Arrangements of cationic starch of varying hydrophobicity on hydrophilic and hydrophobic surfaces Katri S. Kontturi a, * , Susanna Holappa a , Eero Kontturi a , Leena-Sisko Johansson a , Sari Hyvärinen b , Soili Peltonen b , Janne Laine a a Laboratory of Forest Products Chemistry, Helsinki University of Technology, P.O. Box 6300, FIN-02015 TKK, Finland b VTT, P.O. Box 21, FIN-05201 Rajamäki, Finland article info Article history: Received 29 December 2008 Accepted 21 March 2009 Available online 31 March 2009 Keywords: Hydrophobized cationic starch Model surfaces Film deposition abstract Arrangements of cationic starches hydrophobized by acetylation (CS-acet) deposited from aqueous elec- trolyte solutions onto hydrophilized and hydrophobized silica surfaces were investigated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. The inﬂu- ence of electrolyte composition, acetylation degree of cationic starch (DS ACET ), and deposition method was examined. On a hydrophobic substrate, CS-acet formed an open ﬁlm with structure strongly dependent on DS ACET and electrolyte composition of the solution. On a hydrophilic substrate, acetylation increased the hydrophobicity of the ﬁlm when the starch was deposited by adsorbing. When deposition was made by spin coating, i.e., the CS-acet was forced onto the substrate, the hydrophobic tuning by acetylation was lost. The ﬁlms deposited by spin coating were thicker and considerably rougher than those deposited by adsorbing. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Biopolymer ﬁlms of various thicknesses and morphologies have recently gained considerable attention. For instance, ﬁlms of chito- san [1–3], cellulose [4–6], cellulose derivatives [7–10], and starch [11–13] for diverse purposes have been widely studied. Chemical modiﬁcation of the biopolymer has a direct effect for ﬁlm forma- tion, both in case of thick ﬁlms [14] as well as in the ultrathin re- gime [8,15]. Film formation depends strongly also on the deposition method. For example, in case of cellulose, ﬁlms depos- ited by coagulation/regeneration [16] are very different from those prepared by spin coating [17] or Langmuir–Blodgett technique [18] and subsequent regeneration. In this fundamental study, the effect of chemical modiﬁcation and deposition method on thin ﬁlm for- mation of cationic starch has been investigated. Starch is a natural and sustainable precursor for surface modify- ing polymers because of its availability, low price, biodegradability, non-toxicity, and chemical modiﬁability [19–21]. It is widely used e.g., in food, medical, textile, and paper industries [19]. Under- standing the fundamental interactions taking place in starch ﬁlm formation is essential for developing further applications for more complex systems. For instance, ultrathin layers of modiﬁed starch have a potential to substitute conventional surface sizing in paper- making [22]. To study the effect of chemical modiﬁcation on ultrathin ﬁlm formation, we have applied three cationic starch derivatives with varying degrees of hydrophobicity as coating materials. Arrange- ments of these starches were studied on hydrophilic and hydro- phobic silica surfaces, which are well-deﬁned substrates in surface science. Two distinct deposition methods, free adsorbing and forced spin coating, are likely to induce different molecular arrangements of the polymer on the substrate. Here, correlations between the molecular arrangements and ﬁlm formation as well as the properties of ﬁlm were evaluated. Also the inﬂuence of added electrolyte in starch solution on ﬁlm formation was studied. 2. Materials and methods 2.1. Materials 2.1.1. Cationic starches Three cationic potato starch derivatives with constant degree of cationization (0.15) and varying degrees of acetylation were used. Potato starch is a (1 ? 4) linked poly-a-D-glucan and is a native blend of linear amylose (25%) and the (1 ? 6) branched amylo- pectin (75%). Due to the high M w of amylopectin, M w of starch may reach tens of millions. Modiﬁcation, such as cationization or acetylation, generally decreases the M w of starch to some extent. The choice of samples in this study is summarized in Table 1. Cationic unacetylated potato starch (CS-acet0) was supplied by Ciba Specialty Chemicals (Raisio, Finland). A batch of cationic 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.03.026 * Corresponding author. Fax: +358 94514259. E-mail address: katri.kontturi@tkk.ﬁ (K.S. Kontturi). Journal of Colloid and Interface Science 336 (2009) 21–29 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis