In the Laboratory
A Lab Experience To Illustrate the Physicochemical Principles of Detergency J. A. Poce-Fatou* Departamento de Química Física, Universidad de Cádiz, Facultad de Ciencias, Polígono del Río San Pedro, 11510 Puerto Real, Cádiz, Spain; *
[email protected] M. Bethencourt-Núñez Departamento de Ciencias de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Universidad de Cádiz, Facultad de Ciencias del Mar y Ambientales, Polígono del Río San Pedro, 11510 Puerto Real, Cádiz, Spain C. Moreno and J. J. Pinto-Ganfornina Departamento de Química Analítica, Universidad de Cádiz, Facultad de Ciencias del Mar y Ambientales, Polígono del Río San Pedro, 11510 Puerto Real, Cádiz, Spain F. J. Moreno-Dorado Departamento de Química Orgánica, Universidad de Cádiz, Facultad de Ciencias, Polígono del Río San Pedro, 11510 Puerto Real, Cádiz, Spain
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Equipment The equipment needed to perform the washing tests consists of a magnetic stirrer with heating, a 500 mL beaker, and a Teflon-coated magnetic stir bar. This simple device has the same basic characteristics as any industrial or domestic washing machine but on a small scale. The washing tests were performed in both distilled water and tap water. We used 2.0 × 2.5 cm white polyester pieces of fabric as the substrate (S) and linseed oil as dirt (D). Linseed oil has a wide absorption band between 425 and 500 nm (Figure 1), so the reflectance of the samples impregnated with this oil, measured at a wavelength inside this range, represents a valid reference to quantify its presence. An Ocean Optics spectrophotometer and integrating sphere were used to measure the reflectance.
100
Reflectivity (%)
Before 1933, most people used soap flakes to clean fabrics. But soap presented a problem. In hard water, it left scum, also referred to as curds in the detergent industry. This scum could not be rinsed off of the clothes. After 1933, scum formation was avoided by substituting the soap with alkyl sulfate surfactants and a substance called builder (1). The main purpose of a builder is to counter the detrimental effects of polyvalent cations that may lead to the precipitation of the surfactant, to increase the efficiency of the surfactants, and to supplement their beneficial effects on soil removal (2, 3). After 1941 sodium tripolyphosphate was widely regarded as the best builder. However since phosphates stimulate growth of algae in wastewaters they have been replaced in most laundry detergents by the sodium form of zeolite A (4). In this article we present an experimental procedure to analyze the role of a surfactant (sodium dodecyl sulfate, SDS) and a builder (zeolite A) in laundry detergent efficiency. To study the surfactant activity we performed washing tests in distilled water using different concentrations of the surfactant and to analyze the role of the builder the tests were repeated in tap water containing zeolite A and various surfactant concentrations. Efficacy was assessed by diffuse reflectance measurements of a polyester cloth impregnated with linseed oil and washed. The experimental results have been discussed from a physicochemical point of view and can be used as complementary material in the introduction or development of concepts typically included in a lecture on surface chemistry such as surfactant, adsorption, surface tension, work of adhesion, Gibbs isotherm, and electrical double layer, emulsion, or micelle. To expand this experiment we have included detailed information about the syntheses of the organic and inorganic components of the detergent, measurements of water hardness and surface tension, and the mathematical treatment of experimental errors in the online supplement.
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Wavelength / nm Figure 1. Diffuse reflectance spectrum of a white polyester piece of fabric impregnated with linseed oil.
Journal of Chemical Education • Vol. 85 No. 2 February 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
Procedure
Hazards
The polyester samples were soaked in linseed oil for 1 minute and the excess linseed oil was removed by blotting with filter paper. The reflectance of the “dirty” sample (RD) was measured at 456.8 nm with an integrating sphere. Then the polyester sample was washed for 15 minutes in a stirred bath containing either surfactant or surfactant and builder at 25 °C. When the washing was finished, the sample was taken out, rinsed with distilled water, and dried using a hairdryer. The reflectance of the sample was measured again to obtain RC, the reflectance of the “cleaned” sample. The detersive efficiency, DE, was estimated by
Sodium dodecyl sulfate and zeolite A cause eye and skin irritation and may cause respiratory tract irritation. Sodium dodecyl sulfate is a flammable solid.
DE
RC R D R * RD
10 0%
(1)
where R* (=100) is the diffuse reflectance of the white polyester fabric before treatment with linseed oil. We analyzed the influence of the SDS mass concentration in the range 0.00–0.70% with steps of 0.05%. The surfactant was dissolved in 200 mL distilled water using magnetic stirring and ultrasound for 15 minutes. Then the linseed-impregnated polyester cloth sample added and washed using magnetic stirring for 15 minutes. To analyze the builder, a 1 g zeolite A suspension was prepared in 200 mL tap water with constant stirring for 5 minutes prior to the addition of SDS and the dirty cloth. According to the measurements conducted to determine the hardness of tap water (see the online supplement) with this zeolite mass and this period of time, the hardness decreased 97.3%, which practically guarantees the absence of precipitation problems with the surfactant. Reflectance measurements were carried out in triplicate, and for each SDS concentration, three independent washing tests were performed. The results obtained in these tests are shown in Figure 2.
Results and Discussion The main aim of the detersive process is to separate the dirt (D) from a substrate (S). This operation implies an energetic cost that, from a thermodynamical point of view, can be denoted as the work of adhesion (Wad), Wad H BD H BS H DS (2) where γBD, γBS, and γDS are the interface tensions of the boundaries made up by D, S, and the bath (B) (5, 6). If we assume that the solutions we have used to wash the samples are ideally dilute, we can introduce another useful thermodynamic tool to analyze the experimental results, Gibbs isotherm, vH vc
50 40 30
tap water without zeolite
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distilled water tap water + zeolite A
10 0 0.0
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Mass Concentration SDS (%) Figure 2. Detergency data obtained at 456.8 nm for samples washed in distilled water and tap water. Solid points represent the average of three independent washing tests.
Surface Tension / (mN/m)
Detergent Efficiency (%)
60
(3)
tap water without zeolite
80
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T
(RT c
where c is the surfactant concentration, R is the gas constant, T is the temperature, and Γ the surface excess, a positive value for solutes accumulating preferentially in the interface (7, 8). As surfactants have amphipathic structures, SDS accumulates into the BD and BS interfaces. This implies a decrease in the corresponding interface tensions (eq 3) and as a consequence a decrease in Wad (eq 2), which explains the increase observed in DE along with the increasing concentration of SDS. The surface tensions (γAB, where A means air) of SDS solutions prepared in distilled water and tap water with zeolite were measured using the drop weight method as shown in Figure 3
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distilled water tap water + zeolite A
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Mass Concentration SDS (%) Figure 3. Surface tension (γAB) of SDS solutions prepared in distilled water and tap water.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 2 February 2008 • Journal of Chemical Education
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In the Laboratory
(details in the online supplement). Although γAB does not appear in the definition of Wad, the AB interface is similar to the BD and BS interfaces because they all separate a hydrophilic medium from a hydrophobic one. As SDS also accumulates in this type of interface the information provided by γAB may be considered as an approximation of the γBD and γBS values in eq 2. Nevertheless, although the decreases of γAB in tap and distilled water are similar and imply logical increases in the detersive efficiencies showed in Figure 2, DE evolves in different ways depending on the type of water. Once the dirt has been detached from the substrate, the linseed oil stays in suspension forming an emulsion. In contrast with distilled water, tap water contains inorganic electrolytes. The limited adsorption of anionic species at the oil–water interface stabilizes its weak electrical double layer. This retards the coalescence of individual drops (9) and explains the detersive activity detected in the samples washed in tap water without SDS and without zeolite (Figure 2). Detersive activity was also observed in the sample with zeolite and no SDS. To understand why a detersive activity takes place again in spite of the absence of a surfactant we simply have to consider the fact that the very finely divided solid particles of zeolite also contribute to the stabilization of the linseed oil emulsion in water. Emulsion stabilization by solid particles relies upon the specific location of the particles at the interface to produce a strong, rigid barrier that prevents or inhibits the coalescence of drops that may also impart a degree of electrostatic repulsion that enhances the overall stabilizing power of the system (3, 9). This explanation also accounts for the high DE obtained in tap water relative to the values obtained in distilled water below a SDS concentration of 0.4%. Examining the data in Figure 2 we can appreciate how DE seems to stabilize near a maximum value as the SDS concentration approaches 0.70%. This fact is related to the minimization of γBD and γBS and to the presence in the solution of a molecular aggregate called micelle, formed by surfactant monomers that do not accumulate in interfacial regions but in the bulk. The presence of these micelles represents a limit to the decrease obtained by Wad. For that reason, contrary to what one might think, micelles are a detriment more than a benefit for obtaining better values of DE (3, 7, 10).
