Aqueous Solutions of Ethyl (Hydroxyethyl) Cellulose and Hydrophobic Modified Ethyl (Hydroxyethyl) Cellulose Polymer: Dynamic Surface Tension Measurements

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

193, 41–49 (1997)

CS974990

Aqueous Solutions of Ethyl (Hydroxyethyl) Cellulose and Hydrophobic Modified Ethyl (Hydroxyethyl) Cellulose Polymer: Dynamic Surface Tension Measurements Suh-Ung Um, E. Poptoshev, 1 and R. J. Pugh 2 Institute for Surface Chemistry, Box 5607, Stockholm, Sweden Received January 24, 1997; accepted May 16, 1997

rheological properties. They may be easily hydrophobically modified by grafting small amounts of nonyl phenol groups to the hydrophilic chains. Since this is usually achieved by a heterogeneous reaction, regional variations in the degree of crystallinity/amorphous of the polymer result. This gives rise to diversifications in the hydrophobic/hydrophilic nature of the polymer backbone. One of the major advantages of these polymers is that the high-molecular-weight versions are capable of producing high-viscosity aqueous solution even at low concentration. They are also surface-active agents and adsorb strongly at the solid/water, air/water, and oil/water interfaces, and this has resulted in their application as speciality chemicals in mineral processing (flotation depressants), in ceramic processing (binders and dispersants), and in polymerization (protective colloids). Recently, the dynamic interfacial characteristics of these polymers have been determined on a time scale extending from 30 s to about 17 h using the pendant drop technique (1, 2). From these studies, it was reported that the polymer molecules slowly diffuse and adsorb at the interface and these processes are followed by configuration changes. Also, it was shown that the surface tension-versus-time isotherms follow a sigmoidal pattern, and from these data it was possible to identify separate consecutive kinetic regions: the induction period, the surface coverage, and finally a mesophase region. In the mesophase region, slow equilibration occurred which involved a progressive ordering of the polymer segments within the surface layer. In fact, a constant steady-state equilibrium interfacial tension value could not be achieved, although measurements were made over time scales extending over several days. In addition to these relatively slow dynamic (approach to equilibrium) measurements, characteristic surface tension values over shorter time scales are also relevant to many industrial applications. For example, the performance of ethyl (hydroxyethyl) cellulose (EHEC) in flow capillaries, porous media, and foams and the spreading velocity of water-based paints are strongly dependent on ‘‘far from equilib-

The dynamic surface tension of aqueous solutions of ethyl (hydroxyethyl) cellulose (EHEC) and hydrophobic modified ethyl (hydroxyethyl) cellulose (HM-EHEC) were determined using the maximum bubble pressure method. Values were monitored over surface lifetimes ranging from 0.15 to 2.5 s (after dead time corrections). In the low concentration range (õ100 ppm) HM-EHEC was shown to be more surface active, and the presence of salt (0.1 M) was shown to increase the surface activity of both polymer systems. The results were compared with surface tension measurements carried out over longer time scales, determined using the du Nouy ring technique. From these results, isotherms were constructed relating surface tension to surface aging time. Although the results could not be directly correlated to interfacial diffusion models, the isotherms were found to be comparable to previous data reported for lower-molecular-weight poly(oxyethylene ether) surfactants and a higher-molecular-weight EHEC polymer determined using the pendant drop technique. Essentially, the isotherms could be divided into distinct regions: induction period, fast fall region where surface coverage occurs fairly rapidly, and finally the meso-equilibrium region. From the Gibbs equation, the number of segments of polymer per unit surface area at saturated adsorption levels was calculated over regions of different aging times, and the results are discussed in terms of configuration changes of the polymer at the interface. q 1997 Academic Press Key Words: ethyl (hydroxylethyl) cellulose; hydrophobically modified; dynamic interfacial tension measurements; maximum bubble pressure method.

INTRODUCTION

Water-soluble hydroxyethyl cellulose derivatives are widely used throughout industry in numerous colloidal formulations. For example, they are used in food, in cosmetics, and also in aqueous based paint systems to improve the 1 Visiting Scientist from the Department of Physical Chemistry, Faculty of Chemistry, University of Sofia, Sofia, Bulgaria. 2 To whom correspondence should be addressed. E-mail: bob.pugh@surf chem.kth.sc.

