Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 Surfactant mediated adsorption of negatively charged latex particles to a cellulose surface L. Schaffner a, G. Br¨ugger a, R. Nyffenegger a, R. Walter a, J. Riˇcka a,∗, J. Kleimann b, J. Hotz c, Ch. Quellet c a Institute of Applied Physics, Department of Biomedical Photonics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland b Department of Inorganic, Analytical, and Applied Chemistry, University of Geneva, Science II, 30 Quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland c Fragrance Research/Delivery Systems, Givaudan Schweiz AG, CH-8600 D¨ubendorf, Switzerland Received 5 December 2005; received in revised form 16 February 2006; accepted 28 February 2006 Available online 18 April 2006 The adsorption of anionic, carboxyl functionalized latex particles, recharged by a cationic surfactant acting as fabric softener/conditioner, to a cellulose surface was investigated with evanescent wave video microscopy. This technique allows to monitor the deposition and release of individualparticles in real-time with an excellent selectivity and sensitivity. Since the recharged particles and the conditioner compete for the free surface,the initial deposition rate and final surface coverage are found to be strongly dependent on the ratio of particle and conditioner concentrations.
2006 Elsevier B.V. All rights reserved.
Keywords: Cellulose; Evanescent wave microscopy; Particle deposition; Cationic surfactant; Adsorption; Recharging; Competitive adsorption and desorption processes. The adsorption kinetics of polymerlatex and cationic surfactants on their own have been investigated The addition of specially modified polymer particles to laun- in great detail in respectively.
dry detergents and conditioners offers an interesting way to pro- In this paper, we report the use of a "micro laundry" cell and vide functional ingredients, such as bioactive substances, to tex- the technique of evanescent wave video microscopy (EWVM) tile fabrics during laundering, allowing them to be released over to investigate in real-time the deposition and release of fluo- a prolonged period of time after the wash, rinse and drying pro- rescent latex particles on a flat cellulose surface during a sim- cesses have been completed. To fulfill this task, the particles ulated laundering cycle. The micro laundry cell provides an must deposit to a significant extent and remain on the fabric environment for monitoring the dynamics of particles under during drying, storage and use by consumers. In this paper, we laminar shear flow conditions in the immediate vicinity of the consider the case of conditioner-mediated adsorption of latex particles during the rinsing phase.
Videomicroscopy is an established and powerful technique The magnitude of the desired effect is directly related to the in colloid science and, thanks to the improvement in sensitivity number of particles that remain adsorbed on the fabrics after and resolution of CCD cameras, able to provide more and more the final rinsing and drying phases. However, another important information about the adsorption process involved (see requirement is the ability of depleted particles to be removed references therein). The EWVM-technique requires substrates during a subsequent wash cycle. To achieve this behaviour and having a good optical flatness and transparency, which is not the optimize the balance between deposition and removability, it is case for conventional textile fabrics. Hence, for this study, we necessary to understand the factors underlying both deposition use a cellulose film to mimic cotton which is of special interestfor our application as cotton is the most commonly used fabricmaterial worldwide. The interaction of colloidal particles with ∗ Corresponding author. Tel.: +41 31 631 8904.
cellulose surfaces is also of considerable interest in paper making E-mail address: (J. Riˇcka).
0927-7757/$ – see front matter 2006 Elsevier B.V. All rights reserved.

L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 Principally, both positively and negatively charged latex par- ticles can be used in the process. In the case of negatively chargedparticles, however, the interactions with the initially negativelycharged cellulose are a priori not favorable for particle adsorp-tion. This study shows that the cationic surfactant present in theconditioner forms a complex with the anionic particles, whichleads to charge inversion and allows deposition of the particles.
However, during the rinsing phase, the remaining free surfactant Fig. 1. Cellulose film spin-cast onto a quartz substrate. The image was taken compete with the afore mentioned complex for the free cellu- after evaporation of the DMA solvent but before washing out remaining LiCl lose surface, thus making the system extremely sensitive to the crystals with deionized water, after which the film becomes nearly transparent.
relative and absolute concentrations of latex particles and sur-factant. Such competitive adsorption has also been investigated the adhesion of a new cellulose coating. However, the chromic acid treatment requires subsequent removing of all chromiumtraces by submerging the substrates in 1 M hydrochloric acid 2. Materials and methods
for 1 h. Again, the acid treatment was followed by rinsing withdeionized water. Finally the excess water was blown away with 2.1. Preparation and characterization of the model nitrogen and the substrates were dried in an oven at 90 ◦C for After cooling down to room temperature, the substrate was Since the EWVM technique requires a reasonable optical placed on the spin casting apparatus, consisting of a dc motor flatness and transparency of the probed surface, a spin casting and a rotating disk where the substrate may be mounted. To pro- technique was used to produce a thin film of cellulose on a duce a film, an aliquot of about 32 mg cellulose solution was quartz glass substrate We used a 5 g/l cellulose solution dropped onto the center of the rotating quartz substrate with a which was obtained by dissolving cellulose powder (supplier: standard 1 ml syringe. The dc motor was driven at constant rota- Fluka Switzerland, acid-washed from cotton linters, degree of tion frequency of about 3000 rpm. Thirty seconds after placing polymerization: 1400) in a mixture of N,N-dimethylacetamide the drop the power was cut and the plate was left spinning until (DMA, supplier: Merck, for synthesis) and lithium chloride (LiCl, supplier: Merck, pro analysis). All chemicals had to Next, the substrate was placed in the oven again (90 ◦C, 2 h), be dried before use. DMA was dried with a molecular sieve in order to evaporate the cellulose solvent. However, at this stage (0.3 nm). LiCl and the cellulose powder were dried at elevated some LiCl always remains in the cellulose film, probably in the temperature in vacuum for 2 h. The temperature was set to 130 form of small crystals which give the film a visible opacity. By and 100 ◦C for LiCl and cellulose, respectively.
carefully soaking the substrate in a beaker with flowing deion- To dissolve the cellulose, 0.5 g of the powder were first dis- ized water for 10 min the LiCl can be washed out without peeling solved in 94 g DMA for 2 h at a temperature of 130 ◦C while away the still very fragile cellulose film. After a final drying in stirring under dry argon in a closed vessel. To achieve a complete the oven (90 ◦C, 1 h) the cellulose surface was almost transpar- dissolution, 10 g LiCl at a temperature of 100 ◦C were added.
ent and showed sufficient adhesion as not to be washed away (This is method A described in ut using a ten times higher when performing the flow cell experiments.
LiCl concentration with respect to the cellulose.) After letting The thickness of the cellulose layer was estimated with scan- the solution cool down to room temperature under constant stir- ning angle null ellipsometry A typical value was 50 nm, ring, it was filtered (again under dry argon) through a 0.5 ␮m but the actual thickness may vary from sample to sample. The PTFE-membrane to remove LiCl crystals that had formed during technique also allows the determination of the refractive index.
The resulting value of 1.49 at a wavelength of 633 nm is in good Prior to spin casting, the quartz substrates were submerged in agreement with literature chromic acid for 1 h, followed by rinsing with deionized water.
The surface structure of the cellulose films was investigated The acid treatment was necessary to completely remove residual by atomic force microscopy in tapping mode. As can be seen in surfactant molecules and other impurities which could impair the surface of films spin cast from DMA/LiCl solutionis quite homogeneous and flat over large areas. At nanometer-scales, however, a characteristic texture appears ( 1 A second cellulose solvent system consisting of a mixture of N- Note that the scale on the horizontal axis is different from the methylmorpholine-N-oxide (NMMNO) and dimethyl sulfoxide (DMSO) was scale on the vertical axis. While the typical horizontal dimen- tested but found to be unsuitable, even though the cellulose was dissolved muchmore easily and efficiently, because the resulting spin cast cellulose surfaces sions of the characteristic dips and peaks on the surface are on did not adhere well to the quartz substrate and peeled off easily. In addition the the order of 200 nm, vertical irregularities probed by the AFM surface turned out to be much rougher than with cellulose dissolved in DMA tip are not larger than about 20 nm. However, as the AFM profile which lessened contrast with the EWVM-technique considerably. Furthermore, is a convolution of the real surface with the AFM tip this the cellulose solution turned out to be unstable, evidenced by a slow change of does not exclude the existence of up and down structures that color to brown and eventually showing precipitate of cellulose after approxi-mately 8 days, making it necessary to prepare the solution freshly before each are deeper and narrower than the height and width of the tip. The origin of the nanoscale structure of the cellulose films remains

L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 Fig. 3. AFM image of the surface topography of model fabric made of cotton.
The observed area was 2 ␮m ×2 ␮m.
process applied and the cellulose surface can only provide anapproximation of the real textile surface at micrometer lengthscales. In particular the possible influence of the micro-fibersoccurring on natural cotton fibers cannot be investigated withthis model. Nevertheless, cellulose appears to be the simplestavailable model for the present study, combining chemical sim-ilarities with cotton fibers and a given surface structure, whilestill showing an acceptable optical transparency, as required bythe measurement set-up.
2.2. Latex particles Fig. 2. AFM images of spin-cast cellulose films: (top) AFM picture of a cellulose The anionic carboxylated polystyrene latex spheres used in film coated from a DMA/LiCl solution on a quartz substrate (observed area: our measurements were custom-synthesized by in-situ emul- 100 ␮m ×100 ␮m) and (bottom) Nanoscale structure of the cellulose coating sion polymerization of styrene and methacrylic monomers in (observed area: 2 ␮m ×2 ␮m).
the presence of both the active substance ingredients and flu- unexplained, as is the role of adhesion in the generation of these orescent probe molecules. After considering and testing many structures. We assume that the characteristic dips and peaks ob- different fluorescent dyes, fluorescent Yellow 3G (FY3G) was served in films casted from LiCl/DMA solutions is due to the chosen since it shows good incorporation into the particles and formation of small LiCl crystals during solvent evaporation.
is expected to reside mainly in the outer shell of the polystyrene This cellulose surface is to be compared with the surface of spheres, owing to its amphiphilic character. The final particle real textile fabric. The architecture and superstructure of com- suspension had a solid content of 43.22 g/ml and a polymer con- mon fabrics can be understood in terms of a well-organized hier- tent of 35.8 g/ml. The latter was used for the calculation of the archy of fibrillar entities is built-up of micro-fibers number of particles.
(so called fibrilla) which assemble to form the cotton fiber itself.
We determined the hydrodynamic radius and the radius of This smallest morphological unit has a diameter in the range of gyration by dynamic and static light scattering on a ALVCGS- 3–20 nm, depending on the origin of the cellulose. ws 8F light scattering instrument (ALV GmbH) at a wavelength of an AFM image of a model fabric made of cotton. At the first 532 nm. The particle diameters were found to be 232 nm (hydro- glance the surface structure is not very similar to our spin-cast dynamic) or 228 nm (gyration) with a polydispersity of 9%. Par- cellulose films. However, while the surface topography of cotton ticle charge densities were determined through electrophoretic is smoother and seems to be ordered along one direction in com- mobility measurements at different ionic strengths, based on the parison to our cellulose surface, the height of the characteristic theory developed by O'Brien and White resulting sur- surface structures is still of similar magnitude (20 nm).
face charge was found to be (40 ± 5) mC/m2.
While cellulose is the main constituent of cotton, it must be w high resolution pictures of particles ad- kept in mind that the range of different textile fabrics is very sorbed to the surface obtained with electron and atomic force broad and significant amounts of other chemicals used during microscopy. The quartz substrates were investigated after a par- finishing processes may also be present on or in these mate- ticle deposition measurement on the cellulose. The particle di- rials. In addition to that, the structure and surface of the tex- ameter, as determined from the electron microscopic image in tile fibers may vary significantly depending on the production well with the one determined by light scattering.

L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 preventing the gelation of Rewoquat aggregates (lamellae, lipo-somes).
Laundering is usually done with the local tap water of vary- ing ionic strength and mineral content. To be closer to real-lifewhile maintaining standard conditions, we used a standardizedtap water consisting of a 1 mM CaSO4 in deionized water (fromnow on referred to as "tap water"). This solution was also usedfor rinsing the cellulose surface during the simulated launderingcycles.
Samples of varying particle and conditioner concentrations were prepared by mixing the desired amount of the primaryparticle suspension described in the previous section with theconditioner and tap water.
2.4. Evanescent wave video-microscopy (EWVM) ws the setup employed for the measurements. De- tails on the optical features and data evaluation procedures, in- Fig. 4. Electron microscope image of particles adsorbed to cellulose surface. Theparticles appear bigger than they actually are due to being covered with a gold cluding corrections for the limited optical and finite time resolu- surface of around 30 nm thickness for the EM-imaging. The quartz substrate was tion are discussed in detail elsewhere The technique allows investigated after a particle deposition measurement with a conditioner concen- the detection of individual particles adhering to the probed sur- tration of 1.0 × 10−4 g/ml and a particle concentration of 0.94 × 1010/ml. The face with an excellent sensitivity and selectivity. The surface final surface coverage was determined from the EM-image to 682 × 103/mm2 coverage, the influx and outflux of the particles to and from the and with the EWVM-technique to 631 × 103/mm2. This represents 2/3 of thehighest coverage we have found in our experiments.
surface, as well as the distribution of times the particles spend onthe surface can be monitored in real-time, providing a detailed The image also indicates that the colloids may get close and analysis of the dynamics of particle/substrate exchanges.
eventually form aggregates on the surface. Also the height de- A Lexel 85 argon-ion laser with a power of 150 mW (of termined by AFM agrees reasonably well with the measured which 44 mW remain after coupling through the single mode particle diameters, but sometimes the particle may penetrate the fiber mentioned below) at a wavelength of 488 nm serves as surface as shown in light source for the evanescent wave illumination. The laserbeam is coupled to a single mode fiber, whose other end is 2.3. Conditioner and standard tap water fixed on a rotating arm to allow adjusting the angle of incidenceon the prism and thus indirectly the angle of incidence at the Stock solution of conditioner was obtained by mixing 5 g glass/cellulose–water interface θi. As long as this angle is of dihydrogenated Tallowethylammonium methosulfate (Re- higher than the critical angle of total internal reflection θc (For woquat WE 18, supplied by Degussa) and 0.5 g of a mixture of the quartz–water interface θc = 47.7◦) an evanescent wave will C9–C11 linear fatty alcohol - polyethylene glycol ethers having be generated with an intensity decaying exponentially away an average content of 8 mol ethylene oxide per mole of alco- from the interface with a typical length of a few dozen nm.
hol (Neodol 91-8E, ex Shell) and 94.5 ml deionized water. The The exact depth δ can be adjusted by the angle of incidence active substance is the cationic surfactant Rewoquat, a quater- θi our case δ = 141 nm, which is in the range of the nary esther of triethanolamine. Neodol acts as a colloid stabilizer particle radius. Because of the small penetration depth of Fig. 6. Evanescent Wave Video Microscopy Setup. A flow cell, fed by a sy-ringe pump and incorporated into a modified Zeiss Axiovert 100TV inverted Fig. 5. Atomic force microscope image of single adsorbed particle. Observed microscope allows to observe particle adsorption and desorption to a glass– area: 2 ␮m × 2 µm.
cellulose/water interface illuminated by an evanescent wave in real-time.

L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 the evanescent wave, only particles adsorbed to the cellulosesurface film on the quartz substrate are illuminated (To better discriminate the particles from surface structures ofcomparable size they are additionally fluorescently labelled.
The fluorescence light emitted from the particles is guided along the optical paths of the microscope (a Zeiss Axiovert100TV) and can either be observed through the eyepiece orrecorded with a digital CCD camera (Kappa CF 8/1 DX). Anedge filter was set in the optical path to filter out scattered laserlight. The CCD camera is controlled by a computer which alsosteers a shutter that is only opened when recording a picture tominimize bleaching of the fluorescent dye. The resulting highsensitivity and selectivity allows to monitor the individual ad-sorption and desorption of each single particle. We are thus able Fig. 7. Parallel plate flow cell setup. Particles adsorbed to the cellulose surface to record the number of resident particles R(t) on the observed are illuminated by the evanescent wave generated by the total internal reflection area as a function of time.
at the glass/cellulose–water interface. The prism above the upper plate is nec-essary to achieve reflection angles above the critical angle on the glass–water 2.5. Sample cell: micro laundry The dynamics of adsorption and removal of latex particles certain length lcrit = 0.044 × Re × 2h in order for all influences during both rinse and wash cycles are expected to be governed of the inlet to dissipate and laminar flow to develop. With a by two groups of physico-chemical factors. The first group deals flow cell of dimensions 2h = 0.55 mm (thickness), w = 8 mm with particle–substrate interactions and encompasses attractive (width), l = 45 mm (length) and a flow rate of Q = 1 ml/min van der Waals forces, repulsive or attractive electrostatic forces both criteria are fulfilled.
and steric interactions with the substrate and/or with other ad-sorbed species such as surfactants. The second group deals with 2.6. Laundering cycle the transport of the particles to and from the surface which resultsfrom the interplay of diffusion and advection of the particles in The idea behind our micro laundry was to mimic the ac- the flowing liquid.
tual laundering process of a common laundry machine. This Even though the flow field in a real laundry machine is ex- usually involves a sequence of three basic processes: During tremely complex, the dynamics of attachment and detachment the main wash the textile fabric is cleaned with detergent at of the latex particles is governed by processes taking place a chosen temperature. This is followed by several rinse/spin in a very thin layer of fluid adjacent to the textile fiber sur- cycles with water where detergent and soil are removed. Dur- face, which justifies splitting up the hydrodynamic processes ing a final rinsing (conditioning) the conditioner is added to into two components, specifically laminar shear flow parallel the water and its cationic surfactant deposits onto the textiles.
to the surface and a jet flow impinging perpendicularly to the Since, in the present study, the conditioner was the preferred medium for the particle deposition, special attention was paid Both types require their own flow cell geometry, a parallel to the conditioning cycle. In particular, the effect of condi- plate chamber with laminar shear flow perpendicular to the ob- tioner to particle concentration ratio were investigated in order served surface or a stagnation point flow cell with a jet flow to achieve a high final coverage of particles on the cellulose impinging perpendicularly on the probed surface, respectively.
For this paper we have concentrated mainly on the laminar flow The micro laundry flow cell can be used to simulate the tran- conditions, since the parallel plate cell has already been used and sition from wash cycle to rinse and conditioning cycle by simply tested extensively additional measurements with the switching the fluid feed from a detergent solution (4% Deconex, impinging jet cell have been made, but comparing the results supplied by Borer Chemie AG, in deionized water) to tap water with those achieved under laminar flow conditions yield a very and then to a conditioner/latex mixture as described above. By similar adsorption behaviour concerning final particle coverage.
turning the feed back to detergent the removal of deposited par- The main difference is that the saturation level is reached much ticles during subsequent wash cycles can be investigated. This earlier, so that the initial particle deposition rate is higher by a whole simulated laundering cycle can be repeated several times.
factor of up to ten.
As it was shown that a few particles (up to 60 on the observed A predefined mixture of latex, conditioner and tap water is area of 200 ␮m ×150 ␮m) always remained on the surface even pumped with constant velocity trough the flow cell. Thus, the after flushing with Deconex, the cellulose surface substrate has mixture always acts on the "laundry load" (cellulose film) in a to be replaced after 5–10 simulated laundering cycles. Also in well defined flow pattern imposed by the geometry of the sample some rare cases the cellulose surface itself peeled away from the cell. There are two main pre-conditions for laminar flow quartz substrate.
first the Reynold's number Re must be well below 2000 and All experiments were conducted at room temperature (20 ◦C), second, according to the chamber length l must exceed a the flow rate was set to Q = 1 ml/min. This is a discrepancy to

L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 a real wash cycle where usually temperatures of 45, 60 or even90 ◦C are used. However, using higher temperatures did not onlyimprove the removal of the latex particles but also of the cellulosefilm itself.
2.7. Measurement procedure With the EWVM-technique we now recorded the number of particles adsorbed to the cellulose surface, R(t), during thesimulated conditioning cycle. Observations were done with a20× objective (Zeiss LD A-Plan, N.A. 0.3) which resulted inan observed area A of 200 ␮m ×150 ␮m on the cellulose film.
The chosen magnification is a compromise between the opti-cal resolution and the field of view required for a sufficientcounting statistics. However, when the distance between the ad- Fig. 8. Example of a typical measurement. Particle concentration: 0.94 × sorbed particles decreases below the resolution, the spots rep- 1010/ml, conditioner concentration: 1.0 × 10−4 g/ml. The two grayscale images resenting the particles overlap and the counting algorithm be- show pictures taken with an exposure time of 210 ms and an observed area of gins to fail. Eventually the CCD sensor becomes overexposed.
200 ␮m ×150 ␮m. Since the counting algorithm will fail for too dense cover-ages (right grayscale image) the integral fluorescent signal from whole observed To overcome this problem we record the images alternatingly area was alternatingly recorded to determine the final coverage.
with an exposure time of 4 and 210 ms and combine two eval-uation techniques: pictures recorded with the longer exposuretime allow us to count the adsorbed particles directly from the rameters of interest are the initial deposition rate ˙ Γ (0) (deter- image with the counting algorithm the short expo- mined from the particle counting) and the final coverage Γ (∞) sure time is used to simply monitor the change in the overall (derived from the integral intensity measurements).
intensity, which is also proportional to the number of adsorbedparticles. The first 10–20 images with clearly distinguishable 3. Results and discussion
particles are used for calibration. This calibration is repeated forevery single conditioning/deposition cycle. Thus, different con- ws sample results of the deposition curves for dif- ditions between cycles (pH, ionic strength, concentrations, etc.) ferent combinations of conditioner and particle concentrations.
are taken care of. During one single deposition cycle the condi- As can be seen, the adsorption behaviour depends strongly on tions do not change since the feed of the conditioner/particle the composition and can result in drastically different final cov- dispersion remains constant during the whole deposition erages. To better understand this adsorption behaviour, a series of preliminary measurements has been made.
The light intensity given by a single particle varies between First the general adsorption behaviour of the particles was particles on the same cellulose surface since the cellulose surface investigated for different particle surface charges and additives.
is not perfectly homogenous and the adsorption of the condi- In "anionic" refers to the negatively charged particles tioner to the particles is most probably not homogenous as well.
described in Section "cationic" are latex spheres of Around 3000–5000 particles in the observation area (see similar size (diameter: 196 ± 5 nm) but with a positive surface can still be discerned with particle counting, which shouldgive a good estimate of the average light intensity from a singleparticle on the surface.
To test the validity of the light intensity calibration, the par- ticle density was also determined from the electron microscopeimage in The surface coverage as determined from thelight intensity method and direct counting from the EM-imageagree very well. The highest particle densities reached on thesurface were below 5% surface coverage. And even thoughthe single particles can no longer be distinguished optically,the total light they emit is still collected. Thus, linearity isassured.
ws sample pictures and a typical measurement of R(t) for one conditioning cycle. From time 0 forth the samplecell is rinsed with a predefined mixture of latex, conditioner andtap water as described in Sections The number of Fig. 9. Sample measurements. The number of resident particles R(t) on thecellulose in an observed area of 200 ␮m ×150 ␮m as a function of time during particles adsorbed to the surface will at first increase nearly lin- simulated wash cycles for various particle and conditioner concentrations. The early and then approach saturation. From these measurements examples where chosen to show the scope of possible results in terms of final we determine the particle density Γ (t) = R(t)/A. The two pa- coverage and initial deposition rates.
L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 charge (charge density: 16 ± 2 mC/m2). It becomes obvious thatnegatively charged particles on their own do not adsorb to thesurface at all, since they are repelled electrostatically from thenegatively charged cellulose surface, whereas positively chargedparticles are attracted and consequently show strong adsorption.
Using tap water instead of deionized water to dilute the particlesuspension partially screens the charges, which allows a slowbut steady adsorption of negatively charged particles while theadsorption of the positively charged particles is reduced.
The most interesting aspect for the application is the addition of the conditioner. Combination of conditioner and negativelycharged particles has the effect of dramatically increasing boththe initial deposition rate and final coverage of the particles com-pared to tap water. The behaviour of the positively charged par- Fig. 11. Adhesion time distribution functions. P(τ t) for negatively charged par- ticles is different: The addition of conditioner may cause a slight ticles applied in tap water or conditioner. Particle concentration: 1.3 × 1010/ml, increase of the initial deposition rate, but a dramatic decrease of conditioner concentration: 2.5 × 10−4 g/ml. The P(τ t) displayed here is av- the final coverage compared to water. The plateau of the positive eraged over a certain range of t as indivcated in the legend. For tap water theadhesion time distribution remains more or less the same for all t. For conditioner particles is even lower than for the negative particles.
the adhesion time distributions differs before and after reaching the adsorption To further improve our insight, we exploit the possibilities of plateau which happened at around t = 160 s.
the EWVM technique which allows to determine the adhesiontime distribution function P(τ t). This P(τ t) is the probability In the presence of the conditioner P(τ t) is no longer inde- that a particle which arrived at the surface in an infinitesimal pendent of t. There are two different time regions with a clearly time interval at time t continues being observed between t and different τ-dependence triangles). At the beginning, t + τ (Note: In general, P(τ t) may depend on both, τ and during the initial slope of R(t), the relaxation rate is quite small.
t, because of changing conditions of the sorption surface during Consequently, particles that adsorbed in the early stages of a the experiment, in particular P(τ = 0 t) = 1.) In the case of sorption experiment remain on the surface for a long time (ir- particles in tap water with a relative slow adsorption dynamics reversible adsorption). However, particles that arrived after the (see triangles pointing downwards), the adhesion time saturation level has been reached adhere to the surface only for distribution curves did not show any significant dependence of a short period. Thus, the plateau does not represent an equilib- their shape on time t circles). The τ-dependence of rium between particles arriving and leaving, but the number of P(τ t) seems to be highly non-exponential with a long stretched irreversibly adsorbed particles.
Since one of the main purposes of laundry conditioner is to produce a dense coating of surfactant molecules on textilethe observations from be explained easily:The conditioner begins to adsorb onto the cellulose surface incompetition with the colloid particles until there is no more spaceleft for the particles to adsorb to. This is evidenced further bya measurement of particle adsorption after a preconditioningof the surface with conditioner (no adsorption at all isobserved since the surface is already covered by the conditioner.
3.1. Modelling the adsorption process our current interpretation of the observed facts. In our working model for the adsorption the cellulose sur-face as well as the particles are initially charged negatively inwater, repelling each other. As soon as the latex particles aremixed with the laundry conditioner, a cationic surfactant, theirsurface charge is reversed by a surrounding double layer. Nowthe particles readily adsorb to the bare cellulose surface. How- Fig. 10. Effects of particle charge and additives. R(t) in an observed area of ever, the remaining surfactant will also adsorb and compete with 200 ␮m ×150 ␮m for negatively and positively charged particles. The latex par-ticles were applied in pure water, standard tap water and conditioner. Conditioner the recharged particles for the free surface. As soon as the sur- concentration was 2.1 × 10−4 g/ml. The polymer content of 1.25 × 10−4 g/ml face itself is covered with a double layer of conditioner there can was the same for all measurements, which corresponds to a particle concentra- be no further adsorption of latex particles. This blocking effect tion of 2.0 × 1010/ml for the anionic and 3.2 × 1010/ml for the cationic particles.
makes the system highly sensitive to the relative concentrations Note: For better visibility the plot is cut off above 0.5, but P(τ = 0 t) = 1 for of particles and conditioner.
L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 Fig. 12. Effect of previous conditioning. R(t) for negatively charged particlesapplied in conditioner. Prior to the experiment the surface was treated either withpure water or with conditioner. Particle concentration was 0.9 × 1010/ml and Fig. 14. Particle charge reversal. Electrophoretic measurements evidencing conditioner concentration 1.0 × 10−4 g/ml. The adsorbed conditioner prevents charge reversal of negatively charged particles with increasing conditioner con- all particle adsorption.
centration. Particle concentration was kept at 0.353 × 1010/ml in case of both,cationic and anionic particles, and softener concentration was varied in the range We measured the electrophoretic mobility of particle/condi- from 0 to 4 × 10−4 g/ml.
tioner-formulations to confirm the charge reversal of the nega- of particles that can be adsorbed to the surface. b is the bulk con- tively charged particles (Zetasizer 2000, Malvern Instruments centration of particles in the solution above the surface and f (t) Ltd., based on laser Doppler velocimetry). ws a the fraction of free surface as a function of time. Since there is change of sign at a conditioner concentration of ca. 0.5 × a sufficiently fast constant flow of the particle suspension above 10−4 g/ml, while cationic particles seem to remain unaffected. It the surface, the bulk concentration b can be assumed to be time should be noted that the exact dynamic process of the recharge is independent. It is reasonable to assume (and the measurements unknown and the change of sign of the electrophoretic mobility confirm this) that the conditioner adsorbs much faster to the sur- has to be attributed in part to the movement of the conditioner face than the recharged particles, effectively being the limiting (in the form of vesicles or micelles).
factor for the adsorption and thus the dominating effect on the The most simple kinetic model for the adsorption of the fraction of free surface f (t). To derive f (t) we must therefore recharged particles comprises only the two basic processes of look at the adsorption of conditioner on its own, starting at the adsorption and desorption (a classical Langmuir model same basic equation (the subscript c denotes the conditioner): The rate of particle adsorption can thus be written as: dΓ (t) = kacΓmcf (t)bc − kdcΓc(t), aΓmf (t)b kdΓ (t) where ka and kd are the intrinsic adsorption and desorption rate constants for the recharged particles. Γm is the maximum amount (t) = 1 − Inserting f (t) in Eq. reforming the result, we obtain: dΓc(t) = kacΓmcbc − (kdc + kacbc)Γc(t) The differential equation can now be solved to: c(t) = Γmc kdc + kacbc A further simplification is possible by neglecting desorption(kdc → 0). This is legitimate for our present study since par-ticles and conditioner adsorb irreversibly to the surface (as seenin Thus, f (t) becomes:f (t) = e−kacbct Inserting this result into Eq. and again neglecting des-orption (kd → 0) the differential equation is easily solved by Fig. 13. Working model. Negatively charged latex spheres recharged by a double layer of conditioner on their surface. Since, in our model, the cellulose surface is eventually also covered with a double layer of conditioner, the particle adsorption Γ (t) = Γm (1 − e−kacbct) depends on the ratio of conditioner and particle concentration.
L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 The resulting expression has the same qualitative behaviour asour measurements, meaning an initially linear increase that lev-els off. From this we get simple expressions for the characteristicproperties initial deposition rate ˙ Γ (0), final coverage Γ (∞): Γ (0) = Γmkab (∞) = Γm We also introduce the characteristic time tc: Note that the ratio tc = Γ (∞)/ ˙ Γ (0) is much less sensitive to the heterogeneity of the observed cellulose surface than Γ (∞) and Γ (0) themselves.
Of course this simple model neglects a number of features of the real system. For example, the conditioner will form aggre-gates and micelles of different sizes depending on the total bulkconcentration of the conditioner, which makes the dependenceof the adsorption kinetics on the bulk conditioner concentra-tion much more complex, since the adsorption does not only Fig. 15. Initial deposition rate. ˙ Γ (0) in dependence of concentrations of particles happen in the form of single conditioner molecules, but also mi- and conditioner. A polynomial fit of second order visualizes the rough trend. In celles and aggregates (usually resulting in steps in the adsorption accordance with the simple model, Eq. t is mainly proportional to the particleconcentration.
isotherm). However, for the modelling it does not matter whetherthe surfactant adsorbs as single particles or aggregates. The only surfaces which defy our control, despite best efforts to adhere important factor is the amount of free cellulose surface that is closely to the synthesis and spin casting procedures. In extreme covered by it and the rate at which this happens. There are in- cases this results in measurements that amount to half or double dications the adsorption rate of cationic surfactants the magnitude achieved in previous measurements. Therefore, may remain roughly proportional to the bulk concentration of we have listed outliers or even groups of outliers at the points conditioner (and exhibit no steps at all) in the presence of high separately. A single number without error represents a single ionic strengths. This is the case in our system due to the model measurement outlier, errors after a number signify an average of at least three measurements. While it may not make much sense Also, we have assumed that the particles are always fully to indicate a standard deviation for three or four measurements recharged and that the intrinsic adsorption constants are indeed we did it to separate them visually from outliers.
constant, whereas in reality they depend in a nontrivial way of Reproducibility was especially poor close to the black trian- the concentrations of the other component (i.e. the incomplete gle and even more so in the lower left corner where both con- recharging of the particles depending on the conditioner con- ditioner and particle concentrations are very low. Also, since centration, Another concern is that we neglected theeffective conditioner concentration in the bulk which is the to-tal conditioner concentration minus the conditioner moleculesbound to the particles. However, as will be seen the model agreessurprisingly well with our measurements.
3.2. Exploring the parameter space In have summarized the measure- ments of the initial deposition rate, final coverage and charac-teristic time for various combinations of conditioner and par-ticle concentrations according to Section For a better vi-sualization of the overall trends we have underlaid them with atwo-dimensional polynomial fit of second order. However, sincethe fit is only a rough interpolation and polynomial fits tend toproduce artifacts at the boundary one should be careful wheninterpreting the graphs.
Fig. 16. Initial deposition rate compared with fast deposition limit. ˙ Γ (0) in de- Even more important in this context is the strong variability pendence of particle concentration. The dotted line represents the fast depositionlimit from Eq. a slope of 1001 × 10−10 m/s. Apart from the large vari- of the observed properties. It seems that there are still significant ability the observed initial deposition rate follows the trend of the fast deposition differences between different batches of the spin cast cellulose L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 First we compare the results for the initial deposition rate (with our prediction for ˙ Γ (0) from Eq. In accor- dance with the model one observes an increase that is roughlyproportional to the particle concentration. In the left half ofthe graph (up to a conditioner concentration of 1.5 × 10−4)there is also a slight dependence on the conditioner concen-tration visible, which indicates that the particles are not yetfully recharged in this regime. These findings comply withthe change of sign at a conditioner concentration of around0.5 × 10−4 g/ml for a latex particle concentration of 0.35 ×1010/ml as observed in the electrophoretic mobility measure-ment Incomplete recharging may also offer an ex-planation for the large variability near the black triangle. Eventhough the dynamics of the recharge process are unknownit is at least plausible that the equilibrium between condi-tioner free in the solution and adsorbed to the particles is eas-ily disturbed by outside factors in this regime of incompleterecharge.
In the parallel plate channel fast deposition limit the initial rate is simply proportional to the concentration of the adsorbent Fig. 17. Final coverage. Γ (∞) in dependence of concentrations of particles (Smoluchowski–Levich approximation) (see e.g. and conditioner. A polynomial fit of second order visualizes the rough trend.