The experimental device and the methodology explained in this article adapt easily to modifications. In this way we can analyze the influence of parameters other than the surfactant concentration, for example, bath temperature, stirring intensity, washing time, type of surfactant, type of dirt, type of substrate, nature of its weave. As noted before, these changes will depend on the learning context but also on the instructor’s own inventiveness to modify and improve this contribution. As detergency is a phenomenon to which we are exposed in our daily lives, we think that this experience can be also appreciated by students as an intriguing, attractive, and important challenge, which forms a significant part of a Natural Critical Learning environment (11).
Conclusions
http://www.jce.divched.org/Journal/Issues/2008/Feb/abs266.html
The experimental results obtained in this article have been explained using physicochemical principles. For this reason this experience could be useful as a complement to a lecture on surface chemistry. Additional activities such as the syntheses of the detergent components, measurements of water hardness and surface tension, and the mathematical treatment of experimental errors are included in the online supplement. This complementary information can be used totally, partially, or even ignored, to achieve the objectives established by the instructor beyond the area of physical chemistry.
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Literature Cited 1. Raber, L. Chem. Eng. News. 2006, 84 (47), 92; http://pubs.acs.org/ cen/acsnews/84/8447news2.html (accessed Oct 2007). 2. Kevelam, J. Polymer-Surfactant Interactions: Aqueous Chemistry of Laundry Detergents. Universiteitsbibliotheek Groningen. 1998; pp 4–7. http://irs.ub.rug.nl/ppn/173813488 (accessed Oct 2007). 3. Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989; pp 1, 305, 371–372. 4. Smoot A. L.; Lindquist D. A. J. Chem. Educ. 1997, 74, 569– 570. 5. Poce-Fatou, J. A. J. Chem. Educ. 2006, 83, 1147–1151. 6. Shaw, D. J. Colloid and Surface Chemistry, 4th ed.; ButterworthHeinemann Ltd: Oxford, 1992; pp 93–94. 7. Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: 1990; pp 75–84, 513. 8. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry, 7th ed.; Oxford University Press: Oxford, 2002; p 759. 9. Myers, D. Surfactant Science and Technology, 2nd ed.; VCH Publishers, Inc.: New York, 1992; pp 213–215. 10. Schramm, L. L.; Stasiuk, E. N.; Marangoni, D. G. Annu. Rep. Prog. Chem., Sect. C 2003, 99, 3–48. 11. Bain, K. Lo que Hacen los Mejores Profesores Universitarios; Publicacions de la Universitat de València: València, Spain, 2005; pp 114–124 (in Spanish). Bain, K. What the Best College Teachers Do; Harvard University Press: Cambridge, MA, 2004.
Supporting JCE Online Material Abstract and keywords
Full text (PDF) Links to cited URLs and JCE articles Supplement Details on the syntheses of organic and inorganic detergent components Analytical techniques to measure water hardness Surface tension measurements Mathematical treatment of experimental errors
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