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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.

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rium’’ dynamic interfacial tension properties. In the present study, we have carried out dynamic surface tension measurement with EHEC and hydrophobically modified (HM)EHEC polymers using the maximum bubble pressure method. This enabled surface aging to be studied over a range from 0.15 to 2.5 s. This asset was found to be particularly useful in studying the relatively slow diffusion and adsorption of the bulky EHEC surfactant molecules. In addition, with this technique, since a new gas/water interface is generated with every bubble, the method is less sensitive to impurities or contamination than more conventional methods. The results were compared with other dynamic surface tension data determined over longer time scales using the ring tensiometry method. From these experiments, the amount of polymer adsorbed, the area per segment, and the conformation changes that occur in the surface could be estimated and compared under a wide range of dynamic conditions.

tial experiments were carried out with both EHEC and HMEHEC polymers dissolved in deionized (Milli-Q) water; however, since in many industrial systems it is common to have electrolye in the system, further experiments were carried out with addition of electrolyte (NaCl) giving a constant ionic strength solution (0.1 M). Viscosity measurements were carried out using the Ubbelohde viscometer. For the low-viscosity systems, a 0.95-mmdiameter tube (kinematic viscosity range, 5–50 mm2 /s) was used, whereas for the higher-viscosity systems the 1.13-mmdiameter tube (kinetic viscosity, 10–100 mm2 /s) was found more convenient. The samples were thermostated in the viscometer held in a water bath for 15 min before measurements were made. Viscosity data may be expressed as the relative viscosity ( hsp /c), where c is the polymer concentration and hsp , the specific viscosity, is defined by the equation hsp Å ( h 0 h0 )/ h0 ,

[1]

MATERIALS AND EXPERIMENTAL TECHNIQUES

EHEC polymers (MW 100,000) were manufactured and supplied by Berol Nobel AB, Stenungsund, Sweden. Both unmodified and hydrophobically modified polymers were used in this study. The degree of substitution (DS) of the ethyl and hydroxyl ethyl was expressed in terms of ethyl (average number of ethyl groups per anhydroglucose unit of the polymer) and the molecular substitution (MSEO ) refers to the average number of hydroxyl ethyl groups per anhydroglucose unit of the polymer. Values given by the manufacturer were DSethyl Å 0.6–0.7 and MSEO Å 1.8. The HM polymer was manufactured by grafting nonyl phenol chains to the cellulose backbone. The degree of hydrophobic substitution of the HM-EHEC was 1.7 mol% relative to repeating units of the polymer as previously determined (3). This represented the only difference between the two polymers. The structure of the polymers is shown in Fig. 1. The cloud points of the polymers were 657C and they showed no tendency of thermo-induced gelation. The polymer samples were purified (dialyzed to remove electrolyte) and freezedried as previously described (4). They were stored at 87C in a desiccator prior to use. Considerable care was taken to prepare the aqueous polymer solutions by a reproducible method, since time-dependent dissolution effects for EHEC polymers had been reported earlier (5). Each polymer solution were freshly prepared separately from the solid (after direct removal from the desiccator) and dissolved in water by stirring overnight. This eliminated any error involved with solubility time-dependent effects which may occur in the use of stock polymer solution. All the polymer solutions had a pH between 6 and 7 after the dissolution process was complete. Polymer concentrations were expressed in parts per million (with good precision, milligrams per liter). Ini-