In accordance with Eq. is roughly proportional to the ratio of particles to h 1/3 conditioner concentration ratio.
jfast = 0.49b Pe1/3 the deposition rate was very low in this regime, the saturation where b is the particle bulk concentration, 2h = 0.55 mm the level of adsorbed particles was sometimes not reached at all thickness of the laminar flow chamber, x ≈ 22.5 mm the distance during one measurement of approximately half an hour dura- of the observation place from the inlet and Pe = 4 vmh/D = tion. The black triangle indicates a regime where measurements 2, 23 × 106 the P´eclet number for laminar plate flow with were impossible because of an instability of the latex/conditioner vm = Q/2hw = 3.79 mm/s mean flow velocity. The diffusion mixture. This is due to an incomplete recharging of the par- constant D is calculated with the Stokes–Einstein relation D = ticles, which lessens their mutual repulsion and gives rise to kbT/6πηa = 1.87 × 10−12 m2/s from the particle radius a = 115 nm. A comparison with our measured initial rate makessense since at the beginning the surface is still uncovered andthe initial particle adsorption rate is thus still independent ofthe conditioner concentration (see also Eq. our sim-ple model). The result is shown in As can be seen theaverage initial deposition rate agrees well with the fast depo-sition approximation. Still, the variability is considerably highas already stated. However, in the case of the initial depositionrate there are additional uncertainity factors such as the suddenswitch of the inlet to the conditioner/particle dispersion resultingin non-equilibrium conditions at the beginning of the depositonprocess.
ws the concentration dependence of the final cov- erage Γ (∞), which is expected to be proportional to the ratio ofthe particle to conditioner concentrations (recall Eq. In otherwords Γ (∞) should be constant along straight lines through theorigin and increase from the lower right to the upper left. Generaltrends of the data do not contradict this prediction but a conclu-sion is difficult because of the large variability. A conspicuousfeature is the distinctive triangular area of low final coverage inthe lower right corner of the graph which indicates the "block-ing regime", i.e. fast blocking of the particle adsorption by the Fig. 18. Characteristic adsorption time tc in dependence of concentrations of rapidly adsorbing conditioner. The most interesting regime for particles and conditioner. A polynomial fit of second order visualizes the rough the application is a range of concentrations along the diagonal trend. In accordance with the simple Eq. is mostly inversely proportionalto the conditioner concentration.
of the figure: the particles are completely recharged but there is L. Schaffner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006) 39–50 A simple model system of a laundry machine has been used for systematic investigations of sorption processes on textilestaking place during a common wash process. The adsorptionbehaviour of carboxyle functionalized latex particles to a cel-lulose surface with laundry conditioner as a cationic surfactantwas investigated in dependence of the respective concentrationsof particles and conditioner. While all measurements show thesame qualitative behavior of an initially linear increase of ad-sorbed particles up to a saturation maximum there is a great vari-ation in the observed initial deposition rates as well as the finalcoverages. The adsorption behaviour was shown to be governedby an adsorption competition between particles and conditioner, Fig. 19. Characteristic adsorption time t dominated by the conditioner concentration.
c for conditioner (circles) and recharged latex particles (triangles). Darker triangles are averages of multiple measure- The method provides a powerful tool to determine an es- ments and thus weighted more strongly. The dotted line represents a fit to the timate for the optimum particle to conditioner ratio for maxi- data according to Eq. mal coverage. It allows further to optimize this ratio in termsof performance to material level ratio, i.e. to minimize theamount of both latex particles and conditioner to achieve ac- not yet enough conditioner for the blocking effect to come into ceptable in-use performance. A diagonal line from the lower left to the upper right in such a trade-off sit- However, we also notice that the maximum density of parti- uation. Moderate particle concentrations are already sufficient cles on the surface is about ten times lower than random se- to achieve satisfactory results. Another advantage is that this quential adsorption with an effective hard sphere radius due regime showed the best reproducibility and gives the most sim- to particle-particle repulsion ould predict. While the ilar results along the line (in other words different dilutions particles do repel each other, the strong ion concentration of of this conditioner/latex ratio achieve similar final coverages).
the tap water screens the repulsive potential (Debye length: The triangle in the lower right of the diagram corresponds to 4.8 nm, screening parameter κa: 23.83, effective particle diam- the so-called blocking regime where the competition between eter: 267 nm, expected surface coverage: 40.6%, actually ob- nano-particles and conditioner deposition is in favor of the served maximum surface coverage: 4.65%). This is another con- firmation that the remaining free conditioner always acts in a The results presented in this study show clearly that mod- blocking manner, at least in the concentration window we have erate conditioner and high latex particle concentrations favor a consistently sufficient final coverage, which is the most impor- Finally, we examine the behaviour of the characteristic time tant parameter for the industrial application of these particles tc This quantity is expected to be inversely propor- as an additive to conditioner for the delayed release of active tional to the conditioner concentration (Eq. which agrees well with the measurementxcept for a slight slantof the contours of constant time. To confirm that the condi- tioner is the limiting factor in the adsorption process, we mon-itored the adsorption of the conditioner on its own: The surfac- We would like to thank M. Matthys of Givaudan for the syn- tant adsorbs in the form of aggregates which may incor- thesis work and E. K¨ahenb¨uhl of the University of Bern for the porate a fluorescent dye in their hydrophobic region. Labelling AFM images. We also acknowledge the financial support by the is achieved by adding a small amount of a concentrated Yel- Swiss Innovation Promotion Agency CTI.
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