Langmuir 1993,9, 3085-3092
3086
Sorption/Desorption of Ions by Dynamic Electrokinetic and Permeability Analysis of Fiber Plugs J. Jachowicz,* S. Maxey, and C. Williams Clairol, Inc., 2 Blachley Road, Stamford, Connecticut 06922 Received January 7, 1993. I n Final Form: August 3,1993@ Sorptionldesorption of ions on human hair was investigated by using a new instrument which can perform simultaneous measurements of the streaming potential, conductivity, and permeability of the fiber plugs. The apparatus allows, first, the surface of a newly formed plug of fibers to be characterized and then the dynamics of change in its ionic character after treatment with solutions of surfactants and/or polymers, and during rinsing with the test solution, to be followed. The sensitivity of this technique is demonstrated by the analysis of anionic and cationic surfactants as well as cationic polymers. Anionic surfactants such as sodium, ammonium, diethanolammonium, and triethanolammonium lauryl sulfates were shown to bind to hair transiently by surface adsorption and, possibly, penetration into the bulk of the fiber. Their affinity to hair was found to be related to the nature of the counterion as judged from the time dependence in electrokinetic parameters. Cationic surfactants were found to bind to the fiber surface, resulting in reversal of the sign of the streaming potential and reduction of the plug conductivity. The former effect was also temporary, and the initially high and positive {potential was shown to gradually decrease as a result of the removal of the cationic species from the fiber surface during rinsing with the test solution. The treatment of the hair with cationicpolymersproduced more durable surfacemodifications, with the extent of change in the electrostatic character of the surface being related to the charge density of the adsorbed polymer. Introduction Streaming potential measurements are frequently employed to characterize the surface properties of fibers and minerals.lt2 The technique has particular importance in textile science since the electrochemical double layer influences the interactions of fibers with detergents, polymers, and dyes.s7 Streaming potential measurements are typically carried out by the use of fiber plugs and electrolyte solutions at a constant ionic strength and specified pH. In a typical experiment, untreated fibers are loaded into a streaming potential cell and their f potential is measured by passing the test solution through the cell at one or several different pressures, and at one or various densities of packing. In order to determine the effect of a treatment, the fibers are processed outside the cell, reloaded, and remeasured. This gives a value of the potential for modified fibers which can be compared with that obtained for untreated fibers. The results obtained in this manner are frequently impaired by considerable scatter of experimental data, and lack of reproducibility. Some of these problems were traced to nonuniform pad formation, air bubble entrapment in the ~ ~ also been observed pad, or electrode p o l a r i ~ a t i o n .It~has that parameters such as the streaming potential, conductivity, and plug permeability are frequently time-dependent, and can change considerably even during the rinsing of the fibers with the test solution.' This was ascribed to several factors such as the surface hydration, rearrange-
r
e Abstractpublishedin Advance ACSAbstracts, October 15,1993.
(1)(a) Kittaka, S.;Furusawa, K.; Ozaki, M.; Morimoto,T.; Kitahara, A. In Electrical Phenomena at Interfaces; Kitahara, A., Watanabe, A., Eds.; Surfactant Science Series; Marcel Dekker: New York, 1984;Vol. 15, p 183. (b) Suzawa, T.Ibid.; p 299. (2)Jacobasch, H.; Baubock, G.; Schurz, J. Colloid Polym. Sci. 1985, 263 (l),834. (3)Cook, H.D.; Smith, I. T. Appl. Polym. Symp. 1971,18,663. (4)Parreira, H.C. J. Colloid Interface Sei. 1980,75 (l),212. (5)Goddard, E. D.;Harris, W. C. Proceedings of IFSCC XIVth, Barcelona, 1986; p 1039. (6) Jachowicz, J.;Berthiaume,M.; Garcia, M. Colloid Polym. Sci. 1985, 263,847. (7)Onabe, F. J. Appl. Polym. Sci. 1979,22,3495. (8) Ball, B.;Furstenau, D. W. Miner. Sci. Eng. 1973,5,267.
ment of surface layers, and desorption of surfadants,lipids, polymers, etc. This variability of the streaming potential with time of exposure to the test solution adds to the uncertainty in the single-point determination of the { potential and justifies the necessity of measuring the complete, time-dependent characteristics of electrokinetic parameters for both intact as well as modified fibers. Several researchers have performed studies of the adsorption of surfactants, polymers, and proteins on fibers, capillaries, or glass plates by incorporating the adsorbent in the streaming solution.9 The interpretation and comparison of these data are, however, more complicated because the concentration of electrolytes in the solution as well as that of adsorbed species, both of which determine the magnitude and sign of the f potential, is dependent upon the adsorptionldesorption equilibria between fibers and the test solution. This paper presents new instrumentation for performing simultaneous electrokinetic and permeability measurements in fiber plugs. In order to alleviate the sources of irreproducibility related to pad formation, and to obtain additional information about the kinetics of the sorption and desorption processes, the streaming potential and conductivity measurements are performed in a dynamic mode (as a function of rinsing time) before and after online treatments of the fiber plug with various active agents. The electrokinetic data are supplemented by permeability measurements which can detect changes in the fiber arrangement or swelling, and can also be affected by the adsorbed layers formed as a result of fiber interaction with the treatment solution. The latter assumption is based on both experimental and theoretical studies demonstrating the reduction in the flux of solvent through capillaries, capillary arrays, and porous membranes when a polymer is adsorbed on the walls of the flow channel.1° Preliminary experiments with hair fibers and solutions of surfactants ~
~
(9) (a) Norde, W.; Rouwendal, E. J. Colloid Interface Sci. 1990,139 (11,169.(b) Shirahama,H.; Lyklema, J.; Norde, W. Ibid. 1990,139(l),
177. (c) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Phys. Chem. 1964,68,3562.
0743-7463/93/2409-3085$04.00/0 0 1993 American Chemical Society
3086 Langmuir, Vol. 9, No. 11, 1993 and polymers, frequently used as components of cosmetic formulations, are presented in order to demonstrate the sensitivity of the technique.
1
Experimental Section Instrumentation. The valve and control system diagrams of the experimental setup are presented in Figure 1. The device consistsof a streaming potential cell, conductivitymeter, pressure transducer, test and treatment solution reservoirs, flow interrupter, electronic balance, and several electric and manual valves which control the flow of solutions through the measurement cell, maintain air pressure in the system, and allow easy handling of solutions. The key features of the instrumentation are the following: (i) on-line positioning of test and treatment solution reservoirs,allowing fiber treatment within the streamingpotential cell (this minimizesthe error associated with changes in the plug geometry, introduction of air bubbles into the fiber pad or streaming liquid, etc., since the same plug is used throughout the experiment), (ii) pulse method of meas*ing the streaming potential (the timiig of the pulses is affected by the flow interrupter), (iii) simultaneous measurement of the streaming potential, conductivity, and flow rate (permeability of the plug), (iv) special software allowing flexible design of the experiment, Le., timing of treatment and test cycles, controlof pressure,control of timing of the flow interrupter, forward and backward flow of the solutions through the plug, and data collection. The measuring cell consisted of a three-piece polycarbonate body with two silver electrodes of 1.27-cm diameter. Each electrode was perforated with 16 holes of 0.1-cm diameter. The supporting rods were 0.63 cm in diameter and were made of fine silver to minimize the polarization effects; Ag/AgCl electrodes were prepared in a manner described by Onabe.7 One gram of hair fibers chopped into pieces 2-4 mm in length was used to form the plug. The distance between the electrodes could be adjusted in the range from 1.0 to 1.5 cm which corresponds to hair plug densities of 0.72 and 0.48 g/cms, respectively. The dynamic electrokineticand permeability measurements described in this paper were obtained for the hair plugs at the concentration of 0.58 g of hair/cms. The conductivity of the solution in the plug was measured by means of an Orion Model 101 conductivity meter at a frequency of 1kHz. The cell constant of an empty cell (without the fibers) was determined by using leaM KC1 solution and was found to vary from 0.68 (1-cm distance between electrodes) to 0.99 (1.5cm distance between the electrodes; 0.88 for the distance of 1.30 cm at which the dynamic electrokinetic measurements were performed). In all experiments the aqueous solutions were prepared from water purified by using a Barnstead NANOpure system. It was characterized by an initial conductivity of 5 X 10-8 mho/cm. The operation of the instrument and data collection and handling were performed by using an IBM AT computer. A typical experimental protocol employed in this study included the following steps: (1) the measurement period of a newly formed hair plug consisting of 5 pulses of 5 X 106 M KCl solution at 12 cmHg during 5 min in an alternating sequence, flow 30son, flow 30 s off; (2) on-line treatment of hair in the plug with solutions of surfactants or polymers consisting of 5 pulses of the treatment solution at 12 cmHg during 5 min in an alternating sequence, flow 30 s on, flow 30 s off; (3) the measurement period of the treated hair consisting of 30 pulses of 5 X 106 M KCl at 12 cmHg during 30 min in an alternating sequence, flow 30 s on, flow 30 s off; (4) on-line retreatment of hair in the plug with the appropriate solutions consisting of 5 pulses of the treatment solution at 12 cmHg during 5 min in an alternating sequence, flow 30 s on, flow 30 s off; ( 5 ) the measurement period of the retreated hair consisting of 30 pulses of 5 X 1od M KC1 at 12 cmHg during 30 min in an alternating sequence, flow 30 s on, flow 30 s off. (10) (a) Cohen, Y.;Metzner, A. B.Macromolecules 1982,15 (5), 1425. (b) Cohen Stuart, M. A.; Waajen, F.H.W.H.; Coegrove, T.; Vincent, B.; Crowley,T. L.Ibid. 1984,17(9), 1826. (c) Rowland,F. W.; Eirich, F. R. J. Polym. Sci., Part A-1 1966,4,2033. (d)Rowland,F.W.; Eirich, F. R.
Ibid. 1966,4,2401.
Jachowict et al. BALLAST
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Figure 1. Schemes of the apparatus performing dynamic
electrokinetic and permeability measurements of fiber plugs: (a, top) valve diagram, (b, bottom) control system diagram. The first measurement period provides information about the surface and the permeability characteristics of a newly formed hair plug. These parameters are important for the data interpretation becauseof the considerable sample-to-samplevariations in hair properties as well as the difficulty in reproducible plug formation. In the second measurement period, the kinetics of
SorptionlDesorption of Ions sorption and desorption of ions can be followed by the streaming potential and the conductivity measurements. Note that while the streaming potential data reflect the state of the fiber surface, conductivity is related to the presence of free ions in the test solution. In addition to this, the changes in the flow rates may be indicative of either the variations in the volume of the fibers, Le., their swelling or shrinking, or deposition of surfactant or polymer on the fiber surface. The third measurement period allows assessment of the effect of multiple treatments on hair. Materials. Samples of Hair. All fiber samples were commerciallyblended virgin brown hair purchased from DeMeo Brothers, New York. The hair waa precleaned with a nonionic surfactant, Triton X-100,and thoroughly rinsed under deionized water. Hair samples were then soaked in three 500-mL water baths for 15 min each to ensure complete removal of any residual surfactant. Hair swatches were dried at 37 OC. Surfactants and Polymers. Sodium lauryl sulfate (SLS, purity > 98%) was obtained from Fluka. Ammonium lauryl sulfate (ALS),triethanolammonium dodecyl sulfate (TEALS), and diethanolammonium dodecyl sulfate (DEALS)were commercial aamples, in the form of 28%, 40%, and 33.8% aqueous solutions, respectively,and were obtained from Stepan Chemical co. Cetyltrimethylammoniumbromide (CTAB,purity >95% ) was purchased form Aldrick Stearyldimethylbenzylammoniumchloride (SDBAC,purity >SO%) was obtained from Lonza Chemical Co. (purity >85 %), and recrystallized from water prior to use in the experiments. Polymers were commercial samples obtained from Calgon Corp. and were used without additional purification. The homopolymer poly(dimethy1diallyla"onium chloride) (Merquat 100) was in the form of a 40% aqueous solution, and had a molecular weight of 106-106. The copolymer poly(acrylamideco-dimethyldiallylammoniumchloride) (Merquat 550,monomer ratio 5050)waa in the form of an 8.5 % aqueous solution, and had a molecular weight of 5 X 108. The copolymer poly(dimethy1diallylammonium chloride-co-acrylicacid) (Merquat 280)was a 35% aqueoussolution,and was characterized by acationic:anionic monomer ratio 80.20 and molecular weight of 2 X 106.
Langmuir, Vol. 9, No. 11, 1993 3087 0.007
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Figure 2. Kozeny-Cman plot for untreated keratin fibers: (m) 12 cmHg, (+) 8 cmHg, (a) 4 cmHg.
a slow cleavage of peptide bonds and formation of amino acids or small polypeptide fragments. Overnight soaking in deionized water, immediately prior to the experiment, produces lower and more stable conductivity readings, as well as a higher { potential. Hydrodynamicpermeability of the hair plugs at various concentrations of hair in the cell, obtained by varying the distance between the electrodes, could be described by the Kozeny-Carman equation:12
KC^)"^ = (1/5.55~~)'/~(1(UC) K = &&/PA
(1)
(2)
where K is the permeability coefficient of the pad, (U is the hydrodynamic specific volume (cm3/g), u is the specific surface area (cm2/g), Q is the volume rate of streaming flow (cm3/s), L is the length of the pad (cm), P is the pressure difference across the pad (dyn/cm2),A is the crosssectional area of the pad (cm2),and p is the viscosity of the liquid (P). A typical plot of (Kc2)'/3as a function of plug concentration for untreated hair, at three driving pressures of 4, 8,and 12 cmHg, is shown in Figure 2. The hydrodynamic specific volume and the specific surface area of the fiber, determined from the slope and c = 0 intercept, were 0.80 f 0.10 and 691 f 50 cm2/g, respectively. These values represent an average of the data collected on four different hair plugs and the measurements performed at three pressures. They are lower than those reported previously which were obtained from experiments performed using a larger flow cell.6 The value of the hydrodynamicspecific volume is close to that calculated for dry hair (0.77 cm3/g) while the specific surface area was found to be higher than that calculated from simple geometrical considerations (500cm2/g). Reynolds numbers were calculated according to the following equation:l3
Results and Discussion The results of initial investigations indicated a considerable plug-to-plug variation in electrokinetic and permeability parameters. Thus, each experiment included a measurement period of a newly formed hair plug which could serve as a reference point for the subsequent analysis of the effect of treatments. Generally, unmodified, nonionic surfactant-cleaned, and thoroughly rinsed hair was characterized by a streaming potential in the range from -180 to -280 mV. Concomitant changes in plug conductivity ranged from 7.5 to 4.8 mho/cm, and the { potentials calculated from the Smoluchowski equation varied from -8 to -15 mV. The values of electrokinetic parameters were found to depend primarily on the method of hair preparation before the experiment. Even carefully purified hair, washed with a dilute nonionic surfactant solution followed by extensive rinsing with deionized water, and then stored at room temperature for several weeks, would show adrift in conductivity and streaming potential. Typically, the starting conductivity of the plug was around 7-8 mho/cm and slowly decayed to about 5 mho/cm after 30-60 min of rinsing in the streaming potential cell with the test solution. This might be related to a slow release of low molecular weight species from the bulk of the fiber, such as residues of surfactants, loosely bound lipids," or amino acids formed as a result of some kind of a degradative process such as, for example, peptide hydrolysis. The equilibrium water content in hair under ambient conditions (40-6095RH) is about 10-12 7% which should enable
where D , is the equivalent diameter of fiber particles, which was calculated to be equal to 245 pm (for 0.2" pieces of 70-pm fiber diameter; calculated sphericity 0.43),u is the superficial velocity determined by taking the ratio of the volumetric flow rate and the cross-sectional area of the pad, p is the solution density, p is the solution viscosity, and e is the pad porosity defined as the ratio of the volume of voids to the total volume of the plug. For experiments
(11) (a) Shaw,D. A. Int. J. C o m e t . Sci. 1979,1,291. (b)Breuer, M. M. J. SOC.C o m e t . Chem. 1981,32,437. (c) Koch,J.; Aitzetmuller, K.; Bittorf, G.; Waibel, J. J. SOC.Cosmet. Chem. 1982, 33, 317.
(12) Robertson, A. A.;Mason, S. G. Pulp Pap. Mag. Can. 1949,50,103. (13) Horn, J. M.; Onoda, G. Y . J. Colloid Interface Sci. 1977,61 (2), 272.
(3)
3088 Langmuir, Vol. 9,No.11,1993
Jachowicz et al.
performed at various compressions of hair in the pad, at the driving pressure of 12 cmHg, the Reynolds number was found to vary from 3.70 (at the highest concentration of hair in the plug of 0.72 g/cm3) to 21.1 (at the lowest concentration of hair in the pad of 0.48 g/cm3). The creeping or Darcy flow region with a linear relationship between flow rate and pressure corresponds to Re below around 10, while the higher Reynolds numbers characterize a nonlinear relationship and non-Darcy flow. The dynamic electrokinetic and permeability analysis described in this report was performed at a constant concentration of hair, with a plug density of 0.58 g/cm3, and 12 cmHg pressure. The flow rates through the newly formed plug were reproducible within f15% in the range from 3.25 to 3.75 cm3/s which corresponds to Reynolds numbers in the range from 12 to 14, slightly above the linear-nonlinear flow threshold. We also observed an increase in the plug permeability during the flow of the test solution within the first 5-10 min of the experiment which may be related to flow-induced rearrangement of the fibers in the plug. The permeability of the plug, and consequently the flow rate of the test solution at a given pressure, can also change significantly after exposure of the hair to the treatment solution. This may be caused by the following factors. Higher viscosity of the treatment solution which would produce a decrease in permeability usually evident during the first few cycles after switching to the test solution. This effect disappears as soon as the residual treatment solution is rinsed out of the plug. Deposition of a surfactant layer on the fiber surface, resulting in reduction of the size of flow channels in the plug, and consequently a decrease in plug permeability. The effective thickness of the adsorbed layer, 6,, can be estimated by using the following relation:1° (4)
where Q and QB are the flow rates before and after adsorption, respectively, and R is the radius of a channel through which the solvent is passed. For a plug of fibers, whose geometry can be treated in approximation as cylinders, the mean hydraulic radius of channels can be calculated from the following equation:14
R=-- t r
(5) 1-t2 where r is the radius of a cylinder (radius of hair). For a hair plug concentration of 0.58 g/cm3 (experimental condition monitored in this paper) R calculated is equal to 21.4 pm. Penetration of a surfactant into the bulk of the fibers which results in their swelling, and thus in the reduction of the size of flow channels and a decrease in plug permeability. Primary electroviscous effect which may result in a flow retardation if an increase in the absolute value of the streaming potential occurs during the flow of an aqueous solution through the pores of a membrane.16 In experiments described in this paper, however, the pore radius (21.4 pm) is large compared to the thickness of the diffuse part of the double layer (0.0689 pm for 5 X 1 W M KCl (14) McCAbe, W. L.; Smith, J. C. Unit Operations of Chemical Engineering; McGraw-Hill: New York, 1976;p 148. For noncircularcrosssections, the hydraulic radius of a channel is defied as the ratio of the crose-sectional =ea of the channel to its wetted perimeter. Equation 5 can be derived from this definition as shown in the reference. Although an oversimplification,thie approach is justified by predicting the form of flow equations verified by experiment. (15) Gieselman, M. J. Langmuir 1992,8,1342.
solution), so the effect of the electroosmotic backflow should be insignificant. Figure 3 presents the data obtained in a control experiment in which 5 X 106 KC1 was used both as a test and as a treatment solution. As expected, there is virtually no change in the values of the streaming potential (Figure 3d) and conductivity (Figure 3c), and consequentlyin the { potential (Figure 3a) calculated according to the Smoluchowski equation. As mentioned previously, a small increase is observed in the plug permeability (Figure 3b), indicated by an increase in the flow rate. It is probably caused by the flow-induced rearrangement of hair in the plug. Such an increase was observed in all the experiments within the first 5-10 streaming cycles. Effect of Anionic Detergents. The analysis of interactions of anionic detergents with hair was performed by using lauryl sulfates with various counterions such as sodium (SLS),ammonium (ALS), diethanolammonium (DEALS),and triethanolammonium (TEALS) (Figure 4). Although the { potential curves were calculated from the streaming potential data, both curves are presented in Figure 4 for illustrative purposes. The rpotential data do not provide evidence for binding of SLSto the hair surface (Figure 4a). After the treatment, the { potential of hair becomes less negative (a decrease in the absolute value) due to the high concentration of free surfactant in the test solution (and, consequently, its high conductivity (Figure 4c)), and then gradually decreases after extended rinsing, reaching a value characteristic for untreated hair (-14 to -15 mV). The variations in {potentials are accompanied by a slow decrease in the conductivity of the plug from a relatively high value of about 14 pmho/cm observed immediatelyafter the treatment to about 7 pmho/cm after 30 min of rinsing with the test solution. This, together with the {potential data, suggests continuous desorption of the surfactant from the bulk of the fibers, rather than from their surface. These results are consistent with other studies on sorption of SDS, which indicate that keratin fibers such as hair and wool can bind significant amounts of the surfactant in a relatively short period of exposure time.16J7 In addition to this, a study of the cross-sections of fiber soaked in solutions of fluorescently-labeled alkyl sulfate surfactants demonstrated that deposition occurs in the bulk, particularly throughout the intercellular and cell remnant regions of the fiber.'* Although the hair cortex is shielded by a larger number of cuticle layers than wool, and thus it is less accessible to even low molecular weight compounds, it is reasonable to expect a similar pattern of deposition for hair keratin. The sorption/desorption characteristics are different for ammoniumlauryl sulfate. The {potential of hair becomes less negative immediately after the treatment due to the high concentration of residual free surfactant in the test solution, then decreases to more negative values as compared to those obtained for untreated hair, and finally increases again after extended rinsing with the test solution. This produces a broad peak on the curve of the { potential as a function of time with the maximum at 26 and 63 min for the fist and second runs, respectively. The decrease in conductivity is slow, albeit a little faster than in the case of SLS. As mentioned above, the conductivity dependence on time should be related to the rate of surfactant desorption from hair. For SLS and ALS, this process could be approximated by first-order kinetics in (16) Ohbu, K.; Tamura, T.; Muuhima,N.;Fukuda, M. Colloid Polym. Sci. 1986,264, 798. (17) Robbins, C.; Scott, G. V.; Barnhunt, J. D. Text. Res. J. 1968,38, 11 97. ---..
(18) Holt, L. A.; Stapleton,I. W. J. SOC.Dyers Colour. 1988,104,387.
Langmuir, Vol. 9,No. 11, 1993 3089
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Figure 6. { potential (a, top left), fl?w rate (b, top right), conductivity (c, bottom left), and streaming potential (d, bottom right) as a function of time for hair treated wlth 0.5 % w/w cetyltrimethylammonium bromide and stearylbenzyldimethylammoniumchloride: ( 0 ) CTAB, (+) SDBAC. a higher concentration of SDBAC molecules on the hair surface. Also, CTAB and SDBAC streaming and t potential curves are slightly s h h d v e m u s each other which
reflects the effect of a delayed conductivity decrease after the treatment for CTAB. The difference in adsorption/ desorption equilibria for these two surfactanta may be
SorptionlDesorption of Ions related to (1) the higher hydrophobicity of SDBAC, connected with a larger number of methylene groups in the main chain as well as the presence of a benzyl group, both of which which should increase hydrophobic interactions with the keratin surface as compared to CTAB, and (2) formation of crystalline (lamellar) precipitates in supercooled aqueous solution of SDBAC versus clear micellar solutions of CTAB; positively charged dispersions of SDBAC should have high affinity to hair and result in high uptakes of this surfactant on hair. A similar increase in the equilibrium values of the { potential as a function of the chain length was observed for a series of alkyltrimethylammonium halides with alkyl groups containing from 12 to 18 carbon atoms (results not reported in this paper). The conductivity of the plug returned to the baseline, or an even smaller value, immediately after the treatment with SDBAC. For CTAB, because of a significantly reduced flow rate, there is a delay of a few rinsing cycles after the treatment, before the plug conductivity reaches the baseline value. Other experiments, performed with solutions of various cationic surfactants, such as alkyltrimethylammonium halides and dialkyldimethylammonium halides with various alkyl and halide groups, also revealed similar fast reduction in conductivity after the treatment cycle to, usually, a slightly below-baselinevalue. These data suggest that the fibers do not release appreciable amounts of ad(b)sorbed surfactant into the test solution although the streaming potential data clearly demonstrate the disappearance of the initially adsorbed surfactant molecules from the surface. It is possible, thus, that the initially adsorbed surfactant penetrates into the inside of the fiber rather than desorbing into the test solution. This behavior of cationic surfactants contrasts that observed for anionic detergents which, wherein a slow decrease in plug conductivity is observed, are slowly desorbed from hair into the test solution after the treatment cycle. The permeability of the plug is affected to some extent by the interaction of cationic surfactants with the fibers. In the case of cetyltrimethylammonium bromide, the reduction of the flow rate of the test solution through the plug is transient, and vanishes after rinsing with the test solution. It is primarily caused by an increase in viscosity due to the treatment although a small contribution of surface deposition cannot be excluded. For stearyldimethylbenzylammonium chloride (SDBAC),the reduction in plug permeability is durable, independent of rinsing, and thus clearly related to the deposition of a layer of surfactant on the fiber surface. The second application of the SDBAC solution leads to a further, small decrease in the flow rate, suggesting deposition of a second layer of the surfactant. The estimated thickness of deposited SDBAC layers (according to eq 4) after the first and the second treatments are 0.75 and 0.99 pm, respectively. The calculated thickness of the surfactant layer refers to its wet, hydrated, swollen state, probably containing the lamellar phase. In the dry state, the thickness of such surface deposits should be considerablyreduced. It is also noteworthy that the permeability measurements correlate well with the streaming and {potential data which indicate a nearly complete removal of CTAB after rinsing, and the permanence of hair surface modification with SDBAC. Effect of Cationic Polymers. Cationic polymers adsorb a t oppositely charged surfaces at multiple sites in a nearly irreversiblefashion. Their substantivity is related to factors such as the density of charges, molecular weight, flexibility of the chain, and type of counterion. Figure 6
Langmuir, Vol. 9, No. 11, 1993 3091
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Figure 6. t potential (a, top), flow rate (b, middle), and conductivity (c, bottom) as a function of time for hair treated with 0.5% w/w Merquat 100, Merquat 280, and Merquat 550 ( 0 ) Merquat 100, (+) Merquat 280, (X) Merquat 550. presents the potential, flow rate, and conductivity data obtained for two cationic and a zwitterionic polymer, with the overall cationic charge densities of the polymer chain decreasing in the following order: Merquat 100 (poly(dimethyldiallylammonium chloride)) > Merquat 280 (poly(acry1icacid-co-dimethyldiallylammoniumchloride)) > Merquat 550 (poly(acry1amide-co-dimethyldiallylammonium chloride)). In all three cases, following the treatment with a polymer solution, there is a reversal of the polarity of the rpotential as a result of the adsorption of positively charged species. For Merquat 100 and 550, the {potential remains high and positive throughout the rinsing cycle. On the other hand, the { potential of hair treated with Merquat 280 decreases to low positive values as a result of rinsing with the test solution. Adsorption of cationic polymers does not affect the permeability of the hair plug except immediately after the treatment with viscous solutions such as 0.5% Merquat 100 and 0.5% Merquat 550. This indicates that the adsorbed layer is relatively thin, and probably does not exceed 50 nm. The