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where h and h0 are the viscosities of the solution and solvent, respectively. Surface tension was measured using a SensaDyne 6000 tensiometer with Version 4.0 software (Chem-Dyne Research Corp., USA) The technique is a commercial refinement of the maximum bubble pressure method. Highly purified nitrogen gas is bubbled through two glass capillaries of different radius which are immersed in the solution (the diameters of the tubes used in the present study were 4.0 and 0.5 mm). The bubbling of nitrogen through the probes produces a differential pressure which is recorded electronically and related directly to the surface tension. By keeping the two probes at the same immersion depth, the effect of liquid level is canceled. In addition, all the other effects influencing the pressure value such as liquid density, radius of the bubble, tube orifice, gravitational constant, and depth of bubble formation are completely and accurately identified, measured, and resolved in the measurement. After preparing the polymer solutions, surface tension measurements were carried out following standard operating procedures. After stabilization of all parameters at 257C (to within {0.27C) the excess pressure in the system and the time interval between subsequent bubbles are recorded. This time interval is referred to as the bubble interval ( tB ) and can be adjusted to give a range of dynamic surface tension values. It is the bubble interval (expressed in seconds) between the detachment of consecutive bubbles from the small capillary (the reciprocal values are expressed as bubbles per second). The reliable operating range for the our instrument was 2.5 s (0.4 bubbles s 01 ) to 0.1 s (10 bubbles s 01 ). From the measurement of the bubble interval the ‘‘so-called,’’ the surface aging lifetime of the bubble ( t ) can be determined. The bubble interval is the sum of the surface aging lifetime

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FIG. 1. Structure of ethyl (hydroxyethyl) cellulose ether (EHEC) and hydrophobically modified ethyl (hydroxyethyl) cellulose ether (HM-EHEC). Both polymers have a molecular weight of 100,000. Also, HM-EHEC has a nonyl phenol substitution of about 1.7 mol%.

and the dead time ( tD ). At the initial stages of the bubble formation process (the growth period) the bubble radius is greater than the capillary radius; however, as the bubble grows, the bubble radius decreases until a hemispherical meniscus is formed and the radius becomes equal to the capillary radius. It is this bubble growth period t that is the surface lifetime. Providing the volume in the system is relatively small, it is at this moment that the pressure reaches its maximum value according to the measuring system. The dead time followed the attainment of maximum pressure. It is considered to be the time of explosive bubble growth, counting from the hemisphere of the bubble up to the point of cavity collapse or detachment. In recent years, there has been considerable discussion in the literature on how to judge the effective surface age with different types of equipment and measuring techniques (6–10). Several workers have shown that there is also a decay period (time) of declining pressure that occurs directly after the maximum pressure value but preceding the dead time (this has been called the decay time). In the present study using the SensaDyne apparatus only the dead time was observed. A typical series of waveforms for the polymer solutions are shown in Fig. 2. The region of increasing pressure (the bubble age) and the dead time are clearly indicated. Typical results showing a range of waveforms obtained in different polymer solutions, up to concentrations as high as 10,000

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ppm, at a range of bubble speeds are shown in Fig. 2. Each large peak represents the point of discrete bubble release from the orifice. From these data it can be seen that for the low-frequency bubble rate, the dead time is relatively small, but in the case of fast bubble speeds, the dead time becomes extremely significant with these polymer systems. From these data, correction must be made so that the dead time is subtracted from the bubble period. This enables a good estimate of the true surface age of the interface to be made. For fast speeds, it has been recommended by the manufacturers that a program be introduced so that the dead time could be determined from the waveform which is electronically recorded; however, in the present study since a limited number of speeds were used, it was found convenient to compute the data directly from the transient recorder traces and carry out the neccesary corrections to determine the surface age of the bubbles. From Fig. 2 it can be seen that correction ranged from about 5% for the slow bubble speeds ( Ç2 s) up to about 30% in the case of the higher bubble speeds (0.15 s). The highest dead times were observed for the higher concentrations of hydrophobically modified polymer at high bubble speeds. Corrections have been proposed that enable the influence of viscosity (which results from the hydrodynamic resistance of the moving bubble in the viscous liquid) to be eliminated (7). The correction parameter has been expressed

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in terms of the viscosity of the liquid, the surface lifetime of the bubbles, and the radius of the capillary. In the present study, measurements were carried out in the range where the viscosity changes occurring in the polymer solutions were not drastic so the corrections were small (i.e., for the EHEC from 10 to 10,000 ppm and for the HM-EHEC from 10 to 6000 ppm). Beyond these regions where the viscosity increased decisively, the correction factor was found to be inadequate. The precision of the technique was thoroughly checked by measuring the surface tension of Milli-Q water and ethanol seven times, alternating between each solution. The mean surface tension of water was 72.37 mN m01 with a standard deviation of 0.03. Surface tension of the polymer solutions was measured relative to water by measuring the surface tension of water and replacing the water with the polymer solution. In addition to the maximum bubble pressure measurements, for comparison purposes, the surface tensions of the polymer solutions were also measured over longer time scales using the du Nouy ring technique. The polymer solutions were introduced into the sample vessel and stirred for 60 s. The ring was then lowered into the polymer solution

