{"id":69,"date":"2025-07-02T18:05:38","date_gmt":"2025-07-02T18:05:38","guid":{"rendered":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/chapter\/activity-2\/"},"modified":"2026-03-16T14:20:58","modified_gmt":"2026-03-16T14:20:58","slug":"activity-2","status":"publish","type":"chapter","link":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/chapter\/activity-2\/","title":{"raw":"Activity 2 - Surface Potential and Colloidal Stability","rendered":"Activity 2 – Surface Potential and Colloidal Stability"},"content":{"raw":"This activity evaluates the effects of background electrolyte (salt) concentration and a polymer coating on the collloidal stability of citrate-stabilized gold nanoparticles (synthesized in Activity 2<\/strong>). This activity is intended to reinforce the concepts from Chapters 5, 6, and 7<\/strong>. Given a thorough understanding of these concepts, one should be capable of predicting the outcome of the experiments before performing them!<\/span>\n

Materials<\/strong><\/span><\/h3>\n
    \n \t
  • Deionized (DI) water<\/strong><\/li>\n \t
  • Citrate-stabilized gold nanoparticles<\/b>, from\u00a0Activity 1<\/strong> or purchased<\/li>\n \t
  • Polyvinylpyrrolidone (PVP)<\/strong>, K30, 40 kDa, 10 g\/L in DI water<\/li>\n \t
  • NaCl<\/b>, 1 M in DI water<\/li>\n<\/ul>\n

    Initial expectations<\/strong><\/span><\/h3>\nThe chemical structure and pK<\/em>a<\/sub> values for citrate were presented in\u00a0Activity 1\u00a0<\/strong>and are presented again here as\u00a0Figure A2.1<\/strong> for convenience. We will presume the samples are at pH 7, such that the citrate is fully deprotonated.\n\n[caption id=\"attachment_67\" align=\"aligncenter\" width=\"680\"]\"Chemical Figure A2.1.<\/strong> Chemical structure of citric acid and its pK<\/em>a<\/sub> values.[\/caption]\n\nThe chemical structure of the polymer used here, polyvinylpyrrolidone (PVP) is presented in Figure A2.2<\/strong>; as listed above, the average molecular weight is 40 kDa for the PVP used in this activity.\n\n[caption id=\"attachment_67\" align=\"aligncenter\" width=\"121\"]\"Chemical Figure A2.2.<\/strong> Chemical structure of polyvinylpyrrolidone.[\/caption]\n\n
    \n\nConsidering the information above, <\/b>characterize the surface coatings; then predict whether they should provide electrostatic or steric stabilization of the gold nanoparticles.\n\nThe original version of this chapter contained H5P content. You may want to remove or replace this element.\n\nThe original version of this chapter contained H5P content. You may want to remove or replace this element.\n\nThe original version of this chapter contained H5P content. You may want to remove or replace this element.\n\n<\/div>\n

