it should be considered that the saturation of adsorption capacity of a given surface is reached but not necessarily that all the solid surface is covered by surfactant. Effect of the Polar Chain Nature The NP1P4E surfactant, on all samples, is adsorbed in a greater amount than NP4P1E, as can be observed from Fig. 5, where adsorption isotherms of NP1P4E and NP4P1E at 298 K on dolomite are represented. The adsorption isotherms corresponding to other solids are somewhat similar, and are not reported as figures for the sake of brevity. It is now generally accepted that the adsorption of nonionic surfactants on silica involves hydrogen bonds between the oxygen atoms of their oxyethylenic chains and the silanol groups (47–49), and when the polar chain of nonylphenols increases, a systematic decrease in the amount of adsorbed surfactant is observed (50). However, it seems that the decrease of the adsorbed amount of NP4P1E in relation to NP1P4E is due not only to the increase of the polar chain but also to the presence of additional –CH2 – in the propoxy group. This ‘‘extra’’ –CH2 – probably diminishes the polar-
coidas
AP: Colloid
577
LIQUID-PHASE OXYETHYLENIC SURFACTANT ADSORPTION
TABLE 5 Adsorbed Amount (G) at the ‘‘Plateau’’ and Apparent Surface Area (sp,exp) per Molecule of NP1P4E 273 K
298 K
308 K
Sample
G (mmol/m2)
˚ 2) sp (A
G (mmol/m2)
˚ 2) sp (A
G (mmol/m2)
˚ 2) sp (A
QA QB QC Kaolin Dolomite
0.74 1.64 1.60 0.44 2.46
224 101 103 3.77 67
1.60 2.77 3.30 3.52 8.10
103 60 50 47 20
9.24 3.64 3.61 9.73 34.83
18 46 46 17 5
ity of the propoxy groups with respect to the ethoxy group. Consequently, when the propoxy groups are predominant as in the case of NP4P1E, the adsorbed amount decreases. In the case of nonylphenols, adsorption capacity follows the order dolomite ú kaolin ú QC ú QB ú QA, which is opposite to that found for Triton adsorption. As is obvious, this behavior indicates that dolomite and kaolin are less polar than quartz. This is consistent with the above-mentioned transition of the isotherm shapes from S to L. The trend of the three quartz samples is also expected; the surface of QC is less polar than those of the other quartz samples because the thermal treatment reduces the number of surface OH groups. However, the fact that the trend between the quartz samples is the same with nonylphenols as with Tritons indicates that the predominant factor in adsorption processes is the nature of the hydroxyl groups. Although in the case of nonylphenols theoretical values of sp,ag cannot be calculated because the aggregation number has not been measured, experimental values of sp,exp were obtained. The adsorbed amounts of NP1P4E are comparatively very high (Table 5), so the possibility of bilayer formation could be considered. Assuming that the steric constraints in a bilayer are due to the alkyl chains (including the phenol group), the volume of a bilayer portion of N molecules can be estimated as (42) Va Å Nr£a Å pR 2r(2La ),
˚ 2 ). values close to the theoretical sp,ag at 298 K ( sp Å 20 A Thus, it could be considered that a bilayer is formed at 298 K on dolomite and at 308 K on QA and kaolin. For dolomite, ˚ 2 ) is too low; at 308 K, the experimental value of sp (5 A it may be that a multilayer is formed on the dolomite surface. Adsorption of Anionic Surfactants Figure 6 shows the amount of the sulfated nonylphenols adsorbed on kaolin at 298 K. As in the case of the Tritons,
[3]
where R is the radius of the disk, La is the minor semiaxis of the aliphatic core, £a is the average volume of the nonyl chain, and N is the aggregation number. In this case sp,ag would be (42) sp,ag Å pR 2 /Nr Å £a /2Larr,
[4]
where r Å 1 for an infinite bilayer and r Å 0.907 for a close˚ , and £a Å 387 A ˚2 packed arrangement of disks, La Å 12 A 2 ˚ (51). Values for sp,exp of 17–18 A have been obtained. If theoretical and experimental values are compared (Table 5), it can be observed that, at 308 K, QA quartz and kaolin have ˚ 2 , while dolomite presents some sp,exp values of 17–18 A
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
FIG. 6. Adsorption isotherms of ( j ) NP4S, ( l ) NP10S, and ( m ) NP25S on kaolin at 298 K.