3092 Langmuir, Vol. 9,No.11,1993
conductivity of the plug, which increased during the treatment stage due to the electrolytic character of the polymer, is reduced to a value below that of the untreated hair within a few rinsing cycles. This ,is similar to the behavior of cationic surfactants which also lower the plug conductivity to below baseline values. There can be two possible explanations of this result: (i) hair with an adsorbed layer of cationic polymer or surfactant has a lower surface conductivity than untreated hair, and thus results in a decrease in the overall conductivity of the hair plug, or (ii) the adsorbed layer of cationic polymer or surfactant creates an electrostatic barrier against the diffusion of electrolytic (conductive) species from the fiber interior which can also contribute to some extent in the measured plug conductivity. Which of these two factors plays a decisive role cannot be determined on the basis of the above-presented data. It will be shown, however, in subsequent papers that the treatment of hair with other materials such as silicone emulsionsor nonaqueous solvents can also produce a similar effect. Conclusions The most important result of this work is the demonstration of the dynamics of changes in the ionic character of the fiber surface upon exposure to solutions of surfactants and polymers. The use of the streaming potential technique in the dynamic mode, consisting of the treatment of the substrate with a solution of adsorbate followed by intermittent electrokinetic measurements as a function of rinsing time, allows one to observe the kinetics of desorption, and enables comparison of the affinity of various colloids to the fibers. In addition to this, the permeability measurements give information about the thickness of adsorbed layers on the fibers as well as about the rheological properties of the treatment solutions. This feature of the instrumentation was not fully exploited in this paper, since most of the investigated systems did not produce thick deposits on the fiber surface. It will be shown in subsequent papers, exploring the effect of complete cosmetic compositions such as shampoos or conditioners, that the permeability measurements are very useful in characterizing the process of deposition and desorption.
Jachowicz et al.
Anionic surfactants of the lauryl sulfate series were shown to bind to the hair transiently, and rinse off slowly by a dilute, aqueous KC1 solution. Their affinity to hair, and consequently the rate of desorption, was found to be dependent upon the nature of the counterion. Both { potential and conductivity measurements suggest slow desorption of sodium or ammonium sulfates and fast desorption of the DEA and TEA salts. Cationic Surfactants reverse the sign of the {potential from negative to positive by adsorption, as observed immediately after the treatment, and, according to { and streaming potential data, are gradually removed from the surface as the test solution is passed through the plug. Since this process is not accompanied by changes in the conductivity of the plug (as in the case of anionic detergents), it is postulated that the adsorbed surfactant diffuses inside the fiber rather than desorbing into the test solution. This ability of the adsorbed cationic surfactant to rearrange from the surface into the bulk of the hair was suggested earlier by Finkelstein et aLZoCationic polymers, on the other hand, bind to the surface of hair stronger with little desorption, and because of the large size of their molecules cannot diffuse into the bulk of the fiber. As a result, streaming and potentials show less variation as a function of time, and permanent reversal of the ionic character from negative to positive. We believe that the herein described technique can be widely used to study the sorptionldesorption phenomena in a variety of systems including fibers and minerals as substrates in combination with solutions of detergents and polymers. An important practical implication is that the presented methodology can be used to predict the performance of surfactants or polymers as cleaning or conditioning agents. We shall demonstrate in our forthcoming papers that the technique is not only very useful in analyzing simple, model systems, but can also be used for quantitative “fingerprinting” of the sorptionldesorption characteristics of multicomponent, commercial formulations such as hair shampoos and conditioners. (20) Finkelstein, P.; Laden, K. Appl. Polym. Symp. 1971,18,673.