    Procedure<\/strong><\/span><\/h3>\n
      \n \t
    • Prepare three cuvettes, each with 1.6 mL of citrate-stabilized<\/strong> gold nanoparticles.<\/li>\n \t
    • Label the cuvettes as CONTROL<\/strong>,\u00a0CIT<\/strong>, and\u00a0PVP<\/strong>, and prepare as follows:\n
        \n \t
      • CONTROL:\u00a0<\/strong>Add 0.4 mL of\u00a0DI water<\/strong> and mix. This sample will not have any salt added and will serve as a color comparison to evaluate changes in the citrate- and PVP-coated nanoparticles.<\/li>\n \t
      • CIT:<\/strong> Add 0.2 mL of deionized water to the nanoparticles and mix. This sample will have NaCl solution incrementally added (up to 0.2 mL) to observe how the citrate-coated nanoparticles respond to salts.<\/li>\n \t
      • PVP:\u00a0<\/strong>Add 0.2 mL of the PVP solution to the nanoparticles and mix. This sample will have NaCl solution incrementally added (up to 0.2 mL) to observe how the PVP-coated nanoparticles respond to salts.<\/li>\n<\/ul>\n<\/li>\n \t
      • Add the 1 M NaCl to the CIT <\/strong>and PVP<\/strong> cuvettes in aliquots of 40 \u03bcL. The NaCl concentration has been calculated and presented in the table below for an initial volume,\u00a0V<\/em>initial<\/sub>,\u00a0of 1.8 mL for the CIT <\/strong>and PVP<\/strong> samples, and 40 \u03bcL additions of NaCl solution. Record the color of the sample<\/strong> after each addition.<\/li>\n<\/ul>\n\n\n\n\n\n\n\n\n\n
        Aliquot #<\/strong><\/td>\nCumulative volume of NaCl solution,\u00a0V<\/em>1<\/sub> (mL)<\/strong><\/td>\nTotal volume, V<\/em>total<\/sub> = V<\/em>initial<\/sub> + V<\/em>1<\/sub> (mL)<\/strong><\/td>\nNaCl concentration (mM)<\/strong><\/td>\nObservations<\/strong><\/td>\n<\/tr>\n
        Citrate-coated gold nanoparticles<\/strong><\/td>\nPVP-coated gold nanoparticles<\/strong><\/td>\n<\/tr>\n
        1<\/strong><\/td>\n0.040<\/td>\n1.840<\/td>\n21.7<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n
        2<\/strong><\/td>\n0.080<\/td>\n1.880<\/td>\n42.6<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n
        3<\/strong><\/td>\n0.120<\/td>\n1.920<\/td>\n62.5<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n
        4<\/strong><\/td>\n0.160<\/td>\n1.960<\/td>\n81.6<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n
        5<\/strong><\/td>\n0.200<\/td>\n2.000<\/td>\n100<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

        Observations<\/strong><\/span><\/h3>\n
          \n \t
        • Based on changes in the color of the gold nanoparticle samples, note at approximately what NaCl concentration the particles were destabilized:<\/strong>\n
          Expected observations (click to reveal)<\/strong>:<\/summary>\n

          The initial suspension of synthesized gold nanoparticles will have a pink or red color. Figure A2.3<\/strong> below shows the results of the experiment: the citrate-coated gold nanoparticles are destabilized beyond the first aliquot of salt addition (i.e., at salt concentrations higher than \u2248 25 mM NaCl), whereas the PVP-coated gold nanoparticles maintain stability after all five aliquots have been added (> 100 mM NaCl).<\/p>\n \n\n[caption id=\"attachment_68\" align=\"alignnone\" width=\"1024\"]\"Results Figure A2.3.<\/strong> Results of the colloidal stability experiment for the control sample (left) and the citrate-coated gold nanoparticles (middle) and PVP-coated gold nanoaprticles (right) with increasing NaCl additions.[\/caption]\n\n<\/details><\/div><\/li>\n<\/ul>\n

          Brief discussion of the experimental results<\/strong><\/span><\/h3>\nAs discussed in Chapter 6, surface charges on the particle itself or small charged adsorbates are expected to impart only electrostatic forces, whereas polymeric coatings can form a thick adsorbed layer on the particle surface and impart steric forces. The magnitude of the repulsive energy barrier determines whether the particles are likely to agglomerate or remain colloidally stable (separated), as discussed in Chapter 7.\n\nElectrostatic forces are screened upon addition of salts into the solution, resulting in a loss of colloidal stability. This charge screening effect is clearly observed in the experiment, as the citrate-coated gold nanoparticles were destabilized upon addition of relatively low concentrations of NaCl. The agglomeration of the nanoparticles impacts their interaction with light, resulting in the change in color. The agglomerates also settle more rapidly out of suspension.\n\nSteric forces are established by the physical bulk of the coating layer, rather than charge effects. This contrast is also clearly observed in the experiment, as the PVP-coated gold nanoparticles do not show any appreciable destabilization even in high concentrations of NaCl. In fact, you can keep adding as many aliquots of NaCl as you would like, and should not observe destabilization!","rendered":"