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
578
TABLE 6 Adsorbed Amounts of Sulfated Nonylphenols, in mmol/m2, at the ‘‘Plateau’’ at 298 K Sample
NP4S
NP10S
NP25S
QB Kaolin
0.074 0.650
0.055 0.421
0.032 0.271
when the oxyethylenic chain length increases the adsorbed amount diminishes. The anionic sulfated nonylphenols are negatively charged as well as the surfaces of oxides (52– 55) and so our samples also have a negative sign at our given pH values (56). This means that very small amounts of these surfactants should be adsorbed (Table 6). This is true in the case of quartz QB; however, kaolin adsorbs eight times more (of the same order as Tritons; see Table 2). This behavior can be attributed to positive charges on the kaolin edges (57). For enhanced oil recovery (EOR) applications it is convenient to have very low adsorption of the surfactant on the solid surface. The reason for this is that, in this way, the main part of the surfactant could form a microemulsion with oil. It is obvious that a surfactant which has the same type of charge that an adsorbent has on its surface will be less adsorbed. However, ionic surfactants are more expensive that nonionic ones and they can precipitate in a saline medium, such as an oil reservoir. This is the reason for searching for a nonionic surfactant. As can be observed in Fig. 7,
FIG. 8. Adsorbed amount at the ‘‘plateau’’ on kaolin at 298 K.
where the adsorbed amounts of all the surfactants at the plateau for the QB sample at 298 K are presented, the sulfated nonylphenols are less adsorbed in comparison with the rest of the nonionic surfactants. However, for kaolin (Fig. 8) TX-305 is the surfactant less adsorbed on the solid surface. Therefore, due to differences in the nature of the samples studied, finding a surfactant that is optimum for all types of surfaces involved is a difficult task. Comparison with an Apolar Solvent In Fig. 9 adsorption isotherms of TX-100 on kaolin at 298 K, using water and decane as solvents, are compared. In decane, on the kaolin surface, TX-100 is adsorbed four times more than in water. This behavior is easily explained by the Traube rule (58), which states that a polar (apolar) adsorbent preferentially adsorbs the more polar (apolar) component from the apolar (polar) solution. However, for EOR applications, where the solvent is normally a mixture of oil and water, it is difficult to apply this kind of rule. CONCLUSIONS
FIG. 7. Adsorbed amount at the ‘‘plateau’’ on QB at 298 K.
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
At the ‘‘plateau’’ of the adsorption isotherms, only the QC quartz surface is completely covered (or almost covered in the case of TX-305) by the four Tritons studied. QA quartz is covered by É31%, QB quartz by É50%, kaolin by É25%, and dolomite by É16%. So, when maximum adsorption is reached, it should be considered simply that
coidas
AP: Colloid
LIQUID-PHASE OXYETHYLENIC SURFACTANT ADSORPTION
579
ACKNOWLEDGMENTS We thank the group of Professor J. Rouquerol from CTM of CNRS of Marseille, and especially Dr. L. Luciani for measurements of cmc of surfactants. We also thank the group of Professor P. Sermon from Brunel University. The authors acknowledge financial support from Project JOULE by the E.U. (Contract CT-91-0062).
REFERENCES
FIG. 9. Adsorption isotherms of TX-100 ( m ) in water and ( j ) in decane on kaolin at 298 K.
the adsorption capacity of a given surface is saturated and not necessarily that the entire solid surface is covered by the surfactant. On the QC surface there is a conformational change of adsorbed surfactant molecules, from a ‘‘meander’’ to a ‘‘coil’’ conformation, when the length of the oxyethylenic chain increases. For a given series of surfactants, which are distinguished by their oxyethylenic length, it is possible to predict which sample will be more adsorbed by studying the adsorption behavior of only one of the surfactants of the series. NP1P4E seems to form a bilayer on QA quartz and kaolin at 308 K and on dolomite at 298 K. However, when the amount of propoxy groups in the oxyethylenic chain increases, the adsorbed amount on the solid surface diminishes. Sulfated nonylphenols are adsorbed in smaller amounts on quartz than on the rest of the surfactants; this may be due to the negative charges of these surfactants and of the solid surface. By contrast, kaolin exhibits a positive charge on its edges, and in this case the amounts of sulfated nonylphenols adsorbed are similar to those of TX-100, TX-165, and TX-305. On kaolin, using an organic solvent (decane) TX-100 is adsorbed four times more than in water, probably due to the fact that there is no (or less) competition between solvent and surfactant for the solid surface groups.
