Table 4.

Theoretical and experimental surface concentrations Γ and number z of adsorbed silica layers calculated from QCM-D and FTIR-ATR data (±σ)

	QCM-D
	Mass, m [ng cm−2]	Thickness, d [nm]	Surface concentration, Γtheor [mol cm−2] (MSiO2 = 60.1 g mol−1)	Surface concentration, Γtheor [mol cm−2] (MH4SiO4 = 96.1 g mol−1)	Number zQCM-D of adsorbed Si—O—Si layers
pH 7.2, K2HPO4/KH2PO4
EDC	1355 ± 32	6.8 ± 0.9	22.5 × 10−9	14.1 × 10−9	21.5 ± 0.5
EDC/EDA	502 ± 14.2	2.5 ± 0.4	8.35 × 10−9	5.22 × 10−9	8.0 ± 0.5
Non-activated	370 ± 10.6	1.9 ± 0.3	6.16 × 10−9	3.85 × 10−9	5.5 ± 0.5
pH 7.2, KOH
EDC	219 ± 5.3	1.2 ± 0.2	3.64 × 10−9	2.28 × 10−9	4
EDC/EDA	142 ± 3.5	0.6 ± 0.1	2.36 × 10−9	1.48 × 10−9	2
Non-activated	46 ± 1.8	0.2 ± 0.1	0.77 × 10−9	0.48 × 10−9	1.0
			FTIR-ATR
			Surface concentration, Γtheor [mol cm−2]	Surface concentration, Γexp [mol cm−2]	Number zQCM-D of adsorbed Si—O—Si layers
pH 7.2, K2HPO4/KH2PO4
Non-activated			9.04 × 10−9	3.69 × 10−9 (⊥)	4.0 ± 0.5
				3.87 × 10−9 (II)

As basis for the surface concentration Γ a densely spherical package of SiO₄ tetrahedra with a Si—O—Si distance of 0.3058 nm (r_sphere = 0.1529 nm) was assumed.

QCM-D: surface concentration Γ_theor = m/M, number of adsorbed Si—O—Si layers zQCM-D = d/2r_Si—O (r_Si—O = 0.1529 nm).

FTIR-ATR: surface concentration Γ_theor = 4/(N_a·a²) with a²: smallest unit cell (a² = Π·r², r_Si—O = 0.1529 nm), N_a: Avogadro constant, surface concentration Γ_meas = A·d/(ε·N·ν·d_e) with A: integrated absorbance of the Si—O—Si band, d: thickness of the adsorbed layer, ε: molar extinction coefficient, N: middle number of intern active reflections, ν: number of functional groups/molecule, d_e: penetration depth. The molar extinction coefficient ε was calculated from transmission measurements of a (SiO₂)_x·yH₂O gel with the help of the Lambert–Beersche law A = ε·c·d_Fringes.c: concentration of methanol, d_Fringes: cell thickness determined by “Fringes” [32], number of adsorbed Si—O—Si layers z_FTIR = Γ_exp/Γ_theor.