          This activity evaluates the effects of background electrolyte (salt) concentration and a polymer coating on the collloidal stability of citrate-stabilized gold nanoparticles (synthesized in Activity 2<\/strong>). This activity is intended to reinforce the concepts from Chapters 5, 6, and 7<\/strong>. Given a thorough understanding of these concepts, one should be capable of predicting the outcome of the experiments before performing them!<\/span><\/p>\n

          Materials<\/strong><\/span><\/h3>\n
            \n
          • Deionized (DI) water<\/strong><\/li>\n
          • Citrate-stabilized gold nanoparticles<\/b>, from\u00a0Activity 1<\/strong> or purchased<\/li>\n
          • Polyvinylpyrrolidone (PVP)<\/strong>, K30, 40 kDa, 10 g\/L in DI water<\/li>\n
          • NaCl<\/b>, 1 M in DI water<\/li>\n<\/ul>\n

            Initial expectations<\/strong><\/span><\/h3>\n

            The chemical structure and pK<\/em>a<\/sub> values for citrate were presented in\u00a0Activity 1\u00a0<\/strong>and are presented again here as\u00a0Figure A2.1<\/strong> for convenience. We will presume the samples are at pH 7, such that the citrate is fully deprotonated.<\/p>\n

            \"Chemical
            Figure A2.1.<\/strong> Chemical structure of citric acid and its pK<\/em>a<\/sub> values.<\/figcaption><\/figure>\n

            The chemical structure of the polymer used here, polyvinylpyrrolidone (PVP) is presented in Figure A2.2<\/strong>; as listed above, the average molecular weight is 40 kDa for the PVP used in this activity.<\/p>\n

            \"Chemical
            Figure A2.2.<\/strong> Chemical structure of polyvinylpyrrolidone.<\/figcaption><\/figure>\n
            \n

            Considering the information above, <\/b>characterize the surface coatings; then predict whether they should provide electrostatic or steric stabilization of the gold nanoparticles.<\/p>\n

            The original version of this chapter contained H5P content. You may want to remove or replace this element.<\/p>\n

            The original version of this chapter contained H5P content. You may want to remove or replace this element.<\/p>\n

            The original version of this chapter contained H5P content. You may want to remove or replace this element.<\/p>\n<\/div>\n

            Procedure<\/strong><\/span><\/h3>\n
              \n
            • Prepare three cuvettes, each with 1.6 mL of citrate-stabilized<\/strong> gold nanoparticles.<\/li>\n
            • Label the cuvettes as CONTROL<\/strong>,\u00a0CIT<\/strong>, and\u00a0PVP<\/strong>, and prepare as follows:\n
                \n
              • CONTROL:\u00a0<\/strong>Add 0.4 mL of\u00a0DI water<\/strong> and mix. This sample will not have any salt added and will serve as a color comparison to evaluate changes in the citrate- and PVP-coated nanoparticles.<\/li>\n
              • CIT:<\/strong> Add 0.2 mL of deionized water to the nanoparticles and mix. This sample will have NaCl solution incrementally added (up to 0.2 mL) to observe how the citrate-coated nanoparticles respond to salts.<\/li>\n
              • PVP:\u00a0<\/strong>Add 0.2 mL of the PVP solution to the nanoparticles and mix. This sample will have NaCl solution incrementally added (up to 0.2 mL) to observe how the PVP-coated nanoparticles respond to salts.<\/li>\n<\/ul>\n<\/li>\n
              • Add the 1 M NaCl to the CIT <\/strong>and PVP<\/strong> cuvettes in aliquots of 40 \u03bcL. The NaCl concentration has been calculated and presented in the table below for an initial volume,\u00a0V<\/em>initial<\/sub>,\u00a0of 1.8 mL for the CIT <\/strong>and PVP<\/strong> samples, and 40 \u03bcL additions of NaCl solution. Record the color of the sample<\/strong> after each addition.<\/li>\n<\/ul>\n\n\n\n\n\n\n\n\n\n
                Aliquot #<\/strong><\/td>\nCumulative volume of NaCl solution,\u00a0V<\/em>1<\/sub> (mL)<\/strong><\/td>\nTotal volume, V<\/em>total<\/sub> = V<\/em>initial<\/sub> + V<\/em>1<\/sub> (mL)<\/strong><\/td>\nNaCl concentration (mM)<\/strong><\/td>\nObservations<\/strong><\/td>\n<\/tr>\n
                Citrate-coated gold nanoparticles<\/strong><\/td>\nPVP-coated gold nanoparticles<\/strong><\/td>\n<\/tr>\n
                1<\/strong><\/td>\n0.040<\/td>\n1.840<\/td>\n21.7<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n
                2<\/strong><\/td>\n0.080<\/td>\n1.880<\/td>\n42.6<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n
                3<\/strong><\/td>\n0.120<\/td>\n1.920<\/td>\n62.5<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n
                4<\/strong><\/td>\n0.160<\/td>\n1.960<\/td>\n81.6<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n
                5<\/strong><\/td>\n0.200<\/td>\n2.000<\/td>\n100<\/strong><\/td>\n?<\/span><\/td>\n?<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