AID
JCIS 4294
/
6g12$$$623
07-04-96 03:05:01
1. Chiu, Y. C., Chen, L. J., and Pien, W. Y., Colloids Surf. 34, 23 (1988/ 1989). 2. Lim, J. C., and Miller, C. A., Langmuir 7, 2021 (1991). 3. Schambil, F., and Schwuger, M. J., Colloid Polym. Sci. 265, 1009 (1987). 4. Dawe, R. A., J. Chem. Tech. Biotechnol. 51, 361 (1991). 5. Somasundaran, P., Snell, E. D., Fu, E., and Xu, Q., Colloids Surf. 3, 49 (1992). 6. Sobisch, T., Colloids Surf. 66, 11 (1986). 7. Bo¨hmer, M. R., Koopal, L. K., Janssen, R., Lee, E. M., Thomas, R. K., and Rennie, A. R., Langmuir 8, 2228 (1992). 8. Scheutjens, J. M. H. M., and Fleer, G. J., J. Phys. Chem. 83, 1619 (1979). 9. Cases, J. M., and Villieras, K., Langmuir 8, 1251 (1992). 10. Nagarajan, R., and Ruckenstein, E., Langmuir 7, 2934 (1991). 11. Lijour, Y., Calves, J. Y., and Saumagne, P., J. Chem. Soc. Faraday Trans. 1 83, 3283 (1987). 12. Rouquerol, J., Partyka, S., and Rouquerol, F., in ‘‘Adsorption at the Gas–Solid and Liquid–Solid Interface’’ (J. Rouquerol and K. S. W. Sing, Eds.). Elsevier, Amsterdam, 1982. 13. Howard, G. J., and McConnell, P., J. Phys. Chem. 71, 2974 (1967). 14. Boomgaard, Th van den., Tadros, Th. F., and Lyklema, J., J. Colloid Interface Sci. 116, 8 (1987). 15. Levitz, P., Ph. thesis. University of Orleans, France, 1985. 16. Gaufres, R., Sportouch, S., Ammour, J., and Maillols, J., J. Phys. Chem. 94, 4635 (1990). 17. Rudzinski, W., Dominko, A., Partyka, S., and Bruin, B., Adsorpt. Sci. Technol. 2, 207 (1985). 18. Wormuth, K. R., Langmuir 7, 1622 (1991). 19. Cummins, P. G., Staples, E., Penfold, J., and Heenan, R. K., Langmuir 5, 1195 (1989). 20. Corti, M., Minero, C., and Degiorgio, V., J. Phys. Chem. 88, 309 (1984). 21. Karlstro¨m, G., J. Phys. Chem. 89, 4962 (1985). 22. Everett, D., Pure Appl. Chem. 58, 967 (1986). 23. Nevskaia, D. M., Rojas Cervantes, M. L., Guerrero RuıB z, A., and Lo´pez Gonza´lez, J. D., J. Chem. Technol. Biotechnol. 63, 249 (1995). 24. ASTM Powder Diffraction Files ed., Joint Commitee on Powder Diffraction Standards, Pennsylvania, 1979, Files 14-164 and 11-78. 25. Kiselev, A. V., in ‘‘Infrared Spectra of Surface Compounds,’’ Chap. 4. Publishing House, Jerusalem, 1975. 26. Van der Voort, P., Gillis-D’Hamers, Y., Vrancken, K. C., and Vansant, E. F., J. Chem. Soc. Faraday Trans. 1 87, 3899 (1991). 27. Kno¨ringer, H., and Ratnasamy, P., Catal. Rev. Sci. Eng. 17, 31 (1978). 28. Gillis-D’Hamers, I., Cornelissens, I., Vrancken, K. C., Van der Voort, P., and Vansant, E. F., J. Chem. Soc. Faraday Trans. 1 88, 723 (1988). 29. Brindey, G. W., and Lamaitre, J., in ‘‘Chemistry of Clays and Clay Minerals’’ (A. C. D. Newman, Ed.). Wiley, London, 1987. 30. Thomas, F., Bottero, J. Y., Partyka, S., and Cot, D., Thermochim. Acta 122, 197 (1987). 31. Partyka, S., Lindheimer, M., Zaini, S., Keh, E., and Brun, B., Langmuir 2, 101 (1986).