FIG. 3. Specific viscosity versus concentration for ( s ) ethyl (hydroxyethyl) cellulose ether, ( h ) hydrophobically modified ethyl(hydroxyethyl) cellulose ether, ( L ) ethyl (hydroxyethyl) cellulose ether / 0.1 M NaCl, and ( 1 ) hydrophobically modified ethyl (hydroxyethyl) cellulose ether / 0.1 M NaCl.

and held for a relaxation period corresponding to the aging time. The ring was then raised to the interface and the surface tension recorded following standard procedures. Calibration of the instrument was carried out using solutions of known surface tension. RESULTS

1. Viscosity

FIG. 2. Examples of bubble pressure–time waveforms produced for the hydrophobically modified ethyl (hydroxyethyl) cellulose ether systems. (a) 50 ppm, t Å 0.15 s, tD Å 0.050 s; (b) 250 ppm, t Å 0.15 s, tD Å 0.056 s; (c) 1000 ppm, t Å 0.14 s, tD Å 0.063 s; (d) 4000 ppm, t Å 0.14 s, tD Å 0.062 s; (e) 8000 ppm, t Å 0.11 s, tD Å 0.094 s; (f ) 10,000 ppm, t Å 0.10 s, tD Å 0.10 s; (g) 12.5 ppm, t Å 2.3 s, tD Å 0.048 s; (h) 8000 ppm, t Å 2.3 s, tD Å 0.11 s; (i) 10,000 ppm, t Å 2.2 s, tD Å 0.15 s. The bubble period ( tB ), surface age ( t ), and dead time ( tD ) are indicated in the upper waveform.

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In Fig. 3, the relative viscosity of the EHEC and HMEHEC polymers are shown over a range of concentrations up to 10,000 ppm with and without the NaCl. As the concentration of the polymers increases to about 2000 ppm there is only a slight increase in viscosity; however, at higher concentrations around 5000 ppm, the increase becomes more pronounced. Clearly, HM polymer shows a greater increase in viscosity compared with EHEC. In fact for EHEC, the viscosity is 3-fold higher at 10,000 ppm (compared with values at 2000 ppm), whereas for HM-EHEC, there is a 150-fold higher increase over the same concentration range. In addition, from these results it can be concluded that the presence of NaCl had only a marginal effect on the viscosity of the two polymer systems. These results confirm earlier studies, which established that HM polymers have a drastic effect on viscosity compared with unmodified polymers of the same molecular weight (11). This has been explained by aggregation of the polymer units (12, 13). Essentially, the formation of different types of aggregated structures in solution depends on the architecture and hydrophilic/hydrophobic balance of the polymer. At low polymer concentrations, the EHEC samples form only small aggre-

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FIG. 4. Dynamic surface tension versus concentration for ethyl (hydroxyethyl) cellulose ether. Maximum bubble pressure method. ( h ) Surface age 0.14 s, ( 1 ) surface age 2.4 s, ( L ) surface age 0.98 s, ( s ) surface age 1140 s (du Nouy ring method).

gates which behave macroscopically like single polymer chains; however, with the HM-EHEC polymer system, two different types of aggregation are possible: intraaggregate, caused by interaction of hydrophobic tails on the same polymer unit, and interaggregate, caused by interactions with hydrophobic tails of different polymer units. At low polymer concentrations, intraaggregation interactions usually dominate, resulting in smaller polymer coils compared with the random coil of the unmodified polymer. At higher polymer concentrations, however, intermolecular aggregation occurs so that clusters are rapidly formed with crosslinks between different polymer chains causing growth into a loose threedimensional network of infinite size. This causes pronounced increases in viscosity. The addition of NaCl (0.1 M) appeared to have little effect on the specific viscosity and aggregation process, except at high polymer concentrations, where the presence of electrolyte causes a slight reduction in viscosity.

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Also beyond 5000 ppm, the difference in values at different surface aging times decreases until approximately the same surface tension values are reached (corresponding to 55 mN/ m). From these dynamic results, no great significant characteristic differences in the general behavior between the HM and unmodified polymers was detected apart from the fact that the HM-EHEC polymer shows a significantly shorter induction time and the drop in surface tension occurs at concentrations below 100 ppm at surface aging times between 1 and 2.4 s, which indicates a difference in adsorption dynamics. Recently, dynamic interfacial tension studies have been reported by Nahringbauer (1, 2) with a higher-molecularweight EHEC (MW 480,000). Data were collected at the air/water interface using the pendant drop technique (1). Interfacial aging measurements were reported from about 30 s up to several days. From these results, it was found that these longer aging times were required in dilute solution to achieve a true surface equilibrium. A characteristic curve similar to Figs. 4 and 5 was constructed by Nahringbauer [Ref. (1), Fig. 5] for the high-molecular-weight EHEC polymer after a 17-h surface aging period. The point where a break in the curve occurred corresponded to where the surface tension reached a plateau (at about 40 mN/m corresponding to about 5 ppm polymer). It is interesting to see that this point correlates with a pronounced increase in viscosity as shown in Fig. 3. Figures 4 and 5 illustrate the dynamic surface tension results derived from the De Nouy ring (after 19-min aging time). The results show much lower surface tension, with a gradual decrease occurring with increase in polymer concentration; however, there is no evidence of the ccc (critical concentration of condensation) transition in these plots. This can be explained, according

2. Dynamic Surface Tension of EHEC and HM-EHEC In Figs. 4 and 5 the change in dynamic surface tension is shown as a function of the increase in polymer concentration. These experiments were carried out at a range of surface aging times (0.15–2.5 s). Clearly, surface aging has a pronounced effect on the surface tension. With fast surface aging, at EHEC polymer concentrations up to 100 ppm, the initial surface tension does not deviate significantly from the surface tension of water. Over the concentration range 100 to about 2000 ppm, surface tension decreases sharply, but on a further increase beyond these values, relatively smaller decreases occur. It is interesting to note that these falls in surface tension are more pronounced at lower bubble speeds.

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FIG. 5. Dynamic surface tension versus concentration for hydrophobically modified ethyl (hydroxyethyl) cellulose ether. Maximum bubble pressure method. ( L ) Surface age 0.16 s, ( / ) surface age 2.4 s, ( h ) surface age 1.0 s, ( s ) surface age 1140 s (du Nouy ring method).

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FIG. 6. Dynamic surface tension versus concentration for ethyl (hydroxyethyl) cellulose ether / 0.1 M NaCl. Maximum bubble pressure method. ( 1 ) surface age 0.14 s, ( L ) surface age 2.1 s, ( h ) surface age 0.97 s, ( s ) surface age 1140 s (du Nouy ring method).

to Nahringbauer (1), by the fact that the ccc occurs at concentrations below 10 ppm under these slow dynamic conditions. Also from these data, it would appear that the HMEHEC polymer is more surface active than the EHEC polymer. This result is more or less as expected, since the introduction of a hydrophobic side chain to the polymer decreases solubility and increases surface activity; however, it is of interest to note from these results that this difference was detected only under relatively slow dynamic conditions. 3. Influence of Electrolyte on the Dynamic Surface Tension of EHEC and HM-EHEC In Figs. 6 and 7 the change in dynamic surface tension for the EHEC and HM-EHEC polymers in NaCl (0.1 M) is

FIG. 7. Dynamic surface tension versus concentration for hydrophobically modified ethyl (hydroxyethyl) cellulose ether / 0.1 M NaCl. Maximum bubble pressure method. ( 1 ) surface age 0.16 s, ( L ) surface age 1.8 s, ( h ) surface age 0.98 s, ( s ) surface age 1140 s (du Nouy ring method).