                Observations<\/strong><\/span><\/h3>\n
                  \n
                • Based on changes in the color of the gold nanoparticle samples, note at approximately what NaCl concentration the particles were destabilized:<\/strong>\n
                  \n
                  \nExpected observations (click to reveal)<\/strong>:<\/summary>\n

                  The initial suspension of synthesized gold nanoparticles will have a pink or red color. Figure A2.3<\/strong> below shows the results of the experiment: the citrate-coated gold nanoparticles are destabilized beyond the first aliquot of salt addition (i.e., at salt concentrations higher than \u2248 25 mM NaCl), whereas the PVP-coated gold nanoparticles maintain stability after all five aliquots have been added (> 100 mM NaCl).<\/p>\n

                   <\/p>\n

                  \"Results
                  Figure A2.3.<\/strong> Results of the colloidal stability experiment for the control sample (left) and the citrate-coated gold nanoparticles (middle) and PVP-coated gold nanoaprticles (right) with increasing NaCl additions.<\/figcaption><\/figure>\n<\/details>\n<\/div>\n<\/li>\n<\/ul>\n

                  Brief discussion of the experimental results<\/strong><\/span><\/h3>\n

                  As discussed in Chapter 6, surface charges on the particle itself or small charged adsorbates are expected to impart only electrostatic forces, whereas polymeric coatings can form a thick adsorbed layer on the particle surface and impart steric forces. The magnitude of the repulsive energy barrier determines whether the particles are likely to agglomerate or remain colloidally stable (separated), as discussed in Chapter 7.<\/p>\n

                  Electrostatic forces are screened upon addition of salts into the solution, resulting in a loss of colloidal stability. This charge screening effect is clearly observed in the experiment, as the citrate-coated gold nanoparticles were destabilized upon addition of relatively low concentrations of NaCl. The agglomeration of the nanoparticles impacts their interaction with light, resulting in the change in color. The agglomerates also settle more rapidly out of suspension.<\/p>\n

                  Steric forces are established by the physical bulk of the coating layer, rather than charge effects. This contrast is also clearly observed in the experiment, as the PVP-coated gold nanoparticles do not show any appreciable destabilization even in high concentrations of NaCl. In fact, you can keep adding as many aliquots of NaCl as you would like, and should not observe destabilization!<\/p>\n","protected":false},"author":1,"menu_order":9,"template":"","meta":{"pb_show_title":"","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-69","chapter","type-chapter","status-publish","hentry"],"part":17,"_links":{"self":[{"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/pressbooks\/v2\/chapters\/69","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/wp\/v2\/users\/1"}],"version-history":[{"count":1,"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/pressbooks\/v2\/chapters\/69\/revisions"}],"predecessor-version":[{"id":70,"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/pressbooks\/v2\/chapters\/69\/revisions\/70"}],"part":[{"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/pressbooks\/v2\/parts\/17"}],"metadata":[{"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/pressbooks\/v2\/chapters\/69\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/wp\/v2\/media?parent=69"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/pressbooks\/v2\/chapter-type?post=69"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/wp\/v2\/contributor?post=69"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/wp-json\/wp\/v2\/license?post=69"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}