coidas
AP: Colloid
´ PEZ-GONZA ´ LEZ NEVSKAIA, GUERRERO-RUIB Z, AND LO
580
32. Keh, E., Partyka, S., and Zaini, S., J. Colloid Interface Sci. 129, 363 (1989). 33. Lindheimer, M., Keh, E., Zaini, S., and Partyka, S., J. Colloid Interface Sci. 138, 83 (1990). 34. Denoyel, R., Rouquerol, F., and Rouquerol, J., in ‘‘Adsorption from Solution’’ (C. H. Rochester and A. L. Smith, Eds.), p. 225. Academic Press, London. 1983. 35. Evers, O. A., Scheutjens, J. M. H. M., and Fleer, G. J., Macromolecules 23, 5221 (1990). 36. Kjellander, R., and Florin, E., J. Chem. Soc. Faraday Trans. 1 77, 2053 (1981). 37. Denoyel, R., and Rouquerol, J., J. Colloid Interface Sci. 143, 555 (1991). 38. Saito, Y., and Sato, T., J. Phys. Chem. 89, 2110 (1985). 39. Giles, C. H., D’Silva, A. P., and Easton, I. A., J. Colloid Interface Sci. 43, 766 (1974). 40. Partyka, S., Lindheimer, M., and Faucompre, B., Colloids Surf. A: Phys. Eng. Asp. 76, 267 (1993). 41. Denoyel, R., Ph. thesis, Marseille University, Marseille, 1987. 42. Levitz, P., and Van Damme, H., J. Phys. Chem. 90, 1302 (1986). 43. Rosch, M., in ‘‘Nonionic Surfactants, Surfactant Science Series’’ (J. Schick, Ed.), Vol. 1, p. 753. Dekker, New York, 1967.
AID
JCIS 4294
/
6g12$$$624
07-04-96 03:05:01
44. Robson, R. J., and Dennis, E. A., J. Phys. Chem. 81, 1075 (1977). 45. Flory, P. J., ‘‘Principles of Polymer Chemistry,’’ p. 399. Cornell Univ. Press, Ithaca, NY, 1953. 46. Tanford, C., Nozak, V., and Rohde, N. J., J. Phys. Chem. 81, 16 (1977). 47. Rochester, C. H., and Gellan, A., J. Chem. Soc. Faraday Trans. 1 81, 2235 (1985). 48. Furlong, D. N., and Aston, J. R., Colloids Surf. 12, 255 (1984). 49. Scamehorn, J. F., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 85, 494 (1982). 50. Trochet-Mignard, L., Taylor, P., Bognolo, G., and Tadros, Th. F., Colloids Surf. A: Phys. Eng. Asp. 95, 37 (1995). 51. Levitz, P., Langmuir 7, 1595 (1991). 52. Casey, W. H., J. Colloid Interface Sci. 163, 407 (1994). 53. Yates, D. E., and Healy, T. W., J. Colloid Interface Sci. 55, 9 (1976). 54. Axelos, M. A. V., Tchoubar, D., and Bottero, J. Y., Langmuir 5, 1185 (1989). 55. Kosmulski, M., Colloids Surf. A: Phys. Eng. Asp. 95, 81 (1995). 56. Nevskaia, D. M., Guerrero-Ruiz, A., and Lo´pez-Gonza´lez, J. de D., in preparation. 57. Peng, F. F., and Pingkuan, D., J. Colloid Interface Sci. 164, 229 (1994). 58. Traube, I., Ann. 265, 27 (1891).
coidas
AP: Colloid