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FIG. 8. Dynamic surface tension versus concentration for ethyl (hydroxyethyl) cellulose ether (EHEC) and hydrophobically modified ethyl (hydroxyethyl) cellulose ether (HM-EHEC) in 0.1 M NaCl (du Nouy ring method). ( 1 ) EHEC surface age 240 s, ( h ) HM-EHEC surface age 240 s, ( L ) EHEC surface age 1140 s, ( s ) HM-EHEC surface age 1140 s.

shown as a function of the increase in polymer concentration at a range of bubble speeds. The results show a similar characteristic set of the curves as the unmodified polymer; however, the most significant difference in behavior can be observed at concentrations below 100 ppm, where the induction time is shortened, and this is particularly pronounced in the 1- to 2.5-s surface aging results. In Fig. 8, the results obtained from the longer-term surface aging experiments carried out using the de Nouy ring method after 240 and 1140 s with salt are presented. With the EHEC system, the greatest decrease in surface tension occurs for extended aging time (1140 s). With HM-EHEC, the extended aging times did not appear to have a less significant effect on the surface tension. The curves indicate that EHEC is more sensitive to longer-term interfacial aging and salt effects than is the HM polymer. These results can be discussed in the light of earlier studies where it was shown that the addition of salt has a pronounced effect on the bulk solution properties of HM-EHEC and EHEC. In fact, it has been shown that salts can decrease or increase the cloud point (CP) and viscosity of HM polymers. For salts with small anions and small molecular radii such as chloride, a decrease in CP was reported (11). This was explained by the addition of a salt causing the ion to prefer the aqueous environment, resulting in the solvent having a higher effective polarity. This causes a lowering of the chemical potential of water or an increase in the surface tension between polymer and water. Polymer will further deplete NaCl from the interface giving a force to minimize the polymer/solvent interaction (3). In bulk NaCl solution, the polymer molecules cluster together, giving lower solubility and causing a phase separation at a lower temperature; however,

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these earlier reported studies were carried out with high salt and polymer concentrations. With relatively low concentrations of polymer up to a maximum of 10,000 ppm, the present study indicates dynamic surface tension measurements can detect reductions in the interfacial tension of both polymers caused by NaCl (0.1 M NaCl). 4. Surface Tension/Time Plots The dynamic adsorption of polymers and surfactants at the air/water interface is practically important and fundamentally challenging. This has led to the development of many different types of theoretical models based on molecular transfer rates, the equilibrium of the adsorbed layers, and changes in polymer conformation that occur in the interface. Extensive reviews of these models are presented in the literature (14, 15). Many workers have attempted to relate their results (usually presented in the form of surface tensionversus-time isotherms) to these models. Unfortunately, this approach has led to a limited amount of success and there are few cases where a clear-cut interpretation was reached. If we assume a pure diffusion model where there is little desorption from the surface to the subsurface, then the surface concentration can be related to the adsorption time by the diffusion coefficient (16). The equation can be expressed in the form G(t) Å 2c0 (Dt/ p ) 1 / 2 ,

[2]

where D (m2 /s) is the diffusion coefficient, c0 (g/m 2 ) is the bulk concentration, G(t) (g/m 2 ) is the surface excess, and t is time. In the diffusion-controlled adsorption model, it is assumed that there is no activation barrier in the transfer of polymer between the subsurface and the surface so that diffusion is the only mechanism needed in establishing adsorption equilibrium. According to this classical approach, the surfactant concentration in the interface increases directly with the increase in the square root of time and this model was found to be adequate for many simple surfactant systems under dilute conditions. In fact, it has been widely used to analyze dynamic surface tension data; for example, Graham and Phillips (17) used this model to study the kinetics of the initial stages of the adsorption of protein at the air/water interface. Further theoretical development has led to so-called mixed diffusion–kinetic-controlled models where the adsorption step is combined with a transfer mechanism. For example, the rate equations from the Langmuir, Frumkin, Henrys, and Langmuir isotherms have been combined with the classical diffusion process (15). In the present study, since data were collected by two different experimental techniques which give a relatively wide difference in the range of time scales, it was only

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q

FIG. 9. Dynamic surface tension versus surface age s for ( s ) EHEC 12.5 ppm, ( h ) EHEC 25 ppm, ( L ) EHEC 100 ppm, ( 1 ) EHEC 500 ppm, ( / ) HM-EHEC 12.5 ppm, ( n ) HM-EHEC 25 ppm, ( l ) HM-EHEC 100 ppm, ( j ) HM-EHEC 500 ppm. For comparison, curves for polyoxyethylene n-dodecyl ethers C12En (5 1 10 5 ppm) with n Å 31 (curve A) and n Å 43 to 53 (curve B) and a higher-molecular-weight EHEC (MW Å 480,000) (5 ppm, curve C, and 2 ppm, curve D) taken from the literature (1, 18) are shown.

possible to sketch an isotherm relating surface tension versus square root of time (Fig. 9). With this limited amount of data, a complete theoretical analysis was not attempted; however, these results do indicate a strong deviation from diffusion theory. This is not altogether surprising, since for the diffusion equation to be valid, the surface needs to be empty so that each molecule arriving at the surface can arrive at an empty site, and this can only occur in the initial stages of the process. As the process proceeds and the surface becomes crowded, a local equilibrium between the surface and subsurface becomes established. The rate-determining step is then controlled by the mechanism which involves the transfer of polymer segments between the solute and adsorbed state. In these circumstances, the polymer can create a space in the existing film only by penetration and rearrangement in the surface. This process is kinetically controlled and governed by an activation energy. For infinitely long chains consisting of several hundred segments, adsorption can occur only when the adsorption energy exceeds some critical value which is related to the difference between the free energy of transfer of a segment from the bulk polymer in the subsurface to the surface and that of a transition of a solvent molecule from the pure solvent to the surface. With the EHEC, it is also important to consider the influence of the polydispersity of the systems. Polydispersed systems are essentially multicomponent mixtures from which preferential adsorption of certain species occurs. Under the diffusion-controlled theory, the smaller polymer molecules are more rapidly adsorbed in the interface; however, in general, the higher-molecular-weight species will be adsorbed preferentially to the low-molecular-weight species, so with

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an increase in time the larger molecules will gradually replace the smaller molecules in the interface. Since the polymer concentration in the interface region is high, conformation changes and entanglement will occur and this will be a slow process since the EHEC polymer has a branched structure. Nevertheless, from the results presented in the form of the relationship between surface tension and the square root of aging time, some interesting observations can be made between previously reported data. In Fig. 9, it can be seen that the isotherms generally follow a sigmoidal pattern, indicating a strong dependence on bulk concentration. The systems with higher bulk concentration show a more rapid approach to equilibrium at low aging times, whereas the lowconcentration polymer systems show a pronounced induction period. It is interesting to note that these results appear to show similar characteristics to earlier reported results, for aqueous solutions of low molecular weight non-ionics surfactant systems and also for a higher molecular weight EHEC. For example, the typical dynamic curves presented by Tamura et al. (18) for polyoxyethylene n-dodecyl ethers show a strong resemblance to these results. For comparison purposes, the results from the system C12En with n Å 31 to 53 are also presented in Fig. 9. These curves show the characteristic fall in surface tension occurring over much shorter aging times. From the shape of the curve Nahringbauer (1) suggested the process could be subdivided into different stages. Initially, the process involves the diffusion of the molecule to the very thin region of the bulk solution, immediately below the surface or the so-called subsurface region. This is then followed by structural rearrangements and, in the case of polymers, involves spreading and unfolding of the chains. At the same time attachment of polymer segments to the surface can occur, reducing the surface energy. Finally, rearrangement of the adsorbed polymer segments can occur (between the surface and subsurface). The use of empirical curve fits enabled Hua and Rosen (19, 20) to evaluate a series of basic parameters to characterize such isotherms for surfactant systems. In retrospect, the similarities in the curves are not entirely unpredictable since cellulose ethers such as methylcellulose and ethyl (hydroxylethyl) cellulose belong to the poly(ethylene oxide) surfactant family. With regard to the higher-molecular-weight EHEC polymer studied by Nahringbauer (1) the characteristic fall in surface tension occurred over longer aging time scales. These trends clearly illustrate the influence of polymer size, diffusion coefficient, and configuration changes on dynamic surface tension. 5. Gibbs Adsorption Isotherms De Feijter and Benjamins (21) and Nahringbauer (1) have shown that the Gibbs adsorption isotherm is applicable to

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FIG. 10 Surface coverage and number of segments per unit area versus surface aging time.

polydispersed polymer systems. According to these workers, the Gibbs adsorption equation can be expressed in the form Gmax Å 0

S D dg d ln c

max

Mseg . RT

[3]

Here Gmax is the concentration of polymer at the interface at the saturated adsorption level, and Mseg (g/mol) is the average molecular weight of one polymer segment, which can be calculated from the structural formula of the polymer (for the present work, it is equal to 266 g/mol). The polymer concentration is expressed in weight per unit volume. Values were obtained for the maximum slopes of g-versus-ln c curves for EHEC and HM-EHEC using the data from Figs. 4 and 5. Use of the Gibbs equation in the above form enables the surface coverage as well as the number of adsorbed polymer segments per unit surface area at surface saturation Ns to be quantified using the equation Ns Å

Gmax Na , Mseg

[4]

where Na is Avogadro’s constant. These results relating Gmax to surface aging time are illustrated in Fig. 10 and show that at low surface aging times, the saturated coverage is about 0.55 mg/m 2 (1.3 1 10 18 segments/m 2 ), whereas at longer aging times, the coverage is considerably increased to 0.8 to 0.9 mg/m 2 (2 1 10 18 segments/m 2 ), with the HM polymer giving the lower value. These results give an indication of the physical dimensions of the polymer coils. With increase in surface aging time, conformation changes (uncoiling, stretching, etc.) take place in the surface following the model outlined by Nahringbauer (1). Generally, the Gibbs equation is applicable for equilibrium conditions; however, in this presentation the Gibbs equation has been used in an attempt to show the trend of Gmax for different surface ages. That is why the values presented for Gmax are lower than the

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values obtained under equilibrium conditions as presented by Nahringbauer (1). Finally, it is interesting to note that the cross-sectional area of an EHEC polymer (MW 25,000) adsorbed on a smooth hydrophobic substrate, as determined using ellipsometry experiments, was found to have a greater value (2.3 mg/m 2 ), suggesting a more stretched structure (22). CONCLUSIONS

Dynamic surface tension measurements were carried out with aqueous solutions of ethyl (hydroxyethyl) cellulose and hydrophobic modified ethyl (hydroxyethyl) cellulose using the maximum bubble pressure technique (surface lifetimes 0.15–2.5 s). From the results at low polymer concentration, it was shown that HM-EHEC was the most surface active, and addition of NaCl (0.1 M) increased the surface activity of the polymer systems. At these low polymer concentrations, significant differences in values of the surface tension were recorded due to slow transport of the polymer to the interface. With an increase in polymer concentration, a general decrease in surface tension was detected until the surface tension-versus-concentration plot approached a plateau region. Similar behavior for high-molecular-weight EHEC polymers had been reported earlier by Nahringbauer (1) using the pendant drop, where the break in the surface tension-versus-concentration plot was designated the ‘‘critical concentration of condensation.’’ It was suggested that a phase change of the polymer in the interface occurred. The results from the present study were compared with measurements carried out over a longer time scale (several hours) determined using the du Nouy ring method. In the latter case, much lower surface tension values were obtained over the same concentration range. Marked differences in dynamic surface tension could also be detected between the EHEC and HM-EHEC polymer systems over longer time scales. Similar characteristic features of dynamic surface tension-versus-concentration plots had been earlier reported for polyoxyethylene n-dodecyl ethers surfactant systems, although no aging effects had been reported. Finally, from the Gibbs adsorption isotherm, the number of segments per unit area at the interface at saturated adsorption levels was calculated for the polymer systems and this value was shown to increase as a function of aging time. Although the present

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study has limitations in that the dynamic surface tension data also depend to some extent on the bulk surfactant concentration, the results clearly illustrate the profound effects caused by the adsorption of high-molecular-weight polymers at the air/aqueous solution interface. ACKNOWLEDGMENTS The research was financed by the Swedish Institute, which provided a scholarship for E. Poptoshev, and also the Swedish Technical Foundation (TFR), which provided a Ph.D. Scholarship for Suh-Ung Um. We thank Docent Inger Nauhringbauer for advice and helpful discussion. We also acknowledge Dr. Krister Thurenson, Lund University, Sweden, for supplying the polymer and advice on the solution properties. Finally, thanks to Dr. Peter Weissenborn for help with the instrumentation.

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